Subwavelength resonant structures designed for long-wave infrared (LWIR) absorption have been integrated with a
standard vanadium-oxide microbolometer. Dispersion of the dielectric refractive index provides for multiple
overlapping resonances that span the 8-12 μm LWIR wavelength band, a broader range than can be achieved using the
usual quarter-wave resonant cavity engineered into the air-bridge structures. Experimental measurements show a 49%
increase in responsivity for LWIR and a 71% increase across a full waveband as compared to a similar device designed
for only LWIR absorption, using a 300°C blackbody at 35 Hz chopping rate. Increased thermal time constant due to
additional mass is shown to lessen this enhancement at higher chopping rates.
We design, fabricate, and characterize a frequency-selective surface (FSS) with directional thermal emission and absorption for long-wave infrared wavelengths. The FSS consists of an array of patch antennas connected by microstrips, the ensemble of which supports leaky-wave-type modes with forward and backward propagating branches. The branches are designed to intersect at 9.8 μm and have a broadside beam with 20-deg full width at half maximum at this wavelength. The absorption along these branches is near unity. Measurement of the hemispherical directional reflectometer shows good agreement with simulation. The ability to control the spectral and directional emittance/absorptance profiles of surfaces has significant applications for radiation heat transfer and sensing.
Resonantly absorbing thin films comprising periodically sub-wavelength structured metal surface, dielectric spacer, and metal ground plane are a topic of current interest with important applications. These structures are frequently described as “metamaterials”, where effective permittivity and permeability with dispersion near electric and magnetic resonances allow impedance matching to free space for maximum absorption. In this paper, we compare synchrotron-based infrared spectral microscopy of a single isolated unit cell and a periodic array, and we show that the resonances have little to do with periodicity. Instead, the observed absorption spectra of usual periodically structured thin films are best described as due to standing-wave resonances within each independent unit cell, rather than as due to effective optical constants of a metamaterial. The effect of having arrays of unit cells is mainly to strengthen the absorption by increasing the fill factor, and such arrays need not be periodic. Initial work toward applying the subject absorbers to room-temperature bolometer arrays is presented.
Patterned highly absorbing gold black film has been selectively deposited on the active surfaces of a vanadium-oxide-based infrared bolometer array. Patterning by metal lift-off relies on protection of the fragile gold black with an evaporated oxide, which preserves gold black’s near unity absorption. This patterned gold black also survives the dry-etch removal of the sacrificial polyimide used to fabricate the air-bridge bolometers. Infrared responsivity is substantially improved by the gold black coating without significantly increasing noise. The increase in the time constant caused by the additional mass of gold black is a modest 14%.
We present a design for a low-noise bolometer linear array based on the temperature-dependent conductivity of a VOx- Au film. Typical thin film bolometers must compromise between low resistivity to limit Johnson noise and high temperature coefficient of resistivity (TCR) to maximize responsivity. Our vanadium oxide is alloyed with a small concentration of gold by co-sputtering, which gives very low resistivity and very high TCR simultaneously. The film is fabricated on an air bridge device having high thermal conductivity and small thermal time constant optimized for 30 to 60 Hz frame rates. The linear array functions as a low-power profile sensor with a modulated bias. For 1 V bias, we predict responsivity exceeding 1200 V/W. Johnson noise dominates with predicted NEP values as low as 1.0 × 10-11 W/Hz1/2. Preliminary device testing shows film resistivity below 2.5 Ω-cm with TCR exceeding -2.0%. Preliminary measurements of NEP and D* are reported.
Apertureless scattering-type Scanning Near-field Optical Microscopy (s-SNOM) has been used to study the electromagnetic response of infrared antennas below the diffraction limit. The ability to simultaneously resolve the phase and amplitude of the evanescent field relies on the implementation of several experimentally established background suppression techniques. We model the interaction of the probe with a patch antenna using the Finite Element Method (FEM). Green's theorem is used to predict the far-field, cross-polarized scattering and to construct the homodyne amplified signal. This approach allows study of important experimental phenomena, specifically the effects of the reference strength, demodulation harmonic, and detector location.
We design, fabricate, and characterize a Frequency Selective Surface (FSS) with directional thermal emission and
absorption for long-wave infrared wavelengths (LWIR). The FSS consists of an array of patch antennas connected by
microstrips, the ensemble of which supports leaky-wave type modes with forward and backward propagating branches.
The branches are designed to intersect at 9.8 μm, and have a broadside beam with 20° FWHM at this wavelength. The absorption along these branches is near-unity. Measurement of the hemispherical directional reflectometer (HDR)
shows good agreement with simulation. The ability to control the spectral and directional emittance/absortpance profiles
of surfaces has significant applications for radiation heat transfer and sensing.
Metal-Oxide-Metal diodes offer the possibility of directly rectifying infrared radiation. To be effective for sensing or energy harvesting they must be coupled to an antenna which produces intense fields at the diode. While antennas significantly increase the effective capture area of the MOM diode, it is still limited and maximizing the captured energy is still a challenging goal. In this work we investigate integrating MOM diodes with a slot antenna Frequency Selective Surface (FSS). This maximizes the electromagnetic capture area while minimizing the transmission line length which helps reduce losses because metal losses are much lower at DC than at infrared frequencies. Our design takes advantage of a single self-aligned patterning step using shadow evaporation. The structure is optimized at 10.6 µm to have less than 2% reflection (polarization sensitive) and simulations predict that 70% of the incident energy is dissipated into the oxide layer. Initial experimental results fabricated with e-beam lithography are presented and the diode coupled FSS is shown to produce a polarization sensitive unbiased DC short circuit current. This work is promising for both infrared sensing and imaging as well as direct conversion of thermal energy.
Mid-infrared polarimetry remains an underexploited technique; where available it is limited in spectral coverage from
the ground, and conspicuously absent from the Spitzer, JWST and Herschel instrument suites. The unique characteristics
of SOFIA afford unprecedented spectral coverage and sensitivity in the mid-infrared waveband. We discuss the
preliminary optical design for a 5-40μm spectro-polarimeter for use on SOFIA, the SOFIA Mid-InfraRed Polarimeter
(SMIRPh). The design furthers the existing 5-40μm imaging and spectroscopic capabilities of SOFIA, and draws on
experience gained through the University of Florida's mid-IR imagers, spectrometer and polarimeter designs of T-ReCS
and CanariCam. We pay special attention to the challenges of obtaining polarimetric materials suitable at both these
wavelengths and cryogenic temperatures. Finally, we (briefly) present an overview of science highlights that could be
performed from a 5-40μm imaging- and spectro-polarimeter on SOFIA. Combined with the synergy between the
possible future far-IR polarimeter, Hale, this instrument would provide the SOFIA community with unique and exciting
science capabilities, leaving a unique scientific legacy.