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Counter-propagating ultrafast pulses can disrupt the phase of harmonic generation, and offer a means to achieve quasi-phase matching in processes like high-order harmonic generation. Optimizing this process requires accurate modeling. Using second harmonic generation (SHG) as a simpler and more accessible proxy, we compare the results of two numerical simulations to experimental measurements of SHG with counter-propagating pulses. The first follows previous theoretical work in assuming a quasi-CW pulse and solving the nonlinear wave equation in the time-domain. However, we find that adapting a frequency-domain model to account for the broadband nature of ultrafast pulses better reproduces the salient features we observe in our experimental results. 

We present a Monte Carlo model that simulates the effects of non-equilibrium carrier-carrier scattering and the presence of layers of ErAs nanoislands in a GaAs terahertz antenna detector. To minimize computing time, we split the model into two simulations on numerical grids with optimized resolutions. First, we calculate the effects of the ErAs nanoislands on carrier lifetime in a high resolution volume of GaAs. We then incorporate those results into a larger, lower resolution, two-dimensional simulation that models the antenna detector. The computational results match experimental data presented by Kadow et al. [Appl. Phys. Lett. 75, 3548–3550 (1999)] and show that the lifetime of the carriers is closely linked to the periodicity of the nanoisland layers. Our results also highlight how the periodicity of the nanoisland layers affects the sensitivity and bandwidth of the terahertz detector, information that can be used to create custom devices with optimal parameters.