The energy band structure analysis and 2 μm Q-switched laser application of layered rhenium diselenide

Layered rhenium diselenide (ReSe2) has triggered strong interest because of its outstanding optical and electrical properties. In this paper, we prepared a high-quality multilayer ReSe2 saturable absorber with a liquid-phase exfoliation method and characterized its saturable absorption properties around 2 μm. During the Q-switching regime, a maximum average output power of 1.7 W was obtained. A shortest pulse width of 925.8 ns was measured and the corresponding single pulse energy and peak power were 17.6 μJ and 19.0 W, respectively. The results indicate that layered ReSe2 is a promising alternative as a nonlinear optical modulator near the 2 μm region.


Introduction
Short laser pulses, especially in the eye-safe range of 2 mm, 1 have constituted an active research topic in recent years because of their wide variety of applications in the elds of atmospheric gas analysis, 2 wind lidar, 3 laser surgery, 4 and so on. Passive Qswitching (PQS) and mode-locking lasers are the essential technology for generating short laser pulses with pulse width ranging from the nanosecond to femtosecond timescale. In such passive Q-switching lasers, the saturable absorber (SA) is the key element and thus the characteristics of output laser beams strongly depend on the nature of the SA materials. Up to now, semiconductor saturable absorber mirrors (SESAMs), 5 Cr:ZnS 6 , Cr:ZnSe 7 , carbon nanomaterials, 8,9 topological insulators (TI) 10,11 and black phosphorus (BP), 12 have been recognized as reliable SAs for 2 mm lasers. However, the utilization of SESAMs is restricted by reasons of the complicated and expensive fabrication process as well as narrow absorption bandwidth. The Cr-doped crystals, like Cr:ZnS and Cr:ZnSe, have played pivotal roles at 2 mm region lately. 13 Nevertheless, the lack of quality and high-cost are the key factor which constrain them further development. The extraordinary performances such as high carrier mobility, easy-preparation, cost-effective, remarkable electrical and thermal conductivity make the graphene attractive candidate as SAs during the PQS regime. But the low absorption efficiency may limit its applications. Hence, more efforts should be made to explore novel nonlinear optical materials for SAs in the 2 mm region.
Owing to possessing excellent saturable absorption properties and environmental stability, transition metal dichalcogenides (TMDs), which have layered structure of the X-M-X form (where M denotes a transition metal and X denotes a chalcogen), have been achieved encouraging results at 2-3 mm laser operations. [14][15][16][17][18][19] Rhenium diselenide (ReSe 2 ), one of the relatively unexplored members of the TMDs family, has attracted a signicant attention because of its unusual electronic and optical behaviors. [20][21][22][23][24] It crystallizes in a distorted octahedral (1T) diamond-chain structure at the triclinic symmetry. It demonstrates anisotropic response arising from its in-plane anisotropic structure. Rhenium atom contain an extra electron in the d-orbitals, 25,26 which is different from other 2H phase TMDs (e.g., MoS 2 , WS 2 ). ReSe 2 lms can be easily exfoliated from its bulk crystal because of the relatively weaker van der Waals forces between interlayers. The recovery time of ReSe 2 on the order of 10 ps, which is benecial for the ultrafast pulse generation. In addition, the layered ReSe 2 samples show no obvious signs of degradation while exposed to exterior circumstance for several weeks in our study. All the characteristics mentioned above indicate that the layered ReSe 2 is an excellent material for nonlinear optical modulate and it has been applied as SA at 1-2 mm regions. [27][28][29] In 2019, Li et al. reported a ReSe 2 Q-switched Tm:YLF laser, however, the laser pulse width and pulse repetition rate were only 1.61 ms and 28.78 kHz, respectively. 29 According to excellent properties of the ReSe 2 material, the laser pulse characteristics at 2 mm would be further improved. In this paper, a high-quality large-area multilayer ReSe 2 SA was successfully prepared and the saturable absorption properties at 2 mm region were investigated by the open-aperture Zscan method. Based on ReSe 2 as SA, a stable all-solid-state Qswitched Tm:YAP crystal laser was realized. Under an absorbed pump power of 8.5 W, a continuous-wave (CW) output power of 2.9 W was generated, corresponding to a slope efficiency of 44%. In the ReSe 2 Q-switching regime, a maximum average output power of 1.7 W was obtained, giving a slope efficiency of 37%. The shortest pulse duration of 925.8 ns was generated at a repetition rate of 89.4 kHz, resulting in the maximum single pulse energy and peak power of 17.6 mJ and 19.0 W, respectively.

Theoretical analysis of the energy band structure
A monolayer supercell of ReSe 2 with 16 Re atoms and 32 Se atoms and the same structures with one and two Se atomic vacancies are constructed. The ratios (R) between Re and Se atoms for the three structures are 1 : 2, 1 : 1.937 and 1 : 1.875. The rst principle calculation of band structure is based on the Vienna ab initio simulation package (VASP). 30 The Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) is adopted to account for the exchange and correlation part. 31 A plane wave basis set with an energy cutoff of 550 eV and the PAW pseudopotential implemented in VASP are applied. The Brillouin zone is sampled by a Monkhorst-Pack k-point mesh of 21 Â 21 Â 1. In order to avoid the interactions among ReSe 2 layers, the vacuum space thickness along the c axis is set to 20Å.
The calculated band structures corresponding to the atomic structures mentioned above are shown in Fig. 1(a-c) respectively. From Fig. 1(a), it can be seen that the monolayer supercell ReSe 2 with no vacancy has a bandgap of $1.22 eV, which is in good agreement with previous calculations of ReSe 2 . 25 While in Fig. 1(b) and (c), the bandgaps of the structures with one and two Se atomic vacancies are $0.75 eV and $0.50 eV respectively. With the increase of Se atomic vacancies, the bandgap of monolayer ReSe 2 decreases from 1.22 eV to 0.50 eV, mainly due to the reconstruction of atoms around the vacancies perturbs the periodic potential. The corresponding absorption wavelength edge for the structures mentioned above extends from 1 mm to 2.48 mm. To identify the existence of the vacancies in the ReSe 2 sample, the energy-dispersive X-ray spectroscopy (EDS) was measured. Fig. 2(a) presents the ratio of Re to Se was 1.877, which conrmed that some Se vacancies existed in the ReSe 2 sample.
These calculation results of monolayer ReSe 2 are analogous to that of monolayer ReS 2 as in ref. 18. Moreover, the bandgap of certain ReSe 2 conguration is smaller than that of ReS 2 with the same vacancy concentration which mainly attribute to the distortion of Re-Se bond is larger than that of Re-S bond. It can be concluded that the introduction of Se vacancies in monolayer ReSe 2 makes it a promising candidate as a broad band saturable absorber.

Fabrication and optical characteristics of the ReSe 2 -SA
As shown in Fig. 2(b), the ReSe 2 -SA was prepared with LPE method, which has been extensively applied in BP and other 2D materials [17][18][19]32 fabrication. This method enables large-scale exfoliation and uniform dispersion in the exfoliation medium. First, the bulk ReSe 2 with purity of 99.995%, purchased from HQ Graphene Inc, was ground to powder in a mortar. Then, the ReSe 2 powder was dispersed into ethyl alcohol followed by 4 hours sonication. To avoid feature changes of the ReSe 2 under high temperature, the ultrasonic process was operated at a suitable interval and ice was added during the ultrasonic process. Aer that, the as-prepared ReSe 2 solution was centrifuged at 4000 rpm for 10 min with the aim of removing the large-size ReSe 2 sheets and the supernatant was collected for standby. Finally, the processed ReSe 2 solution was spin coated on an output mirror (T ¼ 5% @ 1.9-2.1 mm), and dried under an infrared oven lamp.
In order to investigate the thickness and distribution of the ReSe 2 akes, the atomic force microscopy (AFM, Veeco Dimension Icon) was employed to study the morphology of the as-prepared ReSe 2 -SA. Fig. 3(a) shows an AFM scan image of the  sample in a large area, because of the weak interlayer coupling of the ReSe 2 . The average thickness of the transferred layers on the treated SAM is approximately 10 nm, as presented in Fig. 3(b), corresponding to the layer number of $14 since the height of monolayer ReSe 2 is 0.7 nm. 24 The Raman spectra of ReSe 2 -SA was measured by a Raman microscope with a 532 nm laser as the excitation source, as shown in Fig. 3(c). Due to the weak lattice coupling between the layers, these vibrational intense of the low-frequency modes are stronger and the lowfrequency modes are easier to be observed. Three typical Raman peaks at 119 cm À1 , 161 cm À1 and 174 cm À1 , respectively, were observed, which is consistent with the previous reported results. 22,27,29 The saturable absorption properties of ReSe 2 -SA were also characterized with an open aperture Z-scan method based on a home-made SESAM mode-locked 3 ps Tm:YAP laser at 1940 nm with a repetition rate of 92.5 MHz. The transmittances of the ReSe 2 -SA at different incident uences were measured and tted by the following formula: where T(F) is the transmission rate, A is the normalization constant, DT is the modulation depth, F is the input uence, F sat is the saturation uence and a ns is the non-saturable loss. By tting the curve as shown in Fig. 3(d), the modulation depth and saturation uence at 2 mm wavelength is 7.3% and 4.3 mJ cm À2 , respectively.

Experimental
To investigate the saturable absorption properties of ReSe 2 in 2 mm region, a compact concave-plano cavity with a length of 24 mm was employed, and the experimental setup is shown in Fig. 4. An high quality a-cut Tm:YAP crystal with both sides high-transmission (HT) coated at 793 nm and 1980 nm was employed as the gain medium. The pump source was a 793 nm ber coupled LD with a core diameter of 200 mm and a numerical aperture (N.A.) of 0.22. The pump beam was collimated into the Tm:YAP crystal with a spot radius of 100 mm through a 1 : 1 focusing system. The mirror M 1 was a concave mirror with a curvature radius of 300 mm coated for anti-reection (AR) at 780-810 nm on the entrance surface, and high-reectivity (HR) at 1.9-2.1 mm on the cavity surface. The output coupler M 2 was a at mirror with a transmission of 5% at around 2 mm. On behalf of removing the heat, the Tm:YAP crystal was wrapped with an indium foil and mounted in a copper block sustained at 17 C by circulating water. In the measurement of laser output power, a dichroic beamsplitter was applied to dissipate the leaking pump light from the laser cavity.    First, CW laser operation regime was achieved by using an output mirror (OM, T ¼ 5%) without the ReSe 2 -SA. To protect the gain medium, the maximum absorbed pump power was limited to 8.5 W. A maximum CW output power of 2.9 W was obtained with a slope efficiency of 44%. Then, by substituting ReSe 2 SAM for the plane OM and carefully aligning its position, the PQS operation of the Tm:YAP laser at 2 mm was realized. At the absorbed pump power of 6.4 W, the highest PQS average output power of 1.7 W was achieved, corresponding to a slope efficiency of 37%. While increasing pump power, the ReSe 2 sample got damaged. Fig. 5(a) shows the output power of the CW and PQS laser operations versus the absorbed pump power, respectively. Compared to the CW laser operation, the PQS laser threshold was slightly raised from 1.68 W to 2.55 W and the slope efficiency was dropped from 44% to 37% on account of insertion loss.
The Q-switched pulse trains and proles were detected by a fast InGaAs photodetector (Newport, Model 818-BB-51) with a rise time of 35 ps and recorded with a digital oscilloscope (Tektronix DPO 7104C, 1 GHz bandwidth, 20 Gs s À1 sampling rates). Fig. 5(b) presents the pulse repetition rate and width as a function of absorbed pump power. In the stable PQS regime, the pulse repetition rate and width varied from 19.5 kHz to 89.4 kHz and 1.8 ms to 925.8 ns, respectively. Under the highest absorbed pump power, the maximum pulse energy was reached to 17.6 mJ, corresponding to the maximum peak power of 19 W. The relationships between the pulse energy and peak power on the absorbed pump power are described in Fig. 5(c). The measured spectrum was inset in Fig. 5(c), it obviously illustrates that the central wavelength of CW laser is 1987.5 nm while it is 1937.8 nm for the Q-switched laser. The blueshi phenomenon were might be due to the high intracavity loss introduced by the ReSe 2 -SA. The typical pulse width of 925.8 ns and pulse repetition rate of 89.4 kHz are shown in Fig. 5(d). Fig. 6 shows the radio frequency (RF) spectrum of the output laser, and the signal-to-noise ratio (SNR) of the rst beat was 30 dB at 89.4 kHz, indicating that the Q-switched laser pulse was relatively stable. Table 1 summarizes the 2 mm PQS Tm:YAP laser results ever achieved by employing nanomaterials as SAs. The BP Qswitched 2 mm laser in ref. 33 had higher peak power, pulse energy and shorter pulse width. However, the shortcoming of instability in air limited the application of BP in generating long-time stable pulsed laser. Compared with the other Nano-SAs, such as graphene, MoTe 2 , MoS 2 and ReS 2 , our results could be considered superior than that. We believe that a better laser performance would be achieved by further optimizing the parameters of laser cavity and the characteristics of ReSe 2 -SA.

Conclusions
In summary, the band structure of the ReSe 2 with different ratios of Re and Se was calculated by VASP and the results were that Se atomic defects have the tendency to reduce the bandgap. The few-layered ReSe 2 -SA was prepared by LPE method and the saturable absorption effect was characterized around 2 mm. Using the as-prepared ReSe 2 -SA, a PQS 2 mm Tm:YAP laser was demonstrated for the rst time, to the best of our knowledge. A maximum PQS average output power of 1.7 W was obtained, corresponding to an optical-to-optical and a slope conversion efficiency of 26.6% and 37%, respectively. The shortest pulse duration of 925.8 ns was realized with a pulse repetition rate of 89.4 kHz, resulting in the maximum single pulse energy and peak power of 17.6 mJ and 19.0 W, respectively. The results indicate that the feasibility of layered ReSe 2 as SA for achieving high-efficient 2 mm pulsed lasers.

Conflicts of interest
There are no conicts to declare.