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A compact saturable absorber mirror (SAM) based on multi-layered black phosphorus (BP) nanoplatelets was fabricated and successfully used as an efficient saturable absorber (SA) in a passively Q-switched Tm:YAP laser at 1.9 μm. With the BP SAM, Q-switched pulses with duration of 181 ns and average output power of 3.1 W were generated at a pulse repetition rate of 81 kHz, resulting in a pulse energy of 39.5 μJ, to the best of our knowledge, which is the record among the reports on BP SA-based Q-switched lasers. In addition, the simultaneous dual-wavelength Q-switched operation at both 1969 and 1979 nm has been observed. The results indicate the promising potential of multi-layered BP nanoplatelets as SAs for achieving efficient pulsed lasers at around 2 μm.
© 2016 Optical Society of America
1. Introduction
Pulsed Tm3+ (3F4→3H6) doped lasers operating in the 1.9 - 2.0 μm spectral regime, which is well within the eye-safe region, are promising light sources for photonics applications [1]. Passive Q-switching with saturable absorbers (SAs) is a general technique to generate energetic pulses from crystal or fiber lasers. For 1.9 −2.0 μm operation, semiconductor saturable absorber mirrors (SESAMs) [2], Cr:ZnSe and Cr:ZnS [3] have been extensively employed as a consequence of their excellent mechanical stability and compactness. However, the application of SESAMs is limited by the shortcomings of complicated and expensive manufacturing as well as the narrow absorption bandwidths. Because of the large absorption cross-sections, Cr:ZnSe and Cr:ZnS have become powerful SAs for 2 μm lasers. In 2015, solid-state passively Q-switched Tm:YAP lasers at 1935 nm with Cr:ZnSe and Cr:ZnS polycrystalline SAs were achieved with single pulse energies of 1.55 mJ and 1.85 mJ, respectively, however, the pulse repetition rates were only several hundred Hz [4].
In recent years, two dimensional (2D) layered materials, including graphene [5], transition metal dichalcogenides (TMDs) [6] and black phosphorus(BP) [7, 8]), have attracted much attention in generating ultrafast lasers because of their excellent saturable absorption properties, such as ultrafast recovery time, controllable modulation depth, and low-cost preparation. Compared with graphene and TMDs, monolayer BPs with direct bandgap [9], high in-plane anisotropy [10], high carrier mobility [11] and large on/off ratios at the room temperature [12] have become an alternative 2D semiconductor material suitable for electronic and photonics applications in very recent years. Moreover, the direct bandgap of BPs can be tuned from 0.35 eV for bulk to 2.0 eV for monolayer structure [13], well bridging the zero bandgap of graphene and large bandgap of TMDs. Until now, BP has been successfully employed as SAs in bulk lasers at different wavelengths of 639 nm [14], 1.0 μm [15], 2.0 μm [16], 2.72 μm [17], and in fiber lasers at various wavelengths of 1.5 μm [18–20],1.9 μm [21], 2.8 μm [22]. In 2015, our group successfully demonstrated the generation of 6.1 ps pulses from a BP mode-locked bulk laser at 1064 nm for the first time [23]. Very recently, we reported a BP based passively Q-switched bulk laser at 2.4 μm [24]. However, the average output powers from the above mentioned lasers were only at milliwatts-level and the pulse energies were limited below 10 μJ.
In this paper, a compact multi-layered BP SAM was fabricated with high quality and successfully employed to realize a diode-pumped passively Q-switched Tm:YAP bulk laser. Pulses with the shortest pulse width of 181 ns and maximum average output power of 3.1 W were delivered, corresponding to the highest single pulse energy of 39.5 μJ, which holds the record among the reports on BP SA-based Q-switched lasers, to the best of our knowledge. In addition, simultaneous dual-wavelength Q-switched operation at both 1969 and 1979 nm has been observed. The experimental results indicate that multi-layered BP is a kind of efficient SAs suitable for generating nanosecond solid-state laser pulses in the mid-IR spectral region.
2. Preparation and characterization of BP SAM
The liquid phase exfoliation method, which has been extensively developed for graphene and other 2D materials [7], was employed to produce high quality BP-based SAM in our experiment. This method enables large-scale exfoliation of BPs and uniform dispersion in the exfoliation medium. Firstly, the BP powder grinded from bulk crystal was dispersed into the isopropyl alcohol (IPA) and ultra-sonicated for 4 hours. Secondly, the dispersion was centrifugated with a speed of 2000 rpm for 30 minutes in order to remove the large-size phosphorous sheets and the top 50% of the solution was collected. Then the processed BP solution was dropped upon output coupler (OC), which was partial reflectivity coated at 1.9-2.1 μm (T = 2%). Finally, the treated OC was dried in vacuum at room temperature for 24 hours and then soaked in alcohol followed by 5-min sonication to remove the IPA. Thus the BP SAM was prepared successfully.
As shown in Fig. 1, the morphology of the deposited layers of the BP SAM was investigated by Atomic Force Microscopy (AFM). The average thickness of the transferred layer on SAM was measured to be ~10 nm, corresponding to ~17 layers thick BP sheets [25]. Based on the BP bandgap formula Eg ≈(1.7/n0.73 + 0.3) eV (n is the number of layers) [10, 13], the band gap of the as-prepared BP SAM was estimated to be about 0.515 eV, corresponding to a peak absorption wavelength shorter than 2.4 μm.
To further verify the layer structure of the as-prepared BP SAM, Raman spectroscopy was carried out and shown in Fig. 2(a). Three characteristic Raman spectral peaks at the wave numbers of 361.2 cm−1, 437.1 cm−1, and 464.4 cm−1 were observed, corresponding to the out-of-plane vibration mode Ag1, in-plane vibration modes B2g and Ag2 of phosphorus atoms, respectively [26]. According to Ref [8], the Ag1 and Ag2 modes will shift towards each other with the thickness of BP sheets increased. Compared with the typical Raman spectrum of bulk BP, the measured space of 103 cm−1 between the Ag1 and Ag2 modes indicates that the BP powder has been exfoliated down to several layers, which agrees well with the measured AFM image of Fig. 1.
Fig. 2 (a) Raman spectrum of the prepared multi-layered BP film at room temperature; (b)Transmittance versus incident optical power intensity on BP SAM.
The saturable absorption behavior of as-prepared BP SAM was measured by a home-made SESAM mode-locked 1.85 ps Tm:YAP laser at 1940 nm with a repetition rate of 88 MHz. The measured transmissions of the BP SA at different incident fluences were plotted as dots in Fig. 2(b). Fitted by the nonlinear transmission formula of T(F) = Aexp(-ΔT/(1 + F/Fsat)), where T(F) is the transmission rate, A is the normalization constant, ΔT is the modulation depth, F is the input fluence, and Fsat is the saturation fluence, the modulation depth and saturation fluence of our BP SAM sample were simulated to be 19.6% and 3.1 μJ/cm2, respectively. The relatively large modulation depth and low saturation power intensity imply the potential of the as-prepared BP SAM suitable for generating low threshold and short Q-switched pulses in bulk lasers.
3. Experimental results and discussions
The schematic of the diode-pumped BP SAM passively Q-switched Tm:YAP laser is shown in Fig. 3, where a two-mirror resonator was employed. The pump source was a fiber-coupled diode laser working at 790 nm, with a numerical aperture of 0.22 and fiber core diameter of 200 μm. With an optical re-focusing system, the pump light was delivered into the laser crystal through the input mirror with a concave curvature radius of 100 mm, which is anti-reflectivity (AR) coated for the pump wavelength and high-reflectivity (HR) coated for the lasing wavelength. The whole laser cavity length was 25 mm. An a-cut 3 at.% Tm:YAP crystal had a size of 3 × 3 × 7 mm3, and was mounted in copper heat sink with circulating water cooled at 18 ◦C to remove the accumulated heat inside. Both surfaces of the Tm:YAP crystal were polished and high transmission coated for both pump and lasing wavelengths.
By carefully aligning the laser cavity and optimizing the position of BP SAM, stable Q-switched operation was achieved. The temporal pulse trains were recorded by an oscilloscope (Tektronix 1 GHz bandwidth) and a fast InGaAs photodetector with a rise time of 35 ps, (EOT, ET-5000, USA). Initially, the laser ran in the continuous-wave (CW) operation regime, from which disordered self-pulses with significant pulse jitter were observed, as shown in Fig. 4(a). This phenomenon was also observed in other Tm3+ doped lasers and consistent with nonlinear dynamical chaos [27]. When the absorbed pump power was increased to 1.95 W, the laser emission suddenly changed from the CW operation to the Q-switched operation, and the corresponding average output power was about 280 mW. Stable passively Q-switched pulses were achieved as shown in Fig. 4(b). Further increasing the absorbed pump power to 7.52 W, the average output power reached a maximum value of 3.1 W, under which the shortest Q-switched pulse of 181 ns was obtained, which temporal shape was shown in Fig. 5. The corresponding pulse train with a pulse repetition rate of 81 kHz is shown in the inset of Fig. 5.
Fig. 4 (a) The disordered self-pulse trains of Tm:YAP laser under the output power at 0.15 W. (b) The stable pulse trains of Q-switched Tm:YAP laser under the output power at 0.28 W.
Fig. 5 Typical Q-switched pulses at different time scale under the 7.52 W absorbed pump power of the Q-switched laser.
Figure 6 shows the dependence of the average output powers on the absorbed pump powers, which demonstrates that the Q-switched Tm:YAP laser delivers a slope efficiency of 41%. By using a laser spectrometer (AvaSpec-NIR256) with a resolution bandwidth of 3.7 nm to measure the output spectrum, a typical dual-wavelength operation under the absorbed power of 7.52 W was observed as shown in the inset of Fig. 6. Two peak wavelengths were located at 1969 and 1979 nm, respectively. Considering the absorption spectra of water, the dual-wavelength laser can be potentially used as a laser scalpel with a variable cutting depth in high-water-content tissues for medical surgery [28]. By using the partial reflectivity coated mirror (T = 2%) instead of BP SAM, a CW laser operation was realized for comparison to estimate the unsaturable loss of the BP SAM. As shown in Fig. 6, the maximum average output power for CW operation was 3.42 W under an absorbed pump power of 7.52 W, corresponding to a slope efficiency of 48%. The M 2 factors of the pulsed laser beam were measured by using the 90.0/10.0 scanning-knife-edge method and the measured data is shown in Fig. 7. At the maximum average output power of 3.1 W, the M 2 factors of the BP SAM Q-switched laser were best-fitted to be 1.23 in tangential direction and 1.45 in sagittal direction, respectively.
Figure 8 shows the dependence of the pulse durations and repetition rates on the absorbed pump powers. When the absorbed pump power increased from 1.95 W to 7.52 W, the pulse width dropped from ~720 ns down to ~181 ns, and the pulse repetition rate increased from ~41 kHz to 81 kHz. Under higher absorbed pump power than 7.52W, the generated Q-switched pulse train became unstable, where disordered pulses with significant pulse jitter were observed. With the measured average output powers and pulse repetition rates, the single pulse energies and pulse peak powers were calculated and shown in Fig. 9. A maximum Q-switched pulse energy was calculated as 39.5 μJ, which was higher than any other result ever achieved with BP SA [14–24] and a corresponding maximum pulse peak power of 218 W was obtained. Considering the laser mode radius on the BP SAM, which was estimated to be about 116 μm by using ABCD matrix theory, the intracavity fluence on the BP SAM was calculated to be about 4.38 J/cm2. Even so, no obvious optical damage to the BP SAM or degradation of the Q-switched laser property was observed within our pump range.
4. Conclusions
In summary, we have experimentally demonstrated an efficient diode-pumped passively Q-switched bulk laser exploiting BP film as the saturable absorber. The Q-switched pulses with a maximum average output power of 3.1 W, the shortest pulse width of 181 ns and the highest single pulse energy of 39.5 μJ were generated. In addition, simultaneous dual-wavelength Q-switched operation at both 1969 and 1979 nm has been observed. The results, to the best of our knowledge, are records among the reports on BP SA-based Q-switched lasers and indicate that BP is a kind of promising SA for generating high efficiency and energy pulses with dozens kHz repetition rates. Due to the high modulation depth of the BP SAM, continuous wave mode-locking operation was not observed. Predictably, thinner BP layers with low modulation depth could support mode locking operation of bulk lasers at 2 μm, and further efforts on this aim are on the way.
Acknowledgments
This work is supported partially by the National Natural Science Foundation of China (NSFC) (Grant No: 61575110, 61275142, 61308042, 51321091, and 61475088).
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