Metallic Nanoantenna Arrays Research Articles

Polarization optical switching based on the molding of coherent light scattering via surface lattice resonances

We study the molding of coherent light scattering in the planes of Au metallic nanoantenna arrays (in-plane light scattering) using surface lattice resonances. The nanoantennas are considered to have similar periodicities and footprints (lateral shapes and sizes) but their heights are varied from 20 to 80 nm. We measure the spectra and relative intensities of the scattered light along the edges of the substrates supporting periodic arrays of such nanoantennas. Our results show that in the case of nanoantennas of high degree of flatness (20 nm height), the variation of the incident light polarization can lead to binary partition of scattered light energy between two modes at two different wavelengths, offering a polarization optical switching process in the near infrared range. This happens as the nature of the light scattering process changes from plasmonic to coherent diffraction associated with the formation of “‘edge-mode’ surface lattice resonances. As the heights of the nanoantennas are increased, the energy transfer process becomes more continuous, offering mixture of the two scattering processes.

Geometrically Tunable Beamed Light Emission from a Quantum‐Dot Ensemble Near a Gradient Metasurface

AbstractOptical metasurfaces have been widely investigated in recent years as a means to tailor the wavefronts of externally incident light for passive device applications. At the same time, their use in active optoelectronic devices such as light emitters is far less established. This work explores their ability to control the radiation properties of a nearby continuous ensemble of randomly oriented incoherent dipole sources via near‐field interactions. Specifically, a film of colloidal quantum dots is deposited on a plasmonic metasurface consisting of a 1D array of metallic nanoantennas on a metal film. The array is designed to introduce a linear phase profile upon reflection, and a bi‐periodic nanoparticle arrangement is introduced to ensure adequate sampling of the desired phase gradient. Highly directional radiation patterns are correspondingly obtained from the quantum dots at an enhanced emission rate. The underlying radiation mechanism involves the near‐field excitation of surface plasmon polaritons at the metal film, and their selective diffractive scattering by the metasurface into well‐collimated beams along predetermined geometrically tunable directions. These results underscore the distinctive ability of metasurfaces to control radiation properties directly at the source level, which is technologically significant for the continued miniaturization and large‐scale integration of optoelectronic devices.

Multi-order surface lattice resonances and dark mode activation in metallic nanoantenna arrays

Surface lattice resonances (SLRs) in arrays of metallic nanoantennas are formed via hybridization of their localized surface plasmon resonances with the Rayleigh Anomaly. In this paper, we study the impact of inter-nanoantenna plasmonic coupling on such resonances, demonstrating a significant departure from the cases where the nanoantennas are only coupled to the diffraction modes of the arrays. For this, we study SLRs in a series of metallic nanoantenna arrays wherein their inter-nanoantenna spacings (lattice constants) along their short axes are varied between the limits where transverse modes of the nanoantennas are efficiently coupled to each other to the case where they are well separated. Our results show that for the latter case, when the incident light is polarized along the short axes of the nanoantennas, SLRs are formed via first order parallel coupling. As we reach the limit of inter-nanoantenna plasmonic coupling, however, the nature of SLRs is changed, becoming a second order orthogonal coupling. Our results also show that in the presence of inter-nanoantenna plasmonic coupling, the forbidden quadrupole state of nanoantennas can strongly couple to light, becoming the dominant optical feature of the arrays. For light polarized along the long axes of the nanoantennas, we demonstrate the Rayleigh Anomaly splitting of first order infrared and second order near-infrared SLRs under oblique incident angles. Wavelength multiplex optical filter application of such diffraction orders is discussed.

Collective and local energy transfer in biologically-hybridized systems of semiconductor quantum dots and metallic nanoantenna arrays

We study polarization-dependent energy transfer and exciton dynamics in hybrid systems consisting of quantum dot bioconjugates and arrays of metallic nanoantennas. The size distribution of the quantum dots are used to investigate how excitons with different transition energies interact with the localized surface plasmon resonances (LSPRs) and collective surface lattice resonances (SLRs) supported by the arrays. We show that, depending on the projections of the electric dipoles of the quantum dots along the nanoantennas, they can interact with these resonances differently. This leads to polarization-dependent radiative and non-radiative decay of excitons, highlighting such systems can support two types of energy transfer mechanisms: (i) collective wavelength-dependent transfer of excitation energy from quantum dots to the arrays of the metallic nanoantennas via SLR (photonic-plasmonic path), and (ii) the local energy transfer to individual nanoantennas via localized surface plasmons (direct path). Our results also show that quantum dots with larger cores can have stronger interaction with SLRs. This suggests that such resonances can act as an energy drain that enhances cascaded energy transfer between quantum dots.

Metasurface for Reciprocal Spin-Orbit Coupling of Light on Waveguiding Structures

Metasurfaces exploiting optical spin Hall effects present an efficient means to control light, and suggest new functionalities in integrated optics. The authors show that an array of metallic nanoantennas can be used not only to tailor the polarization of light extracted from a waveguide with the direction of the wave, but also to control the number of output directions and their polarizations. This general approach can be extended to various frequencies and applied to other systems like silicon waveguides or photonic platforms, and could be a building block for multiplexing, chiral sensing, and polarization-encoded optical quantum computing.

Ultrahigh refractive index sensitivity via lattice-induced meta-dipole modes in flat metallic nanoantenna arrays

We investigate control of plasmonic-photonic coupling in flat metallic nanoantenna arrays. We demonstrate that when the nanoantennas are packed together along their short axis (transverse lattice constant) and the incident light polarization is along their long axis, they can support lattice-induced plasmonic resonance coupled to a super-photonic mode that densely fills the superstrate volume. Our results show that at a certain wavelength, this resonance joins the plasmonic tip modes of the nanoantennas, forming meta-dipole modes. These modes have field profiles similar to those of the natural plasmonic dipole modes of individual nanoantennas, but they occur at much shorter wavelengths and offer a very high bulk refractive index sensitivity (925 ± 12 nm/RIU). We show that with an increase in the transverse lattice constant, such a sensitivity decreases as the meta-dipole modes disappear. Under this condition, the refractive index sensitivity supported by natural modes of the nanoantennas increases, as the plasmonic edge mode suppression caused by charge rearrangement decreases.

Optically saturated and unsaturated collective resonances of flat metallic nanoantenna arrays

We study collective optical properties of arrays of flat gold nanoantennas, demonstrating they can support optically saturated and unsaturated plasmonic lattice modes when the incident light is polarized along their short axes. The saturated mode is nearly immune to the variation of the refractive index of the environment, while the unsaturated mode undergoes a large red shift without degradation as the refractive index increases. Our results show that when the incident light becomes polarized along the long axes of the nanoantennas, an increase of the refractive index of the superstrate leads to the formation of secondary plasmon peaks. These peaks are spectrally narrow and can detect variations of the ambient refractive index with a sensitivity of up to 620 nm/RIU (refractive index unit). The results suggest that the periodic arrays of flat metallic nanostructures can support hybridization of their multipolar plasmonic resonances with diffraction orders with distinct similarities and differences compared to those seen in cases of arrays of metallic nanorods.

Turning on plasmonic lattice modes in metallic nanoantenna arrays via silicon thin films.

We study control of optical coupling of plasmon resonances in metallic nanoantenna arrays using ultrathin layers of silicon. This technique allows one to establish and tune plasmonic lattice modes of such arrays, demonstrating a controlled transformation from the localized surface plasmon resonances of individual nanoantennas to their optimized collective lattice modes. Depending on the polarization and incident angle of light, our results support two different types of the silicon-induced plasmonic lattice resonances. For s-polarization these resonances follow the Rayleigh anomaly, while for p-polarization an increase in the incident angle makes the lattice resonances significantly narrower and slightly blueshifted.

Coherent Control of the Optical Absorption in a Plasmonic Lattice Coupled to a Luminescent Layer.

We experimentally demonstrate the coherent control, i.e., phase-dependent enhancement and suppression, of the optical absorption in an array of metallic nanoantennas covered by a thin luminescent layer. The coherent control is achieved by using two collinear, counterpropagating, and phase-controlled incident waves with wavelength matching the absorption spectrum of dye molecules coupled to the array. Symmetry arguments shed light on the relation between the relative phase of the incident waves and the excitation efficiency of the optical resonances of the system. This coherent control is associated with a phase-dependent distribution of the electromagnetic near fields in the structure which enables a significant reduction of the unwanted dissipation in the metallic structures.

Electronically controlled optical beam-steering by an active phased array of metallic nanoantennas

An optical phased array of nanoantenna fabricated in a CMOS compatible silicon photonics process is presented. The optical phased array is fed by low loss silicon waveguides with integrated ohmic thermo-optic phase shifters capable of 2π phase shift with ∼ 15 mW of applied electrical power. By controlling the electrical power to the individual integrated phase shifters fixed wavelength steering of the beam emitted normal to the surface of the wafer of 8° is demonstrated for 1 × 8 phased arrays with periods of both 6 and 9 μm.

Light-Emitting Waveguide-Plasmon Polaritons

We demonstrate the generation of light in an optical waveguide strongly coupled to a periodic array of metallic nanoantennas. This coupling gives rise to hybrid waveguide-plasmon polaritons (WPPs), which undergo a transmutation from plasmon to waveguide mode and vice versa as the eigenfrequency detuning of the bare states transits through zero. Near zero detuning, the structure is nearly transparent in the far-field but sustains strong local field enhancements inside the waveguide. Consequently, light-emitting WPPs are strongly enhanced at energies and in-plane momenta for which WPPs minimize light extinction. We elucidate the unusual properties of these polaritons through a classical model of coupled harmonic oscillators.