400 research outputs found
Resonant Absorption in GaAs-Based Nanowires by Means of Photo-Acoustic Spectroscopy
Semiconductor nanowires made of high refractive index materials can couple the incoming light to specific waveguide modes that offer resonant absorption enhancement under the bandgap wavelength, essential for light harvesting, lasing and detection applications. Moreover, the non-trivial ellipticity of such modes can offer near field interactions with chiral molecules, governed by near chiral field. These modes are therefore very important to detect. Here, we present the photo-acoustic spectroscopy as a low-cost, reliable, sensitive and scattering-free tool to measure the spectral position and absorption efficiency of these modes. The investigated samples are hexagonal nanowires with GaAs core; the fabrication by means of lithography-free molecular beam epitaxy provides controllable and uniform dimensions that allow for the excitation of the fundamental resonant mode around 800 nm. We show that the modulation frequency increase leads to the discrimination of the resonant mode absorption from the overall absorption of the substrate. As the experimental data are in great agreement with numerical simulations, the design can be optimized and followed by photo-acoustic characterization for a specific application
Second harmonic generation on self-assembled GaAs/Au nanowires with thickness gradient
Here we investigated the SH generation at the wavelength of 400 nm (pump laser at 800 nm, 120 fs pulses) of a "metasurface" composed by an alternation of GaAs nano-grooves and Au nanowires capping portions of flat GaAs. The nano-grooves depth and the Au nanowires thickness gradually vary across the sample. The samples are obtained by ion bombardment at glancing angle on a 150 nm Au mask evaporated on a GaAs plane wafer. The irradiation process erodes anisotropically the surface, creating Au nanowires and, at high ion dose, grooves in the underlying GaAs substrate (pattern transfer). The SHG measurements are performed for different pump linear polarization angle at different positions on the "metasurface" in order to explore the regions with optimal conditions for SHG efficiency. The pump polarization angle is scanned by rotating a half-wave retarder plate. While the output SH signal in reflection is analyzed by setting the polarizer in s or p configuration in front of the detector. The best polarization condition for SHG is obtained in the configuration where the pump and second harmonic fields are both p polarized, and the experiments show a SH polarization dependence of the same symmetry of bulk GaAs. Thus, the presence of gold contributes only as field localization effect, but do not contributes directly as SH generator
Hybrid thermal Yagi-Uda nanoantennas for directional and narrow band long-wavelength IR radiation sources
We investigate the possibility of spatially and spectrally controlling the thermal infrared emission by exploitation of the Yagi-Uda antenna design. Hybrid antennas composed of both SiC and Au rods are considered and the contributions of emission from all the elements, at a given equilibrium temperature, are taken into account. We show that the detrimental effect due to thermal emission from the not ideal parasitic elements drastically affect the performances of conventional thermal Au antennas in the 12 ÎĽm wavelength range. Nevertheless, our results show that the hybrid approach allows the development of efficient narrow-band and high directivity sources. The possibility of exploiting the Yagi-Uda design both in transmission and in reception modes, may open the way to the realization of miniaturized, efficient, robust and cheap sensor devices for mass-market applications. 2020 Optical Society of America
Second and Third Harmonic Generation in Metal-Based Nanostructures
We present a new theoretical approach to the study of second and third
harmonic generation from metallic nanostructures and nanocavities filled with a
nonlinear material, in the ultrashort pulse regime. We model the metal as a
two-component medium, using the hydrodynamic model to describe free electrons,
and Lorentz oscillators to account for core electron contributions to both the
linear dielectric constant and to harmonic generation. The active nonlinear
medium that may fill a metallic nanocavity, or be positioned between metallic
layers in a stack, is also modeled using Lorentz oscillators and surface
phenomena due to symmetry breaking are taken into account. We study the effects
of incident TE- and TM-polarized fields and show that a simple re-examination
of the basic equations reveals additional exploitable dynamical features of
nonlinear frequency conversion in plasmonic nanostructures.Comment: 33 pages, including 11 figures and 74 references; corrected
affiliations and some typo
Gap solitons in a nonlinear quadratic negative index cavity
By integrating the full Maxwell's equations we predict the existence of gap solitons in a quadratic, Fabry-Perot negative index cavity. An intense, fundamental pump pulse shifts the band structure that forms when magnetic and electric plasma frequencies are different so that a weak, second harmonic pulse initially tuned inside the gap is almost entirely transmitted. The process is due cascading, which occurs far from phase matching conditions, and causes pulse compression. A nonlinear polarization spawns a dark soliton, while a nonlinear magnetization produces a bright soliton
Enhanced second harmonic generation from resonant GaAs gratings
We study second harmonic generation in nonlinear, GaAs gratings. We find
large enhancement of conversion efficiency when the pump field excites the
guided mode resonances of the grating. Under these circumstances the spectrum
near the pump wavelength displays sharp resonances characterized by dramatic
enhancements of local fields and favorable conditions for second harmonic
generation, even in regimes of strong linear absorption at the harmonic
wavelength. In particular, in a GaAs grating pumped at 1064nm, we predict
second harmonic conversion efficiencies approximately five orders of magnitude
larger than conversion rates achievable in either bulk or etalon structures of
the same material.Comment: 8 page
Optimization of highly circularly polarized thermal radiation in -MoO/-GaO twisted layers
We investigate a bi-layer scheme for circularly polarized infrared thermal
radiation. Our approach takes advantage of the strong anisotropy of
low-symmetry materials such as -GaO and -MoO. We
numerically report narrow-band, high degree of circular polarization (over
0.85), thermal radiation at two typical emission frequencies related to the
excitation of -GaO optical phonons. Optimization of the degree
of circular polarization is achieved by a proper relative tilt of the crystal
axes between the two layers. Our simple but effective scheme could set the
basis for a new class of lithography-free thermal sources for IR bio-sensing.Comment: 11 pages, 6 figure
Quantitative evaluation of emission properties and thermal hysteresis in the mid-infrared for a single thin film of vanadium dioxide on a silicon substrate
We present a comparative study of the emission properties of a vanadium dioxide thin film (approximately 200 nm) deposited on a silicon wafer in different sub-spectral-ranges of the mid-infrared, with particular attention to the windows of transparency of the atmosphere to the infrared radiation (i.e., 3–5 μm, 8–12 μm). The infrared emission properties of the structure are closely related to the well-known phase transition of the first order, from semiconductor to metal, of the vanadium dioxide around the temperature of 68 °C. The characterization of the emissivity in the sub-regions of the mid-infrared was carried out both in the front configuration, that is on the VO2 film side, and in the rear configuration on the silicon wafer side, and showed a strong difference in the hysteresis thermal bandwidth, in particular between the short wave region and the long wave region. The bandwidth is equal to 12 °C for the front and 15 °C for the rear. The emissivity behaviors as a function of temperature during the semiconductor-metal transition in the mid-infrared subregions were analyzed and explained using the theories of the effective medium of Maxwell Garnett and Bruggeman, highlighting the greater functionality of one theory with respect to the other depending on the spectral detection band
Control of Au nanoantenna emission enhancement of magnetic dipolar emitters by means of VO2 phase change layers
Active, ultra-fast external control of the emission properties at the nanoscale is of great interest for chip-scale, tunable and efficient nanophotonics. Here we investigated the emission control of dipolar emitters coupled to a nanostructure made of an Au nanoantenna, and a thin vanadium dioxide (VO2) layer that changes from semiconductor to metallic state. If the emitters are sandwiched between the nanoantenna and the VO2 layer, the enhancement and/or suppression of the nanostructure’s magnetic dipole resonance enabled by the phase change behavior of the VO2 layer can provide a high contrast ratio of the emission efficiency. We show that a single nanoantenna can provide high magnetic field in the emission layer when VO2 is metallic, leading to high emission of the magnetic dipoles; this emission is then lowered when VO2 switches back to semiconductor. We finally optimized the contrast ratio by considering different orientation, distribution and nature of the dipoles, as well as the influence of a periodic Au nanoantenna pattern. As an example of a possible application, the design is optimized for the active control of an Er3+ doped SiO2 emission layer. The combination of the emission efficiency increase due to the plasmonic nanoantenna resonances and the ultra-fast contrast control due to the phase-changing medium can have important applications in tunable efficient light sources and their nanoscale integration
Properties of entangled photon pairs generated in one-dimensional nonlinear photonic-band-gap structures
We have developed a rigorous quantum model of spontaneous parametric
down-conversion in a nonlinear 1D photonic-band-gap structure based upon
expansion of the field into monochromatic plane waves. The model provides a
two-photon amplitude of a created photon pair. The spectra of the signal and
idler fields, their intensity profiles in the time domain, as well as the
coincidence-count interference pattern in a Hong-Ou-Mandel interferometer are
determined both for cw and pulsed pumping regimes in terms of the two-photon
amplitude. A broad range of parameters characterizing the emitted
down-converted fields can be used. As an example, a structure composed of 49
layers of GaN/AlN is analyzed as a suitable source of photon pairs having high
efficiency.Comment: 14 pages, 23 figure
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