Introduction

Passive Q-switching technologies have been extensively applied in industry, medical science, and scientific research because of its noticeable advantages with respect to simple structure and considerable efficiency [1,2,3,4]. Various materials have been used as saturable absorbers, in which the most common one is the semiconductor saturable absorber [5,6,7]. Compared with SESAM, two-dimensional (2D) materials show great potential owing to the broad bandwidth, low cost, and easy fabrication. In recent years, 2D materials like black phosphorus, graphene, and transition mental dichalcogenides (TMDs), have been widely adopted as SAs in the passive Q-switching lasers [8,9,10,11,12]. Among these reported TMDs, such as MoS2, MoSe2, and WS2, one characteristic is its indirect-to-direct bandgap change occurs when going from bulk to monolayer [13, 14].

Unlike those abovementioned TMDs, ReS2 has a direct bandgap, whose value remains ~ 1.5 eV in both bulk and monolayer forms [15]. Furthermore, the photoelectric properties of ReS2 are similar from bulk to monolayer [16]. As a semiconductor, ReS2 exhibits strong nonlinear absorption, so that ReS2 as SA has been experimentally used in solid lasers in 1.5-μm, 2.8-μm, and 3-μm wavelength [17,18,19]. Recently, ReS2 based on sapphire substrate has been reported as a saturable absorber in 1-μm laser [20]. However, the ReS2 saturable absorber was adhered to the sapphire substrate with the weak van der Waals forces, which is easily cleaved from the substrate [20]. Up to date, GaAs has been generally applied in Nd-doped solid-state lasers for Q-switching at 1 μm [21]. However, GaAs can also be combined with other semiconductors into heterostructures, such as MoS2/GaAs, MoSe2/GaAs, and PtSe2/GaAs [22]. So far, the heterostructure semiconductor MoS2/GaAs SA has been used to get shorter pulses [23], convincing us that the similar heterostructure could be attractive for the pulsed operation. The chemical vapor deposition (CVD) technology can precisely control the deposition thickness and generate cleanly lattice-matched surface. In comparison with the ReS2 on sapphire substrate, semiconductor ReS2/GaAs heterostructures as quantum well can confine the carrier and greatly improve the population inversion. The performance of the heterostructure ReS2/GaAs saturable absorber could be expected.

In this paper, the heterostucture semiconductor ReS2/GaAs is firstly fabricated. As saturable absorber, a passively Q-switched Nd:YVO4 solid-state laser was demonstrated with heterostucture ReS2/GaAs. In comparison with the ReS2 saturable absorber or GaAs semiconductor saturable absorber, the laser performance was greatly enhanced with the heterostucture ReS2/GaAs saturable absorber. The experimental results reveal that the ReS2/GaAs saturable absorber could be of great interest for passive Q-switching operation.

Methods/Experimental

Recently, the ReS2 saturable absorber is prepared by liquid phase exfoliation (LPE) owing to the low cost. However, ReS2 monolayer in our experiment was synthesized by CVD because we can precisely control the thickness of ReS2. Here, sulfur powder and ammonium perrhenate (NH4ReO4) were used as the precursors for growth. The ReS2 monolayer was grown on a clean sapphire wafer. During the deposition process, argon was employed as the carrier gas for sulfur. Then, we transferred the CVD grown ReS2 monolayer to a 110-μm-depth GaAs wafer with a dimension of 10 × 10 mm2 to make up the heterostructure. The total procedure was shown in Fig. 1.

Fig. 1
figure 1

a, b The fabrication procedure of the ReS2/GaAs heterostructure

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To make sure the layer number of the prepared ReS2/GaAs heterostructure, we investigated the Raman shift of the prepared sample (Fig. 2). The Ag modes located at 134 and 141 cm−1, while the Eg modes located at 150.7, 160.6, 210.7, and 233 cm−1. The difference of III-I peaks was 16.7 cm−1, which was considered as monolayer [24].

Fig. 2
figure 2

Raman spectroscopy of the heterostructure ReS2/GaAs

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Figure 3 shows the schematic of the passively Q-switched laser with the ReS2/GaAs heterostructure saturable absorber. A 0.1%-Nd-doped c-cut Nd:YVO4 was employed as the laser crystal, whose dimensions were 3 × 3 × 10 mm3. The passively Q-switched laser was end-pumped by a fiber-coupled diode laser at 808 nm. The pump beam was then focused into the crystal with a refocus module with a spot on the gain medium with 400-μm in diameter. A concave mirror M1 was used as the input mirror, which had antireflection (AR) coating at 808 nm on two sides and high-reflection (HR) coating at 1064 nm inside the resonator. The curvature radius of M1 was 200 mm. A flat mirror M2 worked as output coupler (OC) with the transmission at 1064 nm of 10%. A short and linear cavity with a length of about 30 mm was formed. The ReS2/GaAs (or GaAs) was then inserted into the cavity working as saturable absorber and put near the output coupler.

Fig. 3
figure 3

Schematic of the Q-switching laser cavity

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Results and Discussion

The pulse duration and repetition rate were recorded with a digital phosphor oscilloscope (DPO 7104C) via a fast InGaAs photodiode. As shown in Fig. 4 and Fig. 5, with increasing the input power from 0.5 to 2.26 W, the pulse duration from the ReS2/GaAs passively Q-switched laser decreased from 322 to 51.3 ns, while the repetition rate increased from 139 to 452 kHz. In comparison, we also set up the GaAs Q-switched laser. We can see from Figs. 4 and 5 that the ReS2/GaAs heterostructure is contributed to shortening the pulse width and lower the pulse repetition rate.

Fig. 4
figure 4

Pulse duration of the Q-switched laser versus the incident pump power

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Fig. 5
figure 5

Repetition rate of the passively Q-switched laser versus the incident pump power

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Figure 6 shows the profiles of Q-switching pulses at the pump power of 2.26 W with different semiconductor saturable absorbers. The output pulses with the pulse width of 51.3 ns and the pulse energy of 465 nJ can be achieved with the ReS2/GaAs heterostructure saturable absorber. In contrast, the output pulse duration from the GaAs Q-switched laser was 63.2 ns with the pulse energy of 435 nJ, which was shown in the inset picture. Figure 6 also implies that the symmetry of the ReS2/GaAs Q-switched pulse is comparatively much better.

Fig. 6
figure 6

Profile of the Q-switching laser based on ReS2/GaAs or GaAs at the incident pump power of 2.26 W

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The pulse energy and peak power versus the incident pump power are demonstrated in Fig. 7. With increasing pump power, there was a rapid increase in peak power. In addition, the peak power and pulse energy of the ReS2/GaAs Q-switched laser are higher than those of GaAs-based Q-switched laser at the same conditions. And for ReS2/GaAs Q-switched laser, the maximum peak power of 9.1 W and the highest pulse energy of 465 nJ can be achieved at 2.26 W pump power.

Fig. 7
figure 7

Pulse energy (a) and peak power (b) of the Q-switching laser

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We also compared our experimental results with the previous work [20] with the ReS2 saturable absorber on the sapphire substrate. The shortest pulse duration from the ReS2 Q-switched 1-μm laser was 139 ns with a repetition rate of 644 kHz, corresponding to a peak power of 1.3 W. As a consequence, the heterostucture ReS2/GaAs saturable absorber can obviously improve the laser performance, especially in terms of pulse duration, pulse energy, and peak power, when compared with the ReS2 Q-switched lasers or GaAs Q-switched lasers.

Conclusions

In summary, the heterostructure ReS2/GaAs saturable absorber was first fabricated. Based on the ReS2/GaAs heterostructure saturable absorber, the passively Q-switched Nd:YVO4 laser was demonstrated. At the pump power of 2.26 W, the minimum pulse duration of 51.3 ns with a repetition rate of 452 kHz was achieved, corresponding to the highest pulse energy of 465 nJ and the peak power of 9.1 W. Our results confirm that the heterostucture ReS2/GaAs is beneficial to improving the Q-switching performance in comparison with the semiconductor ReS2 or GaAs saturable absorbers.