Quantum theory for cold avalanche ionization in solids

Effect of annealing on nonlinear optical properties of 70% deuterated DKDP crystals at 355 nm

Thermal annealing decreased the nonlinear absorption and refraction, which could lead to an improvement of the laser-induced damage threshold.

Traceless mitigation of laser damage precursors on a fused silica surface by combining reactive ion beam etching with dynamic chemical etching

HF-based etching has been successful in mitigating damage precursors on the surface of fused silica optics used in high power lasers. However, wet etching generally leaves an etching trace leading to surface roughness, which seriously degrades laser beam quality (e.g., transmission loss and wave-front degradation). A way of addressing this issue is to apply plasma etching as a preprocessing step before HF etching, but so far very few studies have provided a practical scheme for engineering applications. In this work, we proposed a novel two-step scheme by combining reactive ion beam etching with dynamic chemical etching techniques. We demonstrate the combined scheme is capable of tracelessly mitigating the laser damage precursors on a fused silica surface. The 0% probability damage threshold obtained by combined etching is 1.4 times higher than that obtained by HF-based etching. The study opens a new approach towards high damage-resistant optics manufacturing and provides the potential possibility of exploring extreme interactions between high-power lasers and matter.

RIBE and DCE techniques can be combined to tracelessly mitigate laser damage precursors on a fused silica surface.

Quantitative correlation between facets defects of RDX crystals and their laser sensitivity

In this work, the {210} facets of cyclotrimethylenetrinitramine (RDX) single crystals with different quality were studied by scanning electron microscopy and atomic force microscopy. Their laser sensitivity was then assessed using a direct laser ignition test irradiated with ultraviolet laser (wavelength: 355nm, pulse width: 6.4ns). Quantitative relationships between laser sensitivity and surface defects of RDX (210) and (2¯1¯0) facets were investigated. It is determined that the laser sensitivity exhibits significant correlation with the surface roughness, size of which is comparable with scales of laser wavelength. 3D FDTD simulations disclose that this relationship can be well explained with light intensity modulation effects induced by micro-defects on the initial plane wave.

Ultraviolet Laser-induced ignition of RDX single crystal

The RDX single crystals are ignited by ultraviolet laser (355 nm, 6.4 ns) pulses. The laser-induced damage morphology consisted of two distinct regions: a core region of layered fracture and a peripheral region of stripped material surrounding the core. As laser fluence increases, the area of the whole crack region increases all the way, while both the area and depth of the core region increase firstly, and then stay stable over the laser fluence of 12 J/cm². The experimental details indicate the dynamics during laser ignition process. Plasma fireball of high temperature and pressure occurs firstly, followed by the micro-explosions on the (210) surface, and finally shock waves propagate through the materials to further strip materials outside and yield in-depth cracks in larger surrounding region. The plasma fireball evolves from isotropic to anisotropic under higher laser fluence resulting in the damage expansion only in lateral direction while maintaining the fixed depth. The primary insights into the interaction dynamics between laser and energetic materials can help developing the superior laser ignition technique.

Formation of broadband antireflective and superhydrophilic subwavelength structures on fused silica using one-step self-masking reactive ion etching

Fused silica subwavelength structures (SWSs) with an average period of ~100 nm were fabricated using an efficient approach based on one-step self-masking reactive ion etching. The subwavelength structures exhibited excellent broadband antireflection properties from the ultraviolet to near-infrared wavelength range. These properties are attributable to the graded refractive index for the transition from air to the fused silica substrate that is produced by the ideal nanocone subwavelength structures. The transmittance in the 400–700 nm range increased from approximately 93% for the polished fused silica to greater than 99% for the subwavelength structure layer on fused silica. Achieving broadband antireflection in the visible and near-infrared wavelength range by appropriate matching of the SWS heights on the front and back sides of the fused silica is a novel strategy. The measured antireflection properties are consistent with the results of theoretical analysis using a finite-difference time-domain (FDTD) method. This method is also applicable to diffraction grating fabrication. Moreover, the surface of the subwavelength structures exhibits significant superhydrophilic properties.

Effects of γ-ray irradiation on optical absorption and laser damage performance of KDP crystals containing arsenic impurities

The effects of γ-irradiation on potassium dihydrogen phosphate crystals containing arsenic impurities are investigated with different optical diagnostics, including UV-VIS absorption spectroscopy, photo-thermal common-path interferometer and photoluminescence spectroscopy. The optical absorption spectra indicate that a new broad absorption band near 260 nm appears after γ-irradiation. It is found that the intensity of absorption band increases with the increasing irradiation dose and arsenic impurity concentration. The simulation of radiation defects show that this absorption is assigned to the formation of AsO₄⁴⁻ centers due to arsenic ions substituting for phosphorus ions. Laser-induced damage threshold test is conducted by using 355 nm nanosecond laser pulses. The correlations between arsenic impurity concentration and laser induced damage threshold are presented. The results indicate that the damage performance of the material decreases with the increasing arsenic impurity concentration. Possible mechanisms of the irradiation-induced defects formation under γ-irradiation of KDP crystals are discussed.

Suppression of transverse stimulated Raman scattering with laser-induced damage array in a large-aperture potassium dihydrogen phosphate crystal

A laser-induced damage array composed of numerous pinpoints is generated in a large-aperture KDP crystal to suppress the transverse stimulated Raman scattering (TSRS). The 36 cm × 8.5 mm × 7 mm damage array is used to block the propagation of the TSRS photons within the crystal and decrease the TSRS gain length. Then several series of experiments were conducted on a large-aperture laser system to test this method and experimental results show that the amplification of TSRS is significantly suppressed by the laser-induced damage array.

Femtosecond crystallographic experiment in wide-bandgap LiF crystal

We report a femtosecond crystallographic study of the dependence of the free-carries generation to the alignment of a crystalline sample to the laser polarization. The probe pulse transmission exhibits a π/2 modulation that is shown to be correlated with the direction dependence of the effective electron mass. This observation suggests that nonlinear ionization is the first channel for free electron generation during the laser pulse. Moreover, the temporal evolution of the probe pulse transmission indicates the dominance of the avalanche ionization and that nonlinear ionization provides the initial seed electrons for avalanche."