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Abstract
We demonstrate high-repetition-rate fundamentally Q-switched mode-locked Nd:YAG waveguide laser modulated by platinum diselenide (PtSe2) saturable absorber. The laser operation platform is a femtosecond laser-written monolithic Nd:YAG waveguide, and the saturable absorber is large-area few-layer PtSe2 that possesses relatively lower saturation intensity and higher modulation depth in comparison with graphene. With the superb ultrafast nonlinear saturable absorption properties of as-synthesized PtSe2, the waveguide laser could operate at ~8.8 GHz repetition rate and ~27 ps pulse duration, while maintaining a relatively high slope efficiency of 26% and high stability with signal-to-noise ratio (SNR) up to 54 dB. Our work indicates the promising applications of laser-written Nd:YAG waveguides and atomically thin PtSe2 for on-chip integration of GHz laser sources toward higher repetition rates and shorter pulse duration.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The discovery of graphene in 2004 has opened a new paradigm of layered two-dimensional (2D) materials [1]. With exceptional photonic and electronic properties, the atomically thin 2D materials have arouse great scientific interests toward a vast number of applications that promised to be the key building blocks in the next-generation optoelectronics and electronics [2]. Transition metal dichalcogenides (TMDCs), denoted as MX2 (M: Mo, W, or Pt and X: S, Se, or Te), are a main category of emerging 2D materials that offers distinct material options with their diverse physical properties. Ignited by the groundbreaking work on graphene based pulsed laser, increasing number of research efforts have been made to unfold the nonlinear optical properties as well as its corresponding applications of emerging 2D materials [3–8]. Based on the nonlinear saturable absorption effects, TMDCs has exhibited great potential to be effective saturable absorbers which could compensate for few shortcomings of graphene and have been widely applied in the generation of pulsed lasers in bulk, fiber, and waveguide platforms [9–17]. As a newly-developed layered 2D material, platinum diselenide (PtSe2) has exhibited remarkable optical properties and is stable in ambient conditions [18–20]. It is intriguing to unfold its ultrafast nonlinear responses and reveal its applications in ultrafast and ultrahigh-repetition-rate pulsed lasing in the waveguide platform.
Waveguides play crucial role of routing light within a photonics chip in integrated optics [21,22]. Femtosecond (fs) laser writing is a powerful technique to implement 3D micro-structured waveguides in glasses and various crystals [23,24]. Nonlinear effects or laser performances could be significantly enhanced within the microscale waveguides by confining the propagating light waves in a desired and very small volume. Recently, waveguide lasers have emerged as miniature and compact laser sources that could be on-chip integrated within photonics circuits [25–30]. Based on the waveguide platform and nanomaterials as saturable absorbers, both Q-switching or mode-locking operation have been demonstrated in various laser gain media [31–35]. In recent years, mode-locked (i.e., Q-switched mode-locked or continuous-wave mode-locked) lasers operating at ultrahigh repetition rates up to multi-GHz regime have gained increasing research interest [36–52], which could find potential applications in precision metrology, ultrafast nonlinear spectroscopy, and high-speed optical communication. Based on the waveguide platform, Q-switched mode-locked lasers have been achieved mainly with graphene saturable as well as several other low-dimensional materials, achieving repetition rate up to 1.5, 2, 5.9, 6.5, and 7.8 GHz [36–44]. Continuous-wave mode-locking have also been demonstrated with low-dimensional SA by several group, achieving repetition rate up to 4.9, 6.5, 11, 15.2, and 21.25 GHz [45–48]. To achieve this goal, most of related works focus on harmonic mode-locking which is well-developed with multiple pulses in the laser cavity [52,53]. Among which, mode locking of waveguide lasers operating at the fundamental pulse repetition frequencies up to gigahertz (GHz) repetition rates are favorable with higher stability and better spectral purity, which is essential for high-precision metrology and related applications. However, GHz fundamentally mode-locked lasers required very compact cavity and therefore also technologically challenging.
In this work, we report on high-repetition-rate fundamentally Q-switched mode-locked lasers based on femtosecond laser-written Nd:YAG waveguides, achieving fundamental repetition rate as high as 8.820 GHz and mode-locked pulse duration as short as 27 ps pulses. The physical properties large-area few-layer PtSe2 sample and the mode-locked lasing performances have also been investigated in detail. This is also the first demonstration of Q-switched mode-locked laser based on Nd:YAG waveguides.
2. Sample preparation and characterization
The large-area few-layer PtSe2 sample used in this work was synthesized directly on a 10 × 10 mm2 optically polished sapphire substrates using the chemical vapor deposition (CVD) method. The Raman spectroscopy is carried out under the excitation laser wavelength of 633 nm at room temperature. As indicated in Fig. 1(a), there are three prominent Raman peaks, at ∼179 cm−1, ∼207 cm−1, ∼236 cm−1, corresponding to the in-plane vibration mode, the out-of-plane mode, and longitudinal optical (LO) modes, respectively, indicating the high quality of the sample by CVD technique. As illustrated in Fig. 1(b), the linear optical absorption properties of PtSe2 sample are investigated by a UV-Vis-NIR Spectrophotometer (UV-1800, Shimadzu) after excluding the effects of substrate, showing from the visible towards near infrared band. The atomic force microscopy (AFM, Bruker Dimension Icon system) is carried out to measure the surface morphology and thickness of PtSe2 film. The tapping mode is set to minimize the damage to the sample. Figure 1(c) shows the AFM topological image, revealing the as-synthesized PtSe2 is a continuous thin film as thin as few atomic layers with high homogeneity. The height profile in Fig. 1(d) indicate the PtSe2 sample is only few atomically layers. The elemental composition and the chemical states of as-synthesized CVD PtSe2 are further confirmed by Xray photoelectron spectroscopy (Thermo Scientific, ESCALAB 250Xi) utilizing monochromated Al Kα X-rays source and an analyzer pass energy of 30 eV. Figures 2(a) and 2(b) illustrate the high-resolution XPS spectra of the binding state for Pt 4f and Se 3d, in which the prominent peaks at 76.6 and 73.3 eV in Fig. 2(a) are attributed 4f5/2 and 4f7/2 orbitals of Pt and prominent peaks at 55.4 and 54.5 eV in Fig. 2(b) are attributed 3d3/2 and 3d5/2 orbitals of Se, which is in good agreement with previous results [54]. The survey data is also presented in Fig. 2(c). In addition, the ratio of Pt and Se atoms are approximately to be stoichiometric 1:2, indicating the PtSe2 is well synthesized.
Fig. 1 Sample characterizations of large-area few-layer PtSe2 with high homogeneity. (a) The Raman spectra and characteristic Raman peaks. (b) The linear absorption spectrum. (c) The nanoscale surface topographic image of the as-synthesized PtSe2 on sapphire substrate. The white scale bar is 2 μm. (d) The corresponding height profiles derived from the AFM image.
Fig. 2 The high-resolution XPS spectrum of as-synthesized PtSe2 (a) Pt 4f. (b) Se 3d. (c) Survey spectrum.
In order to testify the potential applications of as-synthesized PtSe2 for ultrafast and ultrahigh-repetition-rate laser generation, especially in compact systems of microscales, we fabricated the low-loss cladding optical waveguide by direct femtosecond laser writing in an optically polished Nd:YAG laser crystal with dimensions of 10(x) mm × 9.097(y) mm × 2(z) mm. During the fabrication process, the laser source was an amplified Ti:Sapphire fs-laser (Spitfire, Spectra Physics) with 120 fs pulse duration at 795 nm central wavelength. The configuration of laser irradiated tracks was controlled by a PC-controlled XYZ translation stage and the laser parameters were optimized to obtain low-loss waveguides. More details about Nd:YAG waveguide fabrication can be found in [55].
3. Nonlinear optical properties of the as-synthesized PtSe2 sample
To investigate the ultrafast nonlinear optical properties of the as-synthesized large-scale PtSe2 sample in the near infrared regime, the open-aperture Z-scan measurements are carried out to obtain the intensity-dependent saturable absorption features as demonstrated in Fig. 3. The ultrafast laser excitation source is a mode-locked laser system operating at 1030 nm wavelength, with pulse duration of 340 fs and repetition rate of 100 Hz. The pump laser is Gaussian beam with waist diameter of 30 μm. During the experiment, the PtSe2 was placed on the PC-controlled translation stage as the sample moves along the laser propagation direction (Z-axis) from non-focus to on-focus with different light intensities. The real-time power of the reference and transmitted light is monitored by two detectors, the incident pulse energy is tuned by placing an attenuation slice.
Figure 4 demonstrates the typical Z-scan curves of the as-synthesized PtSe2 sample, exhibiting obvious ultrafast saturable absorption response at 1 μm. The transmission increases as the rise of incident pulse energy and the relationship between normalized transmission and sample position Z under the excitation pulse energy of 10 nJ is illustrated in Fig. 4(a), showing a good symmetry along the z = 0 direction. Figure 4(b) depicts the transmittance as a function of the excitation light intensity by taking into account of the beam waist of the Gaussian beam. In order to obtain the saturation intensity IS and nonlinear absorption coefficient β, we fitted the experimental data with the nonlinear absorption model using the following formula [56–59],
where z′ is the laser propagation distance in the sample, α0 is linear absorption coefficient, Is is the saturation intensity, and β is the nonlinear absorption coefficient. Through fitting by excluding the influence of substrate, the non-saturable loss, modulation depth and saturation intensity of as-synthesized PtSe2 sample is measured to be 1.6%, 1.9% and 0.47 GW/cm2, which possess much lower saturation intensity and higher modulation depth than monolayer graphene under the same measuring condition. The β is measured to be −135818 cm/GW and the negative value indicate the saturable absorption. The ultrafast nonlinear optical response of PtSe2 sample ensure it as a potential candidate for ultrafast photonics applications.Fig. 4 The ultrafast nonlinear optical properties of PtSe2 sample measured by an open-aperture Z-scan system. (a) Normalized transmission as a function of sample position Z. (b) Normalized transmission as a function of incident laser intensity. The scatters are experimental data and solid lines are the fitting curves.
4. Pulsed laser operation in a compact waveguide platform
Based on the intriguing ultrafast nonlinear optical responses of CVD-grown PtSe2 at 1 μm, we further investigated its capability for mode-locked pulse generation in near infrared region by incorporating it into the waveguide cavity. The cavity length was approximately 9.4 mm. The waveguide laser system is pumped by a wide-tunable Ti:sapphire laser (Coherent MBR-110) with excellent beam quality of TEM00 and superior stability. The pump wavelength is then set to be 808 nm to achieve the best efficiency. In order to couple the pump laser into the microscale cladding waveguide efficiently, as shown in the schematic illustration of Fig. 5, end-face coupling arrangement is employed with 25-mm focal length spherical convex lens. The input mirror is anti-reflection (AR) coated at 808 nm (>99% transmission) and high-reflection (HR) coated at 1064 nm (>99.9% reflectivity). An objective lens (N.A. = 0.4) are used to collect the emitted laser after eliminating influence of unwanted pump laser by longpass filter with 850 cut-off wavelength. The filtered light beam propagating in free space is then efficiently coupled into a single mode fiber connected with a High-Speed InGaAs Photodetector (New focus, 1414 model) and a 25 GHz wide-bandwidth real time digital oscilloscope (Tektronix, MSO 72504DX). During this experiment, the infrared CCD, power meter and spectrometer (SGM100, 0.05 nm resolution) have also been employed.
Fig. 5 Schematic experimental set-up for laser generation of mode-locked pulses in a Nd:YAG cladding waveguide based on the PtSe2 saturable absorber.
Modulated by CVD-grown PtSe2 sample as a saturable absorber, 1-μm mode-locked pulsed laser operation have been achieved based on a Nd:YAG cladding waveguide. As shown in Fig. 6(a), the threshold pump power of pulse generation is obtained to be 74 mW after linear fitting. The maximum outpower is 127 mW and the corresponding slope efficiencies is ~26%. It can be seen that no obvious polarization-dependent effects have been observed under different pump polarizations. Figure 6(b) shows the laser emission spectrum, in which the full width at half maximum (FWHM) is 1.22 nm and the central wavelength is 1064 nm corresponding to the main laser oscillation line of Nd3+ ions.
Fig. 6 (a) Output power as a function of launched power. (b) The emission waveguide laser emission spectrum modulated by PtSe2.
Figure 7 shows the mode-locked performances modulated by PtSe2 sample. Figure 7(a) shows the single envelope constituted by numbers of mode-locked pulses in the timescale of 40 ns/div under the pump power of 238 mW. The mode-locked pulse trains are shown in Fig. 7(b) on picosecond timescale (400 ps/div). Figure 7(c) demonstrates the single mode-locked trace with pulse duration as short as 27 ps. As depicted in Fig. 7(d), the radio frequency (RF) spectrum of the mode-locked pulses has also been measured, indicating the central repetition rate is as high as 8.820 GHz with the signal-to-noise ratio (SNR) up to 54 dB, indicating the Q-switched mode-locked waveguide laser could operate in high repetition rate while retaining high stability. During the experiments, no degradation of waveguide laser performance is observed, indicating the optical damage threshold of PtSe2 is beyond the maximum pump intensity.
Fig. 7 (a) Q-switched envelope on a nanosecond scale, the inset is on microsecond timescale. (b) Mode-locked pulse trains on picosecond timescale. (c) Single pulse profile of the output laser (d) Radio frequency spectrum of the mode-locked pulses.
Table 1 summarizes the Q-switched mode-locked waveguide lasers based on low-dimensional materials saturable absorbers. It can be seen that the mode-locked waveguide laser in this work have highest repetition rate and very short pulse duration. This is to our knowledge the first demonstration of Q-switched mode-locked laser in a Nd:YAG waveguide platform, which opens a vast number of possibilities of Nd:YAG waveguides for future on-chip ultrafast photonics. This this also the highest repetition rate of Q-switched mode-locked laser ever achieved in a waveguide in the existing literature. It is worth mentioning that the actual pulse duration in this work could be even shorter due to the rise time of the equipment during the measurement. In this work Q-switched mode-locked laser have been achieved due to the pulse energy of the output laser is within the QML instabilities limit, which could be defined as stability criterion (Esat,LEsat,AΔR)1/2. One advantage of the Q-switched mode-locked laser is that it could achieve much higher pulse energy with regularly delayed groups of pulses than that of continuous-wave mode-locked lasers. In future works, one would also expect continuous-wave mode-locking by further reducing the loss of the system or adding additional equipment to control the dispersion such as Gires–Tournois interferometer. The fundamental repetition frequency of the mode-locked waveguide laser could be estimated by the following equations:
where c is the speed of light, n is the refractive index of the waveguide, l is the cavity length, and λ is the wavelength. With high-resolution spiral micrometer measurement, the total length of cavity is approximately 9.396 mm, and the refractive index at 1064 nm is calculated to be 1.8147 with sellmeier equation. It is worth mentioning that the repetition rate could be precisely calculated and only dependent on the cavity length and the refractive index. The fundamental repetition frequency is estimated to be ~8.8 GHz, showing the calculated results are in good agreement with the experimental data and the laser in this work operates in fundamental repartition rate.
Table 1. Comparisons of Q-Switched Mode-Locked Waveguide Lasers Based on Low-Dimensional Materials
5. Conclusion
In conclusion, we investigated the ultrafast nonlinear optical properties of wafer-scale and high-quality few-layer PtSe2 at 1 μm and demonstrated its applications in ultrahigh-repetition-rate Q-switched mode-locked laser generation up to multi-GHz in a monolithic waveguide platform. By employing the Ultrafast Z-scan technique, PtSe2 exhibited superior nonlinear optical properties, such as lower saturation intensity and high modulation depth compared with monolayer graphene. The physical properties of PtSe2 layer are also characterized in details. Modulated by PtSe2, 8.82 GHz fundamentally mode-locked laser has been achieved in monolithic Nd:YAG waveguide system, indicating the great potential of PtSe2 in future on-chip ultrafast photonics.
Funding
National Natural Science Foundation of China (NSFC) (61775120, 61522510); Project 111 of China (B13029); STCSM Excellent Academic Leader of Shanghai (17XD1403900).
Acknowledgments
Portions of this work were presented at the Advanced Solid State Lasers Conference (ASSL) in 2018, paper AM6A.10.
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