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We reported on the passively Q-switched waveguide lasers based on few-layer transition metal diselenide, including molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2), as saturable absorbers. The MoSe2 and WSe2 membranes were covered on silica wafers by chemical vapor deposition (CVD). A low-loss depressed cladding waveguide was produced by femtosecond laser writing in a Nd:YAG crystal. Under optical pump at 808 nm, the passive Q-switching of the Nd:YAG waveguide lasing at 1064 nm was achieved, reaching maximum average output power of 115 mW (MoSe2) and 45 mW (WSe2), respectively, which are corresponding to single-pulse energy of 36 nJ and 19 nJ. The repetition rate of the Q-switched waveguide lasers was tunable from 0.995 to 3.334 MHz (MoSe2) and 0.781 to 2.938 MHz (WSe2), and the obtained minimum pulse duration was 80ns (MoSe2) and 52 ns (WSe2), respectively.
© 2016 Optical Society of America
1. Introduction
Recently, the two-dimensional (2D) nanomaterials, e.g. graphene and layered analogous to graphene, have received much attentions from the researchers in many areas due to the intriguing electronic and optical features [1–3]. Transition metal dichalcogenide (TMDC) possesses excellent properties for generations of photoluminescence, electroluminescence, ultrafast nonlinear absorption, second and third harmonics, which have been widely applied as key components in electronics and photonics [4,5]. In this family of 2D materials, Group VI transition metal (Mo, W and so on) diselenide as well as disulfide are the important members of the TMDCs owing to their direct band gap, with the structure of chalcogen atoms in two hexagonal planes separated by a plane of metal atoms [6].
Optical waveguides have been widely applied in many areas, such as modern telecommunication, quantum computing, information storage, and bio-sensing [7–9]. Due to the diffraction-free light propagation inside the waveguides, the beam manipulation could be achieved efficiently. A number of nonlinear optical applications (e.g., discrete solitons) might be realized in waveguides with diverse configurations that are absent in bulk systems [10]. Waveguides based on gain media have been utilized as the key components to obtain small-size, cost-effective miniature light sources in integrated photonic circuits [11]. Waveguide lasers, including continuous-wave (CW) and pulsed laser operation regimes, have been produced in a number of systems towards a broad range of photonic applications [12–17]. A significant first step of construction of waveguide laser systems is to fabricate waveguides in various gain media [17]. Recently, the femtosecond laser writing has become a powerful technique to manufacture waveguides with versatile geometries in more than 50 optical materials owing to its unique capability for 3D microstructuring and wide applicability for diverse materials [16,17]. The laser written waveguide lasers in CW regimes have achieved excellent performances, such as high output powers, enhanced efficiencies, and reduced lasing thresholds [16–21]. More recently, research on pulsed waveguide lasers have also become more intriguing due to the rapid exploration of nanomaterials as broadband saturable absorbers (SAs) [22,23].Based on the nonlinear saturable absorption of nanomaterials, the passive Q-switching or mode-locking may be realized to achieve stable pulses through the waveguide cavities. A few nanomaterials, including graphene, single-wall carbon nanotubes (SWCN), disulfides (MoS2 and WS2), and black phosphorous, have been applied to achieve pulsed waveguide lasers in all-solid-state systems [22–25]. Particularly, in a laser-written Nd:YAG crystal waveguide with depressed cladding geometry, we have implemented Q-switched waveguide laser based on few-layer MoS2 as SA at wavelength of 1064 nm [25]. Diselenides (both of MoSe2 and WSe2), contrast to disulfide TMDCs, have recently been shown to have excellent direct-gap semiconducting and non-linear optical properties, for instance a narrower gap semiconductor is required [4]. MoSe2 and WSe2 have been applied as SA materials in the Q-switched fiber lasers [26,27]. However, diselenides have not been used as SAs in waveguide laser systems up to now.
In this work, we reported on the all-solid-state Q-switched waveguide lasers in Nd:YAG based on few-layer MoSe2 and WSe2 thin-film mirrors. The lasing performance of the systems was investigated in details. A reasonable comparison was made between the MoSe2 and WSe2 waveguide laser systems, revealing clear discrepancy of the nonlinear absorption properties of two thin films. This work paves the way on the applications of 2D transition metal diselenides as excellent SAs in integrated waveguide laser platforms.
2. Experimental details
The Nd:YAG crystal (doped by 1 at.% Nd3+ ions) sample used in this work was cut into a wafer with dimension of 10 × 9 × 2 mm3 and optically polished. The depressed cladding waveguide were written by an amplified Ti:Sapphire laser system, with pulse duration of 120 fs at a central wavelength of 800 nm with 1 kHz repetition rate. Details of Nd:YAG waveguide has been reported in [25].
The MoSe2 and WSe2 thin films were fabricated by the chemical-vapor-deposition (CVD) technology on fused silica wafers, which were commercial products (6Carbon Technology, Shenzhen, China). Few layers layout of MoSe2 and WSe2 membranes were utilized for ensuring the spatial integrity of the films and overspread the whole wafers. The photographs of wafers coated the MoSe2 and WSe2 thin films are exhibited in Figs. 1(a) and 1(d). The surface topography was characterized by an atomic force microscope (AFM) (shown in Figs. 1(b) and 1(e), respectively). The Raman spectra of MoSe2 and WSe2 membranes depict the shifted peaks of the in-plane E1 2g vibration mode and out-plane A1g vibration mode, which appear at 244.0 cm−1 and 254.7cm−1 in MoSe2, and at 252.7 cm−1 and 260.3 cm−1 in WSe2, respectively, as shown in Figs. 1(c) and 1(f).
Fig. 1 Images of CVD MoSe2 thin film (a) and WSe2 (d) coated glass wafers. AFM images of MoSe2 thin film (b) and WSe2 (e). Raman spectra of the MoSe2 thin film (c) and WSe2 (f).
The nonlinear absorption coefficients of the MoSe2 and WSe2 films were measured by the Z-scan technique with a 1064-nm laser with 22-ps pulse duration and 0.5-μJ energy and a lens (400mm-focal-distance). The experimental setup and process are same as [24]. The measured nonlinear transmissions of MoSe2 and WSe2 films are shown in Fig. 2(a). The relation between the transmission (T) and the excitation energy (I) has been fitted by the following equation
where TN is the nonsaturable absorbance, ΔT is modulation depth, and Isat is saturable intensity. The values of ΔT and Isat are 11.4% (5.4%) and 0.006 GW/cm2 (0.007 GW/cm2) for MoSe2 (WSe2). The damage thresholds of MoSe2 and WSe2 are higher than 0.05 GW/cm2.Fig. 2 (a) The nonlinear transmission as a function of the probe light intensity of MoSe2 (red) and WSe2 (blue), and (b) schematic of the experimental setup for the Q-switched waveguide laser generation.
We utilized a linearly polarized light beam at 808 nm generated from a tunable CW Ti:Sapphire laser (Coherent MBR PE) as the pump source. A thin film was coated on end face towards the pump source with parameters of high reflectivity at ~1064 nm and high transmission at ~808 nm. 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 MoSe2 and WSe2 films were set as an output coupler mirror, the method was same as [24,25]. The detective devices were located after objective lens at the end of experimental system, including an infrared CCD for imaging, an oscilloscope, and a spectrometer. Figure 2(b) depicts the experimental setup of waveguide laser generation.
3. Results and discussion
Figures 3(a)-3(c) show the experimental results of the waveguide lasing based on both MoSe2 and WSe2 SAs. The average output power is illustrated as function of incident power in Fig. 3(a). From the linear fit of the experiment data, the slope efficiencies are determined to 26.4% and 26.5% and maximum output average power values are 115.1 mW and 105.7mW at TE- and TM-polarized (rectangle and circular symbols, respectively) pump via using MoSe2-based SA (red lines and points). Whilst with WSe2 SA (blue lines and points), the slope efficiencies are 7.4% and 7.0% and maximum output average power values are 45.7 mW and 42.7 mW, respectively. The minimum lasing thresholds are 189.7 mW (WSe2) and 275.1 mW (MoSe2).
Fig. 3 The average output power (a) and repetition rate (b) of the Q-switched waveguide lasers as MoSe2 (red lines and points) and WSe2 (blue lines and points) based SAs at TE- (rectangle symbols) and TM-polarization (circular symbols); the Q-switched waveguide laser emission spectrum (c), based on MoSe2 SA. The inset pictures show the output modal profiles from waveguide at TE- and TM-polarized light pump.
The repetition rates of the Q-switched waveguide laser system are exhibited in Fig. 3(b), as function of incident pump power, which are tunable ranging from 0.995 to 3.334 MHz (MoSe2) and 0.781 to 2.938 MHz (WSe2) as the incident power into waveguide increases from 190 mW to 720 mW. Figure 3(c) shows the laser emission spectrum from Nd:YAG crystalline waveguide with MoSe2 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 that 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 ~0.5 nm. In case of WSe2 SA, the same laser spectrum at main emission line of 1064 nm has been obtained for TE and TM polarized pump. The inset two images present modal profiles of the two systems of waveguide lasers.
Using MoSe2 as SA, the Q-switched laser details are depicted in Figs. 4(a) and 4(b) at room temperature, including pulse energy, pulse duration, and pulse train. The maximum single pulse energy is 35.9 nJ at TM-polarized light pump (the value is 31.7 nJ at TE-polarization). The pulse durations are in a range from 80 ns to 290 ns at TM-polarized light pump (the value from 86 ns to 265 ns corresponding to TE-polarization), as shown in Fig. 4(a). The typical oscilloscope traces of the Q-switched pulse train, at the incident power of ~667 mW TE-polarized pumping, have been shown in Fig. 4(b).
Fig. 4 Based on MoSe2 SA, the Q-switched waveguide laser pulse energy and duration (a), and the pulse trains (b).
Figures 5(a) and 5(b) show the details of Q-switched lasers based on WSe2 SA under the same experimental conditions of MoSe2. Figure 5(a) reveals that the maximum single pulse energy is 19.0 nJ at TE-polarized light pump (15.9 nJ at TM-polarization). The pulse durations are ranging from 52 ns to 400 ns at TE-polarized light pumping (from 135 ns to 301 ns at TM-polarization). The typical oscilloscope traces of the Q-switched pulse train, at the incident power of ~457.8 mW TE-polarized pumping, have been shown in Fig. 5(b).
Fig. 5 Based on WSe2 SA, the Q-switched waveguide laser pulse energy and duration (a), and the pulse trains (b).
Based on the results above, the modulation depths of Q-switched laser based on the two materials as SAs can be calculated, which was expressed as [28,29]
where EP is the pulse energy; hνL is the photon energy at the laser wavelength; σL is the emission cross section of the laser material; A is the cross section of pumping volume; q is the modulation depth of SA; ηout is the coupling efficiency. As all parameters of the material and the waveguide, the values of modulation depth (q) can be calculated, which are 2.3% and 2.6% at TE- and TM-polarization pumping modulated by MoSe2, corresponding 4.9% and 4.0% modulated by WSe2.
A comparison of Q-switched waveguide lasers based on different SA materials has been demonstrated in Table 1. It is worth mentioning that the waveguide laser based MoS2 as SA (in Table 1) is on a same design of waveguide and cavity as this work. Compared with MoS2, the waveguide lasers based on MoSe2 and WSe2 of SAs have been obtained shorter pulse durations and ~3 times repetition rate at a same Q-switched waveguide platform. For different waveguide laser designs, as [25], the deeper modulation depths were achieved in SAs of MoSe2 and WSe2. In addition, the WSe2-based laser is with higher threshold but superior lasing performance than MoSe2-based system.
Table 1. Comparison of Q-switched Waveguide Lasers Based on Different Nanomaterials as SAs
Compared with Q-switched fiber lasers, all waveguide lasers have shown obvious advantage based on TMDCs of SAs. More optimized laser parameters, including megahertz-level repetition rates, nanosecond-level pulse durations, and hundred-milliwatt-level output powers, can be achieved under roughly equivalent pump light powers on the waveguide laser platform, compared with fiber lasers [26,27].
4. Summary
In conclusion, by using MoSe2 and WSe2 as SAs, the passively Q-switched waveguide lasers at 1064 nm have been implemented in laser written Nd:YAG depressed cladding waveguide. The waveguide lasers were with tunable repetition rates, low lasing thresholds, and nanosecond-level pulse durations. The Q-switched lasers based on transition metal diselenide as SAs imply more compact sizes of the waveguide laser systems that may be used as miniature light sources in photonic chips for diverse applications.
Acknowledgments
The work is supported by the National Natural Science Foundation of China (NSFC) (No. 11274203) and Junta de Castilla y León under Project SA086A12-2. Support from the Centro de Láseres Pulsados (CLPU) is also acknowledged.
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