Based on the 1.1 μm laser radiation obtained by parametrically amplifying Yb:YAG laser using frequency doubling of Nd:YAG laser, a 220 nm deep-UV coherent source was systematically presented via the efficient fifth-harmonic generation (FiHG) in NH4H2PO4 (ADP) and KD2PO4 (DKDP) crystals. In ADP crystals, noncritical phase-matching (NCPM) fourth-harmonic generation (FHG) and FiHG were realized at 109.4 and 31.6 °C, respectively. For DKDP crystals, we demonstrated the critical phase-matching FHG at 57.5 °C and verified that NCPM FiHG can be achieved at 38.7 °C, which was the first demonstration of the FiHG using the DKDP crystal to our knowledge. The energy-dependent conversion efficiencies, angular acceptances, and temperature acceptances of these nonlinear processes were systematically measured. The highest total conversion efficiencies from 1.1 μm to the fifth harmonic in ADP and DKDP crystals were 17.5% and 23.6%, respectively. Owing to the large-aperture availability of KDP-family crystals, this work paves the way for the generation and application of high-energy and high-peak-power deep-UV laser radiation.

Coherent light sources generated by solid-state lasers have been widely used in numerous important fields, including scientific research,1,2 photolithography manufacturing,3 biomedicine,4,5 and communication,6 and continue to promote the development and technological update of these fields. With the expansion of the application range, the laser wavelengths that can be directly emitted by stimulated radiation are difficult to satisfy the increasing demands, especially in the deep-UV and vacuum-UV wavebands,7–10 which have been dealing with a lack of suitable laser gain media. For example, in the study of high-energy-density physics, an important application is to use the high-energy deep-UV laser in the wavelength range of 180–230 nm, which usually requires an ideal energy level of 10–100 J, as a probe beam to diagnose the plasma based on optical Thomson scattering (OTS).8,9 However, the lack of available deep-UV sources with high enough fluxes and coherence properties has limited the availability of OTS for characterizing dense plasma conditions. In the biomedical field, deep-UV sources are also gradually showing huge application potential.10–12 With the outbreak of Coronavirus disease 2019 and the appearance of highly infectious Delta and Omicron variants, the epidemic is continuing to spread, and prevention is becoming more difficult. Under the background that China adheres to the “dynamic zero-COVID” policy and tackles both imported and domestic infections, the deep-UV sources may play an important role, especially UV light in the waveband of 220–240 nm.12–15 They can be almost completely absorbed by skin keratin, cornea, and tear layer, and have low penetration to biological cells, which not only avoid the spectral region that is harmful to the human body but also maintain efficient inactivation of viruses and bacteria.

At present, based on the laser source with a fixed wavelength, it is a highly effective way to generate new laser radiation in the UV waveband using nonlinear optical techniques, including second-, third-, fourth-, fifth-harmonic generation, and sum-frequency generation (SHG, THG, FHG, FiHG, and SFG),16–24 which considerably broaden the laser wavelength range and application scenarios. Theoretically, the most direct way to obtain deep-UV laser radiation based on nonlinear optical technology is to generate the second harmonic of the UV laser. Since the UV laser has high single photon energy, its generation requires a high threshold pump power and a gain medium with large transition levels. Currently, only a few laser diodes and rare earth ion-doped gain materials can directly emit the UV laser beam, and the output power is also limited.25–27 On the other hand, because of the more serious dispersion of materials in the UV waveband, the phase matching of the SHG cannot be satisfied in the most commonly used crystals.28 Only a few nonlinear materials represented by KBe2BO3F2 (KBBF) crystal can actually be used for the SHG of UV lasers. However, the KBBF is very difficult to grow because of its strong layered structure, and typical crystal sizes obtained are about a millimeter range in thickness and less than 30 mm in aperture,29 which greatly limits the available energy and power improvement of the deep-UV laser radiation due to severe laser damage of optical materials in a high-intensity regime. Hence, the lack of high-performance UV gain medium and satisfying phase-matching nonlinear materials with large aperture makes it very difficult to obtain deep-UV radiation by the direct SHG of UV lasers, especially for the demand of high energy and high power.

In contrast, for the infrared laser, there are a series of gain media (e.g., Ti:sapphire, Nd:glass, Nd:YLF, Nd:YAG, and Yb:YAG) with high optical performance that can emit laser pulses with high energy and high power.25,30,31 For the typical lasers with wavelengths of ∼1 μm, there have been a large number of reports on the SHG, THG, and FHG, and a series of nonlinear materials, such as borate and KH2PO4 (KDP)-family crystals, can be well employed.16–19 Unfortunately, for the deep-UV laser generation near 200 nm via the FiHG, only β-BaB2O4 (β-BBO) and CsLiB6O10 (CLBO) crystals are mainly used,21,22 but their growth sizes are also limited. For the KDP-family crystals, they can be obtained with meter-scale growth sizes and become indispensable nonlinear optical materials for a series of high-power laser facilities, such as the National Ignition Facility, LMJ, and Shenguang.32–34 However, they are difficult to achieve the FiHG of the aforementioned lasers at room temperature due to the limitation of phase matching.23,24 Presently, the FiHG in NH4H2PO4 (ADP) crystal was demonstrated only in cryogenic conditions,23 while the FiHG in KD2PO4 (DKDP) crystal was never reported. Due to the harsh low-temperature conditions, the FiHG using KDP-family crystals was not attracted much attention and wide applications, and many scientific experiments that require high-energy or power deep-UV lasers are also greatly limited.

KDP-family crystals represented by ADP, KDP, and DKDP are a class of nonlinear optical materials with excellent performance. Especially for DKDP crystal, it has a unique advantage that its optical properties can be adjusted by controlling the proportion of deuterium atoms replacing hydrogen atoms in KDP crystal. Although they have been used in many applications, there is no report on the detailed data and direct comparison of transmittance characteristics over the entire deep-UV to vacuum-UV spectral region. Hence, we prepared four polished and uncoated nonlinear crystal samples of β-BBO, CLBO, ADP, and DKDP (deuterium content >98%) and measured their absolute transmittances in the range of 160–290 nm using a vacuum-UV spectrophotometer, as shown in Figs. 1(a) and 1(b). Furthermore, we obtained the UV cut-off wavelengths (λcut) defined by the “0” transmittance level and calculated the bandgap energies of these crystals according to the measurement results and Tauc plot method,35 as shown in Fig. 1(c) and Table I.

FIG. 1.

(a) 5-mm-thick β-BBO, CLBO, ADP, and DKDP crystal samples. (b) Measured absolute transmittance spectra in the spectral region of 160–290 nm. (c) Calculated bandgap energies of β-BBO, CLBO, ADP, and DKDP crystals by the Tauc plot method.

FIG. 1.

(a) 5-mm-thick β-BBO, CLBO, ADP, and DKDP crystal samples. (b) Measured absolute transmittance spectra in the spectral region of 160–290 nm. (c) Calculated bandgap energies of β-BBO, CLBO, ADP, and DKDP crystals by the Tauc plot method.

Close modal
TABLE I.

Parameters of the β-BBO, CLBO, ADP, and DKDP crystal samples, and experimentally measured UV cut-off edges and calculated bandgap energies.

Crystal β-BBO CLBO ADP DKDP 
Length 5 mm 5 mm 5 mm 5 mm 
Orientation θ = 52° θ = 70° θ = 90° θ = 90° 
ϕ = 0° ϕ = 45° ϕ = 45° ϕ = 45° 
UV λcut (nm) 190.3 179.2 179.0 171.2 
Bandgap energy (eV) 6.41 6.84 6.83 7.13 
Crystal β-BBO CLBO ADP DKDP 
Length 5 mm 5 mm 5 mm 5 mm 
Orientation θ = 52° θ = 70° θ = 90° θ = 90° 
ϕ = 0° ϕ = 45° ϕ = 45° ϕ = 45° 
UV λcut (nm) 190.3 179.2 179.0 171.2 
Bandgap energy (eV) 6.41 6.84 6.83 7.13 

It can be seen from Fig. 1 and Table I that KDP-family crystals, especially the DKDP crystal, have excellent UV properties, including a short cut-off edge, large bandgap energy, and high UV transmittance. Besides, since they can obtain a large growth size, KDP-family crystals are very suitable for high-energy and high-peak-power deep-UV laser generation. The key issue of deep-UV laser generation in the KDP-family crystals is the realization of phase matching. For the aforementioned ∼1 μm lasers, the phase matching of FiHG can only be satisfied in physical conditions with low temperatures, while the FiHG of ∼1.1 μm lasers can well be achieved in KDP-family crystals under noncryogenic conditions according to our theoretical analysis. In addition, it is worth noting that the FiHG has been demonstrated in both KDP and ADP crystals whether at low or high temperatures,23,24 but there is no report on the FiHG in DKDP crystal at present. Although it can be predicted theoretically, it is still impossible to give convincing conclusions and true and accurate data due to the lack of practically experimental verification.

Here, we demonstrated the generation of a 220 nm laser source in ADP and DKDP crystals via the FiHG of 1101 nm laser radiation, which was generated based on optical parametric amplification (OPA) by using an Nd:YAG laser and a Yb:YAG laser. The maximum conversion efficiencies of 17.5% and 23.6% from 1101 to 220 nm were achieved in ADP and DKDP crystals, respectively. Meanwhile, we systematically measured the energy-dependent conversion efficiencies, angular acceptances, and temperature acceptances of these nonlinear processes. As far as we know, this is the first experiment to demonstrate the FiHG in ADP and DKDP crystals under noncryogenic conditions, and more importantly, our experimental results show that the phase-matching temperature (38.7 °C) actually required for the FiHG in DKDP crystal is well below (more than 100 °C) the SNLO predicted and our calculated results (SNLO 137, 139.6, and 167.9 °C) based on the data of DKDP crystal reported in the current literature.28,36,37 This means that the current temperature-dependent dispersion equations of the DKDP crystal in the deep-UV spectral region are far from reality. Compared with the lasers with wavelengths of ∼260 nm, the fifth harmonic has more significant advantages in numerous fields owing to its shorter wavelength, higher single-photon energy, and superior space–time resolution. Therefore, the realization of efficient FiHG in KDP-family crystals near room temperature demonstrated here has great practical significance for the improvement of the energy and power of deep-UV laser sources, which can be applied to fields, such as laser physics, material processing, and plasma experiments.1,3,7–9

Figure 2 shows the experimental scheme of 220 nm deep-UV laser generation. A single-longitudinal-mode Nd:YAG laser (1064 nm) and a Yb:YAG laser (1029.5 nm) were used. The beam output from the Nd:YAG laser was a Gaussian pulse with a 7.5 ns pulse duration (FWHM), and the transverse distribution was a flat-topped circular spot with a diameter of 7 mm. The maximum output energy and repetition rate were 1 J and 10 Hz, respectively. The Nd:YAG laser beam passed through a 25-mm-thick LiB3O5 (LBO, θ = 90°, φ = 7.5°, type-I phase matching) crystal to obtain a pump light (532 nm, ωp) used for subsequent OPA, and the temperature of LBO crystal was always controlled at 100 °C. The generated pump light was separated from the remaining 1064 nm laser by a dichroic mirror (1064 nm transmission and 532 nm reflection) and then narrowed and collimated to a beam with a diameter of 3.5 mm by using a 4f-telescope lens system. The output energy of pump light (Ep) was 386 mJ, and the corresponding conversion efficiency (ηp = Ep/E) was 77.2% at an incident energy of 500 mJ (E).

FIG. 2.

(a) Schematic illustration of 220 nm deep-UV laser generation. DM, dichroic mirrors; HR, high reflector; L1 (f = 250 mm), L2 (f = 500 mm), L3 (f = 300 mm), and L4 (f = 300 mm), lens; VSF, vacuum spatial filter; λ/2, half-wave plate; and PBS, polarization beam splitter. (b) Measured spectra of Nd:YAG laser, pump light, and Yb:YAG laser. (c) Measured spectra of fundamental wave, second harmonic, fourth harmonic, and fifth harmonic of 1101 nm laser radiation.

FIG. 2.

(a) Schematic illustration of 220 nm deep-UV laser generation. DM, dichroic mirrors; HR, high reflector; L1 (f = 250 mm), L2 (f = 500 mm), L3 (f = 300 mm), and L4 (f = 300 mm), lens; VSF, vacuum spatial filter; λ/2, half-wave plate; and PBS, polarization beam splitter. (b) Measured spectra of Nd:YAG laser, pump light, and Yb:YAG laser. (c) Measured spectra of fundamental wave, second harmonic, fourth harmonic, and fifth harmonic of 1101 nm laser radiation.

Close modal

For Yb:YAG laser, it was used as a seed light (ωseed). ωseed was a pulse with Gaussian distribution in both space and time, the pulse duration (FWHM) was 12 ns, and the diameter was 5 mm defined as the measurement of 10% of the peak irradiance point. The maximum output energy and repetition rate were 4.5 mJ (Eseed) and 10 Hz, respectively. Both the Nd:YAG laser and Yb:YAG laser adopted an external trigger mode to realize precise time synchronization of pump light and Yb:YAG laser for OPA. The ωp and ωseed were combined in a collinear geometry using a dichroic mirror (ωp reflection and ωseed transmission) and were incident into a 30-mm-thick LBO crystal (θ = 20.5°, φ = 90°, type-II phase matching). The phase matching of OPA was achieved at 84.3 °C, and this temperature was always maintained during the whole experiment. In the OPA process, the seed light was amplified to 133 mJ (signal light, ωs = ωseed, Es), and new laser radiation (idler light, ωi) with a wavelength of 1101 nm and energy of 120.5 mJ (Ei) was generated. The energy conversion efficiency from pump light to idler light [ηi = Ei/(Ep + Eseed)] was 31%. Subsequently, the pump light was separated from the signal light and the idler light using a dichroic mirror (532 nm reflection and 1029.5 and 1101 nm transmission), and the idler light was separated from the signal light via a polarization beam splitter (PBS, 1029.5 nm reflection and 1101 nm transmission).

The separated idler light passed through another 4f-telescope lens system and was used as the fundamental wave (ω1 = ωi) to generate the 275 and 220 nm laser radiations (ω4 and ω5). The diameter of the idle light was 3.2 mm. The employed ω1 energy was controlled by a half-wave plate and PBS. Subsequently, three χ(2) nonlinear optical processes were cascaded to obtain the deep-UV laser radiation, which were ω + ω → 2ω, 2ω + 2ω → 4ω, and 1ω + 4ω → 5ω, respectively. Another LBO crystal with a thickness of 25 mm (θ = 90°, φ = 7.5°, type-I phase matching) was used for the SHG of 1101 nm laser radiation, and the phase matching was achieved at 52.1 °C. The second harmonic (ω2) energies (E2) and efficiencies (η2 = E2/E1) as a function of the fundamental wave energy (E1) are shown in Fig. 3. E1 gradually increases from zero to 120 mJ with a step size of ∼10 mJ. The SHG efficiency reaches the maximum value of 76.8% when the incident energy is 120.6 mJ, and a 550.5 nm laser with an energy of 92.6 mJ was obtained. In this work, the three LBO crystals involved in this paper were coated with dielectric antireflective films. In the whole experiment, the involved spectra of Nd:YAG laser, pump light, Yb:YAG laser, fundamental wave, second harmonic, fourth harmonic, and fifth harmonic are shown in Figs. 2(b) and 2(c).

FIG. 3.

Efficiencies and energies of generated second harmonic as a function of input fundamental wave energy. The solid and dashed lines denote the calculated results of SHG efficiency and energy, respectively; the circle and triangle symbols denote the experimental results of SHG efficiency and energy, respectively.

FIG. 3.

Efficiencies and energies of generated second harmonic as a function of input fundamental wave energy. The solid and dashed lines denote the calculated results of SHG efficiency and energy, respectively; the circle and triangle symbols denote the experimental results of SHG efficiency and energy, respectively.

Close modal

Based on the above experimental platform, we first demonstrated the FHG and FiHG of the 1101 nm laser radiation in ADP crystals. For the FHG, a 20-mm-thick ADP crystal (θ = 90°, φ = 45°, type-I phase matching) was used and mounted in a temperature-controlled oven with a tuning range of 25–200 °C and a control precision of 0.1 °C. The generated fourth harmonic was separated from the second harmonic and the fundamental wave by a dichroic mirror (275 nm reflection and 550.5 and 1101 nm transmission). The variation of FHG efficiency with the crystal angle and temperature was shown in Figs. 4(a) and 4(b), respectively. It can be seen from the diamond symbols that noncritical phase-matching (NCPM) FHG in ADP crystal was realized at 109.4 °C, and the temperature acceptance bandwidth (ΔTADP-FHG) was 0.31 °C, as defined by the FWHM. When the temperature of the ADP crystal was fixed at 109.4 °C, the measured angular acceptance bandwidth (ΔθADP-FHG) was 24.9 mrad.

FIG. 4.

Angular and temperature characteristics of (a) and (b) NCPM FHG in a 20-mm-thick ADP crystal and (d) and (e) NCPM FiHG in a 15-mm-thick ADP crystal. Efficiencies and energies of generated (c) fourth harmonic and (f) fifth harmonic as a function of input pulse energy. In (a), (b), (d), and (e), all efficiencies are normalized to the maximum value at the initial phase-matching condition.

FIG. 4.

Angular and temperature characteristics of (a) and (b) NCPM FHG in a 20-mm-thick ADP crystal and (d) and (e) NCPM FiHG in a 15-mm-thick ADP crystal. Efficiencies and energies of generated (c) fourth harmonic and (f) fifth harmonic as a function of input pulse energy. In (a), (b), (d), and (e), all efficiencies are normalized to the maximum value at the initial phase-matching condition.

Close modal

In the FiHG experiment, the temperature of the ADP crystal used to generate the fourth harmonic was always maintained at 109.4 °C, and another ADP crystal (θ = 90°, φ = 45°, type-I phase matching) with a thickness of 15 mm was employed to generate the fifth harmonic. The obtained fifth harmonic was separated from the fundamental wave, second, and fourth harmonic by a dichroic mirror coated with specially designed films (220 nm reflection and 275, 550.5, and 1101 nm transmission). Figures 4(d) and 4(e) plotted the angular and temperature characteristics of FiHG efficiency, respectively. The NCPM temperature of FiHG was 31.6 °C, as shown by the five-pointed star symbols in Fig. 4(e). The measured angular and temperature acceptance bandwidth (ΔθADP-FiHG and ΔTADP-FiHG) were 22.5 mrad and 0.26 °C, respectively.

Furthermore, we also demonstrated the efficiency and energy variation of the fourth and fifth harmonic output from ADP crystals under different input-pulse energies, and the experimental results are shown in Figs. 4(c) and 4(f). For the FHG experiment, the energy of the 550.5 nm laser radiation gradually increases from zero to 90 mJ with a step size of ∼5 mJ. The FHG efficiency (η4 = E4/E2) reaches a maximum value of 55.7% when the incident ω2 energy is 56 mJ, and the corresponding total conversion efficiency was 40.1%. A 275 nm laser radiation with an energy of 45.1 mJ was obtained at the maximum 550.5 nm laser radiation energy of 90.3 mJ, and the corresponding efficiency was 50%. As for the FiHG, the energy of the 1101 nm laser radiation gradually increases from zero to 120 mJ with a step size of ∼10 mJ. The FiHG efficiency (η5 = E5/E1) reached a maximum value of 17.5% when the incident ω1 energy was 100 mJ. At input 1101 nm laser radiation energy of 120.7 mJ, a 220 nm laser radiation with an energy of 20.3 mJ was generated, and the corresponding conversion efficiency was 16.8%. Based on experimental parameters, we simulated the FHG and FiHG processes in ADP crystals, and the results are shown by the lines in Figs. 4(a)4(e). It can be seen that the numerically simulated results are in good agreement with the experimental results. The change curves of the FiHG efficiency and energy with the fundamental wave energy in Fig. 4(f) were the fitting results due to the lack of crystal data at 220 nm, such as linear absorption and nonlinear two-photon absorption (TPA) coefficients.

According to the theoretically calculated results of phase-matching temperature and the highest withstand temperature of DKDP crystal,24 the FHG of 1101 nm laser radiation can only be achieved with critical phase matching (CPM) in the DKDP crystal. Therefore, a DKDP crystal with a thickness of 20 mm and cutting angles of θ = 75.6° and φ = 45° (type-I phase matching) was used to generate the fourth harmonic, and the phase matching was realized at 57.5 °C. The variation of CPM FHG efficiency with the angle and temperature in DKDP crystal is shown in Figs. 5(a) and 5(b), respectively. The measured angular and temperature acceptance bandwidths (ΔθDKDP-FHG and ΔTDKDP-FHG) were 0.616 mrad and 1.02 °C, respectively.

FIG. 5.

Angular and temperature characteristics of (a) and (b) CPM FHG in a 20-mm-thick DKDP crystal and (d) and (e) NCPM FiHG in a 15-mm-thick DKDP crystal. Efficiencies and energies of generated (c) fourth harmonic and (f) fifth harmonic as a function of input pulse energy. In (a), (b), (d), and (e), all efficiencies are normalized to the maximum value at the initial phase-matching condition.

FIG. 5.

Angular and temperature characteristics of (a) and (b) CPM FHG in a 20-mm-thick DKDP crystal and (d) and (e) NCPM FiHG in a 15-mm-thick DKDP crystal. Efficiencies and energies of generated (c) fourth harmonic and (f) fifth harmonic as a function of input pulse energy. In (a), (b), (d), and (e), all efficiencies are normalized to the maximum value at the initial phase-matching condition.

Close modal

For the FiHG, theoretically, both CPM and NCPM can be achieved in DKDP crystal. Here, we prepared two type-I phase-matching DKDP crystals with the same thickness of 15 mm to generate the fifth harmonic, their cutting angles were θ = 83°, φ = 45° and θ = 90°, φ = 45°, respectively. The temperature of the DKDP crystal used to generate the fourth harmonic was always maintained at 57.5 °C. We first demonstrated the CPM FiHG, and the phase matching can be achieved near 80 °C in theory, but this is not the case. The fifth harmonic was not observed during the whole process of the crystal temperature rising from room temperature to 100 °C. After excluding the possible reasons, such as transmittance and cutting angles of the crystal, the 220 nm laser radiation was still not detected in the second experiment. We speculated that the theoretical calculation result was wrong and the phase matching of this crystal can only be achieved in an environment below room temperature, and the subsequent experimental results of NCPM FiHG also support our speculation.

Likewise, NCPM FiHG in DKDP crystal was demonstrated. The SNLO predicted NCPM temperature was 137 °C, and our calculated results were 139.6 and 167.9 °C. Experimentally, the NCPM was achieved at 38.7 °C, as shown by the five-pointed star symbols in Fig. 5(e). This result confirmed that all theoretically calculated phase-matching temperatures based on the currently reported data were noticeably higher and far from the experimental results. This proves that the previous CPM FiHG experiment cannot be achieved at conditions above room temperature, and, inevitably, the 220 nm laser radiation cannot be obtained. Meanwhile, it also shows that the currently reported refractive-index data of DKDP crystal near 220 nm are quite different from the actual situation. The measured angular and temperature acceptance bandwidths were 23.2 mrad and 0.97 °C, respectively. Figures 5(d) and 5(e) plotted the angular and temperature characteristics of NCPM FiHG efficiency, respectively.

For different incident energies of 550.5 and 1101 nm laser pulses, the conversion efficiency and energy variation of CPM FHG and NCPM FiHG in DKDP crystals were measured, as shown in Figs. 5(c) and 5(f). The obtained maximum FHG efficiency was 68.2% when the input 550.5 nm laser radiation energy was 90 mJ, and the corresponding total conversion efficiency was 52.3%. For the FiHG, the energy of the 1101 nm laser radiation gradually increases from zero to 120 mJ with a step size of ∼10 mJ. The FiHG efficiency reaches a maximum value of 23.6% when the incident ω1 energy is 109.5 mJ, and a 220 nm laser radiation with an energy of 27.3 mJ was obtained at the 1101 nm laser radiation energy of 120.3 mJ, and the conversion efficiency from the 1101 nm laser radiation to the fifth harmonic was 22.7%. Similarly, we simulated the angular characteristics, temperature characteristics, and energy-dependent conversion efficiencies of the CPM FHG in DKDP crystal, as shown by the lines in Figs. 5(a)5(c), which show a good agreement between the calculated and experimental results. Since the temperature-dependent dispersion equations of DKDP crystal far deviates from reality and there is a lack of absorption loss data near 220 nm,28,38–40 the curves in Figs. 5(e) and 5(f) were the fitted results.

Comparing the experimental results, it can be found that both the FHG and FiHG efficiencies in DKDP crystals were higher than those of ADP crystals, although the effective nonlinear coefficient of ADP crystal was slightly higher than that of DKDP crystal.28 The main reason is that the energy loss caused by the absorption in the DKDP crystal is lower than that of the ADP crystal. Meanwhile, since the DKDP crystal has larger bandgap energy than the ADP crystal, the TPA effect in the DKDP crystal is weaker than that in the ADP crystal. In addition, it is worth noting that the experimental results of both FHG and FiHG in ADP crystals are close to the theoretical results in terms of the phase-matching temperature, while the experimental results of FiHG in DKDP crystal are quite different from the theoretical results, which indicates that the current thermo-optical coefficients of DKDP crystal greatly deviate from the actual value.28 

As laser technology keeps developing and application fields continuously expanding, the demand for laser output energy and power is constantly increasing. However, the generation of high-energy, high-intensity, and high-peak-power laser radiations has been facing severe challenges, especially in the deep-UV waveband, optical materials are severing serious laser damage, which gravely hinders the applications and performance improvement of deep-UV laser sources. To overcome this problem, the use of large-aperture crystals is one of the most practical and effective ways at present. KDP-family crystals are the only class of nonlinear crystal materials that can obtain extremely large growth sizes. Their growth sizes can reach the meter level and far exceed that of any other water-soluble crystals. Owing to high purity and optical uniformity and excellent UV optical properties, they are suitable for generating deep-UV laser sources. Especially the DKDP crystal is expected to play an important role in high-intensity deep-UV laser sources.

In conclusion, the basic optical properties of commonly used deep-UV nonlinear materials (β-BBO, CLBO, ADP, and DKDP) were characterized, and their absolute optical transmittances in the deep-UV and vacuum-UV wavebands and bandgap energies were directly compared. Subsequently, by parametrically amplifying Yb:YAG laser using frequency doubling of Nd:YAG laser to generate the 1.1 μm laser radiation, we methodically demonstrated NCPM FHG and FiHG in ADP crystals and CPM FHG and NCPM FiHG in DKDP crystals. The highest conversion efficiencies in ADP and DKDP crystals from 1101 nm laser radiation to 220 nm were 17.5% and 23.6%, respectively, and the results prove the effectiveness and feasibility of obtaining deep-UV laser radiation near 200 nm via the FiHG in KDP-family crystals. Here, we not only demonstrated the FiHG in DKDP crystal for the first time but also found that the experimental results significantly deviate from the theoretical expectations. The required phase-matching temperature of FiHG deviates from the theoretical value by more than 100 °C. The temperature-dependent dispersion equations of DKDP crystal near 220 nm still need to be further studied. Nevertheless, the results show that the NCPM FiHG of 1.1 μm laser with large angular acceptance in both ADP and DKDP crystals can be efficiently achieved near room temperature, which has important practical significance, and this work also paves the way for obtaining high-energy and high-peak-power deep-UV and even vacuum-UV laser sources based on KDP-family large-aperture crystals.

This work was supported by the National Natural Science Foundation of China (Grants Nos. 12004404 and 61975218), Shanghai Sailing Program (Grant No. 18YF1425900), “Strategic Priority Research Program” of Chinese Academy of Sciences (Grant No. XDA25020202), and Youth Innovation Promotion Association CAS (Grant No. 2018282). The authors wish to thank Professor Weili Zhang and Dr. Hu Wang from Shanghai Institute of Optics and Fine Mechanics, CAS for the helpful discussion.

The authors have no conflicts to disclose.

Zijian Cui: Conceptualization (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal). Mingying Sun: Resources (lead); Supervision (lead); Validation (equal). Chao Wang: Resources (equal). Bin Shen: Resources (equal). Xu Zhang: Resources (equal). De’an Liu: Supervision (equal); Validation (equal); Writing – review & editing (equal). Jianqiang Zhu: Supervision (equal); Validation (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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