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.

Counterpropagating ultrafast pulses can disrupt phase matching of harmonic generation, allowing all-optical QPM. We report on improved theoretical understanding of this disruption based on multiple nonlinear three-wave processes.

Here we show an explicit measurement of the microscopic disruption to the phase-matching conditions for second-harmonic generation caused by a counterpropagating light field. This microscopic phase disruption has been assumed as the mechanism behind successful implementations of all-optical quasi-phase matching of high-order harmonic generation but has never been previously observed. Numerical simulations reproduce the features of the observed disruption and indicate pathways for implementing in situ probing and quasi-phase matching of second-harmonic generation with counterpropagating fields. (C) 2016 Optical Society of America

We study the effects of electron density and temperature, both experimentally and numerically, on the Coulomb screening process responsible for saturation of the terahertz generation process at the surface of InAs. We use a pair of ultrafast pulses with adjustable time delay to generate terahertz radiation and compare the results to a 1D drift-diffusion equation model modified to include radial diffusion and cooling of the electrons through scattering. We demonstrate the necessity of these modifications by implementing the original drift-diffusion equation model, as reported by Liu et al. [Phys. Rev. B 73, 155330 (2006)], and show that it underestimates saturation by an order of magnitude. We find excellent agreement between our experimental and numerical results, confirming the validity of our improved model and demonstrating its potential use in a number of applications, such as terahertz pulse shaping with an ultrafast pulse train. (C) 2016 Optical Society of America

Experimental and numerical results verify the microscopic influence of coun- terpropagating light on the phase of the nonlinear polarization wave, providing detailed understanding of an all-optical method for quasi-phase matching.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text E. Gagnon and A. L. Lytle, "Saturation effects on terahertz generation at the surface of InAs," in Frontiers in Optics 2015, OSA Technical Digest (online) (Optica Publishing Group, 2015), paper FW4A.6. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

We present a new technique to study materials' response to broadband, optical excitation using upconversion through excited state absorption. We use the broadband spectrum from an ultrafast laser coupled to a spectral shaper to manipulate our excitation bandwidth and selectively control which transitions occur in the ground state and excited state absorptions. By analyzing the effects of spectral shaping of the excitation laser on the emission yield of fluorescent light, we can test models of the electron population densities and transition probabilities to recover electron energy-level-specific information about the system being studied. Here, we apply this technique to Y2O3:Er3(+). (C) 2015 Optical Society of America.

We have observed modulation of the conversion efficiency of second harmonic generation by 1-2% of the spectral intensity using a single counterpropagating pulse to locally disrupt the phase of the nonlinear polarization wave.

We present a novel scheme for studying up-conversion through excited state absorption (ESA) by using a broadband excitation source with spectral shaping capabilities. Up-conversion processes have typically been investigated using a single, narrowband excitation source, when the two steps of the process are coincident in frequency, which is often made possible by broadening mechanisms of the intermediate excited state manifolds. Thus, narrowband sources are limited in the systems they can excite and what material information they can provide. With broadband light, we are able to drive up-conversion with non-coincident frequencies as well. Finally, by windowing the spectrum, we determine the optimal excitation bandwidth for low-concentration (1%) Y2O3:Er3+ nanocrystals.

Recent high-precision measurements in a three-slit diffraction experiment [Sinha et al., Science 329, 418 (2010)] have been performed as an explicit test of the validity of Born's rule for quantum probabilities. This experiment aims to establish an upper limit to the possibility of higher-order interference, which, if observed, could support generalization of quantum probability theory. We reproduce this three-slit experiment using position-resolved detection, compare our results to a computational model, and find significant limitations to the normalization scheme proposed by Sinha et al. that influence interpretation. We further show that the dependence of the measurements on detector size and position must be taken into account for proper interpretation of results and meaningful comparison with other experimental schemes.

By combining laser pulse self-compression and high harmonic generation within a single waveguide, we demonstrate high harmonic emission from multiply charged ions for the first time. This approach enhances the laser intensity and counteracts ionization-induced defocusing, extending the cutoff photon energy in argon above 500 eV for the first time, with higher spectral intensity and cutoff energy than He for the same input laser parameters. This Letter demonstrates a pathway for extending high harmonic emission to very high photon energies using large, multiply charged, ions with high ionization potentials.

By combining pulse self-compression and high harmonic generation within a single waveguide, we demonstrate harmonic emission from a multiply ionized gas, extending the cutoff photon energy in Ar to > 500 eV.