Sub-wavelength
arrays for novel sensors
Spectroscopic gas sensors have greater sensitivity and stability than conventional electrochemical sensors. Spectroscopic sensors of toxic gases rely on the fact that each gas has a unique absorption line in the infrared portion of the spectrum, arising from the molecular stretching or rotational modes. For example CO has a sharp absorption at 4.7 m, NO absorbs at 5.3 m whereas nerve gases and toxic serin have absorption features near 10 m. Spectroscopic infrared gas sensors offer very high sensitivity for conclusive detection of individual species since each gas has unique absorption lines in the infrared spectrum. Spectroscopic sensors are lightweight, battery-powered, low-maintenance and low-cost- essential attributes for counter-terrorism applications.
The basic sensor operation has been developed by Ion-Optics (Waltham, MA) consisting of a MEMS based source that is electrically heated to emit a narrow emission spectrum and placed opposite a mirror. The emitting device acts as a detector. When a gas is present that absorbs the IR wavelength, the temperature and resistance of the MEMS device changes, from which the concentration of the source gas is determined. (animation).
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MEMS Source & schematic device Top view of hexagonal subwavelength lattice
An on-going collaboration
between MRC and Ion-Optics Inc, (Waltham, MA) was very successful in modeling
the infrared emission from periodic structures of trenches in a metal coated
silicon wafer produced through MEMS. When such a patterned surface is heated it
emits in a narrow band at a wavelength close to the lattice spacing. The width
and strength of the emission is controlled by the two-dimensional photonic
lattice.
The objective of this project is to design and develop a new generation of
high-sensitivity infrared sensors, whose critical component is the tunable high
power narrow-band infrared source. We develop an infrared source based on the
sharp absorption of surface plasmons in a two-dimensional metallic photonic
lattice, residing on a semiconductor substrate. We recently found that very
narrow band emitters can be achieved utilizing the narrow absorption profile of
surface plasmon (SP) modes in a metal-coated periodic lattice. The two
dimensional lattice of holes is a sub wavelength array which allows only a
certain wavelength to pass through it. The unique aspect of this work is the
coupling of a subwavelength array to a photonic crystal to generate a narrow
resonant absorption feature. When these structures are heated they emit in a
narrow emission band. The photonic crystal is critical in producing a strong
resonant absorption and emission.
We simulated the optical properties of these lattices by developing a rigorous electromagnetic simulation approach based on the S-matrix technique. The reflection, transmission and absorption are calculated in Fourier space with a plane wave basis. Excellent comparison is achieved between simulation and measurement for the optical properties. A resonant absorption mode is found at the lattice spacing of the silicon photonic crystal that matches the plasmon-enhanced absorption of the metal lattice. This enhances the emission at this wavelength. Simulation/design performed at Iowa State has new predictive ability for design and fabrication of new devices at Ion-Optics.
Faculty: Rana Biswas
Staff: Changgeng Ding (Postdoctoral fellow)
Collaborators: I. Puscasu, E. Johnson, M. Pralle, A. Greenwald, J. Daly (Ion-Optics)
Funding: NSF(DMR), NSF(DMI)
Summary of recent results
Calculated emission intensity from the sub-wavelength array. Calculations were performed with the S-matrix scattering approach
Emission intensity for resonant mode from the sub-wavelength array. The field intensity just below the top of the hole array is plotted
