Expedited laser damage profiling of KDxH(2-x)PO4 with respect to crystal growth parameters

Performance characterization of freeform finished surfaces of potassium dihydrogen phosphate using fluid jet polishing with a nonaqueous slurry

Potassium dihydrogen phosphate (KDP) and its deuterated analog (DKDP) are unique nonlinear optical materials for high power laser systems. They are used widely for frequency conversion and polarization control by virtue of the ability to grow optical-quality crystals at apertures suitable for fusion-class laser systems. Existing methods for freeform figuring of KDP/DKDP optics do not produce surfaces with sufficient laser-induced-damage thresholds (LIDT's) for operation in the ultraviolet portion of high-peak-power laser systems. In this work, we investigate fluid jet polishing (FJP) using a nonaqueous slurry as a sub-aperture finishing method for producing freeform KDP surfaces. This method was used to selectively polish surface areas to different depths on the same substrate with removals ranging from 0.16 μm to 5.13 μm. The finished surfaces demonstrated a slight increase in roughness as the removal depth increased along with a small number of fracture pits. Laser damage testing with 351 nm, 1 ns pulses demonstrated excellent surface damage thresholds, with the highest values in areas devoid of fracture pits. This work demonstrates, for the first time, a method that enables fabrication of a waveplate that provides tailored polarization randomization that can be scaled to meter-sized optics. Furthermore, this method is based on FJP technology that incorporates a nonaqueous slurry specially designed for use with KDP. This novel nonaqueous FJP process can be also used for figuring other types of materials that exhibit similar challenging inherent properties such as softness, brittleness, water-solubility, and temperature sensitivity.

Progress on deuterated potassium dihydrogen phosphate (DKDP) crystals for high power laser system application

In this review, we introduce the progress in the growth of large-aperture DKDP crystals and some aspects of crystal quality including determination of deuterium content, homogeneity of deuterium distribution, residual strains, nonlinear absorption, and laser-induced damage resistance for its application in high power laser system. Large-aperture high-quality DKDP crystal with deuteration level of 70% has been successfully grown by the traditional method, which can fabricate the large single-crystal optics with the size exceeding 400 mm. Neutron diffraction technique is an efficient method to research the deuterium content and 3D residual strains in single crystals. More efforts have been paid in the processes of purity of raw materials, continuous filtration technology, thermal annealing and laser conditioning for increasing the laser-induced damage threshold (LIDT) and these processes enable the currently grown crystals to meet the specifications of the laser system for inertial confinement fusion (ICF), although the laser damage mechanism and laser conditioning mechanism are still not well understood. The advancements on growth of large-aperture high-quality DKDP crystal would support the development of ICF in China.

Combined modulation of incident laser light by multiple surface scratches and their effects on the laser damage properties of KH2PO4 crystal

Manufacturing-induced surface defects are deemed as a potential source, leading the laser-induced damage threshold (LIDT) of the actual KDP crystal optics to be much lower than the intrinsic one. However, the underlying mechanisms have not been fully recognized. We explore the combined modulation of incident laser light by multiple scratches and their effects on laser damage performance of KDP optics by modeling the light intensifications and performing a laser damage test. Under the combined modulation of multiple scratches, enhanced hot spots are generated due to the focusing effects of convex lens profiles surrounded by the neighboring scratches. The combined modulation actions are much stronger than that of a single scratch. The relative light intensities (I_Rs) caused by multiple scratches can reach up to two times, and the number of hot spots (IPs) are four times as large as those by a single scratch. The I_Rs exhibit a general, increasing tendency as the scratch density increases. But for the case of double total reflections of rear-surface scratches, the totally reflected lights are transmitted through neighboring scratches, resulting in decreasing tendency of I_Rs. The tested LIDTs and optical transmittances of multiple scratches present a gradual, decreasing tendency with the increase of scratch density, which agrees with the simulation results. Besides, the simulated light intensifications could well explain the locations of laser damage, which further verify the role of multiple scratches in lowering the laser damage resistance.

Influence of surface cracks on laser-induced damage resistance of brittle KH₂PO₄ crystal

Single point diamond turning (SPDT) currently is the leading finishing method for achieving ultra-smooth surface on brittle KH(2)PO(4) crystal. In this work, the light intensification modulated by surface cracks introduced by SPDT cutting is numerically simulated using finite-difference time-domain algorithm. The results indicate that the light intensification caused by surface cracks is wavelength, crack geometry and position dependent. Under the irradiation of 355 nm laser, lateral cracks on front surfaces and conical cracks on both front and rear surfaces can produce light intensification as high as hundreds of times, which is sufficient to trigger avalanche ionization and finally lower the laser damage resistance of crystal components. Furthermore, we experimentally tested the laser-induced damage thresholds (LIDTs) on both crack-free and flawed crystal surfaces. The results imply that brittle fracture with a series of surface cracks is the dominant source of laser damage initiation in crystal components. Due to the negative effect of surface cracks, the LIDT on KDP crystal surface could be sharply reduced from 7.85J/cm(2) to 2.33J/cm(2) (355 nm, 6.4 ns). In addition, the experiment of laser-induced damage growth is performed and the damage growth behavior agrees well with the simulation results of light intensification caused by surface cracks with increasing crack depths.

Photoluminescence characterization of KH₂PO₄ crystal: application to three-dimensional growth-sector identification

In this work, rapidly grown KH2PO4 (KDP) crystals extracted from the prismatic and the pyramidal growth sectors of crystal boules were analyzed using photoluminescence measurements. From the spectra, we deduced a robust criterion to discriminate between both growth sectors in an unknown KDP plate. Moreover, spatially resolved photoluminescence was shown to enable a local probing of different planes in the bulk of the material leading to accurate and nondestructive three-dimensional mapping of the sector boundary, which is often the weakest point in terms of laser-damage resistance in rapidly grown KDP crystals.

Investigation of the electronic and physical properties of defect structures responsible for laser-induced damage in DKDP crystals

Laser-induced damage at near operational laser excitation conditions can limit the performance of potassium dihydrogen phosphate (KH(2)PO(4), or KDP) and its deuterated analog (DKDP) which are currently the only nonlinear optical materials suitable for use in large-aperture laser systems. This process has been attributed to pre-existing damage precursors that were incorporated or formed during growth that have not yet been identified. In this work, we present a novel experimental approach to probe the electronic structure of the damage precursors. The results are modeled assuming a multi-level electronic structure that includes a bottleneck for 532 nm excitation. This model reproduces our experimental observations as well as other well-documented behaviors of laser damage in KDP crystals. Comparison of the electronic structure of known defects in KDP with this model allows for identification of a specific class that we postulate may be the constituent defects in the damage precursors. The experimental results also provide evidence regarding the physical parameters affecting the ability of individual damage precursors to initiate damage, such as their size and defect density; these parameters were found to vary significantly between KDP materials that exhibit different damage performance characteristics.

Nanosecond laser induced damage in RbTiOPO4: the missing influence of crystal quality

Nanosecond laser induced damage in RbTiOPO(4) (RTP) an isomorphic material to the more widely known KTiOPO(4) (KTP) is studied in crystals with varying properties. The ionic conductivity along the z-axes of the tested crystals ranged from 1.5 10(-9) S/cm to 1.1 10(-12) S/cm. Further, different growth sectors with different absorption in the range of hundreds of ppm/cm and differing zones in inhomogeneous crystals have been investigated. Despite these important differences in crystal quality, no significant difference could be observed in the laser damage resistance at 1064 nm. Thus growth induced defects only play a minor role in nanosecond laser induced damage in RTP. Transient, laser induced defects are discussed in analogy with KTP as possible laser damage precursors.

Laser damage performance of KD2-chiHchiPO4 crystals following X-ray irradiation

We investigate the laser-induced damage performance of KD(2-chi)H(chi)PO(4) crystals following exposure to X-ray irradiation. Two important issues addressed by our study are i) the performance of the material when operational conditions lead to its exposure to ionizing irradiation and ii) the way the radiation-induced transient defects interact with the pre-existing precursor defects responsible for laser-induced damage. Our results indicate that the damage performance of the material is affected by exposure to X-rays. This behavior is attributed to a change in the physical properties of the precursors which, in turn, affect their ability to initiate damage following interaction with X-ray generated defects.