Nitromethane (NM), a flammable liquid, has been a model system for the shock-to-detonation transition in homogeneous condensed-phase explosives for over 50 years, but we do not understand the fast processes at the molecular scale in the detonation front at the molecular scale. That is largely because prior studies triggered detonations in bomb-sized charges with input shock durations and times-to detonation that were typically microseconds, which made it impossible to observe the faster processes in real time. We studied NM shocked with 4 ns duration input pulses using a tabletop apparatus with laser-launched flyer plates and arrays of tiny disposable optical cuvettes, where the pressure and temperature were probed in real time (1 ns) with photon Doppler velocimetry, optical pyrometry, and high-speed video. Using a 4 ns shock with an input pressure close to the von Neumann spike pressure of 19 GPa, we achieved the minimum time-to-detonation, about 12 ns, where the time-to-detonation is controlled by fundamental molecular processes. We demonstrated the reproducibility of our detonations and showed that they had the same properties as in bomb-sized charges: our detonation velocity, von Neumann spike and Chapman-Jouguet pressures, temperatures, and reaction zone lengths were the same as in bomb-sized charges. Being able to trigger realistic reproducible detonations from a short pulse makes it possible to investigate molecular and fluid dynamics in the detonation by measuring transient responses in real time. We found that it took 6 ns for the temperature to reach 3430 K. The high pressure was observed at about 8 ns, when there was a volume explosion to nearly twice the von Neumann spike pressure before settling down to a steady detonation.

Experimental studies of high-velocity (1-4 km/s) impacts of laser-launched flyer plates with polycrystalline, glassy or polymer windows were performed, with the intent of understanding and assessing the use of optical windows as high-speed solid-state shock wave detector materials. The problem is that the detector material undergoes radical physical transformations during the measurement. Here we present a simplified model where the detector material transformations are described as velocity and time dependent changes in shock impedance. The window materials were used to study the detonation shock produced by nitromethane, which is about 20 GPa and 10 ns. Reference experiments producing shocks in this pressure and duration range characterized how the shock impedances changed with impact velocity and shock duration. Sapphire, CaF₂ and LiF windows had the fastest response to the detonation shocks, while the polymer windows had a sluggish response. Pyrex and BK7 were the best of the glassy windows.

Lowering threshold and enhancing intensity of stimulated Raman scattering (SRS) have been a major focus of nonlinear optics. In our experiments, we combined SRS generation and amplification techniques to enhance the SRS signal. The amplification was achieved through a process called resonance effect, where the frequency difference between the pump laser and the Stokes light emitted from the working medium had self-matched with the vibrational energy level. The performance improvements included a higher SRS conversion efficiency, reaching up to 5.31%, approximately three times the value of a single sample cell efficiency of 1.739%, and realization of a cascaded SRS up to four orders extending from 461 to 997 nm. These Stokes and anti-Stokes coherent lights were generated coaxially to the pump beam as axial and conical ring beams, respectively, which can be explained by the four-wave mixing. This technique has the potential to contribute to the development of a multi-wavelength cascade Raman laser.

We are reporting an esoteric method to determine the optical bandgap of direct gap materials by employing Urbach’s rule. The latter, which describes the slope of the band tail absorption in semiconductors, in its original version, cannot be employed to pinpoint the optical bandgap. Herein, however, we show that a modified form of Urbach’s rule defines the optical bandgap, and therefore, enables the accurate determination of the optical bandgap energy, which turns out to be identical with the threshold energy for the band tail absorption. The model further produces an explicit expression for the absorption coefficient at the optical bandgap energy.

We present a computational approach using a multimaterial, arbitrary Lagrangian–Eulerian code termed ALE3D to model the nanosecond/micrometer dynamics of the launch of 0.5–4.5 km/s laser-driven metal flyer plates and the impact with stationary targets of Pyrex and fused silica glasses, and Lexan and Plexiglas polymers, producing pressures in the target in the range of 5–20 GPa. The simulations are compared to experimental results where the flyer velocity profile and the velocity profile imparted to the target material were measured with high-speed velocimetry. The experimental flyer launch by a high-intensity pulsed laser is modeled by depositing heat into a thin vaporizable layer under the flyer plate. This model produces a flyer plate that has not been exposed to the laser pulse, allowing us to compare the properties of the real flyer to a simulated ideal flyer. The simulations of target impact are generally in good agreement with the experiment except at the highest impact velocities where the shock release process in the experiment is slower than that in the simulation. The cause of this disagreement is attributed to an inadequate description of the shock viscosity during the nanosecond unloading process.

The work addresses an unresolved topic in solid-state physics, i.e., the dependence of the Debye temperature (T_D) on the energy band gap (E_g) of semiconducting materials. The systematic calculation of T_D by using the ratio of sound velocity and lattice constant from the literature resulted in the relation T_D∝exp(E_g). The exponential relationship is confirmed by a theoretical model based on the microscopic analysis of the electrical conductivity in metals and semiconductors.