Christopher M. Roberts

Near-field infrared absorption of plasmonic semiconductor microparticles studied using atomic force microscope infrared spectroscopy

We report measurements of near-field absorption in heavily silicon-doped indium arsenide microparticles using atomic force microscope infrared spectroscopy (AFM-IR). The microparticles exhibit an infrared absorption peak at 5.75 μm, which corresponds to a localized surface plasmon resonance within the microparticles. The near-field absorption measurements agree with far-field measurements of transmission and reflection, and with results of numerical solutions of Maxwell equations. AFM-IR measurements of a single microparticle show the temperature increase expected from Ohmic heating within the particle, highlighting the potential for high resolution infrared imaging of plasmonic and metamaterial structures.

Engineering absorption and blackbody radiation in the far-infrared with surface phonon polaritons on gallium phosphide

We demonstrate excitation of surface phonon polaritons on patterned gallium phosphide surfaces. Control over the light-polariton coupling frequencies is demonstrated by changing the pattern periodicity and used to experimentally determine the gallium phosphide surface phonon polariton dispersion curve. Selective emission via out-coupling of thermally excited surface phonon polaritons is experimentally demonstrated. Samples are characterized experimentally by Fourier transform infrared reflection and emission spectroscopy, and modeled using finite element techniques and rigorous coupled wave analysis. The use of phonon resonances for control of emissivity and excitation of bound surface waves offers a potential tool for the exploration of long-wavelength Reststrahlen band frequencies.

Diffractive interface theory: nonlocal susceptibility approach to the optics of metasurfaces

We present a formalism for understanding the electromagnetism of metasurfaces, optically thin composite films with engineered diffraction. The technique, diffractive interface theory (DIT), takes explicit advantage of the small optical thickness of a metasurface, eliminating the need for solving for light propagation inside the film and providing a direct link between the spatial profile of a metasurface and its diffractive properties. Predictions of DIT are compared with full-wave numerical solutions of Maxwells equations, demonstrating DITs validity and computational advantages for optically thin structures. Applications of the DIT range from understanding of fundamentals of light-matter interaction in metasurfaces to efficient analysis of generalized refraction to metasurface optimization.

Metamaterials-based Salisbury screens with reduced angular sensitivity

We demonstrate that the incorporation of nonlocal nanowire metamaterials into Salisbury screens allows for a substantial reduction of the dependence of incident angle on the absorption maximum. Realizations of angle-independent Salisbury screens for the near-IR, mid-IR, and GHz frequencies are proposed and their performances are analyzed analytically and numerically. It is shown that nonlocal effective medium theory adequately describes the angular dependence of nanowire-based Salisbury screens.

Multiplexed infrared photodetection using resonant radio-frequency circuits

We demonstrate a room-temperature semiconductor-based photodetector where readout is achieved using a resonant radio-frequency (RF) circuit consisting of a microstrip split-ring resonator coupled to a microstrip busline, fabricated on a semiconductor substrate. The RF resonant circuits are characterized at RF frequencies as function of resonator geometry, as well as for their response to incident IR radiation. The detectors are modeled analytically and using commercial simulation software, with good agreement to our experimental results. Though the detector sensitivity is weak, the detector architecture offers the potential for multiplexing arrays of detectors on a single read-out line, in addition to high speed response for either direct coupling of optical signals to RF circuitry, or alternatively, carrier dynamics characterization of semiconductor, or other, material systems.

Metasurface-enhanced transparency

We consider the problem of light transmission from a high refractive index medium into a low-index environment. While total internal reflection severely limits such transmission in systems with smooth interfaces, diffractive metasurfaces may help out-couple light that enters an interface at blazing angles. We demonstrate that the profile of the structured interface can be numerically optimized to target a specific emission pattern. Our study suggests that while metasurfaces can help to out-couple light from a range of incident directions, there exists a universal limit for total transmission efficiency that depends only on the dielectric properties of the two materials and is independent of the profile and the composition of the metasurface coupler.

Mid-infrared epsilon-near-zero modes in ultra-thin phononic films

We demonstrate strong, narrow-band selective absorption and subsequent selective thermal emission from ultra-thin planar films of polar materials at mid-infrared wavelengths. Our structures consist of AlN layers of varying thicknesses deposited upon molybdenum ground planes. We demonstrate coupling to the Berreman mode at frequencies at, or near, the longitudinal optical phonon energy of AlN. Samples are characterized experimentally by temperature-, angle-, and polarization-dependent Fourier transform infrared reflection and emission spectroscopy and modeled using a transfer matrix method approach. Strong, spectrally selective thermal emission, with near angle-independent spectral position, is demonstrated from an AlN layer with thickness t

Interscale mixing microscopy: far-field imaging beyond the diffraction limit

Optical microscopy is widely used to analyze the properties of materials and structures, to identify and classify these structures, and to understand and control their responses to external stimuli. The extent of available applications is determined largely by the resolution offered by a particular microscopy technique. Here we present an analytic description and an experimental realization of interscale mixing microscopy, a diffraction-based imaging technique that is capable of detecting and characterizing wavelength/10 objects in far-field measurements with both coherent and incoherent broadband light. This technique is aimed at analyzing subwavelength objects based on far-field measurements of the interference created by the objects and a finite diffraction grating. A single measurement, analyzing the multiple diffraction orders, is often sufficient to determine the parameters of the object. The presented formalism opens opportunities for spectroscopy of nanoscale objects in the far field.

Mid-IR Plasmonics with Engineered Semiconductor Metals

We investigate the utility of heavily doped semiconductors as plasmonic materials for mid-IR applications. The wavelength flexibility and design-ability of these materials allow for the demonstration of nanophotonic structures and devices for long-wavelength IR light.

Diffractive Interface Theory: nonlocal polarizability approach to the optics of metasurfaces: erratum

A typo in the software implementation of Diffractive Interface Theory [Opt. Express23, 2764 (2015)10.1364/OE.23.002764] was found during subsequent research. The typo was corrected, yielding better-than-originally-reported agreement between Diffractive Interface Theory and full-wave numerical solutions of Maxwell equations.