Confinement and enhancement of light in structures only few atomic diameters across may enable a new class of devices with functionalities in the macroscopic continuum. Pushing toward the ultraviolet range calls for renewed studies of metals, semiconductors, and conductive oxides. For example, aluminum is inexpensive, stable, abundant, has a unique spectral response, and is compatible with metal-oxide-semiconductor technology. We use a hydrodynamic-Maxwell approach to study harmonic generation in nanolayers and gratings in the femtosecond regime and compare with predictions of harmonic generation from gold and silicon gratings, and a cadmium oxide nanofilm arranged in a Kretschmann configuration.
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.
In high power regimes, broad area semiconductor lasers usually suffer from poor beam quality due to their asymmetric beam divergence, large beam quality factor (M2) and from the absence of any intrinsic filtering mechanism that can be integrated inside the cavity. In this work, we present a compact photonic crystal spatial filter, fabricated by periodically modulating refractive index media on a glass substrate using tightly focused femtoseconds laser. This filter work by deflecting the higher angular field components in a given frequency range. We demonstrate the spatial filtering effect when placed in an extended cavity configuration for a single BAS emitter, with transverse width of 400-μm and cavity length 1500-μm. We report a decrease of the laser M2 value along the slow axis with the introduction of the photonic crystal inside the cavity, together with a brightness enhancement by a factor of 1.5 compared to that of an unfiltered case. These results were compared with those obtained in the far-field domain, with a conventional spatial filter consisting of an intra-cavity slit.
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 developed a 1kW cw fiber-coupled diode laser at 9XX nm by using beam combining of eight high power diode laser bars. To achieve beam combining, we employ Lyot-filtered optical reinjection from an external cavity, which forces lasing of the individual diode laser bars on intertwined frequency combs with overlapping envelopes and enables a high optical coupling efficiency. Unlike other spectral beam combining techniques that are based on the use of grating elements, this technique is insensitive to the thermal drift of the laser diodes. In addition to this, the FWHM spectral width at 1 kW output power is only around 7 nm, which is convenient for wavelength sensitive applications such as pumping.
In the context of electromagnetism and nonlinear optical interactions damping is generally introduced as a phenomenological, viscous term that dissipates energy, proportional to the temporal derivative of the polarization. Here, we follow the radiation reaction method presented in [G. W. Ford and R. F. O'Connell, Phys. Lett. A, 157, 217 (1991)], which applies to non-relativistic electrons of finite size, to introduce an explicit reaction force in the Newtonian equation of motion, and derive a hydrodynamic equation that offers new insight on the influence of damping in generic plasmas, metal-based and/or dielectric structures. In these settings, we find new damping-dependent linear and nonlinear source terms that suggest the damping coefficient is proportional to the local charge density, and nonlocal contributions that stem from the spatial derivative of the magnetic field that under the right conditions could modify both linear and nonlinear responses.
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.
We discuss a dynamical model of harmonic generation that arises from surfaces that demarcate two different metals or conductors, or the electron cloud that spills outside a simple metal surface and the interior bulk, having different electron densities, for example a noble metal such as gold, and indium tin oxide (ITO). While in general two adjacent materials may contain free and bound charges that determine their respective dielectric constants, the transition region may be characterized by a large discontinuity in the free electron density, , epsilon-near-zero conditions, or multiple, nested plasmonic resonances. For example, , while the free-electron cloud that spills outside a noble metal surface decreases as a function of position from the hard ionic surface. These discrepancies lead to the prediction that the angular dependence of second harmonic generation (SHG) from a simple planar structure is direction-dependent, and highlights the sensitivity of both SHG and third harmonic generation to the makeup of the surface and what surrounds it. Our calculations also suggest that the nonlinear optical analysis of more complicated, hybrid structures, such as metal/oxide nanoantennas or metasurfaces, should always be performed by including effects that are generally overlooked, such as nonlocal effects (viscosity and pressure); the presence of linear and nonlinear quantum tunneling currents in the nano- and sub nano-gaps between the nanoantenna and embedded nanoparticles; linear and nonlinear contributions of bound (inner-core) electrons to the dielectric constant.
We present an indirect and non-destructive optical method for domain statistic characterization in disordered nonlinear crystals, having a spatially random distribution of ferroelectric domains with homogeneous refractive index. This method, based on a combination of numerical simulations and experimental measurements, analyses the wavelengthdependent second harmonic spatial distribution. We apply this technique to the characterization of two different random media, with drastically different statistical distributions of ferroelectric domains.
We propose and demonstrate a novel functionality of chirped mirror for monochromatic light beams: a diffraction control in reflections resulting in focusing or imaging of beams. The chirped mirrors, commonly used for manipulating temporal profiles of pulses, here are applied for manipulating the spatial dispersion of a monochromatic beam. By penetrating into dielectric layers of chirped mirror, the monochromatic beam experiences the negative diffraction, therefore the beam diverge propagating in front and behind the structure in normal diffraction region can be compensated inside this structure with negative diffraction. The result is focusing or imaging of the reflected beam from a flat interface of chirped mirror without optical axis.
We report and analyze experimental observation of the formation of a narrow, well collimated laser beam at optical
frequency behind the woodpile photonic crystal fabricated using a femtosecond laser multi-photon polymerization
technique. We show that the collimation depends on the input laser beam focusing conditions. We discuss the
experimental results and give theoretical interpretation.
We report first experimental evidences of spatial filtering of light beams in three-dimensional photonic crystals. The
photonic crystals were fabricated in a glass bulk, where refractive index was modified by applying femtosecond laser
pulses. We observe the modification of the angular spectra (the far field) in the central diffraction maximum of the
transmitted radiation in accordance with the theory of spatial filtering.
We show that photonic crystals made of materials with normal dispersion allow simultaneous broad angular range and
broad spectral range phase matching in nonlinear wave mixing processes, in particular second harmonic generation. The
configuration proposed ensures subdiffractive propagation regimes for both interacting waves and the slopes of the two
dispersion curves (for the fundamental and second harmonic waves) at the phase-matching frequencies are similar (at
frequencies slightly shifted from the phase-matched one the phase-mismatch remains very small). The numerical
simulations confirm both the broad angular range (by the use of narrow beams, with the width of few wavelengths) and
the broad spectral range (by the spectral width of the generated second harmonic beam) for the phase matching, showing
a significant increasing of the conversion efficiency with respect to the case of plane waves in homogeneous materials.
We demonstrate multifunction operation in a 2D PC slab showing that the same structure may be used for lasing, frequency shifting and switching under appropriate stimulus. Our analytical model, based on a coupled modes nonlinear approach closely describes the main experimental features. The experimental results constitute a first step towards an active reconfiguration of photonic crystal all-optical circuits.
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