Femtosecond laser written waveguides with MoS 2 as satuable absorber for passively Q-switched lasing

: This work reports on the passively Q-switched waveguide laser system based on Nd:YAG crystal and MoS 2 satuable absorber. A depressed cladding waveguide with circular cross-sectional geometry has been produced in Nd:YAG crystal by direct femtosecond laser writing at low-repetition rate. The confocal microscopic investigation of the structure reveals the well-preserved microphotoluminescence features in the waveguide core. With chemical-vapor-deposition (CVD) MoS 2 membrane as satuable absorber, the passive Q-switching of the Nd:YAG waveguide system has been achieved under optical pump, reaching maximum average output power of 85.2 mW, corresponding to single-pulse energy of 112 nJ, at wavelength of 1064 nm. The repetition rate of the pulsed waveguide laser system is tunable from 0.51 to 1.10 MHz, and the obtained minimum pulse duration is 203 ns. 89 nJ and minimum pulse duration of 203 ns. This work paves the way of MoS 2 based solid state waveguide lasers for photonic applications.


Introduction
Optical waveguides have been widely applied in a broad range of areas, such as optical telecommunications, quantum computing, biophotonic sensing, and information processing, etc [1][2][3][4][5][6]. Recently, optically pumped solid-state waveguide lasers, serving as intriguing miniature light sources, have attracted more and more attentions due to a number of advantages of the systems [7][8][9][10]. Based on the compact waveguiding geometries, enhanced intracavity intensities in the waveguide cores enable more efficient generation of lasers in gain media [8,11]. In addition, the small scale of the waveguide components matches the diameters of optical fibers, which shows the potential capability to construct complex photonic systems based on the waveguide chips and fibers [12].
Direct femtosecond laser writing/inscription has emerged as a powerful technique to fabricate waveguide devices in transparent optical materials [13][14][15][16][17][18]. Due to the efficient micro-engineering of the refractive index of the focal planes induced by the femtosecond laser pulses, either positive or negative index modifications could be achieved, resulting in the implementation of waveguiding structures with diverse configurations [8,19]. In Nd:YAG laser crystal, a number of applications have been obtained based on the platform of laser-written waveguides, e.g., waveguide splitters and modulators in passive regime, and waveguide amplifications and lasers in active regime [12,20,21]. Owing to the excellent fluorescence, physical and thermal properties, the Nd:YAG crystal has widely deployed to solid-state lasers as gain media. With Nd:YAG as the gain media, waveguide lasers have been realized in a mass of compact continuous-wave (CW) systems manufactured by epitaxial growth, ion beam implantation/irradiation, and femtosecond laser writing [22,23]. Pulsed lasers on waveguide platforms permit compact cavity design with high optical intensities, which are intriguing light sources for on-chip integration [24,25].
MoS 2 (molybdenum disulfide) is a novel two-dimensional (2D) nanomaterial, which consists of two hexagonal planes of S atoms coordinated through ionic-covalent interactions with the Mo atoms. It has distinctive electronic, optical, and catalytic properties with the geometry of the ultrathin layer, indicating interesting applications for electronic and optical devices [26,27]. The saturable absorption (SA) is one of the most significant characteristics for MoS 2 as a member of nanosheet materials [28]. Owing to the SA feature, other nanomaterials, e.g., carbon nanotubes, black phosphorus, and graphene, have been successfully applied towards various applications in pulsed laser systems [24,[29][30][31]. With MoS 2 bulk as well as few-layer nanoplate as the SA in solid-state laser systems, pulsed lasers have obtained based on fibers and bulk materials [32][33][34]. In consideration of the bandgap of laminar MoS 2 nanosheet, the theories based on edge/defect state absorption can extended the bandgap from visible/near-visible spectral region to the near-infrared (NIR), enriching applications of solid laser systems with MoS 2 SA [35]. Nevertheless, in solid-state waveguide frameworks, the Q-switched lasers have not been achieved using MoS 2 as SA in the systems, although a few works have been performed on the SA via graphene or carbon nanotubes [36,37].
In this work, we report on the lasing characteristics, by employing a CVD MoS 2 thin film as SA, in Q-switched regimes of a laser written Nd:YAG cladding waveguide, of which the details have been investigated by a confocal micro-photoluminescence spectroscopy.

Experimental details
The Nd:YAG crystal (doped by 1 at.% Nd 3+ ions) sample used in this work was cut into a wafer with dimension of 10 × 9 × 2 mm 3 and optically polished. An amplified Ti:Sapphire laser system, to generate linearly-polarized 120 fs pulses at a central wavelength of 800 nm with 1 kHz repetition rate, was utilized to write the depressed cladding waveguides with the facility of Universidad de Salamanca, Spain. The pulse energy for micro-process was set with a calibrated neutral density filter, a half-wave plate and a linear polarizer. The beam of fabricated fs laser was focused by the largest sample surface (10 × 9 mm 2 ) at a depth of 150 μm through a microscope objective (Leica 40 × , N.A. = 0.65) with a pulse energy of 2.52 μJ. The wafer was placed on a motorized 3-axes stage controlled by computer programming and drove with a constant speed of 0.5 mm/s during the irradiation along the direction perpendicular to both the fs laser propagation and the polarization. The parallel scans of cladding structure (with ~3 μm separation between adjacent tracks) were performed at different depths of the sample (from bottom to top of the sample in order to avoid the previously written tracks shielding the incident pulses), which consist of cladding structure. The cross sections of the cladding guiding structures in the Nd:YAG wafer can be seen in Fig.  1(a).
The homemade confocal microscope was used for the micro-fluorescence investigation of the waveguides. The 488-nm CW laser, as the optical excitation source, was focused in the sample by a 50 × microscope objective (N.A. ~0.55), yielding a spot with a diameter of approximately 0.53 μm. The subsequent emission of Nd 3+ ions was collected by the same microscope objective, after passing through filters, lenses and pinholes, and was analyzed by a high resolution spectrometer. The software LabSpec and WSx were utilized to analyze and fit the spectra and process the fluorescence images in terms of the spectral intensity, energy shift and width of the fluorescence lines. The CVD MoS 2 film coated glass wafer was a commercial product. The surface topography is characterized by an AFM (shown in Fig. 1(b)). The Raman spectrum of MoS 2 nanosheet depicted the shifted peaks appearing at 406 cm −1 and 381 cm −1 , which were corresponding to the in-plane E1 2g vibration mode and out-plane A 1g vibration mode, respectively ( Fig. 1(c)). For ensuring the spatial integrity of the film, hybrid monolayer and few-layer (approximately 2~3 layers) of MoS 2 membrane consisted of the whole film. The Qswitching of waveguide lasing was realized by using an end-pump system. 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 convex lens with a focal length of 30mm was used to couple the pump laser beam into the waveguide. A thin film was coated on end face towards the pump source with parameters of high reflectivity at ~1064nm and high transmission at ~808nm. The MoS 2 film was set as an output coupler mirror, by exerting mechanical bindings to make sure the SA was closely attached to the output facet of waveguides. The inset in Fig. 1(d) shows the microphotograph of the MoS 2 SA surface. It was 1000 × magnified and imaged by the optical microscope (Axio Imager, Carl Zeiss) operating in reflection mode. The generated waveguide lasers were collected by utilizing a 20 × microscope objective lens (N.A. = 0.4) and imaged by using an infrared CCD. We used a spectrometer with resolution of 0.2 nm to analyze the emission spectra of the laser beam from the waveguide. The schematic plot of the experimental setup for the pulsed laser oscillations are depicted in Fig. 1(d).

Results and discussion
The modifications of the fs-laser inscription in the waveguide with 100μm-diameter have been shown in the fluorescence images included in Figs. 2, which illustrated modified the micro-structural and fluorescence properties in the filaments (fs laser focus). Figures 2(a)-2(c) shows the 2D spatial distributions of intensity, induced energy shift, as well as FWHM of the 4 F 3/2 → 4 I 9/2 inter-Stark level emission line of Nd 3+ ions around 940 nm (this line has been found to be hyper-sensitive to slight changes in the Nd 3+ environment, such as damage, volume changes and disorder) [8,38,39]. Figure 2(a) shows that the fluorescence intensity generated by Neodymium ions is significantly reduced at the laser irradiated volumes. This fact unequivocally reveals the efficient creation of defects at the waveguide contour (i.e. partially damaged areas) as a result of the localized optical breakdown. And it is further supported by the fluorescence image obtained in terms of the spatial variation of the fluorescence linewidth (Fig. 2(c)) that reveals a clear increment in the lattice disorder at the damage tracks. Laser irradiation does not only lead to the appearance of well-localized and disorder in the YAG network. The fluorescence map obtained in terms of the induced spectral shift of emission line reveals a remarkable luminescence shift towards lower energies (redshift) at the waveguide contour (damage tracks) that, according to previous works, could be attributed to a local lattice compression at damage tracks. It is worth noting that this well agrees with previous works dealing with femtosecond fabrication of waveguides in YAG crystals and ceramics. It seems that the network damage created at the focal volume is companied by a relevant lattice compression that, in turns, is mechanically compensated by the appearance of dilated volumes located at the apex of damage filaments. Such mechanical compensation is also evidenced in the fluorescence map of Fig. 2(b) where a slight shift towards larger energies is observed at the bottom of the structure. The results in Fig. 2 reveal that a locally modification has been induced in a relevant damage only at the waveguide contour and the fluorescence properties at waveguide volume are well preserved.   Figure 3(d) shows the laser emission spectrum from Nd:YAG waveguide at room temperature, as the absorbed pump power is above the oscillation threshold. The central wavelength for the Qswitched pulsed waveguide lasers is ~1063.9 nm, which clearly denote the laser oscillation line that corresponds to the main fluorescence of 4 F 3/2 → 4 I 11/2 transition of Nd 3+ ions, the full width at half maximum (FWHM) value of this spectrum is about 0.4 nm. The inset two images present waveguide modal profiles of lasers at ~1064nm for TE-and TM-polarization. The average output power of the Q-switched waveguide laser was 83.3 mW and 84.0 mW along TM and TE polarizations, respectively. The corresponding energy of single pulse was 89 nJ. The pulse duration and repetition rate was 203 ns and 1.10 MHz, respectively. By varying the pumping power, the repetition rate of the waveguide pulsed laser could be tuned from 0.51 MHz to 1.10 MHz, as the absorbed power increases from 447 to 617 mW(see Figs. 4(a) and 4(b)). The more compact size of the waveguide laser system may be used as miniature light sources in photonic chips for diverse applications.

Summary
In conclusion, by using MoS 2 as SA, the Q-switched waveguide lasers at 1064nm have been implemented in laser written Nd:YAG depressed cladding waveguide. The pulsed waveguide laser have a repetition rate ranging from 0.51 MHz to 1.10 MHz, with max average power of 84.0 mW, pulse energy of up to 89 nJ and minimum pulse duration of 203 ns. This work paves the way of MoS 2 based solid state waveguide lasers for photonic applications.