No Arabic abstract
Strong interaction between light and matter waves, such as electron beams in electron microscopes, has recently emerged as a new tool for understanding entanglement. Here, we systematically investigate electron-light interactions from first principles. We show that enhanced coupling can be achieved for systems involving slow electron wavepackets interacting with plasmonic nanoparticles, due to simultaneous classical recoil and quantum mechanical photon absorption and emission processes. For slow electrons with longitudinal broadenings longer than the dimensions of nanoparticles, phase-matching between slow electrons and plasmonic oscillations is manifested as an additional degree of freedom to control the strength of coupling. Our findings pave the way towards a systematic and realistic understanding of electron-light interactions beyond adiabatic approximations, and lay the ground for realization of entangled electron-photon systems and Boson-sampling devices involving light and matter waves.
The bulk photovoltaic effect (BPVE) has attracted an increasing interest due to its potential to overcome the efficiency limit of traditional photovoltaics, and much effort has been devoted to understanding its underlying physics. However, previous work has shown that theoretical models of the shift current and the phonon-assisted ballistic current in real materials do not fully account for the experimental BPVE photocurrent, and so other mechanisms should be investigated in order to obtain a complete picture of BPVE. In this Letter, we demonstrate two approaches that enable the ab initio calculation of the ballistic current originating from the electron-hole interaction in semiconductors. Using BaTiO$_3$ and MoS$_2$ as two examples, we show clearly that for them the asymmetric scattering from electron-hole interaction is less appreciable than that from electron-phonon interaction, indicating more scattering processes need to be included to further improve the BPVE theory. Moreover, our approaches build up a venue for predicting and designing materials with larger ballistic current due to electron-hole interactions.
Slow-light media are of interest in the context of quantum computing and enhanced measurement of quantum effects, with particular emphasis on using slow-light with single photons. We use light-in-flight imaging with a single photon avalanche diode camera-array to image in situ pulse propagation through a slow light medium consisting of heated rubidium vapour. Light-in-flight imaging of slow light propagation enables direct visualisation of a series of physical effects including simultaneous observation of spatial pulse compression and temporal pulse dispersion. Additionally, the single-photon nature of the camera allows for observation of the group velocity of single photons with measured single-photon fractional delays greater than 1 over 1 cm of propagation.
The Fresnel-Fizeau effect of transverse drag, in which the trajectory of a light beam changes due to transverse motion of the optical medium, is usually extremely small and hard to detect. We observe transverse drag in a moving hot-vapor cell, utilizing slow light due to electromagnetically induced transparency (EIT). The drag effect is enhanced by a factor 360,000, corresponding to the ratio between the light speed in vacuum and the group velocity under EIT conditions. We study the contribution of the thermal atomic motion, which is much faster than the mean medium velocity, and identify the regime where its effect on the transverse drag is negligible.
Matter manipulation with optical forces has become commonplace in a wide range of research fields and is epitomized by the optical trap. Calculations of optical forces on small illuminated particles typically neglect multiple scattering on nearby structures. However, this scattering can result in large recoil forces, particularly when the scattering includes directional near-field excitations. Near-field recoil forces have been studied in the case of electric, magnetic and circularly polarized dipoles, but they exist for any type of directional near-field excitation. We use the force angular spectrum as a concise and intuitive analytical expression for the force on any dipole near planar surfaces, which allows us to clearly distinguish the effect due to the dipole, and due to the surface. We relate this directly to the coupling efficiency of surface or guided modes via Fermis golden rule. To exemplify this, a near-field force transverse to the illumination is computationally calculated for a Huygens dipole near a metallic waveguide. We believe this formalism will prove insightful for various nanomanipulation systems within areas such as nanofluidics, sensing, biotechnology and nano-assembly of nanostructures.
The emerging field of on-chip integration of nanophotonic devices and cold atoms offers extremely-strong and pure light-matter interaction schemes, which may have profound impact on quantum information science. In this context, a long-standing obstacle is to achieve strong interaction between single atoms and single photons, while at the same time trap atoms in vacuum at large separation distances from dielectric surfaces. In this work, we study new waveguide geometries that challenge these conflicting objectives. The designed photonic crystal waveguide is expected to offer a good compromise, which additionally allows for easy manipulation of atomic clouds around the structure, while being tolerant to fabrication imperfections.