Electromagnetic radiation in the terahertz (THz) frequency range is a fascinating spectroscopic tool that provides resonant access to fundamental modes, including the motions of free electrons, the ...rotations of molecules, the vibrations of crystal lattices and the precessions of spins. Consequently, THz waves have been extensively used to probe such responses with high sensitivity. However, owing to recent developments in high-power sources, scientists have started to abandon the role of pure observers and are now exploiting intense THz radiation to engineer transient states of matter. This Review provides an overview and illustrative examples of how the electric and magnetic fields of intense THz transients can be used to control matter and light resonantly and nonresonantly.
The ideas of topology have found tremendous success in closed physical systems, but even richer properties exist in the more general open or dissipative framework. We theoretically propose and ...experimentally demonstrate a bulk Fermi arc that develops from non-Hermitian radiative losses in an open system of photonic crystal slabs. Moreover, we discover half-integer topological charges in the polarization of far-field radiation around the bulk Fermi arc. Both phenomena are shown to be direct consequences of the non-Hermitian topological properties of exceptional points, where resonances coincide in their frequencies and linewidths. Our work connects the fields of topological photonics, non-Hermitian physics, and singular optics, providing a framework to explore more complex non-Hermitian topological systems.
Melt-driven erosion in microparticle impact Hassani-Gangaraj, Mostafa; Veysset, David; Nelson, Keith A ...
Nature communications,
11/2018, Volume:
9, Issue:
1
Journal Article
Peer reviewed
Open access
Impact-induced erosion is the ablation of matter caused by being physically struck by another object. While this phenomenon is known, it is empirically challenging to study mechanistically because of ...the short timescales and small length scales involved. Here, we resolve supersonic impact erosion in situ with micrometer- and nanosecond-level spatiotemporal resolution. We show, in real time, how metallic microparticles (~10-μm) cross from the regimes of rebound and bonding to the more extreme regime that involves erosion. We find that erosion in normal impact of ductile metallic materials is melt-driven, and establish a mechanistic framework to predict the erosion velocity.
We report a demonstration of two-dimensional (2D) terahertz (THz) magnetic resonance spectroscopy using the magnetic fields of two time-delayed THz pulses. We apply the methodology to directly reveal ...the nonlinear responses of collective spin waves (magnons) in a canted antiferromagnetic crystal. The 2D THz spectra show all of the third-order nonlinear magnon signals including magnon spin echoes, and 2-quantum signals that reveal pairwise correlations between magnons at the Brillouin zone center. We also observe second-order nonlinear magnon signals showing resonance-enhanced second-harmonic and difference-frequency generation. Numerical simulations of the spin dynamics reproduce all of the spectral features in excellent agreement with the experimental 2D THz spectra.
Intense single-cycle THz pulses resonantly interacting with molecular rotations are shown to induce field-free orientation and alignment under ambient conditions. We calculate and measure the degree ...of both orientation and alignment induced by the THz field in an OCS gas sample, and correlate between the two observables. The data presents the first observation of THz-induced molecular alignment in the gas phase.
Melting Can Hinder Impact-Induced Adhesion Hassani-Gangaraj, Mostafa; Veysset, David; Nelson, Keith A ...
Physical review letters,
2017-Oct-27, Volume:
119, Issue:
17
Journal Article
Peer reviewed
Open access
Melting has long been used to join metallic materials, from welding to selective laser melting in additive manufacturing. In the same school of thought, localized melting has been generally perceived ...as an advantage, if not the main mechanism, for the adhesion of metallic microparticles to substrates during a supersonic impact. Here, we conduct the first in situ supersonic impact observations of individual metallic microparticles aimed at the explicit study of melting effects. Counterintuitively, we find that under at least some conditions melting is disadvantageous and hinders impact-induced adhesion. In the parameter space explored, i.e., ∼10 μm particle size and ∼1 km/s particle velocity, we argue that the solidification time is much longer than the residence time of the particle on the substrate, so that resolidification cannot be a significant factor in adhesion.
We study supersonic impact of individual metallic microparticles on metallic substrates, that is, the unit process of materials buildup in cold spray coatings/additive manufacturing. We resolve the ...moment of impact bonding through real-time observations of single particle impacts with micron-scale and nanosecond-level resolution. We offer the first in-situ observation of a material-dependent threshold velocity, above which the particle undergoes an impact-induced jet-like material ejection and adheres to the substrate. We report direct measurements of critical velocities for structural metals, which unlike in nozzle experiments, are not affected by process-related complexities obscuring particles' kinetic and thermal histories.
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Strong, long-range Coulomb interactions can lead to correlated motions of multiple charged particles, which can induce important many-body effects in semiconductors. The exciton states formed from ...correlated electron-hole pairs have been studied extensively, but basic properties of multiple-exciton correlations-such as coherence times, population lifetimes, binding energies and the number of particles that can be correlated-are largely unknown because they are not spectroscopically accessible from the ground state. Here we present direct observations of high-order coherences in gallium arsenide quantum wells, achieved using two-dimensional multiple-quantum spectroscopy methods in which up to seven successive light fields were used. The measurements were made possible by the combination of a reconfigurable spatial beam-shaper that formed multiple beams in specified geometries and a spatiotemporal pulse-shaper that controlled the relative optical phases and temporal delays among pulses in all the beams. The results reveal triexciton coherences (correlations of three excitons or six particles), whose existence was not obvious because the third exciton spin is unpaired, and the values of their coherence times and binding energies. Rephasing of biexcitons, triexcitons and unbound two-exciton coherences was demonstrated. We also determined that there are no significant unbound correlations of three excitons and no bound or unbound four-exciton (eight-particle) correlations. Thus, the limits, as well as the properties, of many-body correlations in this system were revealed. The measurement methods open a new window into high-order many-body interactions in materials and molecules, and the present results should guide ongoing work on first-principles calculations of electronic interactions in semiconductor nanostructures.
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