Abstract

We experimentally investigate crystals doped with tetravalent chromium or divalent cobalt as saturable absorbers for passive Q-switching of visible solid-state lasers. The recovery time of the ground-state and excited-state absorption cross sections are determined by pump-probe and Z-scan measurements, respectively. We provide saturation intensities, useful wavelength ranges of the investigated materials, and advices to realize passive Q-switching of visible lasers using these crystals as saturable absorbers.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction

Visible lasers are of considerable interest for laser displays with great color reproduction characteristics [1], ophthalmic medical treatments [2,3], and processing of metals with reduced reflectivity in the visible such as copper and gold [4]. Solid-state lasers directly emitting in the visible based on rare-earth-doped gain media have been recently developed [5]. This progress was driven by the advance of novel blue emitting pump sources, such as indium-gallium-nitride (InGaN) laser diodes (LDs) [6] and frequency-doubled optically pumped semiconductor lasers (2ω-OPSLs) [7]. Crystals doped with praseodymium (Pr) are currently the leading laser materials for direct visible emission because of their strong emission at many lines in the visible region. Many groups reported Pr lasers using the above-mentioned blue pump sources [825]. In addition to Pr, fluoride crystals doped with trivalent terbium (Tb) pumped by 2ω-OPSLs [2628] or laser diodes [29] enabled efficient green and, more interestingly, yellow laser operation.

For NIR lasers passive Q-switching is well established. However, there are limited materials recognized as saturable absorbers for the visible region, since the history of directly emitting visible solid-state lasers is relatively short compared to that of NIR lasers. We summarized the parameters of the materials previously reported to be suitable saturable absorbers in the visible region in Table 1 [3041].

Yttrium aluminum garnet (Y3Al5O12, YAG) doped with tetravalent chromium (Cr4+) is a representative Q-switching material for 1-µm lasers [42]. Many groups intensively studied its saturation characteristics [4353] and demonstrated e.g. microchip Nd3+:YAG/Cr4+:YAG lasers generating high peak power pulses [5456]. We previously reported the saturation parameters of the visible absorption in Cr4+:YAG, and demonstrated passive Q-switching of a diode-pumped Pr:LiYF4 (YLF) laser at 640 nm [30]. Spinel (MgAl2O4) crystals doped with divalent cobalt (Co2+) are known as saturable absorbers for 1.5-µm lasers, e.g. based on Er3+ [57,58]. Demesh et al. reported on the saturation of the visible absorption in Co2+:MgAl2O4 31]. This saturable absorber Q-switched Pr:YLF lasers at 523, 607, and 640 nm pumped by 2ω-OPSL [31] or blue LDs [32]. GaInP-based SESAMs enabled Q-switching, as well as mode-locking, of Pr lasers [3335]. However, these SESAMs showed relatively large non-saturable losses in the visible region compared to those for the NIR region. CdTe/CdS-quantum dots (QDs) also exhibit saturable absorption for visible light. Xu et al. applied these QDs for passive Q-switching of Pr:YLF lasers at 607, 640, and 720 nm, and obtained pulses of below 300 ns duration [36]. Graphene, in theory, shows saturable absorption at any wavelength due to its zero-bandgap structure [59], often referred to as ‘Dirac cones’. Since the saturation intensity or fluence significantly increases for shorter wavelengths, the application of graphene as a saturable absorber has been mostly limited to NIR lasers. Yamashita et al. presented an empirical law stating that the saturation intensity of monolayer graphene is proportional to λ-6, where λ is the wavelength [60]. Despite the resulting very high saturation intensity of graphene in the visible region, Kajikawa et al. reported Q-switching of a Pr3+ doped fluoroaluminate fiber laser using few-layer graphene [37]. Transition metal dichalcogenides (TMDs) exhibit a wide direct bandgap resonantly absorbing in the NIR to visible region [61]. There are reports on Q-switching of Pr:ZBLAN fiber lasers at 635 and 607 nm using TMDs in a polyvinyl alcohol (PVA) film [38,39]. Gold nanoparticles and black phosphorus are also suitable saturable absorbers to Q-switch Pr fiber lasers at 635 nm [40,41]. It should be noted, that the above-mentioned reports on passively Q-switched visible lasers are all based on fluoride crystals or glasses doped with Pr. There are no reports on Q-switched Tb3+ lasers which can potentially generate high energy pulses thanks to their very long upper state lifetime, typically ≈5 ms in fluorides [29].

In this paper, we present the characterization of transition-metal-doped crystals as saturable absorbers in the visible region. Crystals exhibit higher damage thresholds than films or layers; therefore, they sustain higher pulse energies and peak power during Q-switching. Crystalline saturable absorbers allow very good control of the parameters determining the performance of passively Q-switched lasers: the initial transmission and the modulation depth. These parameters can be tailored just by adapting the absorber’s length and/or the doping concentration of transition metal ions, while the control of these parameters in thin-film or thin-layer saturable absorbers is more challenging. To the best of our knowledge, Cr4+:YAG and Co2+:MgAl2O4 are the only crystalline saturable absorbers applied in the visible region reported so far. Transition metal ions in a variety of hosts exhibit different saturation characteristics since the crystal field of the host strongly influences their unshielded 3d-electrons. We investigated saturable absorption of Cr4+ in YAG and Mg2SiO4 (forsterite) as well as Co2+ in MgAl2O4, ZnGa2O4, LiGa5O8, YAG, and Gd3Ga5O12 (GGG). We focused on the characteristics at the three major emission wavelengths of Pr:YLF lasers at 523, 607, and 640 nm, as well as the green emission wavelength of Tb doped fluorides at 545 nm to investigate the applicability of these crystals for passive Q-switching.

Tables Icon

Table 1. Overview of previously reported saturable absorbers for visible lasers.

2. Methods

Dynamics of saturable absorbers are in general described by a four-level model schematically shown in Fig. 1(a). This model is strictly valid only if there is no intermediate level between the ground and excited states, i.e. a single exponential decay of the population change in the excited state occurs. The four-level system has three parameters: recovery time of the ground state τ (referred to as ‘recovery time’ in the following), ground-state absorption (GSA) cross section σgs, and excited-state absorption (ESA) cross section σes. The transmission of a saturable absorber is then given as:

$$T = \exp [{ - ({{\sigma_{gs}}{n_{gs}} + {\sigma_{es}}{n_{es}}} ){l_{SA}}} ]$$
where nes is the population density in the excited state and lSA is the thickness of the saturable absorber. The relaxation from the meta-stable level E3 to the excited state E2 is assumed to be very fast, thus the population density of the meta-stable level E3 is always considered to be zero. The lifetime of the upper level E4 is considered to be infinitely short. Therefore, a non-zero σes represents residual absorption, i.e. non-saturable losses, which cannot be bleached by optical excitation. The initial transmission, or small-signal transmission, T0 and the saturated transmission Tsat, corresponding to the transmission when perfectly bleached, are respectively written as follows:
$${T_0} = \exp ({ - {\sigma_{gs}}{n_{tot}}{l_{SA}}} )$$
$${T_{sat}} = \exp ({ - {\sigma_{es}}{n_{tot}}{l_{SA}}} )$$
where ntot is the total density of the absorption centers.

 figure: Fig. 1.

Fig. 1. (a) Four-level model of a saturable absorber. (b) Experimental setups of pump-probe and Z-scan measurements.

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We measured the recovery time of the saturable absorbers by the pump-probe technique schematically shown in Fig. 1(b). The pump laser was a 10-Hz optical parametric oscillator (OPO) (versaScan, GWU-Lasertechnik) tunable from 410 to 2550 nm. The pulse energy of the OPO was more than 10 mJ and the pulse duration was ≈5 ns in the wavelength range of 400–700 nm. We used a continuous-wave (CW) helium-neon (HeNe) laser at 632.8 nm as the probe because all the investigated crystals exhibit a GSA at this wavelength. We focused the pump and probe beams spatially overlapping into the crystal. We kept the probe beam smaller than the pump beam to ensure a strong modulation of the probe beam, which was detected by a fast Si-photodetector (PD). A narrow bandpass filter transmitting only at 632.8 ± 1.5 nm suppressed unwanted scattered or reflected pump light. The time-dependent transmission of a saturable absorber is expressed as follows:

$$T(t )= \exp \left[ {\left\{ { - {\sigma_{gs}}({{n_{tot}} - {n_{es}}({t = 0} )} )\exp \left( { - \frac{t}{\tau }} \right) - {\sigma_{es}}{n_{es}}({t = 0} )\exp \left( { - \frac{t}{\tau }} \right)} \right\}{l_{SA}}} \right].$$
This equation considers the optical excitation taking place instantaneously. The first and second term on the right side represent the GSA and ESA, respectively. Equation (4) can be rewritten as follows:
$$\ln \left[ {\ln \left( {\frac{{T(t)}}{{{T_0}}}} \right)} \right] ={-} \frac{t}{\tau } + \ln [{({{\sigma_{gs}} - {\sigma_{es}}} ){n_{es}}{l_{SA}}} ].$$
This equation shows that the recovery time can be determined from the slope −1/τ of ln[ln(T(t)/T0)].

We performed Z-scan measurements with numerical fits to determine GSA and ESA cross sections of the saturable absorbers under investigation (see Fig. 1(b)). Again, we used the OPO as the pump source. A 1:1 beam splitter (BS) split the pump beam, and one part was directed to an energy meter (PE25-C, Ophir) to monitor fluctuations of the pump pulse energy. The other part was focused into the crystal, and a second energy meter measured the energy of the transmitted pulse. A motor-controlled stage (HPS-170, PI miCos) translated the crystal along the optical axis to vary the input pulse fluence. We recorded the transmission-versus-position curve (hereafter called ‘Z-scan curve’) for every 0.5-mm step using an automated LabVIEW (National Instrument) program. The program averaged the energy for hundred pulses at each position to suppress noise caused by pump energy fluctuations.

The GSA and ESA cross sections were determined by fitting the Z-scan curve with the model described in [50]. The model regards the spatial profile of the pump beam to be elliptical Gaussian because the model overestimates ESA cross sections if a top-hat beam profile is assumed [50]. The variation of the pump beam size in the crystals was taken into account, but the crystals were significantly shorter than the confocal length of the pump beam. Therefore, the fluence could be defined as the pulse energy divided by the beam cross section area averaged among a crystal. We separately measured the small signal absorption coefficient α by a double-beam absorption spectrometer (LAMBDA1050, Perkin Elmer) because the pump pulses in the Z-scan measurements were too energetic to measure the small signal transmission. We defined the figure-of-merit (FOM) of saturable absorbers as the ratio of GSA and ESA cross sections σgs/σes. An ideal saturable absorber with zero residual absorption has then a FOM of infinity.

3. Results and discussion

3.1 Tetravalent chromium-doped crystals as saturable absorbers

The first material system under investigation was Cr4+:YAG. Its lattice provides two different Al3+ sites: 60% of the Al3+ ions occupy tetrahedral sites, the remaining are found on octahedral sites. Cr ions substitute for the Al3+ on both sites. Since the Cr ions form Cr3+ at the Al3+ sites in the absence of oxygen vacancies, divalent charge compensating ions, usually Ca2+ or Mg2+ substituting respectively for the Y3+ or Al3+ ions, are codoped to enhance the formation of Cr4+. We prepared two (111)-cut Cr4+:YAG crystals: a 0.80 mm long Cr4+(0.08%):YAG sample and a 0.66 mm long Cr4+(0.11%):YAG sample. The doping levels were calculated by the absorption coefficient at 1064 nm and the GSA cross section of 6.1×10−19 cm2 reported in [50]. Figure 2 shows the absorption spectra of both Cr4+:YAG samples. The broad absorption band centered at 1 µm corresponds with the well-known transition of Cr4+ on tetrahedral sites (upward black arrow in the energy diagram of Cr4+:YAG in Fig. 3). Cr4+ ions also exhibit strong absorption peaks at 615 and 647 nm (upward orange-red arrow in Fig. 3), and passive Q-switching of Pr:YLF lasers in this range was demonstrated using Cr4+:YAG [30]. Although the absorption around 1 µm is purely owing to Cr4+, the visible absorption is due to three different types of Cr ions: Cr3+ and Cr4+ on octahedral sites (Cr3+[Oh] and Cr4+[Oh]) as well as Cr4+ on tetrahedral sites (Cr4+[Td]). Cr3+[Oh] exhibits absorption around 600 nm, which overlaps with the Cr4+[Td] absorption in the orange-red region [51]. The strong absorption below 500 nm can be attributed to Cr4+[Oh], and its broad tail at the longer wavelength side is still not zero in the orange-red region. Thus, the presence of Cr3+[Oh] and Cr4+[Oh] gives rise to unwanted absorption not contributing to the saturable absorption process, which apparently increases the non-saturable losses. Since the amount of these unwanted absorption centers depends on the growth conditions as well as the post-growth treatment, e.g. annealing, the absorption spectra of Cr4+:YAG crystals are in general different. To estimate the amount of absorption due to Cr4+ [Td], we resolved the absorption spectra using the data of the peak positions and widths due to Cr4+[Td] and Cr4+[Oh] presented in [51]. Note that the absorption of Cr3+[Oh] was subtracted in the data reported here. We determined the amount of Cr4+[Td] by fitting the absorption around 1 µm, and found that the measured orange-red absorption was stronger than that expected by the data in [51]. Thus, both Cr4+:YAG samples contain a non-negligible visible absorption due to Cr3+[Oh]. Table 2 lists the estimated absorption coefficients at 607 and 640 nm due to Cr3+ and Cr4+ for the two samples.

 figure: Fig. 2.

Fig. 2. Absorption spectra of the two Cr4+:YAG samples and a Cr4+:Mg2SiO4 (forsterite) sample. The Fresnel reflections on the uncoated facets were taken into account using the Sellmeier equations of undoped YAG [68] and undoped forsterite [69].

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 figure: Fig. 3.

Fig. 3. Energy diagrams of tetrahedrally coordinated Cr4+ in YAG (D2d symmetry when the closest ligands are only taken into account) [51] and forsterite (Cs symmetry) [70], as well as tetrahedrally coordinated Co2+ in spinel or garnet crystals [76].

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Tables Icon

Table 2. Determined parameters of Cr4+:YAG and Cr4+:forsterite crystals.

Tables Icon

Table 3. Determined parameters of Co2+ doped MgAl2O4 spinel crystal.

The ground-state recovery time of Cr4+(0.08%):YAG and Cr4+(0.11%):YAG samples was estimated to be 3.8 ± 0.6 µs and 3.9 ± 0.9 µs when excited at 607 nm, respectively (see Fig. 4(a) for Cr4+(0.08%):YAG). We did not observe a visible change in the recovery time when exciting at 640 nm. The slope −1/τ was determined by fitting to the range except the first 1 µs from the excitation since the decay curves deviated from an exponential function due to a fast quenching observed just after the excitation. The quenching was more pronounced in the sample of higher doping concentration, Cr4+(0.11%):YAG. These recovery times are in good agreement with the fluorescence lifetime of the NIR emission around 1.4 µm, and we could not detect any fluorescence in the visible region; therefore, the excited ions non-radiatively relax to the 3B2(3T2) level and then relax to the ground state by emitting in the NIR. We tuned the excitation wavelength to determine the applicable wavelength range for passive Q-switching. Figure 4(b) shows the transmission change of the Cr4+(0.08%):YAG sample for different pump wavelength between 580 and 430 nm. We conclude that Cr4+:YAG is practically useful at wavelengths down to ≈580 nm because of a strong transient color center formation below this wavelength. The long-time-window transmission change shown in Fig. 4(c) reveals the lifetime of this color center to be as long as ≈7 ms. Except the range between 520 nm and 530 nm, bleaching was still observed at shorter wavelengths, where the color formation was clearly visible (see Fig. 4(b)). Even at 450 nm saturable absorption was detected. This observation is consistent with a dip around 525 nm in the excitation spectrum of Cr4+:YAG presented by K­ück et al. [62].

 figure: Fig. 4.

Fig. 4. (a) Time-dependent normalized transmission of Cr4+(0.08%):YAG and Cr4+:forsterite, pumped at 607 and 570 nm, respectively. The curve of Cr4+:forsterite is also shown ten-times magnified in the linear scale. (b) Time-dependent transmission change of Cr4+(0.08%):YAG when pumped at wavelengths between 580 and 430 nm. (c) Time-dependent transmission change of Cr4+(0.08%):YAG pumped at 430 nm recorded over 15 ms.

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Figure 5(a) shows the saturation curves, i.e. fluence vs. transmission, of the Cr4+:YAG samples at 607 and 640 nm. Both samples exhibit large non-saturable losses, as these saturation curves converge well below 50%. One of the reasons is the existence of unwanted absorption centers, namely, Cr3+[Oh] and Cr4+[Oh]. We determined the GSA and ESA cross sections by numerical fitting to these saturation curves. The resulting parameters minimizing the fitting error are summarized in Table 2. To evaluate the saturation properties of Cr4+ ions, we excluded the absorption due to Cr3+. For example, in the Cr(0.08%):YAG sample, we found that ≈40% of the peak absorption coefficient of 17.9 cm−1 at 607 nm can be attributed to Cr3+ absorption. This means, the non-bleachable Cr3+ absorption causes significant losses in this sample, reducing the saturated transmission to below 60%. We found the GSA cross sections of the two Cr4+:YAG samples to be almost identical, but the ESA cross sections of the Cr4+(0.11%):YAG sample were 30–50% larger than for the lower doped Cr4+(0.08%):YAG sample. The difference in ESA cross sections might be due to the ratio of Cr4+[Oh] and Cr4+[Td] ions, but also, partially, the inaccurate evaluation of the Cr3+ density. Note that the inaccuracy in the Cr3+ density only influences the determination of ESA cross sections, not GSA cross sections. As a consequence, to achieve a suitable FOM of Cr4+:YAG saturable absorbers for the visible, great care has to be taken on growth conditions and post-growth processing to reduce the amount of detrimental Cr3+ and Cr4+[Oh]. Note that a thin slice of ∼100 µm needs to be prepared if we apply the investigated samples for Q-switching experiments.

 figure: Fig. 5.

Fig. 5. Saturation curves of (a) Cr4+:YAG and (b) Cr4+:forsterite crystals.

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It is well-known that the 1-µm absorption in Cr4+:YAG exhibits a polarization dependence due to the local symmetry of the Al3+ on tetrahedral sites being reduced from Td to S4. The CrO42- tetrahedrons stretched along <100> show different optical response [43]. We could, however, not observe the polarization dependence in the Z-scan measurement, since we used (111)-cut crystals in which the dependence is much less pronounced.

Mg2SiO4 (forsterite) doped with Cr4+ is a laser gain medium emitting at ≈1.2 µm [63], and also enables passive Q-switching of 1-µm lasers [50,64]. Forsterite crystals comprise Mg2+ on octahedral sites and Si4+ on tetrahedral sites. Cr ions substitute for both sites and respectively form Cr3+[Oh] and Cr4+[Td] [65], despite the large differences in ionic radii [71]. We prepared a 1.08 mm Cr4+:forsterite sample cut perpendicular to the c-axis. Figure 2 shows its absorption spectra for E||a- and E||b-polarization. Cr4+[Td] exhibits absorption peaks at 570 nm (E||a) and at 730 nm (E||b). The weak but broad absorption around 460 nm and the broad tail in the absorption for E||a at 600–800 nm were assigned to Cr3+[Oh] with inversion and mirror symmetry, respectively, according to the site-selective excitation spectroscopy in [65].

The recovery time of Cr4+:forsterite when pumped at 570 nm (E||a) was measured to be 2.6 ± 0.1 µs as shown in Fig. 4(a), which is in reasonable agreement with the fluorescence lifetime of the 1.2 µm emission [66]. The excited ions may undergo a fast non-radiative relaxation to the NIR emitting level, thus the fluorescence lifetime of the emission mainly determines the recovery time of the ground state.

We performed Z-scan measurements for E||a-polarization at 523, 545, and 570 nm (see Fig. 5(b). Bleaching was observed for all wavelengths and the fit-results are summarized in Table 2. The ESA cross sections were determined to be 1.1–1.4 × 10−18 cm2, corresponding to a saturated transmission of ≈83% at all three wavelengths. As the estimated ESA cross sections do not strongly depend on the wavelength, we attribute the residual absorption to Cr3+[Oh], not contributing to the saturable absorption action. Cr4+:forsterite exhibited the highest FOM at the absorption peak, but similar to the Cr4+:YAG crystals, the saturation characteristics of Cr4+:forsterite should also depend on the growth conditions and post-growth treatment reducing the amount of Cr3+. Chen et al. studied the formation mechanisms of Cr4+ and Cr3+ in forsterite and stated that the amount of Cr3+ can be suppressed in low Cr doped crystals [67].

We also investigated the saturation characteristics of Cr4+:forsterite for E||b-polarization at 640 nm. The GSA and ESA cross sections were determined to be (0.95 ± 0.15) × 10−18 cm2 and (0.55 ± 0.15) × 10−18 cm2, respectively. Even stronger saturation was observed for longer wavelengths, e.g. 720 nm. For passive Q-switching laser in the orange-red region, E||c polarization seems to be best suited due to its strong absorption peaking at 650 nm (upward red arrow in Fig. 3) [70], however, no samples with access to E||c polarization were available for our experiments.

3.2 Divalent cobalt-doped crystals as saturable absorbers

We also prepared samples of Co2+ doped MgAl2O4, ZnGa2O4, LiGa5O8, YAG, and GGG. The thickness of the samples was 2.98, 1.30, 0.75, 3.05, and 0.60 mm, respectively. These crystals were polished and uncoated. All crystals possess a cubic lattice. The first three crystals have spinel structure and the others have garnet structure.

A spinel structure crystal provides trivalent octahedral and divalent tetrahedral cation sites. In the case of MgAl2O4 and ZnGa2O4, the octahedral sites are occupied by Al3+ and Ga3+ and the tetrahedral sites by Mg2+ and Zn2+, respectively. Doped Co ions substitute for both sites in the respective ionization state, thus Co3+[Oh] and Co2+[Td] ions are formed, respectively. The small mismatch in their ionic radii with the substituted ions [71] makes both configurations likely. In LiGa5O8, more precisely written as Ga2(Li,Ga3)O8, the tetrahedral sites are occupied by Ga3+, while the octahedral sites are shared by Li+ and Ga3+ with a ratio of 1:3. To distinguish from normal spinels, the structure of LiGa5O8 is called inverse-spinel. Co ions in LiGa5O8 substitute for octahedral and tetrahedral Ga3+, but not for octahedral Li+ due to the large mismatch in their ionic radii. To support the formation of Co2+ on the trivalent sites, Si4+ was codoped for charge compensation.

In the cubic garnet structure, doped Co ions substitute for octahedral and tetrahedral Al3+ and Ga3+, and Co3+[Oh] and Co3+[Td] are formed. Also in the garnets, Si4+ was codoped for charge compensation.

Figure 6 shows the absorption spectra of the Co2+ samples. In all cases, the broad NIR and visible absorptions correspond to the transitions 4A2(4F)→4T1(4F) and 4A2(4F)→4T1(4P), respectively (see Fig. 3, right). Only in the spectrum of Co2+:YAG, we recognized additional small absorption centered around 700 nm and 1050 nm. This is due to residual Co3+ [72]. According to the Tanabe-Sugano diagram [73] the positions of the absorption peaks of transition metals are strongly influenced by the crystal field strength. In a crystalline host with a stronger crystal field, the energy levels blue-shifted. We thus conclude, that LiGa5O8 has the weakest crystal field among the investigated crystals. The position of the NIR absorption band may reflect a stronger crystal field in the aluminum oxides (MgAl2O4 and YAG) than gallium oxides (ZnGa2O4, LiGa5O8, and GGG), which is explained by the smaller ionic radius of Al3+ in any coordination compared to Ga3+ [71]. The reduction from Td symmetry causes a further splitting of the energy levels and results in broader absorption. It was however not evident in the measured absorption spectra, though the local symmetry of tetrahedral sites in the garnet crystals is reduced to S4 [43].

 figure: Fig. 6.

Fig. 6. Absorption spectra of Co2+ doped MgAl2O4, ZnGa2O4, LiGa5O8, YAG, and GGG.

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Figure 7 and 8 show the time-dependent normalized transmission of all Co2+ samples excited at 607 nm from the pump-probe measurements. We clearly observed bleaching in all samples. The recovery times of the garnets in the few nanosecond range were found to be significantly shorter than those of the spinels in the hundreds of nanosecond range.

 figure: Fig. 7.

Fig. 7. Time-dependent normalized transmission in Co2+ doped (a) MgAl2O4, (b) ZnGa2O4, and (c) LiGa5O8. The excitation wavelength was 607 nm for all the three samples.

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 figure: Fig. 8.

Fig. 8. Time-dependent normalized transmission in Co2+ doped (a) YAG, and (b) GGG. The excitation wavelength was 607 nm for both samples.

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This significant difference is related to the relaxation process from the excited 4T1(4P) level (see Fig. 3, right). We excited the Co2+ samples at 532 nm and observed three emission bands around 700 nm, 850 nm, and 1200 nm in the spinels, corresponding to the transitions 4T1(4P)→4A2(4F), 4T1(4P)→4T2(4F) and 4T1(4P)→4T1(4F), respectively (dashed arrows in Fig. 3). The 700-nm fluorescence was two orders of magnitude stronger than the NIR fluorescence. We could not detect any fluorescence from the Co2+ doped garnets, which indicates that the 4T1(4P)-level is rapidly quenched non-radiatively after the excitation. This may be explained by the higher phonon energies of garnet crystals compared to spinels [74,75], causing a stronger coupling of vibrational wave functions with phonons and thus stronger multiphonon relaxation.

We performed Z-scan measurements with Co2+:MgAl2O4 and Co2+:GGG. The three other crystals were not suited for this measurement due to their small aperture below 4 mm2. The results are shown in Fig. 9. The determined parameters are summarized in Table 3.

 figure: Fig. 9.

Fig. 9. Saturation curves of the (a) Co2+:MgAl2O4 and (b) Co2+:GGG samples.

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The Co2+:MgAl2O4 strongly bleached when excited at 523, 545, 607, and 640 nm. We recorded the largest transmission change at 607 nm. Although the initial transmission at 607 nm was lower than at 640 nm, this ratio changed when bleached, indicating a smaller ESA cross section at 607 nm than 640 nm. It is remarkable that the transmission at 607 and 640 nm exceeded 97% when bleached. Despite the unknown GSA and ESA cross sections, we could estimate the FOM of Co2+:GGG from the initial and saturated transmission by the equation:

$$FOM = \frac{{{\sigma _{gs}}}}{{{\sigma _{es}}}} = \frac{{{\sigma _{gs}}{n_{tot}}{l_{SA}}}}{{{\sigma _{es}}{n_{tot}}{l_{SA}}}} = \frac{{\ln {T_0}}}{{\ln {T_{sat}}}}.$$
The resulting FOM amounted to 22 and 9 at 607 and 640 nm, respectively. The transmission of the Co2+:MgAl2O4 also increased when excited at 523 and 545 nm; however, the transmission decreased again in the higher fluence range.

The Co2+:GGG bleached at 607 and 640 nm but not at 545 nm or shorter. When we excited at 545 nm, the transmission quickly dropped by >15% even at low fluences of 0.5 J/cm2. The decrease in transmission was even more pronounced for shorter pump wavelengths, e.g. 523 nm (not shown in Fig. 9). The saturation characteristics at 607 and 640 nm strongly change around 0.2 and 0.25 J/cm2, respectively. This is due to the short recovery time of Co2+:GGG of 12 ± 2 ns (see Fig. 8(b)), which does not longer allow to consider the 5-ns pump pulse as instantaneous excitation. Taking this into account for the calculation of GSA and ESA cross sections of the Co2+:GGG was beyond the capacity of our model.

The Co2+:MgAl2O4 excited at 523 and 545 nm and Co2+:GGG at 640 nm showed a roll-over at high excitation fluences, which can also not be explained by our model and would require taking into account further energy levels. We performed further Z-scan measurements of these samples with longer pulses from a frequency-doubled Q-switched Nd:YLF laser at 527 nm (Evolution-15, Coherent) with a longer full-width half-maximum pulse duration of 280 ns. We also observed bleaching in both samples, but no roll-over even at the highest fluences of 10 J/cm2. Thus, the roll-over may be due to energy upconversion or ESA to the conduction band of the hosts, i.e. charge transfer, due to the high density of excited ions under high intensity 5-ns excitation. The corresponding parity allowed transition is expected to have larger cross sections than the 3d intrashell transitions. This may also explain the decrease Co2+:GGG’s transmission under excitation at 545 nm, but further investigations are required for fully understand this phenomenon.

We could also estimate the absorption cross sections of Co2+:MgAl2O4 and Co2+:LiGa5O8 in the visible using the absorption spectra (Fig. 6) and their reported GSA cross section values at 1.54 µm [58]. The GSA cross section of Co2+:MgAl2O4, for instance, at 607 nm was estimated to be (10.9 ± 1.9) × 10−19 cm2 which is in good agreement with our results. In the same way, the GSA cross section of Co2+:LiGa5O8 at 607 nm was estimated to be (11.4 ± 1.8) × 10−19 cm2. It is thus conceivable that Co2+ in other spinel structure crystals, including ZnGa2O4, exhibits similar values of GSA cross sections.

3.3 Considerations on other crystalline hosts

There is a large number of possible crystalline hosts to incorporate Cr4+ or Co2+ ions, and it is unrealistic to investigate all of these to find the best one. Despite the limited number of crystals investigated here, our results still have useful implications to search for further good saturable absorbers for the visible based on Cr4+ or Co2+.

For Cr4+, host crystals comprising a single substitution ion on a tetrahedral site are advantageous. Such crystals may exhibit a high FOM, as they suppress unwanted absorption due to Cr3+ and/or Cr4+[Oh]. The Cr4+:Mg2SiO4 (forsterite) crystal showed the best FOM at its peak absorption wavelength; however, we still observed residual absorption by Cr3+[Oh] substituting Mg2+ ions. In this respect, for instance, LiAlO2 is an attractive host crystal for chromium because its lattice only comprises tetrahedral substitution sites.

For Co2+, we need to choose crystalline hosts with low phonon energies to feature a long recovery time; otherwise, the saturation intensity becomes too high. It is however not trivial to evaluate or predict the interaction between phonons and Co2+ ions. We can instead qualitatively check the recovery time of the red fluorescence of a given Co2+ doped crystal when exciting the visible absorption band. The presence of red fluorescence indicates that radiative relaxation dominates over phonon-induced non-radiative relaxation, and such crystals may have recovery times up to a microsecond, as seen for the spinel crystals. On the other hand, the absence of red fluorescence indicates very short recovery times due to multiphonon processes as observed in the garnets. The validity of this rule for other lattice structures is supported by our investigations of a Co2+:Ca2MgSi2O7 (åkermanite) crystal. The absorption spectrum for the polarization parallel to the crystal’s a-axis was similar to the spectra shown in Fig. 6, but we could not observe any fluorescence from the crystal when the visible band was excited and the recovery time was, as expected, as short as 5 ns.

3.4 Passive Q-switching of Pr and Tb visible lasers using the saturable absorbers

For the application of our saturable absorber materials in passively Q-switched lasers, we need to consider appropriate resonator designs for each saturable absorber. The saturation intensity of the saturable absorber plays a critical role to obtain efficient Q-switching. Passive Q-switching is obtained only if the saturable absorber bleaches prior to the gain medium; otherwise, the laser operates in CW mode. The properties of the saturable absorbers investigated here are summarized in Table 4. In this section, we present resonator design considerations for two representative visible laser gain media: Pr:YLF and Tb:LiLuF4 (LLF). In the case of Pr:YLF laser [5], the saturation intensities of the visible emission at 523, 607, and 640 nm are calculated to be ≈360, ≈60, and ≈36 kW/cm2, respectively. In the case of Tb:LLF laser [5], the saturation intensity at 545 nm is calculated to be ≈46 kW/cm2.

Tables Icon

Table 4. Summarized properties of the investigated saturable absorbers.a

It was shown, that Cr4+:YAG can be utilized for Q-switching Pr:YLF lasers at 607 and 640 nm [30]. Since the saturation intensity of Cr4+:YAG is roughly half as high as in Pr:YLF, Cr4+:YAG bleaches prior to this gain medium, if the beam sizes in both elements are comparable. Cr4+:forsterite is useful to Q-switch green lasers, i.e. Pr:YLF at 523 nm and Tb:LLF at 545 nm. The saturation intensity at 523 nm is lower for Cr4+:forsterite than for Pr:YLF. This condition loosens resonator design restrictions, because a strong focus in the Cr4+:forsterite is typically not required. Therefore, it is conceivable to miniaturize the laser resonator to shorten the pulse duration to few-ns in a so-called microchip laser [77]. Cr4+:forsterite may also be suitable to Q-switch Tb:LLF laser at 545 nm as well. Thanks to the long upper state lifetime of Tb, high energy, high peak power pulses are expected from such a Q-switched Tb:LLF laser. Note that high energy pulses tend to damage components in resonators, particularly coatings of crystals and mirrors. Thus, a resonator configuration which does not strongly focus the beam on these components is required.

Crystals doped with Co2+ exhibit a broad visible absorption band covering the green to red region, which makes them suitable to Q-switch both Pr:YLF and Tb:LLF lasers at all transitions in this range. All the investigated Co2+ crystals have, however, more than an order of magnitude shorter recovery times than the Cr4+ crystals, which results in high saturation intensities. For Co2+:MgAl2O, the saturation intensity was calculated to be ≈360 kW/cm2 at 607 nm and thus nearly an order of magnitude higher than in Pr:YLF and Tb:LLF. Accordingly, Co2+:MgAl2O4 saturable absorbers should be placed in a strong focus to bleach them prior to the gain medium. This is applicable to the other Co2+ spinels since they are expected to have saturation intensities of comparable scale. The Co2+ garnets must have higher saturation intensities owing to their short recovery times, which put restrictions on the resonator design. The observed decrease in transmission at high fluence level (Fig. 9) further indicates that the performance of lasers Q-switched with Co2+ doped garnet saturable absorbers can degrade when the intensity in the crystal is too high. This must be further studied in forthcoming laser experiments.

4. Conclusion

We investigated the saturation characteristics of the visible absorption in oxide crystals doped with Cr4+ or Co2+. These transition-metal-doped crystals were found to exhibit useful properties as saturable absorbers to Q-switch visible lasers. The absorption in crystals doped with Cr4+ shows a strong dependence on the choice of the host crystal, as applicable wavelengths are significantly differing between YAG and forsterite. On the other hand, the absorption in crystals doped with Co2+ is less sensitive to the host crystal. Instead, the recovery times of Co2+ ions in different host crystals differ from a few nanosecond to a microsecond. This results in saturation intensities varying over two or three orders of magnitude. Co2+:MgAl2O4 is a very suitable saturable absorber to Q-switch orange-red lasers compared to Cr4+:YAG due to its higher FOM. Co2+:MgAl2O4 can also enable a better Q-switching performance at 523 nm than Cr4+:forsterite because the FOM of Cr4+:forsterite is limited by its low GSA at this wavelength. Cr4+:forsterite, in turn, should be a better choice for green lasers of longer wavelength, e.g. at 545 nm, and yellow lasers. Other transition metal ions of divalent, trivalent, or tetravalent charge states exhibiting an absorption in the visible region may also be suitable for the application as a saturable absorber in the visible and should be investigated in future. We expect that this report guides the further research on passively Q-switched visible solid-state lasers. First experiments in this respect are in progress in our lab and will be presented in a separate report.

Funding

Ministry of Education, Culture, Sports, Science and Technology; Bundesministerium für Bildung und Forschung (13N14192).

Acknowledgment

The authors thank Albert Kwasniewski for measuring the crystal orientation by the X-ray diffraction.

Disclosures

The authors declare no conflicts of interest.

References

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References

  • View by:

  1. K. V. Chellappan, E. Erden, and H. Urey, “Laser-based displays: A review,” Appl. Opt. 49(25), F79–F98 (2010).
    [Crossref]
  2. N. S. Kapany, N. A. Peppers, H. C. Zweng, and M. Flocks, “Retinal Photocoagulation by Lasers,” Nature 199(4889), 146–149 (1963).
    [Crossref]
  3. M. A. Mainster, “Wavelength selection in macular photocoagulation. Tissue optics, thermal effects, and laser systems,” Ophthalmology 93(7), 952–958 (1986).
    [Crossref]
  4. M. Naeem, “Laser Processing of Reflective Materials,” Laser Tech. J. 10(1), 18–20 (2013).
    [Crossref]
  5. C. Kränkel, D.-T. Marzahl, F. Moglia, G. Huber, and P. W. Metz, “Out of the blue: semiconductor laser pumped visible rare-earth doped lasers,” Laser Photonics Rev. 10(4), 548–568 (2016).
    [Crossref]
  6. S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, and Y. Sugimoto, “InGaN-Based Multi-Quantum-Well-Structure Laser Diodes,” Jpn. J. Appl. Phys. 35(Part 2, No. 1B), L74–L76 (1996).
    [Crossref]
  7. S. Calvez, J. E. Hastie, M. Guina, O. G. Okhotnikov, and M. D. Dawson, “Semiconductor disk lasers for the generation of visible and ultraviolet radiation,” Laser Photonics Rev. 3(5), 407–434 (2009).
    [Crossref]
  8. A. Richter, E. Heumann, E. Osiac, G. Huber, W. Seelert, and A. Diening, “Diode pumping of a continuous-wave Pr3+-doped LiYF4 laser,” Opt. Lett. 29(22), 2638–2640 (2004).
    [Crossref]
  9. K. Hashimoto and F. Kannari, “High-power GaN diode-pumped continuous wave Pr3+-doped LiYF4 laser,” Opt. Lett. 32(17), 2493–2495 (2007).
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2020 (1)

E. Castellano-Hernández, S. Kalusniak, P. W. Metz, and C. Kränkel, “Diode-Pumped Laser Operation of Tb3+:LiLuF4 in the Green and Yellow Spectral Range,” Laser Photonics Rev. 14(2), 1900229 (2020).
[Crossref]

2019 (1)

2018 (3)

2017 (5)

M. Demesh, D.-T. Marzahl, A. Yasukevich, V. Kisel, G. Huber, N. Kuleshov, and C. Kränkel, “Passively Q-switched Pr:YLF laser with a Co2+:MgAl2O4 saturable absorber,” Opt. Lett. 42(22), 4687–4690 (2017).
[Crossref]

S. Kajikawa, M. Yoshida, S. Motokoshi, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible ns-pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fibre with SESAM,” J. Eng. 1(7), 407–409 (2017).
[Crossref]

B. Xu, S. Luo, X. Yan, J. Li, J. Lan, Z. Luo, H. Xu, Z. Cai, H. Dong, J. Wang, and L. Zhang, “CdTe/CdS Quantum Dots: Effective Saturable Absorber for Visible Lasers,” IEEE J. Sel. Top. Quantum Electron. 23(5), 1–7 (2017).
[Crossref]

D. Wu, Z. Cai, Y. Zhong, J. Peng, Y. Cheng, J. Weng, Z. Luo, and H. Xu, “Compact Passive Q-Switching Pr3+-Doped ZBLAN Fiber Laser With Black Phosphorus-Based Saturable Absorber,” IEEE J. Sel. Top. Quantum Electron. 23(1), 7–12 (2017).
[Crossref]

P. W. Metz, D.-T. Marzahl, G. Huber, and C. Kränkel, “Performance and wavelength tuning of green emitting terbium lasers,” Opt. Express 25(5), 5716–5724 (2017).
[Crossref]

2016 (7)

P. W. Metz, D.-T. Marzahl, A. Majid, C. Kränkel, and G. Huber, “Efficient continuous wave laser operation of Tb3+-doped fluoride crystals in the green and yellow spectral regions,” Laser Photonics Rev. 10(2), 335–344 (2016).
[Crossref]

S. Luo, X. Yan, Q. Cui, B. Xu, H. Xu, and Z. Cai, “Power scaling of blue-diode-pumped Pr:YLF lasers at 523.0, 604.1, 606.9, 639.4, 697.8 and 720.9 nm,” Opt. Commun. 380, 357–360 (2016).
[Crossref]

C. Kränkel, D.-T. Marzahl, F. Moglia, G. Huber, and P. W. Metz, “Out of the blue: semiconductor laser pumped visible rare-earth doped lasers,” Laser Photonics Rev. 10(4), 548–568 (2016).
[Crossref]

K. Iijima, R. Kariyama, H. Tanaka, and F. Kannari, “Pr3+:YLF mode-locked laser at 640 nm directly pumped by InGaN-diode lasers,” Appl. Opt. 55(28), 7782–7787 (2016).
[Crossref]

Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

W. Li, J. Peng, Y. Zhong, D. Wu, H. Lin, Y. Cheng, Z. Luo, J. Weng, H. Xu, and Z. Cai, “Orange-light passively Q-switched Pr3+-doped all-fiber lasers with transition-metal dichalcogenide saturable absorbers,” Opt. Mater. Express 6(6), 2031–2039 (2016).
[Crossref]

K. F. Mak and J. Shan, “Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides,” Nat. Photonics 10(4), 216–226 (2016).
[Crossref]

2015 (2)

2014 (5)

2013 (4)

2012 (2)

2011 (3)

2010 (3)

2009 (2)

S. Calvez, J. E. Hastie, M. Guina, O. G. Okhotnikov, and M. D. Dawson, “Semiconductor disk lasers for the generation of visible and ultraviolet radiation,” Laser Photonics Rev. 3(5), 407–434 (2009).
[Crossref]

M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, “Visible cw laser emission of GaN-diode pumped Pr:YAlO3 crystal,” Appl. Phys. B 97(2), 363–367 (2009).
[Crossref]

2008 (1)

V. Ostroumov and W. Seelert, “1 W of 261 nm cw generation in a Pr3+:LiYF4 laser pumped by an optically pumped semiconductor laser at 479 nm,” Proc. SPIE 6871, 68711K (2008).
[Crossref]

2007 (3)

2006 (2)

2004 (2)

Y. Kalisky, “Cr4+-doped crystals: Their use as lasers and passive Q-switches,” Prog. Quantum Electron. 28(5), 249–303 (2004).
[Crossref]

A. Richter, E. Heumann, E. Osiac, G. Huber, W. Seelert, and A. Diening, “Diode pumping of a continuous-wave Pr3+-doped LiYF4 laser,” Opt. Lett. 29(22), 2638–2640 (2004).
[Crossref]

2003 (3)

R. Feldman, Y. Shimony, and Z. Burshtein, “Dynamics of chromium ion valence transformations in Cr,Ca:YAG crystals used as laser gain and passive Q-switching media,” Opt. Mater. (Amsterdam, Neth.) 24(1-2), 333–344 (2003).
[Crossref]

R. Feldman, Y. Shimony, and Z. Burshtein, “Passive Q-switching in Nd:YAG/Cr4+:YAG monolithic microchip laser,” Opt. Mater. (Amsterdam, Neth.) 24(1-2), 393–399 (2003).
[Crossref]

W. Chen and G. Boulon, “Growth mechanism of Cr:forsterite laser crystal with high Cr concentration,” Opt. Mater. (Amsterdam, Neth.) 24(1-2), 163–168 (2003).
[Crossref]

2002 (2)

Z. Burshtein and Y. Shimony, “Refractive index dispersion and anisotropy in Cr4+:Mg2SiO4,” Opt. Mater. 20(2), 87–96 (2002).
[Crossref]

K. V. Yumashev, I. A. Denisov, N. N. Posnov, N. V. Kuleshov, and R. Moncorgé, “Excited state absorption and passive Q-switch performance of Co2+ doped oxide crystals,” J. Alloys Compd. 341(1-2), 366–370 (2002).
[Crossref]

2001 (2)

A. Sennaroglu, “Analysis and optimization of lifetime thermal loading in continuous-wave Cr4+-doped solid-state lasers,” J. Opt. Soc. Am. B 18(11), 1578–1586 (2001).
[Crossref]

N. Pavel, J. Saikawa, S. Kurimura, and T. Taira, “High Average Power Diode End-Pumped Composite Nd:YAG Laser Passively Q-switched by Cr4+:YAG Saturable Absorber,” Jpn. J. Appl. Phys. 40(Part 1, No. 3A), 1253–1259 (2001).
[Crossref]

2000 (1)

A. Okhrimchuk and A. Shestakov, “Absorption saturation mechanism for YAG:Cr4+ crystals,” Phys. Rev. B 61(2), 988–995 (2000).
[Crossref]

1999 (4)

M. Riley, E. Krausz, N. Manson, and B. Henderson, “Selectively excited luminescence and magnetic circular dichroism of Cr4+-doped YAG and YGG,” Phys. Rev. B 59(3), 1850–1856 (1999).
[Crossref]

G. Xiao, J. H. Lim, S. Yang, E. Van Stryland, M. Bass, and L. Weichman, “Z-scan measurement of the ground and excited state absorption cross sections of Cr4+ in yttrium aluminum garnet,” IEEE J. Quantum Electron. 35(7), 1086–1091 (1999).
[Crossref]

K. V. Yumashev, “Saturable absorber Co2+:MgAl2O4 crystal for Q switching of 1.34-µm Nd3+:YAlO3 and 1.54-µm Er3+:glass lasers,” Appl. Opt. 38(30), 6343–6346 (1999).
[Crossref]

J. J. Zayhowski, “Microchip lasers,” Opt. Mater. (Amsterdam, Neth.) 11(2-3), 255–267 (1999).
[Crossref]

1998 (3)

D. E. Zelmon, D. L. Small, and R. Page, “Refractive-index measurements of undoped yttrium aluminum garnet from 0.4 to 5.0 µm,” Appl. Opt. 37(21), 4933–4935 (1998).
[Crossref]

N. N. Il’ichev, A. V. Kir’yanov, and P. P. Pashinin, “Model of passive Q switching taking account of the anisotropy of nonlinear absorption in a crystal switch with phototropic centres,” Quantum Electron. 28(2), 147–151 (1998).
[Crossref]

Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, and M. R. Kokta, “Excited-state absorption studies of Cr4+ ions in several garnet host crystals,” IEEE J. Quantum Electron. 34(2), 292–299 (1998).
[Crossref]

1997 (1)

N. V. Kuleshov, V. G. Shcherbitsky, V. P. Mikhailov, S. Hartung, T. Danger, S. Kück, K. Petermann, and G. Huber, “Excited-state absorption and stimulated emission measurements in Cr4+:forsterite,” J. Lumin. 75(4), 319–325 (1997).
[Crossref]

1996 (2)

S. Kück, K. Petermann, U. Pohlmann, and G. Huber, “Electronic and vibronic transitions of the Cr4+-doped garnets Lu3Al5O12, Y3Al5O12, Y3Ga5O12 and Gd3Ga5O12,” J. Lumin. 68(1), 1–14 (1996).
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S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, and Y. Sugimoto, “InGaN-Based Multi-Quantum-Well-Structure Laser Diodes,” Jpn. J. Appl. Phys. 35(Part 2, No. 1B), L74–L76 (1996).
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1995 (1)

Y. Shimony, Z. Burshtein, and Y. Kalisky, “Cr4+:YAG as Passive Q-switch and Brewster Plate in a Pulsed Nd:YAG laser,” IEEE J. Quantum Electron. 31(10), 1738–1741 (1995).
[Crossref]

1994 (1)

H. Eilers, U. Hömmerich, S. M. Jacobsen, W. M. Yen, K. R. Hoffman, and W. Jia, “Spectroscopy and dynamics of Cr4+:Y3Al5O12,” Phys. Rev. B 49(22), 15505–15513 (1994).
[Crossref]

1993 (1)

N. V. Kuleshov, V. P. Mikhailov, V. G. Scherbitsky, P. V. Prokoshin, and K. V. Yumashev, “Absorption and luminescence of tetrahedral Co2+ ion in MgAl2O4,” J. Lumin. 55(5-6), 265–269 (1993).
[Crossref]

1992 (2)

H. Eilers, K. R. Hoffman, W. M. Dennis, S. M. Jacobsen, and W. M. Yen, “Saturation of 1.064 µm absorption in Cr,Ca:Y3Al5O12 crystals,” Appl. Phys. Lett. 61(25), 2958–2960 (1992).
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M. I. Demchuk, V. P. Mikhailov, N. I. Zhavoronkov, N. V. Kuleshov, P. V. Prokoshin, K. V. Yumashev, M. G. Livshits, and B. I. Minkov, “Chromium-doped forsterite as a solid-state saturable absorber,” Opt. Lett. 17(13), 929–930 (1992).
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1991 (1)

W. Jia, H. Liu, S. Jaffe, W. M. Yen, and B. Denker, “Spectroscopy of Cr3+ and Cr4+ ions in forsterite,” Phys. Rev. B 43(7), 5234–5242 (1991).
[Crossref]

1988 (1)

H. R. Verdun, L. M. Thomas, D. M. Andrauskas, T. McCollum, and A. Pinto, “Chromium-doped forsterite laser pumped with 1.06 µm radiation,” Appl. Phys. Lett. 53(26), 2593–2595 (1988).
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1986 (1)

M. A. Mainster, “Wavelength selection in macular photocoagulation. Tissue optics, thermal effects, and laser systems,” Ophthalmology 93(7), 952–958 (1986).
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1968 (1)

J. P. Hurrell, S. P. S. Porto, I. F. Chang, S. S. Mitra, and R. P. Bauman, “Optical phonons of yttrium aluminum garnet,” Phys. Rev. 173(3), 851–856 (1968).
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1967 (1)

D. L. Wood, “Optical absorption of tetrahedral Co3+ and Co2+ in garnets,” J. Chem. Phys. 46(9), 3595–3602 (1967).
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1963 (1)

N. S. Kapany, N. A. Peppers, H. C. Zweng, and M. Flocks, “Retinal Photocoagulation by Lasers,” Nature 199(4889), 146–149 (1963).
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1954 (1)

Y. Tanabe and S. Sugano, “On the Absorption Spectra of Complex Ions. I,” J. Phys. Soc. Jpn. 9(5), 753–766 (1954).
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Andrauskas, D. M.

H. R. Verdun, L. M. Thomas, D. M. Andrauskas, T. McCollum, and A. Pinto, “Chromium-doped forsterite laser pumped with 1.06 µm radiation,” Appl. Phys. Lett. 53(26), 2593–2595 (1988).
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Bass, M.

G. Xiao, J. H. Lim, S. Yang, E. Van Stryland, M. Bass, and L. Weichman, “Z-scan measurement of the ground and excited state absorption cross sections of Cr4+ in yttrium aluminum garnet,” IEEE J. Quantum Electron. 35(7), 1086–1091 (1999).
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Bauman, R. P.

J. P. Hurrell, S. P. S. Porto, I. F. Chang, S. S. Mitra, and R. P. Bauman, “Optical phonons of yttrium aluminum garnet,” Phys. Rev. 173(3), 851–856 (1968).
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Bellancourt, A.-R.

Blau, P.

Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, and M. R. Kokta, “Excited-state absorption studies of Cr4+ ions in several garnet host crystals,” IEEE J. Quantum Electron. 34(2), 292–299 (1998).
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Bonaccorso, F.

F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nat. Photonics 4(9), 611–622 (2010).
[Crossref]

Boulon, G.

W. Chen and G. Boulon, “Growth mechanism of Cr:forsterite laser crystal with high Cr concentration,” Opt. Mater. (Amsterdam, Neth.) 24(1-2), 163–168 (2003).
[Crossref]

Bu, Y.

Burshtein, Z.

R. Feldman, Y. Shimony, and Z. Burshtein, “Dynamics of chromium ion valence transformations in Cr,Ca:YAG crystals used as laser gain and passive Q-switching media,” Opt. Mater. (Amsterdam, Neth.) 24(1-2), 333–344 (2003).
[Crossref]

R. Feldman, Y. Shimony, and Z. Burshtein, “Passive Q-switching in Nd:YAG/Cr4+:YAG monolithic microchip laser,” Opt. Mater. (Amsterdam, Neth.) 24(1-2), 393–399 (2003).
[Crossref]

Z. Burshtein and Y. Shimony, “Refractive index dispersion and anisotropy in Cr4+:Mg2SiO4,” Opt. Mater. 20(2), 87–96 (2002).
[Crossref]

Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, and M. R. Kokta, “Excited-state absorption studies of Cr4+ ions in several garnet host crystals,” IEEE J. Quantum Electron. 34(2), 292–299 (1998).
[Crossref]

Y. Shimony, Z. Burshtein, and Y. Kalisky, “Cr4+:YAG as Passive Q-switch and Brewster Plate in a Pulsed Nd:YAG laser,” IEEE J. Quantum Electron. 31(10), 1738–1741 (1995).
[Crossref]

Cai, Z.

D. Wu, Z. Cai, Y. Zhong, J. Peng, Y. Cheng, J. Weng, Z. Luo, and H. Xu, “Compact Passive Q-Switching Pr3+-Doped ZBLAN Fiber Laser With Black Phosphorus-Based Saturable Absorber,” IEEE J. Sel. Top. Quantum Electron. 23(1), 7–12 (2017).
[Crossref]

B. Xu, S. Luo, X. Yan, J. Li, J. Lan, Z. Luo, H. Xu, Z. Cai, H. Dong, J. Wang, and L. Zhang, “CdTe/CdS Quantum Dots: Effective Saturable Absorber for Visible Lasers,” IEEE J. Sel. Top. Quantum Electron. 23(5), 1–7 (2017).
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Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
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W. Li, J. Peng, Y. Zhong, D. Wu, H. Lin, Y. Cheng, Z. Luo, J. Weng, H. Xu, and Z. Cai, “Orange-light passively Q-switched Pr3+-doped all-fiber lasers with transition-metal dichalcogenide saturable absorbers,” Opt. Mater. Express 6(6), 2031–2039 (2016).
[Crossref]

S. Luo, X. Yan, Q. Cui, B. Xu, H. Xu, and Z. Cai, “Power scaling of blue-diode-pumped Pr:YLF lasers at 523.0, 604.1, 606.9, 639.4, 697.8 and 720.9 nm,” Opt. Commun. 380, 357–360 (2016).
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D. Wu, J. Peng, Z. Cai, J. Weng, Z. Luo, N. Chen, and H. Xu, “Gold nanoparticles as a saturable absorber for visible 635 nm Q-switched pulse generation,” Opt. Express 23(18), 24071–24076 (2015).
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Z. Liu, Z. Cai, S. Huang, C. Zeng, Z. Meng, Y. Bu, Z. Luo, B. Xu, H. Xu, C. Ye, F. Stareki, P. Camy, and R. Moncorgé, “Diode-pumped Pr3+:LiYF4 continuous-wave deep red laser at 698 nm,” J. Opt. Soc. Am. B 30(2), 302–305 (2013).
[Crossref]

Calmano, T.

Calvez, S.

S. Calvez, J. E. Hastie, M. Guina, O. G. Okhotnikov, and M. D. Dawson, “Semiconductor disk lasers for the generation of visible and ultraviolet radiation,” Laser Photonics Rev. 3(5), 407–434 (2009).
[Crossref]

Camy, P.

Castellano-Hernández, E.

E. Castellano-Hernández, S. Kalusniak, P. W. Metz, and C. Kränkel, “Diode-Pumped Laser Operation of Tb3+:LiLuF4 in the Green and Yellow Spectral Range,” Laser Photonics Rev. 14(2), 1900229 (2020).
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E. Castellano-Hernández, P. W. Metz, M. Demesh, and C. Kränkel, “Efficient directly emitting high-power Tb3+:LiLuF4 laser operating at 587.5 nm in the yellow range,” Opt. Lett. 43(19), 4791–4794 (2018).
[Crossref]

Chang, I. F.

J. P. Hurrell, S. P. S. Porto, I. F. Chang, S. S. Mitra, and R. P. Bauman, “Optical phonons of yttrium aluminum garnet,” Phys. Rev. 173(3), 851–856 (1968).
[Crossref]

Chellappan, K. V.

Chen, N.

Chen, W.

W. Chen and G. Boulon, “Growth mechanism of Cr:forsterite laser crystal with high Cr concentration,” Opt. Mater. (Amsterdam, Neth.) 24(1-2), 163–168 (2003).
[Crossref]

Cheng, Y.

D. Wu, Z. Cai, Y. Zhong, J. Peng, Y. Cheng, J. Weng, Z. Luo, and H. Xu, “Compact Passive Q-Switching Pr3+-Doped ZBLAN Fiber Laser With Black Phosphorus-Based Saturable Absorber,” IEEE J. Sel. Top. Quantum Electron. 23(1), 7–12 (2017).
[Crossref]

W. Li, J. Peng, Y. Zhong, D. Wu, H. Lin, Y. Cheng, Z. Luo, J. Weng, H. Xu, and Z. Cai, “Orange-light passively Q-switched Pr3+-doped all-fiber lasers with transition-metal dichalcogenide saturable absorbers,” Opt. Mater. Express 6(6), 2031–2039 (2016).
[Crossref]

Cui, Q.

S. Luo, X. Yan, Q. Cui, B. Xu, H. Xu, and Z. Cai, “Power scaling of blue-diode-pumped Pr:YLF lasers at 523.0, 604.1, 606.9, 639.4, 697.8 and 720.9 nm,” Opt. Commun. 380, 357–360 (2016).
[Crossref]

Danger, T.

N. V. Kuleshov, V. G. Shcherbitsky, V. P. Mikhailov, S. Hartung, T. Danger, S. Kück, K. Petermann, and G. Huber, “Excited-state absorption and stimulated emission measurements in Cr4+:forsterite,” J. Lumin. 75(4), 319–325 (1997).
[Crossref]

Dawson, M. D.

S. Calvez, J. E. Hastie, M. Guina, O. G. Okhotnikov, and M. D. Dawson, “Semiconductor disk lasers for the generation of visible and ultraviolet radiation,” Laser Photonics Rev. 3(5), 407–434 (2009).
[Crossref]

Demchuk, M. I.

Demesh, M.

Demirbas, U.

Denisov, I. A.

K. V. Yumashev, I. A. Denisov, N. N. Posnov, N. V. Kuleshov, and R. Moncorgé, “Excited state absorption and passive Q-switch performance of Co2+ doped oxide crystals,” J. Alloys Compd. 341(1-2), 366–370 (2002).
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Denker, B.

W. Jia, H. Liu, S. Jaffe, W. M. Yen, and B. Denker, “Spectroscopy of Cr3+ and Cr4+ ions in forsterite,” Phys. Rev. B 43(7), 5234–5242 (1991).
[Crossref]

Dennis, W. M.

H. Eilers, K. R. Hoffman, W. M. Dennis, S. M. Jacobsen, and W. M. Yen, “Saturation of 1.064 µm absorption in Cr,Ca:Y3Al5O12 crystals,” Appl. Phys. Lett. 61(25), 2958–2960 (1992).
[Crossref]

Diening, A.

Dong, H.

B. Xu, S. Luo, X. Yan, J. Li, J. Lan, Z. Luo, H. Xu, Z. Cai, H. Dong, J. Wang, and L. Zhang, “CdTe/CdS Quantum Dots: Effective Saturable Absorber for Visible Lasers,” IEEE J. Sel. Top. Quantum Electron. 23(5), 1–7 (2017).
[Crossref]

Eilers, H.

H. Eilers, U. Hömmerich, S. M. Jacobsen, W. M. Yen, K. R. Hoffman, and W. Jia, “Spectroscopy and dynamics of Cr4+:Y3Al5O12,” Phys. Rev. B 49(22), 15505–15513 (1994).
[Crossref]

H. Eilers, K. R. Hoffman, W. M. Dennis, S. M. Jacobsen, and W. M. Yen, “Saturation of 1.064 µm absorption in Cr,Ca:Y3Al5O12 crystals,” Appl. Phys. Lett. 61(25), 2958–2960 (1992).
[Crossref]

Erden, E.

Fechner, M.

D.-T. Marzahl, F. Reichert, P. W. Metz, M. Fechner, N.-O. Hansen, and G. Huber, “Efficient laser operation of diode-pumped Pr3+,Mg2+:SrAl12O19,” Appl. Phys. B 116(1), 109–113 (2014).
[Crossref]

F. Reichert, D.-T. Marzahl, P. W. Metz, M. Fechner, N.-O. Hansen, and G. Huber, “Efficient laser operation of Pr3+, Mg2+:SrAl12O19,” Opt. Lett. 37(23), 4889–4891 (2012).
[Crossref]

F. Reichert, F. Moglia, D.-T. Marzahl, P. W. Metz, M. Fechner, N.-O. Hansen, and G. Huber, “Diode pumped laser operation and spectroscopy of Pr3+:LaF3,” Opt. Express 20(18), 20387–20395 (2012).
[Crossref]

M. Fechner, F. Reichert, N.-O. Hansen, K. Petermann, and G. Huber, “Crystal growth, spectroscopy, and diode pumped laser performance of Pr, Mg:SrAl12O19,” Appl. Phys. B 102(4), 731–735 (2011).
[Crossref]

Feldman, R.

R. Feldman, Y. Shimony, and Z. Burshtein, “Passive Q-switching in Nd:YAG/Cr4+:YAG monolithic microchip laser,” Opt. Mater. (Amsterdam, Neth.) 24(1-2), 393–399 (2003).
[Crossref]

R. Feldman, Y. Shimony, and Z. Burshtein, “Dynamics of chromium ion valence transformations in Cr,Ca:YAG crystals used as laser gain and passive Q-switching media,” Opt. Mater. (Amsterdam, Neth.) 24(1-2), 333–344 (2003).
[Crossref]

Ferrari, A. C.

F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nat. Photonics 4(9), 611–622 (2010).
[Crossref]

Fibrich, M.

M. Fibrich, J. Šulc, and H. Jelínková, “Pr:YAlO3 laser generation in the green spectral range,” Opt. Lett. 38(23), 5024–5027 (2013).
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M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, “Visible cw laser emission of GaN-diode pumped Pr:YAlO3 crystal,” Appl. Phys. B 97(2), 363–367 (2009).
[Crossref]

Flocks, M.

N. S. Kapany, N. A. Peppers, H. C. Zweng, and M. Flocks, “Retinal Photocoagulation by Lasers,” Nature 199(4889), 146–149 (1963).
[Crossref]

Fujimoto, Y.

S. Kajikawa, M. Yoshida, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible Q-switched pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fiber with graphene,” Opt. Commun. 424, 13–16 (2018).
[Crossref]

S. Kajikawa, M. Yoshida, S. Motokoshi, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible ns-pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fibre with SESAM,” J. Eng. 1(7), 407–409 (2017).
[Crossref]

Fujita, S.

Gaponenko, M.

Graf, T.

H. Ridderbusch and T. Graf, “Absorption in Cr4+:YAG Crystals,” IEEE J. Quantum Electron. 43(2), 168–173 (2007).
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Guina, M.

M. Gaponenko, P. W. Metz, A. Härkönen, A. Heuer, T. Leinonen, M. Guina, T. Südmeyer, G. Huber, and C. Kränkel, “SESAM mode-locked red praseodymium laser,” Opt. Lett. 39(24), 6939–6941 (2014).
[Crossref]

S. Calvez, J. E. Hastie, M. Guina, O. G. Okhotnikov, and M. D. Dawson, “Semiconductor disk lasers for the generation of visible and ultraviolet radiation,” Laser Photonics Rev. 3(5), 407–434 (2009).
[Crossref]

Gün, T.

Hansen, N.-O.

Härkönen, A.

Hartung, S.

N. V. Kuleshov, V. G. Shcherbitsky, V. P. Mikhailov, S. Hartung, T. Danger, S. Kück, K. Petermann, and G. Huber, “Excited-state absorption and stimulated emission measurements in Cr4+:forsterite,” J. Lumin. 75(4), 319–325 (1997).
[Crossref]

Hasan, T.

F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nat. Photonics 4(9), 611–622 (2010).
[Crossref]

Hashimoto, K.

Hastie, J. E.

S. Calvez, J. E. Hastie, M. Guina, O. G. Okhotnikov, and M. D. Dawson, “Semiconductor disk lasers for the generation of visible and ultraviolet radiation,” Laser Photonics Rev. 3(5), 407–434 (2009).
[Crossref]

Henderson, B.

M. Riley, E. Krausz, N. Manson, and B. Henderson, “Selectively excited luminescence and magnetic circular dichroism of Cr4+-doped YAG and YGG,” Phys. Rev. B 59(3), 1850–1856 (1999).
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Heuer, A.

Heumann, E.

Hirosawa, K.

Hoffman, K. R.

H. Eilers, U. Hömmerich, S. M. Jacobsen, W. M. Yen, K. R. Hoffman, and W. Jia, “Spectroscopy and dynamics of Cr4+:Y3Al5O12,” Phys. Rev. B 49(22), 15505–15513 (1994).
[Crossref]

H. Eilers, K. R. Hoffman, W. M. Dennis, S. M. Jacobsen, and W. M. Yen, “Saturation of 1.064 µm absorption in Cr,Ca:Y3Al5O12 crystals,” Appl. Phys. Lett. 61(25), 2958–2960 (1992).
[Crossref]

Hömmerich, U.

H. Eilers, U. Hömmerich, S. M. Jacobsen, W. M. Yen, K. R. Hoffman, and W. Jia, “Spectroscopy and dynamics of Cr4+:Y3Al5O12,” Phys. Rev. B 49(22), 15505–15513 (1994).
[Crossref]

Huang, S.

Huber, G.

P. W. Metz, D.-T. Marzahl, G. Huber, and C. Kränkel, “Performance and wavelength tuning of green emitting terbium lasers,” Opt. Express 25(5), 5716–5724 (2017).
[Crossref]

M. Demesh, D.-T. Marzahl, A. Yasukevich, V. Kisel, G. Huber, N. Kuleshov, and C. Kränkel, “Passively Q-switched Pr:YLF laser with a Co2+:MgAl2O4 saturable absorber,” Opt. Lett. 42(22), 4687–4690 (2017).
[Crossref]

P. W. Metz, D.-T. Marzahl, A. Majid, C. Kränkel, and G. Huber, “Efficient continuous wave laser operation of Tb3+-doped fluoride crystals in the green and yellow spectral regions,” Laser Photonics Rev. 10(2), 335–344 (2016).
[Crossref]

C. Kränkel, D.-T. Marzahl, F. Moglia, G. Huber, and P. W. Metz, “Out of the blue: semiconductor laser pumped visible rare-earth doped lasers,” Laser Photonics Rev. 10(4), 548–568 (2016).
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F. Reichert, D.-T. Marzahl, and G. Huber, “Spectroscopic characterization and laser performance of Pr,Mg:CaAl12O19,” J. Opt. Soc. Am. B 31(2), 349–354 (2014).
[Crossref]

P. W. Metz, F. Reichert, F. Moglia, S. Müller, D.-T. Marzahl, C. Kränkel, and G. Huber, “High-power red, orange, and green Pr3+:LiYF4 lasers,” Opt. Lett. 39(11), 3193–3196 (2014).
[Crossref]

D.-T. Marzahl, F. Reichert, P. W. Metz, M. Fechner, N.-O. Hansen, and G. Huber, “Efficient laser operation of diode-pumped Pr3+,Mg2+:SrAl12O19,” Appl. Phys. B 116(1), 109–113 (2014).
[Crossref]

M. Gaponenko, P. W. Metz, A. Härkönen, A. Heuer, T. Leinonen, M. Guina, T. Südmeyer, G. Huber, and C. Kränkel, “SESAM mode-locked red praseodymium laser,” Opt. Lett. 39(24), 6939–6941 (2014).
[Crossref]

F. Reichert, T. Calmano, S. Müller, D.-T. Marzahl, P. W. Metz, and G. Huber, “Efficient visible laser operation of Pr,Mg:SrAl12O19 channel waveguides,” Opt. Lett. 38, 2698–2701 (2013).
[Crossref]

F. Reichert, D.-T. Marzahl, P. W. Metz, M. Fechner, N.-O. Hansen, and G. Huber, “Efficient laser operation of Pr3+, Mg2+:SrAl12O19,” Opt. Lett. 37(23), 4889–4891 (2012).
[Crossref]

F. Reichert, F. Moglia, D.-T. Marzahl, P. W. Metz, M. Fechner, N.-O. Hansen, and G. Huber, “Diode pumped laser operation and spectroscopy of Pr3+:LaF3,” Opt. Express 20(18), 20387–20395 (2012).
[Crossref]

T. Gün, P. W. Metz, and G. Huber, “Power scaling of laser diode pumped Pr3+: LiYF4 cw lasers: efficient laser operation at 522.6 nm, 545.9 nm, 607.2 nm, and 639.5 nm,” Opt. Lett. 36(6), 1002–1004 (2011).
[Crossref]

M. Fechner, F. Reichert, N.-O. Hansen, K. Petermann, and G. Huber, “Crystal growth, spectroscopy, and diode pumped laser performance of Pr, Mg:SrAl12O19,” Appl. Phys. B 102(4), 731–735 (2011).
[Crossref]

N.-O. Hansen, A.-R. Bellancourt, U. Weichmann, and G. Huber, “Efficient green continuous-wave lasing of blue-diode-pumped solid-state lasers based on praseodymium-doped LiYF4,” Appl. Opt. 49(20), 3864–3868 (2010).
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A. Richter, E. Heumann, G. Huber, V. Ostroumov, and W. Seelert, “Power scaling of semiconductor laser pumped Praseodymium-lasers,” Opt. Express 15(8), 5172–5178 (2007).
[Crossref]

A. Richter, E. Heumann, E. Osiac, G. Huber, W. Seelert, and A. Diening, “Diode pumping of a continuous-wave Pr3+-doped LiYF4 laser,” Opt. Lett. 29(22), 2638–2640 (2004).
[Crossref]

N. V. Kuleshov, V. G. Shcherbitsky, V. P. Mikhailov, S. Hartung, T. Danger, S. Kück, K. Petermann, and G. Huber, “Excited-state absorption and stimulated emission measurements in Cr4+:forsterite,” J. Lumin. 75(4), 319–325 (1997).
[Crossref]

S. Kück, K. Petermann, U. Pohlmann, and G. Huber, “Electronic and vibronic transitions of the Cr4+-doped garnets Lu3Al5O12, Y3Al5O12, Y3Ga5O12 and Gd3Ga5O12,” J. Lumin. 68(1), 1–14 (1996).
[Crossref]

Hurrell, J. P.

J. P. Hurrell, S. P. S. Porto, I. F. Chang, S. S. Mitra, and R. P. Bauman, “Optical phonons of yttrium aluminum garnet,” Phys. Rev. 173(3), 851–856 (1968).
[Crossref]

Iijima, K.

Il’ichev, N. N.

N. N. Il’ichev, A. V. Kir’yanov, and P. P. Pashinin, “Model of passive Q switching taking account of the anisotropy of nonlinear absorption in a crystal switch with phototropic centres,” Quantum Electron. 28(2), 147–151 (1998).
[Crossref]

Ishii, O.

S. Kajikawa, M. Yoshida, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible Q-switched pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fiber with graphene,” Opt. Commun. 424, 13–16 (2018).
[Crossref]

S. Kajikawa, M. Yoshida, S. Motokoshi, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible ns-pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fibre with SESAM,” J. Eng. 1(7), 407–409 (2017).
[Crossref]

Iwasa, N.

S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, and Y. Sugimoto, “InGaN-Based Multi-Quantum-Well-Structure Laser Diodes,” Jpn. J. Appl. Phys. 35(Part 2, No. 1B), L74–L76 (1996).
[Crossref]

Jacobsen, S. M.

H. Eilers, U. Hömmerich, S. M. Jacobsen, W. M. Yen, K. R. Hoffman, and W. Jia, “Spectroscopy and dynamics of Cr4+:Y3Al5O12,” Phys. Rev. B 49(22), 15505–15513 (1994).
[Crossref]

H. Eilers, K. R. Hoffman, W. M. Dennis, S. M. Jacobsen, and W. M. Yen, “Saturation of 1.064 µm absorption in Cr,Ca:Y3Al5O12 crystals,” Appl. Phys. Lett. 61(25), 2958–2960 (1992).
[Crossref]

Jaffe, S.

W. Jia, H. Liu, S. Jaffe, W. M. Yen, and B. Denker, “Spectroscopy of Cr3+ and Cr4+ ions in forsterite,” Phys. Rev. B 43(7), 5234–5242 (1991).
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Appl. Opt. (6)

Appl. Phys. B (3)

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Figures (9)

Fig. 1.
Fig. 1. (a) Four-level model of a saturable absorber. (b) Experimental setups of pump-probe and Z-scan measurements.
Fig. 2.
Fig. 2. Absorption spectra of the two Cr4+:YAG samples and a Cr4+:Mg2SiO4 (forsterite) sample. The Fresnel reflections on the uncoated facets were taken into account using the Sellmeier equations of undoped YAG [68] and undoped forsterite [69].
Fig. 3.
Fig. 3. Energy diagrams of tetrahedrally coordinated Cr4+ in YAG (D2d symmetry when the closest ligands are only taken into account) [51] and forsterite (Cs symmetry) [70], as well as tetrahedrally coordinated Co2+ in spinel or garnet crystals [76].
Fig. 4.
Fig. 4. (a) Time-dependent normalized transmission of Cr4+(0.08%):YAG and Cr4+:forsterite, pumped at 607 and 570 nm, respectively. The curve of Cr4+:forsterite is also shown ten-times magnified in the linear scale. (b) Time-dependent transmission change of Cr4+(0.08%):YAG when pumped at wavelengths between 580 and 430 nm. (c) Time-dependent transmission change of Cr4+(0.08%):YAG pumped at 430 nm recorded over 15 ms.
Fig. 5.
Fig. 5. Saturation curves of (a) Cr4+:YAG and (b) Cr4+:forsterite crystals.
Fig. 6.
Fig. 6. Absorption spectra of Co2+ doped MgAl2O4, ZnGa2O4, LiGa5O8, YAG, and GGG.
Fig. 7.
Fig. 7. Time-dependent normalized transmission in Co2+ doped (a) MgAl2O4, (b) ZnGa2O4, and (c) LiGa5O8. The excitation wavelength was 607 nm for all the three samples.
Fig. 8.
Fig. 8. Time-dependent normalized transmission in Co2+ doped (a) YAG, and (b) GGG. The excitation wavelength was 607 nm for both samples.
Fig. 9.
Fig. 9. Saturation curves of the (a) Co2+:MgAl2O4 and (b) Co2+:GGG samples.

Tables (4)

Tables Icon

Table 1. Overview of previously reported saturable absorbers for visible lasers.

Tables Icon

Table 2. Determined parameters of Cr4+:YAG and Cr4+:forsterite crystals.

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Table 3. Determined parameters of Co2+ doped MgAl2O4 spinel crystal.

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Table 4. Summarized properties of the investigated saturable absorbers.a

Equations (6)

Equations on this page are rendered with MathJax. Learn more.

T = exp [ ( σ g s n g s + σ e s n e s ) l S A ]
T 0 = exp ( σ g s n t o t l S A )
T s a t = exp ( σ e s n t o t l S A )
T ( t ) = exp [ { σ g s ( n t o t n e s ( t = 0 ) ) exp ( t τ ) σ e s n e s ( t = 0 ) exp ( t τ ) } l S A ] .
ln [ ln ( T ( t ) T 0 ) ] = t τ + ln [ ( σ g s σ e s ) n e s l S A ] .
F O M = σ g s σ e s = σ g s n t o t l S A σ e s n t o t l S A = ln T 0 ln T s a t .

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