DKDP Crystals Research Articles

High-efficiency fourth harmonic generation and polarization smoothing based on an orthogonal cascade of DKDP crystals: publisher's note.

This publisher's note corrects an error in Appl. Opt.63, 1411 (2024)APOPAI0003-693510.1364/AO.512229.

High-efficiency fourth harmonic generation and polarization smoothing based on an orthogonal cascade of DKDP crystals.

Because of the high efficiency of frequency conversion and beam-target coupling, a fourth harmonic (4ω) laser has a splendid application prospect in a high-power laser facility. The polarization smoothing (PS) crystal is preferably after the frequency conversion crystal to flexibly obtain the best uniformity illumination of the target. However, as a high irradiance 4ω laser beam propagates through the PS crystal, the transverse stimulated Raman scattering (TSRS) effect of the PS crystal will be stronger, resulting in significant energy dissipation and crystal damage. This paper proposes a novel, to the best of our knowledge, fourth harmonic generation (FHG) scheme based on an orthogonal cascade of the DKDP crystals. This orthogonal cascaded FHG (OC-FHG) scheme employs two cascaded FHG crystals with orthogonal optical axes, and the PS crystal is in the middle. The PS crystal can rotate the polarization direction of the 2ω laser by 90°, while the polarization direction of the 4ω laser is maintained to a great extent. This OC-FHG scheme realizes the FHG by two steps, and the laser intensity at the PS crystal cuts down nearly 50%. The output intensity of the 4ω laser can be increased from 1.8G W/c m 2 to about 3.6G W/c m 2 under the condition of effectively inhibiting the TSRS effect. Meanwhile, the output 4ω laser contains two orthogonal polarized beams realizing in-beam polarization smoothing instantaneously. In addition, the novel FHG scheme can also have a high conversion efficiency and bandwidth tolerance.

Characteristics of broadband OPCPA based on DKDP crystals with different deuterations for the SEL-100 PW laser system.

We present the performances of a broadband optical parametric chirped pulse amplification (OPCPA) using partially deuterated potassium dihydrogen phosphate (DKDP) crystals with deuteration levels of 70% and 98%. When pumped by a Nd:glass double frequency laser, the OPCPA system using the 98% deuterated DKDP crystal achieves a broad bandwidth of 189 nm (full width at 1/e2 maximum) from 836 nm to 1025 nm. For the DKDP crystal with length of 43 mm, the pump-to-signal conversion efficiency reaches 28.4% and the compressed pulse duration is 13.7 fs. For a 70% deuterated DKDP crystal with a length of 30 mm, the amplified spectrum ranges from 846-1021 nm, the compressed pulse duration is 15.7 fs, and the conversion efficiency is 25.5%. These results demonstrate the potential of DKDP crystals with higher deuteration as promising nonlinear crystals for use as final amplifiers in 100 Petawatt (PW) laser systems, supporting compression pulse duration shorter than 15 fs.

Effect of deuterium content on the structural, optical, and thermal properties of DKDP crystals: a systematic analysis.

Deuterated potassium dihydrogen phosphate (DKDP) crystals with different deuterium contents have a wide range of applications, such as frequency conversion in high power lasers, electro-optic modulation, and Q-switching crystals for Pockels cells. However, there is a lack of systematic research on the effect of deuterium content on the fundamental structure and properties of these DKDP crystals. To this end, in this study, a series of DKDP crystals with different deuterium contents have been grown using the "point-seed" rapid growth method, and the structure and properties of the crystals have been characterized. The results indicate that as the deuterium content increases, the cell parameter along the a(b)-axis direction gradually increases, and the transmittance gradually increases in the infrared range. A small amount of doping (low H or D ratio) reduces the structural integrity of the crystal, and the crystals at intermediate deuterium concentrations have better crystallinity. The thermal properties of the crystals do no change significantly with the variation in the deuterium content. Overall, these findings can serve as a useful reference for boosting the application of DKDP crystals with various deuterium contents.

Investigation and analysis of pin-point damage and damage growth characteristics in KDP and DKDP crystals.

In this paper, we report the damage and damage growth in potassium dihydrogen phosphate and its deuterated analog crystals. A time-resolved shadow imaging system was used to investigate the damage behavior in the bulk and on the rear surface. The damage images show differences in the damage sizes of the crystals with different deuterization rates. Theoretical simulations demonstrated that this may be due to differences in the crystallographic defects. The experimental results showed that the development of crystal damage was not only manifested as the expansion of damage on the rear surface of the crystal but also as an increase in pin-point density and size within the crystal. Crystals with higher deuterization rates had higher probability of the increasing of initial damage size, rather than the increasing of pin-point density.

Final amplifier of an ultra-intense all-OPCPA system with 13-J output signal energy and 41% pump-to-signal conversion efficiency.

Optical parametric chirped-pulse amplification (OPCPA) using high-energy Nd:glass lasers has the potential to produce ultra-intense pulses (>1023 W/cm2). We report on the performance of the final high-efficiency amplifier in an OPCPA system based on large-aperture (63 × 63-mm2) partially deuterated potassium dihydrogen phosphate (DKDP) crystals. The seed beam (180-nm bandwidth, 110 mJ) was provided by the preceding OPCPA stages. A maximum pump-to-signal conversion efficiency of 41% and signal energy up to 13 J were achieved with a 52-mm-long DKDP crystal due to the flattop super-Gaussian pump beam profile and flat-in-time pulse shape.

Formation mechanism of DKDP surface in single point diamond fly-cutting process and the resulting degradation of laser-induced damage performance.

Laser damage performance of DKDP (KD2xH2(1-x)PO4) crystal is largely determined by the surface microstructures generated in the manufacturing process, more specifically, single point diamond fly-cutting process. However, because of the lack of knowledge about the formation mechanism and damage performance of the microstructures, laser induced damage of DKDP crystal remains a key issue limiting the output energy of the high power laser systems. In this paper, the influence of fly-cutting parameters on the generation of DKDP surface and the underlying material deformation mechanism have been investigated. Except for cracks, two kinds of new microstructures, namely micro grains and ripples, have been found on the processed DKDP surfaces. GIXRD, nano-indentation and nano-scratch test results prove that the micro grains are generated by the slip motion of the crystal, while the simulation results show that the cracks are induced by the tensile stress formed behind the cutting edge. Moreover, the formation of micro grains can facilitate the plastic chip flow through the mechanism of grain boundary sliding, which will further lead to a periodic fluctuation of the chip separation point and the formation of micro ripples. Finally, laser damage test results demonstrate that cracks will degrade the damage performance of DKDP surface significantly, while the formation of micro grains and micro ripples has little impact. The results of this study can deepen the understanding of the formation mechanism of the DKDP surface during the cutting process and provide guidance to improve the laser-induced damage performance of the crystal.

Phase-stabilized UV light at 267 nm through twofold second harmonic generation.

Providing phase stable laser light is important to extend the interrogation time of optical clocks towards many seconds and thus achieve small statistical uncertainties. We report a laser system providing more than 50 µW phase-stabilized UV light at 267.4 nm for an aluminium ion optical clock. The light is generated by frequency-quadrupling a fibre laser at 1069.6 nm in two cascaded non-linear crystals, both in single-pass configuration. In the first stage, a 10 mm long PPLN waveguide crystal converts 1 W fundamental light to more than 0.2 W at 534.8 nm. In the following 50 mm long DKDP crystal, more than 50 µW of light at 267.4 nm are generated. An upper limit for the passive short-term phase stability has been measured by a beat-node measurement with an existing phase-stabilized quadrupling system employing the same source laser. The resulting fractional frequency instability of less than 5×10-17 after 1 s supports lifetime-limited probing of the 27Al+ clock transition, given a sufficiently stable laser source. A further improved stability of the fourth harmonic light is expected through interferometric path length stabilisation of the pump light by back-reflecting it through the entire setup and correcting for frequency deviations. The in-loop error signal indicates an electronically limited instability of 1 × 10-18 at 1 s.

Optical and laser damage properties of 98% deuterium DKDP crystal in different crystal orientations

Served as electro-optic switch, frequency converter and the optical parametric chirped pulse amplifier (OPCPA), highly deuterated DKDP crystals are critical optical elements in high power laser systems. In our work, samples taken in Z direction and Ⅰ-type phase-match direction from an as-grown high deuterium DKDP crystal, which was grown by the rapid growth method with continuous filtration. Thermo-gravimetric apparatus (TGA) measurement was applied to obtain the deuterium content of the DKDP crystal. the results showed the deuterium level of the samples in our experiments exceeded 98%. The ultraviolet–visible–near-infrared transmittance spectrum showed that the transmittance of Z-cut at 532 nm and Ⅰ-type at 1064 nm were 92.99% and 96.07%, respectively. The 0% laser-induced damage threshold of Z-cut in 1064 nm was about 40J/cm2, exceeding much more than the application requirement in certain high power laser systems. The polarization state of laser had little effect on the laser-induced damage characteristics of Ⅰ-type sample. Then the laser-induced damage morphologies of 98% DKDP crystals induced by the 1064 nm and 532 nm lasers were investigated. The results indicated that the "micro-explosion" of different kinds of defects in the bulk led to different damage morphologies, and many cracks in the damage pinpoints presented the regular orientation with better consistency in view of the Ⅰ-type phase-match direction. Our research can provide a reference for the applications of highly deuterated DKDP crystals in high power laser systems.

220 nm deep-UV coherent source based on fifth-harmonic generation in ADP and DKDP crystals

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.

Research on the Nonlinear Absorption Coefficient of 98% Deuterated DKDP Crystal at Fourth-Harmonic-Generation Wavelength

A series of 98% deuterated DKDP crystals were grown in solutions with different pD values (2.9, 3.3, 3.8, and 4.3) using the rapid growth method. Samples were cut along the z-direction and fourth-harmonic-generation (FHG) direction which contained both pyramidal and prismatic regions. The nonlinear absorption (NLA) coefficient β of 98% deuterated DKDP crystals was obtained using the Z-scan method operated at the FHG wavelength (266 nm) of a picosecond Nd:YAG laser. According to the results, the nonlinear absorption at 266 nm could be identified as two-photon absorption. The β values of crystal grown in the solution with 3.3 pD value were higher than those of crystals grown in solutions with other pD values under higher laser fluence. The results also indicated that FHG device samples should be prepared from the pyramidal region due to its lower β value. This work will help optimizing the application of 98% DKDP crystals as FHG elements in high-power laser systems.

Ultraviolet laser-induced damage characteristics of 70% deuterated potassium dihydrogen phosphate crystals

Bulk damage of deuterated potassium dihydrogen phosphate (DKDP, KD2PO4) crystal can be induced by intense laser irradiation, severely restricting the output of high- power laser energy and the usage of DKDP crystals in high-power laser systems. In this paper, laser-induced damage threshold (LIDT) and damage growth characteristics of DKDP crystal under 355 nm laser irradiation were systematically studied. The bulk laser-induced damage (LID) density of the crystal increased exponentially with the increase of the laser irradiation fluence. LID closely relates to the growth defects called precursors. Laser conditioning could effectively enhance the LID resistance by modifying the precursors. The LID density of DKDP crystals decreases after laser conditioning, and the increasing conditioning fluence can reduce the LID density. Damage growth after the initial LID by 1-on-1 measurement is different from that by R-on-1 measurement since the process of R-on-1 measurement leading to initial LID is equal to the laser conditioning procedure. The study in this paper provides a reference for improving the application of DKDP crystal in high-power laser systems.

Noncritical phase-matching fourth- and fifth-harmonic generation of 1077 nm laser using KDP-family crystals.

We systematically demonstrated the angular and temperature acceptances of noncritical phase-matching (NCPM) fourth- and fifth-harmonic generation (FHG and FiHG) of a 1077 nm laser in NH4H2PO4 (ADP), KH2PO4 (KDP), and KD2PO4 (DKDP) crystals. In this work, a new, to the best of our knowledge, laser frequency with a wavelength of 1077 nm was generated by optical parametric amplification, in which the pump light (526.3 nm) was generated by the frequency doubling of a Nd:YLF laser (1052.7 nm), and the signal light was a Yb:YAG laser (1029.5 nm). Subsequently, the 1077 nm laser was used as the fundamental wave for FHG and FiHG to obtain a deep-ultraviolet laser source. For ADP and DKDP crystals, NCPM FHG of a 1077 nm laser was realized at 74.0∘C and 132.5∘C, respectively, and large angular acceptances of 59.8 and 61.6 mrad were measured. For the FiHG, NCPM was realized in a KDP crystal at 48.5∘C with an angular acceptance of 56.4 mrad. The results pave the way for high-energy and high-power deep-ultraviolet laser generation using KDP-family crystals under noncryogenic conditions.

Enhancing the temporal contrast and peak power of femtosecond laser pulses.

It is shown that a nonlinear polarization interferometer and a chirped mirror enable enhancement of the contrast of high-power laser pulses with a duration of tens and hundreds of femtoseconds by several orders of magnitude and simultaneously a several-fold reduction of their duration. Different variants of interferometers based on cubic nonlinearity in KDP and DKDP crystals are considered. The interferometer and chirped mirror parameters are optimized aimed at enhancing the peak power of the compressed pulse.

Rapid growth technology and optical properties of a large aperture high-deuterium DKDP crystal

In this paper, a concept of establishing a uniform temperature field range and a “double-layer four-stage continuous filtration circulation system (DF-CFCS)” during crystal growth have been proposed.

Study on directional rapid growth and properties of 90% DKDP crystal

Due to the obvious pyramid-prism boundary in the deuterated potassium dihydrogen phosphate (DKDP) crystal grown by “point-seed” rapid growth method, the crystal uniformity and utilization rate were poor. In this paper, a DKDP crystal with deuterium concentration of about 90% has been grown using a new crystal carrier, and its deuterium concentration, extinction ratio and transmission spectrum have been measured. At the same time, the optical uniformity of this crystal was compared with that grown by the “point-seed” crystal rapid growth method, and the influence of these two methods on the pyramid-prism boundary of the crystal was studied. The result showed that the average deuterium concentration of the directionally grown DKDP crystal was 90.27%, which met the requirements of the experimental plan. Meanwhile, it was shown that the extinction ratio was up to 4623:1, and both the pyramidal sector and the prismatic sector of the crystal had good transmission performance. The optical uniformity test chart indicated that there was obvious pyramid-prism boundary in the crystal samples obtained by “point-seed” rapid growth method, while there was no pyramid-prism boundary in those by directional rapid growth method.

Optimizing sub-nanosecond laser conditioning of DKDP crystals by varying the temporal shape of the pulse.

We propose a strategy to optimize the laser conditioning of DKDP crystals by varying the temporal shape of sub-nanosecond pulses. Four sub-ns temporally shaped pulses with nearly the same full width at half maxima of ∼600 ps but different rising-falling statuses were designed to conduct laser-induced damage (LID) and laser conditioning experiments on DKDP crystals. The shape of the pulse substantially influences the damage pinpoints size and LID threshold (LIDT) of the crystals in the sub-nanosecond range. After sub-nanosecond laser conditioning, the ns R-on-1 LIDT showed that slow-rising fast-falling pulse (R400-F200 and High-foot pulses) conditioning achieved a 14%-20% LIDT enhancement than the traditional Gaussian pulse (R300-F300 pulse). The 8-ns laser damage morphologies after slow-rising fast-falling pulse conditioning showed cracks, whereas those after fast-rising slow-falling pulse (R200-F400 pulse) conditioning were pinpoint core, as usual. These results suggest that the rising front plays an important role in the LID and laser conditioning of the DKDP crystals. A pulse with a slower rising front is beneficial for thermal modification, thereby leading to better LID properties. This strategy greatly expands and enriches the manipulation methods to improve the LIDT of DKDP crystals, and sheds light on understanding the laser damage mechanisms.

Optical parametric amplification at critical wavelength degeneracy—a proposed approach for 100-PW class femtosecond laser development

At signal critical wavelength degeneracy (CWD), super-broad parametric gain bandwidths (SBGBs) can be obtained in nonlinear crystals. A 280 nm SBGB, for ∼60 small signal peak gain, has been calculated considering a collinear optical parametric chirped pulse amplification (OPCPA) at CWD in a 40 mm long, 58.3% partially deuterated KDP (P-DKDP) crystal, pumped by a frequency doubled nanosecond Nd:glass laser at 1 GW/cm2 pump intensity. This gain bandwidth in the 1 µm spectral range is significantly broader compared to the 150 nm gain bandwidth, calculated under similar conditions, for the broad-band non-collinear (NC) OPCPA in DKDP crystals in the 900 nm spectral range. It can support the amplification of significantly shorter femtosecond pulses, giving rise to a higher peak power at the same amplified pulse energy. The spectral profile of the gain bandwidth can be shaped by slightly adjusting the deuteration ratio (DR) of the P-DKDP crystals. A small-angle non-collinear OPCPA near the CWD can be used for the spatial separation of the signal and idler beams with the same polarization, keeping at the same time an ultra-broad gain bandwidth. A SBGB of 255 nm FWHM has been calculated for a non-collinear OPCPA stage with a 40 mm long P-DKDP crystal, 10.3 mrad signal-pump wavevectors internal angle, 57.8% DR, pumped by a few-ns 527 nm wavelength laser at 1 GW/cm2 pump intensity. The half-meter size P-DKDP crystals, pumped by frequency doubled few-ns multi-kJ Nd:glass lasers, are able to amplify super broad-band nanosecond stretched pulses up to the kJ energy level. They might be appropriate amplifying media for the development of single-beam 100-PW class femtosecond laser systems based on OPCPA at CWD.

Numerical analysis of the DKDP-based high-energy optical parametric chirped pulse amplifier for a 100 PW class laser.

An optical parametric chirped pulse amplifier (OPCPA) based on a large-aperture DKDP crystal and pumped by a 10 kJ level Nd:glass laser can serve as the final amplifier for a 100 PW level laser. A comprehensive numerical investigation on such a high-energy OPCPA is presented in this work. The effects on the efficiency-bandwidth product induced by the deuteration level, absorption loss, temperature variation, and optimization of zero-phase-mismatch wavelength (ZPMW) are analyzed in detail. Based on the analysis above, a three-dimensional numerical simulation taking into account the effects of pumping depletion, diffraction, and walk-off shows that, by optimization of ZPMW, broadband (over 210 nm spectral width in FWHM) and high efficiency (${\gt}37\%$) amplification can be realized in the DKDP crystal even with a moderate deuteration level of 70%, which can relax the requirement of a high deuteration level in a large-aperture DKDP crystal. The numerical analysis can provide meaningful guidance for the design and construction of 100 PW class laser systems.

Characterization of partially deuterated KDP crystals using two-wavelength phase-matching angles

Partially deuterated potassium dihydrogen phosphate (DKDP) allows for the optical parametric chirped-pulse amplification of high-energy broadband optical pulses to generate ultrashort, high-peak-power pulses. Modeling and experimental optimization of the noncollinear interaction geometry require the combination of a model for the deuteration-dependent optical indices and knowledge of the deuteration level of the DKDP crystal being used. We study and demonstrate a novel experimental technique that determines the deuteration level of a DKDP crystal consistent with a specific index model, based on the phase-matching angles measured at two wavelengths. Simulations and experiments show that the simple two-wavelength technique allows for consistent results with three different index models, and in particular, for the characterization of four crystals with deuteration levels ranging from 70% to 98%.