Laser heating of free electrons in wide-gap optical materials at 1064 nm

Ultraviolet Laser Damage Dependence on Contamination Concentration in Fused Silica Optics during Reactive Ion Etching Process

The reactive ion etching (RIE) process of fused silica is often accompanied by surface contamination, which seriously degrades the ultraviolet laser damage performance of the optics. In this study, we find that the contamination behavior on the fused silica surface is very sensitive to the RIE process which can be significantly optimized by changing the plasma generating conditions such as discharge mode, etchant gas and electrode material. Additionally, an optimized RIE process is proposed to thoroughly remove polishing-introduced contamination and efficiently prevent the introduction of other contamination during the etching process. The research demonstrates the feasibility of improving the damage performance of fused silica optics by using the RIE technique.

Theoretical and experimental research on laser-induced damage of cylindrical subwavelength grating

We designed and fabricated two-dimensional cylindrical subwavelength gratings (SWGs) on fused silica for use in 1064 nm laser system. The transmittance and laser-induced damage threshold (LIDT) under the irradiation of 1064 nm pulses were performed, and a lower LIDT compared to blank fused silica was obtained. To understand damage mechanism quantitatively, macro-temperature on the SWGs integrated fused silica and micro-Electric field, micro-thermal-stress distribution on SWGs during the laser irradiation process were investigated by Finite Element Analysis, and the comparison between theoretical and experimental research indicated the presence of absorption centers and the non-uniform thermal mechanical distribution of SWGs contributed to the LIDT reduction on SWGs integrated fused silica.

Bulk and surface laser damage of silica by picosecond and nanosecond pulses at 1064 nm

We measured bulk and surface dielectric breakdown thresholds of pure silica for 14 ps and 8 ns pulses of 1064 nm light. The thresholds are sharp and reproducible. For the 8 ns pulses the bulk threshold irradiance is 4.75 +/- 0.25 kW/microm2. The threshold is approximately three times higher for 14 ps pulses. For 8 ns pulses the input surface damage threshold can be made equal to the bulk threshold by applying an alumina or silica surface polish.

Microscale nanosecond laser-induced optical breakdown in water

Microscale optical breakdown induced in bulk pure water by high-power nanosecond KrF laser pulses was studied using optical transmission and contact broadband photoacoustic techniques. The breakdown has been identified as a sharp transmission drop coinciding with the appearance of unipolar compressive acoustic pulses, both indicating a thresholdlike rise of local intrinsic absorption in the micrometer-scale laser focal volume. The acoustic pulses, which are much broader than the exciting laser pulse and show a strongly reduced far-field diffraction effect, result from breakdown-induced millimeter-sized steam bubbles. The acoustic pulse amplitudes exhibit a sub-linear ( proportional, variantI(3/4)) pressure dependence on the laser intensity I characteristic of subcritical electron-ion plasma and demonstrating the avalanche enhancement of two-photon ionization above the breakdown threshold until the appearance of the critical plasma. In the critical plasma regime, where the transmission and the acoustic signals slowly vary as a function of laser intensity, the main acoustic pulse is preceded by nanosecond and sub- micros prepulses, where the first one represents a GPa-level plasma-driven shock wave and the second one adjacent to the main pulse appears due to weak submillimeter-long heating of water surrounding the hot plasma by its bremsstrahlung radiation, indicating significant dissociation of water molecules in the plasma.

Imaging of laser-induced reactions of individual defect nanoclusters

The response of individual defect nanoclusters located in the bulk of a dielectric material following exposure to 355-nm, 3-ns high-power laser irradiation is investigated by use of microscopic fluorescence imaging. Experiments were carried out on KH(2)PO(4) crystals. We provide direct imaging of the reaction to an external stimulus of individual defect clusters and demonstrate a novel method of studying the dynamic behavior of bulk defects.

Microscopic fluorescence imaging of bulk defect clusters in KH(2)PO(4) crystals

A microscopic fluorescence imaging system is used to detect optically active centers located inside a transparent dielectric crystal. Defect centers in the bulk of KH(2)PO(4) crystals are imaged based on their near-infrared emission following photoexcitation. The spatial resolution of the system is 1mum in the image plane and 25mum in depth. The experimental results indicate the presence of a large number of optically active defect clusters in different KH(2)PO(4) crystals, whereas the concentration of these clusters depends on the crystal sector and growth method.

Mechanisms of Excimer Laser Ablation of Wide Band-Gap Materials: the Role of Defects in Single Crystal MgO

Laser ablation has important applications in surface modification, materials analysis, and thin film deposition. We have been examining the details of processes that lead to the emission and formation of particles (atomic/molecular ground state neutrals, excited neutrals, tions, electrons) when wide band gap materials are irradiated with pulsed UV laser light. Etching and deposition of wide bandgap materials is of particular interest due to their excellent insulating and optical properties. Our studies bear directly on achieving control of emission intensities and particle characteristics for use in film deposition and materials analysis. In model wide bandgap materials such as single crystal alkali halides and MgO (nominally transparent materials), exposure to repeated pulses of 248 nm excimer laser radiation of a few J/cm2 results in substantial interaction including extensive biaxial deformation and cleavage. Significant surface heating also occurs, consistent with strong free-carrier/laser interactions. We present strong evidence that achieving intense emission of atomic, molecular, and ionic particles actually depends on point defect production by laser-induced deformation and fracture. Defect production via dislocation motion yields orders of magnitude increases in laser vaporization of these wide bandgap materials, including cluster ion formation. The dependence of the laser-material interaction on dislocation density and mobility, as well as point defect density, suggests several novel strategies for the enhancing the ablative response or preventing laser damage.