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Abstract
Ultraviolet (UV) beam generation at 266 nm using the sum-frequency (SFG) method with CsB3O5 (CBO) crystals was first suggested in 1997 [Opt. Lett. 22, 1840 (1997). [CrossRef] ]; however, there has been no further research in the past 25 years. Herein, by sum-frequency mixing in CBO crystals, we obtained a high conversion efficiency picosecond (ps) and a high-power nanosecond (ns) 266 nm UV beam output. First, a ps laser device with simultaneously radiated wavelengths of 1064 and 355 nm and repetition frequency of 10 Hz was used as the fundamental laser source, and the conversion efficiency from 1064 + 355 nm to 266 nm reached 20.35%. We then used a 1064 nm ns laser with a high output power and repetition frequency of 10 kHz as the pump source. We accurately modified the optimal phase matching direction of the CBO crystal, and the achieved output power at 266 nm reached 5.32 W.
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1. Introduction
The 266 nm ultraviolet (UV) laser is widely used in scientific research, industrial production, and medical treatment. A common method for obtaining 266 nm UV beam sources is the fourth-harmonic generation (FHG) of Nd3+-based lasers using nonlinear optical (NLO) crystals, such as β-BaB2O4 (BBO) [1–6], CsLiB6O10 (CLBO) [7–12], KBe2BO3F2 (KBBF) [13], and NaSr3Be3B3O9F4 (NSBBF) [14–17]. However, some disadvantages of these crystals limit their commercial applications. For instance, BBO has a large walk-off effect, small acceptance angle, hygroscopic characteristics, and two-photon absorption during FHG. The main disadvantage of the CLBO is hygroscopic, making it unstable for long-term use at room temperature. KBBF has a serious layered habit and is difficult to grow thick. NSBBF is prone to crack during laser experiments. To date, there is still a lack of excellent NLO crystals to generate 266 nm beams. Therefore, it is of great significance to generate a 266 nm beam by another route in existing NLO crystals with excellent properties.
In fact, a 266 nm beam can also be obtained by the sum-frequency-generation (SFG) method (ω + 3ω → 4ω). In 1994, Wu et al. successfully realized a 270 mW 266 nm beam using three LBO crystals [18]. In 2014, high conversion efficiency (28%) and high-power (1 W) 266 nm beam was obtained by Gabriel Mennerat et al. [19]. The highest output power of the 266 nm beam obtained by the SFG method using an LBO crystal reached 3.3 W, and the conversion efficiency reached 14% [20]. In 2022, high-energy (133 mJ) and sub-nanosecond (∼270 ps) UV pulses at 266 nm using the FHG method in LBO crystals were reported by Wang et al., and the conversion efficiency was 13.3% [21].
The CsB3O5 (CBO) crystal can also generate a 266 nm beam using the SFG method. CBO crystals have a high damage threshold and wide transmittance range, and can easily be grown in high quality and large sizes [22]. More importantly, CBO has a large effective NLO coefficient (deff), much larger than that of LBO, in 266 nm beam generation by the SFG method. For the LBO crystal, deff can reach its maximum value of 0.47 pm/V when the phase matching direction is θ = 90°, φ = 60.6° for type I phase matching. For the CBO crystal, we calculated that the maximum value of deff can be as high as 1.32 pm/V in the type I phase matching condition (θ = 52.4°, φ = 90°), that is, nearly three times as large as that of the LBO crystal. As presented in Table 1, we compared the NLO characteristics of LBO and CBO crystals in 266 nm beam generation using the SFG method and found that CBO can also be compared with LBO in other characteristics such as laser damage threshold, walk-off angle, and acceptance angles. Hence, we believe that the CBO crystal is a suitable choice for 266 nm beam generation by the SFG method. Early in 1997, the SFG scheme with the CBO crystal was suggested by Wu YiCheng et al. [23], but because the nonlinear coefficient matrix of the CBO crystal was not accurately measured at that time, the calculation of the deff of the CBO crystal was not sufficiently accurate. The experimental details and output power of the 266 nm beam have not been provided yet. Since then, no further studies on 266 nm beam generation using CBO crystals have been reported.
Table 1. Comparison of nonlinear optical (NLO) characteristics between LBO and CBO crystals in 266 nm beam generation by the sum-frequency-generation (SFG) method
In this study, we obtained an effective 266 nm UV output using a CBO crystal through the SFG method. First, a picosecond (ps) laser with a low repetition rate was used as the fundamental laser source, and the conversion efficiency from 1064 + 355 nm to 266 nm was up to 20.35%. This result directly verifies that the CBO crystal has great potential for realizing 266 nm UV beam output using the SFG method. Nevertheless, the CBO crystal we used (θ = 52.4°, φ = 90°) deviated from its optimal phase matching (OPM) direction. Then, using a nanosecond (ns) laser with high output power and a much higher repetition rate as the pump source, we accurately modified the OPM direction of the CBO crystal using our experimental data and the thermal refractive index coefficient data of the CBO crystal reported in 2013 [24], and a maximum output power of 3.79 W at 266 nm was obtained. We reprocessed the CBO crystal device according to the modified direction and finally increased the 266 nm output power to 5.32 W. These results will serve as an important reference for future experimental research.
2. High conversion efficiency picosecond 266 nm beam generation
To realize a high conversion efficiency of 266 nm beam radiation, a high-peak-power density ps laser was used as the pump source. The experimental setup is shown in Fig. 1. The pulse widths of the 1064 and 355 nm beam sources with vertical polarization were 25 and 18 ps, respectively. The repetition frequency was set at 10 Hz. Both the 1064 and 355 nm beams could be simultaneously radiated from the pump source. The 1064 and 355 nm light beams were reflected by two 45° high-reflection (HR) mirrors, M1 (HR@1064 nm) and M2 (HR@355 nm), respectively. To optimize the conversion efficiency of SFG, two apertures with the same diameters of 3 mm (A1 and A2) were used to make the 1064 and 355 nm beams with the same beam size. The 1064 nm laser was then passed through a λ/2 wave plate at 1064 nm (HWP1) and a Glan–Taylor prism (GTP). They played two roles in this experiment: one was to adjust the power of the 1064 nm laser as the incidence of the CBO crystal during the sum-frequency experiment, and the other was to ensure that the 1064 nm laser in the experiment was strictly of single polarization (horizontally polarized). Then, reflected by a 45° HR@1064 nm mirror (M3), the 1064 nm laser was overlapped and aligned carefully with the 355 nm beam at M4 (45° HR@355 nm and high-transmittance (HT) at 1064 nm). The CBO crystal we used was type-I angular phase-matching; thus, a λ/2 wave plate at 355 nm (HWP2) was used to adjust the polarization of the 355 nm beam and consistently maintained the polarization of the 355 and 1064 nm beams. M5, M6, M7, and M8 are 45° HR@355 nm mirrors, and M7 and M8 were fixed on an offset stage to fine-tune the optical path length (OPL). The smallest displacement of the offset stage was 0.1 mm. Finally, the 355 nm beam was overlapped with the 1064 nm laser at M4 as the incidence of the CBO crystal.
A CBO crystal with dimensions of 4 mm × 4 mm × 19.26 mm (length) was cut along θ = 52.4° and φ = 90°. The two end faces were optically polished, but uncoated. Finally, a Brewster-angle fused-silica prism was employed to separate the 266 nm beam from the residual 355 and 1064 nm beams. A power meter (FL 300 A, OPHIR) was used to measure the 266 nm output power.
In the high-efficiency experiment, a low-repetition-rate laser device that can generate 1064 and 355 nm ps beams simultaneously was used as the fundamental laser. The two outputs were adjusted separately. Therefore, in the experimental process, we can regulate the power of the 1064 and 355 nm beams separately to obtain the highest 266 nm beam output power. However, ultrafast pulses can cause a phase mismatch between the 355 and 1064 nm beams during sum-frequency owing to differences in the OPL. Therefore, we used an offset stage that can make the displacement sufficiently small (0.01 mm minimum) to fine-tune the OPL. During the experiment, we found that the CBO crystal was not precisely cut along the OPM direction; the crystal was slightly tilted when the optimal 266 nm output was achieved. Thus, we placed the crystal on a rotating stage and measured an inclination angle of approximately 4° (external angle). Owing to the angular deviation of the crystal, Fresnel reflection losses must occur at the 1064 and 355 nm beams on both the incident surface and the exit surface of the CBO crystal. These losses were added to the data in all the subsequent experiments with the CBO crystal cut along θ = 52.4° and φ = 90°.
The results are shown in Fig. 2. A maximum efficiency of 20.35% was obtained when the total pump power of the 1064 and 355 nm beams was 35.08 mW and the 266 nm beam output power was 7.14 mW. Then, further increasing the pump power, a maximum 266 nm beam power of 8.65 mW was observed with a lower efficiency of 18.25%, and the corresponding total pump power was 47.40 mW. This indicated that the conversion efficiency reached a saturation state. The high conversion efficiency indicates that the CBO crystal is suitable for 266 nm UV beam generation by the SFG method.
Fig. 2. 266 nm picosecond (ps) beam output power and conversion efficiency vs. 1064 and 355 nm input power.
3. High-power nanosecond 266 nm beam generation
As shown in Fig. 3, a Q-switched Nd:YAG laser (EdgeWave IS161-E) at 1064 nm with a pulse width of 10 ns and a repetition rate of 10 kHz was used as the fundamental laser source. The beam shape was rectangular with a size of 5 × 5 mm2. A lens system, including two lenses with focal lengths of −50 and +150 mm, was used to collimate and narrow the beam diameter at a ratio of 3:1. A type-I phase-matching LBO crystal (4 mm × 4 mm × 15 mm (length)) with θ = 90° and φ = 11.3° was used for the second-harmonic generation. Both the entrance and exit surfaces of the LBO crystal were anti-reflection (AR) at 1064 and 532 nm. Subsequently, a type-II phase-matching LBO crystal (4 mm × 4 mm × 12 mm (length)) with θ = 42.6° and φ = 90° was used for third-harmonic generation. The entrance and exit surfaces are not coated. A mirror (M) was used to separate the 532 nm beam from the 1064 and 355 nm beams. The mirror (M) was 45° HR-coated at 532 nm and HT-coated at 1064 and 355 nm. Subsequently, the 1064 and 355 nm beams were focused by a lens (f) and passed through the CBO crystal. The CBO crystal (4 mm × 4 mm × 19.26 mm) was held in a homemade heating oven. The same Brewster-angle fused-silica prism as that shown in Fig. 1 was used to separate the 266 nm beam. A power meter (FL 300 A, OPHIR) was used to measure the 266 nm output power.
To confirm the ratio between the 1064 and 355 nm beam powers used as the pump source of 266 nm beam generation, the powers of the 1064 and 355 nm beams were measured before the sum-frequency experiment. A maximum power of 34.40 W of the 355 nm beam was obtained. The corresponding efficiency from 1064 to 355 nm was 31.16%, and the highest conversion efficiency was 32.60%.
In our previous experiments, we found that the temperature of the CBO crystal increased easily when the pump power was excessively high, which could cause the OPM direction to deviate from the cutting direction of the CBO. Therefore, a heating oven was used to stabilize the crystal temperature, and we set the temperature at 50 °C during the experiments.
To achieve the optimal experimental conditions, lenses of different focal lengths were used to control the beam diameter incident on the CBO crystal, and the results are shown in Fig. 4. The 266 nm output power and conversion efficiency were the highest when f = 200 mm (solid line). When the 1064 nm pump power was 55.18 W, the 266 nm output power was as high as 3.79 W, and the corresponding conversion efficiency from 1064 to 266 nm was 6.87%. The highest conversion efficiency was 7.62% when the 1064 nm pump power was 43.2 W. Because the conversion efficiency no longer increased, the pump power was not further increased.
Fig. 4. 266 nm ns beam output power and conversion efficiency with different focal lengths under 50 °C
Because the CBO crystal we used had some deviation from its OPM direction, we carried out the following experiment to modify the OPM direction of the CBO crystal. The thermal refractive index coefficients of the CBO crystal were measured within a temperature range from 40 °C to 190 °C in Ref. [24]; therefore, combined with these data, we need to measure the inclination angles of the CBO crystal accurately for achieving the highest 266 nm output at different temperatures. Table 2 lists the inclination angles (external angles) of the CBO crystal from 25 °C to 90 °C, and the inclination angle increased from 3.93° to 5.29°. Using the refractive-index dispersion equations of the CBO crystal at different temperatures obtained in Ref. [24], we converted the inclination angles into internal deviations. Combined with the thermal refractive index coefficient data, the OPM direction of the CBO crystal in the 266 nm beam generation by the sum-frequency method can be modified. Table 3 presents the revised OPM angle θ of the CBO crystal at different temperatures.
Table 2. Inclination angles (external angles) of the CBO crystal for achieving the highest 266 nm output at different temperatures
Table 3. Revised optimal phase matching (OPM) angle θ of the CBO crystal at different temperatures
According to the data in Table 3, we reprocessed the CBO crystal along θ = 55.2°, φ = 90° (at 70 °C) with dimensions of 5 mm × 5 mm × 14.70 mm (length), and under the focusing condition of f = 200 mm, the experiment of high power 266 nm UV beam output was carried out again. As shown in Fig. 5, when the 1064 nm pump power was 67.76 W, the 266 nm output power measured was up to 5.32 W, and the corresponding conversion efficiency from 1064 to 266 nm was 7.85%. The highest conversion efficiency was 8.02% when the 1064 nm pump was 59.63 W. This is the highest output power ever achieved for CBO crystals at 266 nm. Figure 6 shows a set of beam spot images of different wavelengths, where (a) and (b) are the spot images of the 1064 and 355 nm beams incident on the CBO crystal measured using laser beam diagnostics (Spiricon SP620U), and (c) is the spot image of the 266 nm beam (because we did not have a suitable laser beam diagnostics for this wavelength, we acquired an image of it with just a camera).
Fig. 6. beam spot images: (a) 1064 nm and (b) 355 nm, obtained by laser beam diagnostics; (c) 266 nm, acquired with a camera.
In the process of the experiment, we found that the beam shape of the 266 nm beam was slightly elliptical, which was due to the walk-off effect of the CBO crystal during the sum-frequency. This can affect the output power and conversion efficiency of the 266 nm UV beam. In future experiments, α-BBO will be used to compensate for the walk-off effect. The output power and conversion efficiency of the 266 nm UV beam can be further improved.
4. Conclusions
The 266 nm beam output performance of the CBO crystal under different experimental conditions was studied. In the high-peak-power density ps laser experiment, a high conversion efficiency of 20.35% from 1064 + 355 nm to 266 nm was obtained. In the high average power laser experiment, we modified the OPM angle of the CBO crystal on the 266 nm generation using the ω+3ω→4ω method. The CBO crystal was reprocessed according to the corrected direction, and using the newly processed CBO crystal, we finally increased the output power of the 266 nm UV beam from 3.79 W to 5.32 W. All these data show that the CBO crystal has a huge advantage and potential for 266 nm beam realization through the SFG method. Our future research will focus on improving the beam quality and power stability of the 266 nm beam. With further improvements in the CBO crystal growth process and further optimization of our experimental conditions, the output power of the 266 nm UV beam will be further increased. We believe that CBO crystals can be used for high-power and high-stability 266 nm UV beam production.
Funding
Beijing Municipal Science & Technology Commission, Administrative Commission of Zhongguancun Science Park (Grant No. Z211100004821006); National Natural Science Foundation of China (Grant No. 51972314); Chinese Academy of Sciences (Grant No. YJKYYQ20210014).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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