266 nm laser output in NaSr3Be3B3O9F4 crystal by the fourth harmonic generation process with a picosecond mode-locked Nd-based YAG laser has been done for the first time. When the input pumping energy was 870 μJ at 532 nm, a 280 μJ 266 nm UV laser was obtained and the corresponding conversion efficiency was 35.9%. Further investigations identified that NaSr3Be3B3O9F4 has a large acceptance angle width of 0.47 (mrad • cm), a small walk-off angle of 35.43 mrad and a large deff as 0.62 pm/V for the fourth harmonic generation. These results indicate that NSBBF is applicable for high-power 266 nm laser generation.
© 2017 Optical Society of America
UV lasers have a wide range of applications in many fields, such as scientific research, medical treatment, micro-processing, and industrial manufacturing for its preeminent properties like high resolution and high photon energy . So far, there’re mainly three kinds of lasers, all solid state laser, semiconductor laser and gas laser, to produce the desired UV laser. Typical gas laser used for UV light generation is excimer laser . The output power of excimer laser is very high, but the laser is very difficult to use for its large size, extremely strict working conditions, and expensive maintenance fee . For the semiconductor lasers, they can be very small in size and very large in output power, but the quality of the laser beam is not so competitive for its bad monochromaticity sourcing from its working mechanism . Compared with gas laser and semiconductor laser, high-power of all-solid-state UV lasers which produce UV light by a cascade frequency conversion process with a nonlinear optical (NLO) crystal are more popular for their superior properties like narrow beam width, easy-handling size and high reliability. There are mainly two wavelengths UV all-solid-state lasers as 355 nm and 266 nm. So far, the major NLO crystals for 355 nm generation include LiB3O5 (LBO) [5,6] K3B6O10Br (KBB) [7,8] and CsB3O5 (CBO) [9,10] and the output power for LBO can be as high as 160W . As for the 266 nm laser, it requires a simpler fourth harmonic generation (FOHG) process, while the photon energy of 266 nm light is much higher than that of 355 nm light which leads to a wider application. Therefore, 266 nm laser is of greater importance and more efficiency than 355 nm laser, and considerable attentions have been paid to the development of 266 nm all-solid-state laser. But, there are still no mature products of high-power 266 nm all-solid-state laser in the commercial market up till now. And the lacking of suitable NLO crystals has been the key factor to this situation. To date, the commonly used 266 nm NLO crystals are borates, including β-BaB2O4 (BBO) , and CsLiB6O10 (CLBO) . They can produce watt-level 266 nm laser light by the FOHG of Nd-based lasers. Unfortunately BBO suffers from two vital problems as being photorefractive and possessing two-photon absorption phenomenon [14–16], which restricts its output power. Though CLBO can produce a maximum 28.4 W 266 nm laser , the crystal is highly hygroscopic and the working temperature is rigorously restricted to about 150 °C, which is not convenient for use. On the other hand, the deep-UV NLO crystals like KBe2BO3F2 (KBBF) [18–20] and RbBe2BO3F2 (RBBF) [21–23] are capable of producing 266 nm laser light by a direct FHOG process too, but these crystals are rather difficult to grow due to a serious layer growth habit. Some other new borate crystals including YAl3(BO3)4 (YAB) , K2Al2B2O7 (KABO) [25,26] and Ga-doped BaAlBO3F2  have been developed for the FOHG at 266 nm in recent years. Among them, YAB has excellent NLO properties but the crystal is hard to overcome its twin structure which seriously affects the UV laser generation. Also KABO owns good NLO properties, but it has abnormal absorptions in the 200-300 nm region which seriously reduces the output power of the 266 nm laser. The last one, Ga-doped BaAlBO3F2, has abnormal UV absorption too, and the crystal growth is difficult. In a word, the great demands for high power of 266 nm laser and the lacking of excellent UV NLO crystals make it urgent to search for new UV NLO crystals for commercial applications. The basic requirements for new UV NLO crystals could be summed up as good NLO properties, favorable growth habit and easy-handling physical properties.
In 2011, our group developed a new UV NLO crystal, NaSr3Be3B3O9F4 (NSBBF) . This crystal crystallizes in the space group R3m. It exhibits a relatively large nonlinear optical response which is about three times larger than that of KH2PO4 (KDP) for the perfect alignment of the BO3 groups in its structure. Also, unlike KBBF and some other mentioned crystals above, NSBBF shows no sign of layered growth tendency and other structure problems, which are beneficial for bulk crystal growth and high-power generation. Following experiments identified NSBBF is non-hygroscopic, which ensures it being easy-handling in the daily use, and the cut-off wavelength of NSBBF is down to 170 nm. In 2014, Wang et al.  succeeded in growing a bulk crystal with the size of 13 Ч 14 Ч 20 mm3 from a NaF-LiF-B2O3 flux system by the top-seeded solution growth (TSSG) method and measured its refractive index. The results supported that NSBBF is able to produce 266 nm laser by a FOHG process. To obtain better quality crystals, we optimized the growth conditions from a modified NaF-LiF-B2O3 flux system by TSSG method. Finally we succeeded in growing one large crystal with the size of 24 Ч 23 Ч 13 mm3. In this letter, we report the 266 nm laser generation in this NSBBF crystal for the first time and further evaluate its capacity for FOHG process.
2. Basic optical parameters of NSBBF for 266 laser generation
As reported by Wang et al. , the Sellmeier equations of NSBBF are as follows (λ in μm):30]:Eqs. (1) and (2), we can get the fact that the shortest wavelength that can be achieved by Type-I PM of NSBBF is down to 233 nm. For 266 nm laser generation, the values of no(ω), no(2ω), ne(2ω) are 1.6041, 1.6605, 1.5892 respectively and ƟPM is calculated to be 62.04°. The effective nonlinear optical coefficient can be written as:Eq. (3) and the previously calculation of the nonlinear optical coefficient d22 , NSBBF yields a relatively large value of deff as 0.62 pm/V for the FOHG process, when θ = 62.04° and φ = 30°.
Also the walk-off angle and the angular acceptance are two vital parameters to assess a nonlinear optical crystal’s ability for high power of laser generation. In the negative uniaxial crystal like NSBBF, its walk-off angle satisfies Eq. (4), NSBBF exhibits a small walk-off angle of 35.43 mrad for 266 nm laser generation. Using the plane wave approximation and neglecting the depletion of the fundamental wave, the intensity of SHG satisfies 32]
3. Experiment and results of 266 nm laser generation in NSBBF
NSBBF crystal was grown from a modified NaF-LiF-B2O3 flux system by using  oriented seed with a cooling rate of 0.5 °C/d. The growth process is about one month. As shown in Fig. 1(a), the as-grown NSBBF crystal shows optically transparence without inclusions inside the crystal. The crystalline quality of this as-grown crystal was evaluated by the X-ray rocking curve with a centimeter-scale and optically polished plate. And the full-width at half-maximum (FWHM) is measured to be 0.011°, as shown in Fig. 2. In order to realize the FOHG, the device must be cut along the PM angle (θ = 62.04°, φ = 30°). First the as-grown face is indexed by X-ray orientation technique. The (001) plane needs to be manually ground. Then the crystal was cut along the PM angle and a device with size of 4 Ч 4 Ч 4.3 mm3 was obtained as shown in Fig. 1(b). The two surfaces for light path were optically polished with CeO2 polishing powder. The experimental setup for 266 nm light generation is shown in Fig. 1(d). The fundamental laser source is a frequency-doubled picosecond mode-locked Nd:YAG laser (PL2140, EKSPLA), together with an LBO crystal inside to output the fundamental 532 nm green laser with a pulse width of 30 ps at a repetition rate of 10Hz. By using a diaphragm, we minimized the incident 532 nm light spot to a restricted round laser spot with a size of Φ 3 mm and pumped it directly into the NSBBF crystal which was placed on a rotation stage. To separate the mixed residual 532 nm and 266 nm laser light, a quartz prism cut at the Brewster angle was adopted. And the final 266 nm laser output power was recorded by a laser power meter. On the right side of Fig. 1(c), a blue bright spot which is identified as 266 nm laser spot was clearly observed and recorded.
Having been noticed, the polished NSBBF crystal was not coated to reduce the reflectance of light. Therefore, to calculate the conversion efficiency of the 266 nm laser light, the effect of the reflected 532 nm fundamental light from the front incidence plane of NSBBF and the reflected 266 nm harmonic light from the back emergent face of NSBBF can’t be neglected. As both of them are almost in a normal incidence condition and the NSBBF crystal is a negative uniaxial crystal, the conversion efficiency can be written asEqs. (1) and (7) and the recorded data, Fig. 3 illustrates the input energy of incident 532 nm laser light versus the output energy of 266 nm laser light, and the corresponding conversion efficiency. It is clear that the conversion efficiency climes to a highest value of 35.9% (the surface reflection has been considered) with an output energy of 280 μJ when the intensity of the incident 532 nm laser reaches to 870 μJ. It is the first time to realize the fourth harmonic generation by using a NSBBF crystal and the conversion efficiency is pretty high among these 266 nm NLO crystals.
4. Determination of the angular acceptance angle of NSBBF
For further determination of the acceptance angle of NSBBF, we conducted the following experiment with the same crystal, as shown in Fig. 4. Differently, the laser source is changed into a nanosecond Q-switched Nd:YAG laser (Edgewave, IS161-E) with 10 ns width and 10 kHz repetition rate at 1064 nm. Also, an LBO crystal was used to produce 532 nm green light, and the rotation stage was changed into a motorized one with a precision of 0.0025° to record the rotated angles. Here we adopted an attenuation system to regulate the output power of 1064 nm manually and a lens system to minimize the size of the incident laser spot with a ratio of 1:3, meanwhile, two dichroic mirrors, M1 and M2 were employed to separate the generated 532 nm laser from the residual 1064 nm laser. Then, the 532 nm laser was pumped into the NSBBF crystal for 266 nm laser generation. Both measured results and fixed curve were plotted as black hollow dots and black line in Fig. 5.
Figure 5 illustrates the experimental records for the determination of the external angular acceptance. From the fixed curve, the external angular acceptance of NSBBF was measured to be 1.075 mrad • cm, while the internal angular acceptance is determined to be 0.67 mrad • cm which is larger than the calculated value. This dilated angular acceptance may be caused by both the less competitive optical quality of this used crystal bar and the focused laser beam brought by the adopted lens system.
5. Comparisons of the major 266 nm NLO crystals
The value of an applicable UV NLO crystal for FOHG is mainly determined by its physical and optical properties. For comparisons, the optical characteristics of the major NLO crystals for 266 nm laser generation are listed in Table 1. As we know, a large acceptance angle, a large SHG coefficient and a small walk-off angle are very important factors for high power of laser generation. Clearly from Table 1, the walk-off angle of NSBBF is much smaller than that of BBO and comparable to CLBO. Its angular acceptance is larger than that of BBO and comparable to that of CLBO. Also, its SHG coefficient is comparable to that of CLBO and YAB. It has some advantages compared with BBO as NSBBF exhibits a wider bandgap, so no obvious two-photon absorption and photo refractive were observed yet, it also has no extrinsic absorption in the 200-300 nm region. Moreover, it is not hygroscopic which is quite important for real applications. All of these optical and physical properties make NSBBF a promising candidate for the high power 266 nm laser generation.
In conclusion, we demonstrated 266 nm laser generation with a NSBBF crystal for the first time. A high conversion efficiency as 35.9% was obtained when the intensity of the incident 532 nm laser reaches to 0.41 GW/cm2. And further investigations on the 266 nm generation properties like walk-off angle, angular acceptance and effective nonlinear optical coefficient identified that NSBBF is a good candidate for high power of 266 nm generation.
National Natural Science Foundation of China (NSFC) (No. 51402316); National Key Research and Development Program of China (2016YFB0402103)
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