Irina Puscasu

Photonic crystal enhanced narrow-band infrared emitters

We have experimentally and theoretically developed a unique thermally stimulated midinfrared source that emits radiation within a narrow range of wavelengths (δλ/λ⩽0.2). The emission wavelengths are defined by the periodicity of a metal coated silicon–air photonic crystal etched into the emitter surface. The lattice of the holes in the metal mediate the coupling of light into discrete surface plasmon states. This yields surfaces with spectrally nonuniform infrared reflection properties where over much of the IR 90+% of photons are reflected yet, in a narrow spectral region, 90% absorption is observed. Transfer matrix calculations simulate well the position and strength of the absorption features. This technology will afford tunable infrared emitters with high power in a narrow spectral band that are critical for sensing, spectroscopy, and thermophotovoltaic applications.

Narrow-band, tunable infrared emission from arrays of microstrip patches

We demonstrate through a combination of theory and experiment that an array of microstrip patches leads to a surface with sharp and tunable emission bands. The physical mechanisms and locations for various emission peaks are described via both analytical theory and numerical simulations. These predictions agree well with our experimental data, taken on systems designed to emit strongly in the infrared. The main peak, which arises from plasmons trapped under a patch, can be well separated from other spectral structures, narrow in wavelength, but broad in angular distribution.

Modeling parameters for the spectral behavior of infrared frequency-selective surfaces

Comparisons of experiment and theory are presented for transmission spectra over the range 2-15 mum of a set of frequency-selective surfaces consisting of arrays of simple dipole patches of aluminum on or in silicon. The arrays are fabricated by direct-write electron-beam lithography. Important parameters controlling the spectral shape are identified, such as dipole length, spacing, resistance, and dielectric surroundings. The separate influence of these variables is exhibited. Encouraging agreement between simple model calculations and the measurements is found.

Optical resonances in periodic surface arrays of metallic patches

The transmission of light along the surface normal through an air-quartz-glass interface covered with a periodic array of thin, rectangular gold patches has been studied over the visible to infrared range. The various structures that are observed can be qualitatively understood as arising from standing-wave resonances set by the size and surroundings of the metal patches. A method-of-moments calculational scheme provides simulations in good quantitative agreement with the data. It is shown how the standing-wave picture provides a useful conceptual framework to understand and exploit such systems.

Refractive-index and element-spacing effects on the spectral behavior of infrared frequency-selective surfaces.

Transmission and reflection characteristics of inductive-mesh frequency-selective surfaces were measured in the 4-12-microm range. Specific issues investigated include the effect of interelement spacing on the location and width of the resonance and the influence of superstrate and substrate refractive indices on the spectral response of the structure.

Mechanisms underlying extraordinary transmission enhancement in subwavelength hole arrays

Extraordinary transmission in subwavelength hole arrays has been interpreted by surface-plasmon models and diffraction-based models. To understand controversial mechanisms of transmission enhancement, we simulate hole arrays, using a rigorous Fourier-space scattering matrix simulation. At the enhanced transmission maximum there are large evanescent diffracted fields above the metal surface. These evanescent fields are decomposed into longitudinal and transverse components. Both components are comparable in magnitude. The longitudinal field is 15%-20% larger in the square lattice. Transverse fields are slightly larger in the triangular lattice. The longitudinal and transverse evanescent surface fields are related to bound surface modes of the hole array.

Resonant enhancement of emission and absorption using frequency selective surfaces in the infrared

We investigate the infrared properties of frequency selective surfaces consisting of aluminum patches on silicon substrates. Resonant behavior is found not only in the transmission and reflection, but also in the absorption and emission of these surfaces. The resonance location is a controllable function of the surface pattern. Simple model calculations reproduce well the qualitative behavior of our samples.

Comparison of infrared frequency selective surfaces fabricated by direct-write electron-beam and bilayer nanoimprint lithographies

We report on the fabrication of crossed-dipole resonant filters by direct-write electron-beam and nanoimprint lithographies. Such structures have been used as spectrally selective components at visible, microwave, and infrared wavelengths. Imprinting is accomplished in a modified commercial hot press at 155 °C. The replica is then etched in oxygen plasma and developed in chlorobenzene to selectively dissolve the poly(methylmethacrylate and methacrylic acid) and poly(methylmethacrylate) bilayer resist. This step enhances undercut and improves lift-off metalization. Infrared fourier transform spectroscopy was performed to characterize the transmission response of the frequency selective surfaces (FSSs) fabricated. The resonant behavior for the direct-write FSS was found to be 5.3 μm and for the nanoimprinted FSS to be 6 μm. The shift towards longer wavelengths is consistent with the dimensions obtained for the FSSs elements in both cases.

Sharp thermal emission and absorption from conformally coated metallic photonic crystal with triangular lattice

A metallic photonic crystal consisting of a triangular lattice of holes in a silicon layer coated with gold is fabricated at a lattice pitch of 3.75 μm using conventional lithographic methods. The photonic crystal exhibits a deep reflection minimum and sharp thermal emission peak near the lattice spacing. Scattering matrix simulations agree well with measurements. This simple structure with a single patterned metallic layer has no emission sidebands and can be scaled to other lattice spacings to tune the wavelength of the absorption and emission peak.

Photonic crystals enable infrared gas sensors

Sensors of trace gases are of enormous importance to diverse fields such as environmental protection, household safety, homeland security, bio-hazardous material identification, meteorology and industrial environments. The gases of interest include CO for home environments, CO2 for industrial and environment applications and toxic effluents such as SO2, CH4, NO for various manufacturing environments. We propose a new class of IR gas sensors, where the enabling technology is a spectrally tuned metallo-dielectric photonic crystal. Building both the emitting and sensing capabilities on to a single discrete element, Ion Optics’ infrared sensorchip brings together a new sensor paradigm to vital commercial applications. Our design exploits Si-based suspended micro-bridge structures fabricated using conventional photolithographic processes. Spectral tuning, control of bandwidth and direction of emission were accomplished by specially designed metallo-dielectric photonic crystal surfaces.