Oliver Edwards

Infrared surface plasmons on heavily doped silicon

wavelengths, respectively. The permittivity spectra were used to calculate SPP mode heights above the silicon surface and SPP propagation lengths. Reasonable merit criteria applied to these quantities suggest that only the heaviest doped material has sensor potential, and then mainly within the wavelength range 6 to 10 lm. Photon-to-plasmon coupling resonances, a necessary condition for sensing, were demonstrated near 10 lm wavelength for this material. The shape and position of these resonances agree well with simple analytic calculations based on the theory of Hessel and Oliner (1965). V C 2011 American Institute of Physics. [doi:10.1063/1.3672738]

Infrared surface plasmon resonance biosensor

A Surface Plasmon Resonance (SPR) biosensor that operates deep into the infrared (3-11 μm wavelengths) is potentially capable of biomolecule recognition based both on selective binding and on characteristic vibrational modes. A goal is to operate specifically at wavelengths where biological analytes are strongly differentiated by their IR absorption spectra and where the refractive index is increased by dispersion. This will provide enhanced sensitivity and selectivity, when biological analytes bind reversibly to biomolecular recognition elements attached to the sensor surface. This paper describes work on the optical and materials aspects of IR surface plasmon resonances. First, three possible coupling schemes are considered: hemicylindrical prisms, triangular prisms, and gratings. Second, materials with plasma frequencies one order of magnitude smaller than for noble metals are considered, including doped semiconductors and semimetals.

Intracavity laser absorption spectroscopy using mid-IR quantum cascade laser

Intracavity Laser Absorption Spectroscopy (ICLAS) at IR wavelengths offers an opportunity for spectral sensing with sufficient sensitivity to detect vapors of low vapor pressure compounds such as explosives. Reported here are key enabling technologies for this approach, including multi-mode external-cavity quantum cascade lasers and a scanning Fabry-Perot spectrometer to analyze the laser mode spectrum in the presence of a molecular intracavity absorber. Reported also is the design of a compact integrated data acquisition and control system. Applications include military and commercial sensing for threat compounds, chemical gases, biological aerosols, drugs, and banned or invasive plants or animals, bio-medical breath analysis, and terrestrial or planetary atmosphere science.

Plasmon resonance response to millimeter-waves of grating-gated InGaAs/InP HEMT

Tunable resonant absorption by plasmons in the two-dimensional electron gas (2DEG) of grating-gated HEMTs is known for a variety of semiconductor systems, giving promise of chip-scale frequency- agile THz imaging spectrometers. In this work, we present our approach to measurement of electrical response to millimeter waves from backward-wave oscillators (BWO) in the range 40-110 GHz for InP-based HEMTs. Frequency-modulation of the BWO with lock-in amplification of the source-drain current gives an output proportional to the change in absorption with frequency without contribution from non-resonant response. This is a first step in optimizing such devices for man-portable or space-based spectral-sensing applications.

Infrared Intracavity Laser Absorption Spectrometer

A spectral sensing method with sufficient sensitivity to detect vapors of low vapor-pressure compounds such as explosives would have great promise for defense and security applications. An opportunity is Intracavity Laser Absorption Spectroscopy (ICLAS) at IR wavelengths. Our approach is based on multi-mode external-cavity quantum cascade lasers and a scanning Fabry-Perot spectrometer to analyze the laser mode spectrum in the presence of a narrow band intracavity absorber. This paper presents results of numerical solution of laser rate equations that support feasibility of kilometer effective active-cavity path lengths and sensitivity to concentrations of 10 ppb. This is comparable to the saturated vapor pressure of TNT. System design considerations and first experimental results are presented at 10 and 70 μm wavelengths.

Infrared surface waves on semiconductor and conducting polymer

Conductors with infrared plasma frequencies are potentially useful hosts of surface electromagnetic waves with sub-wavelength mode confinement for sensing applications. Such materials include semimetals, semiconductors, and conducting polymers. In this paper we present experimental and theoretical investigations of surface waves on doped silicon and the conducting polymer polyaniline (PANI). Resonant absorption features were measured in reflection from lamellar gratings made from doped silicon for various p-polarized CO2 laser wavelengths. The angular reflectance spectra for doped silicon was calculated and compared with the experiments using experimental complex permittivities determined from infrared (IR) ellipsometry data. Polyaniline films were prepared, optical constants determined, and resonance spectra calculated also. A specific goal is to identify a conductor having tight mode confinement, sharp reflectivity resonances, and capability to be functionalized for biosensor applications.

MEMS clocking-cantilever thermal detector

We present performance calculations for a MEMS cantilever device for sensing heat input from convection or radiation. The cantilever deflects upwards under an electrostatic repulsive force from an applied periodic saw-tooth bias voltage, and returns to a null position as the bias decreases. Heat absorbed during the cycle causes the cantilever to deflect downwards, thus decreasing the time to return to the null position. In these calculations, the total deflection with respect to absorbed heat is determined and is described as a function of time. We present estimates of responsivity and noise.

Effects of Polymer Infusion and Characteristic Length Scale on Gold-Black Long-Wave and Far-Infrared Absorbance

Infrared absorbance is investigated in gold-black, a porous nano-structured conducting film. A two level full factorial optimization study with evaporation-chamber pressure, boat current, substrate temperature, and degree of polymer infusion (for hardening) was performed. Polymer infusion was found generally to reduce absorbance in the long wave IR but has little effect at THz wavelengths, although for samples with the highest absorbance there is a slight improvement in the absorbance figure of merit (FOM) in both wavelength regimes. The characteristic length scales of the structured films vary considerably as a function of deposition parameters, but the IR FOM is found to be weakly correlated with these distributions, which are determined by wavelet analysis of scanning electron micrographs.

Finesse of silicon-based terahertz Fabry-Perot spectrometer

This paper considers factors that affect achievable finesse for a recently demonstrated silicon-based scanning Fabry- Perot transmission filter at millimeter and sub-millimeter wavelengths. The mirrors are formed by alternating quarter-wave optical thicknesses of silicon and air in the usual Bragg configuration. Fundamental loss by lattice and free carrier absorption are considered. Technological factors such as surface roughness, bowing, and misalignment are considered for various proposed manufacturing schemes.

Scanning Fabry-Perot filter for terahertz spectroscopy based on silicon dielectric mirrors

A scanning Fabry-Perot transmission filter composed of a pair of dielectric mirrors has been demonstrated at millimeter and sub-millimeter wavelengths. The mirrors are formed by alternating quarter-wave optical thicknesses of silicon and air in the usual Bragg configuration. Detailed theoretical considerations are presented for determining the optimum design. Characterization was performed at sub-mm wavelengths using a gas laser together with a Golay cell detector and at mm-wavelengths using a backward wave oscillator and microwave power meter. High resistivity in the silicon layers was found important for achieving high transmittance and finesse, especially at the longer wavelengths. A finesse value of 411 for a scanning Fabry-Perot cavity composed of three-period Bragg mirrors was experimentally demonstrated. Finesse values of several thousand are considered to be within reach. This suggests the possibility of a compact terahertz Fabry-Perot spectrometer that can operate in low resonance order to realize high free spectral range while simultaneously achieving a high spectral resolution. Such a device is directly suitable for airborne/satellite and man-portable sensing instrumentation.