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DOI: 10.1039/D5CE00610D (Paper) CrystEngComm, 2025, Advance Article

Precise deuterium content measurement of K(HxD1−x)2PO4 crystals based on deep-UV generation

Yiqun Shiab, Zijian Cui*a, Junze Xua, Mingying Sun*a, Xiuqing Jianga, Li Yina, Xinglong Xiea, De'an Liuc and Jianqiang Zhua
aKey Laboratory of High Power Laser and Physics, Shanghai Institute of Optics and Fine Mechanics, CAS, Shanghai 201800, China. E-mail: cuizijian@siom.ac.cn; sunmy@siom.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
cZhangjiang Laboratory, Shanghai, 201210, China

Received 16th June 2025 , Accepted 26th July 2025

First published on 29th July 2025

Abstract

Large-diameter K(HxD1−x)2PO4 (DKDP) crystals have been widely applied for optical parametric chirped pulse amplification and broadband UV generation in superintense ultrafast lasers and inertial confinement fusion facilities. Herein, we systematically investigated the noncritical phase-matching wavelength properties of KDP-family crystals at room temperature and demonstrated a precise characterization method for the deuterium contents of large-diameter DKDP crystals based on the ∼245 and ∼215 nm deep-UV sum-frequency generation (SFG). This SFG measurement method overcomes the low accuracy disadvantage of the Raman spectroscopy, especially for the DKDP crystals with deuterium contents of >90%. Full-range (0–100%) measurement with a precision of ∼0.148% cm was achieved, which is an enhancement of more than one order of magnitude over conventional spectroscopic schemes. This work not only has the potential to be a standard method for the deuterium content characterization of DKDP crystals, but also paves the way for energy improvement and applications of tunable deep-UV radiation by fully utilizing large-diameter characteristics of KDP-family crystals.

1. Introduction

Due to the excellent optical properties of low half-wave voltage, large electron-optic coefficient, wide transmission bands from near-IR to deep-UV, large nonlinear optical coefficients, and high laser damage thresholds, the KH2PO4 (KDP) crystal and its isomorph, the K(HxD1−x)2PO4 (DKDP) crystal, have been widely used to make electron-optic switches and various laser frequency converters in the fields of inertial confinement fusion (ICF) and superintense ultrafast lasers.1–5 The DKDP crystal is a crystal that forms when the hydrogen atoms (1H) present in a KDP crystal are partially or completely substituted with deuterium atoms (2H or D). Parameter x represents the average proportion of hydrogen sites on the PO4 groups in the crystal occupied by deuterium atoms. The chemical and physical properties of DKDP crystals are closely related with the deuterium content, which enables researchers to meet application requirements in different fields by adjusting the deuterium content in the solution during the growth of DKDP crystals.6–8

Because of the limitation of laser damage thresholds, large-diameter DKDP crystals are indispensable for high-energy ICF laser devices. Compared with the KDP crystal, DKDP crystals have lower transverse stimulated Raman scattering (TSRS) coefficients, which is beneficial for avoiding TSRS damage.9–11 For the χ(2) nonlinear optical process, such as the second-, third-, fourth-harmonic generation (SHG, THG, and FHG), and optical parametric chirped pulse amplification, phase-matching conditions are strongly affected by the deuterium content and ordinary and extraordinary refractive indices of nonlinear crystals.12–15 For the THG, a uniformity of 2% deuterium content is required. And for the type-I noncritical phase-matching (NCPM) FHG, the deuterium content change cannot exceed 0.4%.16 The deuterium-content uniformity of the large-diameter DKDP crystals will directly affect the harmonic generation efficiency and the beam quality of high-power laser devices. Furthermore, the broadband low-coherence light is expected to overcome the laser-plasma instability in ICF.17,18 The broadband third-harmonic generation based on the gradient deuterium DKDP crystal with refractive index gradient variation is a novel approach.19 Therefore, the precise determination of the deuterium content changes of the gradient deuterium DKDP crystals is the key to the practical application.

Several methods are commonly employed, each with its unique advantages and limitations. Neutron powder diffraction (NPD) is considered as a standard method to measure the deuterium content of DKDP crystals, yet its application necessitates access to expensive neutron sources.20 Thermal gravity analysis (TGA) provides favorable accuracy. However, it is destructive to crystal samples.21 Loiacono et al. proposed the variation in ferroelectric transition temperature (FFT) of DKDP crystals to measure the deuterium content.22 However, this method cannot obtain the distribution of deuterium content. Both IR and Raman spectroscopy are non-destructive, operationally convenient, and capable of investigating the distribution of deuterium content.23 The accuracy of IR spectroscopy is 1.68% cm−1 in highly deuterated DKDP crystals, which may limit the ability to detect minor compositional inhomogeneities. For micro-Raman spectroscopy, one wavenumber variation in the Raman shift could be attributed to 2.64% deuteration, and it has a large error in highly deuterated crystals. C. Dorrer et al. demonstrated a method based on the phase-matching angles measured at two wavelengths (TW).24 However, this method was limited with deuterium content ranging from 70% to 98%. Considering the generation of the involved wavelengths (850 nm, 915 nm, and 976 nm), it is difficult to popularize. In addition, Lisong Zhang et al. modulated the NCPM temperature (TNCPM) with deuterium content and fundamental wavelength (T-FHG), which can be applied in the 60–100% deuterium content range in a non-cryogenic environment.25 To satisfy the increasing demand for high-quality large-diameter DKDP crystals, high-precision nondestructive deuterium content measurements are needed for further studies.

In this work, based on the SFG between the wavelength-tunable IR radiation supplied by optical parametric oscillation (OPO) and the fixed (third and fourth) harmonic, the noncritical phase matching (NCPM) wavelength tuning characteristics of DKDP crystals at ∼245 nm and ∼215 nm were verified in the experiment. Furthermore, a measurement scheme was demonstrated based on the relationship between the tunable wavelength and the deuterium content of DKDP crystals. Compared with conventional spectroscopic measurements, the accuracy achieves an enhancement more than one order of magnitude and the full-range (0–100%) capability overcomes the limitation of Raman spectroscopic measurements, especially for highly deuterated DKDP crystals. This SFG measurement method has the potential to serve as a standard to characterize the deuterium content uniformity in large-diameter DKDP crystals precisely and non-destructively.

2. Theoretical analysis

In the χ(2) nonlinear optical frequency conversion, the phase-matching condition and dispersion equations of the nonlinear crystals are the base of the parameter calculation. For the SFG, the phase-matching condition was:
 
n3(D)/λ3 = n1(D)/λ1 + n2(D)/λ2 (1)
The dispersion equations of the DKDP crystals, n(D), are related with the deuterium content, which can be written an expression for the Sellmeier formula at an arbitrary deuterium content (D).26 Therefore, the λ2 at NCPM conditions (θpm = 90°) can be determined by a fixed λ1.

As illustrated in Fig. 1(a), the blue and red solid curves respectively delineate the correlation between λ1 and λ2 in a pure KDP crystal and DKDP under type-I conditions. Herein, λ1 is systematically varied across the deep-UV to IR (210–1200 nm). Concurrently, the dots curve represents this relationship under type-II conditions. Considering the established dependence of the refractive index on the deuterium content, the calculated curves for intermediate deuterium levels (D) are expected to reside between these two boundary curves. Notably, each fixed λ1 corresponds to a unique λ2 value that is linked to the specific deuterium content of the crystal. This characteristic enables a quantification scheme: via the SFG, where λ1 is fixed and λ2 is tunable, the deuterium content of DKDP crystals can be measured through the precise measurement of the phase-matched λ2 wavelength.


image file: d5ce00610d-f1.tif
Fig. 1 (a) The NCPM wavelength couples at 298 K of KDP and DKDP crystals; (b) the relationship between the fixed radiation λ1 and Δλ2; (c) the calculated relationships between λ2 and the deuterium content when λ1 was 1064 nm, 532 nm, 355 nm, and 266 nm, respectively.

In order to fully improve the measurement accuracy of this method, the selection of the fixed λ1 requires rigorous optimization to ensure maximal spectral separation between the phase-matched λ2 of pure KDP and 100% deuterated DKDP crystals, which has been marked by the orange arrows in Fig. 1(a). The spectral separation was defined as:

 
Δλ2 = |λDKDP2 − λKDP2|. (2)
As shown in Fig. 1(b), the maximum of Δλ2 occurred at λ1 = 336 nm. Considering the availability of laser wavelengths, the harmonic generation based on an Nd:YAG laser was selected to obtain the fixed λ1. When λ1 was set to 1064 nm, 532 nm, 355 nm, and 266 nm (FW, SHG, THG, and FHG), the maximum spectral separation Δλ2 was achieved when λ1 operated at the THG.

Furthermore, theoretical calculations were performed to establish the functional relationship between the tunable λ2 and the deuterium content in DKDP crystals. The results under type-I conditions are presented in Fig. 1(c), which indicates that the relationship can be well fitted linearly with high coefficients of determination (R2). This proves the reliability of the linear fitting. When the fixed λ1 is set at 355 nm, λ2 should be tunable at near-IR (from 760 nm to 820 nm). When the fixed λ1 is set at deep-UV (266 nm), λ2 should be tunable at IR (from 1080 nm to 1150 nm). Herein, the tunable λ2 can be obtained by the OPO pumped at 532 nm. Consequently, ∼245 nm and ∼215 nm radiations will be generated via the SFG between the fixed λ1 and the tunable λ2.

In addition, the simulation result shows that one wavenumber change in the tunable range could be attributed to a 0.159% variation in the deuterium content of the DKDP crystal in the 355 nm and tunable near-IR SFG scheme (355 nm scheme) and a 0.588% variation in the 266 nm and tunable IR SFG scheme (266 nm scheme), which achieves an enhancement more than one order of magnitude over conventional spectroscopic measurements. This wavelength-tunable SFG method has the potential for non-destructive characterization of gradient deuterium distributions in large-diameter DKDP crystals.

3. Experimental section

3.1. Sample preparation

In this experiment, seven DKDP samples were prepared. These crystals were grown from different deuterium content solutions (D): 0%, 30%, 45%, 65%, 85%, 98%, and 99.9%. All the samples were processed along the type-I NCPM direction (θ = 90°, ϕ = 45°) with the size of 15 × 15 × 25 mm for further deep-UV laser generation. Based on the previous calculation results, type-I phase matching can obtain shorter SFG wavelengths than type-II phase matching, which indicates more application potential in high-energy deep-UV laser generation. Besides, the effective nonlinear coefficient under type-II phase matching conditions (deoe) will vanish for θ = 90° and ϕ = 45°. Therefore, the measurement under type-II conditions can not be achieved by these samples. To verify our results, both the 355 nm scheme and 266 nm scheme were performed.

In order to establish the dependence between λ2 and deuterium contents, the Raman spectra of all DKDP crystal samples were measured. Firstly, a single Si crystal was applied as a standard to calibrate the spectrograph before the spectra were obtained. Then, the Raman spectra were obtained with an excitation wavelength of 532 nm, and the spectra were analyzed by fitting the PO4 Raman peak with a single Lorentzian to determine the peak position. As shown in Fig. 2, the Raman shifts of the PO4 vibration peak of the KDP crystal at 920.12 cm−1 were observed. As the Raman shifts toward a shorter wavenumber of 886.45 cm−1, the deuterium content increases to 99.9%. Finally, according to the equation reported by F. Liu et al.,23 the deuterium content of a series of DKDP crystals was determined.


image file: d5ce00610d-f2.tif
Fig. 2 The Raman results of a series of DKDP crystal samples with different deuterium contents.

3.2. Experimental setup

According to the preceding discussion, when the THG and OPO pumped by the SHG based on an Nd:YAG laser source were selected as the fixed λ1 and the wavelength-tunable radiation λ2, the OPO output wavelength should be tunable from 760 to 820 nm. And when the FHG and OPO pumped by the SHG based on an Nd:YAG laser source were selected as the fixed λ1 and the wavelength-tunable radiation λ2, the OPO output wavelength should be tunable from 1080 to 1150 nm. The experimental setup of the 355 nm scheme and the 266 nm scheme are shown in Fig. 3(a) and (b), respectively.
image file: d5ce00610d-f3.tif
Fig. 3 (a) Experimental setup of the 355 nm scheme; (b) experimental setup of the 266 nm scheme. HWP, half-wave plate; PBS, polarization beam splitter; BS, beam splitter.

The fundamental laser at 1064 nm was supplied by an Nd:YAG nano-second laser source with a pulse duration of 7 ns (FWHM) and a repetition rate of 10 Hz. The fundamental laser energy varied from 0 to 100 mJ by a half-wave plate and a polarizing beam splitter. The measured diameter of the fundamental laser was ∼3 mm.

3.2.1. 355 nm scheme. To obtain the THG, the cascaded LBO crystals were used for the SHG and THG of the 1064 nm laser, respectively. Both surfaces of the SHG LBO crystal were coated with 532 nm and 1064 nm anti-reflection films, while the surfaces of the THG LBO crystal were coated with 355, 532, and 1064 nm anti-reflection films. Additionally, the 355 nm radiation was reflected by a BS2 (355 nm reflection, 1064 and 532 nm transmission) and other beams were collected by a block.

In the tunable λ2 generation stage, the pump light of the OPO was obtained by the SHG (via the other SHG LBO crystal) of the reflected s-polarized 1064 nm radiation and separated by the BS1 (532 nm reflection, 1064 nm transmission). The surfaces of the BBO crystal were coated with 532 and 750–850 nm anti-reflection films.

For the ∼245 nm NCPM SFG, the tunable λ2 and 355 nm were mixed by BS3 (355 nm reflection, 750–850 nm transmission). The DKDP crystal to be measured was placed in a temperature controller with an accuracy of 0.1 °C, which was mounted on an electric turntable with an adjustment precision of 8 μrad. The ∼245 nm deep-UV laser was reflected by BS4 (240–250 nm reflection, 355 and 750–850 nm transmission) and collected by an energy meter.

3.2.2. 266 nm scheme. As shown in Fig. 3(b), the pump light of the OPO was obtained by the SHG (via the SHG LBO crystal). Both surfaces of the SHG LBO crystal were coated with 532 and 1064 nm anti-reflection films. The surfaces of the OPO BBO crystal were coated with 532 and 1070–1170 nm anti-reflection films. Next, the FHG was obtained by the residual 532 nm laser after the OPO in a 12 × 12 × 20 mm DKDP crystal maintained at 40.5 °C (θ = 90°, ϕ = 45°, type-I phase matching). For the ∼215 nm NCPM SFG, similarly, the DKDP crystals to be measured were placed in the same temperature controller and the electric turntable. The ∼215 nm deep-UV laser was reflected by BS (210–220 nm reflection, 266, 532, and 1070–1170 nm transmission) and collected by an energy meter.

3.3. Experimental procedure

First of all, the incident angle ensured that the laser beams entered the DKDP crystal vertically by the retro-reflection position to aim the direction at θ = 90°. Then, the output wavelengths of the OPO were tuned via adjusting the incident angles of the BBO crystal. Meanwhile, the ∼245 and ∼215 nm laser energies were measured at different OPO wavelengths. In this way, the relationship between the λ2 and the SFG energies was obtained. The maximum wavelength acceptance bandwidth (ΔλNCPM) of NCPM at 30 °C can be obtained from the full-width at half-maximum (FWHM) of the wavelength tuning curve.

In order to ensure that the SFG efficiencies were not affected by the laser energy, the 355 nm energy and near-IR OPO energy were controlled at ∼10 mJ and ∼3.5 mJ in the 355 nm scheme experiment, respectively. In the 266 nm scheme experiment, the 266 nm energy and IR OPO energy were controlled at ∼7 mJ and ∼1.6 mJ, respectively.

4. Results and discussion

4.1. Tunable deep-UV laser generation near 245 and 215 nm

In the 355 nm scheme experiment, the spectra of the generated ∼245 nm deep-UV, 355 nm, and tunable near-IR OPO are shown in Fig. 4(a). In the 266 nm scheme experiment, the spectra of the generated ∼215 nm deep-UV, 266 nm, and tunable IR OPO are shown in Fig. 4(b). The UV and IR spectra were obtained using dedicated spectrometers for each spectral range (Ocean Optics HR4 and NR1.7). The wavelength tuning curves of different deuterium content DKDP crystals in the 355 nm scheme at 30 °C are shown in Fig. 5(a). Based on the inherent phase-matching behavior of the χ(2) nonlinear frequency conversion process, the experimental data are normalized and fitted by sin2[thin space (1/6-em)]x/x2 to measure the NCPM wavelength λ2 and wavelength acceptance ΔλNCPM, as defined by the FWHM. The fitted peak λ2, measured λ3, and calculated ΔλNCPM are listed in Table 1. In the same way, the NCPM SFG experiment of 266 nm scheme was performed. The wavelength tuning curves of different deuterium content DKDP crystals in the 266 nm scheme at 30 °C are shown in Fig. 5(b). And the experimental data were processed in the same way as the 355 nm scheme, which are listed in Table 1. After calculation, the photon conversion efficiencies of ∼215 nm and ∼245 nm (η215 = P215/POPO, η245 = P245/POPO) were 4.9% and 1.3%, respectively. Therefore, the conversion efficiency at 215 nm is higher in this experiment. Meanwhile, the conversion efficiency was affected by the laser power, beam profile, and beam divergence angle. If the laser energy and beam quality were improved using the lens, the conversion efficiency can be increased significantly.
image file: d5ce00610d-f4.tif
Fig. 4 (a) The spectra of the generated ∼245 nm deep-UV, 355 nm, and tunable near-IR; (b) the spectra of the generated ∼215 nm deep-UV, 266 nm, and tunable IR.

image file: d5ce00610d-f5.tif
Fig. 5 The wavelength tuning curves of different deuterium content DKDP crystals in the 355 nm scheme (a) and 266 nm scheme (b) at 30 °C.
Table 1 The key experimental results of the 355 nm scheme and the 266 nm scheme
D 355 nm scheme 266 nm scheme
λ2 (nm) λ3 (nm) ΔλNCPM (nm) λ3 energy (μJ) λ2 (nm) λ3 (nm) ΔλNCPM (nm) λ3 energy (μJ)
0.0% 771.635 242.704 0.580 143.8 1085.352 213.640 0.622 401.2
30.0% 782.134 243.733 0.657 136.1 1100.621 214.226 0.704 338.2
45.0% 786.711 244.175 0.955 142.9 1107.355 214.479 0.773 449.3
65.0% 794.271 244.898 0.934 190.3 1118.553 214.896 0.811 500.3
85.0% 806.428 246.043 1.068 194.9 1135.709 215.522 0.721 419.0
98.0% 810.563 246.426 0.989 197.7 1141.538 215.730 0.919 437.8
99.9% 814.544 246.796 0.923 155.6 1147.281 215.935 1.067 404.2

As reported in ref. 23, Raman spectroscopy could be used to determine the deuteration level of DKDP crystals for crystals grown from solutions with a deuteration level of less than 92%. Therefore, the relationship between the deuterium content and λ2 in the 355 nm scheme at 30 °C was linearly fitted based on the Raman results of 0%, 30%, 45%, 65%, 85%, and 98% DKDP crystals. As shown in Fig. 6(a), the black dots represent the relationship between the fitted λ2 and Raman results of the seven samples in the 355 nm scheme. The blue solid line is the linear fitting curve. Based on the results, the shortest near-IR wavelength that can achieve the SFG with 355 nm in random deuterium content DKDP crystals can be predicted. The dependence can be described as:

 
D (%) = 2.30289λ2 − 1776.98983. (3)


image file: d5ce00610d-f6.tif
Fig. 6 The linear relationship between the deuterium content and λ2 at 30 °C in the (a) 355 nm scheme and (b) 266 nm scheme.

In Fig. 6(b), the black dots represent the relationship between the fitted λ2 and Raman results of the seven samples in the 266 nm scheme. And the purple solid line is the linear fitting curve. Similarly, the linear relationship between the deuterium content based on the Raman results and λ2 in the 266 nm scheme at 30 °C can be described as:

 
D (%) = 1.59494λ2 − 1731.07148. (4)

Based on the results, the shortest IR wavelength that can achieve the SFG with 355 nm and 266 nm in random deuterium content DKDP crystals can be predicted.

4.2. Deuterium content characterization

Furthermore, based on eqn (3) and (4), the deuterium content of DKDP crystals can be determined by the precise measurement of matched λ2. The Raman, 355 nm scheme, and 266 nm scheme measurement results are summarized in Table 2. The deuterium content of the DKDP crystal grown from solutions with a deuteration level of 99.9% can be calculated according to the formula. Compared with the Raman result at 88.97%, the result at 98.80% obtained by our schemes is obviously more reasonable and persuasive. In addition, the results of the 355 nm scheme and 266 nm scheme agree well but there is a deviation from the Raman results, which indicates that the results of Raman spectroscopy have larger errors and the actual deuterium content of the samples should be corrected according to the average of our results.
Table 2 Deuterium content results of Raman, 355 nm scheme, and 266 nm scheme measurements
D (%) 0.00% 30.0% 45.0% 65.0% 85.0% 98.0% 99.9%
Raman 0.00% 21.65% 34.08% 53.96% 77.93% 89.62% 88.97%
355 nm 0.00% 24.18% 34.72% 52.13% 80.13% 89.65% 98.82%
266 nm 0.00% 24.35% 35.09% 52.95% 80.32% 89.61% 98.77%
Average 0.00% 24.27% 34.91% 52.54% 80.23% 89.63% 98.80%

Besides, it can be inferred from the slope of eqn (3) and (4) that the accuracy of the 355 nm method is 2.303% nm−1 and the accuracy of the 266 nm method is 1.595% nm−1. To compare with Raman spectroscopy, after converting the wavelengths to wavenumbers, the accuracies of the 355 nm method and 266 nm method are 0.148% cm and 0.199% cm, respectively. Limited by the resolution of the spectrometer, only a little valid data can be used to fit the curves, which is the main source of the error in our method. Besides, it can be observed from Fig. 4 that the spectral base widths of the OPO radiation are ∼3 nm. The FWHM of the tunable OPO radiation and the generated deep-UV radiation were ∼1.5 nm and ∼0.5 nm, respectively. By using the volume Bragg grating and injection seeding techniques, the OPO spectral width can be narrowed to <10 pm, which will obviously improve the accuracy of our methods.27,28 If a higher-resolution spectrometer could be applied and the linewidth could be further narrowed, the error can be expected to decrease to 0.1%.

The comparison of the deuterium content measurements is shown in Table 3. Firstly, it is convincing that our method is an enhancement of more than one order of magnitude over conventional spectroscopic measurements and overcomes the disadvantage of the Raman spectroscopic measurements for highly deuterated DKDP crystals. Besides, compared with the neutron powder diffraction, the cost of this method is greatly reduced, and the destruction of samples is also avoided. Due to the measured diameters of the ∼245 nm and ∼215 nm laser being ∼2.5 mm, the uniformity of the deuterium content in DKDP crystals can be measured by scanning the crystal at intervals of 3 mm along the X and Y directions respectively and measuring the deuterium content at the corresponding positions, which is unachievable for the destructive methods and temperature-dependent methods. For large-diameter crystals, the scanning interval can be expanded to 3 cm. In addition, this wavelength-tunable SFG method can be applied in the deuterium content measurement of ADP and DADP crystals due to the similar refractive index dispersion characteristics.

Table 3 Comparison of the deuterium content measurements
Method Accuracy D (0–100%) Uniformity Non-destructive Cost
355 nm 0.148% cm ✓ ✓ ✓ Low
266 nm 0.199% cm ✓ ✓ ✓ Low
NPD20 0.5% ✓ ✗ ✗ High
TGA21,29 0.8% ✓ ✗ ✗ Mid
FTT22 0.93% K−1 ✓ ✗ ✗ High
Raman23 2.64% cm 0–92% ✓ ✓ Low
IR23 1.68% cm ✓ ✓ ✓ Low
TW24 1.18% mrad−1 70–98% ✓ ✓ Mid
T-FHG25 0.45% K−1 60–100% ✗ ✓ Low

4.3. Discussion

As shown in Fig. 6, the matched λ2 can agree well with the theoretical calculation in KDP crystals. However, it can be observed that as the deuterium content in the DKDP crystal increases, the discrepancy between theoretical calculations and experimental verification becomes increasingly significant. In particular, the gap in the 266 nm scheme is obviously larger than the 355 nm scheme. When the deuterium content of the DKDP crystal was 100%, the λ2 that can achieve the SFG with 355 nm and 266 nm should be 815.098 nm and 1148.050 nm, respectively. Therefore, Δλ2 in the 355 nm and 266 nm scheme should be 43.463 nm and 62.698 nm, respectively. This result validates our expectations for the 266 scheme in theoretical analysis that Δλ2 actually keeps increasing when λ1 goes toward a shorter wavelength. In contrast to Fig. 1(b), experimental results demonstrate that no maximum point was predicted to exist at λ1 = 336 nm. This result presented that the Sellmeier equation at the deep-UV waveband significantly deviates from the actual refractive index.

It is assumed that the 98.8% DKDP crystal ordinary refractive indices based on ref. 26 at 266 nm, 355 nm, 814.544 nm, and 1147.281 nm are accurate, and the extraordinary refractive index corrections at 246.796 nm and 215.935 nm are given as 1.5141 and 1.5422, respectively. Meanwhile, the simulation results of the extraordinary refractive indices at 246.796 nm and 215.935 nm were 1.6852 and 1.6853, which were significantly larger than our experiment results. This correction is applicable to random deuterium content DKDP crystals. The refractive index corrections at the UV range of the KDP-family crystals will provide an important data reference for the non-cryogenic high-energy ∼245 nm and ∼215 nm deep-UV laser generation and promote the exploration of the KDP-family crystal deep-UV laser generation edges. Besides, this work will enrich deep-UV laser sources, which is helpful in measuring the refractive index, laser-induced damage threshold, and absorption properties in the deep-UV spectral region for evaluating the crystal quality.

5. Conclusion

Based on the tunable deep-UV laser generation, we obtained the NCPM SFG wavelength characteristics and analyzed the deep-UV SFG ability at ∼245 nm and ∼215 nm of DKDP crystals. According to the linear formula between the deuterium content and the OPO wavelengths, the shortest IR wavelength that can achieve the SFG with 355 nm and 266 nm in random deuterium content DKDP crystals can be predicted. The results presented the near FiHG edges of KDP-family crystals at room temperature. Furthermore, a novel large-diameter DKDP crystal deuterium content measurement scheme was demonstrated. Full-range (0–100%) deuterium content measurements accuracy is better than 0.2% cm−1, which is an enhancement of more than one order of magnitude over conventional spectroscopic measurements. And this scheme overcomes the disadvantage of the Raman spectroscopic measurements for highly deuterated DKDP crystals and is promising to be applied in the large-diameter KDP-family crystal selection for the superintense ultrafast lasers and ICF facilities as a standard method. In conclusion, this work will improve the output energy of high-power deep-UV laser devices and pave the way for the application in high-energy-density physics, plasma diagnostics, and materials science.

Author contributions

Yiqun Shi: conceptualization, methodology, investigation, data curation, and writing – original draft; Zijian Cui: conceptualization, methodology, investigation, and writing – review & editing; Junze Xu: investigation and data curation; Mingying Sun: investigation, validation, and resources; Xiuqing Jiang: investigation and data curation; Li Yin: investigation and data curation; Xinglong Xie: project administration and writing – review & editing; De'an Liu: writing – review & editing; Jianqiang Zhu: supervision and writing – review & editing.

Conflicts of interest

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.

Acknowledgements

The authors would like to thank Dr. Xingping Li, Prof. Song Liu, and Prof. Qinghui Li from the Center of Sci-Tech Archaeology, Shanghai Institute of Optics and Fine Mechanics, CAS for assisting the Raman experiment. The work was supported by the National Natural Science Foundation of China (Grants No. 12004404); the Shanghai Rising-Star Program (Grant No. 18YF1425900); the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grants No. XDA25020202 and No. XDA25020101); and the Youth Innovation Promotion Association CAS (Grant No. 2018282).

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