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Abstract
In this work, we report on the synthesis and characterization of a functionalized graphene modified by Ag based nanoparticles (Ag2S@Ag) and the applications as an optical modulator for pulsed laser generation at 1μm in the compact waveguide platform written by a femtosecond laser. The nonlinear optical response of the Ag2S@Ag nanoparticle modified graphene has been explored by the Z-scan technique, which exhibits an enhanced ultrafast saturable absorption response with higher modulation depth and lower saturation intensity. The Q-switching and Q-switched mode locking have been realized in a waveguide laser system by using Ag nanoparticle modified graphene as a saturable absorber (SA) mirror. Fundamental Q-switched mode-locked waveguide lasers with a repetition rate of 6.44 GHz have been achieved, reaching shorter pulse duration (33 ps) in comparison to that (52 ps) through unmodified graphene SA. This work suggests potential applications of functionalized graphene in ultrafast photonics.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As emerging layered two-dimensional (2D) materials, graphene and graphene-based materials have recently attracted remarkable research interest in the field of optoelectronics and photonics [1]. In the realm of photonics, graphene has been proven to be an effective saturable absorber (SA) in different platforms and various operation regimes [2–5] for its exceptional nonlinear optical characteristics such as strong light-matter interaction, broadband saturable absorption and ultrafast recovery time [6–8]. The outstanding optical properties combined with the ease of integration with other photonic devices such as fiber or waveguide enable graphene a promising candidate for miniature integrated photonic circuits [9, 10]. Typically, in order to meet the application-oriented demands for graphene, it could be functionalized or patterned to possess desired functionalities through a variety of modification techniques, such as ion beam modification, chemical functionalization or laser processing [11–14]. In particular, functionalization of 2D materials with the combination of other nanomaterials has attracted increasingly research interests. Silver (Ag) is a kind of noble metal and silver sulfide (Ag2S) is a narrow bandgap chalcogenide semiconductor. As novel semiconductor–noble metal heterostructures, the Ag2S@Ag nanostructure have been successfully synthesized and favorable charge transfer process from Ag to Ag2S may be appeared exhibited intriguing electrical, photocatalytic, and surface plasmon resonance (SPR) properties [15,16]. With noble metal induced SPR, the nonlinear optical (NLO) performances could be enhanced significantly and superior performance could be expected in pulsed lasing [17]. In previous work, we have reported the fabrication and characterization of this kind of hybrid nanoparticles and exhibited excellent adsorption performance indicating its potential to be developed for effective dye removal and low-cost water purification [18]. To achieve functionalization of graphene in this work, we constructed a composite system of graphene and Ag2S@Ag nanoparticles, which may provide a novel platform for the applications in photonics and optoelectronics.
Optical waveguides with diverse guiding structures provide a multifunctional platform for integrated photonic circuits [19]. Based on the waveguide platform, a variety of photonic applications can be achieved such as waveguide lasers, frequency converters, optical switches and so on. Among developed waveguide fabrication techniques, femtosecond (fs) laser writing has been proven to be an efficient method to fabricate various low-loss 3D waveguiding structures in a broad range of optical materials [20–22]. With the tight confinement of light field, laser oscillations in the waveguiding structure may possess reduced lasing thresholds and enhanced slope efficiency compared with those of bulk laser systems. The modal profile could also be tailored in a flexible manner [23,24]. Generally, to achieve device functionalities (e.g., photodetectors or lasers), 2D materials could be integrated with the waveguide by evanescent field interaction or direct incorporation [25–27], in which direct integration of 2D thin films onto the waveguide end-facets is an easy and convenient way to enable functionality. Up to now, waveguide lasers have been recognized as compact and reliable laser sources and pulsed laser operation have been realized based on a variety of 2D materials or SESAMs under different operation regimes, either Q-switching or mode-locking, in various gain medium with broad spectral region from the visible towards the mid-infrared light [28–35].
In the case of waveguide laser, Q-switched operation based on graphene SA have been achieved by He et al. and Li et al. previously [36,37]. Mode-locking lasers modulated by graphene SA have also been achieved in waveguide configuration. In particular, mode-locked lasers with fundamental repetition rate up to multi-GHz regime are of great research interest and have been widely used in a variety of applications such as frequency combs generation, nonlinear microscopy and high-speed optical processing [38]. Based on graphene SA, Q-switched mode-locked (QML) lasers have been demonstrated by a few groups under the waveguide platform with repetition rate of 1.5 GHz, 5.9 GHz and 7.8 GHz at different operation wavelength [39–41]. Based on other 2D materials, 6.5 GHz QML in the waveguide system has been reported by Li et al. with MoS2 and Bi2Se3 as SAs [42]. Continuous-wave mode-locked (CWML) laser has also been demonstrated in the waveguide system by Okhrimchuk et al [43].
In this work, we report on the synthesis and characterization of Ag2S@Ag nanoparticle modified graphene and its applications as a superior optical modulator in pulsed laser generation under compact waveguide platform. The physical properties as well as the nonlinear optical properties of the nanomaterial are measured by performing Raman spectroscopy, atomic force microscope (AFM), and Z-scan technique. We further applied this novel material into a waveguide platform as SA and achieved Q-switched and 6.44 GHz fundamental Q-switched mode-locked laser operation. The lasing performances have also been investigated in details.
2. Preparation and materials characterization
The depressed cladding waveguide is fabricated by femtosecond (fs) laser writing in an optically polished Nd:YVO4 crystal (doped by 1 at.% Nd3+ ions), which was cut into a wafer with dimensions of 10(x) × 10(y) × 2(z) mm3. The detailed laser parameters are the same as previous work [36]. During the writing process in this work, the sample is placed on a PC-controlled XYZ translation stage and scanned with the velocity of 750 μm/s, forming the 50 μm-diameter depressed cladding waveguiding structure with well-preserved physical properties.
The graphene/Ag2S@Ag nanocomposite sample is fabricated and characterized in our work. First, graphene monolayer, served as bottom layer, is synthesized directly on sapphire substrate by chemical vapor deposition (CVD) process. Then, the Ag2S@Ag nanoparticles is synthesized by laser-induced fabrication technique and details about fabrication and characterization are reported previously [18]. Following, the as-prepared Ag2S@Ag nanoparticles is drop-casted onto the pre-grown graphene and annealing process is carried out to remove the organic impurities of the nanocomposite. Figure 1(a) is a schematic demonstration of the fabrication process of graphene/Ag2S@Ag nanocomposite and shows an optical photograph of the as-prepared sample on sapphire substrate. Atomic force microscopy of the fabricated graphene/Ag2S@Ag nanocomposite is performed in tapping mode by employing a Bruker Dimension Icon system with ScanAsyst and the nanoscale surface topographic image as well as the height profiles of the sample are shown in Fig. 1(b) with the measured average thicknesses to be 16.03 nm. As shown in Fig. 1(c), the Raman signal of graphene exhibited large enhancement induced by the deposition of Ag2S@Ag nanoparticles, suggesting surface-enhanced Raman scattering (SERS) of graphene. The Raman enhancement factor of the 2D (2687 cm−1) and G (1585 cm−1) band is 4.6 and 6.4, respectively. The reduction in the intensity ratio of 2D and G band could be attributed to the doping effect of graphene. The increase of D band (D for disorder, 1345 cm−1), which is the Raman signature for disorder and lattice defect, indicates the presence of defects in graphene during the fabrication process of graphene/Ag2S@Ag nanocomposite.
Fig. 1 (a) Schematic illustration of the formation of Ag2S@Ag nanoparticle modified graphene on sapphire substrate (b) The AFM topographic image and height profile. (c) The measured Raman spectrum of graphene and fabricated nanocomposite.
The linear absorption properties from the visible to near infrared spectral range is investigated by employing a UV–Vis–NIR Spectrophotometer (UV-1800, Shimadzu). Figure 2(a) shows the ultraviolet–visible–NIR absorption spectra of Ag2S@Ag nanoparticle modified graphene, pristine graphene and the sapphire substrate, respectively. The modified graphene exhibited an enhanced broadband linear absorption, which is superior for the applications of broadband optical modulator. The relative absorption curve shown in Fig. 2(b) is obtained by using the absorbance values of graphene/Ag2S@Ag subtracted by that of pristine graphene and substrate, indicating the presence of near field enhancement effect lead by SPR absorption. At the wavelength of 1064 nm, the complex synergistic effects between graphene and Ag2S@Ag nanoparticles could also give the contribution to the enhancement in addition to SPR effect. As demonstrated in the insert of Fig. 2(b), the near field distribution at the wavelength of 1064 nm influenced by SPR effect has also been calculated to obtain solutions of Maxwell’s equations using DDSCAT software package with discrete dipole approximation (DDA) method [44], indicating the enhancement of the near field in the surrounding areas of the nanoparticles.
Fig. 2 (a) The absorption spectrum of modified graphene, pristine graphene and the sapphire substrate acquired by a spectrophotometer. (b) The relative absorption spectrum indicating the occurrence of SPR absorption. The insert is the calculated near field enhancement at 1064 nm by DDA method.
In order to better understand the nonlinear optical response of Ag2S@Ag nanoparticle modified graphene, we designed and performed the typical open-aperture Z-scan system as illustrated in Fig. 3(a). The open-aperture regime allows one to investigate the intensity-dependent absorption features. The graphene and modified graphene samples are excited by a mode-locked laser with beam waist radius of 30 μm operating at 1030 nm (340 fs) with 100 Hz repetition rate. The reference and transmitted light is monitored by Detector A and Detector B as the sample gradually moved on a motorized translation stage along the laser propagation direction (Z axis). The incident pulse energy is tuned by placing an attenuation slice and is set to be increased from 30 to 150 nJ, corresponding to the on-focus light intensity from 6.37 to 31.86 GW/cm2.
Fig. 3 (a) Schematic representation of the experimental setup of open-aperture Z-scan system. (b) Normalized transmission as a function of sample position measured by an open-aperture Z-scan system. The scatters are experimental data and solid lines are the fitting curves.
The normalized transmittance of Ag2S@Ag nanoparticle modified graphene and pristine graphene samples are measured by varying the sample position Z with different on-focus light intensities and the experimental data was fitted by the following nonlinear absorption model [45,46],
where z′ is the laser propagation distance within the sample, and α0 is the linear absorption coefficient at the corresponding wavelength. As shown in Fig. 3(b), the nanocomposite has exhibited stronger nonlinear optical response. The typical saturable absorption curves displayed a good symmetrical peak shape, indicating the absorption of graphene/Ag2S@Ag tuned to be saturated as the increase of light intensity. The modulation depth and saturation intensity of single graphene is measured to be 1.05% and 72.39 GW/cm2, which is within the zone of reasonableness under the same measuring condition. By fitting these curves, the corresponding modulation depth and saturation intensity of the nanocomposite are obtained to be 19.7% and 1.28 GW/cm2. The nonlinear absorption coefficient β is found to be ranges from −95908 to −127306 cm/GW with the increase in pulse energy. The bleaching owing to saturation may be caused by the occupation of valence band and conduction band after constantly excitation of electrons. The enhanced nonlinear responses of graphene/Ag2S@Ag may be attributed to the SPR effect from the Ag2S@Ag nanoparticles as well as the synergistic effects between graphene and Ag2S@Ag nanoparticles. The superb ultrafast saturable absorption properties of graphene/Ag2S@Ag as well as lower saturation intensity and higher modulation depth are intriguing for photonics applications.Table 1 summarizes the linear and nonlinear optical parameters of several typical low-dimensional materials. Different material fabrication methods and excitation laser could strongly influence the linear and nonlinear parameters. Under the same conditions, it can be seen that silver-based nanoparticle modified graphene process superior properties in comparison with pristine graphene. The functionalization of optical properties toward optoelectronic applications could also be expected to expand to other 2D materials or carbon nanotubes.
Table 1. The linear and nonlinear optical properties of several low-dimensional materials
3. Pulsed laser operation
On the basis of the nonlinear optical properties, we further investigated the potential applications of Ag2S@Ag nanoparticle modified graphene as a SA. We applied this novel material into the typical end-face coupling arrangement to perform the Q-switched waveguide laser generation. The schematic configuration for the generation of Q-switched waveguide laser is demonstrated in Fig. 4(a) and the insert is the cross-sectional image of the fabricated waveguide imaged by an optical microscope (Axio Imager, Carl Zeiss). The pump source is linearly polarized light at 808 nm emitted from a tunable CW Ti:sapphire laser (Coherent MBR-PE). The waist radius of the launched laser beam is approximately 0.75 mm at 1/e2 position. The polarization state of launched laser is controlled by placing a half-wave plate. The generated laser is then coupled into the cladding waveguide through a spherical convex lens with a focal length of 25 mm. An input mirror M1 (99% transmission of at 808 nm and >99.9% reflectivity at 1064 nm) and graphene/Ag2S@Ag thin film (SA) are integrated onto the input and output waveguide end-facets, respectively, constituting the Fabry-Perot Resonator with Nd:YVO4 waveguide as a gain media. The generated pulsed laser is collected through a 20 × long work-distance microscope objective lens (N.A. = 0.4) and pass through a longpass filter (Thorlabs, FEL0850) to avoid the transmission of unwanted pump light. The filtered output laser is then coupled into a fiber which is connected directly with a 25 GHz High-Speed Fiber-Optic Photodetector (New focus, 1414 model) and a 25 GHz wide-bandwidth digital oscilloscope (Tektronix, MSO 72504DX) with the rise time as fast as 16 ps. The generated output laser is also measured by utilizing power meter, infrared CCD and spectrometer.
Fig. 4 (a) Schematic representation of Q-switched waveguide laser generation. (b) output power as a function of launched power; The insert is measured near-field modal profile of the output laser. (c) The Q-switched emission waveguide laser emission spectrum modulated by graphene/Ag2S@Ag; The insert is the pulse trains of generated laser.
The typical characteristics of Q-switched waveguide laser modulated by graphene/Ag2S@Ag SA are characterized under the same condition as demonstrated in Fig. 4(a). Through linear fitting, it can be found that the Q-switched waveguide laser operation can be achieved as the launched power exceeded the threshold value of 0.12 W. As shown in Fig. 4(b), the maximum output power has reached about 176 mW (93 mW) and the corresponding slope efficiency is 21.8% (12.4%) at TE and TM polarization. The difference of output power at TE and TM polarization could be associate with the polarization dependent emission cross section of the Nd:YVO4 crystal. The inset shows the measured near-field modal profile of the pulsed laser, showing the fundamental mode. Figure 4(c) illustrates the measured emission laser spectrum with the full width at half maximum (FWHM) to be ~1.5 nm and the central wavelength is 1064 nm, which denotes the main laser oscillation line of 4F3/2 to 4I11/2 transition of Nd3+ ions. The inset shows the typical pulse trains of the Q-switched laser measured by an oscilloscope.
Figure 5 exhibits the Q-switched laser parameters (e.g., pulse energy, peak power, repetition rate, and pulse duration) as functions of the launched power, in which the red symbols and lines represent the left axes and blue ones corresponds to the right axes. As illustrated in Fig. 5(a), the repetition rate of the pulsed laser is raised from 2.16 to 6.95 MHz (0.95 to 4.08 MHz) and the pulse duration rapidly decreased into a relatively stable value of 27.3 ns (35 ns) as the ascending of the launched power. With the increase of launched power, the pulse energy climbed from 7.1 to 19.6 nJ (2.7 to 17.2 nJ), corresponding to the peak power from 75 to 685 mW (20 to 498 mW). By comparing with single graphene SA under the same Nd:YVO4 waveguide platfom [32], with higher modulation depth and lower saturation intensity, it can be found that Q-switched laser modulated by graphene/Ag2S@Ag SA exhibited relatively shorter pulse duration and higher pulse peak power.
By carefully adjusting the laser cavity and the SA position, Q-switched mode-locked lasers modulated by graphene/Ag2S@Ag and single graphene have been achieved in the waveguide platform under the same conditions. Figure 6 shows the typical characteristics of the Q-switched mode-locked waveguide lasers modulated by graphene/Ag2S@Ag SA. The single Q-switched envelope involving mode-locked pulses recorded in 2 μm/div is illustrated in Fig. 6(a). The pulse duration of the Q-switched envelope is measured to be 31 ns and the corresponding pulse energy is 17 nJ with the output power of 82 mW. Figure 6(b) shows the recorded mode-locked pulse trains with the timescale of 400 ps/div. As shown in Fig. 6(c), the pulse duration of the output mode-locked pulses is measured to be as short as 33 ps, which is much shorter than that of the single graphene as shown in the insert. The shorter pulse duration could be associated with the larger modulation depth of graphene/Ag2S@Ag SA. The radio frequency (RF) spectrum of the QML laser is depicted in Fig. 6(d) and the exhibit a sharp peak with a fundamental repetition rate of 6.44 GHz and signal-to-noise ratio (SNR) up to 58 dB. During the experiment, The QML operation could keep stable for hours and no damage of graphene/Ag2S@Ag SA has been observed within our pump range. The fundamental repetition frequency in this experiment is estimated to be ~6.5 GHz by the following formula: frep = c/2nl, which is consistent with the experimental results.
Fig. 6 (a) Q-switched envelope on a nanosecond scale (b) Mode-locked pulse trains on picosecond timescale. (c) Single pulse profile of the output laser modulated by Ag2S@Ag nanoparticle modified graphene, the inert is the pulse profile modulated by single graphene. (d) the RF spectrum.
4. Conclusions
In summary, we proposed a functionalized graphene by combining graphene with Ag2S@Ag nanoparticles and the synthesized nanomaterial exhibits enhanced nonlinear optical responses with higher modulation depth and lower saturation intensity. Based on the compact waveguide platform, we have realized both Q-switched and 6.44 GHz Q-switched mode-locking operation with superior performances compared with single graphene. This work illuminates a pathway of functionalized graphene toward the applications of on-chip ultrafast photonics.
Funding
National Natural Science Foundation of China (Nos. 61775120, 61522510); Strategic Priority Research Program of CAS (XDB16030700); Key Research Program of Frontier Science of CAS (QYZDB-SSW-JSC041); STCSM Excellent Academic Leader of Shanghai (17XD1403900).
Acknowledgment
The authors would like to thank Rang Li for fruitful discussions and valuable input in the analysis of near-field enhancement. The authors also acknowledge the technical support from Prof. Haohai Yu.
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