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Continuous wave and SnSe2/PdSe2 passively Q-switched Nd:GdNbO4 laser under direct pumping

https://doi.org/10.1016/j.optlastec.2020.106558Get rights and content

Highlights

  • CW and pulsed1066 nm Nd:GdNbO4 laser under direct pumping was demonstrated.

  • The comparison between indirect pumping and direct pumping was performed.

  • Pulsed SnSe2/Nd:GdNbO4 and PdSe2/Nd:GdNbO4 lasers were obtained for the first time.

Abstract

In this paper, a continuous wave (CW) and two-dimensional (2D) tin diselenide (SnSe2) and palladium diselenide (PdSe2) passively Q-switched Nd:GdNbO4 laser under direct pumping was demonstrated for the first time. The CW laser performance under the pump at 879 nm was compared to that using traditional 808 nm pumping. The 2D SnSe2 and PdSe2 nanosheets were synthesized by chemical vapor deposition (CVD) technique. The optical properties and microstructures of two nanosheets were investigated. In the passively Q-switched operation, the highest repetition rate, narrowest pulse width, maximum pulse energy and peak power of SnSe2/Nd:GdNbO4 laser were 160 kHz, 555 ns, 2.3 μJ and 4.2 W, respectively. And for PdSe2/Nd:GdNbO4 laser, the corresponding values were 143 kHz, 530 ns, 3.7 μJ and 7.0 W, respectively.

Introduction

Continuous wave (CW) and pulsed lasers are widely used in several applications, such as laser sensing, laser ignition, laser processing and laser ranging [1], [2], [3], [4], [5], [6]. Laser diode-pumped solid-state lasers (DPSSLs) have advantages of compact structure, high efficiency and strong reliability [7]. The research on the gain medium is important for DPSSLs. Since the crystal of Nd:GdNbO4 was successfully grown in 2017, it has been reported as an excellent laser gain medium with excellent optical properties [8], [9]. A c-cut Nd:GdNbO4 crystal possesses large absorption cross section of 9.5 × 10-20 cm2 and wide absorption bandwidth (FWHM) of 5 nm at the wavelength of 808 nm [8]. The emission cross section of Nd:GdNbO4 at 1.06 μm is 9.3 × 10-20 cm2. In contrast with the usually used Nd:YAG (230 μs), Nd:GdNbO4 has the shorter excited state’s fluorescence lifetime of 178 μs, which could facilitate the generation of a pulsed laser with high repetition rate [10]. Therefore, it is expected to produce a high-performance 1.06 μm laser using Nd:GdNbO4 crystal.

Since Nd:GdNbO4 crystal has a strong absorption peak at 808 nm, a laser diode (LD) with emission wavelength of 808 nm has been adopted to pump the 4F5/2 level. Under this condition, quantum defect and thermal relaxation occur at the transition between 4F5/2 level and laser upper level of 4F3/2. Direct pumping is an effective method to avoid thermal relaxation process coming from no-radiation transition [11], [12]. Under direct pumping with 879 nm, the particles at ground state are directly pumped into the 4F3/2 level and laser emission will occur at the transition from 4F3/2 to 4I11/2 level. Compared with indirect pumping of 808 nm, it reduces the quantum defect rate from 24% to 17% and the thermal loading of laser crystal is approximately reduced by 28% at 1.06 μm [13]. Therefore, direct pumping is beneficial to improve the beam quality, slope efficiency and pulsed laser performances [14], [15].

Since the first development of two-dimensional (2D) graphene in 2004 [16], various 2D materials such as transition metal dichalcogenides (TMDs) have been reported. TMDs have the merits of ultra-fast recovery time, ultra-thin thickness and wide spectral range [17], [18], [19], [20], [21], [22]. Palladium diselenide (PdSe2) and tin diselenide (SnSe2) are new members in TMDs family. Compared with other 2D materials like black phosphorus (BP), they could maintain long-term stability in the air. Because of the layer-dependent bandgap, they could be suitable for the near-infrared laser. Different from the usually studied TMDs, SnSe2 and PdSe2 replace the transitional-metal W and Mo atoms with Sn and Pd atoms, which makes them possess a similar structure but different characteristics in electronics and optics. SnSe2 is a kind of narrow bandgap material because of metal Sn with outer ρ electrons, leading to indirect bandgap of around 1 eV [23], [24]. For PdSe2, the high variation value (1.3 eV) of bandgap between monolayer and bulk possesses the obvious advantage over WS2 (0.8 eV) and MoS2 (0.7 eV) [25]. Furthermore, the electron mobilities of SnSe2 and PdSe2 are 8.6 cm2V-1S-1 and 200 cm2V-1S-1, respectively [26], [27]. The relatively high values might make a positive effect on recovery time, which is advantageous for using as saturable absorbers (SAs). Both PdSe2 and SnSe2 belong to 2D layered diselenide materials, and thus the rich metal reserves on earth are provided with dominant advantages in its application fields at lower cost.

In this paper, a CW and SnSe2/PdSe2 passively Q-switched Nd:GdNbO4 laser under direct pumping was demonstrated for the first time. The CW laser performance under the pump at 879 nm was compared to that using traditional 808 nm pumping. The SnSe2 and PdSe2 nanosheets were prepared by chemical vapor deposition (CVD) technique. The optical characteristics and microstructures of SnSe2 and PdSe2 nanosheets were researched, respectively. The pulsed output performances of SnSe2/PdSe2 Nd:GdNbO4 lasers were further investigated.

Section snippets

Preparation of SnSe2 and PdSe2 nanosheets

SnSe2 and PdSe2 nanosheets were synthesized by using CVD technique. Stannous oxide (SnO) was heated to 550 °C, while selenium (Se) powder was heated to 250 °C. The carrier gas (300 sccm argon and 30 sccm hydrogen) was injected into the container to transport evaporated SnO and Se to the 500 °C area. The generation process lasted for 10 min and SnSe2 nanosheet was finally grown on the sapphire substrate after cooling to room temperature. As for PdSe2, palladium dichloride (PdCl2) was heated to

Experimental setup

Fig. 5 showed the experimental sketch of Nd:GdNbO4 laser. Two fiber coupled LDs with emission wavelength of 808 nm and 879 nm were used as pump sources in the CW operation, of which the fiber core diameters were 400 μm, respectively. The pump light emitted by LD was reshaped through a pair of plano-convex lenses L1 and L2. The focusing radius of pump beam in the crystal was ~320 μm. The input mirror M1 was coated for high reflectivity at 1.06 μm and high transmission at 808 nm and 879 nm. M2

CW operation

CW Nd:GdNbO4 laser was carried out under indirect pumping with 808 nm. The absorption coefficient for 808 nm pumping was measured to be 2.65 cm−1. The results were displayed in Fig. 6, where it could be seen that the CW output power increased with absorbed pump power. For indirect 808 nm pumping, the maximum CW output power reached 1.2 W, 1.63 W, and 1.5 W at absorbed pump power of 4.8 W in terms of the transmission of output mirror with 5%, 10%, and 15%, respectively. The corresponding slope

Conclusions

In conclusion, a CW and SnSe2/PdSe2 passively Q-switched Nd:GdNbO4 laser under direct pumping was demonstrated for the first time. The optical characteristics and microstructures of SnSe2 and PdSe2 nanosheets were investigated, respectively. The thickness of SnSe2 and PdSe2 nanosheets were 12 nm and 4 nm, corresponding to the layer number of 16 and 7, respectively. The CW laser performance under the pump at 879 nm was compared to that using traditional 808 nm pumping. The maximum CW output

CRediT authorship contribution statement

Yufei Ma: Conceptualization, Supervision. Shanchun Zhang: Investigation. Zhonghua Zhang: Validation. Xiaoxu Liu: Writing - original draft. Shoujun Ding: Data curation. Wenpeng Liu: Methodology. Qingli Zhang: Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This research was supported by National Natural Science Foundation of China (Grant Nos. 61875047, 61505041 and 51702322), Natural Science Foundation of Heilongjiang Province of China (Grant No. YQ2019F006), Fundamental Research Funds for the Central Universities, Financial Grant from the Heilongjiang Province Postdoctoral Foundation (Grant No. LBH-Q18052), University Natural Science Research Project of Anhui Province of China (Grant No. KJ2019ZD06), Joint Fund of Chinese Academy of Sciences

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