We present experimental measurements of SHG and THG in the visible and UV ranges from crystalline silicon membranes 200nm to 2microns in thickness, and use the physical parameters retrieved to predict the properties of silicon metasurfaces. Our theoretical model predicts well both spectral and angular responses and efficiencies of the membranes. Silicon is centrosymmetric but possesses a third order nonlinearity that is dispersive and relatively large in the UV range. The results also suggest that judicious exploitation of the nonlinear dispersion of ordinary semiconductors has the potential to transform device physics well into the UV range.
We discuss traditional second and third harmonic generation from metallic mirrors, gratings, and novel nonlinear optical properties of metal/vacuum interfaces. The boundary is a spillout region composed of free electrons having exponentially decreasing density that vanishes within an atomic diameter. Classical electrodynamics cannot discern field variations either between atoms or over the distance of a decaying wave function. The boundary consists of a single spatial discretization step, a function of unknown average density. This layer acts like an epsilon-near-zero material that enhances the local field by more than three orders of magnitude, alongside a dramatic decrease of nonlinear thresholds.
We report contrasted experimental and theoretical results on second and third harmonic generation from an amorphous silicon nanolayer. We perform this study by analysing the harmonic optical signals generated when fs and ps pulses tuned at 800nm and 1064nm, respectively, interact with the material. We measure second and third harmonic efficiencies as functions of angle of incidence and compare with numerical simulations based on a microscopic hydrodynamic model that accounts for different possible contributions to the nonlinear polarization. We measure and estimate the efficiencies of the nonlinear signals using no assumptions about effective surface or volume nonlinearities, enabling proper explanations about the microscopic origins of SH and TH signals arising from the amorphous silicon nanolayer.
Metal nanostructures are largely used to enhance light-matter interactions and the overarching optical response by means of plasmonic resonances and surface-enhanced spectroscopy. They can also be designed to improve efficiency of nonlinear optical processes like second and third harmonic generation, despite their high absorption in the visible and near infrared range. When the metallic layer thickness is reduced to a few nanometers in thickness, light-matter interactions can display new phenomena where conventional approximations may not always be applicable. While planar structures are generally the simplest to fabricate and easier to simulate, the efficiency of the harmonic signals are generally quite small but can be enhanced when resonant nanostructures are implemented. In this work we design a gold nanograting with resonant features around 800nm. When the fundamental beam is tuned around the resonance wavelength in the near-infrared region we obtain a second and third harmonic signal tuned in the opaque region of gold. We firstly measure TM and TE components of the SH signal at 400 nm as a function of the fundamental beam polarization, tuning the fundamental wavelength around the resonance. Then we experimentally estimate the relative enhancement induced by the grating with respect to SHG from planar gold, finding a maximum enhancement of 800 for the central resonant wavelength. We calculate the predicted harmonic conversion efficiencies of the grating employing our microscopic hydrodynamic approach to model light-matter interactions. This model relies on temporal and spatial derivatives and mere knowledge of the effective electron mass to determine the relative magnitudes of surface and volume. Our simulations predict an enhancement factor close to 1000, of the same order of magnitude as our measurements. The same model predicts a 500-fold enhancement for the THG with respect to the plain gold layer.
Methods currently used to determine nonlinear optical constants like n2 or chi3 rely on open and closed z-scan techniques. The study of optics at the nanoscale in the femtosecond regime requires new tools and approaches to extract linear and nonlinear dispersions exhibited by matter. We present a practical approach that amounts to numerical ellipsometry that utilizes experimental harmonic generation conversion efficiencies to retrieve complex, nonlinear dispersion curves. We provide examples of retrieved linear and nonlinear dispersions for a variety of materials, and show that for Silicon the numerical retrieval method yields chi3~10^(-16) (m/V)^2 and chi33w~10^(-17) (m/V)^2 , and visible and near IR ranges. Similarly, we predict chi3~10^(-17) (m/V)^2 and chi33w~10^(-19) (m/V)^2 for ITO as it exhibits linear and nonlinear anisotropic responses due to nonlocal effects.
Nanostructures made of semiconductors or metals are nowadays routinely integrated in photonic devices. At this scale light-matter interaction displays interesting new phenomena. We report a collection of experimental results of nonlinear harmonic generation in different nanolayers: semiconductors, conductive oxides and metals. The comparison of these experimental results with numerical predictions of our theoretical model identifies, distinguishes and explains the different nonlinear contributions to the harmonics generated by these materials at nanoscale. Our model accounts for surface, magnetic and bulk nonlinearities arising from free and bound charges, preserving linear and nonlinear dispersion, nonlocal effects due to pressure and viscosity.
Currently, nanostructures are routinely fabricated and integrated in different photonic devices for a variety of purposes and applications. For instance, in order to engineer properly nano-antennas or filters, it is important to understand accurately how light interacts with metals, semiconductors, or ordinary dielectrics at the nanoscale. When the nanoscale is reached, light-matter interactions displays new phenomena and conventional approximations may not always be applicable. Thus, new strategies must be sought in order to study and understand light-matter interactions at the nanoscale. In this work, we present experimental results of second and third harmonic generation from gold nanolayers, raveling novel behavior at nanoscale. These measurements are compared with numerical simulations based on a microscopic hydrodynamic model which accounts for surface, magnetic and bulk nonlinearities arising from both free and bound charges, preserving linear and nonlinear dispersion, nonlocal effects due to pressure and viscosity, and an intensity dependent free electron density, to which we refer as hot electrons contribution.
The use of semiconductors, metals and conductive oxides in the process of fabrication of actual nano devices is at the front edge of nowadays technology, exploiting the properties of light propagation and localization at nanometric scale in new and surprising ways. At these scales the usual theory describing the nonlinear (NL) effects of electromagnetic fields should be revisited and analyzed.
We report a collection of experimental results of nonlinear harmonic generation in different nanolayers: semiconductors, conductive oxides and metals. The comparison of these experimental results with numerical predictions of our theoretical model identifies, distinguishes and explains the different nonlinear contributions to the harmonics generated by these materials at nanoscale. Our model accounts for surface, magnetic and bulk nonlinearities arising from free and bound charges, preserving linear and nonlinear dispersion, nonlocal effects due to pressure and viscosity.
The use of semiconductors such as GaAs, GaP or Si in the process of fabrication of actual nano devices is at the front edge of nowadays technology, exploiting the properties of light propagation and localization at nanometric scale in new and surprising ways. At these scales the usual theory describing the nonlinear effects of electromagnetic fields are pushed to the limit of usual approximations and should be revisited and analyzed. Recently, we have studied in detail the generation of the second and third harmonic the opaque region of GaAs and Si, going beyond the previous studies and deeply analyzing the nonlinear process in order to infer which are the different mechanisms leading to the second and third harmonic generation at the surface of these materials. We demonstrate that the bulk nonlinearity is not the only one active term and that we have strong contributions coming from the surface and magnetic Lorentz terms, which usually are either hidden by the bulk contributions or assumed to be negligible. Experimental and theoretical simulations are contrasted, using a hydrodynamic model [1,2] that accounts for all salient aspects of the dynamics, including surface and bulk generated harmonic components. [3] The study, made in detail for GaAs is extended here to other semiconductors as Si and GaP. We also consider resonant structures as gratings and nanowires capable to strongly enhance the nonlinear efficiencies. Although the harmonic generation in this regime and materials still has low efficiency, these findings have significant repercussions and are consequential in nanoscale systems, which are usually investigated using only dispersion less bulk nonlinearities, with near-complete disregard of surface and magnetic contributions and their microscopic origins.
We report a comparative experimental and theoretical study of second harmonic generation from a 20nm-thick indium tin oxide nanolayer in the proximity of the epsilon-near-zero condition. We record the efficiency of the second harmonic signal both as a function of wavelength as well as of the angle of incidence around the epsilon-near-zero crossing point. We compare our experimental results with numerical simulations based on a hydrodynamical model able to capture all major physical mechanisms driving the electrodynamic behavior of conductive oxide layers, with unique aspects of the different nonlinear sources. We found a very good quantitative and qualitative agreement between experiment and theory.
We demonstrate second harmonic generation from a GaAs substrate, well-below the absorption edge. The pump is tuned in the transparency range, at 1064 nm, while the SH is tuned in the opaque spectral range of GaAs, at 532 nm. We work far from the phase matching condition and we find that the phase locked component of the second harmonic propagates trough the opaque material. As expected, we find that the polarization of the generated SH signal is sensitive to the polarization of the pump. We demonstrate different surface and bulk contributions to the SH transmitted signal and we show that the surface-generated SH components can be more intense than bulk-generated SH signals. The experimental results are contrasted with numerical simulations that include these two factors, using a hydrodynamic model, accounting for all aspects of the dynamics, including surface and bulk generated harmonic components.
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