We report on a1.3 μm Q-switched laser operation based on LD-end-pumped Nd:LiYF4 (YLF) crystal firstly using few-layer topological insulator (TI) Bi2Se3 as saturable absorber (SA). The TI Bi2Se3 SA is fabricated by transferring the liquid-phase-exfoliated Bi2Se3nanosheets onto a BK7 glass substrate. After a free-running laser operation, the TI Bi2Se3 SA is inserted into the concave-plano laser cavity and finally a stable Q-switched laser operation is achieved with a maximum average output power of close to 0.2 W corresponding to a pulse repetition rate of 161.3 kHz, a shortest pulse width of 433 ns and a pulse energy of about 1.23 μJ. The results experimentally extend the promising application of the 2D material, few-layer TI Bi2Se3, in solid state lasers (SSLs).
© 2015 Optical Society of America
In the recent ten years, since carbon nanotubes (CNTs) [1,2] and graphene [3–5] have been found to be very promising acting as SA in the field of fiber lasers and solid state lasers, efforts have been made to explore all those high-performance nanosheet materials because of their excellent properties, such as ultra-broadband saturable absorption, low cost and easy fabrication. All these properties lead to such kind of nanosheet materials suitable for mass production not only in the application of SAs but also in various other applications in optoelectronics. However, despite all these advantages of the nanosheet materials, one must recognize that they have not yet reached the maturity of those their counterpart materials, e.g. Cr:YAG and SESAMs. CNTs, for instance, were reported to have cluster-induced losses, and the suitable diameter control of the CNTs is often demanded for broadband saturable absorption . Relatively low modulation depth and damage threshold could prohibit the generation of large pulse energies using graphene. Therefore, these broadband materials seem promising but further works are necessary for a real development.
Recently, some new nanosheet materials grouped as topological insulators have appeared to be potential as SAs and therefore have drawn a lot of attention, such as Bi2Se3 [6–13], Bi2Te3  and Sb2Te3 . They show ultra-broadband saturable absorption like graphene, but they exhibit much higher modulation depth than graphene. Among them, Bi2Se3 has been mostly studied as SA in fiber lasers and SSLs. For example, for fiber laser, in 2013, Luo et al. reported a 1.06 µm Q-switched ytterbium-doped fiber laser and obtained tunable pulse repetition rate from 8.3 to 29.1 kHz . One year later, researchers from the same team demonstrated a diode-pumped Tm3+-doped double-clad fiber laser with the shortest pulse width of 4.18 μs and tunable repetition rate from 8.4 to 26.8 kHz . For SSLs, Yu et al. reported the first TI Bi2Se3-based Q-switched SSL using Nd:GdVO4 as gain medium and obtained 58.5 nJ single pulse energy . Hu et al. demonstrated a 1.6 μs Q-switched Yb:KGW laser and obtained a highest average output power of 439.4 mW, which represents the highest output power to date using topological insulators as SA . TI Bi2Se3-based simultaneously Q-switched dual-wavelength Nd:Lu2O3 laser was also realized by Wang et al. . Very recently, using few-layer Bi2Se3, Jia et al. also demonstrated a dual-wavelength Q-switched Nd:YVO4 laser and they obtained single pulse energy 0.56 μJ and shortest pulse width as 250 ns .
Nd:YLF has some important advantages despite its relatively smaller gain coefficient than Nd:YAG [16–20]. First of all, the spectroscopic properties show it to be superior to YAG for certain application. For instance, it has emission wavelength at 1.053 μm that matches the wavelength of the peak gain in Nd-doped phosphate glass. Moreover, fluorides have negative index variation with temperature which partially compensates the surface bulging due to thermal dilatation. Hence Nd:YLF has reduced thermal effects supporting stable resonators over a wide power range. Furthermore, the lifetime of the upper laser level (4F3/2) is twice longer than that of Nd:YAG, resulting in improved energy storage in the upper laser level, which means Nd:YLF is more favorable for pulsed laser operation than Nd:YAG.
High-peak-power all-solid-state laser sources in the 1.3 µm spectral region are of particular interest in remote sensing, eye-safe optical ranging, fiber sensing, and communication. For example, 1.3 µm lasers with characteristics of high absorption in water and low extinction in blood are being applied in resection of pulmonary metastases and dental applications recently [21,22]. Thus the diode-pump solid-state lasers operating at 1.3 µm wavelength region have attracted a great deal of attention. So far, no report concerns 1.3 μm TI Bi2Se3-based pulsed lasers and also no report concerns TI Bi2Se3-based Nd:YLF pulsed lasers, to the best of our knowledge. In this paper, we present the first 1.3 μmNd:YLF Q-switched laser using few-layer TI Bi2Se3 deposited on a BK7 glass substrate as SA. By inserting the TI Bi2Se3 SA into a concave-plano Nd:YLF laser cavity, we can obtain a stable Q-switched laser operation with laser pulses of 1.23 μJ and 433 ns at a repetition rate of about 161.3 kHz.
2. Preparation and characterization of TI Bi2Se3 SA
The preparation process of few-layer TI Bi2Se3SA is the same as depicted in . Figure 1 shows the characterization of the as-prepared TI Bi2Se3. The bulk Bi2Se3 and the as-prepared few-layer Bi2Se3 are both characterized by X-ray diffraction (XRD) in Fig. 1(a), in which all the labeled peaks of the bulk Bi2Se3 can be easily indexed to rhombohedralBi2Se3 (JCPDs NO. 89-2008). The bulk Bi2Se3 had been successfully exfoliated because the XRD pattern of the few-layer Bi2Se3 shows a high  orientation and some characteristic peaks disappeared. The Raman spectroscopy is shown in Fig. 1(b). The characteristic peaks of the bulk Bi2Se3 are calibrated at 72.0, 128.6 and 172.7 cm−1. Compared with the bulk Bi2Se3, few-layer Bi2Se3 sample shows an obvious red shift of peak. Furthermore, atomic force microscopy (AFM) image was registered for characterizing the thickness of the as-prepared few-layer Bi2Se3 (see Fig. 1(c)). The height profile diagram (see Fig. 1(d)) shows the TI Bi2Se3nanosheets are around 2-4 nm, which indicates the TI Bi2Se3nanosheets are about 3 layers since the thickness of single layer is 0.96 nm . The transmission of the as-prepared TI Bi2Se3was measured to be about 95.1% at 1313 nm using a Perkinelmer Lambda 750 optical spectrometer.
Following the synthesis of the few-layer TI Bi2Se3suspension, it was proceeded to the following steps to simply transfer the TI Bi2Se3nanosheets onto anti-reflection coated BK7 glass plates (0.5 mm in thickness). First, proper amounts of homogeneous TI Bi2Se3 solution were directly dripped onto the BK7 glass substrate. Second, the glass substrate was rotated at low speed to uniformly disperse the TI Bi2Se3 solution. Third, we dried it inside an oven with a constant temperature of 70°C for two hours.
3. Experimental setup
The schematic of the laser experimental setup is shown in Fig. 2.An 18 W laser diode is used as pump source emitting central wavelength around 803 nm at maximum output power with FWHM of about 2.88 nm. Although the greatest absorption for Nd:YLF is obtained near 797 and 806 nm (for σ polarization absorption), as well as 792 and 797 nm (for π polarization absorption), the 803 nm laser diode is only available pump source during the experiment. The laser diode is out-coupled using a fiber with a numerical aperture of 0.22 and a core diameter of 400 μm. To optimize the overlap between the pump spot size and the laser mode, we use a collimating doublet of 50-mm focal length and a focusing doublet of 40-mm focal length. Thus the end face of the coupling fiber was imaged into the laser crystal with a minimum spot radius of about 160 μm.
The laser cavity was a typical concave-plano cavity with total cavity length of about 35 mm. Such cavity is helpful not only in mitigating the thermal lensing effect of the laser crystal because of big laser mode but also, more importantly, in achieving small laser mode at the place of output mirror, which is favorable for saturable absorption of the TI Bi2Se3 SA. The coatings of the input mirror (IM) and of the output coupler (OC) were designed and fabricated in our lab by using a plasma direct-current sputtering technique. The curved dichroic IM has a maximum transmission (~90%) at the pump wavelength and is highly reflective (~99.9%) around 1.31 μm. In addition, in order to avoid lasing at the high gain transition lines around 1.05 μm, a high transmission about 90% at around 1.05 μm was coated. The OC is a flat mirror with transmission of 2.3% at the laser wavelength.
The laser crystal is a nicely polished but uncoated c-cut Nd:YLF crystal with a section of 3 × 3 mm2 and a length of 4 mm along the laser cavity axis. Due to a relatively weaker absorption of the c-cut than a-cut, the used Nd:YLF has a relatively high doping concentration as 1at.%. Thus, the single pass absorption ratio of the Nd:YLF crystal was measured to be about from 46% to 62% due to the shift of the pump wavelength. In addition, to avoid the thermal fracture of the laser crystal, a water cooling device is employed to efficiently remove the generated heat inside the laser crystal, which is wrapped inside a piece of indium foil and mounted inside a copper plate. A pinhole is also used for stopping the residual pump power as much as possible.
4. Laser results and discussion
A free-running continuous-wave (CW) operation was first carried out and we achieved maximum output power up to 1.65 W corresponding to a maximum pump power of 17.8 W. The threshold pump power was recorded to be 1.47 W during the experiment. A linear fit of the CW laser data gives a slope efficiency of about 9.6% with respect to pump power (see Fig. 3(a)). No output power rollover was observed indicating a feasible power scaling by using a pump source with better emitting wavelength to match the peak absorption of the laser crystal or by using longer laser crystal for absorbing more pump power.
We then inserted the as-prepared TI Bi2Se3 SA and a pinhole into the laser cavity for generation of Q-switched laser pulse after the free-running laser operation. By continuously increasing the pump power from the continuous-wave threshold and finely tilting the glass substrate, a stable Q-switched pulsed laser operation was achieved when the pump power reached about 2.88 W. The higher threshold for the case of Q-switched laser operation should be explained by the linear absorption loss of the TI Bi2Se3 SA. Above the threshold pump power, a stable Q-switched laser operation was observed. The average output power is registered also in Fig. 3(a) showing a maximum output power of 0.198 W. Linear fit of the laser data gives a slope efficiency of about 6.5%. The output pulse laser was no longer stable when higher pump power was injected into the laser crystal. Damage can also be observed on the surface of the Bi2Se3saturable absorber with a burning formed dot when further increasing the pump power to about 7.8 W. The laser emission was monitored and measured with a peak wavelength of 1313.04 nm by using an optical spectrum analyzer HP 70951B shown in Fig. 3(b).
The single pulse profile and pulse trains shown in Fig. 4 at maximum output power were both recorded using a digital oscilloscope (Tektronix TDS 1012, 100MHz) and a fast photodetector (Thorlabs, PDA10CF-EC). The shortest pulse duration was measured to be about 433 ns and the corresponding pulse repetition rate was recorded to be 161.3 kHz. Figure 5(a) shows the evolution of the pulse repetition rate and the pulse width. The pulse repetition rate almost linearly increased from 36.5 to 161.3 kHz, while the pulse width shortened from 628 to 433 ns. According to the recorded pulse repetition rate and pulse width, we can approximately calculate the pulse peak power and pulse energy (see Fig. 5(b)). The maximum single pulse energy was estimated to be 1.23 μJ with a corresponding peak power of 2.84 W.
In 2013, Hernando et al. reported a Nd:YLF Q-switched laser at 1047 nm using a mono-layer graphene as SA and they obtained much longer laser pulse as 2.5 µs . A much longer laser cavity as 282 mm and small modulation depth of their mono-layer graphene SA could be the reason for their longer laser pulse than that of ours. Finally, it should be pointed out that, according to , our low transmission coating of the output coupling is also helpful to generations of shorter pulses and higher repetition rate for a fixed cavity length.
After obtaining a 1.65 W continuous-wave laser at 1313 nm in a LD-pumped Nd:YLF laser crystal, we have experimentally demonstrated a passively Q-switched Nd:YLF laser at the same lasing wavelength by inserting a TI Bi2Se3 SA. The maximum average output power of about 0.2 W associated with corresponding laser pulse width 443 ns and pulse energy 1.23 μJ (peak power 2.84 W) at a repetition rate of 161.3 kHz is obtained. Therefore, these laser results definitely reveal that few-layer TI Bi2Se3 could be a very promising SA for the realization of compact and cost-effective Q-switched solid state lasers and also for the potential realization of ultra-short pulsed mode-locked solid state lasers, which will be our next investigation.
The authors wish to thank the financial support from National Natural Science Foundation of China (61275050, 61475129), the Specialized Research Fund for the Doctoral Program of Higher Education (20120121110034, 20130121120043), the Fundamental Research Funds for the Central Universities (2013121022), Natural Science Foundation of Fujian Province of China (2014J01251), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
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