Open Access
Add to BibSonomy
Abstract
A comparative study was conducted to investigate the 3.9 µm mid-IR emission properties of Ho3+ doped NaYF4 and CsCdCl3 crystals as well as Ho3+ doped Ga2Ge5S13 glass. Following optical excitation at ∼890 nm, all the studied materials exhibited broad mid-IR emissions centered at ∼3.9 µm at room temperature. The mid-IR emission at 3.9 µm, originating from the 5I5 → 5I6 transition, showed long emission lifetime values of ∼16.5 ms and ∼1.61 ms for Ho3+ doped CsCdCl3 crystal and Ga2Ge5S13 glass, respectively. Conversely, the Ho3+ doped NaYF4 crystal exhibited a relatively short lifetime of ∼120 µs. Temperature dependent decay time measurements were performed for the 5I5 excited state for all three samples. The results showed that the emission lifetimes of Ho3+:CsCdCl3 and Ho3+:Ga2Ge5S13 were nearly temperature independent over the range studied, while significant emission quenching of the 5I5 level was observed in Ho3+:NaYF4. The temperature dependence of the multi-phonon relaxation rate for 3.9 µm mid-IR emission in Ho3+:NaYF4 crystal was determined. The room temperature stimulated emission cross-sections for all three samples were calculated using the Füchtbauer-Landenburg equation. Furthermore, the results of Judd-Ofelt analysis are presented and discussed.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
There is an increasing demand for mid-IR laser sources for various applications such as remote sensing of bio-chemical agents, free-space communication, spectroscopy, and medical procedures [1–3]. However, finding suitable combinations of active ion and host material that allow for efficient room-temperature mid-IR lasing, with a suitable pumping scheme, remains a challenge for researchers in the field. For active emitters, several trivalent rare-earth (RE3+) ions have been identified as having promising emission transitions in the mid-infrared (mid-IR) spectral region of interest for laser development [1–32]. These include Pr3+, Tb3+, Dy3+, Ho3+, and Er3+, which have an energy level structure offering high potential for mid-IR transitions in the 3–5 µm spectral range. However, achieving laser operation at longer IR wavelengths is challenging in traditional oxide laser hosts, as the inherently small energy transitions are especially susceptible to non-radiative decay through multi-phonon relaxation (MPR). To overcome this, research efforts have shifted towards the use of low maximum phonon energy materials doped with RE3+ ions, in order to mitigate the phonon-assisted non-radiative decay rates between these closely spaced RE3+ energy levels [4–32]. Potential low-phonon materials under investigation include both halide and chalcogenide compositions. Among the RE3+ candidates, the 3.9 µm emission from Ho3+ is of particular interest because of its positioning right in the center of the 3 - 5 µm atmospheric transmission window, making it ideal for atmospheric sensing and free-space communication applications. Additionally, the position of the upper level of this Ho3+ transition raises the possibility of direct diode pumping.
In the search for suitable mid-IR host materials, fluoride (maximum phonon energy ∼300-450 cm-1) and chloride (maximum phonon energy ∼200-250 cm-1) crystals have garnered extensive attention [4–15,26–31]. Specifically, RE3+ doped NaYF4 (NYF) has been widely studied in various forms, such as powders, nanoparticles, ceramics, and glasses, due to its notable upconversion characteristics [32–38]. On the other hand, CsCdCl3 (CCC) doped with RE3+ has received limited examination with regards to mid-IR fluorescence properties, with the majority of research centering around applications in optoelectronics and scintillators [39–43]. Recently, the authors et al. reported the 4.5 µm fluorescence properties of Er3+ doped CsCdCl3 and CsPbCl3 crystals, suggesting their potential as mid-IR laser sources [31].
The recent interest in RE3+ doped chalcogenide glasses has been driven by two key factors: low phonon energies (250-350 cm-1) and advancements in purification and processing techniques [16–24]. Of particular interest are gallium-germanium (GaGe) based chalcogenide glasses, which offer a combination of ease of synthesis coupled with chemical stability [16–24]. To date, most RE3+ doped GaGe materials studied for mid-IR lasing have been selenide (Se)-based glasses, which necessitated pumping at longer wavelengths due to its relatively narrow band gap [17–19]. Figure 1 depicts the transmission spectra of undoped selenide, as well as sulphide, glasses. It can be observed that the absorption edge of the selenide glass is located at approximately 1 µm, whereas sulphide glasses have a wider bandgap than selenide and thus exhibit increased transparency in the 800 to 1000 nm spectra region, making commercial diode pumping possible. Promising mid-IR spectroscopic results were recently reported by the authors et al. for Er3+ doped Ga2Ge5S13 glass [25]. In this work, we present the results of a comparative spectroscopic investigation of novel Ho3+ doped disordered low-phonon fluoride (NaYF4) and chloride (CsCdCl3) single crystals, as well as sulphide glasses with a Ga2Ge5S13 (GGS) composition, with the aim of evaluating their potential as mid-IR laser gain materials.
2. Experimental details
The NYF and CCC materials under study crystallize in the cubic and hexagonal perovskite structures with wide band gaps offering transparency ranges of 0.15-11 µm and 0.3-15 µm, respectively. CCC has a low moisture sensitivity whereas NaYF4 is considered non-hygroscopic. Both Ho3+:NYF and Ho3+:CCC crystals were grown by the Bridgman technique with the former being grown by Crystagon Inc. and the latter being grown at Hampton University using a two-zone crystal growth furnace. Details of the material preparation and crystal growth process for Ho3+:CCC were described elsewhere [41]. The Ho3+:NaYF4 ingot exhibited a slight pinkish coloration and was cloudy throughout large parts of the crystal. A few nearly transparent sample pieces were identified in the ingot and prepared for optical transmission studies. The obtained Ho3+:CCC ingot showed some cracking throughout the boule despite the slow cooling rate. However, crack-free samples with good optical quality were selected and polished for spectroscopic studies. Ho3+:CCC was stable at ambient conditions for extended time periods indicating its low moisture sensitivity. Ho3 + doped GGS glasses were prepared at Brimrose Technology Corporation using a melt-quenching technique. Details of the glass preparation and fabrication were described elsewhere [25]. The physical properties of all investigated materials, including their corresponding maximum phonon energies are listed in Table 1. The Ho3+ concentrations in the studied materials (Table 1) were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES) by Galbraith Laboratories, Inc.
Table 1. Physical characteristics of the studied materials: Ho:NaYF4, Ho:CsCdCl3 crystals, and Ho:Ga2Ge5S13 glass.
Room temperature transmission and absorption spectra were recorded using a Cary 6000i UV-Vis-NIR spectrophotometer and a Nicolet 6700 Fourier-transform infrared spectrometer. Mid-IR fluorescence spectra were recorded after excitation with a continuous-wave Spectra-Physics Tsunami Ti:Sapphire laser. A Princeton Instruments Acton SpectraPro 0.15-m monochromator (λblaze: 4 µm, 150 grooves/mm) was used to collect the mid-IR emission spectra. The emission signal was recorded by an Infrared Associates liquid-nitrogen-cooled InSb detector in conjunction with a Stanford Research Systems SR830 dual-phase lock-in amplifier. Fluorescence decay measurements were carried out using the output of a pulsed (10-ns pulses, 10 Hz) Nd:YAG pumped Optical Parametric Oscillator system. The decay transients were recorded with a LabVIEW-driven National Instruments (USB-6366 DAQ) data acquisition system. For temperature-dependent fluorescence studies, the sample was mounted on the cold finger of a two-stage closed-cycle CTI Cryodyne cryogenic refrigerator.
3. Results and discussion
3.1 Absorption and Judd-Ofelt analysis
Figure 2 (a) shows the room temperature ground state absorption coefficient spectra of Ho3+ doped NYF crystal, GGS glass, and CCC crystal arranged from the top to bottom in the order of decreasing maximum phonon energy. The baseline of each absorption spectrum, attributable to a combination of Fresnel reflection and other background losses (from impurities, scattering, etc.), has been subtracted. As indicated in Fig. 2 (b), characteristic intra-4f absorption bands were assigned to transitions from the 5I8 ground state manifold to the excited manifolds of Ho3+ ions. We observed strong absorption bands in the visible and moderate ones in the infrared region for all samples. While the 5I8 → 5I5 absorption is desirable for resonant pumping of the upper laser level of the ∼3.9 µm Ho3+-laser [4,5], the observed absorptions in this band are relatively weak (circled in Fig. 1 (a) and quantified in Table 2), as in all previously known Ho3+-doped laser materials [1–5]. This low absorption is a laser design challenge, but not an insurmountable one, as it can be mitigated by a number of techniques such as the use of longer samples, higher dopant concentration, or multiple-pass pumping.
Fig. 2. (a) Room temperature absorption coefficient spectra of Ho3+:NaYF4 crystal, Ho3+:Ga2Ge5S13 glass, and Ho3+:CsCdCl3 crystal from the top to the bottom in the order of decreasing maximum phonon energy. The Ho3+ concentration in the studied materials ranged between 1.2 - 4.2 × 1020 cm-3. (b) Energy level diagram showing the corresponding absorption lines from the ground state 5I8.
Table 2. Room-temperature absorption cross-section values of 5I8 → 5I5 transition for the investigated Ho3+ doped materials.
Using the available absorption data, Judd-Ofelt (J-O) analysis was performed for the Ho3+ doped GGS and CCC samples. A J-O calculation was not performed for the Ho3+:NaYF4 crystal due to the availability of J-O analysis results in the literature [34]. Six and seven manifolds were selected to determine the three Ωt parameters, known as the J-O intensity parameters, for Ho3+ transitions in GGS and CCC samples, respectively. Tables 3 and 4 show the average Ho3+ transition wavelengths, integrated absorption coefficients, and the line strengths for Ho3+:GGS and Ho3+:CCC samples, respectively. A single refractive index in the middle of the absorption range for each material was used in the J-O calculations (listed in Table 1). The magnetic dipole contributions were subtracted from the measured line-strengths, as required, obtaining the electric dipole contributions. Table 5 lists the J-O intensity parameters for all three studied samples. The acquired J-O intensity parameters are within the range of values reported for other Ho3+ doped materials [15,23,27,34–38]. The root mean square (rms) errors between measured and calculated line strengths were small enough to indicate good consistency between the calculated and measured values (Table 3 and Table 4). The calculated radiative rates, branching ratios, and radiative lifetimes of the 5I5 level for all three samples are listed in Table 6. It should be noted that the calculated radiative lifetimes are subject to an estimated error of less than 20% due to the limited accuracy of J-O theory.
Table 3. Average transition wavelengths, integrated absorption coefficients and the line strengths of Ho3+:Ga2Ge5S13 glass.
Table 4. Average transition wavelengths, integrated absorption coefficients and the line strengths of Ho3+:CsCdCl3 crystal.
Table 5. Judd-Ofelt intensity parameters of Ho:NaYF4, Ho:Ga2Ge5S13, and Ho:CsCdCl3.
Table 6. Calculated radiative rates, branching ratios, and radiative lifetimes of the 5I5 excited state for the investigated Ho:NaYF4, Ho:Ga2Ge5S13, and Ho:CsCdCl3 materials.
3.2 Emission and decay time studies
Figure 3 (a) depicts the room temperature mid-IR emission spectra for all three studied samples in the spectral range between 2.7 µm and 4.2 µm. This spectral region was chosen to present two Ho3+ transitions: the 5I6 → 5I7 transition centered at ∼2.8 µm and the 5I5 → 5I6 transition centered at ∼3.9 µm. The observed emissions were excited at ∼ 890 nm populating the 5I5 level, and subsequently the 5I6 level through radiative and nonradiative transitions. In displaying both transitions, it is possible to highlight two different emission pathways that occur in the investigated samples. Depending on the specific decay pathways from the 5I5 level, a significant reversal in the relative intensities of the 2.8 µm and 3.9 µm emissions was observed as displayed in Fig. 3 (a). The schematic diagram of the relevant lower lying Ho3+ levels indicating the pump transition as well as the two emission pathways for the fluoride and chloride samples is illustrated in Fig. 3 (b). For the case of the Ho3+:NaYF4 with the maximum phonon energy ∼360 cm-1, we observed significant quenching of the 3.9 µm fluorescence (5I5 → 5I6), accompanied by intense 2.8 µm (5I6 → 5I7) emission. This is attributed to multi-phonon relaxation (MPR) quenching the 5I5 level leading to the majority of excited electrons populating the 5I6 upper emission level as shown in the left side of Fig. 3 (b). For the Ho3+:CCC with the smaller maximum phonon energy ∼252 cm-1, strong 3.9 µm emission dominates while the 2.8 µm emission is relatively much weaker. This marked decrease in 2.8 µm emission is due to the fact that only ∼7% of the excited 5I5 population reaches the 5I6 level through radiative channels. The remaining ∼93%, according to calculated branching ratios (Table 6), bypass the 5I6 level entirely, as illustrated in the right side of Fig. 3 (b).
Fig. 3. (a) Room-temperature mid-IR emission spectra of 5I6 → 5I7 and 5I5 → 5I6 transitions in the same spectral region for all the studied samples. (b) The partial energy level diagram of Ho3+ ions indicating the pump wavelength and corresponding emission transitions. The branching ratios of the 5I5 to the corresponding lower levels are listed for the chloride sample.
Figure 4 depicts the room temperature 5I5 emission decay transients (monitored at ∼ 3.9 µm) of all studied Ho3+ doped samples under pulsed excitation at ∼ 895 nm. The longest emission lifetime of ∼16.5 ms was observed from the Ho3+ doped CCC crystal. To the best of our knowledge, this is in fact the longest lifetime so far demonstrated among Ho3+ doped materials for this transition. The next best was observed by Virey et al. [28] who reported mid-IR fluorescence of ∼3.9 µm for Ho3+ doped CsCdBr3 (CCB) with the emission lifetime of 13 ms for 5I5 level. The Ho3+ doped GGS glass exhibited a mid-IR lifetime of ∼1.61 ms, which was similar to the value of ∼ 1.7 ms from recent results on Ho3+ doped GaLaS glass [24]. Meanwhile, the shortest lifetime (∼120 µs) was measured in Ho3+:NYF crystal - though, to the best of our knowledge, it is the longest 5I5 level lifetime observed in fluorides [1–5]. Indeed, the emission lifetimes of the 5I5 level in the most-studied Ho3+:LiYF4 (Ho3+:YLF) and Ho3+:BaY2F8 (Ho3+:BYF) lasers were measured as ∼18 µs and ∼50 µs, respectively [4,5]. It should be noted that the measured lifetimes of our studied materials may be influenced by energy transfer among Ho3+ ions. In order to determine the degree of this impact, it is necessary to conduct concentration-dependent measurements, with a particular focus on investigating samples with lower concentrations. Regrettably, we do not have access to multiple concentrations for this purpose. However, it is expected that lower concentration samples may exhibit longer lifetimes due to reduced energy transfer interactions.
Fig. 4. Room temperature 5I5 decay transients of the studied Ho3+ doped materials under 895 nm excitation.
To gain a comprehensive understanding of the observed lifetimes from the phonon perspective, Table 7 portrays a list of the measured 5I5 emission lifetimes compared to maximum phonon energies (and therefore the number of phonons needed to bridge the energy gap of ∼2560 cm-1), for the three investigated materials. It is evident from the data that the longest lifetime in the Ho3+:CCC crystal is associated with the highest number of phonons needed to bridge the energy gap, in agreement with the negligible nonradiative decay rates in the chloride host. Both the Ho3+:NaYF4 and Ho3+:GGS samples require fewer phonons to bridge the gap leading to the observed shorter lifetimes. However, the fact that both of these materials need the same number of phonons to bridge the gap, but exhibit very different lifetime values (0.12 ms for NYF and 1.61 ms for GGS), suggests that phonons are not the only sole factor affecting these observed values. This will be further discussed in a later section. The reported decay times are estimated to have an error margin of approximately 10%.
Table 7. Measured lifetime (5I5), maximum phonon energies, and the number of phonons needed to bridge the energy gap of 5I5 to the next lower level for the studied Ho3+ doped materials.
The observed lifetimes have generated interest in investigating the temperature dependence of the decay dynamics of the 3.9 µm. Figure 5 (a) depicts the 5I5 emission lifetime as a function of temperature for all three studied samples. For the Ho3+:CCC crystal, no significant change in emission lifetime was observed between 10 K and room temperature. This suggests that nonradiative processes due to a multi-phonon decay are negligible, as would be expected from the energy-gap law for low phonon energy hosts [25–31]. This is consistent with the fact that the calculated radiative lifetime (16.2 ms) from J-O analysis (Table 6) shows good agreement with the measured lifetime (16.5 ms). In contrast, for the Ho3+:NYF crystal, the 5I5 lifetime exhibited a significant increase with decreasing temperature, reaching a value of approximately 504 µs at 10 K. This corresponded to a quenching of the emission lifetime by ∼75% when going from low to room temperature. It is established that nonradiative decay can occur through several processes including MPR, energy transfer between RE3+ ions at different incorporation sites, and energy transfer to other impurities in the crystal [12,26–29]. To further examine the degree to which MPR was the cause of the observed 3.9 µm emission quenching in the Ho3+:NYF crystal, the temperature dependent lifetime results were compared to the theoretical energy-gap law.
Fig. 5. (a) Temperature dependence of the 5I5 level lifetime values between 10 K and room temperature for all the studied samples. (b) Exponential energy-gap law fitting of the 5I5 level for Ho:NaYF4 crystal.
The energy-gap law, which calculates the rate of nonradiative decay due to MPR (WMPR) between RE3+ energy levels, is expressed by [2]:
where ΔE is the energy spacing of the transition, T is the temperature, ħω is the maximum phonon energy of the host, p (p = ΔE / ħω) is the number of phonons needed to bridge the energy gap, and B and β are both empirically derived host dependent parameters. To determine the parameters for Ho3+:NYF, several Ho3+ transitions with energy gaps in the range of 2500 to 5200 cm-1 were investigated for their decay rates. The experimental and radiative lifetimes (τexp and τrad) of the 5I5, 5I6, and 5I7 states were determined to be (0.12, 9.87), (3.21, 7.76), and (18.12, 19.67) ms, respectively. The WMPR rates for the selected excited states were then calculated using the relation; WMPR = 1/τexp - 1/τrad, where τrad is the radiative lifetime as predicted by J-O analysis [34]. Fitting the measured MPR rates to the energy-gap law using Eq. (1), resulted in B = 0.34 × 108 s-1 and β = 3.72 × 10−3 cm. These experimentally determined parameters were then used to describe the temperature dependence of the measured nonradiative decay rates of the 3.9 µm mid-IR emission, as shown in Fig. 5 (b). A comparison between the experimental results and calculated multiphonon decay rates shows substantial agreement. As a result, it can be concluded that the observed lifetime variation with temperature aligns well with the nonradiative decay via multiphonon emission, which is in accordance with the energy-gap law. Additionally, a J-O analysis [48–50] yielded a radiative lifetime of 9.87 ms for the 5I5 state, indicating the quantum efficiency may be as low as ∼1% at room temperature.For the case of Ho3+:GGS glass, the 5I5 emission lifetimes were observed to be nearly temperature independent throughout the entire temperature range, similar to the results obtained for Ho3+ doped CCC crystal (Fig. 5 (a)). This outcome was unexpected, considering that both Ho3+:GGS and Ho3+:NYF have comparable maximum phonon energies, as indicated in Table 7. This observation may be due to the correlation between the host dependence of electron- phonon coupling and the effective mass of the active cation and its nearest neighbors [51]. Hence, one of the contributing factors could be the weaker electron-phonon coupling between the Ho3+ and its neighboring anions, due to the significantly heavier effective mass of Ho3+:GGS compared to Ho3+:NYF. Using the J-O parameters obtained for Ho3+:GGS glass, a radiative lifetime of 3.3 ms (Table 6) was calculated for the 5I5 level, which yields a radiative quantum efficiency of ∼50% at room temperature. This value is much higher than the value for Ho3+:NYF, and so tends to support the conclusion that nonradiative decay is much weaker in Ho3+:GGS glass. The fact that the inferred quantum efficiency is significantly less than unity but the lifetime changes little from 10 K to room temperature may indicate another nonradiative decay mechanism at work, such as energy transfer. However, as was mentioned earlier, samples of differing Ho3+ concentration would be needed to test that possibility.
3.3 Stimulated emission cross-section
The Füchtbauer-Ladenburg (F-L) equation was employed to determine the stimulated emission cross section (σemiss) of the ∼3.9 µm mid-IR transition [52]. The F-L equation is described by:
Fig. 6. Stimulated emission cross-section spectra for the 5I5 → 5I6 transition in the investigated Ho3+ doped materials, which were calculated using the Füchtbauer-Landenburg equation.
Table 8. Branching ratios, radiative lifetimes (τrad) and measured lifetimes (τexp), quantum efficiency (ηQE), emission cross-sections (σemiss), and sigma-tau (στ) product of the 5I5 → 5I6 excited state for Ho3+:NaYF4, Ho3+:Ga2Ge5S13 and Ho3+ :CsCdCl3.
4. Conclusions
In summary, a comparative spectroscopic analysis was carried out on Ho3+ doped fluoride and chloride crystals as well as sulphide glass, with the aim of evaluating their suitability for mid-IR laser applications. The absorption spectra of the three studied samples displayed the characteristic Ho3+ transitions in the visible and IR spectral region, which were utilized in performing Judd-Ofelt analysis. The 3.9 µm emission properties were investigated under both room temperature and cryogenic conditions. The broad mid-IR emission bands centered at ∼3.9µm (5I5 → 5I6) of Ho3+ were observed in all of the studied materials upon optical excitation into the 5I5 absorption band at ∼890 nm. The significant difference in the room temperature mid-IR (5I5) emission lifetime is a remarkable characteristic that distinguishes the Ho3+ doped materials studied. The Ho3+ in CCC exhibited the longest 5I5 emission lifetime of 16.5 ms, while the Ho3+ in GGS glass displayed a shorter lifetime of 1.6 ms. Despite this variation, both materials provide a sufficient storage time to accommodate laser diode pumping. The shortest lifetime of ∼120 µs was observed in Ho3+:NYF, and this was still the longest 5I5 level lifetime recorded in fluorides. Indeed, the emission lifetimes of the 5I5 level in the best studied Ho:YLF and Ho:BYF lasers were measured as ∼18 µs and ∼50 µs, respectively [4,5]. The Ho:NYF laser, due to much longer Ho3+ storage time in it (∼120 µs), should be able to generate much higher pulsed energy - when pumped with higher pulse energy Cr:LiCAF laser (due to much enhanced Cr3+ storage time, τupper = 170 µs) [53] versus more conventional pumping with a Cr:LiSAF laser (τupper = 67 µs) [53]. Among all the studied materials, Ho3+ doped GGS glass was determined to offer a favorable balance between desirable spectroscopic characteristics and overall optical quality. Of the crystals evaluated, the chloride host displayed superior spectroscopic performance, exhibiting longer lifetimes and higher στ products in comparison to the fluoride host. Despite these advantageous results, the use of chlorides presents significant challenges, including difficulties in crystal growth, low thermal conductivity, and mechanical strength. The growth of CCC crystals remains in the early stages and ongoing efforts towards optimization are in progress to fully realize its potential as an efficient mid-IR laser gain material.
Funding
National Science Foundation (NSF-DMR 1827820); Army Research Office (W911NF1810447, W911NF210200, W911NF2120181).
Acknowledgements
Brimrose Technology Corporation gratefully acknowledges the support of this work by Army Research Office through Cooperative Agreement W911NF2120181. The work at Hampton University was supported by the Army Research Office through grant W911NF1810447 and Cooperative Agreement W911NF210200 and National Science Foundation through grant NSF-DMR 1827820 (PREM).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. M. Eichhorn, “Quasi-three-level solid-state lasers in the near and mid infrared based on trivalent rare earth ions,” Appl. Phys. B 93(2-3), 269–316 (2008). [CrossRef]
2. A. A. Kaminskii, Crystalline Lasers: Physical Processes and Operating Schemes (CRC Press, 1996).
3. I. T. Sorokina, “Crystalline Mid-Infrared Lasers,” in I.T. Sorokina and K.L. Vodopyanov, eds., Solid-State Mid-Infrared Laser Sources. Topics in Applied Physics, vol 89. (Springer, 2003). [CrossRef]
4. A. M. Tabirian, H. P. Jenssen, and A. Cassanho, “Efficient, room temperature mid-infrared laser at 3.9 µm in Ho:BaY2F8,” in: Optical Society of America (OSA) Trends in Optics and Photonics (TOPS), C. Marshall ed., Advanced Solid-State Lasers, 50, 70–176 (2001).
5. 5. L. Esterowitz, R. C. Eckardt, and R. E. Allen, “Long-wavelength stimulated emission via cascade laser action in Ho:YLF,” Appl. Phys. Lett. 35(3), 236–239 (1979). [CrossRef]
6. N. P. Barnes and R. E. Allen, “Room temperature Dy:YLF laser operation at 4.34 µm,” IEEE J. Quantum Electron. 27(2), 277–282 (1991). [CrossRef]
7. S. R. Bowman, S. K. Searles, N. W. Jenkins, S. B. Qadri, E. F. Skelton, and J. Ganem, “Diode pumped room temperature mid-infrared erbium laser,” In: C Marshall, ed., Advanced Solid-State Lasers, Vol. 50 (Optical Society of America, 2001). pp. 154–156.
8. L. Isaenko, A. Yelisseyev, A. Tkachuk, and S. Ivanova, “New Monocrystals with Low Phonon Energy for Mid-IR Lasers,” in: M. Ebrahim-Zadeh and I. T. Sorokina, eds., Mid-Infrared Coherent Sources and Applications, (Springer, 2008), pp. 3–65.
9. A. G. Okhrimchuk, L. N. Butvina, E. M. Dianov, I. A. Shestakova, N. V. Lichkova, V. N. Zagorodnev, and A. V. Shestakov, “Optical spectroscopy of the RbPb2Cl5:Dy3+ laser crystal and oscillation at 5.5 µm at room temperature,” J. Opt. Soc. Am. B 24(10), 2690–2695 (2007). [CrossRef]
10. V. Savitski, P. Schlosser, L. Isaenko, and A. Tarasova, “Diode-pumped Dy:KPb2Cl5 laser at 4.2-4.45 µm,” Laser Congress 2021 (ASSL, LAC) Optical Society of America, Paper number: ATu4A.1. [CrossRef]
11. S. R. Bowman, L. B. Shaw, B. J. Feldman, and J. Ganem, “A 7-µm praseodymium-based solid-state laser,” IEEE J. Quantum Electron. 32(4), 646–649 (1996). [CrossRef]
12. L. D. Merkle, Z. Fleischman, E. Brown, J. L. Allen, U. Hommerich, and M. Dubinskii, “Enhanced mid-infrared emission of Pr3+ ions in solids through a “3-for-1” excitation process – quantified,” Opt. Express 29(24), 39001–39015 (2021). [CrossRef]
13. M. C. Nostrand, R. H. Page, S. A. Payne, and W. F. Krupke, “Room temperature CaGa2S4:Dy3+ laser action at 2.43 and 4.31 µm and KPb2C15:Dy3+ laser action at 2.43 µm,” in OSA TOPS in Optics and Photonics Series, Advanced Solid State Lasers, 26, 441–449 (1999).
14. T. T. Basiev, M. E. Doroshenko, V. V. Osiko, and D. V. Badikov, “Mid IR laser oscillations in new low phonon PbGa2S4:Dy3+ crystal,” Technical Digest of OSA Topical Meetings, Advanced Solid State Photonics, Vienna, Austria (2005) paper TuB10, pp. 75–79. [CrossRef]
15. E. Brown, Z. Fleischman, L. D. Merkle, E. Rowe, A. Burger, S. A. Payne, and M. Dubinskii, “Optical spectroscopy of holmium doped K2LaCl5,” J. Lumin. 196, 221–226 (2018). [CrossRef]
16. A. Berrou, C. Kieleck, and M. Eichhorn, “Mid-infrared lasing from Ho3+ in bulk InF3 glass,” Opt. Lett. 40(8), 1699–1701 (2015). [CrossRef]
17. M. F. Churbanov, B. I. Denker, B. I. Galagan, V. V. Koltashev, V. G. Plotnichenko, M. V. Sukhanov, S. E. Sverchkov, and A. P. Velmuzhov, “First demonstration of ∼5 µm laser action in terbium-doped selenide glass,” Appl. Phys. B 126(7), 117 (2020). [CrossRef]
18. M. F. Churbanov, B. I. Denker, B. I. Galagan, V. V. Koltashev, V. G. Plotnichenko, G. E. Snopatin, M. V. Sukhanov, S. E. Sverchkov, and A. P. Velmuzhov, “Laser potential of Pr3+ doped chalcogenide glass in 5-6 µm spectral range,” J. Non-Cryst. Solids 559, 120592 (2021). [CrossRef]
19. B. I. Denker, B. I. Galagan, S. E. Sverchkov, V .V. Koltashev, V.G. Plotnichenko, M. V. Sukhanov, A. P. Velmuzhov, M. P. Frolov, Yu. V. Korostelin, V. I. Kozlovsky, S. O. Leonov, P. Fjodorow, and Ya. K. Skasyrsky, “Resonantly pumped Ce3+ mid-infrared lasing in selenide glass,” Opt. Lett. 46(16), 4002–4004 (2021). [CrossRef]
20. R. S. Quimby and B. G. Aitken, “Multiphonon energy gap law in rare-earth doped chalcogenide glass,” J. Non-Cryst. Solids 320(1-3), 100–112 (2003). [CrossRef]
21. V. S. Shiryaev and M. F. Churbanov, “Recent advances in preparation of high-purity chalcogenide glasses for mid-IR photonics,” J. Non-Cryst. Solids 475, 1–9 (2017). [CrossRef]
22. M. Li, Y. Xu, X. Jia, L. Yang, N. Long, Z. Liu, and S. Dai, “Mid-infrared emission properties of Pr3+-doped Ge-Sb-Se-Ga-I chalcogenide glasses,” Opt. Mater. Express 8(4), 992–1000 (2018). [CrossRef]
23. C. C. Ye, D. W. Hewak, M. Hempstead, B. N. Samson, and D. N. Payne, “Spectral properties of Er3+-doped gallium lanthanum sulphide glass,” J. Non-Cryst. Solids 208(1-2), 56–63 (1996). [CrossRef]
24. T. Schweizer, B. N. Samson, J. R. Hector, W.S. Brocklesby, D. W. Hewak, and D. N. Payne, “Infrared emision from holmium doped gallium lanthanum sulphide glass,” Infrared Phys. Technol. 40(4), 329–335 (1999). [CrossRef]
25. E. Brown, Z. Fleischman, J. McKay, U. Hommerich, W. Palosz, S. Trivedi, and M. Dubinskii, “Spectroscopic study of Er-doped Ga2Ge5S13 glass for mid-IR laser applications,” Opt. Mater. Express 12(4), 1627–1637 (2022). [CrossRef]
26. E. Brown, Z. Fleischman, J. McKay, and M. Dubinskii, “Spectroscopic characterization of low-phonon Er-doped BaF2 single crystal for mid-IR lasers,” Opt. Mater. Express 11(2), 575–584 (2021). [CrossRef]
27. M. Velazquez, A. Ferrier, J.-L. Doualan, and R. Moncorge, “Rare-earth doped low phonon energy halide crystals for mid-infrared sources,” In: A. Al-Khursan, ed., Solid-State Laser, (IntechOpen, 2012). pp. 119–142.
28. E. Virey, M. Couchaud, C. Faure, B. Ferrand, C. Wyon, and C. Borel, “Room temperature fluorescence of CsCdBr3:RE (RE = Pr, Nd, Dy, Ho, Er, Tm) in the 3 - 5 µm range,” J. Alloys Compd. 275-277, 311–314 (1998). [CrossRef]
29. Yu.V. Orlovskii, B. B. Basiev, K. K. Pukhov, O. K. Alimov, N. A. Glushkov, and V. A. Konyushkin, “Low-phonon BaF2:Ho3+,Tm3+ doped crystals for 3.5-4 µm lasing,” Opt. Mater. 32(5), 599–611 (2010). [CrossRef]
30. E. E. Brown, Z. Fleischman, R. Hawrami, E. Ariesanti, A. Burger, U. Hommerich, S.B. Trivedi, and M. Dubinskii, “Comparative study of rare-earth doped low-phonon fluoride and chloride crystals for mid-IR laser potential,” Proc. SPIE 11724, 11724C (2021). [CrossRef]
31. E.E. Brown, Z.D. Fleischman, J. McKay, U. Hommerich, A. Kabir, J. Riggins, S. Trivedi, and M. Dubinskii, “Mid-IR spectroscopy of Er3+ doped low-phonon CsCdCl3 and CsPbCl3 crystals,” J. Opt. Soc. Am. B 40(1), A1–A8 (2023). [CrossRef]
32. S. Zhao, X. Sun, X. Wang, L. Huang, Y. Fei, and S. Xu, “The influence of phase evolution on optical properties in rare earth doped glass ceramics containing NaYF4 nanocrystals,” J. Eur. Ceram. Soc. 35(15), 4225–4231 (2015). [CrossRef]
33. D. Kumar, S.K. Sharma, S. Verma, V. Sharma, and V. Kumar, “A short review on rare earth NaYF4 upconverted nanomaterials for solar cell applications,” Materials Today-Proceedings 21, 1868–1874 (2020). [CrossRef]
34. S. Yang, X. Haiping, J. Zhang, Y. Jiang, Y. Shi, X. Gu, J. Zhang, Y. Zhang, H. Jiang, and B. Chen, “Growth and spectral properties of Ho3+ doped α-NaYF4 single crystal,” Opt. Mater. 45, 209–214 (2015). [CrossRef]
35. Y. Zhang, B. Chen, S. Xu, X. Li, J. Zhang, J. Sun, X. Zhang, H. Xia, and R. Hua, “A universal approach for calculating the Judd–Ofelt parameters of RE3+ in powdered phosphors and its application for the b-NaYF4:Er3+/Yb3+ phosphor derived from auto-combustion-assisted fluoridation,” Phys. Chem. Chem. Phys. 20(23), 15876–15883 (2018). [CrossRef]
36. M. Luo, B. Chen, X. Li, J. Zhang, S. Xu, X. Zhang, Y. Cao, J. Sun, Y. Zhang, X. Wang, Y. Zhang, D. Gao, and L. Wang, “Fluorescence decay route of optical transition calculation for trivalent rare earth ions and its application for Er3+-doped NaYF4 phosphor,” Phys. Chem. Chem. Phys. 22(43), 25177–25183 (2020). [CrossRef]
37. C. Wang, H. Xia, Z. Feng, Z. Zhang, D. Jiang, J. Zhang, S. He, Q. Tang, Q. Sheng, X. Gu, Y. Zhang, B. Chen, and H. Jiang, “Infrared spectral properties for α-NaYF 4single crystal of various Er3+ doping concentrations,” Opt. Laser Technol. 82, 157–162 (2016). [CrossRef]
38. N. Ojha, M. Tuomisto, M. Lastusaari, and L. Petit, “Upconversion from fluorophosphates glasses prepared with NaYF4:Er3+, Yb3+ nanocrystals,” RSC Adv. 8(34), 19226–19236 (2018). [CrossRef]
39. R. Demirbilek, R. Feile, and C. Bozdogan, “Temperature dependent Raman spectra of CsCdBr3 and CsCdCl3 crystals,” J. Lumin. 161, 174–179 (2015). [CrossRef]
40. A.N. Romanov, A.A. Veber, T. Fattakhova, D.N. Vtyurina, M. Kouznetsov, and V.B. Sulimov, “Spectral properties and NIR photoluminescence of Bi+ impurity in CsCdCl3 ternary chloride,” J. Lumin. 149, 292–296 (2014). [CrossRef]
41. U. Hommerich, S. Uba, A. Kabir, S.B. Trivedi, C. Yang, and E. Brown, “Visible emission studies of melt-grown Dy-doped CsPbCl3 and KPb2Cl5 crystals,” Opt. Mater. Express 10(8), 2011–2018 (2020). [CrossRef]
42. M. Kobayashi, K. Omata, S. Sugimoto, Y. Tamagawa, T. Kuroiwa, H. Asada, T. Takeuchi, and S. Kondo, “Scintillation characteristics of CsPbCl3 single crystals,” Nucl. Instrum. Methods Phys. Res., Sect. A 592(3), 369–373 (2008). [CrossRef]
43. B. Ghebouli, M.A. Ghebouli, M. Fatmi, and M. Bouhemadou, “First-principles study of the structural, elastic, electronic, optical and thermodynamic properties of the cubic perovskite CsCdCl3 under high pressure,” Solid State Commun. 150(39-40), 1896–1901 (2010). [CrossRef]
44. Y. Ledemi, S.H. Messaddeq, I. Skhripachev, S.J.L. Ribeiro, and Y. Messaddeq, “Influence of Ga incorporation on photoinduced phenomena in Ge-S based glasses,” J. Non-Cryst. Solids 355(37-42), 1884–1889 (2009). [CrossRef]
45. S.H. Messaddeq, M. S. Liu, D. Lezal, Y. Messaddeq, S.J.L. Ribeiro, L.F.C. Oliveira, and J.M.D.A. Rollo, “Raman investigation of structural photoinduced irreversible changes of Ga10Ge25S65 chalcogenide glasses,” J. Optoelec. Ad. Mater. 3, 295–302 (2001).
46. V. Nazabal, F. Starecki, J.-L. Doualan, P. Nemec, P. Camy, H. Lhermite, L. Bodiou, M.L. Anne, J. Carrier, and J.L. Adam, “Luminescence at 2.8 µm: Er3+-doped chalcogenide micro-waveguide,” Opt. Mater. 58, 390–397 (2016). [CrossRef]
47. G. Louvet, S. Normani, L. Bodiou, J. Gutwirth, J. Lemaitre, P. Pirasteh, J.-L. Doualan, A. Benardais, Y. Ledemi, Y. Messaddeq, P. Nemec, J. Carrier, and V. Nazabal, “Co-sputtered Pr3+ doped Ga-Ge-Sb-Se active waveguides for mid-infrared operation,” Opt. Express 28(15), 22511–22523 (2020). [CrossRef]
48. B.M. Walsh, “Judd-Ofelt theory: Principles and Practices,” in: B. Di Bartolo and O. Forte, eds., Advances in Spectroscopy for Lasers and Sensing (Springer, 2006), pp. 403–433.
49. B.R. Judd, “Optical absorption intensities of rare-earth ions,” Phys. Rev. 127(3), 750–761 (1962). [CrossRef]
50. G.F. Ofelt, “Intensities of crystal spectra of rare-earth ions,” J. Chem. Phys. 37(3), 511–520 (1962). [CrossRef]
51. Y. V. Orlovskii, T. T. Basiev, K. K. Pukhov, M. E. Doroshenko, V. V. Badikov, D. V. Badikov, O. K. Alimov, M. V. Polyachenkova, L. N. Dmitruk, V. V. Osiko, and S. B. Mirov, “Mid-IR transitions of trivalent neodymium in low phonon laser crystals,” Opt. Mater. 29(9), 1115–1128 (2007). [CrossRef]
52. B.F. Aull and H.P. Jenssen, “Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections,” IEEE J. Quantum Electron. 18(5), 925–930 (1982). [CrossRef]
53. U. Demirbas, “Cr:Colquiriite lasers: current status and challenges for further progress,” Prog. Quantum Electron. 68, 100227 (2019). [CrossRef]
