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The nonlinear optical responses of two-dimensional (2D) few-layer tin diselenide (SnSe2) have been investigated in this work. By applying the Z-scan technique, the saturable absorption of SnSe2 thin film has been observed at a wavelength of ~1μm, which enables SnSe2, a new saturable absorber for pulsed laser generation. A detailed exploration of the nonlinear absorption of SnSe2 film and their dependence on excitation pulse energy is carried out. Our results show that saturation intensity of the nonlinear absorption is ~23.78 GW/cm2 and corresponding nonlinear absorption coefficients are at a range from −13596 to −2967.3 cm/GW. By employing the SnSe2-coated mirror as a saturable absorber, the passively Q-switched lasing has been achieved on a crystalline waveguide platform at ~1 μm.
© 2017 Optical Society of America
1. Introduction
Graphene and other two dimensional (2D) materials attract great attentions of both research communities and industry, and a number of novel ultrathin devices have been manufactured based on the exciting new physics related the atomic-scale layered materials [1–4]. Two-dimensional transition metal dichalcogenides or diselenides (TMDCs or TMDSs), exhibiting excellent electrical and optical properties, have emerged as promising candidates for various applications beyond graphene [5–9]. One of the most exotic features of semiconducting TMDC and TMDS materials is that the band gap engineering may be achieved by varying the number of layers, which enables them for production of on-demand devices due to the layer-dependent properties [7,10–12]. Tin diselenide (SnSe2) belongs to one class of typical two-dimensional layered diselenide materials [10,13,14]. As a kind of narrow band-gap semiconductors with a hexagonal crystal structure, SnSe2 is promising for versatile applications, e.g., infrared optoelectronic devices, solar cells, memory switching devices, and anodes for lithium-ion batteries [15–19]. In addition, the band gaps of SnSe2 show layer-sensitive features: the indirect fundamental band gaps of few-layers and bulk are ranging from 1.07 eV to 1.69 eV, whilst the direct ones are from 1.84 eV to 2.04 eV [20].
The nonlinear optical properties of 2D materials are the basis of their photonic applications. The investigation of the ultrafast nonlinear properties (nonlinear susceptibility, nonlinear refraction and absorption, carrier relaxation lifetime, etc.) is critical for the development of 2D-material-based photonic and optoelectronic devices [21–23]. The nonlinear saturable absorption is essential for generation of ultrafast laser pulses, which has been widely applied in the laser systems of bulk, fiber, and waveguides. Recently, the prominent broadband saturable absorption performance and its applications in layered 2D materials (including graphene, TMDC and TMDS, topological insulators, black phosphorus, h-BN, etc) for ultrafast pulses over a broad wavelength range have been widely reported [25–28]. Nevertheless, the exploration of new 2D material based nonlinear absorption is still necessary because of distinguished saturable absorption requirement for diverse laser systems. In previous works, we have focused on the nonlinear saturable absorption responses and applications on waveguide platforms, in which the passive Q-switching has been achieved to generate efficient pulsed lasers at low pump thresholds. These compact devices also allow non-diffraction beam propagation and steering, similar to the fiber systems [29–32].
In this work, we report, to our best knowledge for the first time, on the nonlinear optical properties of few-layer SnSe2, and the applications as a saturable absorber (SA) for Q-switched lasers at 1μm wavelength. The Raman spectroscopy, absorption spectroscopy and atomic force microscope (AFM) are performed to characterize the features of the SnSe2 thin film. The nonlinear responses and saturable absorption of SnSe2 are measured by Z-scan technique under the excitation of femtosecond (fs) pulses at 1030 nm.
2. Preparation and characteristics of SnSe2
The 2D SnSe2 thin film used in this work was customized from a chemical maker (6Carbon Technology, Shenzhen, China). It was made to be a continuous thin film by chemical vapor deposition (CVD) technology, coated on a 10 × 10 mm2 surface of a sapphire wafer which was optically polished. The topography of thin film is characterized by an atomic force microscope (AFM) (shown in Fig. 1(a)), which was observed the SnSe2 crystal grains stacked one by one for ensuring the spatial integrity of the film and overspread the whole surface. The height difference between the sapphire substrate and the sample surface is approximately 22 nm. As well for investigating the linear absorption region, an absorption spectrum of the SnSe2 thin film sample was measured by using a UV/VIS/NIR spectrophotometer (UV1800, Shimadzu, Japan) from 200 nm to 1100 nm, with resolution of 1 nm. Figure 1(b) shows the linear absorbance as functions of wavelength as well as converting of photon energy. It is revealed that the thin film remained a broad absorbed gap from at least ~1.13 eV (1100 nm) of near-infrared (NIR) to ~6.2 eV (200 nm) of ultraviolet (UV) region as similar as the bulk of SnSe2, of which the indirect band gap was 1.08 eV [21].
Fig. 1 (a) AFM image of SnSe2 thin film and measurement of height, (b) linear absorption as functions of wavelength and photon energy, corresponding to top and bottom axes.
The Raman spectra of the SnSe2 thin film were investigated at wavelength of 532 nm. The 2H polytype for SnSe2 contains three atoms in the unit cell, therefore, there are nine normal modes, three of which are Raman-active, whilst in this work two Raman peaks were observed clearly [20]. The lowest-frequency mode Eg, is doubly degenerate and has been characterized by an in-plane stretching behavior. The other nondegenerate higher frequency Raman mode A1g has been characterized by an out of plane stretching of the selenium atoms. In- and out-plane Raman models were shown in Fig. 2(a), of which the vibration modes induced two main Raman modes [33]. Figure 2(b) shows the measured Raman spectrum of SnSe2 thin film. The Eg and A1g modes have been observed with the positions of 108.4 cm−1 and 189.0 cm−1, respectively. The A1g displays a much larger intensity than the lower-frequency Eg mode. The shift of these two main modes is ~80.6 cm−1, indicating that the count of the layers was estimated to be three, which was in agreement with the result in [20].
3. Nonlinear optical investigation
An open-aperture Z-scan system was employed to investigate the saturable absorption performance of SnSe2, and the arrangement of the devices is depicted in Fig. 3. In this system, transmittance through the sample as a function of the incident laser intensity was measured as the sample gradually moved through the focus of a lens along the laser propagation direction (z axis). The sample was mounted on a motorized translation stage to traverse the focused beam with a 150 mm-focal-length lens. A mode-locked fiber laser operating at 1030 nm, with the pulse width of 340 fs and repetition rate of 100 Hz, was employed for detecting nonlinear optical response. The beam waist radius was ∼32 µm at 1030 nm. The incident energy was set to be a range from 30 nJ to 150 nJ, corresponding from 2.68 GW/cm2 to 13.38 GW/cm2 at z = 0 respectively, for comparing optical response at different intensities.
Figure 4 shows the normalized transmittance in five different testing energy values (dots), which exhibit symmetrical peaks on the focal point (z = 0), indicating clear saturable absorption responses in these different energy values, and the SA behavior presents a good stability as the increase of pulse energy.
Recently, Ding et al. have demonstrated that electronic relaxation time of SnSe2 is less than 300 fs at room temperature [34]. Therefore, SnSe2 can be applied to a fast saturable absorption model with the probe pulse with 340-fs-width in this work. Based on the theory, the propagation equation in the thin film can be written as [35,36]
where z′ is the propagation distance in the samples. And the total absorption α(I) consists of a linear absorption coefficient α0 and a nonlinear absorption coefficient β:For a system with obvious nonlinear saturable absorption response, we can also describe α(I) in Eq. (1), the saturable absorption model, with the form of [37]
where IS is the saturation intensity.From the fitting of Z-scan data in Fig. 4 using Eqs. (1) and (3), the fitting curves are shown as solid lines. The value of Is is calculated to be 23.78 GW/cm2 at incident energy of 30 nJ, are fitted by Eqs. (1) and (3). As we can see, the model agrees well with the Z-scan results and the transmittance varies from ~5.4% to ~6% at different pulse energy excitations, this phenomenon shows a substantial stability under ultrafast laser irradiation.
The fitted values of β are shown as circle symbols in Fig. 5. The values are changed from −13596 cm/GW to −2967.3 cm/GW, which are obtained by fitted the data of Z-scan using Eqs. (1) and (2). However, they appeared a tendency to be saturated while the excitation peak power enlarging, which might reveal that relationship between the reciprocal transmission 1/T and irradiance. In Eqs. (1) and (2), β is assumed to be independent of the irradiance, and this is valid only at low intensities. According to the experimental data, as the saturation occurs, the nonlinear absorption coefficient is supposed to be dependent on the irradiance. Therefore, we employed a mathematical hyperbolic model to fit the β data in Fig. 5. The solid line is the theoretical variation for a hyperbolic irradiance dependence of the nonlinear absorption coefficient, whereas the dashed line is the theoretical variation for a constant coefficient. The fitted curve well agrees with experimental β values, which further verified the previous assumption about relationship between β and the irradiances. These results revealed that the SnSe2 thin film possesses an enormous potential in the non-linear region of weak-intensity excitation power and a prospect for being SA in mode-locking or Q-switched lasers.
Fig. 5 Fitting values of nonlinear coefficient β as functions of the different excitation irradiance, and their fitted curves.
4. Q-switched lasing generation
We utilized a waveguide laser platform to analyze the nonlinear saturable absorption responses of thin film as substantial SA as shown in Fig. 6. The platform is based on a Nd:YAG (doped by 1 at.% Nd3+ ions) crystal waveguide, fabricated by the femtosecond laser writing and coated optical film, of which the set was same as Ref [31]. and [32]. A 30mm-focal-length lens and a 20 × microscope objective lens (N.A. = 0.4) were used to launch the pump beam and collect the output lasers, respectively. The optical pump beam was generated by a linearly polarized light beam at 810 nm generated from a tunable CW Ti:Sapphire laser (Coherent MBR PE). The SnSe2 thin film of SA was set as an output coupler mirror, which was pressed close to the end-face without optical film coated of the waveguide. The detective devices were located after objective lens at the end of experimental system, including an oscilloscope, a spectrometer, etc.
Fig. 6 Schematic of the experimental setup for the Q-switched waveguide laser generation; the insert is the SA of SnSe2.
Based on waveguide laser platform, a Q-switched pulsed laser was achieved under continuous laser pump used SnSe2 thin film as a SA. Figure 7(a) shows the lasing average output power as a function of incident power with the maximum output average power values of 102.3 mW (80.2 mW) at TE- (TM-) polarization. Through linear fitting, laser retained lasing thresholds of ~287 mW (~302 mW) and slope efficiencies of 11.9% (9.5%) at TE (TM) polarization. Figure 7(b) shows the laser emission spectrum from Nd:YAG crystalline waveguide with the SnSe2 thin film as SA. The central wavelength for the Q-switched lasers is 1064 nm for both TE- and TM-polarization, which clearly denotes the laser oscillation line corresponds to the main fluorescence of 4F3/2→4I11/2 transition of Nd3+ ions. The full width at half maximum (FWHM) value of the emission line is ~1 nm. In case of SnSe2 absorber, the same laser spectrum at main emission line of 1064 nm has been obtained for TE- and TM-polarized pump. The typical oscilloscope traces of the Q-switched pulse train, at the incident power of ~797 mW TE- and TM-polarized pump, have been shown in Figs. 7(c) and 7(d).
Fig. 7 The average output power as a function of incident power (a), emission spectrum of Q-switched waveguide laser system (b), and the pulse trains output at (c) TE- and (d) TM-polarized light pump.
Figures 8(a) and (b) show the experimental pulse parameters of the waveguide lasing, of which blue symbols and lines are corresponding to left axes and purple ones are corresponding to right axes. The repetition rates of the Q-switched waveguide laser system were exhibited by blue symbols and lines in Fig. 8(a), which were tunable ranging from 0.337 to 2.294 MHz and from 0.438 to 1.865 MHz as the incident power into waveguide increases in both of TE- and TM-polarizations. The pulse durations were in a range from 183 ns to 299 ns at TM-polarized pump (the value from 129 ns to 241 ns corresponding to TE-polarization), as well shown in Fig. 8(a) by purple lines and symbols. The pulse energy and peak power were shown in Fig. 8(b), which raised obviously with increase in incident pump power, from 6.7 nJ to 44.5 nJ (from 6.5 nJ to 43.1 nJ) and maximum to 347.0 mW (218.0 mW) at TE (TM) polarized orientation, respectively.
Fig. 8 The Q-switched waveguide laser (a) parameters of repetition rates and pulse durations, and (b) values of pulse energy and peak power, in TE- and TM-polarization.
Based on the results above, the modulation depths (q) of Q-switched laser based on the SnSe2 as the SA can be calculated, which was expressed as an equation of pulse energy EP [31]:
where hνL is the photon energy; σL is the emission cross section of the laser material; V is the cross section of pumping volume; ηout is the coupling efficiency. The modulation depth of can be calculated to be 7.2% maximum.Table 1 illustrates the comparison of Q-switched waveguide lasers based on various 2D SAs materials. Please note that the experimental setup of waveguide laser based on novel SnSe2 in this work shares the same waveguide and laser cavity configuration with our previous works. Compared with other 2D materials, the waveguide lasers based on SnSe2 SAs are of comparable performance. In fact, the lasing performances, especially, modulation depth and lasing threshold, with SAs of 2D materials are approaching to those with semiconductor saturable absorber mirrors (SESAMs) [38]. This suggests that the SA and nonlinear responses of 2D few-layer SnSe2 thin film are suitable for pulsed lasing in low-irradiance regimes.
Table 1. Comparison of Q-switched Waveguide Lasers Based on Different 2D Materials
5. Summary
In conclusion, we have investigated the optical nonlinearity of few-layer SnSe2 thin film by employing Z-scan technique at 1030 nm with femtosecond laser pulses. The thin film exhibits superb optical properties with saturation intensity of ~23.78 GW/cm2 and nonlinear absorbed coefficients from −13596 cm/GW to −2967.3 cm/GW with the increase in excitation peak power. Furthermore, the nonlinear absorbed coefficients have exhibited the saturation effect. The thin film was employed as a SA on waveguide laser platform to generate Q-switched pulsed laser at 1.06μm. The lasers work stably with tunable repetition rates, low lasing thresholds, and nanosecond-level pulse durations. The results of Z-scan and waveguide laser system have indicated excellent nonlinear properties of the thin film, especially in weak-intensity excitation peak power. Our results have suggested the layered SnSe2 can be applied as promising SAs in bulk or integrated active devices for ultrafast laser generations.
Funding
National Natural Science Foundation of China (NSFC) (11274203); Strategic Priority Research Program of CAS (XDB16030700); Key Research Program of Frontier Science of CAS (QYZDB-SSW-JSC041).
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