A mid-infrared intracavity laser absorption spectrometer based on an external cavity multi-mode quantum cascade laser
is combined with a scanning Fabry-Perot interferometer is used as tunable narrow band transmission filter to analyze the
laser emission spectrum. Sensitivity as a trace gas detector at 8.1 micron wavelengths has been demonstrated based on a
weak water vapor line at an absorption coefficient of 1 x 10-5 cm-1. For molecules of reasonably strong absorption cross section (10-17 cm2), this corresponds to a detection limit of 40 ppb.
The convergence of silicon photonics and infrared plasmonics allows compact, chip-scale spectral sensors. We report on
the development of a compact mid-IR spectrometer based on a broad-band IR source, dielectric waveguides, a
transformer to convert between waveguide modes and surface plasmon polaritons (SPP), an interaction region where
analyte molecules are interrogated by SPPs, an array of ring resonators to disperse the light into spectral components,
and photodetectors. The mid-IR light source emits into a dielectric waveguide, leading to a region that allows coupling
of the incident photons into SPPs. The SPPs propagate along a functionalized metal surface within an interaction region.
Interactions between the propagating SPP and any analytes bound to the surface increase loss at those wavelengths that
correspond to the analyte vibrational modes. After a suitable propagation length the SPP will be coupled back into a
dielectric waveguide, where specific wavelength components will be out-coupled to detectors by an array of ring
resonators. We have selected a 3.4 micron LED as the IR source, based on both cost and performance. Initial
experiments with circular waveguides formed from GLSO glass include measurement of the loss per mm.
Electrodynamic simulations have been performed to inform the eventual Si taper design of the proposed
photonic/plasmonic transformer. The SPP propagation length necessary for a discernible change in the signal due to
absorption in the interaction region has been estimated to be on the order of 1 mm, well within the bounds of calculated
propagation lengths for SPPs on Au.
Conducting polymers are potentially useful materials in sensor applications. Polyaniline is one of the
most promising of these materials due to high conductivity and plasma frequencies as high as the mid-infrared.
The application of this material is still limited because of low conductivity. In this paper, we
chemically prepared a composite of co-doped polyaniline with hydrochloric acid and MSA (methane
sulfonic acid) in aqueous solution with both colloidal and nano-graphite. Solutions of the composite
material were prepared in m-cresol and NMP (N-mthyle-2-pyrrolidone), which are common organic
solvents. This approach resulted in material with conductivity higher than either intrinsic polyaniline or
graphite alone. The solution of the composite was spin coated on suitable substrates. The thicknesses of
the films were measured using atomic force microscope (AFM). Fourier transform infrared spectra
(FTIR) and micro-Raman spectra were collected to confirm the composition and determine the infrared
thickness. Surface plasmon resonances for grating patterns of this composite material were calculated
using experimental determined infrared (IR) ellipsometry data. The goal is to identify a material which
has potential application for surface plasmons resonance sensing with high sensitivity and selectivity in
IR range.
A quantum cascade laser at IR wavelengths with an open external cavity presents an opportunity for spectral sensing of
molecular compounds that have low vapor pressure. The sensitivity of such a system is potentially very high due to
extraordinarily long effective optical paths that can be achieved in an active cavity. We demonstrate here an external
cavity mid-IR QCL molecular absorption sensor using a fixed Fabry-Perot etalon as the spectrum analyzer. The system is
sensitive to the water vapor present in the laboratory air with an absorption coefficient of just 9.6 x 10-8 cm-1. The system
is sensitive enough to detect the absorption coefficient of TNT vapor at room temperature.
Mid-IR spectrometers with adequate resolution for chemical sensing and identification are typically large, heavy, and
require sophisticated non-stationary optical components. Such spectrometers are limited to laboratory settings. We
propose an alternative based on semiconductor micro-fabrication techniques. The device consists of several enabling
parts: a compact broad-band IR source, photonic waveguides, a photon-to-surface-plasmon transformer, a surfaceplasmon
sample-interaction region, and an array of silicon ring-resonators and detectors to analyze the spectrum. Design
considerations and lessons learned from initial experiments are presented.
R. Peale, Nima Nader Esfahani, Christopher Fredricksen, Gautam Medhi, Justin Cleary, Joshua Hendrickson, Walter Buchwald, Himanshu Saxena, Oliver Edwards, Michael Lodge, Ben Dawson, M. Ishigami
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. We present our calculations of transmission spectra and resonant photoresponse due to plasmons in InPand
graphene-based HEMTs at millimeter and THz wavelengths. Our experimental approach to measurement of
electrical response is also described. Potential applications include man-portable or space-based spectral-sensing.
A Surface Plasmon Resonance (SPR) biosensor that operates deep into the infrared (3-11 μm wavelengths) is potentially
capable of biomolecule recognition based on both selective binding and characteristic vibrational modes. The goal is to
operate such sensors 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 selectivity and sensitivity, when
biological analytes bind reversibly to biomolecular recognition elements attached to the sensor surface. This paper
investigates potentially useful IR surface plasmon resonances hosts on lamellar gratings formed from various materials
with plasma frequencies in the IR wavelength range. These materials include doped semiconductors, CuSnS, graphite
and semimetal Bi and Sb. Theoretical results were compared with the experimental results. Penetration depth
measurement from the experimental complex permeabilities values shows the tighter mode confinement than for usual
Au giving better overlap with biological analytes.
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.
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.
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.
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.
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.
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