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

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

Demonstration of extrinsic chirality of photoluminescence with semiconductor-metal hybrid nanowires

Chiral optical response is an inherent property of molecules and nanostructures, which cannot be superimposed on their mirror images. In specific cases, optical chirality can be observed also for symmetric structures. This so-called extrinsic chirality requires that the mirror symmetry is broken by the geometry of the structure together with the incident or emission angle of light. From the fabrication point of view, the benefit of extrinsic chirality is that there is no need to induce structural chirality at nanoscale. This paper reports demonstration extrinsic chirality of photoluminescence emission from asymmetrically Au-coated GaAs-AlGaAs-GaAs core-shell nanowires fabricated on silicon by a completely lithography-free self-assembled method. In particular, the extrinsic chirality of PL emission is shown to originate from a strong symmetry breaking of fundamental HE 11 waveguide modes due to the presence of the asymmetric Au coating, causing preferential emission of left and right-handed emissions in different directions in the far field

VO2 phase change control of au nanorod emission enhancement of magnetic dipolar emitters

In this work, we combine the enhancement of the emitter efficiency due to the proximity of a resonant nanostructure, and the possibility to modulate it by means of a thin layer of a phase change material (PCM). PCMs have been used as active subwavelength elements that can switch between the phases that differ in electric and optical properties. The phase change results in a modulation of amplitude or phase of transmission or reflection over nanoscale propagation lengths, and it is compatible with fast optical systems [1,2]. Vanadium Dioxide (VO 2 ) is a promising candidate for nanoscale modulation since it shows dramatic contrast in its complex refractive index as it undergoes a structural phase transition from monoclinic (semiconductor) to rutile (metallic) phase at 68\ub0C [3], induced thermally, electrically or optically. The proposed structure can be fabricated as follows: a thin V0 2 layer is deposited on a glass substrate, and covered by a thin spacer layer of silica, which is doped by luminescent ions; above the spacer, Au nanorods are added to provide the plasmonic resonant enhancement. Sandwiched magnetic dipoles feel strong resonance when V0 2 is metallic due to the strong magnetic field arising from the current loops between Au nanorod and V0 2 ; this resonance blue-shifts and decreases when V0 2 is dielectric. We first maximize the absorption change between the two phases at the emission line of Er 3+ , i.e. 1540 nm. With these optimized geometric parameters, we investigate emission of single dipoles in the layer under the nanorod, considering different positions, types, and orientations. We show the power emitted to the far-field by the magnetic dipole, averaged over the positions under the nanorod. We show that the emitted far-field follows the high contrast at the resonant wavelength of the optimized absorption, proving the control of the enhanced emission and its switching off. Finally, we investigate the influence of the periodicity, as the upper part of the design can be fabricated as a patterned 2D array of nanorods. We believe that such an approach can be of great importance for active modulation of efficient light sources at the nanoscale

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\u2019s 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

Rich Near-Infrared Chiral Behavior in Diffractive Metasurfaces

The mimicking of chiral effects at the nanoscale usually requires complex nanostructured systems fabricated by using highly sophisticated techniques. Here, we demonstrate that low-cost diffractive metasurfaces, fabricated by nanosphere lithography, can offer a remarkably rich chiral broadband response in the near-infrared range. We measure the spin-dependent response as a function of the incident angle and wavelength. The resulting extinction maps reveal switching of the resonances governed by extrinsic chirality. We further employ numerical modeling to gain an insight into the electromagnetic behavior over the unit cell. The simulation results agree with experiments, and we further investigate the possibility of tailoring chirality by matching the refractive index below and above the metasurface. Finally, we investigate chiral emission of the ensemble of electric dipoles embedded in the refractive-index-matching layer. This approach opens alternative perspectives for chiral surface-lattice resonances and chiral emission in low-cost materials

Thermal scan of metal based metasurface and evidence of circular dichroism and optothermal anisotropy

Photothermal and photoacoustic techniques are utilized to explore the thermal properties of metal based metasurfaces. We experimentally evidence the role of the symmetry in the heat transport and its relation with the optical circular dichroism

Circular dichroism in low-cost plasmonics: 2D arrays of nanoholes in silver

Arrays of nanoholes in metal are important plasmonic devices, proposed for applications spanning from biosensing to communications. In this work, we show that in such arrays the symmetry can be broken by means of the elliptical shape of the nanoholes, combined with the in-plane tilt of the ellipse axes away from the array symmetry lines. The array then differently interacts with circular polarizations of opposite handedness at normal incidence, i.e., it becomes intrinsically chiral. The measure of this difference is called circular dichroism (CD). The nanosphere lithography combined with tilted silver evaporation was employed as a low-cost fabrication technique. In this paper, we demonstrate intrinsic chirality and CD by measuring the extinction in the near-infrared range. We further employ numerical analysis to visualize the circular polarization coupling with the nanostructure. We find a good agreement between simulations and the experiment, meaning that the optimization can be used to further increase CD

Tunable chiro-optical effects in self-assembled metasurfaces: experiments, simulations, and perspectives

The nanoscale community has proposed various nanostructures for the enhancement of near- and far-field chiro-optical effects. Here we study such effects in asymmetric metasurfaces which can be produced by means of nanosphere lithography (NSL). NSL, combined with tilted plasmonic deposition, is a versatile, self-assembling method for fabrication of different asymmetric nanogeometries. Polystyrene nanospheres (PSN) are first self-assembled on glass, then reduced in diameter, and subsequently covered with a plasmonic layer. By controlling fabrication parameters, we can obtain three types of samples. First sample is based on PSN asymmetrically covered by metal under 45deg. This sample has a high contribution of the plasmonic elliptical nanohole array on the glass. Second sample is a plasmonic elliptical nanohole array obtained by simply removing the PSN from the first sample. Third sample is obtained by increasing the metallic deposition angle to 60deg; this way, nanohole array contribution vanishes, and the metasurface is based on asymmetric plasmonic nanoshells. We report on numerical studies on these three samples, when excited by oblique left or right circular polarization in the near-infrared range. The simulations are in good agreement with previously obtained experimental results, which gives a route to possible optimization of fabrication parameters for different applications. Finally, we comment on the follow-up application for each geometry. We believe that this technique can be used to produce high quality and low-cost substrates for chiral sensing; moreover, with the inclusion of near-field light emitting layer, these metasurfaces could lead to tunable circularly polarized visible or near-infrared light emission

Diffracted Beams from Metasurfaces: High Chiral Detectivity by Photothermal Deflection Technique

Measurements of difference in optical interactions between circularly polarized excitations of opposite handedness (circular dichroism) are highly important for both natural and artificial chiral structures. Here the photothermal deflection technique is proposed as a method to detect the optical chirality of a metasurface, analyzing the diffracted beams by the metasurface itself. Two metasurfaces are investigated, based on Au and Ag. The samples are fabricated by nanosphere lithography, with a tilted deposition of thin metal layer, which produces symmetry-breaking. The unit cell periodicity of these metasurfaces allows for multiple order diffraction in the 450\u2013520\ua0nm range, which encompasses the emission lines of an Ar laser. The metasurfaces are placed on a mirror and excited by an Ar pump beam at different orientations, laser wavelengths, and circular polarization degree; the probe beam scans the absorption of the diffraction orders back-reflected from the underlying mirror. In this way, the chiral investigation is simplified by placing the scanning of absorption-induced thermal effect into the metasurface plane, thus avoiding the transmission/reflection measurements of a specific order, which are done by angular placement of detector. Theoretical and numerical approaches are further developed to reconstruct both thermal and optical behavior of the chirality at the nanoscale