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Register a new userHarmonic generation from gold nanolayers: an old problem under a new light
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Understanding how light interacts at the nanoscale with metals, semiconductors, or ordinary dielectrics is pivotal if one is to properly engineer nano-antennas, filters and, more generally, devices that aim to harness the effects of new physical phenomena that manifest themselves at the nanoscale. We presently report experimental results on second and third harmonic generation from 20nm- and 70nm-thick gold layers, for TE- and TM-polarized incident light pulses. We highlight and discuss for the first time the relative roles bound electrons and an intensity dependent free electron density (hot electrons) play in third harmonic generation. While planar structures are generally the simplest to fabricate, metal layers that are only a few nanometers thick and partially transparent are almost never studied. Yet, transmission offers an additional reference point for comparison, which through relatively simple experimental measurements affords the opportunity to test the accuracy of available theoretical models. Our experimental results are explained well within the context of the microscopic hydrodynamic model that we employ to simulate second and third harmonic conversion efficiencies, and to simultaneously and uniquely predict the nonlinear dispersive properties of a gold nanolayer under pulsed illumination. Using our experimental observations and our model, based solely on the measured third harmonic power conversion efficiencies we predict |chi3|~10^(-18)-10^(-17)(m/V)^2, triggered mostly by hot electrons, without resorting to the implementation of a z-scan set-up.
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Noble metals with well-defined crystallographic orientation constitute an appealing class of materials for controlling light-matter interactions on the nanoscale. Nonlinear optical processes, being particularly sensitive to anisotropy, are a natural and versatile probe of crystallinity in nano-optical devices. Here we study the nonlinear optical response of monocrystalline gold flakes, revealing a polarization dependence in second-harmonic generation from the {111} surface that is markedly absent in polycrystalline films. Apart from suggesting an approach for directional enhancement of nonlinear response in plasmonic systems, we anticipate that our findings can be used as a rapid and non-destructive method for characterization of crystal quality and orientation that may be of significant importance in future applications.
The recent observation of high-harmonic generation from solids creates a new possibility for engineering fundamental strong-field processes by patterning the solid target with subwavelength nanostructures. All-dielectric metasurfaces exhibit high damage thresholds and strong enhancement of the driving field, making them attractive platforms to control high-harmonics and other high-field processes at nanoscales. Here we report enhanced non-perturbative high-harmonic emission from a Si metasurface that possesses a sharp Fano resonance resulting from a classical analogue of electromagnetically induced transparency. Harmonic emission is enhanced by more than two orders of magnitude compared to unpatterned samples. The enhanced high harmonics are highly anisotropic with excitation polarization and are selective to excitation wavelength due to its resonant feature. By combining nanofabrication technology and ultrafast strong-field physics, our work paves the way for designing new compact ultrafast photonic devices that operate under high intensities and short wavelengths.
We re-examine a 50+ year-old problem of deep central reversals predicted for strong solar spectral lines, in contrast to the smaller reversals seen in observations. We examine data and calculations for the resonance lines of H I, Mg II and Ca II, the self-reversed cores of which form in the upper chromosphere. Based on 3D simulations as well as data for the Mg II lines from IRIS, we argue that the resolution lies not in velocity fields on scales in either of the micro- or macro-turbulent limits. Macro-turbulence is ruled out using observations of optically thin lines formed in the upper chromosphere, and by showing that it would need to have unreasonably special properties to account for critical observations of the Mg II resonance lines from the IRIS mission. The power in turbulence in the upper chromosphere may therefore be substantially lower than earlier analyses have inferred. Instead, in 3D calculations horizontal radiative transfer produces smoother source functions, smoothing out intensity gradients in wavelength and in space. These effects increase in stronger lines. Our work will have consequences for understanding the onset of the transition region, the energy in motions available for heating the corona, and for the interpretation of polarization data in terms of the Hanle effect applied to resonance line profiles.
The $Kepler$ $problem$ studies the planar motion of a point mass subject to a central force whose strength varies as the inverse square of the distance to a fixed attracting center. The orbits form a 3-parameter family of unparametrized plane curves, consisting of all conics sharing a focus at the attracting center. We study the geometry and symmetry properties of this family, as well as natural 2-parameter subfamilies, such as those of fixed energy or angular momentum. Our main result is that Kepler orbits form a `flat family, that is, the local diffeomorphisms of the plane preserving this family form a 7-dimensional local group, the maximum dimension possible for the symmetry group of a 3-parameter family of plane curves (a result of S. Lie). The new symmetries are different from the well-studied `hidden symmetries of the Kepler problem, acting on energy levels in the 4-dimensional phase space of the problem. Furthermore, each 2-parameter family of Kepler orbits with fixed non-zero energy admits $mathrm{PSL}_2(mathbb{R})$ as its symmetry group and coincides with one of the items of a classification due to A. Tresse (1896) of 2nd order ODEs admitting a 3-dimensional group of point symmetries. Other items on Tresse list also appear in Keplers problem by considering repulsive instead of attractive force or motion on a surface with (non-zero) constant curvature. Underlying these newly found symmetries is a duality between Keplers plane and Minkowskis 3-space parametrizing the space of Kepler orbits.
High-harmonic generation is one of the most fundamental processes in strong laser-field physics that has led to countless achievements in atomic physics and beyond. However, a rigorous quantum electrodynamical picture of the process has never been reported. Here, we prove rigorously and demonstrate experimentally that the quantum state of the driving laser field, as well as that of harmonics, is coherent. Projecting this state on its part corresponding to harmonic generation, it becomes a superposition of a state, amplitude-shifted due to the quantum nature of light, and the initial state of the laser. This superposition interpolates between a Schr{o}dinger kitten, and a genuine Schr{o}dinger cat state. This work opens new paths for ground-breaking investigations in strong laser-field physics and quantum technology. We dedicate the work to the memory of Roy J. Glauber, the inventor of coherent states.
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