Junya Hattori

Laser-induced stress waves play a crucial role in ultrashort pulse laser (USPL) processing such as laser shock peening. However, previous studies have primarily relied on numerical simulations or two-dimensional measurements, lacking experimental validation of the three-dimensional stress distribution. This study presents what we believe to be a novel optical measurement technique to quantify the three-dimensional axisymmetric shear stress distribution of laser-induced stress waves using time-resolved circular polariscope imaging and integrated photoelasticity. Femtosecond laser pulses were used to generate stress waves in silica glass, and the induced birefringence was captured using a polariscope. The slow-axis angle and the optical retardance were extracted and used to reconstruct three-dimensional distribution of the shear stress component. The reconstructed stress distribution agrees with theoretical predictions and numerical simulations of a previous study. Additionally, we found that higher pulse energies resulted in broader stress wavefronts. This behavior is attributed to the expansion of the ablation region, which alters the wavefront shape. The ability to visualize and quantify the shear stress in three dimensions offers a more comprehensive understanding of stress wave interactions with materials. This study lays the foundation for further advancements in USPL processing and experiments utilizing USPL-induced high pressure.

Femtosecond laser-induced plasma filaments have potential for various applications including attosecond physics, spectroscopy, and microprocessing. However, the use of plasma filaments to generate high-aspect-ratio internal modifications remains low-efficiency. Here, we experimentally demonstrated high-efficiency internal processing using plasma filaments induced by a double-pulse femtosecond laser. The processing mechanism was revealed through an investigation of the ultrafast dynamics of plasma filaments in experiments and simulations. We found that the excitation region of the first pulse (P1) exerted a temporal effect on the propagation and absorption of the second pulse (P2) due to the evolution of excited electrons, thus resulting in different processing characterizations. At a smaller inter-pulse delay (IPD), electrons and self-trapped excitons induced by P1 improved the absorption of P2 in the shallow region. Consequently, the main excitation regions of P1 and P2 were separated, resulting in a lower density of energy deposition and weak modifications. Whereas, at a larger IPD, P2 penetrated a deeper region with the relaxation of electrons and excitons induced by P1, leading to a better overlap of excitation regions between P2 and P1, thus improving the density of energy deposition and achieving efficient microprocessing. Besides, at an infinite IPD, P2 behaved like P1, but no modification was obtained owing to the complete energy diffusion of P1. Therefore, controlling the electron dynamic and energy diffusion contributes to the improvement of modification efficiency. Furthermore, the distribution of electron densities on the cross section was estimated to precisely analyze the microprocessing. These results are expected to aid in a better understanding of the interaction mechanism between dielectrics and intense ultrafast lasers and be useful for microprocessing applications.

Stress waves propagating inside materials play a significant role in femtosecond laser processing. In this study, we measured femtosecond-laser-induced stress waves inside synthetic silica glass using a time-resolved Mach–Zehnder interferometer. A laser pulse with a wavelength of 1030 nm, pulse width of 180 fs, and pulse energy of 100 μJ was used to induce stress waves. The three-dimensional distribution of the refractive-index change of the stress wave was obtained via reconstruction using the inverse Abele transform. This result contributes to the further development of femtosecond laser processing.

Controlling the tissue temperature rise during retinal laser therapy is highly desirable for predictable and reproducible outcomes of the procedure, especially with non-damaging settings. In this work, we demonstrate a method for determining the optical absorption, the thermal conductivity, and the thermal expansion coefficients of RPE and choroid using phase-resolved optical coherence tomography (pOCT). These parameters are extracted from the measured changes in the optical path length (ΔOPL) using an axisymmetric thermo-mechanical model. This allows the calculation of the temperature rise during hyperthermia, which was further validated by imaging the temperature-sensitive fluorescence at the same location. We demonstrate that, with a temperature uncertainty of ±0.9°C and a peak heating of about 17°C following a laser pulse of 20 ms, this methodology is expected to be safe and sufficiently precise for calibration of the non-damaging retinal laser therapy. The method is directly translatable to in-vivo studies, where we expect a similar precision.