1. Introduction

Inertial confinement fusion (ICF), as one of the most probable ways to realize controlled thermonuclear fusion, is expected to provide mankind with nearly unlimited clean energy [1]. The research work in the field of ICF is undoubtedly of great significance to the national economy, and military applications, as well as to the exploration of basic research. The laser driver, as a core ICF device, has attracted much attention and research since it was proposed [2–6]. Potassium dihydrogen phosphate (KH₂PO₄, KDP) and its deuteride (KD_xH_(2-x)PO₄, DKDP) have the advantages of high nonlinear coefficients, wide transmission range, and large size [7], and they are the only nonlinear optical crystal materials that can satisfy the throughput aperture of ICF laser drive devices [8–10]. As the terminal optical component of frequency conversion in ICF, frequency conversion efficiency and laser-induced damage threshold (LIDT) are two important core indexes. At present, the KDP/DKDP crystals have already met the requirements in terms of frequency conversion efficiency, but they are mainly limited by the fact that the LIDT is much lower than the theoretical calculation results of the valence bonding structure, which can't fully satisfy the requirements of the ICF laser driver operating fluence. Researchers have been studying damage in KDP-like crystals for more than half a century [11–16], and have also developed a consistent understanding such as damage in crystals in vivo is mainly induced by defects, but the understanding of the intrinsic properties of the defects of the damage-inducing source is still unclear.

Currently, the operating wavelength of the ICF laser drive source is mostly chosen to be 351 nm, such as the National Ignition Facility (NIF) in the U.S. [17], the Laser Megajoule (LMJ) in France [18], and the SG-III facility in China [19]. Since short wavelengths are more effective in coupling with plasma and are favorable for particle compression, the FHG of 1µm-band lasers is expected to be the preferred candidate for next-generation ICF facilities. The ammonium dihydrogen phosphate (NH₄H₂PO₄, ADP) has a similar crystal structure to KDP and belongs to the tetragonal system [20,21]. Compared with KDP/DKDP crystal, ADP has larger nonlinear optical (NLO) coefficients d_eff (∼ 1.1 times that of KDP), higher LIDT (∼ 2 times that of KDP), and shorter ultraviolet transmission cut-off, which is mostly used to achieve second (2ω), third (3ω), fourth (4ω), and fifth harmonic (5ω) generation [22–24]. For example, Qi et al. demonstrated efficient third-harmonic-generation (THG) conversion at 1064 nm in ADP crystal using a single-crystal birefringent phase-matched cascade THG technique [25]. In 2013, Ji et al. reported non-critical phase-matched (NCPM) fourth-harmonic-generation (FHG) of ADP crystal in a 1053 nm laser with a 2ω-to-4ω conversion efficiency of 70% [24]. Wang et al. realized an efficient NCPM fifth-harmonic-generation (FIHG) for Nd: glass laser with a maximum light conversion efficiency of 14% by temperature-controlled ADP crystal at -75.1 °C [26]. In addition, the crystal size of point-seed rapid-growth ADP has been able to meet the requirements of large-aperture laser devices [27]. In previous reports, only some of the work has focused on the defect-induced damage behaviors of ADP crystals [28,29], and the effect of temperature on the LIDT of ADP crystals in nonlinear frequency transformation has largely not been studied. Therefore, the study of the effect of temperature on the ADP crystal LIDT in nonlinear frequency transformation is crucial for its application in high-power laser systems.

In this work, the study samples were cut from rapid-growth large-size ADP crystals along the room-temperature type-I PM FHG and high-temperature type-I PM FHG directions at a fundamental frequency of 1064 nm, respectively. LIDTs were tested by the R-on-1 method in FHG experiments. The conversion efficiency, output energy, and acceptance angle bandwidth measurements were implemented to characterize ADP crystals’ FHG performance. The study would be beneficial for understanding the effect of temperature on the performance of ADP crystals in FHG, and provide some references for future ICF facilities FHG laser drive technology solutions.

2. Experimental

ADP crystal was grown via the point-seed rapid growth method from supersaturated NH₄H₂PO₄ aqueous solutions. ADP crystals were cut along the room-temperature type-I FHG direction (θ = 79°, ϕ = 45°) and high-temperature type-I FHG direction (θ = 90°, ϕ = 45°), respectively. The detailed cutting style is demonstrated in Fig. 1(a). Both ADP samples had the dimension of 8 × 8 × 10 mm³, and the transmittance faces of the sample were polished but uncoated. With a spectrophotometer (PerkinElmer Lambda 1050+), the transmission spectrum of ADP crystal was measured, and the result was shown in Fig. 1(b). Excellent UV transmittance is the basic condition for realizing a high damage threshold and high conversion efficiency UV frequency conversion.

 

Fig. 1. (a) Processing schematic of ADP samples. (b) Transmission spectrum of ADP samples.

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The experimental setup for FHG of ADP crystal is shown in Fig. 2. The fundamental light source A was a pulsed Nd: YAG laser (Continuum, SLII-10), which can generate 1064 nm, 7 ns pulses at a repetition rate of 10 Hz. To further improve the beam quality of the fundamental light, a shaping diaphragm B was used to intercept the facula. A KDP (θ = 41°, ϕ = 45°, type-I PM) was used as the second harmonic generation (SHG) crystal C. Mirror D was a dichroic mirror with high reflective (HR) at 1064 nm (R = 99%) and high transmitted (HT) at 532 nm (T = 89%). The SHG energy is detected with the sampling system made of beam splitter E (PR @ 532 nm) and energy calorimeter J. The SHG light was focused into FHG crystal G by a convex lens F whose focal length was 500 mm. For the FHG part, the ADP crystal was mounted in a temperature-controlling device that can vary the temperature from room temperature to 180 °C with a control precision of 0.1 °C. The temperature-controlling device was mounted on a precise rotating stage with a resolution of 0.1°. Mirror H serves as the filter of the FHG laser (HR @ 532 nm, HT @ 266 nm). Energy calorimeter I was used to detect FHG laser energy.

 

Fig. 2. Experimental setup for the FHG experiments. A. 1064 nm fundamental laser; B. shaping diaphragm; C. SHG crystal; D., H. dichroic mirror; E. beam splitters; F. convex lens; G. FHG crystal; I., J. energy calorimeters.

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3. Results and discussion

3.1 Fourth harmonic generation performance

In the room-temperature FHG experiments, the temperature is 23 °C, and the corresponding PM angle of ADP crystal is θ = 79° and ϕ = 45° (type-I PM). In the high-temperature FHG experiments, NCPM can be achieved by adjusting the crystal temperature to make the ADP crystal FHG PM angle aim the direction θ = 90°. Theoretical calculations show that the PM angle of the ADP crystal realizing type-I PM FHG increases with increasing temperature, and the PM angle reaches up to 90°, i.e., the NCPM angle, which corresponds to the highest temperature of the ADP crystal realizing FHG (∼50 °C). The optimal NCPM temperature (T_NCPM) was found by recording the variation of the 266 nm output energy with the slow warming of the ADP crystal. Fig. S1 shows the variation of FHG efficiency with temperature, the curve was fitted by sin²x/x² to evaluate the T_NCPM and the FWHM of the curve. The fitted curve shows that the T_NCPM is 48.9 °C with a temperature acceptance bandwidth (ΔT_ADP) of 1.21 °C. Because the crystal length was 10 mm, the temperature bandwidth and temperature phase-mismatch sensitivity (∂Δk/ΔT) were determined to be 1.21 °C·cm and 4.6 °C^-1·cm^-1 for ADP, respectively. The FHG performance of ADP crystals at two temperatures are shown in Fig. 3. In terms of output energy, as shown in Fig. 3(a), the largest 4ω energy of 1.51 mJ appeared at the 2ω energy of 6.91 mJ in the room-temperature FHG experiments, and largest 4ω energy of 3.21 mJ appeared at the 2ω energy of 8.37 mJ in the high-temperature FHG experiments. For FHG conversion efficiency, as shown in Fig. 3(b), the highest 4ω conversion efficiencies in the room-temperature and high-temperature experiments are 22.3% and 38.1%, respectively. Compared to the room temperature, the largest 4ω output energy and maximum 4ω conversion efficiency in the high-temperature are improved by 113% and 71%, respectively. It can be seen that the FHG performance can be greatly improved by elevating the temperature of the ADP crystal. The stability of the 4ω output power of ADP crystals was recorded in Fig. 3(c). The energy volatility over 20 minutes in the room temperature and high-temperature experiments are about 1.9% and 2.7%, respectively. The difference in energy stability between the two experimental conditions is not significant, and the slightly larger energy volatility in the high-temperature experiment is due to the crystal length as well as temperature fluctuations in the temperature-controlled furnace chamber.

 

Fig. 3. The FHG performance of ADP crystals: (a) output energy (Pentagram: energy of damage to the samples), (b) conversion efficiency, and (c) energy stability. (Blue data: room temperature, Red data: high temperature).

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As shown by the pentagram marking in Fig. 3(a), we continued to increase the energy of the 532 nm beam after the FHG output energy had reached its maximum until the ADP crystal showed damage, i.e., the output energy of the 266 nm light decreased significantly. For room-temperature FHG experiments, when the 2ω energy increased to 9.22 mJ, the 4ω output energy decreased to 1.34 mJ, the conversion efficiency decreased to 14.3%, and a damage point appeared on the front surface of the crystal. The optical photograph of the ADP crystal laser damage point is shown in Fig. 4(a), and the diameter of the damaged spot is about 200 µm. For high-temperature FHG experiments, when the 2ω energy increased to 11.01 mJ, the 4ω output energy decreased to 2.56 mJ, the conversion efficiency decreased to 23.3%, and a damage point appeared on the front surface of the crystal. The optical photograph of the ADP crystal laser damage point is shown in Fig. 4(c), and the diameter of the damaged spot is about 200 µm.

 

Fig. 4. The optical photograph of the ADP crystals laser damage point at (a) 23°C (79°, 45°), (b) 23°C (90°, 45°), (c) 48.9°C (90°, 45°).

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3.2 Laser-induced damage threshold

To investigate the effect of temperature on the LIDT during FHG, we tested the LIDT of two tangential ADP crystals at different temperatures using the R-on-1 approach. As mentioned earlier, the calculated LIDT of ADP crystals under room-temperature and high-temperature conditions are 4.19 and 5.01 GW/cm², respectively. Since ADP crystals realized FHG at different temperatures in different tangential directions, the tangential directions also impact the LIDT results [30]. In order to exclude the effect of tangency on the LIDT results of ADP crystals, we tested the LIDT of (90°, 45°)-cut ADP crystal at room temperature. At the same position of the (79°, 45°)-cut ADP crystal, a damaged spot appeared on the front surface of the crystal when the 2ω energy reached 9.5 mJ, and the calculated LIDT was 4.32 GW/cm². Figure 4 shows the optical photographs of the laser damage points of two tangential ADP crystals at different temperatures, and the damage points’ diameters are about 200 µm. Under room temperature, we can see that the LIDTs of (90°, 45°)-cut ADP and (79°, 45°)-cut ADP have little difference, while the LIDT of (90°, 45°)-cut ADP at 48.9 °C is higher than both, with an approximate enhancement of 16% ∼ 20%. It indicates that under high-temperature thermal conditioning, the LIDT of ADP crystal has obvious improvement. We think there are two main reasons for this: (1) Appropriately increasing the temperature of the entire ADP crystal causes the thermal motion of the lattice atoms to become intense, but does not destroy the perfection of the crystal structure. This helps the crystal defects (including impurity defects, vacancies, and dislocations) to adjust to the right position when enough energy is obtained [31], which in turn improves the LIDT. (2) In the ADP crystal FHG experiment, 2ω and 4ω laser conditioning processes were included. Previous studies have shown that subthreshold laser energy density conditioning at short wavelengths is more helpful in improving the LIDT of KDP-like crystals [30]. Compared to CPM at room temperature, NCPM at high temperatures produces higher 4ω energy and the resulting laser conditioning is more effective. In addition, according to previous studies, the LIDT of DADP crystals becomes lower after deuteration, and the trend of change is the same as that of DKDP crystals [7]. This is mainly because the strength of hydrogen bonding becomes weaker after deuteration, and when impurity defects and other defects absorb laser energy, the probability of destruction by thermal effect increases, resulting in a decrease in the LIDT of the crystal [32]. Combined with our experimental results, high-temperature NCPM is the best FHG working condition for ADP crystals.

3.3 Angle tuning

Another non-negligible advantage of high-temperature NCPMs is their large angular acceptance bandwidth, which reduces the requirement for beam wavefront as well as beam pointing stability during the harmonic conversion process. For room temperature, keeping the 4ω output energy at 1.5 mJ, the PM angular sensitivity was measured by monitoring the fourth harmonic output as the ADP crystal rotated in the θ direction through an angle of 79°. As shown in Fig. 5 RT-CPM, the full-width at half-maximum (FWHM) angular acceptance of ADP is 7.3 mrad, which is equivalent to an external angular acceptance of 7.3 mrad·cm^1/2 and an angular phase-mismatch sensitivity (∂²Δk/Δ²θ) of 0.42 mrad^-2·cm^-1. This is consistent with the high angular sensitivity in CPM. For high temperature, keeping the temperature of ADP at 48.9 °C, we measured the PM angular sensitivity by monitoring the fourth harmonic output as the crystal rotated in the θ direction through an angle of 90°. Based on the fitting results in Fig. 5 HT-NCPM, the angular acceptance bandwidth of ADP is 52.8 mrad, which is 6 times higher compared to the room-temperature process. The external angular acceptance and angular phase-mismatch sensitivity of ADP are 52.8 mrad·cm^1/2 and 7.98 × 10⁻³ mrad^-2·cm^-1, respectively. For the angular acceptance bandwidth, our results are 1.4 times the NCPM FHG of the DKDP crystals at 1064 nm previously reported by Zhang et al. [33], which is similar to the results previously obtained at 1053 nm by Ji et al. [24].

 

Fig. 5. External angular tuning of FHG in ADP crystals. (Blue data: room temperature, Red data: high temperature)

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4. Conclusion

In conclusion, we improve the fourth harmonic generation performance by elevating the temperature of ADP crystal. The main parameters and results involved in the whole experiment are shown in Table 1. For high-temperature FHG, the energy, conversion efficiency, and angular acceptance range are increased by 113%, 71%, and 623%, respectively. The LIDT is 5.01 GW/cm², compared to the (79°, 45°)-cut ADP crystals at 23 °C, the LIDT of (90°, 45°)-cut ADP crystals at 48.9 °C has an approximate enhancement of 19.6%. This suggests that by ramping up the temperature, the ADP crystals not only exhibit high conversion efficiency during FHG but also have a higher LIDT. It shows that elevating ADP temperature is an efficient method to improve its deep UV frequency conversion properties, which may also be available to other NLO crystals. This discovery provides a very valuable technology for the future development of UV, deep UV lasers in ICF facilities.

Table 1. A summary of FHG experimental results

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