Alec Rose

Control of Radiative Processes Using Tunable Plasmonic Nanopatch Antennas

The radiative processes associated with fluorophores and other radiating systems can be profoundly modified by their interaction with nanoplasmonic structures. Extreme electromagnetic environments can be created in plasmonic nanostructures or nanocavities, such as within the nanoscale gap region between two plasmonic nanoparticles, where the illuminating optical fields and the density of radiating modes are dramatically enhanced relative to vacuum. Unraveling the various mechanisms present in such coupled systems, and their impact on spontaneous emission and other radiative phenomena, however, requires a suitably reliable and precise means of tuning the plasmon resonance of the nanostructure while simultaneously preserving the electromagnetic characteristics of the enhancement region. Here, we achieve this control using a plasmonic platform consisting of colloidally synthesized nanocubes electromagnetically coupled to a metallic film. Each nanocube resembles a nanoscale patch antenna (or nanopatch) whose plasmon resonance can be changed independent of its local field enhancement. By varying the size of the nanopatch, we tune the plasmonic resonance by ∼ 200 nm, encompassing the excitation, absorption, and emission spectra corresponding to Cy5 fluorophores embedded within the gap region between nanopatch and film. By sweeping the plasmon resonance but keeping the field enhancements roughly fixed, we demonstrate fluorescence enhancements exceeding a factor of 30,000 with detector-limited enhancements of the spontaneous emission rate by a factor of 74. The experiments are supported by finite-element simulations that reveal design rules for optimized fluorescence enhancement or large Purcell factors.

Controlling the second harmonic in a phase-matched negative-index metamaterial.

Nonlinear metamaterials have been predicted to support new and exciting domains in the manipulation of light, including novel phase-matching schemes for wave mixing. Most notable is the so-called nonlinear-optical mirror, in which a nonlinear negative-index medium emits the generated frequency towards the source of the pump. In this Letter, we experimentally demonstrate the nonlinear-optical mirror effect in a bulk negative-index nonlinear metamaterial, along with two other novel phase-matching configurations, utilizing periodic poling to switch between the three phase-matching domains.

Resolution of the Frequency Diverse Metamaterial Aperture Imager

The resolution of a frequency diverse compressive metamaterial aperture imager is investigated. The aperture consists of a parallel plate waveguide, in which an array of complementary, resonant metamaterial elements is patterned into one of the plates. Microwaves injected into the waveguide leak out through the resonant metamaterial elements, forming a spatially diverse waveform at the scene. As the frequency is scanned, the waveforms change, such that scene information can be encoded onto a set of frequency measurements. The compressive nature of the metamaterial imager enables image reconstruction from a significantly reduced number of measurements. We characterize the resolution of this complex aperture by studying the simulated point spread function (PSF) computed using different image reconstruction techniques. We compare the imaging performance of the system with that expected from synthetic aperture radar (SAR) limits.

Comprehensive simulation platform for a metamaterial imaging system

Recently, a frequency-diverse, metamaterial-based aperture has been introduced in the context of microwave and millimeter wave imaging. The generic form of the aperture is that of a parallel plate waveguide, in which complementary metamaterial elements patterned into the upper plate couple energy from the waveguide mode to the scene. To reliably predict the imaging performance of such an aperture prior to fabrication and experiments, it is necessary to have an accurate forward model that predicts radiation from the aperture, a model for scattering from an arbitrary target in the scene, and a set of image reconstruction approaches that allow scene estimation from an arbitrary set of measurements. Here, we introduce a forward model in which the metamaterial elements are approximated as polarizable magnetic dipoles, excited by the fields propagating within the waveguide. The dipoles used in the model can have arbitrarily assigned polarizability characteristics. Alternatively, fields measured from actual metamaterial samples can be decomposed into a set of effective dipole radiators, allowing the performance of actual samples to be quantitatively modeled and compared with simulated apertures. To confirm the validity of our model, we simulate measurements and scene reconstructions with a virtual multiaperture imaging system operating in the K-band spectrum (18-26.5 GHz) and compare its performance with an experimental system.

Large Metasurface Aperture for Millimeter Wave Computational Imaging at the Human-Scale

We demonstrate a low-profile holographic imaging system at millimeter wavelengths based on an aperture composed of frequency-diverse metasurfaces. Utilizing measurements of spatially-diverse field patterns, diffraction-limited images of human-sized subjects are reconstructed. The system is driven by a single microwave source swept over a band of frequencies (17.5–26.5 GHz) and switched between a collection of transmit and receive metasurface panels. High fidelity image reconstruction requires a precise model for each field pattern generated by the aperture, as well as the manner in which the field scatters from objects in the scene. This constraint makes scaling of computational imaging systems inherently challenging for electrically large, coherent apertures. To meet the demanding requirements, we introduce computational methods and calibration approaches that enable rapid and accurate imaging performance.

Wave mixing in nonlinear magnetic metacrystal

We present experimental measurements of three- and four-wave mixing phenomena in an artificially structured nonlinear magnetic metacrystal at microwave frequencies. The sum frequency generation signal for the varactor-loaded split-ring resonator (VLSRR) metamaterial agrees quantitatively with that predicted using an analytical effective medium model describing the VLSRR medium. A resonant enhancement of the nonlinear response is observed near the metamaterial resonance.

Overcoming phase mismatch in nonlinear metamaterials [Invited]

Nonlinear metamaterials have potentially interesting applications in highly efficient wave-mixing and parametric processes, owing to their ability to combine enhanced nonlinearities with exotic and configurable linear properties. However, the strong dispersion and unconventional configurations typically associated with metamaterials place strong demands on phase matching in such structures. In this paper, we present an overview of potential phase matching solutions for wave-mixing processes in nonlinear metamaterials. Broadly speaking, we divide the phase matching solutions into conventional techniques (anomalous dispersion, birefringence, and quasi-phase matching) and metamaterial-inspired techniques (negative-index and index-near-zero phase matching), offering numerical and experimental examples where possible. We find that not only is phase matching feasible in metamaterials, but metamaterials can support a wide range of phase matching configurations that are otherwise impossible in natural materials. These configurations have their most compelling applications in those devices where at least one of the interacting waves is counter-propagating, such as the mirror-less optical parametric oscillator and the nonlinear optical mirror.

Numerical studies of the modification of photodynamic processes by film-coupled plasmonic nanoparticles

The local plasmon resonances of metallic nanostructures are commonly associated with massive local field enhancements, capable of increasing the photoexcitation of nearby quantum emitters by orders of magnitude. However, these same plasmonic structures support high densities of bound and dissipative states, often quenching the nearby emitter or at least offering competitive nonradiative channels. Thus, finding a plasmonic platform that supports massive field enhancements and a high proportion of radiating to nonradiating states remains an active and promising area of research. In this paper, we outline a simple method for numerically studying plasmonic enhancements in fluorescence and apply it to several variants of the film-coupled nanoparticle platform. Film-coupled nanoparticles make excellent candidates for these investigations since the gap dimension between nanoparticle and film—key to the enhancement mechanism—can be precisely controlled in experimental realizations. By correlating the properties of embedded fluorophores with the resonances of the film-coupled nanoparticles, we show quantum yield engineering that is nearly independent of the fluorophore’s intrinsic quantum yield, yielding overall fluorescence enhancements of over four orders of magnitude.

Experimental determination of the quadratic nonlinear magnetic susceptibility of a varactor-loaded split ring resonator metamaterial

This letter presents a quantitative measurement of the second harmonic generated by a slab of varactor loaded split ring resonator metamaterial and the retrieval of the effective quadratic nonlinear magnetic susceptibility χm(2) using an approach based on transfer matrices. The retrieved value of χm(2) is in excellent agreement with that predicted by an analytical effective medium theory model.

Design and Simulation of a Frequency-Diverse Aperture for Imaging of Human-Scale Targets

We present the design and simulation of a frequency-diverse aperture for imaging of human-size targets at microwave wavelengths. Predominantly relying on a frequency sweep to produce diverse radiation patterns, the frequency-diverse aperture provides a path to all-electronic operation, sampling a scene without the requirement for mechanical scanning or expensive active components. Similar to other computational imaging schemes, the frequency diverse aperture removes many hardware constraints by placing an increased burden on processing and analysis. While proof-of-concept simulations of scaled-down versions of the frequency-diverse imager and simple targets can be performed with relative ease, the end-to-end modeling of a full-size aperture capable of fully resolving human-size targets presents many challenges, particularly if parametric studies need to be performed during a design or optimization phase. Here, we show that an in-house developed simulation code can be adapted and parallelized for the rapid design and optimization of a full-size, frequency-diverse aperture. Using files of human models in stereolithography format, the software can model the entire imaging scenario in seconds, including mode generation and propagation, scattering from the human model, and measured backscatter. We illustrate the performance of several frequency-diverse aperture designs using images of human-scale targets reconstructed with various algorithms and compare with a conventional synthetic aperture radar approach. We demonstrate the potential of one aperture for threat object detection in security-screening applications.