Gautam Medhi

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]

Long-wave infrared surface plasmon grating coupler

We present a simplified analytic formula that may be used to design gratings intended to couple long-wave infrared radiation to surface plasmons. It is based on the theory of Hessel and Oliner (1965). The recipe is semiempirical, in that it requires knowledge of a surface-impedance modulation amplitude, which is found here as a function of the grating groove depth and the wavelength for silver lamellar gratings at CO(2) laser wavelengths. The optimum groove depth for photon-to-surface-plasmon energy conversion was found by experiment and calculation to be approximately 10%-15% of the wavelength. This value is about twice what has been reported previously in the visible spectral range for sinusoidal grating profiles.

Infrared surface polaritons on antimony

The semimetal antimony, with a plasma frequency ~80 times less than that of gold, is potentially useful as a host for infrared surface polaritons (SPs). Relevant IR SP properties, including the frequency-dependent propagation length and penetration depths for fields into the media on either side of the interface, were determined from optical constants measured on optically-thick thermally-evaporated Sb films over the wavelength range 1 to 40 μm. Plasma and carrier relaxation frequencies were determined from Drude-model fits to these data. The real part of the permittivity is negative for wavelengths beyond 11 μm. Distinct resonant decreases in specular reflected intensity were observed for Sb lamellar gratings in the wavelength range of 6 to 11 μm, where the real part of the permittivity is positive. Both resonance angles and the angular reflectance spectral line shapes are in agreement with theory for excitation of bound surface electromagnetic waves (SPs). Finite element method (FEM) electrodynamic simulations indicate the existence of SP modes under conditions matching the experiments. FEM results also show that such waves depend on having a significant imaginary part of the permittivity, as has been noted earlier for the case of surface exciton polaritons.

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.

Fano-Resonance Photonic Crystal Membrane Reflectors at Mid- and Far-Infrared

We report here single-layer ultracompact Fano-resonance photonic crystal membrane reflectors (MRs) at mid-infrared (IR) and far-IR (FIR) bands, based on single layer crystalline Si membranes. High-performance reflectors were designed for surface-normal incidence illumination with center operation wavelengths up to the 75-μm FIR spectral band. Large-area patterned MRs were also fabricated and transferred onto glass substrates based on membrane transfer processes. Close to 100% reflection was obtained at the ~ 76-μm spectral band, with a single-layer Si membrane thickness of 18 μm. Such Fano-resonance-based membranes reflectors offer great opportunities for high-performance ultracompact dielectric reflectors at IR and THz regions.

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.

Quantum cascade laser intracavity absorption spectrometer for trace gas sensing

A mid-infrared intracavity laser absorption spectrometer for trace gas sensing is demonstrated. An external-cavity multi-mode quantum cascade laser with central wavelength 8.0 μm was combined with a scanning Fabry-Perot interferometer, which analyzed the change of the laser emission spectrum caused by introducing an analyte inside the cavity. The detection mechanism is based on monitoring the laser spectrum dynamics at adiabatically changing laser conditions in long pulse operation mode. Fast acquisition and vapor exchange allow nearly real-time analyte detection. Sensitivity at the level of 1 × 10−5 cm−1 was demonstrated based on a weak water vapor absorption line.

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.