Publications

High damage threshold gratings are in demand worldwide as critical components for next generation ultrahigh intensity lasers. Here we investigate the pulse-duration dependence of ultrafast laser-induced damage thresholds (LIDT) in hybrid multilayer dielectric gratings, touted to combine superior performance properties of both metallic and multilayer dielectric (MLD) gratings, using a dynamic finite-difference time-domain model incorporated with linear and non-linear absorption models. Simulations agree with reported experimental LIDT values for three representative designs and predict scaling exponents which vary with pulse durations ranging from 10 to 500 fs. The results reveal strong dependence on both material bandgap and grating field distribution, providing guidance for designing high LIDT gratings.

We present a finite-difference time-domain (FDTD) framework for modeling femtosecond (fs) to sub-picosecond (ps) laser interactions in hybrid multilayer optical coatings. A total-field/scattered-field (TFSF) formulation injects obliquely incident, transversely Gaussian pulses without prescribing a full initial field profile, enabling stable simulations beyond the sub-100-fs regime. Material response incorporates Keldysh photoionization and impact ionization described by a multiple-rate-equation (MRE) model, accurately capturing carrier multiplication in wide-bandgap dielectrics. The generated carriers are coupled to a Drude model whose collision frequency evolves in time through the electron temperature, providing a unified description of optical response in both dielectric and metallic layers. The framework is applied to a hybrid SiO₂/HfO₂/Au grating irradiated by a 1057 nm, 500 fs pulse. Simulated carrier density and absorbed energy density predict laser-induced damage thresholds (LIDT) in very good agreement with experiment. The approach is general and applicable to pulse durations from 10 fs to 1 ps.

We present a 2D FDTD–based framework for sub-ps laser interaction with hybrid metal–dielectric gratings, incorporating Keldysh photoionization and impact ionization to predict ultrafast electron dynamics, field enhancement, and damage initiation.

In this work, (201) β-Ga₂O₃ was irradiated with 95 fs, 1030 nm laser pulses in order to investigate ultrafast laser-induced morphological and crystalline defects, which play an important role in the controlled transformation of materials at the nanomicro-scale. Kelvin probe force microscopy and depth-resolved cathodoluminescence spectroscopy (DRCLS) revealed laser-induced subsurface crystallographic defects below the ablation threshold that were undetectable by optical and atomic force microscopy. While DRCLS probed depths of 58–180 nm, scanning transmission electron microscopy provided complementary insights into regions beyond the reach of DRCLS, enabling direct imaging of the crystal structure and defects at or just below the surface. The analysis revealed a depth-dependent modification of the material, with an amorphous layer forming closest to the surface and damage site, transitioning to a defective region exhibiting a phase change to γ-Ga₂O₃, and further transitioning to a region rich in point defects, with defect concentrations decreasing with depth. A Keldysh-ionization-based FDTD simulation of the single-pulse interaction was carried out as well, revealing a high density of carrier generation consistent with the depth scales observed by the measurements. These findings contribute to a deeper understanding of defect formation mechanisms in β-Ga₂O₃ and highlight the potential of ultrashort laser pulses for precision subsurface modification in wide-bandgap semiconductors.

We developed a customized 2D finite-difference time-domain (FDTD) simulation framework to investigate ultrashort laser interactions with dielectric materials and predict the laser-induced damage threshold (LIDT). Unlike traditional FDTD models that only compute electromagnetic field distributions, our implementation includes the Keldysh photoionization model, impact ionization, the Spitzer collision model, and the Drude model for free-carrier response. These enhancements enable dynamic tracking of transient electron density and field enhancement during femtosecond pulse irradiation. Applying this framework, we simulated the interaction of a 2 μm wavelength, 70 fs laser pulse with a multilayer dielectric (MLD) grating fabricated via ion beam sputtering (IBS), a popular architecture for future laser drivers for particle accelerators, and predicted the peak LIDT fluence based on the critical density threshold. The simulation also reveals that damage is initiated in the first HfO₂ layer directly beneath the grating pillars, where the peak electron density occurs.

We present a fully three-dimensional kinetic framework for modeling intense short-pulse lasers interacting with dielectric materials. Our work modifies the open-source particle-in-cell code EPOCH to include new models for photoionization and dielectric optical response. We use this framework to model the laser-induced damage of dielectric materials by few-cycle laser pulses. The framework is benchmarked against experimental results for bulk silica targets and then applied to model multilayer dielectric mirrors through a sequence of simulations with varying laser fluence. This allows us to better understand the laser damage process by providing new insight into energy absorption, excited particle dynamics, and nonthermal excited particle distributions. We compare common damage-threshold metrics based on energy density and excited-electron density.

Ultrahigh-intensity laser systems are crucial for various applications, such as particle acceleration and the study of extreme conditions. Therefore, a higher laser-induced damage threshold (LIDT) of optical components is demanded. To study the mechanisms of ultrafast laser-solid interaction, modeling is important. Traditional modeling of ultrashort laser-solid interaction is based on the finite-difference time-domain (FDTD) method, which can only reveal the electromagnetic field distributions. However, particle-in-cell (PIC) simulation can reveal both the field distributions and the particle dynamics. We used and modified EPOCH with Keldysh photoionization and a model for spatially varying permittivity to capture the thermalization of electrons and plasma generation in a multilayer dielectric mirror irradiated by a few-cycle pulse and to predict the LIDT. The modeling work can provide guidance for the design and manufacturing of optical components.

The strong-field ionized electron motion and energy in the interaction of a few-cycle femtosecond laser pulse with a SiO₂/HfO₂-based multilayer dielectric reflector designed for 800 nm is studied using the PIC method incorporated with the Keldysh photoionization theory.

Multilayer dielectric (MLD) gratings with high diffraction efficiency and a high laser-induced damage (LID) threshold for pulse compressors are key to scaling the peak and average power of chirped pulse amplification lasers. However, surface defects introduced by manufacturing, storage, and handling processes can reduce the LID resistance of MLD gratings and impact the laser output. The underlying mechanisms of such defect-initiated LID remain unclear, especially in the femtosecond regime. In this Letter, we model dynamic processes in interactions of a 20-fs near-infrared (NIR) laser pulse and a MLD grating design in the presence of cylindrically symmetrical nodules and particle contaminants and cracks at the surface. Utilizing a dynamic model based on a 2D finite difference in time domain (FDTD) field solver coupled with photoionization, electron collision, and refractive index modification, we study the simulation results for the damage site distribution initiated by defects of various types and sizes and its impact on the LID threshold of the grating design.

The influence of various defects on the femtosecond laser damage resistance of a HfO₂/SiO₂/Ta₂O₅-based 48-layer grating designed for 800 nm is studied using the finite-difference time-domain method incorporated with the Keldysh photoionization theory.