Metasurfaces composed of planar arrays of subwavelength artificial structures show promise for extraordinary light manipulation. They have yielded novel ultrathin optical components such as flat lenses, wave plates, holographic surfaces, and orbital angular momentum manipulation and detection over a broad range of the electromagnetic spectrum. However, the optical properties of metasurfaces developed to date do not allow for versatile tunability of reflected or transmitted wave amplitude and phase after their fabrication, thus limiting their use in a wide range of applications. Here, we experimentally demonstrate a gate-tunable metasurface that enables dynamic electrical control of the phase and amplitude of the plane wave reflected from the metasurface. Tunability arises from field-effect modulation of the complex refractive index of conducting oxide layers incorporated into metasurface antenna elements which are configured in reflectarray geometry. We measure a phase shift of 180° and ∼30% change in the reflectance by applying 2.5 V gate bias. Additionally, we demonstrate modulation at frequencies exceeding 10 MHz and electrical switching of ±1 order diffracted beams by electrical control over subgroups of metasurface elements, a basic requirement for electrically tunable beam-steering phased array metasurfaces. In principle, electrically gated phase and amplitude control allows for electrical addressability of individual metasurface elements and opens the path to applications in ultrathin optical components for imaging and sensing technologies, such as reconfigurable beam steering devices, dynamic holograms, tunable ultrathin lenses, nanoprojectors, and nanoscale spatial light modulators.

We present a novel plasmonic antenna geometry - the double resonant antenna (DRA) - that is optimized for second-harmonic generation (SHG). This antenna is based on two gaps coupled to each other so that a resonance at the fundamental and at the doubled frequency is obtained. Furthermore, the proximity of the localized hot spots allows for a coupling and spatial overlap between the two field enhancements at both frequencies. Using such a structure, both the generation of the second-harmonic and its re-emission into the far-field are significantly increased when compared with a standard plasmonic dipole antenna. Such DRA are fabricated in aluminium using electron beam lithography and their linear and nonlinear responses are studied experimentally and theoretically.

Significant augmentation of second harmonic generation using Fano resonances in plasmonic heptamers made of silver is theoretically and experimentally demonstrated. The geometry is engineered to simultaneously produce a Fano resonance at the fundamental wavelength, resulting in a strong localization of the fundamental field close to the system, and a higher order scattering peak at the second harmonic wavelength. These results illustrate the versatility of Fano resonant structures to engineer specific optical responses both in the linear and nonlinear regimes thus paving the way for future investigations on the role of dark modes in nonlinear and quantum optics. 1 One such property is the generation of hot spots, whose presence in nanostructures, such as optical antennae, is promising for applications in nonlinear optics, since they require a strong electric near-field.2 Recent studies have reported the observation of different nonlinear optical phenomena in such nanosystems, like, for example, second harmonic generation (SHG), 3 multiphoton luminescence, 4,5 third harmonic generation, 6,7 higher harmonic generation, 8 and four-wave mixing. 9 SHG is significantly dependent on the symmetry of both the material being used and the structure being studied and therefore is a sensitive tool for characterizing plasmonic structures.10−13 SHG from centrosymmetric materials is due to the breaking of inversion symmetry at their surfaces and is therefore used as a surface probe.14 In recent years, strong SHG has been observed in metallic nanostructures such as sharp metallic tips, 15 multiple resonant nanostructures, 16,17 split-ring resonators, 18 metamaterials 19,20 and nanocups, 21 underlining the fast growing interest in the area of nonlinear plasmonics. 22,23 Efficient SHG requires the presence of strong SHG sources, that is, nonlinear polarization currents oscillating at the second harmonic frequency, at the nanostructure surface as well as an efficient scattering of the SHG signal into the far-field. Several strategies have been developed to enhance SHG from metallic nanostructures including using multiple resonances at both the fundamental and the second harmonic wavelengths, 16 breaking the centrosymmetry using noncentrosymmetric nanostructures 20 and even enhancing the electric fields using nanogaps. 24 However, the efficiency of SHG is restricted by two fundamental classes of losses that electromagnetic waves suffer in such metallic structures, namely radiative "losses" like optical scattering and nonradiative losses like the generation of heat. 25 Therefore, to increase the detected SHG in the far-field the structures must decrease the radiative losses at the fundamental wavelength while increasing them at the second harmonic wavelength and at the same time increase the near-field at the fundamental wavelength. Because the SHG yield increases as the square of the fundamental field intensity, 26 a higher near-field at the fundamental is required to increase the near-field at the second harmonic. The wid...

Second harmonic generation from plasmonic nanoantennas is investigated numerically using a surface integral formulation for the calculation of both the fundamental and the second harmonic electric field. The comparison between a realistic and an idealized gold nanoantenna shows that second harmonic generation is extremely sensitive to asymmetry in the nanostructure shape even in cases where the linear response is barely modified. Interestingly, minute geometry asymmetry and surface roughness are clearly revealed by far-field analysis, demonstrating that second harmonic generation is a promising tool for the sensitive optical characterization of plasmonic nanostructures. Furthermore, defects located where the linear field is strong (e.g., in the antenna gap) do not necessarily have the strongest impact on the second harmonic signal. KEYWORDS: Plasmonics, nonlinear optics, surface integral formulation, realistic nanostructures Furthermore, the coupling between several substructures is a convenient way to control SPR properties.2,3 In particular, bringing two nanostructures in close proximity results in an enhancement by several orders of magnitude of the electric field in the hot spot. This geometry is called a nanoantenna, or an optical antenna, since it represents the optical analogous of microwave and radiowave antennas.4−6 The electromagnetic properties of nanoantennas can be tuned by modifying their geometric parameters (length, shape, and gap dimension) and tailored for specific applications.7 For instance, optical antennas have been designed for studying quantum systems at the single emitter level, Optical antennas are also promising for the observation of nonlinear optical effects that require a high electric field such as that observed in the hot spots.19 Several studies have reported the observation of multiphoton excited luminescence, 20−22 second harmonic generation (SHG), 23,24 third harmonic generation, 25,26 high harmonic generation, 27 and four-wave mixing, 28 as well as ultrafast spectroscopy. 29,30 Recently, a new approach has been proposed to enhance nonlinear conversion in nanoantennas, using structures that are resonant at the several wavelengths involved in the frequency conversion. 31−33Among the different nonlinear parametric optical processes, SHG is the simplest one and has the advantage of being sensitive to the symmetry of the plasmonic nanostructures as well as their spatial arrangement. 34−40 Contrary to the other nonlinear optical processes, it was observed that SHG can be significantly suppressed in centrosymmetric gaps, although the fundamental electric field is strongly enhanced. 41 On the other hand, efficient SHG is observed in asymmetric gaps, such as the one formed in noncentrosymmetric T-shaped gold dimers.42 A clear insight into the impact of the nanoantenna shape on their SHG properties, particularly in the far-field, is therefore required to guide further the development of SHG from nanoantennas for practical applications like nonlinear plasmonics sensing. 4...

Objective: Electrical stimulation via cortically implanted electrodes has been proposed to treat a wide range of neurological disorders. Effectiveness has been limited, however, in part due to the inability of conventional electrodes to activate specific types of neurons while avoiding other types. Recent demonstrations that magnetic stimulation from a micro-coil can selectively activate pyramidal neurons (PNs) while avoiding passing axons suggest the possibility that such an approach can overcome some this limitation and here we use computer simulations to explore how the micro-coil design influences the selectivity with which neurons are activated. Methods: A computational model was developed to compare the selectivity of magnetic stimulation induced by rectangular-, V-and W-shaped coil designs. The more promising designs (V-and W-shapes) were fabricated for use in electrophysiological experiments including in vitro patch-clamp recording & calcium imaging (GCaMP6f) of mouse brain slices. Results: Both V-and W-shaped coils reliably activated layer 5 (L5) PNs but V-coils were more effective while W-coils were more selective. Activation thresholds with double-loop coils were approximately one-half those of single-loop coils. Calcium imaging revealed that both V-and Wcoils better confine activation than electrodes. Conclusion: Individual design features can influence both the strength as well as the selectivity of micro-coils and can be accurately predicted by computer simulations. Significance: Our results show how coil design influences the response of cortical neurons to stimulation and are an important step towards the development of next-generation cortical prostheses.

A surface integral formulation for the second-harmonic generation (SHG) from periodic metallic-dielectric nanostructures is described. This method requires the discretization of the scatterers' surface in the unit cell only. All the physical quantities involved in this problem are derived in the unit cell by applying specific periodic boundary conditions both at the fundamental and the second-harmonic (SH) frequencies. Both the fundamental and the SH electric fields are computed using the method of moments and periodic Green's function evaluated with the Ewald's method. The accuracy of the method is carefully assessed using two specific cases, namely the surface plasmon enhancement of SHG from a gold film and the SHG from L-shaped nanoparticle arrays. These two examples emphasize the accuracy and versatility of the proposed method, which can be applied to a broad range of periodic metallic structures, including plasmonic arrays on arbitrary substrates and metamaterials.

ABSTRACT:We report here and experimentally demonstrate an actively controlled gate-tunable plasmonic metasurface operating in the visible region of the electromagnetic spectrum, wherestrikingly -the operating voltages for reflectance modulation are much less than 1V. The electrically tunable metasurface consists of inverse dolmen structures (iDolmen) patterned on silver and chromium on a quartz substrate and subsequently covered with a 5 nm thin layer of Al2O3 followed by a 110 nm indium tin oxide (ITO) layer, which acts as a transparent electrode.Our designed structures show up to 78% change in reflection upon applying small voltages (<1V).We explain this behaviour via ion conductance of silver through Al2O3 and ITO, leading to active

Emission control of colloidal quantum dots (QDs) is a cornerstone of modern high-quality lighting and display technologies. Dynamic emission control of colloidal QDs in an optoelectronic device is usually achieved by changing the optical pump intensity or injection current density. Here we propose and demonstrate a distinctly different mechanism for the temporal modulation of QD emission intensity at constant optical pumping rate. Our mechanism is based on the electrically controlled modulation of the local density of optical states (LDOS) at the position of the QDs, resulting in the modulation of the QD spontaneous emission rate, far-field emission intensity, and quantum yield. We manipulate the LDOS via field effect-induced optical permittivity modulation of an ultrathin titanium nitride (TiN) film, which is incorporated in a gated TiN/SiO2/Ag plasmonic heterostructure. The demonstrated electrical control of the colloidal QD emission provides a new approach for modulating intensity of light in displays and other optoelectronics.