1,720,985 research outputs found
Thin films of phase change materials for light control of metamaterials in the optical and infrared spectral domain
No full textNanophotonic 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
Enhanced Near-Field Chirality in Periodic Arrays of Si Nanowires for Chiral Sensing
Full text linkNanomaterials can be specially designed to enhance optical chirality and their interaction with chiral molecules can lead to enhanced enantioselectivity. Here we propose periodic arrays of Si nanowires for the generation of enhanced near-field chirality. Such structures confine the incident electromagnetic field into specific resonant modes, which leads to an increase in local optical chirality. We investigate and optimize near-field chirality with respect to the geometric parameters and excitation scheme. Specially, we propose a simple experiment for the enhanced enantioselectivity, and optimize the average chirality depending on the possible position of the chiral molecule. We believe that such a simple achiral nanowire approach can be functionalized to give enhanced chirality in the spectral range of interest and thus lead to better discrimination of enantiomers
Demonstration of extrinsic chirality of photoluminescence with semiconductor-metal hybrid nanowires
Full text linkChiral 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
No full textIn 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°C [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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Nanostructured materials for circular dichroism and chirality at the nanoscale: towards unconventional characterization [Invited]
Full text linkIn 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
Extrinsic chirality tailors Stokes parameters in simple asymmetric metasurfaces
Full text linkMetasurfaces tailor electromagnetic confinement at the nanoscale and can be appropriately designed for polarization-dependent light-matter interactions. Adding the asymmetry degree to the desing allows for circular polarizations of opposite handedness to be differently absorbed or emitted, which is of interest in fields spanning from chiral sensing to flat optics. Here, we show that simple, low-cost asymmetric metasurfaces can control Stokes parameters in the transmitted far-field. With only 50 nm of asymmetric plasmonic shells on self-assembled polystyrene nanospheres, our metasurfaces allow for great spectral and incident angle tunability. We first investigated broadband extrinsic chirality in metasurfaces with asymmetric plasmonic semishells; we found high extinction circular dichroism (CD) in the near-infrared range. We then excited it with linear polarization and performed hyperspectral Stokes polarimetry on the transmitted field. We showed that the S3 parameter follows the behavior of CD in extinction, and that the output field position on the Poincaré sphere can be widely controlled by using the incidence angle and wavelength. Furthermore, simulations agreed well with the experiments and showed how the near-field chiro-optical response influences the extrinsic chiral behavior in absorption and the polarization state of the transmitted field
Control of Au nanoantenna emission enhancement of magnetic dipolar emitters by means of VO2 phase change layers
No full textActive, 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
Rich Broadband Chiral Behavior in Low-cost Plasmonic Nanostructures
No full textPlasmonic nanostructures with broken symmetry have been proposed for the control of the circular polarization at the nanoscale, important for applications spanning from chiral sensing to circularly polarized nanosources. We recently demonstrated that the low-cost self-assembling nanosphere lithography, combined with tilted metal deposition, opens new possibilities for the design and fabrication of chiral nanostructures. Starting from polystyrene nanospheres (PNS) covered by asymmetric metallic shell, we measured chirality in absorption at 633 nm [1]. We discovered that the nanohole array (NHA) formed due to the shadow, can exhibit chiral properties in a broad near-infrared range [2] , [3]
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