Open Access
Add to BibSonomy
Abstract
High-energy deep ultraviolet (UV) sources are required for high-density plasma diagnostics. The fifth-harmonic generation of large-aperture neodymium lasers in ammonium dihydrogen phosphate (ADP) can significantly increase UV energies due to the availability of large ADP crystals. Noncritical phase matching in ADP for (ω + 4ω) was achieved by cooling a 65 × 65-mm crystal in a two-chamber cryostat to 200 K. The crystal chamber used helium as the thermally conductive medium between the crystal and the crystal chamber, which was surrounded by a high-vacuum chamber with a liquid nitrogen reservoir. A temperature variation of 0.2 K across the crystal aperture was obtained. The total conversion efficiency from the fundamental to the fifth harmonic at 211 nm was 26%.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1 Introduction
1.1 Reinvigoration of interest in fifth-harmonic generation
Since the fifth-harmonic generation (5HG) of a neodymium laser in a cascade of nonlinear crystals was realized for the first time in 1969 [1], the first peak of 5HG publications occurred in late 1970s and early 1980s. Fifth-harmonic generation with a resulting UV radiation at 211 nm was realized in various types of nonlinear crystals [2–9]. Usually a cascade of three nonlinear processes was used: second-harmonic generation (SHG) to transform the fundamental infrared radiation into green; then another frequency-doubling process or fourth-harmonic generation (4HG); and finally sum–frequency generation from the fourth harmonic and the residual radiation at the fundamental frequency. However, small-aperture beams and relatively low conversion efficiency limited the use of 5HG to low-energy applications [10]. The first detailed analysis of an optimized cascade of crystals was published in 1982 [11] and predicted a high conversion efficiency. Although it was soon experimentally realized [12] with an efficiency of 19%, 5HG remained a topic of academic interest only.
There is now renewed interest in high-energy 5HG, mostly for developing lasers for diagnosing hot dense plasmas, where the higher harmonic provides better penetration of the plasma. The four-color laser, including the fifth harmonic, has been used to investigate Z-pinch [13]. Thomson scattering of a 5ω beam can be used for fusion diagnostics because 5HG fits the spectral region from 180 to 230 nm with less self-generated background from the plasma [14,15]. The sixth harmonic is possible [16] but is extremely difficult to generate and cannot be practically applied. Higher than sixth harmonics cannot be generated in the solid state because of absorption. Inert gases and metal vapors can generate higher harmonics [17], but the conversion efficiency is proportional to media density, i.e., very low. In practice, 5HG is the highest harmonic that can be used effectively.
Recently we demonstrated a record 5HG efficiency of 30%, producing 335 mJ at 211 nm in a 12 × 12-mm beam, using a cesium lithium borate (CLBO) crystal [18]. CLBO has high second-order nonlinearity, can be grown in a relatively large size, and is phase matched at room temperature. An estimated 5ω beam energy of 10 J requires a large-aperture laser and, accordingly, large crystals. While significant progress in understanding 5HG and handling CLBO crystals has been recently achieved [19], this process is still limited by crystal size. Although a CLBO boule could be grown up to 146 × 132 × 118 mm [20], practically, the size of a finished optics does not exceed 5 cm. Furthermore, the extremely hygroscopic property of CLBO crystals requires that they be at high temperatures (∼120°C). Thermal gradients must be minimal to ensure constant phase matching across the crystal for good beam uniformity. Finally, the cost of manufacturing large CLBO crystals is prohibitive for many applications.
Ammonium dihydrogen phosphate (ADP) crystals, which can be easily grown to much larger sizes, are an alternative way of generating a high-energy beam at 211 nm [21]. In this paper, we demonstrate 5HG using a large-aperture ADP crystal and show that the conversion efficiency is comparable to that obtained for CLBO.
1.2 Noncritical phase matching in ADP
Potassium dihydrogen phosphate (KDP) and ADP crystals are popular nonlinear crystals because of their good nonlinear properties, wide range of transmission, and large sizes, which make them most commonly used frequency-conversion crystals for large fusion-class laser systems. For cascade 5HG, however, they have a significant limitation: phase-matching conditions for sum–frequency generation are not met at room temperature. Noncritical phase-matching conditions could be reached by cooling crystals to –140°C (KDP) and –70°C (ADP). This is not trivial, especially for large-aperture crystals, because a definite temperature must be strictly stabilized and maintained across the entire crystal. Any holder that keeps a crystal in the vacuum chamber and maintains a crystal temperature through thermal conductive contact provides some gradient of temperature through a crystal. The most effective way to stabilize an entire crystal under low temperature is a two-chamber cryostat [22], where the internal chamber keeps a crystal almost isolated from a holder but surrounded by a gas atmosphere. The internal chamber is held in the high-vacuum external chamber to minimize heating. ADP is preferable over KDP because of its higher phase-matching temperature.
1.3 Optimization of cascade 5HG
The key point for high-efficiency frequency cascade conversion IR radiation into 5ω is that we must save some portion of the energy at the fundamental frequency (20% in an ideal-case plane wave without any type of absorption and Fresnel reflections), untouched through the first two crystals to mix with the 4ω beam in the third crystal. There are two possible frequency-conversion schemes to optimize 5HG, remembering no Type-II phase matching for 4HG and 5HG exists in KDP and ADP: o1o1→e2:o2o2→e4:o1o4→e5, and o1e1→e2:o2o2→e4:o1o4→e5, where o and e are ordinary and extraordinary waves in crystals.
In the first case, we must detune SHG down from the maximum by adjusting the length of the first crystal, while two other processes (4HG and 5HG) should be maximized. In the second case, the required energy distribution between orthogonal polarizations could be set by rotating the input beam polarization. Here the energy distribution is adjustable for any given input energy, making the second case preferable. A similar analysis was initially developed for third-harmonic generation of fusion lasers [23], and later for 5HG [11].
2. Experiments
2.1 Two-chamber cryostat
The most important part of the experimental setup is the two-chamber cryostat, shown in Fig. 1(a). The tank with liquid nitrogen is connected to the internal chamber through the solid copper (upper) and the hollow stainless-steel (lower) cylinders. The lower hollow cylinder has two 50-W flexible Kapton insulated heaters mounted on the outside surfaces to stabilize the internal chamber temperature. The internal chamber contains the crystal holder. Contacts between the crystal and its holder are minimized to improve the cooling uniformity from the 1 atm of helium that surrounds the crystal. Helium, the main thermal agent between the internal chamber and the crystal, was chosen because of its high (compared with other gases) thermal conductivity. Three silicon diode cryostat temperature sensors are located on two outside points of the internal chamber and on a side of the crystal inside the internal chamber. For better insulation from the thermal radiation, a few (from three to seven) Mylar-aluminum foil layers of multilayer insulation were wrapped around the internal chamber. Two 120-mm-diam, 10-mm-thick fused-silica windows are located on the opposite sides of the internal chamber coaxially to the crystal to pass input and output beams.
Fig. 1. (a) The cross section of the two chamber cryostat with the liquid nitrogen tank (LN), the copper cylinder (Cu), the stainless-steel cylinder (SS) and the internal chamber filled with helium (He) with a crystal (ADP) inside; (b) a photo of an ADP crystal inside the two-chamber cryostat through two input windows.
“Cold flow” goes down from the liquid nitrogen tank to the internal chamber, then reaches the crystal through the helium. As soon as the temperature of the crystal (or the internal chamber wall, depending on which temperature sensor is chosen as the control) reaches a chosen set point temperature, the heaters begin working to maintain that temperature through a temperature-stabilization loop. A high-performance cryogenic temperature controller is used to monitor and control temperature within the internal chamber to better than 0.01°C resolution. Each of the two heaters mounted on the lower cylinder are controlled by a proportional integral derivative feedback loop. The feedback continually adjusts the output power to the heaters in order to keep the chosen temperature constant. The system has high thermal mass and reaches the target 200 K temperature in about 36 h.
The internal chamber is installed into the external chamber and pumped down to a vacuum of better than 5 × 10−7 Torr. The external chamber also has input and output windows, so the two-chamber cryostat has a total of four windows. Two input windows have sol-gel antireflection (AR) coatings at 266 nm (4ω), while the output windows are coated at 211 nm (5ω). The ADP crystal (65 × 65 × 10 mm, Type I, θ = 90°, ϕ = 45°) has AR at lω and 4ω on the input face and uncoated output face. While the ADP crystal was examined periodically at cryo temperatures over two years, no degradation of the crystal was observed.
A special heavy-duty rotation stage was designed and fabricated to carry this large, heavy two-chamber cryostat and rotate it within an angular range of 5° with microradian accuracy. As a result, phase matching could be tuned by both crystal temperature and angle.
The thermal model was developed based on COMSOL Multiphysics and was able to predict the lowest reachable temperature and a cooling time to reach that temperature. Measured (solid) and predicted (dashed) temperatures of the crystal (red) and the internal chamber (blue) are shown in Fig. 2 without temperature stabilization, while the heater remains turned off. The black dashed line shows the non-critical phase-matching temperature for 5HG in ADP.
Fig. 2. Measured (solid) and predicted (dashed) temperatures of the crystal (red) and the internal chamber (blue) at the time of cooling. The black dashed line is the non-critical phase matching temperature for 5HG in ADP.
2.2 Experimental setup
The crystal orientations relative to the input beam polarization are shown in Fig. 3(a). The angle α between input-beam polarization and the horizontal plane was tuned using the HWP before the first crystal, changing the balance of energy between the ordinary and extraordinary axes in the first Type-II doubler and preserving some fraction of the fundamental frequency beam through the first two crystals for the interaction in the last crystal. The first frequency doubler was a deuterated potassium dihydrogen phosphate (DKDP) crystal (30 × 30 × 27 mm), which was chosen instead of KDP to decrease linear absorption at the fundamental frequency. It was cut in a Type-II configuration to convert 1ω → 2ω. A second frequency doubler, a Type-I KDP crystal (30 × 30 × 15.5 mm), was used to convert 2ω → 4ω.
Fig. 3. (a) Schematic showing the orientation of the crystal axes and polarizations. The angle (α) of the 1ω polarization was set using a half-wave plate (HWP) for optimal conversion. (b) Experimental setup showing the input laser beam, conversion crystals, and energy diagnostics for each frequency.
The experimental setup is shown in Fig. 3(b) with the cascade of three nonlinear crystals. We chose the same setup we used for 5HG in CLBO crystals [18] so that the results can be compared. The final crystal, made of ADP and contained within the two-chamber cryostat, was located at the image plane of a Nd:YLF laser [24] that was optimized to produce a flat-topped, square-beam profile with a square pulse (1053 nm, 12 × 12 mm, from 1 ns to 2.8 ns, ≤1.5 J, 5 Hz, 0.1 Hz, or a single shot). The fused-silica prism separates the harmonic beams in space. The input and output beam energies were measured using identical cross-calibrated pyroelectric energy meters. All beam profiles were recorded.
3. Results
The frequency-conversion efficiency from 1ω →2ω was ∼70% at an input beam intensity of 0.55 GW/cm2 in the case of an equal split of 1ω input into the two polarization axes of the doubler crystal. For 5HG it was detuned by input beam polarization rotation. The frequency conversion from 2ω → 4ω was ∼80% at an intensity of the second-harmonic beam of 0.40 GW/cm2, demonstrating a good agreement with plane-wave conversion calculations [18].
Calculations based on Refs. [25] and [26] predict the temperature of noncritical phase matching for 5HG in ADP around 222.4 K. On the contrary, we reached 5HG at –73°C (200 K). It is very close to the temperature of –67.5°C (205.5 K) reported in Ref. [22], which was measured by a different temperature measurement system based on a wire resistance gauge. It is also similar to data from earlier experiments [1]. Therefore, it appears that the reference data of refraction index versus temperature Refs. [25] and [26] are not correct.
The temperature acceptance of 5HG in ADP at a fixed crystal angle was measured (see Fig. 4). Each point was taken without temperature stabilization at a given temperature: 5ω energy was measured while the temperature of the ADP crystal was slowly drifting. “Red” data were taken while the crystal temperature was increasing, and “blue” data were taken while crystal temperature was decreasing. Note that the red curve was shifted down by 0.87 K to match the blue curve. This difference likely comes from the temperature gradient without stabilization and system lag because of the thermal mass.
Fig. 4. Fifth-harmonic energy temperature (T) acceptance at a fixed position angle of the ADP crystal.
An important point here is that the temperature acceptance is only 0.4 K (FWHM). To keep the system close to the maximum (>90%) of 5HG efficiency, an ADP crystal must be temperature stabilized with an accuracy better than 0.1 K. Angular acceptance of 5HG at a given temperature of the ADP crystal was also measured (Fig. 5). The measured acceptance does not agree with the simulations based on the Sellmeier equations from Ref. [26], as previously been mentioned. According to the Sellmeier equations, such calculated dependence was close to the experimental one at the lower calculation temperature.
We optimized the input polarization relative to the crystal axis. Figure 6 demonstrates 5HG’s sensitivity to input polarization rotation. That dependence was taken under maximum input energy and the best α angle of 52° is significantly different from α = 45°, which is the best at low input beam intensities.
To maximize conversion efficiency into 5ω we must realize phase matching over the full beam aperture, which requires temperature stabilization of the ADP crystal with an accuracy better than 0.1 K across a whole crystal. Figure 7 shows beam profiles of the input beam at the fundamental frequency (a) before the first crystals and (b) fifth-harmonic beam after the oven. Compared to the relatively uniform 1ω beam, the 5ω beam is slightly varied spatially. Some residual radial nonuniformity of the 5ω beam was caused by a small phase mismatch inside the ADP and corresponds to a temperature gradient over ADP crystal. Overall conversion efficiency could be better with improving temperature uniformity of ADP crystal.
Fig. 7. Input beam image (a) at the fundamental frequency on the front of the cascade of crystals and fifth-harmonic output beam image (b) after the cryostat.
Because the ADP crystal is isolated in a helium atmosphere with very low direct conductivity to the chamber, even small energy absorbing from interaction laser beams cannot disappear promptly. That determines very important limitation of ADP and other crystals in a gas cryostat; they work well in single-shot regime (about one shot per minute), but cannot work at a high repetition rate because of phase mismatch caused by laser heating. Figure 8 shows the sequence of fifth-harmonic output beam images behind the cryostat taken after different number of shots running at 5-Hz repetition rate. Even a small amount of absorbed laser energy inside a crystal after the first shot makes the crystal warmer in the center and disturbs temperature distribution, resulting in a spatially varying phase mismatch that grows as a series of radial rings. The 5ω beam almost disappears after 24 shots. This effect is negligible provided there is sufficient time for the crystal to thermalize. A one-minute interval between shots is enough to maintain required temperature distribution of 0.1 K over crystal aperture.
Fig. 8. Fifth-harmonic output beam images after the cryostat taken after different number of shots running at 5-Hz repetition rate.
After careful optimization, the 5HG efficiency became as high as 26% at a 0.1-Hz repetition rate. The fifth-harmonic efficiency η(5ω), shown in Fig. 9, is defined as the ratio of 5ω output energy after the chamber with ADP crystal to the 1ω energy at the input of the first (DKDP) crystal. The maximum η(5ω) conversion efficiency of 26% was reached with a 2.4-ns pulse and an input intensity of 0.3 GW/cm2. This definition of efficiency describes the portion of the input 1ω energy that has been transformed into the fifth harmonic and is available at the output of the cascade of crystals for use in any application, and includes linear and nonlinear loss mechanisms.
Fig. 9. Fifth-harmonic efficiency and energy balance measured as a function of input-pulse energy and intensity.
The energy balance B, which is also shown in Fig. 9, is the ratio of the total energy of all beams after the cryostat to the 1ω energy at the input of the first crystal of the cascade. Therefore, B represents a fraction of energy transmitted from input to output through all three crystals and the cryostat. (That is, a lossless system has a B equal to unity.) The initial balance at very low input beam intensity corresponds to passive losses, mostly coming from linear absorptions and Fresnel reflections. It also demonstrates that total conversion efficiency of 5HG could be increased by, for example, better AR coatings and crystalline windows [27]. Significant nonlinear losses of the total energy transmitted through the system at a high level of the input intensity beam result mainly from two-photon absorption (TPA).
Using the 5ω beam from the above describe setup we measured TPA at 211 nm in the air and in the another longer (15 mm) ADP crystal cut at x-plane. Figure 10 presents energy transmission of the 13.5 m distance of the air and of the ADP crystal in air at room temperature. TPA coefficient of ADP at 211 nm was measured as (1.2 ± 0.2) cm/GW. Due to a relatively small dynamic range of the input energy that measurement is not very accurate. Difference in TPAs of ADP in different crystal axis is within the error of measurement. That number is about 10× lower than TPA in ADP from [12], but their numbers were estimated by conversion efficiency, not by direct measurements. We also measured TPA in the air (0.0008 ± 0.0002 cm/GW) and in fused silica (0.5 ± 0.1 cm/GW) at 211 nm and in ADP at 263 nm (0.25 ± 0.1 cm/GW).
Fig. 10. Transmission of the air and the ADP crystal in the air at room temperature measured at 211 nm as a function of intensity of input fifth harmonic.
4. Conclusions
ADP crystals are an alternative to CLBO to generate deep UV radiation for hot dense plasma diagnostics. Two-chamber cryostats can stabilize the temperature across very large-sized nonlinear crystals with accuracy better than 0.1 K. Energy conversion efficiency from the input beam at fundamental frequency into an useful 5ω beam after the cryostat is 26%—comparable to the efficiency measured when using CLBO crystals.
Funding
National Nuclear Security Administration (DE-NA0003856); University of Rochester; New York State Energy Research and Development Authority; Lawrence Livermore National Laboratory (DE AC52 07NA27344).
Acknowledgments
The authors thank Steven Yang, Philip Datte, and Sid Patankar at Lawrence Livermore National Laboratory for the support and helpful discussions. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856, the University of Rochester, and the New York State Energy Research and Development Authority and the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE AC52 07NA27344. This report was prepared as an account of work sponsored by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof.
Disclosures
The authors declare no conflicts of interest.
References
1. A. G. Akmanov, S. A. Akhmanov, B. V. Zhdanov, A. I. Kovrigin, N. K. Podsotskaya, and R. V. Khokhlov, “Generation of coherent radiation at λ = 2120 å by cascade frequency conversion,” J. Exp. Theor. Phys. Lett. 10(6), 154–157 (1969).
2. G. A. Massey, “Efficient upconversion of long-wavelength UV light into the 200–235-nm band,” Appl. Phys. 24(8), 371–373 (1974). [CrossRef]
3. G. Massey, M. Jones, and J. Johnson, “Generation of pulse bursts at 212.8 nm by intracavity modulation of an Nd:YAG laser,” IEEE J. Quantum Electron. 14(7), 527–532 (1978). [CrossRef]
4. M. Jones and G. Massey, “Milliwatt-level 213 nm source based on a repetitively Q-switched, cw-pumped Nd:YAG laser,” IEEE J. Quantum Electron. 15(4), 204–206 (1979). [CrossRef]
5. K. Kato, “Phase matched generation of 2128 å in KB5O8 • 4H2O,” Opt. Commun. 19(3), 332–333 (1976). [CrossRef]
6. A. G. Arutyunyan, V. G. Atanesyan, K. B. Petrosyan, and K. M. Pokhsraryan, “Frequency multiplication of ultrashort light pulses in potassium pentaborate,” Sov. Tech. Phys. Lett. 6(3), 120–121 (1980).
7. K. Kato, “High-efficiency high-power UV generation at 2128 å in urea,” IEEE J. Quantum Electron. 16(8), 810–811 (1980). [CrossRef]
8. C. Chen, Y. X. Fan, R. C. Eckardt, and R. L. Byer, “Recent developments in barium borate,” Proc. SPIE 681, 12–17 (1986). [CrossRef]
9. Y. K. Yap, M. Inagaki, S. Nakajima, Y. Mori, and T. Sasaki, “High-power fourth- and fifth-harmonic generation of a Nd:YAG laser by means of a CsLiB6O10,” Opt. Lett. 21(17), 1348–1350 (1996). [CrossRef]
10. A. Lago, R. Wallenstein, C. Chen, Y. X. Fan, and R. L. Byer, “Coherent 70.9-nm radiation generated in neon by frequency tripling the fifth harmonic of a Nd:YAG laser,” Opt. Lett. 13(3), 221–223 (1988). [CrossRef]
11. S. B. Arifzhanov, A. A. Ghulamov, E. A. Yerofeyev, and T. Usmanov, “Optical beam and pulse profiling in fifth-harmonic generation,” in Proceedings of the International Conference on Lasers ‘82, R. C. Powell, ed. (STS Press, McLean, VA, 1982), pp. 300–303.
12. I. A. Begishev, R. A. Ganeev, A. A. Gulamov, E. A. Erofeev, S. R. Kamalov, T. Usmanov, and A. D. Khadzhaev, “Generation of the fifth harmonic of a neodymium laser and two-photon absorption in KDP and ADP crystals,” Sov. J. Quantum Electron. 18(2), 224–228 (1988). [CrossRef]
13. V. V. Ivanov, A. A. Anderson, and I. A. Begishev, “Four-color laser diagnostics for Z-pinch and laser-produced plasma,” Appl. Opt. 55(3), 498–501 (2016). [CrossRef]
14. J. S. Ross, S. H. Glenzer, J. P. Palastro, B. B. Pollock, D. Price, G. R. Tynan, and D. H. Froula, “Thomson-scattering measurements in the collective and noncollective regimes in laser produced plasmas,” Rev. Sci. Instrum. 81(10), 10D523 (2010). [CrossRef]
15. P. Datte, J. S. Ross, D. Froula, J. Galbraith, S. Glenzer, B. Hatch, J. Kilkenny, O. Landen, A. M. Manuel, W. Molander, D. Montgomery, J. Moody, G. Swadling, J. Weaver, G. Vergel de Dios, and M. Vitalich, “The preliminary design of the optical Thomson scattering diagnostic for the National Ignition Facility,” J. Phys.: Conf. Ser. 717, 012089 (2016). [CrossRef]
16. G. Wang, X. Wang, Y. Zhou, Y. Chen, C. Li, Y. Zhu, Z. Xu, and C. Chen, “12.95 mW sixth harmonic generation with KBe2BO3F2 crystal,” Appl. Phys. B 91(1), 95–97 (2008). [CrossRef]
17. R. A. Ganeev, “Harmonic generation from partially ionized plasma [invited],” J. Opt. Soc. Am. B 31(9), 2221–2231 (2014). [CrossRef]
18. I. A. Begishev, J. Bromage, S. T. Yang, P. S. Datte, S. Patankar, and J. D. Zuegel, “Record fifth-harmonic-generation efficiency producing 211 nm, joule-level pulses using cesium lithium borate,” Opt. Lett. 43(11), 2462–2465 (2018). [CrossRef]
19. S. Patankar, S. T. Yang, J. D. Moody, A. J. Bayramian, G. F. Swadling, D. Barker, P. Datte, G. Mennerat, M. Norton, C. W. Carr, I. A. Begishev, J. Bromage, and J. S. Ross, “Understanding fifth-harmonic generation in CLBO,” Proc. SPIE 10516, 1051603 (2018). [CrossRef]
20. X. Yuan, G. Shen, X. Wang, D. Shen, G. Wang, and Z. Xu, “Growth and characterization of large CLBO crystals,” J. Cryst. Growth 293(1), 97–101 (2006). [CrossRef]
21. I. A. Begishev, M. Romanofsky, S. Carey, R. Chapman, G. Brent, M. J. Shoup III, J. D. Zuegel, and J. Bromage, “High-efficiency, large-aperture fifth-harmonic generation of 211-nm pulses in ammonium dihydrogen phosphate crystals for fusion diagnostics,” Proc. SPIE 10898, 108980N (2019). [CrossRef]
22. I. A. Begishev, R. A. Ganeev, V. V. Gorbushin, A. A. Gulamov, S. R. Kamalov, and T. Usmanov, “System of cryostating wide-aperture nonlinear crystals for generating the fifth harmonic of neodymium-laser radiation,” J. Appl. Spectrosc. 51(5), 1218–1221 (1989). [CrossRef]
23. W. Seka, S. D. Jacobs, J. E. Rizzo, R. Boni, and R. S. Craxton, “Demonstration of high efficiency third harmonic conversion of high power Nd-glass laser radiation,” Opt. Commun. 34(3), 469–473 (1980). [CrossRef]
24. V. Bagnoud, M. J. Guardalben, J. Puth, J. D. Zuegel, T. Mooney, and P. Dumas, “High-energy, high-average-power laser with Nd:YLF rods corrected by magnetorheological finishing,” Appl. Opt. 44(2), 282–288 (2005). [CrossRef]
25. G. G. Gurzadian, V. G. Dmitriev, and D. N. Nikogosian, Handbook of Nonlinear Optical Crystals, 3rd rev. ed., Springer Series in Optical Sciences, Vol. 64 (Springer-Verlag, Berlin, 1999).
26. A. V. Smith, Crystal Nonlinear Optics: With SNLO Examples, 2nd ed. (AS-Photonics, Albuquerque, NM, 2019), p. 782.
27. S. Patankar, S. T. Yang, J. D. Moody, G. F. Swadling, A. C. Erlandson, A. J. Bayramian, D. Barker, P. Datte, R. L. Acree, B. Pepmeier, R. E. Madden, M. R. Borden, and J. S. Ross, “Two-photon absorption measurements of deep UV transmissible materials at 213 nm,” Appl. Opt. 56(30), 8309–8312 (2017). [CrossRef]
