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Abstract
By using few-layer black phosphorus (BP) as saturable absorber, an efficient mode-locked Nd:GdVO4 bulk laser operating at 1.34 μm was realized. An average output power of 350 mW was achieved with a slope efficiency of 15%. The corresponding mode-locking pulse repetition rate, pulse duration and pulse energy were 58.14 MHz, 9.24 ps and 3.0 nJ, respectively. To the best of our knowledge, the pulse width is the shortest among the mode-locked 1.34 μm neodymium lasers ever obtained with other two-dimensional materials saturable absorber. The results clearly indicate the few-layered BP is a kind of promising saturable absorber for ultrafast 1.34 μm lasers.
© 2017 Optical Society of America
1. Introduction
Due to the tremendous increase of practical applications in high-capacity telecommunication, industrial, scientific breakthroughs, ultrafast lasers have attracted increasing attention and stimulated the instantaneous researching in saturable absorbers (SA) for passive mode-locking of lasers. The newly emerging two-dimensional (2D) materials [1–3], such as graphene [4], topological insulators (TIs) and transition-metal dichalcogenides (TMDs) [e.g., tungsten disulfide (WS2)] have shown good performance in generating ultrafast lasers [5–7]. Recently, black phosphorus (BP), the most thermodynamically stable allotrope of phosphorus, has joined in the family of 2D materials [8]. Just like graphene, individual atomic layers are stacked together by Van Der Waals interactions in BP [9]. Different from TMDs only exhibiting direct band-gap in monolayer, BP owns direct band gap independent on its layer number, which makes it have high absorption and adjustable optical modulation depth, particularly at long wavelength range where strong motivations for optical communications and military purposes [10–12]. The band gap of BP can be tuned from ~1.5 eV for phosphorene to ~0.3 eV for bulk BP, which fills up the “blank space” between graphene (zero) and TMDs (1.57-2.0 eV) [11,13] and means that BP have broadband response from visible to mid-infrared spectral region. The combined consequence of the valence band depletion, conduction band filling and ultra-fast intra-band carrier thermalization effects in BP makes the inter-band transmissions easily be saturated under strong illumination. Moreover, Wang et al. measured the fast recovery time of BP to be τs = 24 ± 2 fs at 1550 nm [14]. However, some previous reports reported longer recovery time on the level of few ps (0.024-1.36 ps) [15, 16]. Even though the measurements used by different methods may be different among the results, the relaxation time of BP still be shorter than that of graphene (0.1-1.46 ps) and MoS2 (4-100 ps) [15, 16]. All the above properties make BP become a kind of potential optical modulator for ultrafast lasers.
Up to now, much attention has been paid on the performance of BP in generating ultrafast lasers at 1.0 μm [17–19], 1.5 μm [20–22], 1.9 μm [23] and 2.8 μm [24]. However, no report was found on the application of BP SA in 1.34 μm lasers, which are required for information storage, medicine, remote sensing as well as atmospheric pollution monitoring. Especially, the picosecond solid-state lasers operating at 1.34 μm are regarded as ideal light sources for optical fiber communications on account of the coincidence with the weak dispersion effect and low intrinsic loss of the silica fiber [4,25–27].
In this letter, high quality BP SAM suitable for 1.34 μm laser was fabricated by isopropyl alcohol (IPA) liquid exfoliation method and successfully employed to realize stable mode-locked Nd:GdVO4 laser at 1.34 μm. Pulse width of 9.24 ps was obtained with a maximum output power of 350 mW and a repetition rate of 58.14 MHz. These results indicated that BP is an intriguing mode-locker for the ultrafast lasers at 1.34 μm.
2. Preparation and characterization of BP SAM
The liquid phase exfoliation (LPE) technique was employed to fabricate the few-layer BP nanosheets. The exfoliation flakes from the BP bulk crystal (XFNANO, purity 99.998%) was grinded sufficiently and dispersed into the isopropyl alcohol (IPA). The mixture dispersion was ultra-sonicated for 1 hour and processed by centrifugation (2000 rpm for 20 min). The supernatant was collected for use. Then the as-prepared BP solution was dropped onto a piece of quartz plate with high reflectivity coated at 1300-1400 nm wavelength region and dried in a vacuum oven at room temperature for 24 h. After that, the substrates were soaked in alcohol followed by sonication of 5 min to remove the IPA.
From the 2D topographical measurement of BP SAM as shown in Fig. 1(a) and 1(b) taken by an atomic force microscopy (AFM), the cross-section analysis implied the thickness of the prepared BP sheets was about ~5 nm. Since the height of a single layer phosphorene is approximately 0.6 nm [28], the BP sheets in our experiment are estimated to be ~7 layers thick, corresponding to a band gap of about 0.62 eV [29], which is exactly suitable for the use in 1.34 μm region.
Figure 2(a) exhibits the scanning electron microscopy (SEM) result of the BP SAM, indicating that the BP nanosheets are distributed uniformly on the substrate. From the image of transmission electron microscopy (TEM) shown in Fig. 2(b) and 2(c) which was employed to further characterize the morphology and crystallinity of the phosphorene, we can see that an ultrathin nanosheet of phosphorene with stacked folds clear and uniform lattice fringes were observed from the BP atomic layer (single vacancies cannot be detected), suggesting that the phosphorene produced by basic-IPA-exfoliation method retains the original crystalline state [30]. Moreover, the electron beam destroys the crystalline structure of BP and changes it into amorphous form during data acquisition. Therefore, it is difficult to obtain high-quality selected-area electron diffraction (SAED) patterns [see Fig. 2(d)]. In order to ensure that the BP nanosheets been successfully transferred on the substrate with intact structure, we measured the typical Raman spectrum of the BP film excited by a 532 nm laser source. As shown in Fig. 3(a), it was obvious that there were three main peaks locating at 360.35 cm−1, 436.68 cm−1 and 465.08 cm−1, corresponding to one out-of-plane mode A1 g and two in-plane vibration modes, B2 g and A2 g, respectively [28]. The A1 g and A2 g modes will shift toward each other with the increasing thickness of the phosphorus sheets according to the previous reports [10]. The measured value of 105 cm−1 implied that the BP powder have been exfoliated down to be several layers, which agrees well with the above AFM image. To investigate the nonlinear absorption behavior of BP SA, the transmission curve was measured by a homemade 12 ps Nd:GdVO4 laser at 1341 nm with a repetition rate of 60 MHz, as shown in Fig. 3(b). At higher power regime (above ~59 μJ/cm2), the black phosphorus shows a reverse saturable absorption (RSA) process, where the transmittances reduce with the increase of intensity. Analyses based on the energy band structure shows that the saturable absorption and RSA can be mainly attributed to the bleaching in the ground state absorption and two-photon absorption effect, respectively [31]. The inset of Fig. 3(b) shows the experimental data with the theoretically fitting curve for saturable absorption process. The modulation depth and saturation fluence were determined to be 16% and 1.3 μJ/cm2, respectively. Considering the Fresnel reflection loss of ~6.65% from both sides of the pure quartz substrate, the nonsaturable loss of BP SA was estimate to be 8.35%.
Fig. 2 (a) SEM image of BP SAM. (b) TEM image of BPSAM. (c) High-resolution TEM images of the surface part of BP SAM. (d) SAED pattern of BP.
Fig. 3 (a) Raman spectrum of the few-layer BP. (b) Transmittance versus incident optical energy intensity on phosphorene SA. Inset: the fitting curve of saturable absorption data.
3. Experimental setup and results
To investigate the ultrafast pulse generation ability of the as-prepared BP SAM, a five-mirror resonator with a cavity length of 2.58 m was employed. The experimental setup is schematically shown in Fig. 4. The Nd:GdVO4 crystal was 8 mm in length and 3 mm × 3 mm in cross-section. Both end faces were antireflection (AR) coated around 1342 nm. A fiber coupled laser diode emitting at 808 nm, which had a core diameter of 400 μm and a numerical aperture of 0.22, was used as the pump source. With a 1.8:1 optical collimation system, the pump spot radius focused into the crystal was 110 μm. Using the ABCD matrix propagation theory, we calculated the mode radii of the laser beam were 105 μm inside laser crystal and ~81 μm on BP SAM, respectively. The crystal was wrapped with indium foil and held in a copper block, which was maintained at 17°C by water. Flat mirror M3 with a transmission of 1% at 1342 nm was used as output coupler.
With the BP SAM inserted into the cavity, the CW mode locking (CWML) was realized by adjusting the laser cavity mirrors and absorbed pump power. Initially, CW operation was achieved at an absorbed pump power of 0.52 W. Then the laser operation regime varied from CW to Q-switched mode locking (QML). As shown in Fig. 5(b), a nearly fully modulated QML pulse envelope was yielded within a 750-ns long Q-switched envelope. And the mode-locked pulses within the Q-switched pulse envelope were separated by about 17 ns. The separated time was consistent with the cavity round-trip transmission time and corresponded to a repetition rate of ~58.14 MHz. To achieve stable CWML operation, the intracavity pulse energy should satisfy the theoretical condition [32]
where EP.C is the critical intracavity single pulse energy, Fsat,L is the saturation fluence of the gain medium, which can be described as Fsat,L = ℎυ/(2σl) (σl is the emission cross section of the gain medium), Fsat,A is the saturation fluence of the BP SAM, AL and AA are the laser mode areas on the gain medium and BP SAM, and ΔR is the modulation depth of the BP SAM. The parameters involved in the above formula are listed as follows: σl = 7.6 × 10−19 cm2, Fsat,A = 1.3 µJ/cm2, ΔR = 16%. The value of is calculated to be 377 nJ. When the absorbed pump power reached 1.85 W, stable CWML operation was achieved. At this point, the EP.C was about estimated to be 415 nJ with respect to an average output power of 120 mW. The CWML regime was successfully sustained between 1.85 W to 2.62 W of the absorbed pump power. As the absorbed pump power exceed 2.62 W, no pulse breakup effect was observed, but the laser stepped into unstable QML operation and then became disordered. The pulse train was given in Fig. 5(d). Several factors may be contributed to this phenomenon. First, the high intracavity intensity strongly bleached the black phosphorus, consuming considerable inversion population and introducing Q-switched instabilities. Thus the cavity stability region deviated from the set value for designing cavity to obtain CWML operation. Second, the fragile atomic binding force and the strong activity of phosphorus atoms caused the thermal instability of phosphorene film [33]. Third, the thermal-lensing-effect from the gain crystal also induced cavity instability. However, when the absorbed pump power was lowered back to the previous levels supporting CWML, stable mode-locking operation could be re-established. The average output power of the mode-locked laser depending on the absorbed pump power was plotted in Fig. 5(a). The maximum output power of CWML operation was 350 mW, corresponding to a slope efficiency of 15.4% and optical-to-optical conversion efficiency of 13.4%. The typical CWML pulse trains in nanosecond and millisecond time span were described in Fig. 5(c). Moreover, the BP SAM was replaced with high-reflection plane mirror after the laser measurement, and we observed no mode-locking pulse in full absorbed pump power range.Fig. 5 (a) Average output power as a function of absorbed pump power. (b) Pulse train of QML recorded at 1.7 W absorbed pump power (Pabs). (c) CWML pulses trains recorded at 2.2 W absorbed pump power. (d) Unstable pulses trains of QML recorded at 2.75 W absorbed pump power.
The autocorrelation trace of the mode-locked pulse was recorded by a commercial noncollinear autocorrelator (APE, Pulse Check 150), as shown in Fig. 6(a). If assuming a sech2 pulse shape, the pulse duration was measured to be 9.24 ps, which was the shortest one among 1.34 μm neodymium mode-locked lasers ever obtained by 2D materials SAs [4, 27]. The short pulse width should be attributed to the ultrafast recovery time of BP. The emission wavelength of the CWML laser was measured as depicted in Fig. 6(b), and the emission wavelength peaks were located at 1340.5 nm and 1340.7 nm with FWHM of 0.212 and 0.275 nm for CW and CWML operations, respectively. It was evidently that the spectrum was broadened in the CWML operation. The time bandwidth product was 0.424, which was 1.34 times of the Fourier transform limit value (0.315).
Fig. 6 (a) Normalized autocorrelation trace for 9.24 ps duration. (b). Emission spectra in the CW and CWML regimes.
To further verify the stability of the mode-locked operation, the radio-frequency (RF) spectra at different spans were recorded by a spectrum analyzer (Agilent N9000A). As indicated in Fig. 7, a sharp peak around 58.14 MHz with a signal-to-noise ratio reaching ~70 dB was found at the fundamental beat note, which matched well with the cavity length. And the RF spectrum in a wide span of 1 GHz was also shown in the inset of Fig. 6, implying that clean CWML operation of BP mode-locked laser was achieved. It could be maintained for several hours in the CWML operation under no extra perturbation.
Fig. 7 RF spectrum (resolution bandwidth: 30 Hz). Inset: 1-GHz wide-span spectrum (resolution bandwidth: 11 kHz).
4. Conclusion
In this letter, a diode-pumped CWML laser at 1340.7 nm has been successfully realized by a BP SAM, which, to the best of our knowledge, is the first BP mode-locked solid-state laser in the 1.34 μm spectral region. Mode-locked pulses with duration of 9.24 ps with an average output power of 350 mW were achieved. The obtained pulse duration is also the shortest one among those from mode-locked bulk lasers around 1.34 μm ever obtained by 2D SAs. The results validate that BP is a novel kind of highly promising optical material for generating ultrafast lasers in near-infrared region after the previous 2D SAs such as graphene, TI and TMDS.
Funding
National Natural Science Foundation of China (NSFC) (Grant No: 61575110, 61675116, and 51321091); The Fundamental Research Funds of Shandong University.
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