Heterostructure ReS₂/GaAs Saturable Absorber Enables Ultra‑Fast Passively Q‑Switched Nd:YVO₄ Laser

Abstract

We report the first fabrication of a ReS₂/GaAs heterostructure via chemical vapor deposition and its deployment as a saturable absorber (SA) in a passively Q‑switched Nd:YVO₄ laser. The device achieves a record‑shortest pulse width of 51.3 ns at a repetition rate of 452 kHz, delivering 465 nJ per pulse and a peak power of 9.1 W. Compared with single‑material SAs (ReS₂ or GaAs), the heterostructure delivers shorter pulses and higher pulse energy, confirming its superior performance for high‑efficiency Q‑switching.

Introduction

Passive Q‑switching remains a cornerstone technology for generating high‑energy, short‑duration laser pulses in industrial, medical, and research applications. Conventional semiconductor saturable absorber mirrors (SESAMs) offer excellent performance but are limited by high cost and narrow bandwidth. Two‑dimensional (2D) materials such as graphene, black phosphorus, and transition‑metal dichalcogenides (TMDs) have emerged as promising alternatives, offering broadband operation, low cost, and facile fabrication.

Among TMDs, ReS₂ stands out because it retains a direct bandgap (~1.5 eV) in both bulk and monolayer forms, and its photo‑electric response is remarkably consistent across thicknesses. Previous work has demonstrated ReS₂ SAs in 1.5‑μm, 2.8‑μm, and 3‑μm solid‑state lasers, and recently a sapphire‑based ReS₂ SA at 1 μm. However, the weak van der Waals adhesion to sapphire limits device robustness. GaAs, routinely employed in Nd‑doped lasers, can be integrated with other semiconductors to form heterostructures (e.g., MoS₂/GaAs) that enhance carrier confinement and improve SA performance.

In this study, we synthesize a ReS₂/GaAs heterostructure by transferring CVD‑grown ReS₂ onto a 110‑μm GaAs wafer. We then evaluate its performance as a SA in a Nd:YVO₄ laser and compare it against conventional ReS₂ and GaAs SAs.

Methods / Experimental

ReS₂ monolayers were synthesized by CVD using sulfur and ammonium perrhenate precursors on a clean sapphire substrate. The resulting monolayer was transferred onto a 10 × 10 mm², 110‑μm thick GaAs wafer (cut along the (111) orientation) to form the heterostructure (see Fig. 1). Raman spectroscopy confirmed monolayer thickness via the characteristic A_g modes at 134 and 141 cm⁻¹ and E_g modes at 150.7, 160.6, 210.7, and 233 cm⁻¹, with a III–I peak separation of 16.7 cm⁻¹.

Figure 1. (a) Schematic of the CVD growth process; (b) Transfer of ReS₂ onto GaAs wafer.

The laser cavity consisted of a 0.1 % Nd‑doped c‑cut Nd:YVO₄ crystal (3 × 3 × 10 mm³) end‑pumped by an 808 nm fiber‑coupled diode laser. A 400‑μm pump spot was focused into the crystal. The resonator used a concave input mirror (200 mm radius, HR at 1064 nm, AR at 808 nm) and a flat output coupler (10 % transmission at 1064 nm). The ReS₂/GaAs (or GaAs) SA was positioned near the output coupler inside the ~30 mm long cavity (see Fig. 3).

Figure 2. Raman spectra of the ReS₂/GaAs heterostructure.

Figure 3. Schematic of the Q‑switched laser cavity.

Results and Discussion

Pulse characteristics were recorded with a fast InGaAs photodiode and a digital oscilloscope. As the pump power increased from 0.5 to 2.26 W, the pulse duration shortened from 322 ns to 51.3 ns, while the repetition rate rose from 139 kHz to 452 kHz (Fig. 4–5). The heterostructure SA consistently outperformed both the standalone ReS₂ and GaAs SAs, yielding the shortest pulses and lowest repetition rates.

Figure 4. Pulse duration versus incident pump power.

Figure 5. Repetition rate versus incident pump power.

At the highest pump power (2.26 W), the heterostructure SA produced 51.3 ns pulses with 465 nJ energy and a peak power of 9.1 W (Fig. 6). In contrast, the GaAs SA delivered 63.2 ns pulses at 435 nJ. The pulse shape symmetry was also improved with the heterostructure.

Figure 6. Pulse profiles for ReS₂/GaAs and GaAs SAs at 2.26 W.

Peak power and pulse energy as functions of pump power are shown in Fig. 7. The ReS₂/GaAs SA consistently achieved higher values than the GaAs SA under identical conditions.

Figure 7. (a) Pulse energy; (b) Peak power versus pump power.

Compared with a sapphire‑based ReS₂ SA (139 ns pulses at 1.3 W peak power), the heterostructure demonstrates markedly superior performance across all key metrics.

Conclusions

We have successfully fabricated a ReS₂/GaAs heterostructure SA and integrated it into a passively Q‑switched Nd:YVO₄ laser. At 2.26 W pump power, the laser achieved 51.3 ns pulses, 452 kHz repetition rate, 465 nJ pulse energy, and 9.1 W peak power—outperforming conventional ReS₂ and GaAs SAs. These results confirm that ReS₂/GaAs heterostructures are highly effective for high‑performance Q‑switching applications.

Abbreviations

2D

Two‑dimensional

AR

Antireflection

CVD

Chemical vapor deposition

HR

High reflection

LPE

Liquid phase exfoliation

OC

Output coupler

SESAM

Semiconductor saturable absorber mirror

TMD

Transition‑metal dichalcogenide