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

Nanostructured materials for circular dichroism and chirality at the nanoscale: towards unconventional characterization [Invited]

In this work, we review the last attempts to use nanostructured materials for the enhancement of the chiro-optical effects at the nanoscale. Starting from the numerical design, we review different geometries that exhibit circular dichroic behavior in the far field; we then focus on the new branch of near-field chirality, where numerous nanostructures have been proposed for background-free chiral sensing. The next section reports on nanofabrication methods, with a special focus on self-assembling, cost- and time-efficient techniques. Finally, we review the chiro-optical experiments. Besides conventional extinction-based techniques, we are today able to reveal chiro-optical effects via photothermal behavior and photoluminescence, going down to single nanostructure chirality with sophisticated near-field techniques. We believe that the novel designs, state-of-the-art nanofabrication and modern characterization techniques have come to a stage to provide chiro-optical sensors and light components based on nanostructures

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

Thin films of phase change materials for light control of metamaterials in the optical and infrared spectral domain

Nanophotonic component can be tuned by means of a thin phase change material (PCM) film put in its vicinity; PCM changes phases upon heat stimuli and thus provides a phase or amplitude optical contrast at the nanoscale. Vanadium dioxide (VO2) is a promising material which undergoes semiconductor-to-metal phase transition at about 68 °C. In this work we combine its transition with a metamaterial made of golden nanodiscs, in the infrared spectral range. Metamaterial dimensions can be adjusted to provide the almost unitary absorption in the desired spectral range, which can be switched to zero absorption by changing the VO2 phase. We further propose the optical switching of a large number of unitary cells by means of a near-infrared or visible laser. For given wavelength and power of the laser, we show how the spot-size can be adjusted to ensure that the whole VO2 reaches the transition temperature. We further investigate the substrate influence. We believe that this approach can be generally used to design the metamaterial for a specific application, and find optimum experimental requirements for its switching

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

Circular dichroism in a plasmonic array of elliptical nanoholes with square lattice

Chiral properties of plasmonic metasurfaces, especially related to different absorption of left and right circularly polarized light leading to circular dichroism (CD), are a research hot topic in nanophotonics. There is often a need to understand the physical origin of CD for different chiral metasurfaces, and to get guidelines for the design of structures with optimized and robust CD. In this work, we numerically study CD at normal incidence in square arrays of elliptic nanoholes etched in thin metallic layers (Ag, Au, Al) on a glass substrate and tilted with respect to the symmetry axes. Strong CD arises in absorption spectra at the same wavelength region of extraordinary optical transmission, indicating highly resonant coupling between light and surface plasmon polaritons at the metal/glass and metal/air interfaces. We elucidate the physical origin of absorption CD by a careful comparison of optical spectra for different polarizations (linear and circular), with the aid of static and dynamic simulations of local enhancement of the electric field. Furthermore, we optimize the CD as a function of the ellipse parameters (diameters and tilt), the thickness of the metallic layer, and the lattice constant. We find that silver and gold metasurfaces are most useful for CD resonances above 600 nm, while aluminum metasurfaces are convenient for achieving strong CD resonances in the short-wavelength range of the visible regime and in the near UV. The results give a full picture of chiral optical effects at normal incidence in this simple nanohole array, and suggest interesting applications for chiral biomolecules sensing in such plasmonic geometries

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