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Abstract
We demonstrate the generation of high-energy (133 mJ) and sub-nanosecond (∼270 ps) deep ultraviolet (DUV) pulses at 266 nm by sum-frequency mixing in LiB3O5 (LBO) crystals. The highest 133 mJ pulse energy ever reported corresponds to a peak power of 0.49 GW and an energy conversion efficiency of 13.3% from the infrared at 1064 nm to DUV at 266 nm. This is the highest output energy ever reported for the DUV sub-nanosecond pulses to the best of our knowledge. Higher energy efficiency of 25.7% can be achieved from 1064 nm to 266 nm when the fundamental energy was reduced to 346 mJ. Furthermore, the DUV generations using LBO and typical β-BaB2O4 (BBO) crystals were compared regarding the energy efficiency, and the effects of the nonlinear absorption are discussed.
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1. Introduction
High energy DUV lasers are required for various applications in many fields, including scientific research, medical treatment, and industrial manufacturing, due to their advantages of high photon energy and high resolution [1,2]. The most common method for generating a high peak power ultraviolet (UV) lasers below 300 nm is using nonlinear optical (NLO) crystals based on frequency quadrupling of 1µm wavelength radiation originating from Q-switched Nd:YAG, Nd:YVO4, and Yb:YAG lasers [3–6]. Figure 1 summarizes the typical results of DUV laser at 266 nm and 258 nm in the past decades [7–21]. These generated DUV radiations have pulse durations of nanosecond (ns), picosecond (ps), and femtosecond (fs). Pulses at ns duration DUV sources mainly produced watts or tens of watts level of average power, a few mJ of energy, and kW of peak power. The highest average power reported was 40 W by M. Nishioka et al. in 2003 [7]. Pulse duration at ps region mainly provided watt level of power, several µJ level energy, and MW level peak power. The typical high energy of 2.74 mJ with 4.2 ps pulse duration at 1 kHz was reported by C.-L.Chang et al. in 2015 [12], a higher peak power of 0.56 GW was achieved. Another ps DUV source at 257 nm with 7.6 W average power, less than 1.5 ps pulse duration at 77 kHz was demonstrated in 2019 [10]. Fewer sub-ps or fs DUV sources have been reported. A 4.6 W average power, estimated 150 fs pulse duration at 796 kHz DUV source was generated in 2017 [14]. A 2 mJ energy, 20 W average power, 665 fs pulse duration, 258 nm DUV pulses were reported in 2020 [15]. The short pulse duration gave the source the highest peak power of 3 GW. Based on the above reports, the energy of DUV sources was hovering at several mJ levels. The increasing applications in laser fine manufacturing require DUV sources with higher energy and higher peak power in the sub-nanosecond or picosecond range.
Fig. 1. Performance comparison of DUV laser at 266 nm and 258 nm generations in the literature showing the advantage of our work
The high-performance NLO crystals are the key to generating high peak power UV lasers. The major NLO crystals for FHG at 266 nm are BBO and CsLiB6O10 (CLBO). Although these crystals manifest good performances for UV generation at 266 nm [6,19], they present some disadvantages which limit their practical applications. BBO has large walk-off angles, small angular acceptance bandwidth, and suffers from high two-photo absorption, which seriously restricts the conversion efficiency, detrimental to the stability. CLBO is highly hygroscopic, which is inconvenient to use. In addition, some other new crystals such as YAl3(BO3)4 (YAB) [16,17], NaSr3Be3B3O9F4 (NSBBF) [18], and KBe2BO3F2 (KBBF) [19], also show potential for producing 266 nm by FHG, but most of these crystals are still not mature products in the commercial market so far. The research for new materials and methods to circumvent these difficulties is still ongoing.
Due to excellent operating performance, LBO crystal is successfully used for the generated visible and near UV radiation in commercial laser systems. With the progress of crystal growth technology, large size LBO crystal has high homogeneity [22], which is conducive to frequency conversion at high energy and high intensity. LBO crystal is also phase-matchable to generate the 266 nm through the fundamental (1064 nm) sum-frequency mixing with the generated third-harmonic (355 nm). LBO crystal shows promise for generating high-energy DUV radiation.
There are only two experimental reports on LBO extracavity sum-frequency mixing to produce DUV at 266 nm. Mennerat et al. obtained average power above 1 W of a 266 nm radiation at a 10 kHz pulse repetition rate [23]. Nikitin et al. reported an output power of 3.3 W at 266 nm was achieved with 14% IR-to-DUV conversion efficiency [20]. It shows potential for generating high peak power at 266 nm.
This paper reports the generation of sub-nanosecond radiation at 266 nm with higher energy based on sum-frequency mixing in LBO crystals. A DUV output at 266 nm with an energy of 133 mJ/pulse and a pulse width of 270 ps, corresponding to a peak power of 0.49 GW, has been created. To the best of our knowledge, it is the highest DUV pulse energy at 266 nm ever attained for the hundred-picosecond DUV pulses. Furthermore, we compare the FHG results of LBO and BBO crystals under the same experimental conditions of input fundamental laser and beam size and eventually figured out the reasons for the lower performance of BBO than LBO.
2. Experimental setup for the fourth harmonic generation with LBO crystals
The experimental setup for FHG from 1064 nm to 266 nm using LBO crystals is shown in Fig. 2. A homemade Nd:YAG master oscillator power amplifier (MOPA) laser at 1064 nm was employed as the laser source. The output of the laser source at 1064 nm and their harmonics were all linearly polarized, and the polarization directions of each component were graphically illustrated in the lower part of Fig. 2. This homemade laser has an approximately flat-topped profile and a beam divergence of 1.24 times the value of the diffraction limit. The maximum output energy was 1 J with a 360 ps pulse duration when operated at 1-10 Hz repetition rate. The output beam diameter was approximately 11 mm (D4σ) after a collimator.
Fig. 2. Experimental setup for FHG with LBO crystal and the polarization matching. The colored beam profile is for the 1064 nm beam.
All the LBO crystals were mounted in ovens individually, and the temperature of the oven is controlled to a precision of ± 0.1°C, and they are placed in series. Each of the LBO crystals had a dimension of 14 × 14 × 10 mm3. A type-I noncritical phase-matched LBO crystal (LBO1) was used for second-harmonic generation (SHG) of 532 nm. The LBO1 crystal was cut in the XY-plan with θ=90° and φ=0°. The end surfaces of the LBO1 were antireflection (AR) coated for both 1064 nm and 532 nm. The third harmonic generation (THG) is achieved by sum-frequency generation between the residual fundamental beam at 1064 nm and the second harmonic at 532 nm. The second harmonic at 532 nm and the fundamental at 1064 nm are orthogonally polarized and passed through a type-II LBO crystal (LBO2) for THG of 355 nm. The LBO2 was cut in YZ-plan with θ=44.6° and φ=90°. Only the entrance surface of the LBO2 was AR-coated at 1064 nm and 532 nm. Since LBO crystal cannot be phase-matched for direct frequency doubling of the laser at 532 nm, for the FHG of 266 nm, it can only be achieved by sum-frequency generation between the fundamental beam at 1064 nm and the third harmonic at 355 nm by a type-I LBO (LBO3) cut in XY-plan with θ=90° and φ=61°. The temperature is maintained at 78.5°C. The end surfaces of the LBO3 were uncoated to avoid potential optical damage. A quartz prism was adopted to separate the generated 266 nm wave from other residual waves. The polarization matching of the scheme is easy to achieve. The polarization direction of the beam produced by the previous crystals and the polarization direction of the residual beam is what is required by the next crystal without beam manipulation. The temperature of the crystal was adjusted individually in each stage. In addition to ensuring a general phase matching, a small mismatch is often fine-tuned to meet the optimal energy ratio needed for the next crystal.
3. High peak power fourth harmonic generation with LBO crystals
The experiment results of the green and UV beams are shown in Fig. 3. As illustrated in Fig. 3(a), the maximum energy of 748 mJ at 532 nm was obtained when the 1064 nm energy was 1004 mJ, corresponding to a conversion efficiency of 74.5%, while the temperature of LBO1 was set to 148°C. Figure 3(b) showed the results when the temperature of LBO2 was maintained at 60.2°C, while the LBO1 was kept at 148.8°C. For the THG, the maximum energy conversion efficiency is 42.6%, from 1064 nm to 355 nm. The maximum energy of 355 nm is 428 mJ. As evident from Fig. 3(b), the pumping energy dependence of THG efficiency on the pumping energy at 1064 nm is no longer linear at high input energy, confirming the saturation effect occurred. The saturation effect can be attributed to the absorption-related effect, the back conversion, and the thermal phase-mismatch.
Fig. 3. Dependences of output energy and conversion efficiency for SHG (a) and THG (b) on input energy at 1064 nm. Inset of (a): The beam profile of 532 nm. Inset of (b): The beam profile of 355 nm.
Ideally, for efficient frequency conversion of sum-frequency mixing, the ratio of the interacting photon number would be 1:1. THG via sum-frequency generation between the pulses at 532 nm and 1064 nm implies that to maximize the output at 355 nm, the energy ratio of 1064 nm and 532 nm source is 1:2 considering the photon energy of each component. The energy ratios of 1064 nm and 532 nm waves were adjusted by changing the temperature of LBO1. Figure 4 shows the dependence of SHG and THG conversion efficiency on LBO1 temperature. The maximum THG conversion efficiency was achieved with the energy ratio of 1064 nm, and the 532 nm wave was 1:1.4 in the actual experiment. The deviation from theory is attributed to the disparity in pulse width and overlap time of interacting beams and the difference in the overlap of the interacting spots. In the experiment, the pulse width of 532 nm wave was measured as 300 ps and less than that of 1064 nm wave. When the two pulses interact, part of 1064 nm is not involved, resulting in a reduction of 532 nm consumption. The FHG, implemented by sum-frequency between the waves at 1064 nm and 355 nm, was also optimized in the same way.
The energy dependence of 266 nm and FHG conversion efficiency on 1064 nm input are illustrated in Fig. 5. Under the various experimental conditions, including the pumping energy at 1064 nm, crystal temperature, etc., two LBO crystal lengths (7 mm and 10 mm, respectively) were studied. The temperature of LBO3 was maintained at 78.5°C, while the LBO1 was kept at 149.4°C, and the LBO2 was kept at 59.6°C. Figure 5(a) shows the energy and conversion efficiency for a 7 mm long LBO crystal. The maximum energy at 266 nm was 116 mJ when the maximum pumping energy was used. The corresponding conversion efficiency from IR to DUV is 11.5%. Using the 10 mm LBO crystal, the related maximum energy of 133 mJ at 266 nm, corresponding to an FHG conversion efficiency of 13.3% from 1064 nm to 266 nm, as shown in Fig. 5(b). In comparison, the optimized FHG conversion efficiency is 25.7%, which occurred when the 1064 nm energy was 346 mJ, and the corresponding output energy is only about 89 mJ. As shown in Fig. 5, the significant saturation effect occurs towards high pump fundamental energy. The saturation effect can be attributed to pump depletion, back conversion, and thermal phase-mismatch, which occur at high pumping intensity. Experimentally, we only optimized the temperature of each crystal at the maximum input energy to obtain a maximum 266 nm output. The energy ratio of 1064 nm and 355 nm corresponding to the energy of the fundamental pump at this temperature is shown in Fig. 6(a). The energy ratio of 1064 nm and 355 nm for the maximum conversion efficiency of 25.7% is found out to be 1:1.9, while the theoretical value should be 1:3. The discrepancy can be attributed to the same reasons explained in THG, where the experimental energy ratio is 1:1.4, and it is different from the theoretical value of 1:2. With the decrease or increase of the 1064 nm/355 nm energy ratio, the conversion efficiency also decreases since the photon number of each component will be unmatched, when further increasing the pumping energy at 1064 nm, and the efficiency will be lowered. Figure 6(b) shows the dependence of FHG conversion efficiency on LBO2 and LBO3 temperature. It can be seen that deviating from the optimal temperature causes the conversion efficiency to decrease gradually. FHG conversion efficiency is relatively more tolerant of LBO2 temperature.
Fig. 5. Energy (squares) and conversion efficiency (dots) of FHG as a function of the energy of fundamental pump measured in LBO with crystal lengths of 7 mm (a) and 10 mm (b), respectively.
Fig. 6. (a) The energy ratio between 1064 nm and 355 nm vs. the energy of the fundamental pump. (b) FHG conversion efficiency vs. temperature of LBO2. Inset of (b): FHG conversion efficiency vs. temperature of LBO3.
Figure 7(a) presents the relevant fourth harmonic spectra with the spectrum centered at 266.2 nm. The bandwidth of the FHG is less than 1 nm (FWHM). The far-field spatial profile of the 266 nm beam is shown in the insert of Fig. 7(a). The beam profile is taken based using laser-induced fluorescence on a screen due to the fact that the Si-based CCD is not sensitive to DUV at 266 nm. The intensity distribution of the beam is uniform without a hotspot. The temporal pulse shape for the 266 nm output is shown in Fig. 7(b). The data were measured using a 4 GHz bandwidth oscilloscope (Keysight MSOS404A) with a rise time of 107.5 ps and a fast photodetector (Alphalas UPD-50-UD) whose rise time is 50 ps and spectral range of 170-1100 nm. The FWHM pulse duration is measured to be 297 ps, and the pulse duration of the laser pulse at 266 nm can be found to be ∼270 ps after deconvolution from the rise time of the oscilloscope and photodetector, and the corresponding peak power of DUV is calculated to be 0.49 GW.
Fig. 7. The spectrum (a) and pulse shape (b) of the FHG in LBO. Inset of (a): photographs of the beam spot at 266 nm.
4. Comparison with BBO crystals
4.1 FHG obtained with BBO crystals
Since phase-matching for SHG of 532 nm is possible for BBO crystal, for comparisons, we investigated the generation of FHG in BBO crystal in the high peak power regime. A type-I BBO crystal for frequency quadruple cut at θ=47.7° and φ=0° with dimensions of 13×13×3 mm3 was placed after LBO1. The diameters of the input beam are collimated to 11 mm. Figure 8 shows the energy dependence at 266 nm and FHG conversion efficiency for BBO on 1064 nm input. The maximum output energy at 266 nm was 126 mJ, corresponding to a conversion efficiency of 12.5% from infrared to DUV. The maximum FHG conversion efficiency was 20.2% when the 1064 nm energy was 346 mJ. The results are close to but lower than that of LBO. Even though the deff of BBO is larger than that of LBO, the lower optimized conversion efficiency of BBO than LBO can presumably be attributed to two-photon absorption (TPA).
Fig. 8. The energy and conversion efficiency dependence of FHG on input energy at 1064 nm in a 3 mm-long BBO crystal.
Table 1 lists the major properties of both LBO and BBO crystals. Even if LBO has a relatively small effective nonlinear coefficient deff, it shows a smaller walk-off angle and larger angular acceptance. The larger angular acceptance means a smaller effect of space offset. LBO has the advantages of a high damage threshold, almost non-hygroscopic, high crystal growth yield, and weaker nonlinear absorption in DUV regions.
Table 1. Characteristics of BBO and LBO crystals for 266 nm generation
4.2 Nonlinear absorption
To make sure that the two-photon-absorption (TPA) in BBO could be a reason for a lower conversion efficiency than LBO, we conducted some investigation on the TPA of these two crystals. Since TPA could not be measured directly, measuring the transmittance versus the intensity at 266 nm is more straightforward [19]. When a laser passes through a crystal, its nonlinear absorption is proportional to the square of the instantaneous intensity, and the intensity loss dI/dz is described by the differential equation [24]
where α and β are the linear and TPA coefficient, respectively, and I is the intensity of the laser.According to the formula in [25], the transmittance of BBO and LBO crystals were simulated, and experimental data were measured, as shown in Fig. 9. For BBO crystal, the β-coefficient of 0.9 cm/GW from [26] was adopted, the values of the β are also consistent with our experimental measurements. Due to its larger bandgap, the TPA at 266 nm is weaker for LBO crystal. In addition to the literature [27], few measurements and calculations for β-coefficient of TPA in LBO were found. According to the data results in this work, the values of β could be estimated to be <0.05 cm/GW. To accurately determine the β, a wider range of intensities is required. As evident from Fig. 9, the transmission of BBO decreases nonlinearly with the increase of intensity from 30 to 390 MW/cm2. The transmittance of LBO does not change that significantly in this region. Therefore, one can conclude that BBO has more significant TPA than LBO. The TPA creates color centers, which leads to absorption at the generated UV waves and green waves. Further, it causes thermal phase mismatch and instability of the UV output [28]. In the experiment, we observed that the stability of the produced pulses at 266 nm by LBO is more stable than that by BBO.
Fig. 9. Transmittance of BBO (a) and LBO (b) crystals vs. the intensity at 266 nm. The dots are calculated, and the squares are experimental measurements.
5. Conclusions
In conclusion, we demonstrated a DUV laser at 266 nm that combines high energy, high peak power, and short pulse duration by extracavity sum-frequency mixing in LBO crystal. The maximum output of 428 mJ at 355 nm and 133 mJ at 266 nm were obtained with pump energy of 1004 mJ at 1064 nm. The DUV pulses have a peak power up to 0.49 GW. Moreover, the highest FHG conversion efficiency of 25.7% from the infrared to DUV has been reached with pump energy of 346 mJ at 1064 nm. Furthermore, we compared the FHG conversion properties of LBO and BBO crystals. The LBO crystal has an advantage in high peak power generation of 266 nm due to its lower two-photon absorption for the pump and single-photon absorption of the generated DUV, which also results in a more stable output.
Funding
Scientific Instrument Developing Project of the Chinese Academy of Sciences (YJKYYQ20200001); National Natural Science Foundation of China (62175230, 62175232).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available but may be obtained from the authors upon reasonable request.
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