Justin S. White

Plasmonics for extreme light concentration and manipulation

The unprecedented ability of nanometallic (that is, plasmonic) structures to concentrate light into deep-subwavelength volumes has propelled their use in a vast array of nanophotonics technologies and research endeavours. Plasmonic light concentrators can elegantly interface diffraction-limited dielectric optical components with nanophotonic structures. Passive and active plasmonic devices provide new pathways to generate, guide, modulate and detect light with structures that are similar in size to state-of-the-art electronic devices. With the ability to produce highly confined optical fields, the conventional rules for light-matter interactions need to be re-examined, and researchers are venturing into new regimes of optical physics. In this review we will discuss the basic concepts behind plasmonics-enabled light concentration and manipulation, make an attempt to capture the wide range of activities and excitement in this area, and speculate on possible future directions.

Extraordinary optical absorption through subwavelength slits.

We report on the ability of resonant plasmonic slits to efficiently concentrate electromagnetic energy into a nanoscale volume of absorbing material placed inside or directly behind the slit. This gives rise to extraordinary optical absorption characterized by an absorption enhancement factor that well exceeds the enhancements seen for extraordinary optical transmission through slits. A semianalytic Fabry-Perot model for the resonant absorption is developed and shown to quantitatively agree with full-field simulations. We show that absorption enhancements of nearly 1000% can be realized at 633 nm for slits in aluminum films filled with silicon. This effect can be utilized in a wide range of applications, including high-speed photodetectors, optical lithography and recording, and biosensors.

Nonresonant enhancement of spontaneous emission in metal-dielectric-metal plasmon waveguide structures

We theoretically investigate the spontaneous emission process of an optical, dipolar emitter in metal-dielectric-metal slab and slot waveguide structures. We find that both structures exhibit strong emission enhancements at nonresonant conditions, due to the tight confinement of modes between two metallic plates. The large enhancement of surface plasmon-polariton excitation enables dipole emission to be preferentially coupled into plasmon waveguide modes. These structures find applications in creating nanoscale local light sources or in generating guided single plasmons in integrated optical circuits.

Spectral properties of plasmonic resonator antennas

A theoretical study of the optical properties of metallic nano-strip antennas is presented. Such strips exhibit retardation-based resonances resulting from the constructive interference of counter propagating short-range surface plasmon-polaritons (SR-SPPs) that reflect from the antenna terminations. A Fabry-P erot model was formulated that successfully predicts both the peak position and spectral shape of their optical resonances. This model requires knowledge of the SR-SPP reflection amplitude and phase pickup upon reflection from the structure terminations. These quantities were first estimated using an intuitive Fresnel reflection model and then calculated exactly using full-field simulations based on the finite-difference frequency-domain (FDFD) method. With only three dimensionless scaling parameters, the Fabry-P erot model provides simple design rules for engineering resonant properties of such plasmonic resonator antennas.

Efficient optical coupling into metal-insulator-metal plasmon modes with subwavelength diffraction gratings

We demonstrate efficient optical coupling into metal-insulator-metal (MIM) plasmon modes. Subwavelength grating couplers are used to optically excite the MIM plasmon mode, which is observed with reflection spectroscopy. Coupling efficiencies of up to 28% are measured for insulator thicknesses of 12nm. It is found that the MIM resonance has a significant shift in energy as a function of grating depth. This shift is much larger than that seen from traditional surface plasmon modes. MIM plasmons are promising tools for probing molecular junctions due to strong field confinement and high field intensities within the insulator.

Side-coupled cavity model for surface plasmon-polariton transmission across a groove

We demonstrate that the transmission properties of surface plasmon-polaritons (SPPs) across a rectangular groove in a metallic film can be described by an analytical model that treats the groove as a side-coupled cavity to propagating SPPs on the metal surface. The coupling efficiency to the groove is quantified by treating it as a truncated metal-dielectric-metal (MDM) waveguide. Finite-difference frequency-domain (FDFD) simulations and mode orthogonality relations are employed to derive the basic scattering coefficients that describe the interaction between the relevant modes in the system. The modeled SPP transmission and reflection intensities show excellent agreement with full-field simulations over a wide range of groove dimensions, validating this intuitive model. The model predicts the sharp transmission minima that occur whenever an incident SPP resonantly couples to the groove. We also for the first time show the importance of evanescent, reactive MDM SPP modes to the transmission behavior. SPPs that couple to this mode are resonantly enhanced upon reflection from the bottom of the groove, leading to high field intensities and sharp transmission minima across the groove. The resonant behavior exhibited by the grooves has a number of important device applications, including SPP mirrors, filters, and modulators.

Planar lenses based on nanoscale slit arrays in a metallic film

We experimentally demonstrated planar lenses based on nanoscale slit arrays in a metallic film. Electromagnetic simulations of lens designs and confocal measurements on manufactured structures show excellent agreement, but deviate from simple theory.

Mid-IR plasmonic antennas on silicon-rich oxinitride absorbing substrates: Nonlinear scaling of resonance wavelengths with antenna length

We report on the resonant properties of platinum dipole antennas fabricated on a silicon-rich-oxinitride thin film that exhibits significant absorption in the mid-infrared of the electromagnetic spectrum (λ−1≈1100 cm−1). A nonlinear scaling between the resonant wavelength and the antenna length has been found and quantitatively confirmed by full-field electromagnetic simulations. The resonant wavelength increases linearly with antenna length for small lengths and tends to saturate for large ones (length >4 μm). This saturation effect is attributed to the coupling of a geometrical antenna resonance and an absorption resonance of the substrate material.

Plasmonic solar cells with broadband absorption enhancements

A combined computational and experimental study optimizing plasmon-enhanced absorption in Si thin film solar cells presented. A model system consisting of a 2-dimensional periodic /aperiodic arrays of Ag nanostructures on a silica coated Si film supported by a silica substrate is used in the simulations. We develop basic design rules for the realization of broadband absorption enhancements for such structures, by simultaneously taking advantage of 1) the high near-fields surrounding the nanostructures close to their surface plasmon resonance frequency and 2) the effective coupling to waveguide modes supported by the Si film through an optimization of the array properties.