1,720,982 research outputs found
All-optical tuning of EIT-like dielectric metasurfaces by means of chalcogenide phase change materials
Full text linkElectromagnetically induced transparency (EIT) is a pump-induced narrowband
transparency window within an absorption lin
e of the probe beam sp
ectrum in an atomic
system. In this paper we propose a way to bring together the all-dielectric metamaterials to
have EIT-like effects and to optically tune the
response by hybridizing them with a layer of a
phase change material. We propose a design of the metamaterial based on Si nanoresonators
that can support an EIT-like resonant response. On the top of the resonators we consider a
thin layer of a chalcogenide phase change material, which we will use to tune the optical
response. Our choice is Ge
2
Sb
2
Te
5
(GST), since it has two stable phases at room temperature,
namely amorphous and crystalline, between which it can be switched quickly, nonvolatively
and reversibly, sustaining a large number of
switching cycles. They differ in optical
properties, while still having moderately low losses in telecom range. Since such dielectric
resonators do not have non-radiative losses of metals around 1550nm, they can lead to a high-
Q factor of the EIT-like response in this range
. Firstly, we optimize the starting structure so
that it gives an EIT-like response at 1550 nm wh
en the GST layer is in the amorphous state.
Our starting design uses glass as a substrate, but we also consider implementation in SOI
technology. If we then switch the thin layer of
GST to its crystalline phase, which has higher
losses, the EIT-like response is red shifted, pr
oviding around 10:1 contrast at 1550nm. This
reversible tuning can be done with an ns visible pulsed laser. We discuss the results of the
simulation of the dielectric metasurface for differe
nt configurations and
the tuning possibilit
Plasmonic Elliptical Nanohole Arrays for Chiral Absorption and Emission in the Near-Infrared and Visible Range
Full text linkChiral plasmonic nanostructures with tunable handedness-dependent absorption in the visible and infrared offer chiro-optical control at the nanoscale. Moreover, coupling them with emitting layers could lead to chiral nanosources, important for nanophotonic circuits. Here, we propose plasmonic elliptical nanohole arrays (ENHA) for circularly dependent near-infrared and visible emission. We first investigate broadband chiral behavior in an Au-ENHA embedded in glass by exciting it with plane waves. We then study the coupling of ENHA with a thin emitting layer embedded in glass; we focus on the emission wavelengths which provided high chirality in plane-wave simulations. Our novel simulation set-up monitors the chirality of the far-field emission by properly averaging a large set of homogeneously distributed, randomly oriented quantum sources. The intrinsic chirality of ENHA influences the circular polarization degree of the emitting layer. Finally, we study the emission dependence on the field distribution at the excitation wavelength. We demonstrate the chiral absorption and emission properties for Au-ENHA emitting in the near-infrared range, and for Ag-ENHA which is excited in green range and emits in the Lumogen Red range. The simple geometry of ENHA can be fabricated with low-cost nanosphere lithography and be covered with emission gel. We thus believe that this design can be of great importance for tunable chiral nanosources
Maximum Chirality Empowered by a Bound State in a Continuum in a Plasmonic Metasurface
Full text linkAchieving a strong and robust chiral response in plasmonic metasurfaces is among the key goals of current nanophotonic research. In this work, we theoretically show that the circular dichroism (CD) of a metal metasurface can be maximized by exploiting the concept of a bound state in a continuum (BIC) with symmetry breaking. We consider a gold metasurface with a deformation of circular holes into oval holes. The chiral response at small values of the angle of incidence is dominated by a quasi-BIC, with nearly maximal values of the absorption CD that are almost independent of the deformation. A strong CD in emission is also demonstrated. Symmetry analysis and mode profiles show that the extrinsically chiral response does indeed follow from a symmetry-broken BIC and is associated with a strong enhancement of the local electrical field. The concept of a plasmonic BIC with symmetry breaking provides a robust pathway to increase the chiral response in metal metasurfaces and opens research opportunities in chiral plasmonics that combine narrow resonances with local field enhancement
Fluorescence spectroscopy of enantiomeric amide compounds enforced by chiral light
Full text linkChirality, the absence of mirror symmetry, governs behavior in most biologically important
molecules, thus making the chiral recognition of great importance in the pharmaceutical and
agrochemical industries, as well as medicine. Chiral molecules can be characterized by means of
optical experiments based on chiro-optical excitation of molecules. Specifically, chiral absorptive
materials differently absorb left- and right-circular polarized light, i.e., they possess circular dichroism
(CD). Unfortunately, the natural CD of most molecules is very low and lies in the ultraviolet range.
Fluorescence-detected CD is a fast and sensitive tool for investigation of chiral molecules which
emit light; ultralow CD in absorption can be detected as the difference in emission. In this work, we
perform fluorescence-detected CD on novel chiral amide compounds, designed specifically for visible
green emission; we synthesize two enantiomeric fluorescent compounds using low-cost starting
compounds and easy purification. We investigate different solutions of the enantiomers at different
concentrations, and we show that the fluorescence of the intrinsically chiral compounds depends on
the polarization state of the penetrating light, which is absorbed at 400 nm and emits across the green
wavelength range. We believe that these compounds can be coupled with plasmonic nanostructures,
which further shows promise in applications regarding chiral sensing or chiral emission
Asymmetric hole array: tuning the optical circular dichroism for chiral molecules sensing
No full textOptical enantioselectivity of chiral molecules could be enhanced by depositing them on suitable nanostructured substrates. Different kind of chiral substrates can be developed, but chiral features are in general difficult to fabricate or costly. Self-assembled approach allows realizing plasmonic metasurfaces with a low cost reliable procedure. In this case asymmetric fabrication parameters can induce chiral optical response of the realised substrate. Self-organized polystyrene spheres deposited on glass substrate, are utilised to produce asymmetric hole array on a metal thin film. In our case the spheres (518 nm in diameter) where reduced by selective reactive ion etching and then covered by gold (and other metals), that is evaporated at a glancing angle. After the removing of the spheres an elliptical-hole array is produced forming a circular-dichroic substrate. The circular dichroic response of light interacting with the substrate can be tuned by choosing proper incidence angle and excitation wavelength, while the flat nature of the metasurface is very useful for easy molecular deposition processes. Two new enantiomers (right-handed and left-handed molecules) have been synthesized in order to present a good circular dichroism in the visible range and to be tested on the realized metasurfaces. Different tests were carried out on the samples, investigating the spectral optical properties of the structures with and without chiral molecules on top of them. The results are very promising due to the possibility of easily tuning and optimizing the optical response
Photoacoustic technique for the characterization of plasmonic properties of 2D periodic arrays of gold nanoholes
Full text linkWe apply photo-acoustic (PA) technique to examine plasmonic properties of 2D periodic arrays of nanoholes etched in gold/chromium layer upon a glass substrate. The pitch of these arrays lies in the near IR, and this, under appropriate wave vector matching conditions in the visible region, allows for the excitation of surface plasmon polaritons (SPP) guided along a dielectric – metal surface. SPP offered new approaches in light guiding and local field intensity enhancement, but their detection is often difficult due to the problematic discrimination of their contribution from the overall scattering. Here PA measures the energy absorbed due to the non-radiative decay of SPPs. We report on the absorption enhancement by presenting the spatial mapping of absorption under the incidence angles and wavelength that correspond to the efficient excitation of SPPs. Moreover, a comparison with optical transmission measurements is carried out, underlining the applicability and sensitivity of PA technique
Chiral near-field manipulation in Au-GaAs hybrid hexagonal nanowires
Full text linkWe demonstrate the control of enhanced chiral field distribution at the surface of hybrid metallo-dielectric nanostructures composed of self-assembled vertical hexagonal GaAs-based nanowires having three of the six sidewalls covered with Au. We show that weakly-guided modes of vertical GaAs nanowires can generate regions of high optical chirality that are further enhanced by the break of the symmetry introduced by the gold layer. Changing the angle of incidence of a linearly polarized plane wave it is possible to tailor and optimize the maps of the optical chirality in proximity of the gold plated walls. The low cost feasibility of the sample combined to the simple control by using linearly polarized light and the easy positioning of chiral molecules by functionalization of the gold plates make our proposed scheme very promising for enhanced enantioselective spectroscopy applications
Photo-acoustic technique with widely tuneable laser: Metasurface circular dichroism response
Full text linkChirality, an intrinsic property of certain entities in the universe, is characterized by the absence of mirror symmetry. Understanding chirality is crucial as it influences molecular interactions and properties. Circular dichroism (CD), measured using circularly polarized light, is a standard technique for probing chirality, but its sensitivity is often limited. Here, we explore extrinsic chirality (i.e. a property arising from asymmetric achiral materials when observed from out of normal incidence directions), using photo-acoustic spectroscopy (PAS). PAS allows direct measurement of local absorption, by monitoring the heat produced and transferred to the surrounding air, regardless the transmitted, reflected, and scattered light that flows away from the sample. In conventional techniques, the CD is usually measured by taking into account only the extinction as transmitted (or reflected) light. In this study, we introduce a new PAS setup that employs an oblique-incidence laser to study extrinsic chirality in silver-coated self-assembled metasurfaces. Our experimental results reveal intriguing CD trends dependent on the angle of incidence and wavelength, indicative of extrinsic chirality. This study expands the application of PAS, enabling simultaneous analysis of multiple wavelengths and providing valuable insights into chiral metasurfaces
Self-Phase-Matched Second-Harmonic and White-Light Generation in a Biaxial Zinc Tungstate Single Crystal
Full text linkSecond-order nonlinear optical materials are used to generate new frequencies by exploiting second-harmonic generation (SHG), a phenomenon where a nonlinear material generates light at double the optical frequency of the input beam. Maximum SHG is achieved when the pump and the generated waves are in phase, for example through birefringence in uniaxial crystals. However, applying these materials usually requires a complicated cutting procedure to yield a crystal with a particular orientation. Here we demonstrate the first example of phase matching under the normal incidence of SHG in a biaxial monoclinic single crystal of zinc tungstate. The crystal was grown by the micro-pulling-down method with the (102) plane perpendicular to the growth direction. Additionally, at the same time white light was generated as a result of stimulated Raman scattering and multiphoton luminescence induced by higher-order effects such as three-photon luminescence enhanced by cascaded third-harmonic generation. The annealed crystal offers SHG intensities approximately four times larger than the as grown one; optimized growth and annealing conditions may lead to much higher SHG intensities
Characterization of Chirality in Diffractive Metasurfaces by Photothermal Deflection Technique
Full text linkChirality, a lack of mirror symmetry, is present in nature at all scales; at the nanoscale, it governs the biochemical reactions of many molecules, influencing their pharmacology and toxicity. Chiral substances interact with left and right circularly polarized light differently, but this difference is very minor in natural materials. Specially engineered, nanostructured, periodic materials can enhance the chiro-optical effects if the symmetry in their interactions with circular polarization is broken. In the diffraction range of such metasurfaces, the intensity of diffracted orders depends on the chirality of the input beam. In this work, we combine a photothermal deflection experiment with a novel theoretical framework to reconstruct both the thermal and optical behavior of chiro-optical
behavior in diffracted beams
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