GHz Compact Laser Enabled by GaSb Nanowires as Saturable Absorbers

Owing to the narrow bandgap and excellent optoelectronic properties, III‐Sb nanowires (NWs) enable efficient optical response from near‐infrared to mid‐infrared, making them ideal candidates for broadband optical modulation. Herein, high‐purity GaSb NWs with controlled density are prepared on the transparent substrates of glasses. The phase purity and crystallinity of as‐prepared GaSb NWs are verified by X‐ray diffraction. Z‐scan and I‐scan techniques are adopted to investigate the nonlinear optical modulation properties, displaying a high modulation depth of 40% (at 1 μm wavelength) for the GaSb NWs with growth time of 30 min. Furthermore, the as‐prepared GaSb NWs are used as the saturable absorber elements in a waveguide laser cavity, demonstrating efficient Q‐switched mode‐locked lasers with a repetition rate of 8.2 GHz and a pulse duration of 31 ps (operating at 1 μm wavelength). All results show the great potential of GaSb NWs for the ultrafast laser and nonlinear optical applications.


Introduction
][11] In particular, SA elements play central roles in activating and stabilizing pulse generation.[14][15][16][17][18] Therefore, the pace of pursing novel and excellent SA elements has never stopped in the past few years.21][22][23][24][25] Significantly, the electrical and optical properties of NWs can be effectively tuned by controlling the sizes, shapes, and compositions, providing a viable route for tailoring light-matter interaction and achieving the generation, amplification, propagation, and modulation of light on the nanoscale. [8,25,26]wing to the high carrier mobility, narrow bandgap, tunable light absorption, strong spin-orbit interaction, and considerable subwavelength size effect at room temperature, III-Sb NWs have attracted research attention in the fields of electronics, optoelectronics, and spintronics in the past decades. [27,28]Among III-Sb semiconductors, GaSb has the largest hole mobility of 850-1000 cm 2 V À1 s À1 and a relatively narrow bandgap of 0.72 eV, displaying fantastic applications in fieldeffect transistors, complementary metal oxide semiconductor inverters, and infrared photodetectors. [21,29]Up to now, GaSb thin films have been applied as SA elements for the generation of pulsed lasers at wavelengths from the C-band to midinfrared. [30]enerally speaking, semiconductor NWs always display larger nonlinear susceptibility compared to their bulk counterparts due to the famous Lorentz local field effect. [31,32]However, the study of nonlinear optical absorption of GaSb NWs and the experimental demonstration of their potential applications as SA elements in pulsed laser generation are still missing.
In this work, the optical modulation functionality of GaSb NWs is demonstrated based on a solid-state waveguide laser operating at the pulsed regime.High-purity GaSb NWs are first prepared on a transparent substrate by the surfactant-assisted chemical vapor deposition (CVD) method, [24,33] followed by the investigation of nonlinear optical response.By introducing the surfactant of sulfur in CVD method, the growth orientation, length, diameter, and hole mobility of GaSb NWs have been controlled well in the literatures. [21,26,34,35]When the as-prepared GaSb NWs are used as SA elements, a modulation depth of up to 40% (at 1 μm wavelength) is achieved.By inserting the as-prepared GaSb NWs into the waveguide laser cavity, a stable Q-switched mode-locked laser is experimentally demonstrated.Furthermore, the as-studied efficient Q-switched mode-locked laser shows a repetition rate of 8.2 GHz and a pulse duration of 31 ps operating at 1 μm wavelength.Our experimental results distinctly reveal the nonlinear optical absorption properties of GaSb NWs, which provide a promising solution for the design of III-V NW-based photonic devices for light manipulation.

Results and Discussion
In this work, the transparent glass is adopted as the growth substrate of GaSb NWs, benefiting the study of light modulation.As shown in Figure 1, GaSb NWs with controlled density are prepared using 0.1 nm Au films as growth catalysts.With the growth time increasing from 1 to 30 min, the density of as-prepared NWs increases gradually, resulting in a decrease of light transmission (Figure 1a and Figure S1, Supporting Information).Obviously, GaSb NWs with controlled density can be achieved on the transparent substrates of glasses.The phase purity of as-prepared GaSb NWs is verified by X-ray diffraction (XRD), as shown in Figure 1b.With the growth time longer than 3 min, all the as-prepared GaSb NWs exhibit zincblende crystal structures without the phase peaks of gallium oxide or antimony oxide, indicating high phase purity.Owing to the low density, there are no obvious XRD peaks in the sample with a growth time of 1 min.The sharp peaks in the XRD patterns further verify the good crystallinity of the as-prepared GaSb NWs, promising light modulation.The UV-vis absorption spectra of as-prepared GaSb NWs are shown in Figure 1c.Obviously, the as-prepared GaSb NWs exhibit intensive broadband absorption in the visible and near-infrared (NIR), indicating their potential broadband optical modulation properties.On the other hand, it distinctly reveals that the intensity of absorption increases with the extension of growth time, which is due to the higher density of as-prepared NWs with longer growth time.For the sample with 10 min growth time, full absorption is observed in the NIR, which further elucidates excellent absorption to NIR light.The UV-vis absorption spectra of as-prepared GaSb NWs with the growth times of 20 and 30 min are investigated as well in Figure S1, Supporting Information, exhibiting identical characterization features to that of the sample with growth time of 10 min.
The nonlinear optical absorption of the GaSb NWs on glass with different growth times is studied by a home-built openaperture Z-scan experimental setup.A 1030 nm femtosecond fiber laser (FemtoYL-10, YSL Photonics) with a pulse duration of 400 fs, a pulse energy of 4.2 μJ, and a repetition rate of 25 kHz is employed.The sample (GaSb NWs on glass) is placed on a PC-controlled translation stage during the Z-scan measurement.In this case, Z-scan measurement curves with normalized transmittance as a function of the sample positions are demonstrated in Figure 2. It is obvious that the transmittance gradually increases with the extension of growth time of GaSb NWs.The transmittance curves also display symmetrical peaks centered at the Z = 0 position, indicating the excellent typical saturable absorption characteristic.Nevertheless, there is no obvious saturable absorption in the samples of growth times of 1 and 3 min, as shown in Figure S2, Supporting Information, which may be due to the small density of GaSb NWs on glass substrates.According to the nonlinear absorption model, the modulation depth and the saturation intensity can be obtained by fitting the normalized experimental data according to the following formula. [11,36] where T is the normalized transmittance, ΔR is the modulation depth, I s is the saturation intensity, I 0 is the incident peak intensity, and Z 0 is the Rayleigh length of incident beam.As a result, the modulation depths are simulated to be 24.5%, 31.enhanced interaction between light and NWs would lead to the increased modulation depth and saturable intensity, which is consistent with the result of Z-scan measurement.
To further investigate the nonlinear optical response of the GaSb NWs on the glass substrate, the I-scan measurement is also employed to study the dependence of optical transmission on the incident laser intensity (with a beam waist radius of around 50 μm at the focal position).Figure 3a-d shows the optical transmittance as a function of the laser intensity.Obviously, the optical transmittance of the samples tends to be saturated gradually with the increase of incident light power, confirming the good SA property.The modulation depths and saturation fluences of GaSb NWs with different growth times can be determined using the following formula for fitting the I-scan measurement results. [37,38] where I is the incident laser pulse energy and α ns is the nonsaturable loss.The calculated modulation depths and saturation fluences of different samples are summarized in Figure 3e,f.It is clear to see that both the modulation depth and saturation fluence of GaSb NWs increase with the growth time.They reach their respective peaks at 40.5% and 150.1 mJ cm À2 for a growth time of 30 min.This phenomenon can be attributed to the heightened density of GaSb NWs as the growth duration extends, leading to enhanced light modulation.This observation is consistent with the findings of the Z-scan measurements.Furthermore, the good SA performance is comparable or even better than that of the commonly used 2D layered materials, indicating the great potential application of GaSb NWs in ultrafast lasers. [39]o further investigate the saturable absorption properties, the as-prepared GaSb NWs on the glass substrate are adopted as SA elements in Nd:YAG waveguide laser in this work.Notably, for the pulsed waveguide laser experiment, the GaSb NW with 10 min growth time is used.This choice strikes a balance between achieving a sufficiently high modulation depth (31%) and maintaining a relatively low saturation fluence (27.3 mJ cm À2 ).The former is to ensure stable pulsed laser generation while the latter is required due to the limitation on the optical pumping conditions.For waveguide laser experiment, a Nd:YAG waveguide with a cladding geometry fabricated by femtosecond laser direct writing technique is used.A typical end-face coupling arrangement is used for the waveguide laser characterization (see Figure S4, Supporting Information for the schematic illustration of the experimental setup).A tunable CW Ti:sapphire laser (Coherent MBR) is used as the optical pumping source.A planoconvex lens ( f = 25 mm) and a microscope objective (20 Â, N.A. = 0.4) are used for laser in-and out-coupling.The laser cavity consists of a pump mirror (M1) with high transmittance of 99.8% at 808 nm and high reflectivity of > 99.9% at 1064 nm, along with an output mirror (M2) possessing %60% reflectivity at 1064 nm.Both mirrors are securely attached to the two end facets of the Nd:YAG waveguide.The GaSb NWs with growth time of 10 min are inserted into the waveguide cavity (clamped between the waveguide output facet and the M2 mirror) and used as SA elements for laser modulation.In this way, the Q-switched mode-locked waveguide lasers can be obtained.A high-speed InGaAs photodetector (New focus, 1414 model) coupled by a single-mode fiber after a long-pass filter and analyzed by a digital oscilloscope (Tektronix, MSO 72504DX) is then used to detect the output pulsed laser.
After the success in the construction of GaSb NWs waveguide laser, the performance of the Q-switched mode-locked laser is studied in Figure 4. Figure 4a shows the relationship between the output power and the absorbed pump power.Under continuous wave conditions, a maximum average output power of 205 mW is achieved.During Q-switched mode-locked operation, the peak output laser power reaches 80.5 mW, demonstrating a slope efficiency of 11.7%.The emission peak is centered at 1056.6 nm. Figure 4c displays the single envelope consisted of lots of mode-locked picosecond pulses on the nanosecond timescales.The mode-locked pulse trains within the envelope are shown in Figure 4d.The pulse duration is determined to be 31 ps, as implied by the mode-locked pulse train (see Figure 4e).The corresponding fundamental repetition rate is measured to be 8.2 GHz (see Figure 4f, with a signal-to-noise ratio of 40 dB), which is in good agreement with the theoretical value (the theoretical fundamental repetition rate is calculated according to the formula of f rep = c 2nl À1 , where c is the light speed, n is the effective refractive index of the waveguide, and l is the effective length of the cavity).It is worth noting that the relatively narrow pulse of 31 ps and high repetition frequency of 8.2 GHz reveal the excellent nonlinear optical properties of GaSb NWs, promising ultrafast optical modulation.The SA performance comparisons with other materials are listed in Table 1.Obvioulsy, GaSb NWs reported here show the high modulation depth, short pulse width, and ceteral frequency at 1056.6 nm.The stability of the Q-switched mode-locked pulse train waveforms is also testified for around 1 hour, revealing minimal variation of < 5% in pulse intensity.At high pump powers, slight waveform disturbances emerge due to thermal effects induced by heating SA elements.No damage of the GaSb NWs is observed in the experiment, suggesting that the SA elements used in this work possess relatively high damage threshold.

Conclusion
In this work, we have demonstrated the growth of GaSb NWs on glass substrates by applying surfactant-assisted solid-source CVD method.The materials and optical properties of the prepared NWs are experimentally characterized, showing great potential application in nonlinear optical modulation.Accordingly, a hybrid "waveguides þ GaSb" configuration combined by a waveguide and the as-prepared GaSb NWs is proposed, demonstrating efficient Q-switched mode-locked pulsed laser.The results reported in this work suggest the great potential of GaSb NWs for the applications in next-generation ultrafast lasers.

Experimental Section
GaSb NW Growth: GaSb NWs stuied in this work were prepared by adopting a previously reported surfactant-assisted solid-source CVD method in a dual-zone horizontal tube furnace.The adopted Au catalyst films with a thickness of 0.1 nm were predeposited onto the growth substrate of glasses.Before the NW growth, the source material of 0.5 Â g GaSb powder (99.999% purity) was placed in the upstream zone of the furnace, followed with the surfacntant of 0.5 Â g sulfur (99.99% purity) locats in the middlle of two zones and the Au catalyst film located in the middle of the downstream zone of the furnace.Next, the pressure of the growth chamber was pumped down to 7 Â 10 À3 Torr by a mechanical pump.During the NW growth, the source was heated to 750 °C, while the substrate was heated to a temperature of 560 °C.Hydrogen (99.9995% purity) was used as the carrier gas and flowed through the horizontal quartz tube at the rate of 200 sccm.After a designed growth time, the heating of the source and substrate stopped together and the as-prepared GaSb NWs could be taken out as the furnace was cooled down to room temperature under the hydrogen flow.
Material Characterization: Optical images were taken by optical microscope (OLYMPUS BX53M).The morphology of the as-prepared GaSb NWs was examined by scanning electron microscope (SEM, Nova NanoSEM 450, FEI Company).The crystal phase was determined by XRD (D8 Advance, Bruker).The absorption spectrum was measured by a UV-4100 spectrometer (Hitachi).

Figure 1 .
Figure 1.Controllable growth of GaSb NWs on the transparent substrates of glasses.a) Optical images of as-prepared GaSb NWs on glasses with growth times of 1, 3, 5, and 10 min, below is SEM image of as-prepared GaSb NWs with the growth time of 10 min.b,c) XRD patterns and UV-vis absorption spectra of as-prepared GaSb NWs on glasses with the growth times of 1, 3, 5, and 10 min.

Figure 2 .
Figure 2. The nonlinear optical properties of GaSb NWs.Normalized transmission as a function of sample position Z for GaSb NWs with the growth times of a) 5 min, b) 10 min, c) 20 min, and d) 30 min.

Figure 3 .
Figure 3.The saturable absorption properties of GaSb NWs.Normalized transmission as a function of incident light intensity for GaSb NWs with the growth times of a) 5 min, b) 10 min, c) 20 min, d) 30 min.e,f ) Saturation fluence and modulation depth versus the growth times of GaSb NWs.

Figure 4 .
Figure 4.The Q-switched mode-locked laser modulated by GaSb NWs.a) The output power versus incident pump power for continuous-wave and Q-switched mode-locked operation.b) The emission spectrum of Q-switched mode-locked laser.c) Q-switched envelope on a nanosecond scale.d) Mode-locked pulse trains.e) Single-pulse profile.f ) The measured radio frequency spectrum.

Table 1 .
The SA performance comparison in the literatures.