Brett R. Wenner

Guided-mode resonant coherent light absorbers.

We present a new class of coherent perfect absorbers based on guided-mode resonance in thin semiconductor films. Using particle-swarm optimization methods, we design a thin-film amorphous silicon grating that maximizes coherent modulation of the absorbance. The optimized device exhibits a maximum scattering power of ∼94% and a power absorption limit approaching 100% at the 1550-nm wavelength.

Application of highly conductive ZnO to the excitation of long-range plasmons in symmetric hybrid waveguides

Plasmonics combines attractive features of nanoelectronics and optics enabling highly integrated, subwavelength optical and electronic circuits. The wide application of plasmonic devices hinges on practical demonstrations with low losses at optical and infrared wavelengths. In this frequency regime, noble metals suffer from large losses that are difficult to compensate by adding gain material. Transparent conducting oxides, e.g., ZnO, are good alternatives to metals for plasmonic applications in the optical regime since they exhibit high conductivity and relatively small negative real permittivity values. We study Ga-doped ZnO layers grown on Al 2 O 3 at 200°C by pulsed laser deposition in Ar ambient. The bulk electrical properties, determined by Hall effect, were ρ=2.93×10 −4   Ω-cm ; μ=25.5  cm 2 /V-s ; and n=8.36×10 20   cm −3 . These values of μ and n were used to predict optical properties through the Drude dielectric function. The optical and electrical properties of the material were used to design insulator-metal-insulator (in our case, ZnO embedded in polymer) waveguides for long-range plasmons using full-wave electromagnetic models built with finite element method simulations. The models were used to predict the effect of device geometry on propagation length and losses of the plasmon mode.

Experimental Evidence for Coherent Perfect Absorption in Guided-Mode Resonant Silicon Films

We experimentally verify a new class of coherent absorbers based on guided-mode resonance effects in periodic thin films. We design a silicon-based resonant absorber that is fabricated and tested near the 1300-nm wavelength. Implementing phase control, the device can, in principle, be switched between full absorption and full scattering. The first experimental prototype presented herein shows ~78% absorption in the in-phase state. Nearly total scattering is realized in the out-of-phase state. The experimental results agree reasonably well with theory.

Application of highly conductive ZnO to plasmonics

Plasmonics combines attractive features of nanoelectronics and optics enabling highly integrated, dense subwavelength optical components and electronic circuits which will help alleviate the speed-bottleneck in important technologies such as information processing and computing. The wide application of plasmonic devices hinges on practical demonstrations with low losses at standard optical wavelengths such as near infrared, visible, telecom, etc. Conventional plasmonic devices, based on noble metals, suffer from large losses in these frequency regimes and are difficult to compensate completely by simply adding gain material. Transparent conducting oxides (TCOs) such as ZnO are good alternatives to metals for plasmonic applications in the optical regime since they exhibit high conductivity and relatively small negative real permittivity values necessary for practical plasmonic devices. Ga-doped ZnO layers were grown on Al2O3 at 200 °C by pulsed laser deposition in Ar ambient. The electrical properties, determined by the Hall effect, were: ρ = 2.95x 10-4 Ω-cm; μ = 25.3 cm2/V-s; and n = 8.36 x 1020 cm-3. These values of μ and n were used to predict optical properties through the Drude dielectric function. Reflection measurements confirmed the Hall-effect predictions. The optical and electrical properties of the material were used to design insulator-metal-insulator (in our case, Quartz-ZnO-polymer) waveguides for long range plasmons using full-wave electromagnetic models built with finite element method simulations. The models were used to predict the behavior of ZnO as well as examine the effect of device geometry on propagation length and losses of the plasmon mode.

A computational approach to optimize microring resonators for biosensing applications

Microcavity structures have recently found utility in chemical/biological sensing applications. The appeal of these structures over other refractive index-based sensing schemes, such as those based on surface plasmon resonance, lies in their potential for producing a highly sensitive response to binding events. High-Q devices, characterized by sharp line widths, are extremely attractive for sensing applications because the bound analyte provides an increased optical pathlength, thus shifting the resonant frequency of the device. In this work, we design and simulate resonant microrings using full-wave finite element models. In addition to structure design, integration of the biological recognition element on the resonator is also considered. This is equally important in dictating the sensitivity of the sensing device. To this end, we take a four-step theoretical approach to optimizing the sensor. We begin by using FEM analysis to obtain the characteristic resonant wavelength, line width, and quality factor for bare ring resonators absent of surface functionalization. Next, we simulate the structure with a biorecognition element attached to the surface. The third step is to model the functionalized microring to mimic the interaction with the target analyte. At each step, we derive the transmission spectra, electric field distributions and coupling efficiencies, as well as wavelength dependence using empirical data for the refractive indices of biorecognition element and analyte. Finally, the geometry of the microrings is optimized in conjunction with the constituent material properties and the recognition chemistry using FEM combined with an optimization algorithm to maximize the sensitivity of the integrated biosensor.

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.

Properties of mixed metal-dielectric nanogratings for application in resonant absorption, sensing, and display

Abstract. We treat fundamental resonance effects in hybridized metal–dielectric elements that may find applications in absorption, sensing, and displays. The hybrid structures support guided-mode resonance (GMR) and surface plasmon resonance (SPR) operating independently or in unison. Numerical simulations of periodic resonant films coated in gold that effectively combine principles of both resonance effects show viability of absorbers with equalized spectra and hybrid waveguides. The experimentally measured spectra show qualitative agreement with theoretical models. We introduce a hybrid GMR/SPR refractive-index sensor consisting of a thin aluminum film integrated with a subwavelength silicon-dioxide grating. The sensor operates between the Rayleigh wavelengths of the cover and the substrate. A GMR is excited by TE-polarized light and is subsequently attenuated by the Rayleigh anomaly as the cover index increases. In transverse-magnetic-polarized light, it operates as a Rayleigh sensor with sharp spectral features that would be easily monitored with a spectrum analyzer. As a final device example, we present simulation results pertaining to a one-dimensional color filter utilizing SPR, GMR, and the Rayleigh anomaly and convert it into a polarization insensitive two-dimensional device. With dual periods along orthogonal directions, two resonant peaks are induced within the visible spectrum for unpolarized input light rendering a color-mixing effect. The output color of the dual pixel is tunable with the input polarization state.

A printed dipole reconfigured with magneto-static responsive structures that do not require a directly connected biasing circuit

An initial study of novel Magneto-static Responsive Structures (MRSs) and their application to the frequency reconfigurability of a printed dipole antenna is presented here. The embodiment of the MRSs consisted of a cylindrical cavity with a diameter of 0.9 mm drilled into a 20.0 mil thick 1.5 mm × 1.5 mm TMM4 substrate. The cavities were partially filled with silver coated magnetic particles and covered on the top and bottom with copper tape. The conducting magnetic particles responded to an externally applied magnetic field and formed columns in the direction of the magnetic field lines. The columns connected the top and bottom conducting planes, acting as a switch. It was demonstrated that the electrical length of an antenna could be changed and the resonant frequencies could be reconfigured from 1.5 GHz to 1.9 GHz by incorporating the MRSs into the dipole antenna and controlling the ON and OFF states of the MRS switch. Overall, it was shown that the simulated results agreed well with the measurements. It was also demonstrated that the proposed MRSs do not need directly connected biasing circuitry, making them particularly useful for complex antenna designs.

Infrared Plasmonics via ZnO

Conventional plasmonic devices involve metals, but metal-based plasmonic resonances are mainly limited to λres < 1 μm, and thus metals interact effectively only with light in the UV and visible ranges. We show that highly doped ZnO can exhibit λres ≥ 1 μm, thus moving plasmonics into the IR range. We illustrate this capability with a set of thin (d = 25–147 nm) Al-doped ZnO (AZO) layers grown by RF sputtering on quartz glass. These samples employ a unique, 20-nm-thick, ZnON buffer layer, which minimizes the strong thickness dependence of mobility (μ) on thickness (d). A practical waveguide structure, using these measurements, is simulated with COMSOL Multiphysics software over a mid-IR wavelength range of 4–10 μm, with a detailed examination of propagation loss and plasmon confinement dimension. In many cases, Lplas < λlight, thus showing that IR light can be manipulated in semiconductor materials at dimensions below the diffraction limit.

Efficient broadband energy detection from the visible to near-infrared using a plasmon FET

Plasmon based field effect transistors (FETs) can be used to convert energy induced by incident optical radiation to electrical energy. Plasmonic FETs can efficiently detect incident light and amplify it by coupling to resonant plasmonic modes thus improving selectivity and signal to noise ratio. The spectral responses can be tailored both through optimization of nanostructure geometry as well as constitutive materials. In this paper, we studied various plasmonic nanostructures using gold for a wideband spectral response from visible to near-infrared. We show, using empirical data and simulation results, that detection loss exponentially increases as the volume of metal nanostructure increases and also a limited spectral response is possible using gold nanostructures in a plasmon to electric conversion device. Finally, we demonstrate a plasmon FET that offers a broadband spectral response from visible to telecommunication wavelengths.