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Abstract
In this paper, we report a 946nm double Q-switched laser side pumped by an 808-nm pulse laser diode (LD). A layered tungsten diselenide (WSe2) saturable absorber (SA) together with an MgO doped LiNbO3 electro-optic (EO) modulator is applied to double Q-switch the Nd: YAG laser, producing trains of nanosecond-duration pulses with 500 Hz repetition rate. Such WSe2 saturable absorbers are fabricated by chemical vapor deposition (CVD) in a hot wall chamber and then embedded into a resonant mirror. The achieved pulse energy of double Q-switched laser at 946 nm is approximately 2.63 mJ with 10.8 ns pulse width and the peak power is round 244 kW, corresponding to the beam quality factors of M2x = 3.846,M2y = 3.861. Monolayer WSe2 nanosheets applied in the experiment would be a promising SA for passive Q-switching operation.
© 2017 Optical Society of America
1. Introduction
Pulse laser at 946nm has plenty of potentials in vapor detection, diffraction absorption radar and blue light (i.e. 473 nm) generation [1–6]. However, 946 nm light emitted (4F3/2→4I9/2) from Neodymium ion (Nd3+) is based on a quasi-three level system, which is less effective than the 1064 nm emission via a four-level model (4F3/2→4I11/2). A significant drawback of the 946-nm emission is that the corresponded gain cross section is smaller by about an order of magnitude than that of the 1064 nm, leading to relatively high laser threshold. Moreover, there could be considerable reabsorption losses owing to population in the lower energy level, which is the upper 857 cm−1 crystal-field component of 4I11/2 ground-state transition via Stark splitting [7, 8]. Pulsed lasers at 946 nm have been extensively studied since 1970s, of which most were obtained by end-pumped short gain media to generate only a few tens to hundreds of micro-joules pulse energy [9–11]. However, improvement of the pulse energy generated from quasi-three level system is still not comparable with four-level system due to relatively low pump power and gain of the media. Technical success in producing high quality electro-optic crystals (i.e. BBO, KDP, RPT and LiNbO3) and diode-laser arrays has stimulated studies on pulsed lasers with higher single pulse energy and peak power [12–14]. However, inferiorities like easy deliquescence, high price and low damage threshold have limited the application of BBO, KDP, RPT and pure LiNbO3 crystal. MgO doped LiNbO3 crystal has thus been widely used as Q-switcher due to its advantages of anti-light interference and refraction, relatively high damage threshold and lower half-wave voltage [15].
Passive Q-switching, as another approach to derive pulsed laser, is usually achieved by saturation absorbers (SAs) such as Cr4+: YAG crystals, semiconductor saturable absorber mirrors (SESAMs), graphene and Black Phosphorus (BP) [16–21]. Transition metal dichalcogenides (TMDs), with a general chemical formula MX2, where M and X represent a transition metal and a chalcogen, respectively, have also been discovered as promising saturable absorbers in recent years [22–25]. Layered WSe2 which is known as a member of the TMDs has also been used as saturable absorber for pulse laser generation. However, reports on WSe2 and similar materials used as SAs in passive Q-switched/mode-locked lasers are most at ~1.55 μm [26–28], only a few of them are reported operating at wavelengths under 1 μm [4, 29]. Few of the reports were about WSe2 used as a saturable absorber for passive Q-switched lasers working at 946nm.
Despite of extensive fundamental studies on active or passive Q-switching operation reported before, there still remains inferiorities including relatively long pulse width, unstable pulse train and asymmetric waveform (e.g., fast rising edge and slow falling edge in active Q-switched pulses or slow rising edge and fast falling edge in passively Q-switched pulses). In addition, the pulse energy and peak power are not as high as expected values due to the inferior properties of optical modulators or saturable absorbers [30, 31]. Combination of both active and passive Q-switching operations could achieve output pulses with higher peak power compared with that of individual EO or passive Q-switching operation [32]. Meanwhile, symmetric pulses with shorter pulse width and better beam quality could be achieved by double Q-switched lasers [33].
Here, we demonstrate a double Q-switched Nd: YAG laser at 946 nm based on an MgO: LiNbO3 EO modulator and a monolayer WSe2 saturable absorber mirror. The MgO: LiNbO3 EO modulator and the WSe2 saturable absorber mirror work as active and passive Q-switchers, respectively. Such Q-witched laser with both passive and active Q-switching operating simultaneously could effectively compress the pulse width, get higher peak power and better beam quality. The WSe2 nanosheets is fabricated by chemical vapor deposition (CVD) method in a hot wall chamber and then transfers to a high reflection mirror in order to form a saturable absorption mirror. For the first time, to the best of our knowledge, 946 nm double Q-switched pulses with 244kW peak power, 2.63 mJ pulse energy, 10.8ns pulse width at the repetition rate of 500Hz were obtained. Further, we theoretically calculate the pulsed laser performance with rate equation model by taking the spatial photon density distributions of the pump beam, and the population-inversion density into account. The calculation results are reasonably consistent with the experimental value of the pulse width.
2. Material fabrication and characterization
In the experiment, the CVD method was applied to grown WSe2 films in a hot-wall chamber [34, 35]. The chamber was heated by an external power source and the substrates were heated by radiation from heated chamber walls. Layered WSe2 film was deposited on sapphire substrates by selenizing WO3 powders in a hot-wall CVD chamber [36–38]. The WO3 powders (250 mg) and Se powders (300 mg) were placed in two ceramic boats with ~16 cm separation along the gas stream direction. Several sapphire substrates were placed at downstream where the reaction product precisely deposited. Ar and H2 mixed gases (95% Ar and 5% H2 with flow rate of 50 sccm) were injected into the chamber to help with reaction. Temperature in the chamber was increased to 1000 °C with 30 °C/min ramping rate, and then remained constant for 5 minutes. High quality monolayer WSe2 film could be deposited on the substrates in this process. The film grown on sapphire was then transferred on high reflection mirror to form saturable absorber mirror and other samples remained were prepared for characterization.
Raman spectrum was measured by a confocal Raman spectroscopy (Alpha 500R, WITec GmbH, Germany) excited with a 532-nm laser, as shown in Fig. 1(a), where two characteristic peaks corresponding to E2g1 (256.5 cm−1) and A1g (265.3cm−1) were observed. This verified that the fabricated material was WSe2 and the layer number was monolayer or few-layer [39, 40].
Fig. 1 (a) Raman spectrum for WSe2 nanosheets. Insets: saturable absorber mirror coated with WSe2 film (upper), and optical microscopic morphology of WSe2 sheet on sapphire substrate (lower). (b) Atomic force microscopy (AFM) measured thickness profile of the WSe2 film along the green line in the inset. (c) Linear absorption spectrum of the monolayer WSe2. (d) Normalized nonlinear optical transmittance measured based on Z-scan measurement by using a 1030 nm laser with 340 fs (red dots) and theoretical fitting (green solid line) of the monolayer WSe2.
It was noticed that no characteristic peak was shown at 308 cm−1 in the spectrum, which was known as a fingerprint for monolayer WSe2 corresponding to the vibration between layers, hence the WSe2 nanosheets fabricated was proved to be monolayer, coincided with former studies [37, 39, 41]. Atomic force microscopy (AFM, Nano View-1000, NanoSystem, Korea) was then applied to measure the thickness and morphology of WSe2 film. The image and cross-section profile for WSe2 nanosheets grown on sapphire in Fig. 1(b) showed the morphology and thickness of layered material. Thickness analyzed from the profile was measured to be 0.79 nm [42, 43].
Optical absorption properties of few-layer WSe2 is studied intensively by measuring the coefficient relevant to linear absorption (α) and nonlinear absorption (β). The curve in Fig. 1(c) was linear absorption spectrum (R1, IdeaOptics, China) from visible to near infrared band [44]. It was demonstrated that optical absorption reduced dramatically with increase of wavelength. The linear absorption coefficient (α) was calculated according to the Lambert's Law I = I0e-αL, in which I is the transmitted light intensity, I0 represents the incident light intensity, and L is the film thickness. Taking the values of I0 (T = 99.64% at 946nm) and L (0.79 nm) into account, α was then calculated to be 8.05 × 106 cm−1 at the incident wavelength of 946nm. To measure the nonlinear absorption coefficient of the same material, an open aperture (OA) Z-scan excited with 1030 nm fs pulsed laser was carried out, the result was shown in Fig. 1(d). Theoretical curve fitting was obtained by taking Eq. (1) into account [45, 46]:
where 𝛼0L is the modulation depth, Is is the saturation power intensity, L is the thickness of monolayer WSe2, αns is the non-saturable absorption coefficient, which was calculated to be 13.55% for the WSe2 film. The modulation depth of monolayer WSe2 was ~0.2% and the saturable power intensity was calculated to be 0.014 GW/cm2.3. Double Q-switched 946 nm Nd: YAG laser with monolayer WSe2 saturable absorber
3.1 Laser setup
A linear-cavity was designed with two plain-concave mirrors as reflective and output mirrors, as schematically depicted in Fig. 2. A 0.6 at. % Nd3+ doped Nd: YAG crystal rod (Φ3 × 65 mmrod) was used as the gain medium, which was side pumped by an 808-nm pulsed laser diode array and water cooled to 12 °C. Mirror M1 and M2 were coated for high reflection at 946 nm and anti-reflection at 1064 nm and 1319 nm. The transmittance of the output mirror M2 at 946 nm was measured to be 8%. An EO crystal MgO: LiNbO3 cut for double Brewster's angle (calculated to be 66.9°) was applied to actively modulate the laser for polarized pulses output. Such crystal cut for Brewster's angle could avoid losses induced by other optical elements (e.g. phase retarder and polarizer). A synchronous signal generator was employed to ensure the LD arrays and the MgO: LiNbO3 modulator being triggered synchronously.
Considering the fixed size of Nd: YAG rod and MgO: LiNbO3 crystal, thermal effect in Nd: YAG could possibly induce deterioration of beam quality, stability and even detuning of laser resonance. This requires optimization of resonant cavity to attain thermal insensitivity within a certain range of focal lengths of thermal lens. By revising the formula of thermal focal length [47], the measured value of focal length declined from 150 mm to 80 mm with 70A and 120A pump current, respectively. The structure of resonant cavity was simulated by LSCAD [48] and the parameters were optimized based on the fixed size of gain medium and MgO: LiNbO3 crystal. The radii of curvature for the high reflection mirror M1 and output mirror M2 were calculated to be 2.5 m and 1.0 m, respectively, corresponding to a resonant cavity length of 120 mm. Stability of the sagittal and tangential beams were calculated with cavity length ranging from 80mm to 150mm and the results were as follows:
where s and t represent sagittal and tangential beam spot, respectively. Simulation results in formula (2) and (3) showed that the optimized resonant cavity coefficient was smaller than 1, demonstrating that the cavity was thermal insensitive. The variation of beam spot size versus different thermal focal lengths was also simulated and shown in Figs. 3(a) and 3(b). Radius of the beam at different positions in the resonant cavity was also calculated. The results showed that with 150 mm thermal focal length, spot radius at the reflective mirror and inside the gain media was ~1.7mm, as well as ~1.5mm at the output mirror. Moreover, radius of the beam at output mirror maintained when the focal length was 80 mm yet reduced by 0.1 mm at the reflective mirror and inside the gain media. Such uniformity of laser beam could improve the stability and beam quality of laser output. Larger laser spot size could also reduce the risk of damage to WSe2 film, crystals and optical coatings.Fig. 3 Simulation results for laser-spot diameter variation with resonant cavity at (a) 150 mm and (b) 80 mm focal length of thermal lens.
3.2 Experimental results
In the cavity without insertion of the WSe2 saturable absorber, active Q-switching operation could be initiated by the EO modulator. Likewise, passive Q-switching operation was achieved when the saturable absorber mirror was positioned as end-reflection mirror and EO modulator was turned off. Double Q-switching operation was obtained when EO Q-switcher was turned on together with saturable absorber integrated into the reflection mirror, and thus active and passive optical modulation would dominate the laser pulse generation. Comparative experiments with and without WSe2 integrated were carried out to analyze saturable absorption of monolayer WSe2. Meanwhile, the output properties for active, passive and double Q-switching operations were studied and summarized in Figs. 4(a) and 4(b). Q-switching operation started at 82 A pump current when only EO modulator was used. At the highest pump current of 116 A, 4.85 mJ pulse energy, 22.1 ns pulse width, 219 kW peak power and 500 Hz repetition rate were acquired, respectively. While for passive Q-switching operation at the same pump current, a pulse string with 500 Hz repetition rate, 3.14 mJ pulse energy, ~230 μs pulse width was obtained. Each pulse string contains ~30 sub-pulses, corresponding to 125 kHz repetition rate. The single pulse energy, pulse width and peak power of the sub-pulse was ~0.105 mJ, ~49.5 ns and ~2.11 kW, respectively. After the monolayer WSe2 saturable absorber was coated on the high reflection mirror M1, the laser was turned to double Q-switching operation. Insertion loss of WSe2 slightly increased the Q-switching threshold pump current to 90 A. The maximum output of 2.63 mJ, peak power of 244 kW, pulse width of 10.8 ns and repetition rate of 500 Hz were obtained at 116 A of pump current. Figure 4(b) illustrates the actively, passively and double Q-switched lasers centered at 946.3 nm, corresponding to the line widths of 0.249 nm, 0.214 nm and 0.196 nm, respectively.
Fig. 4 (a) The output energy, pulse width and (b) linewidth of 946 nm pulse laser with EO, passive and double Q-switching operation
Additionally, by using a laser beam quality analyzer without power attenuation (ModeScan1740, Photon USA Inc.), the beam quality factors M2 of the laser were measured as Mx2 and My2 of (6.460, 6.876), (4.651, 4.768) and (3.846, 3.861), corresponding to actively, passively and double Q-switching operation, respectively. The 2D and 3D near field beam profiles of the three Q-switch regimes are shown in Figs. 5(a)-5(c).
Fig. 5 2D and 3D near field beam profiles of (a) EO Q-switching, (b) passive Q-switching and (c) double Q-switching operation.
4. Discussion
From beginning of the CVD fabrication process, the sapphire substrates are exposed to the chamber filled with Se and WO3 precursors. Ar and H2 are the necessary catalyst for the synthesis of WSe2 due to relatively lower chemical reactivity of Se compared to S. In addition, hydrogen is proved to be essential in the senelization of WO3 powders by following chemical reaction Eq [37, 49]:
It should be mentioned that all the measurements of the fabricated materials were carried out with the monolayer WSe2 on sapphire substrate, rather than WSe2 dispersion or flakes derived from centrifugation. Such measurements could ensure all the characterization results were from monolayer format.
Z-scan measurements excited by a femto-pulse laser operation at 1030 nm are carried out to identify the saturable absorption of the monolayer WSe2 film. We then measured the transmittance of the blank sapphire substrate in the same Z-scan setup to exclude the possible nonlinear absorption, the result showed there was no nonlinear response, further confirming that the nonlinear absorption was solely contributed by the monolayer WSe2.
We note that monolayer WSe2 has a band gap of 1.65 eV [42] while it displayed saturable absorption property at 946 nm (1.3108 eV) which had been proved in our experiment. Both experiments have demonstrated that saturation absorption also happens in the case that single photon energy is smaller than the material bandgap. Various explanations have been proposed including crystallographic defect-induced sub-bandgap absorption, coexistence of semiconducting and metallic states, edge state of the materials and two-photon absorption saturation [24, 50–52], however, the ultimate interpretation of the mechanism is still not clear.
It could be observed from the Z-scan measurement (shown in Figs. 1(d)) that in addition to saturable absorption demonstrated, two-photon-absorption (TPA) effect was also observed since the transmittance dropped as the incident power intensity increased [52], considering that the monolayer WSe2 is semiconductor with a bandgap of ~1.65 eV. TPA is a process where two photons are absorbed simultaneously by the material. The electrons in the valence band of WSe2 could absorb two photons simultaneously and transit to the conduction band when is excited by fs pulses at 1030 nm. Despite of this, the monolayer WSe2 still exhibits a typical saturable absorption at relatively low power intensity with photon energy at 1030nm, which is less than the bandgap of monolayer WSe2. Similar phenomena have also been reported by other literatures [51, 53]. Crystallographic defects might provide a proper explanation for such phenomena. Most of the layered materials used as saturable absorbers are in finite area, and as such, they could not be considered as infinite crystal structure. Symmetry broken at the edge of the flacks and unsatisfactory bonds between W and Se atoms at edge might modify the band structure of the atomic planes, creating the edge-states in the bandgap. Moreover, edge-states effect could become pronounced in the sheets with large edge to surface area ratio [54]. This suggests that the edge-states might play an important role in defining the band structure of the WSe2 and resulted in photon density within the bandgap of monolayer WSe2 [55]. Absorptions with sub-bandgap energy could be permitted and thus the electrons are able to transit from valence band to these states with reduced energy induced by the edge-states, which could be saturated to initiate Q-switching operation.
Figures 6(a)-6(f) show the pulse train and single pulse profiles of the EO, passive and double Q-switching operations, respectively. It was clearly seen that the pulse train had much smoother profile after inserting WSe2 saturable absorber into the cavity. Passive Q-switching operation was achieved with monolayer WSe2 imbedded into the reflective mirror, generating a pulse string with repetition rate of ~125 kHz and pulse width of 49.5 ns. However, it was noticed that fluctuations would affect the pulses due to thermal effect of the materials. Pulse width obtained by EO Q-switching operation in the experiment were shorter compared with that of passive Q-switching operation. Stable pulse train could also be achieved with only EO modulator in the experiment, yet initial inferiorities of 946 nm emissions like serious reabsorption losses of the lower energy level, which could possibly result in instability of EO Q-switching operation and deteriorate the beam quality. By cooperating both EO modulator and monolayer WSe2 saturable absorber, pulses with better beam quality and shorter pulse width were achieved.
Fig. 6 Output pulse train at 500Hz repetition rate of (a) EO, (b) passive and (c) double Q-switching operation and corresponding pulse profiles of (d) EO, (e) passive and (f) double Q-switching operation.
In theory, Q-switching operation could be well explained by rate equations [56]. A diode-pumped double Q-switched laser is considered where an Nd: YAG rod, an EO modulator and a saturable absorber are included. Intracavity photon density, inverted population density in the gain media as well as photon density in the saturable absorber are approximately assumed as plane wave represented as 𝜙. If the spontaneous radiation is neglected during the pulse formation, and considering the absorption of WSe2 saturable absorber together with the turnoff time of EO modulator, we could obtain the rate equations of a diode-pumped double Q-switched Nd: YAG laser with EO and saturable absorber:
where tr is the round-trip time of light in the resonator; σ and l are the stimulated-emission cross section and length of the Nd: YAG rod, respectively; n is the spatial distribution of the population-inversion density; n0s is the total population density of WSe2 saturable absorber; ngs and nes are the population density of ground state and excited state, respectively; σgs and σes are the cross section of ground state and excited state of the saturable absorber; n0s is the total population density of the WSe2 saturable absorber; ls is the saturable absorber thickness; B is the coupling coefficient of the TPA [46]; δe is the loss function of the EO Q-switcher defined as δe(t) = cos2[πV(t)/2Vλ/4], where Vλ/4 is the quarter wave voltage [57]; R is the reflective index of output mirror of the resonant; L is the intrinsic loss; c is the velocity of light in vacuum; τ is the stimulated-radiation lifetime of the gain medium; Rin = Pinη/hγp is the pump rate, where Pin is the pump power, hγp is the single photon energy of the pump light; γ is the inversion coefficient which is assumed to be 1 for a four-level system and 2 for a three-level system. EO and passive Q-switching could be achieved respectively if neglecting different terms in the above-mentioned rate equations. Part of the parameters are shown in Table 1. According to the parameters, the pulse profiles of EO, passive and double Q-switching operation are obtained by solving the rate equations. The theoretical pulse profiles of EO, passive and double Q-switching are achieved (shown in Figs. 7(a)-7(c)). It is observed that simulated pulse width of EO, passive and double Q-switching operations were 21.1ns, 50.2 ns and 9.8 ns. Figures 7(a)-7(c) respectively compared with Figs. 6(d)-6(f) and shows that the theoretical simulation and experimental results were reasonably consistent.
Table 1. parameters for Q-switching operations
Absorption property of WSe2 saturable absorber could be used to reduce the offset voltage at the trailing edge of the pulses, further improving the pulse symmetry as well as compressing the pulse width. Pulse width of double Q-switching was thus reduced more than 50% in contrast to EO Q-switching and 75% compared with passive Q-switching operation. Moreover, it was observed that the linewidth of 946nm was reduced by ~0.196 nm in double Q-switching operation. This compression of linewidth might also benefit from saturable absorption properties of WSe2. The pulse width for double Q-switching operation was quite short, thus only a few longitudinal modes close to the center wavelength were allowed to oscillate in the cavity and amplified. The longitudinal modes far from the center wavelength were suppressed because their gain was smaller than the increased threshold, leading to mode number reduction. Thus, the monolayer WSe2 saturable absorber also functions as a longitudinal modes selection device. More detailed comparison of laser performance and beam qualities was also carried out and summarized in Table 2.
Table 2. Comparison of different Q-switching operations at the repetition rate of 500 Hz
The Peak-to-peak instability was calculated by the formula ± (Nmax-Nmin) × 0.5/Naverage, where N represents the duration or energy of output pulses. It was obvious to see that in the cavity with WSe2 saturable absorber, pulse width and line width of the laser were significantly reduced, and the pulse stability was greatly improved. On the contrary, the pulse energy decreased because of the inevitable insertion loss caused by the device. These results demonstrated that WSe2 saturable absorber worked as a pulse filter had better modulation depth compared with sulfides of the same type [25]. It was observed that the energy of single pulse decreased while the oscillation threshold increased at the same time. The reason for such phenomenon was that photons generated from oscillation were absorbed by the electrons of valence-band which had introduced insertion loss as well as the filtering effect. Meanwhile, pulse width was compressed due to the saturable absorption of material, consequently leading to increase of peak power.
Only slight fluctuation of pulse width appeared over the whole range of pump current owing to stable modulation depth of monolayer WSe2 film, which is also advantageous to pulse stability and symmetry. During the experiment, it was observed that pulse train generated and gradually stabilized with increasing input power, whereas fluctuation occurred while continuously increasing pump current and eventually no regular pulse train occurred. The reason for such phenomenon might be that thermal effect produced by continuous operating of laser system could induce slight changes in lattice structure and non-uniform of monolayer WSe2 film.
Generalizing the above discussion, we propose that WSe2 could be used as a SA to Q-switch a solid-state laser operating at 946 nm by directly integrated into the resonant cavity. Direct-bandgap structure of monolayer WSe2 was benefited to passive Q-switching operation which could help with compression of pulse width and increase of output power for the laser.
5. Conclusion
In conclusion, we introduced a double Q-switched Nd: YAG laser operating at 946 nm by using MgO: LiNbO3 EO modulator and hot wall chamber CVD grown monolayer WSe2 saturable absorber. Experimental observations showed that our WSe2 saturable absorber could significantly improve pulse stability and compress pulse width comparing to the active Q-switching operation with EO modulator. Output pulse with 2.63 mJ pulse energy, 10.8 ns pulse width at 500 Hz repetition rate was achieved, corresponding peak-to-peak instabilities of pulse energy and pulse width were ± 3.65% and ± 2.58%, respectively. Meanwhile, the beam quality factor (M2x = 3.846,M2y = 3.861), far-field divergence angle (θx = 3.44 mrad, θy = 3.53 mrad), and beam waist size (dx = 1.510 mm, dy = 1.488 mm) were measured, respectively. These results demonstrated that monolayer WSe2 was an effective saturable absorber for Q-switching at sub-1 μm wavelength region. Double Q-switching operation would be advantageous to simple passive or active Q-switching operation in pulse width compression and beam quality optimization.
Funding and Acknowledgments
This work is partially supported by Key Laboratory Project of Shaanxi (No. 2010JS112) and National Natural Science Foundation of China (No. 61205114)
We thank Six Carbon Company for technical support. We thank Prof. Jinhai Si, Minmin He, Yang Yu and Boyang Zhao for fruitful discussion. We appreciate Shaanxi Key Lab of Information Photonic Technique, School of Electronics and Information Engineering, Xi’an Jiaotong University for using their equipment.
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