Nanosecond and femtosecond lasers based on black arsenic-phosphorus alloys saturable absorber

Shengjun Huang; Yiran Wang; Jingliang He; Xiancui Su; Jie Liu

doi:10.3788/COL202220.021408

Abstract

In this paper, the high-repetition-rate passively Q-switched (PQS) and the femtosecond continuous-wave mode-locked (CWML) lasers are successfully obtained with 2D black arsenic-phosphorus (b-AsP) nanosheets as saturable absorber (SA) at 1 μm for the first time, to the best of our knowledge. The saturable absorption properties and ultrafast carrier dynamics of the 2D b-AsP SA are explored by Z-scan and pump-probe techniques. Moreover, according to the measurement of desired nonlinear optical characteristics of the relaxation time of 27 ps and the modulation depth of 7.14%, the PQS and CWML lasers are demonstrated with the highest repetition rate of 2.26 MHz in the PQS laser and the pulse width of 470 fs in the CWML laser. The results show 2D b-AsP SA has enormous potential for pulse modulation in solid-state bulk lasers.

Keywords

mode-locked lasers nonlinearity passively Q-switched lasers ultrafast lasers

1. Introduction

After the first, to the best of our knowledge, discovery of graphene in 2004, two-dimensional (2D) nanomaterials have attracted extensive attention due to the quantum confinement effect on the unique mechanical, optical, and electronic properties^[1–11]. After graphene, many 2D nanomaterials have sprung up, like transition metal dichalcogenides (TMDs), topological insulators, and black phosphorus (BP), covering an extensive electromagnetic spectral range including insulators, semiconductors, semimetals, and metals^[12–14]. Among these 2D nanomaterials, BP has a widely tunable band gap with different layers from 2.2 eV in monolayer to 0.3 eV in its bulk form, high electron mobility, and excellently strong in-plane anisotropic physical properties^[15–17]. It has been widely applied in optoelectronic devices, energy conversion, medicine, and other fields^[18–25]. However, BP is easy to oxidize in air, which significantly limits its further applications^[26].

One effective way to improve the properties of 2D materials is alloying or doping, which has been demonstrated in a variety of materials like graphene, boron nitride (h-BN), and TMDs^[27–31]. In 1994, Shirotani etal. reported the superconducting properties of the alloys with synthesizing BP and phosphorus-arsenic alloys firstly, to the best of our knowledge^[32]. In 2015, Liu etal. demonstrated the characteristic of layered 2D black arsenic-phosphorus (b-AsP), where the layered b-AsP materials were semiconductors with tunable band gaps from 0.3 eV down to 0.15 eV via tuning the chemical compositions during material synthesis^[33]. Until now, b-AsP has been widely researched in electronic and optoelectronic devices due to its unique photoelectric and anisotropic properties and its better air stability compared with BP^[34–37]. Nevertheless, the nonlinear optical properties of 2D b-AsP have not been reported, especially the saturable absorption properties applied to solid-state bulk lasers.

In this paper, a high-quality 2D b-AsP saturable absorber (SA) is elaborated. Meanwhile, the saturable absorption and ultrafast carrier dynamics properties are explored by the Z-scan and the pump-probe techniques. Furthermore, the passively Q-switched (PQS) and the continuous-wave mode-locked (CWML) solid-state bulk lasers operating at 1.0 µm are achieved based on the b-AsP SA. The findings illustrate that 2D b-AsP SA could potentially be nonlinear optical modulators in solid-state bulk lasers.

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2. Results and Discussion

By using the top-down liquid phase exfoliation (LPE) method, the 2D b-AsP SA is fabricated. The atomic ratio of As to P in the high-purity commercial b-AsP crystals is approximately 41:9 (the corresponding bandgap is approximately 0.16 eV)^[33]. The surface morphology and structure of the prepared b-AsP SA are researched by the Raman spectra and atomic force microscopy (AFM).

Figure 1(a) presents the unique Raman spectra of b-AsP nanosheets stimulated by a 532 nm laser at room temperature. The Raman peaks are constituted by the out-of-plane $A_{g}^{1}$ mode, in-plane $B_{g}^{2}$ mode, and out-of-plane $A_{g}^{2}$ mode of b-As, which locate at 223.2, 233.9, and $253.6 {cm}^{- 1}$ respectively, while the out-of-plane $A_{g}^{1}$ mode, in-plane $B_{g}^{2}$ mode, and $A_{g}^{2}$ mode of BP are located at 350.0, 467.9, and $485.6 {cm}^{- 1}$ , respectively^[33]. Figure 1(b) shows the AFM image, where the dimensions of the prepared 2D b-AsP nanosheets are $3 μm \times 4 μm$ with $\sim 4 nm$ thickness. The corresponding layer number of 2D b-AsP nanosheets is about 4–5.

Figure 1.(a) Raman spectrum of b-AsP nanosheets. (b) AFM image and representative height profile of b-AsP nanosheets.

The Z-scan and pump-probe techniques are used to characterize the saturable absorption and the ultrafast carrier dynamics properties of the prepared b-AsP SA. Firstly, the saturable absorption properties of the b-AsP SA are measured by the Z-scan technique through using a fiber laser as the measured light with the repetition rate of 200 kHz and pulse duration of $\sim 15 ps$ ^[38]. The experimental data is fitted by the following formula^[39]: $T = A \times \exp (- \frac{Δ R}{1 + \frac{F}{F_{sat}}}),$ (1)where $T$ , $A$ , $Δ R$ , $F$ , and $F_{sat}$ refer to the transmission rate, normalization constant, modulation depth, input fluence, and saturation fluence, respectively. As shown in Fig. 2(a), based on the measured data, the $Δ R$ of the b-AsP SA is simulated to be 7.14 %, while the $F_{sat}$ is $5.86 {μJ / cm}^{2}$ .

Figure 2.(a) Transmittance versus incident optical power intensity of b-AsP SA. (b) Time-resolved response of b-AsP SA.

Furthermore, the ultrafast carrier relaxation process of the prepared b-AsP SA is explored by a non-degenerate pump-probe technique with a femtosecond resolution. The probe wavelength and pump wavelength are 1 µm and 800 nm, respectively. The results are shown in Fig. 2(b), in which the experimental data is fitted by an exponential decay function^[40]: $\frac{Δ T}{T_{0}} = A_{1} + A_{2} \times \exp (- \frac{t}{τ}),$ (2)where $T_{0}$ , $Δ T$ , $A_{1}$ , $A_{2}$ , and $τ$ refer to the transmittance of the probe laser beam before pump laser excitation, probe laser transmission variation after pump laser excitation, relative amplitudes of the double temporal components, and the intra-band relaxation time related to electron-electron coupling, respectively. By fitting the measured data, the $τ$ is determined to be 27 ps, indicating that the as-prepared b-AsP SA can be applied for ultrafast photonics devices.

The picosecond relaxation time, small saturation fluence, and large modulation depth demonstrate that the prepared b-AsP SA is a slow SA and can be used in the PQS and CWML lasers. Finally, the saturable absorption properties of the 2D b-AsP SA are demonstrated by the PQS $Nd : {YVO}_{4}$ and CWML Yb:KYW lasers. Figure 3(a) shows the experimental setup of PQS laser, which is a concave-plane cavity with a cavity length of 22 mm. The pump source with the central wavelength of 808 nm is focused on a $3 mm \times 3 mm \times 10 mm$ $Nd : {YVO}_{4}$ crystal (doping concentrations of 0.3% atomic fraction). The input mirror ( $R = 200 mm$ ) is high-reflectivity (HR) coated at 0.95–1.1 µm and anti-reflectivity (AR) coated at 0.75–0.9 µm. Meanwhile, a transmission of 10% at 1064 nm is used as the output mirror.

Figure 3.(a) Experimental setup of PQS. (b) The ratio of output powers to absorbed pump powers under the CW and PQS operations. (c) The emission wavelength of the CW and PQS lasers. (d) Pulse durations and repetition rates versus absorbed pump powers. (e) Pulse energy and peak power versus absorbed pump power. (f) The pulse trains and the shortest pulse profiles with different time scales.

By inserting the b-AsP SA into the laser cavity, a stable PQS laser operation is realized. Figure 3(b) depicts the dependence of output powers on absorbed pump powers. When the absorbed pump power is 5 W, a maximum Q-switched output power of 768 mW is acquired. As shown in Fig. 3(c), the emission wavelength of PQS operation is located at 1064.4 nm. Compared with the CW operation, the full width at half-maximum (FWHM) is narrow, and the central wavelength is blue-shifted, which may be caused by the high insertion loss of the sapphire-based b-AsP SA.

The variations of the pulse durations and repetition rates versus the absorbed pump power are described in Fig. 3(d). The pulse duration decreases from 900 ns to 140 ns, and the pulse repetition rates increase from 2.26 MHz to 140 kHz. As shown in Fig. 3(e), the highest pulse energy is 0.36 µJ with the highest peak power of 7.5 W. Figure 3(f) describes the characteristic pulse trains and the shortest pulse profiles with different time scales under the maximum output power: $F_{sat} Δ R < \frac{{(P T_{R})}^{2}}{F_{sat, L} A_{eff, L} A_{eff, A}} = \frac{{(P T_{R})}^{2} \times m σ_{em, L} λ}{h c π ω_{eff, L}^{2} \times π ω_{eff, A}^{2}},$ (3)where $T_{R}$ , $P$ , $A_{eff, L}$ , and $A_{eff, A}$ refer to round-trip time, laser power in the laser cavity, and laser mode areas on laser crystal and SA, respectively. According to the Eq. (3), the result is that $0.42 μJ / {cm}^{2}$ on the left is far less than $17.2 {μJ / cm}^{2}$ on the right. Therefore, stable CWML lasers can be obtained with the b-AsP SA.

The setup of CWML laser is demonstrated in Fig. 4(a), a fiber-coupled 976 nm LD is focused on an Np-cut Yb:KYW with a size of $3 mm \times 3 mm \times 4 mm$ . In the laser cavity, a Gires–Tournois interferometer (GTI) mirror with a total of $- 500 {fs}^{2}$ group delay dispersion (GDD) per round is used to compensate the normal GDD. Figure 4(b) indicates that the pulse width is measured by an autocorrelation and fitted by ${sech}^{2}$ -pulse shape to be about 470 fs. It is noted that the central wavelength is 1048 nm with the FWHM of 3.8 nm in Fig. 4(c). Compared with the Fourier transform limit value of 0.315, the corresponding time-bandwidth product of 0.448 is slightly larger, indicating that a slight chirping exists.

Figure 4.(a) Experimental setup of CWML laser. (b) CWML laser spectrum. (c) Autocorrelation trace. (d) The frequency spectrum with a wide and narrow span. (e) Pulse trains at the maximum pump power.

A spectrum analyzer records the radio frequency (RF) in Fig. 4(d). The central peak and signal-to-noise ratio are obviously located at 41.8 MHz and up to 58 dB under the resolution bandwidth (RBW) of 1 kHz. The RF spectra show the immaculate and stable CWML operation.

The output laser pulse trains with different time scales are presented in Fig. 4(e), which can remain stable over 24 h in the stability test. Compared with the BP SA applied on the solid-state CWML laser in the previous work^[41], although the pulse width of our b-AsP SA is relatively wide, it can still work normally after a month of resting in air, demonstrating the excellent air stability of our sample.

3. Conclusions

In conclusion, based on the prepared high-quality b-AsP SA, this paper explores the ultrafast carrier dynamics and saturable absorption properties. Furthermore, saturable absorption properties are experimentally demonstrated by the PQS and CWML lasers. The results show that the highest repetition rate of the PQS laser is 2.26 MHz, and the pulse duration of the CWML laser is 470 fs, demonstrating that b-AsP SA could be an excellent nonlinear optical modulator for pulse modulation in solid-state bulk lasers.

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