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Abstract
Broadband nonlinear optical modulators are necessary for versatile applications in optical communication, laser radar, laser manufacturing, etc. Here, we report that the zirconium triselenide (ZrSe3) nanoflakes exhibit robust broadband nonlinear optical absorption and can modulate the Nd:YVO4 solid-state laser and erbium-doped ZBLAN fiber laser to deliver stable nanosecond pulse experimentally. The ZrSe3 nanoflakes have been prepared by the liquid phase exfoliation method successfully and exhibit broadband nonlinear optical absorption in the near-infrared and the mid-infrared regime. With the nonlinear absorption performance of the ZrSe3 nanoflakes, stable pulsed operation has been achieved with the output pulse width as short as 344.68 ns around 1 µm from the Q-switched Nd:YVO4 solid-state laser and 599 ns around 2.8 µm from the erbium-doped ZBLAN fiber laser successfully. The experimental results suggest that ZrSe3 nanoflakes can act as an excellent nonlinear optical modulator towards the mid-infrared regime and may make inroads toward developing high-performance broadband optoelectronic devices.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Nanosecond pulsed lasers have shown significant and attractive applications in material processing [1–4], optical communication [5], environmental remote sensing [6], laser ranging [7], medical treatment [8–10], etc. Traditionally, the nanosecond pulsed lasers can be achieved by the active method by incorporating electro-optical or acousto-optic modulators, or passive method by introducing the saturable absorber (SA) inside the laser cavity [11]. Compared with the active method, the passive method via the Q-switched operation modulated by SA can provide a compact and cost-effective way to deliver the nanosecond laser [12,13].
The SA plays an indispensable role for the passively Q-switched operation, which can act as an intensity-dependent optical modulator with reduced absorption at high optical intensities. With the requirements to develop high power, high efficiency, compact and cost-effective nanosecond lasers, different kinds of SA have been proposed and demonstrated. The commercial semiconductor saturable absorber mirrors (SESAMs) have been adopted in different laser systems [14]. However, it suffers from the limited operating wavelength, and complicated fabrication procedures. With the evolving of the optical material, especially the two-dimensional (2D) material, the SAs have exhibited excellent nonlinear optical response with the reduced dimension. 2D materials, such as graphene [15–19], topological insulators [20–23], black phosphorus [13,24–27], transition metal chalcogenides [28–34], have shown broadband nonlinear optical absorption and ultrafast carrier dynamics, and have been used to modulate the laser system successfully. Graphene is the first 2D material to be used for generating laser pulses, which has broadband absorption (UV to mid-IR, even to terahertz regime) and a fast relaxation lifetime [35]. However, it has low layer-dependent absorption (∼2.3%), and low modulation depth (∼1% at 1 or 1.5 µm) [36]. Thus, research interest has turned to finding other broadband, ultrafast, and robust nonlinear optical materials and related SAs.
Transition metal chalcogenides (TMCs) have aroused wide attention due to their excellent physical and chemical properties and their application potential in optical switches devices, etc. Among them, the group IV–V transition metal trichalcogenides (TMTCs) are a less-explored family of materials, and the transition-metal chalcogenides MX3 are typical VdW-stacked layered materials belonging to the space group P21/m, where M is the transition metal atom belonging to either group IVB (Ti, Zr, Hf) or group VB (Nb, Ta) and X chalcogen atoms from group VIA (S, Se, Te) [37]. They possess a pseudo-one-dimensional (1-D) structure which is a trigonal prism (MX3) growing along the b-axis with a metal atom located at the center and trigonal bases consisting of the chalcogen atoms. Because of the slight change of their chain-like structure, TMTCs show unique physical properties [38]. ZrSe3, one of TMTCs, has an indirect band gap value of 1.1 eV and a direct band gap value of 1.47 eV [39,40], which has been demonstrated to have excellent stability and light response in the visible region [41]. The photodetector based on ZrSe3 nanobelts has been proposed to show good photoresponse with wavelengths ranging from 405 nm to 780 nm [42]. In addition, the ZrSe3 low-dimension materials have shown sensitive optoelectronic response with external applied field, such as high thermoelectric performance from the ZrSe3 monolayer and strain-tunable band-gap transitions of the exfoliated ZrSe3 [43,44]. However, the broadband optical response beyond the visible regime and the optoelectronic applications, especially the nonlinear optical characteristics and applications of the ZrSe3 are still far from thoroughly investigated, which are meaningful for promoting the practical development of 2D-based nonlinear optical devices.
Here, ZrSe3 nanoflakes have been prepared by the cost-effective liquid phase exfoliation (LPE) method, and the detailed material characterizations have been carried out. The nonlinear optical characteristics of the ZrSe3 nanoflakes have been investigated, and its nonlinear optical parameters have been extracted experimentally. In addition, the stable pulsed operation has been achieved with the output pulse width as short as 344.68 ns around 1 µm from the Q-switched Nd:YVO4 solid-state laser, and 599 ns around 2.8 µm from the erbium-doped ZBLAN fiber laser successfully. The results show that the ZrSe3 nanoflakes exhibit great capacity and potential in designing broadband absorption optical devices.
2. Material preparation and characterizations
The film-type ZrSe3 SAs have been prepared by using the LPE method and the spin coating method [45]. The ZrSe3 powders were first dispersed in ethanol, and then sonicated for 2 h using a 120 W ultrasonic cleaner, followed by centrifugation at 3000 rpm for 10 min to remove any large aggregates. Then the 100 µL of the ZrSe3 dispersion solution was spin-coated on the quartz and gold mirror, and then placed in the ambient condition for 12 h to obtain the ZrSe3-based SA. The ZrSe3 nanoflakes have been characterized via the Raman spectroscopy, scanning electron microscopy (SEM) and X-ray spectroscopy (EDX), as shown in Fig. 1. Figure 1(a)-(b) shows the EDX spectroscopy maps of the ZrSe3 nanoflakes to illustrate its elemental composition. The typical peaks of Zr and Se were depicted in Fig. 1(c), and the atomic ratio of Zr and Se is 22.82:77.18, which is 1:3 approximately. The SEM image of the ZrSe3 nanoflakes was obtained with resolution of 500 nm and 200 nm by SEM, as shown in Fig. 1(d). Figure 1(e) shows a typical transmission electron microscopic (TEM) image of an individual ZrSe3 nanoflakes with a width less than 100 nm. Figure 1(f) is an HRTEM image of the nanobelt, which shows the marked lattice fringe spacing of 0.28 nm. AFM in ScanAsyst mode was used to measure the thickness of the products. Figure 1(g) is a typical AFM image of the ZrSe3 nanoflakes, showing a nanoflake shape with a thickness about 3.75 nm. The peaks recorded by the Raman spectrum is shown in Fig. 1(h), and the four typical Raman peaks are corresponding to the modes of Ag3, Ag5 and Ag6, Ag8, respectively, which matches well with previous results [46]. In addition, Fig. 1(i) shows the broadband linear transmission spectrum of the ZrSe3 nanoflakes, which indicates that the sample has broadband absorption towards the mid-infrared region.
Fig. 1. (a)-(b) TEM-EDX maps of a ZrSe3 nanoflakes showing zirconium and triselenide, respectively. (c) The EDS of the ZrSe3 nanoflakes. (d) The high-magnification SEM image of the ZrSe3. (e) TEM image of individual ZrSe3 nanoflakes. (f) The absorption spectrum from the ZrSe3 nanoflakes. (g)AFM image of the ZrSe3 nanoflakes. (h) Raman spectrum of the ZrSe3 nanoflakes. (i) The linear absorption spectrum from the ZrSe3 nanoflakes.
The nonlinear saturation absorption behavior of the ZrSe3 nanoflakes has been measured by the open-aperture (OA) Z-scan technique. To explore the broadband nonlinear saturable absorption of the ZrSe3 nanoflakes, the OA Z-scan measurements were performed utilizing a femtosecond optical parametric amplifier (OPA) pumped by an 800 nm Ti: sapphire amplifier system (Coherent, USA) with output wavelengths of 1064 nm and 2800 nm lasers (repetition rate 1 kHz, pulse duration 35 fs), respectively. The OA Z-scan experimental result with the ZrSe3 nanoflakes was shown in Fig. 2, and we can acquire the saturation intensity and modulation depth of the ZrSe3 nanoflakes by fitting the experimental data with the following formula:
where T, ${\alpha _0}$, I, ${I_s}$, and L represent the normalized transmittance, linear absorption coefficient, incident laser intensity, saturation intensity, and sample thickness coefficient, respectively. By fitting the experimental results, the modulation depth and saturation intensity for the ZrSe3 nanoflakes are 20.07% and 643 GW/cm2 at 1064 nm, 10.52% and 117 GW/cm2 at 2800 nm, respectively.Fig. 2. Nonlinear saturable absorption properties of the ZrSe3 nanoflakes. (a) OA Z-scan trace and (b) the relationship between transmittance and input laser intensity at 1064 nm wavelength. (c) OA Z-scan trace and (d) the relationship between transmittance and input laser intensity at 2800 nm wavelength.
3. Experimental results and discussions
3.1 Nd:YVO4 solid-state laser
To demonstrate the broadband nonlinear optical absorption performance of the ZrSe3 nanoflakes, the Q-switched Nd:YVO4 solid-state laser has been designed elaborately, as shown in Fig. 3. The pump source was an 808 nm fiber coupled laser diode, and the numerical aperture (NA) and core diameter of the pigtail were 0.22 and 105 µm, respectively. The planar parallel polished Nd:YVO4 crystal has a 0.3% doping concentration with a dimension of 3 × 3 × 12 mm3. The Nd:YVO4 crystal was wrapped with indium foil and mounted inside a copper block. The copper block was connected to a water-cooled chiller with temperature obtained at 15 °C. The pump light was collimated by a lens L1 (LA1951-B, coating-B, f = 25.4 mm) and focused by a Lens L2 (LA1608-B, coating-B, f = 75.0 mm). The design of the resonant cavity was a typical F-P cavity, which was composed of a pump input mirror M1 (high reflectivity (> 99.5%) @1000-1100 nm & high transmittance (>97%) @808 nm) and an output coupling mirror M2 (∼95%@1000–1100 nm), and the cavity length was about 13 cm. We have used an optical power meter (1917-R), an optical spectrum analyzer (DSO9404A) and a photodiode (DSO9484A) for the measurements.
The stable Q-switched pulse trains can be delivered from the solid-state laser when the pump power was around 1 W by slowly adjusting the position of ZrSe3-SA. The output spectrum of the ZrSe3 nanoflakes Q-switched solid-state laser was shown in Fig. 4(a), which has the central wavelength at 1064.7 nm. The radio-frequency result of the Q-switched Nd: YVO4 solid-state laser has been measured, as shown in Fig. 4(b). It can be obtained from the figure that the signal-noise-ratio was 34 dB, which confirms the relative stability of Q-switched pulse. We have also obtained the output performance of the passively Q-switched solid-state laser with the slope efficiency 23.6%, as shown in Fig. 4(c). The inset in Fig. 4(c) shows the image of the output beam, from which we could find that the beam spot exhibits a symmetrical Gaussian distribution. With the increase of pump power from 3 to 5.5 W, the pulse repetition rate increases monotonously from 193.63 to 413.47 kHz and the pulse width decreases monotonously from 766.25 to 344.68 ns. The dependence of the output pulse repetition rate and output pulse width as the function of the incident pump power was shown in Fig. 4(d), and the maximum output power could reach up to 755 mW by increasing the incident pump power to 5.5 W. We have also corroborated the stable Q-switched operation from the oscilloscope traces at different scale duration, as shown in Fig. 5. We have not increased the incident pump power above 5.5 W due to the thermal influences on the ZrSe3-SA under high power laser illumination.
Fig. 4. The output characteristics of ZrSe3-based passively Q-switched all-solid-state laser. (a) Optical spectrum. (b) Radio-frequency output spectrum. (c) Output power as a function of the incident pump power, and the inset shows the image of the output beam. (d) Pulse width and repetition rate as a function of the incident pump power.
Fig. 5. Typical Q-switched pulse trains at different time scale (5 µs/div and 1 µs/div) when the laser operating at different incident pump power. (a) and (b) 4.0 W; (c) and (d) 5.0 W; (e) and (f) 5.5 W.
3.2 Er3+:ZBLAN fiber laser
To evaluate the application potential of the as-prepared sample in the mid-infrared regime, we designed the linear cavity configuration of the Er3+-doped ZBLAN fiber laser with ZrSe3-SA as the Q-switcher depicted in Fig. 6. The pump source is a 975 nm fiber coupled laser diode, and the NA and core diameter of the pigtail were 0.2 and 105 µm, respectively. The 3.8 m Er3+-doped ZBLAN fiber has a doping concentration of 70000 ppm, and the diameter and NA of the core are 15 µm and 0.12, respectively. The inner cladding diameter is 240 × 260 µm (NA of 0.4). The fiber end near the pump source side is perpendicular-cleaved to act as output coupler, and the other end of the fiber is cleaved at an angle of 8° to suppress parasitic lasing. The resonator was composed of a perpendicular cleaved optical fiber end surface and a gold mirror M2 integrated with the ZrSe3 nanoflakes. The 975 nm pump laser output from the pigtail was coupled into the gain fiber through a lens group consisting of lens L1 (coating-B, f = 25.4 mm) and lens L2 (CaF2 lens, transmission 94% @ 975 nm, f = 50 mm). The emerging laser beam from the fiber end cleaved at 8° angle was focused on ZrSe3-SA via the lens L3 (uncoated CaF2 lens, f = 20 mm) and L4 (uncoated CaF2 lens, f = 25.4 mm). The dichroic mirror M1 (HT@975 nm, HR@2.8 µm) located between the pump coupling systems was used to separate lasers from the pump laser. With the pump power 0.26 W, stable Q-switched pulse can be obtained by adjusting the position of ZrSe3-SA.
During the experiment, we used the M2 without ZrSe3 nanoflakes on it to feed back the laser, and only the continuous-wave operation can be observed. By substituting M2 with the ZrSe3 nanoflakes modulator, stable Q-switched pulse was initially observed from the Er3+-doped ZBLAN fiber laser when the incident pump power was 0.26 W. We continue to increase the pump power up to 1.7 W, and the pulse operation was still stable. Figure 7(a) depicted the output spectrum of the ZrSe3 Q-switched Er3+-doped ZBLAN fiber laser, which locates at 2790 nm with FWHM 6 nm. At the same time, the signal-noise-ratio was also measured to be 36 dB, which confirms the stable Q-switching operation of the fiber laser, as shown in Fig. 7(b). Figure 7(c) shows the output power as a function of the incident pump power, and we can see that the average output power increased nearly linearly with the incident pump power with the slope efficiency about 7%. The relationship between pulse repetition rate and pulse width versus incident pump power was measured, as shown in Fig. 7(d). With the increase of pump power from 0.26 to 1.69 W, the pulse repetition rate increases monotonically from 23 to 76 kHz and the pulse width decreases monotonically from 5.8 to 0.559 µs. We have also presented the oscilloscope traces of Q-switched pulse in different time scales at the maximum pump power in Fig. 8, from which we can see the excellent pulse stability of the mid-infrared laser pulse.
Fig. 7. The output characteristics of ZrSe3 -based passively Q-switched Er3+:ZBLAN fiber laser. (a) Optical spectrum. (b) Radio-frequency output spectrum. (c) Output power as a function of the incident pump power. (d) Pulse width and repetition rate as a function of the incident pump power.
Fig. 8. Typical Q-switched pulse trains at different time scale (20 µs /div and 5 µs/div) when the laser operating at different incident pump power. (a) and (b) 1.1 W; (c) and (d) 1.3 W; (e) and (f) 1.7 W.
In order to compare the performance of the passively Q-switched laser based on ZrSe3 SA with other materials, we summarized the reported results based on different 2D chalcogenides materials in Table 1. The results show that the ZrSe3 can act as an excellent SA for the pulsed lasers. It is necessary to mention that the pulse duration of the listed Q-switched lasers is mostly in the microsecond time scale, while the ns-level Q-switched pulses can satisfy more in-demand applications. Here, we have achieved the output pulse width as short as 344.68 ns around 1 µm from the Q-switched Nd:YVO4 solid-state laser, and 599 ns around 2.8 µm from the Er3+-doped ZBLAN fiber laser successfully. Notably, the maximum repetition rate range of 413.47 kHz is much higher than that of passively Q-switched lasers at 1064 nm with other 2D materials. Overall, ZrSe3-based SA exhibits excellent physiochemical properties and optical modulation performance in terms of laser parameters like fundamental repetition rate and pulse duration.
Table 1. Comparison of pulsed lasers based on different TMCs
4. Conclusions
In conclusion, we have demonstrated the broadband saturable absorption properties of the ZrSe3 nanoflakes from the near- to mid-infrared regime experimentally. The nonlinear optical measurements at different wavelengths verified that the as-prepared ZrSe3 nanoflakes possesses broadband saturable absorption properties. With the ZrSe3 nanoflakes as SA, we achieved the passively Q-switched Nd:YVO4 laser operation around 1 µm with pulse width 344.68 ns, repetition rate 438 kHz, and the Q-switched Er3+-doped fluoride fiber laser with the output pulse width as short as 559 ns. The experimental results suggest that ZrSe3 nanoflakes can act as an excellent nonlinear optical modulator towards mid-infrared regime, and may make inroads towards developing high performance broadband optoelectronic devices.
Funding
National Natural Science Foundation of China (61975055).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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