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Abstract
Two-dimensional (2D) materials have generated great interest in the past few years opening up a new dimension in the development of optoelectronics and photonics. In this paper, we demonstrate 6.5 GHz fundamentally Q-switched mode-locked lasers with high performances in the femtosecond laser-written waveguide platform by applying graphene, MoS2 and Bi2Se3 as saturable absorbers (SAs). The minimum mode-locked pulse duration was measured to be as short as 26 ps in the case of Bi2Se3 SA. The maximum slope efficiency reached 53% in the case of MoS2 SA. This is the first demonstration of Q-switched mode-locked waveguide lasers based on MoS2 and Bi2Se3 in the waveguide platform. These high-performance Q-switched mode-locked waveguide lasers based on 2D materials pave the way for practical applications of compact ultrafast photonics.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The emerging layered two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDCs, e.g., MoS2, WSe2), and topological insulators (TIs, e.g., Bi2Se3), have generated remarkable research interest in the past few years due to their extraordinary properties in a number of aspects [1–3]. Based on these intriguing 2D materials, optoelectronic devices with novel functionalities and applications can be realized in terms of its unique and distinct optical properties [4–6]. One of the remarkable applications is that 2D materials can be applied as promising saturable absorbers (SAs) in Q-switched lasers and mode-locked lasers under different configurations [7]. As an atomic-scale conjugated sp2 carbon sheet with honeycomb lattice, graphene has been considered as one of the most effective ultra-broadband SAs in ultrashort pulse generation for its exceptional nonlinear optical properties such as broadband saturable absorption and ultrafast recovery time [8–10]. Inspired by the study of graphene-based SA, a growing number of other 2D materials have been demonstrated to be suitable choices of SAs for ultrafast photonic applications. Molybdenum disulfide (MoS2) is one of the representative TMDCs, in which the inner triangular lattice of Mo atoms sandwiched between two layers of S atoms. MoS2 shows strong light–matter interaction and ultrafast saturable absorption, enabling it to be a broadband SA [11–13]. As a novel Dirac material discovered in recent years, topological insulator bismuth selenide (Bi2Se3) has also exhibited excellent saturable absorption properties with narrow bandgap and large modulation depth [14–16].
Optical waveguides with various 3D structures fabricated by femtosecond laser direct-writing could confine light field within microscale volumes and were key components in miniature photonic devices [17, 18]. Among various types of structures, depressed cladding waveguides (so-called Type III configuration), surrounded by a number of low refractive index tracks, are favorable structures for the ease of integration with optical fibers and better confinement of light field. Waveguide lasers have been recognized as compact and integratable laser sources and superior laser performances could be expected such as enhanced slope efficiency and lower lasing threshold [19–23]. Moreover, laser modal profiles could also be manipulated in waveguides with a flexible manner according to the practical requirements [24, 25]. Based on the waveguide platform, pulsed laser operation regimes (both Q-switching and mode-locking) have been realized in various laser gain medium with 2D materials as SAs, obtaining a wide spectral region from the visible towards the mid-infrared for optoelectronic integrated devices [26–31].
Mode-locking lasers with multi-GHz repetition frequencies have attracted great interest of researchers and have found applications in various fields, including optical frequency combs generation, nonlinear microscopy, and high-speed optical communication [32–35]. Much of these studies on the mode locking up to GHz level have been focused on the harmonic of the fundamental frequency [36–38], which could be affected by the pulse-to-pulse timing jitter or supermodes of the coexisted multiple pulses. In particular, mode locking operating at the fundamental pulse repetition frequencies up to gigahertz (GHz) repetition rates is preferable in order to meet the demand of practical applications. Under waveguide configuration, both mode-locking or Q-switching regime have been demonstrated with graphene as a SA. In the waveguide configuration, Mary et al. obtained picosecond Q-switched mode-locking under Yb:BG waveguide with 1.5 GHz repetition rate [39] and Okhrimchuk et al. realized 11 GHz CW mode-locked waveguide laser in Nd:YAG crystal [40]. Ren et al. demonstrate the Q-switched mode-locked waveguide laser operated at a fundamental repetition rate of 7.8 GHz at the wavelength of 2 μm [28]. More recently, Thorburn et al. reported 5.9 GHz fundamentally Q-switched mode-locked waveguide lasers with graphene as a SA at the wavelength of ~2.1 µm [41]. Based on MoS2 or Bi2Se3 SA, only Q-switched lasers were reported under waveguide configuration [16, 26]. However, in the regime of mode-locking, Q-switched mode-locked waveguide lasers modulated by MoS2 and Bi2Se3 have not been reported yet.
In this paper, we demonstrate the high-performance Q-switched mode-locked waveguide lasers near 1 μm modulated by typical 2D materials (i.e., graphene, MoS2 and Bi2Se3) as saturable absorbers that operate at the fundamental repetition rate of ~6.5 GHz under the same waveguide configuration. The detailed mode-locking performances have been investigated in this work, indicating the promising applications of 2D materials in compact and integratable ultrafast photonics
2. Sample preparation and experimental setup
The high-quality monolayer samples used in this work were customized from a 2D materials supplier (6Carbon Technology, Shenzhen, China) and were synthesized to be a continuous film directly on 10 × 10 mm2 optically polished sapphire substrates by chemical vapor deposition (CVD) process. The optical images of the three samples fabricated graphene, MoS2, and Bi2Se3 are shown in Fig. 1.
The waveguide platform with depressed cladding structure used in this work was fabricated by femtosecond laser direct-writing in optically polished Nd:YVO4 crystal (doped by 1at.% Nd3+ ions) wafer with dimensions of 10(x) × 10(y) × 2(c) mm3. Ti:sapphire regenerative amplifier (Spitfire, Spectra Physics) was employed as fs laser source to generate 800 nm linearly polarized 120 fs pulses with measured energy on sample to be 0.28 μJ. During the writing process, the sample was placed on a PC-controlled XYZ translation stage with a constant scanning velocity of 750 μm/s, achieving the desired circular geometry with the diameter of 50 μm. Details of Nd:YVO4 waveguide fabrication have been reported in [31].
The Q-switched mode-locked waveguide laser generation was performed under the typical end-face coupling arrangement as illustrated in Fig. 2. The inset is the cross-sectional image of the fabricated waveguide imaged by an optical microscope (Axio Imager, Carl Zeiss). In this work, the pump source was linearly polarized light operating at the wavelength of 808 nm, emitted from a narrowband tunable CW Ti:sapphire laser (Coherent MBR-110). The waist radius of the pump beam is approximately 0.75 mm at 1/e2 position. The polarization of the pump laser could be controlled by half-wave plate. The generated launched laser was then coupled into the 50-μm diameter cladding waveguide by using a 25-mm focal length of spherical convex lens. The input mirror M1 was coated with antireflection coating (>99% transmission) at 808 nm and highly reflective coating (>99.9% reflectivity) at 1064 nm. The samples in our work were integrated onto the output end-facets of the cladding waveguide. A 20 × microscope objective lens (N.A. = 0.4) was used to collect the generated mode-locked laser and a longpass filter (Thorlabs, FEL0850) with a cut-on wavelength of 850 nm was used to eliminate the unwanted pump light. The laser power was investigated by an integrating sphere photodiode power sensor (Thorlabs, S142C) while taking the Fresnel reflection of the end facets and the coupling efficiency into account. The filtered laser was then focused through microscope objective lens, coupling the free-space laser into a single mode fiber which was connected directly to the photodetector. The obtained Q-switched mode-locked trains were recorded by employing a High-Speed Fiber-Optic InGaAs Photodetector (New focus, 1414 model) with the rise time as fast as 14 ps together with a 25 GHz wide-bandwidth real time digital oscilloscope (Tektronix, MSO 72504DX) with the rise time of 16 ps.
3. Results and discussion
3.1 Q-switched mode-locked waveguide laser modulated by graphene
By employing graphene as a SA, Q-switched mode-locked lasers have been achieved in the waveguide platform. Figure 3(a) demonstrates the measured average output power as a function of pumping power by linear fitting. It can be found that efficient Q-switched mode-locked waveguide laser emission has been achieved when the pump power higher than the laser oscillation threshold of 21 mW (35 mW) with slope efficiencies of 46% (25%), reaching the maximum output power values of 375 mW (198 mW) at TE (TM) polarization. The difference of output power at TE and TM polarization could be associate with the polarization dependent emission cross section. As shown in Fig. 3(b), the spectrum was measured to be centered at the wavelength of 1064 nm for both TE and TM polarization, corresponding to the main laser oscillation line of 4F3/2 to 4I11/2 transition of Nd3+ ions. The insert of Fig. 3(b) demonstrates the measured near-field modal profile of the output laser, in which the light field was well confined in fundamental mode.
Fig. 3 (a) Output power as a function of launched power. (b) The laser emission spectrum modulated by graphene; The insert is measured near-field modal profile of the output laser.
Figure 4 summarizes the typical characteristics of the Q-switched mode-locked waveguide lasers modulated by graphene SA. Figure 4(a) shows single Q-switched envelope constituted by mode-locked components on nanosecond (100 ns/div) time scale, while the inset is the one on microsecond (2 μm/div) time scale. The pulse energy of the Q-switched envelope is measured to be 51 nJ with pulse duration and peak power of 79 ns and 622 mW under the pump power of 650 mW. Figure 4(b) demonstrates the measured mode-locked pulsed trains on the time scale of 400 ps/div. The full width at half-maximum (FWHM) of the individual pulse is as short as 52 ps, as shown in Fig. 4(c). Figure 4(d) illustrates the radio frequency (RF) spectrum and the fundamental repetition rate is measured to be 6.436 GHz with signal-to-noise ratio (SNR) up to 55 dB, indicating that the mode-locked laser operates in a relatively stable regime. Meanwhile, no optical damage of graphene SA has been observed during the experiment, indicating ultra-high optical damage threshold.
Fig. 4 Q-switched mode-locked waveguide lasers modulated by graphene. (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) RF spectrum.
3.2 Q-switched mode-locked waveguide laser modulated by MoS2
By incorporating MoS2 SA into the laser cavity, Q-switched mode-locked waveguide laser was achieved in the waveguide platform for the first time. Figure 5(a) characterized the relation between the input pump power and the measured average output power. Through linear fitting, laser oscillation happened in case of the absorbed pump power exceeds the threshold value of 65 mW (76 mW) and the maximum output power values were measured to be 424 mW (312mW) at TE (TM) polarization, corresponding to the slope efficiencies of 56% (41%). The linear relationship indicates higher output power could be obtained by continually increasing the pump power to the Watt level. The emission spectrum of Q-switched mode-locked waveguide laser was illustrated in Fig. 5(b) centered at the wavelength of 1064 nm and the near-field modal profile was measured to be fundamental mode as shown in the inset.
Fig. 5 (a) Output power as a function of launched power. (b) The emission laser spectrum modulated by MoS2; The insert is measured near-field modal profile of the output laser.
The Q-switched envelope with mode-locked pulse trains on nanosecond (100 ns/div) time scale was shown in Fig. 6(a), and the inset picture is the one on microsecond (2 μm/div) time scale. Under the pump power of 650 mW, the pulse duration of the Q-switched envelope is 126 ns and the corresponding pulse energy and peak power is 75 nJ with pulse duration and and 598 mW. Figure 6(b) demonstrates the mode-locked pulsed trains with the time interval between the adjacent pulses of 154 ps. As shown in Fig. 6(c), the FWHM of single pulse is as short as 43 ps. The corresponding repetition rate is measured to be 6.48 GHz with signal-to-noise ratio of 51 dB, which could be seen in the RF spectrum illustrated in Fig. 6(d). During the experiment, no optical damage of graphene SA has been observed in our pump range.
Fig. 6 Q-switched mode-locked waveguide lasers modulated by MoS2. (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) RF spectrum.
3.3 Q-switched mode-locked waveguide laser modulated by Bi2Se3
Using Bi2Se3 sample as a SA, Q-switched mode-locked laser emission was demonstrated in the waveguide configuration for the first time. Figure 7 shows the average output power versus pumping power through linear fitting. Laser oscillation happens as the pump power higher than the threshold value of 19 mW (38 mW), and the maximum output power was 277 mW (142 mW) with the corresponding slope efficiencies of 33% (18%). Figure 7(b) was the laser emission spectrum of the output laser modulated by Bi2Se3 SA. The central wavelength of the Q-switched mode-locked lasers is 1064 nm, which correspond to the main laser oscillation line 4F3/2 to 4I11/2 of Nd3+ ions. Fundamental mode has also achieved as shown in the insert of Fig. 7(b).
Fig. 7 (a) Output power as a function of launched power. (b) The laser emission spectrum modulated by Bi2Se3; The insert is measured near-field modal profile of the output laser.
Figure 8 demonstrates the experimental results of the Q-switched mode-locked waveguide lasing modulated by Bi2Se3 SA. Under the pump power of 650 mW, the Q-switched envelope containing by mode-locked pulses on nanosecond timescale (100 ns/div) was illustrated in Fig. 8(a) and the inset is on larger timescale (2 μs/div). The pulse width of the envelope is 73 ns with the pulse energy and peak power to be 28 nJ and 387 mW. The measured mode-locked trains were shown in Fig. 8(b) with the time scale of 400 ps/div. Figure 8(c) illustrates the singe mode-locked pulse with FWHM as short as 26 ps. The RF spectrum was shown in Fig. 8(d) and the fundamental repetition rate was 6.556 with signal-to-noise ratio of 59 dB, indicating its good mode-locking stability. Within our pump range, no obvious optical damage to the Bi2Se3 SA has been observed.
Fig. 8 Q-switched mode-locked waveguide lasers modulated by Bi2Se3. (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) RF spectrum.
3.4 Comparisons of mode-locked waveguide laser modulated by 2D materials
Table 1 summarizes the mode-locked lasers based on 2D materials as SAs under the waveguide platform. It can be found that most works have been focused on graphene SA previously, with the pulse width ranges from 1.06 to ~100 ps and repetition frequency ranges from 1.5 to 11.3 GHz. Compared with graphene SA in this work, the Q-switched mode-locked waveguide lasers based on MoS2 and Bi2Se3 have obtained relatively shorter pulse duration which could be associate with the physical properties (e.g., lower saturation intensity and higher modulation depth) of the SA. The minimum mode-locked pulse width was measured to be as short as 26 ps in the case of Bi2Se3 SA. With graphene, MoS2, and Bi2Se3 SA, the time-bandwidth product (TBP) is calculated with the value of 9.6, 7.9 and 4.8, indicating that the mode-locking in the three cases are strongly chirped. It can be seen that the Q-switched envelop is modulated larger with Bi2Se3 in the comparison with other two cases and fully modulated envelop could also be achieved by further optimization of the waveguide. It is worth mentioning that the experimental setup of waveguide laser in our works share the same laser configuration, resulting in the similar repetition rate of 6.5 GHz. The central repetition rate may be slightly varied with different thickness of the sample films. The fundamental repetition frequency of the waveguide Fabry-Perot cavity can be estimated by the following equation:
Table 1. Comparisons of Mode-locked Waveguide Lasers Based on Different 2D Materials
where c is the speed of light, n is the refractive index of the waveguide, and l is the cavity length. With a total cavity length of approximately 11 mm, the fundamental repetition frequency is estimated to be ~6.5 GHz, which is in good agreement of the experimental results in this work.
4. Conclusions
In summary, high-performance Q-switched mode-locked waveguide lasers have been demonstrated by applying graphene, MoS2 and Bi2Se3 as SAs. The output waveguide laser operates at the fundamental repetition rate of 6.5 GHz with tens of picosecond mode-locked pulse duration. This work indicates the potential of novel 2D materials to be developed as promising SA material for compact ultrafast photonic applications in the waveguide platform.
Funding
National Natural Science Foundation of China (NSFC) (61522510); The 111 Project of China (No. B13029)
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