We describe a new class of plasmonic photonic (PPh) crystal structures integrated onto a MEMS platform for high
temperature-intensity, high speed, and high efficiency narrow band emitting and signaling applications in the infrared.
We exploit 2D organized metallo-dielectric surface structures1,2 for angular and spectral control of reflection,
absorption and emission, efficiently in the infrared, with little or no leakage into adjacent visible or near-infrared
bands. The 2D PPh structures are built on a MEMS platform, for thermal isolation from the environment. Recent
advances2 in the design of the 2D PPh structures allows for tremendous performance enhancement: high
temperature/high intensity operation close to 1000 C and high speed (200Hz with 50% modulation), opening new
applications in spectroscopy, infrared imaging, and signaling. Demonstrated wafer-level vacuum sealing improves the
wall plug efficiency dramatically allowing these devices to be portable, light and battery operated. One potential
application for these light-weight, low-power consumption, low-cost IR emitters is signaling and marking in 3-5 or 8-
12 micron thermal bands.
There is a strong desire to create narrowband infrared light sources as personnel beacons for application in infrared
Identify Friend or Foe (IFF) systems. This demand has augmented dramatically in recent years with the reports of
friendly fire casualties in Afghanistan and Iraq. ICx Photonics' photonic crystal enhancedTM (PCETM) infrared emitter
technology affords the possibility of creating narrowband IR light sources tuned to specific IR wavebands (near 1-2
microns, mid 3-5 microns, and long 8-12 microns) making it the ideal solution for infrared IFF. This technology is
based on a metal coated 2D photonic crystal of air holes in a silicon substrate. Upon thermal excitation the photonic
crystal modifies the emitted yielding narrowband IR light with center wavelength commensurate with the periodicity of
the lattice. We have integrated this technology with microhotplate MEMS devices to yield 15mW IR light sources in the
3-5 micron waveband with wall plug efficiencies in excess of 10%, 2 orders of magnitude more efficient that
conventional IR LEDs. We have further extended this technology into the LWIR with a light source that produces 9
mW of 8-12 micron light at an efficiency of 8%. Viewing distances >500 meters were observed with fielded camera
technologies, ideal for ground to ground troop identification. When grouped into an emitter panel, the viewing distances
were extended to 5 miles, ideal for ground to air identification.
Ion Optics has developed a thin silicon membrane MEMS device that replaces the thermal source, IR filter, IR detector and mechanical chopper in conventional non-dispersive infrared gas sensors. The key enabling technology is a 2-D photonic crystal. The center wavelength and bandwidth of emitted radiation from the photonic crystal depends upon the pattern etched into the surface. Previously we reported designs based on hexagonal arrangements of holes about 2 microns diameter. New results for more intricate designs with deliberate photonic crystal "defects" will be presented. Experimental results will be compared to 3-D electromagnetic models. The 2-D photonic crystal structure consists of an array of air rods produced by self-aligned etching into a thin (100nm) conductor on top of a dielectric membrane. We describe fabrication routes via conventional silicon microlithography and novel approaches including nano-imprinting and transfer molding. We present spectral emission and absorption measurements which relate optical intensity to details of photonic crystal design and fabrication.
Light-weight, low-power consumption, low-cost IR sources are required for combat ID (IFF, identify friend or foe), trail markers, pallet markers, etc. They must be visible with conventional viewers at 200 meters in the 3-5 micron or 8-12 micron bands and emit no visible or near infrared radiation. Ion Optics has tested a prototype MEMS IR source that can meet all of these requirements. It uses a hermetically sealed filament with a photonic crystal-enhanced (PCETM) coating that efficiently generates narrowband IR light. The photonic crystal surface structure limits emission to (tunable) predetermined bands (3-5 and 8-12 microns specifically). These devices generate 10mW of IR light in the 3-5 micron band with "wall-plug" efficiency of 10%, 2 orders of magnitude more efficient than conventional IR LED's. This high efficiency enables overnight battery operation. Using traditional 3-5 micron MWIR cameras, we measured visibility ranges of 200 meters. Current research and development on wafer-level packaging of the MEMS device promises to increase device yield, improve reliability, reduce package size and reduce total cost.
A new kind of Identification Friend or Foe (IFF) infrared beacon has been demonstrated. The omni-directional beacon consists of a pyramidal array of 1W pulsIR thermal light sources. Operating at a total power of 84W, the beacon can be used to track and identify surface vehicles and personnel with a recognition range of up to 6 miles on the battlefield and in urban environments or the marine boundary layer. Advanced photonic technology enables the beacon to be seen only while using a 3-5 μm or 8-12 μm thermal imaging system. There is no visible or near-IR emission to betray the location of the beacon. The beacon is rugged and will operate from -40 to 50°C ambient temperature, 0-100% relative humidity, 0 - 10,000 ft altitude, and meets MIL-STD 810F and MIL-STD 461E.
Sensors of trace gases are of enormous importance to diverse fields such as environmental protection, household safety, homeland security, bio-hazardous material identification, meteorology and industrial environments. The gases of interest include CO for home environments, CO2 for industrial and environment applications and toxic effluents such as SO2, CH4, NO for various manufacturing environments. We propose a new class of IR gas sensors, where the enabling technology is a spectrally tuned metallo-dielectric photonic crystal. Building both the emitting and sensing capabilities on to a single discrete element, Ion Optics’ infrared sensorchip brings together a new sensor paradigm to vital commercial applications. Our design exploits Si-based suspended micro-bridge structures fabricated using conventional photolithographic processes. Spectral tuning, control of bandwidth and direction of emission were accomplished by specially designed metallo-dielectric photonic crystal surfaces.
Inexpensive optical MEMS gas and chemical sensors offer chip-level solutions to environmental monitoring, industrial health and safety, indoor air quality, and automobile exhaust emissions monitoring. Previously, Ion Optics, Inc. reported on a new design concept exploiting Si-based suspended micro-bridge structures. The devices are fabricated using conventional CMOS compatible processes. The use of photonic bandgap (PBG) crystals enables narrow band IR emission for high chemical selectivity and sensitivity. Spectral tuning was accomplished by controlling symmetry and lattice spacing of the PBG structures. IR spectroscopic studies were used to characterize transmission, absorption and emission spectra in the 2 to 20 micrometers wavelength range. Prototype designs explored suspension architectures and filament geometries. Device characterization studies measured drive and emission power, temperature uniformity, and black body detectivity. Gas detection was achieved using non-dispersive infrared (NDIR) spectroscopic techniques, whereby target gas species were determined from comparison to referenced spectra. A sensor system employing the emitter/detector sensor-chip with gas cell and reflective optics is demonstrated and CO2 gas sensitivity limits are reported.
A new IR-based sensor technology is introduced for environmental monitoring of industrial pollutants (CO2, CO, NOx, etc.). The design concept exploits Si-based, thermally isolated suspended bridge structures. These devices, which function as both IR emitter and detector, are fabricated using MEMS-based processing methods. Photonic bandgap (PBG) modified surfaces enable narrow band IR emission for high chemical selectivity and sensitivity. Spectral tuning is accomplished by controlling symmetry and lattice spacing of the PBG structures. IR spectroscopic studies were used to characterize transmission, absorption and emission spectra in the 2 to 20 micrometers wavelength range. Device characterization studies measured drive and emission power, temperature uniformity, and black body detectivity. Gas detection was achieved using non-dispersive infrared (NDIR) spectroscopic techniques, whereby target gas species and concentrations were determined from comparison to referenced spectra. A sensor system employing the emitter/detector sensor-chip with gas cell and reflective optics is demonstrated and CO2 gas sensitivity limits are reported. A multi-channel microsensor-array is proposed for multigas (e.g., CO2, CO, and NOx, etc.) detection.
MEMS silicon (Si) micro-bridge elements, with photonic band gap (PBG) modified surfaces are exploited for narrow-band spectral tuning in the infrared wavelength regime. Thermally isolated, uniformly heated single crystal Si micro-heaters would otherwise provide gray-body emission, in accordance with Planck's distribution function. The introduction of an artificial dielectric periodicity in the Si, with a surface, vapor-deposited gold (Au) metal film, governs the photonic frequency spectrum of permitted propagation, which then couples with surface plasmon states at the metal surface. Narrow band spectral tuning was accomplished through control of symmetry and lattice spacing of the PBG patterns. Transfer matrix calculations were used to model the frequency dependence of reflectance for several lattice spacings. Theoretical predictions that showed narrow-band reflectance at relevant wavelengths for gas sensing and detection were then compared to measured reflectance spectra from processed devices. Narrow band infrared emission was confirmed on both conductively heated and electrically driven devices.
A comparative study between theory and experiment is presented for transmission through lossy frequency selective surfaces (FSSs) on silicon in the 2 - 15 micrometer range. Important parameters controlling the resonance shape and location are identified: dipole length, spacing, impedance, and dielectric surroundings. Their separate influence is exhibited. The primary resonance mechanism of FSSs is the resonance of the individual metallic patches. There is no discernable resonance arising from a feed-coupled configuration. The real part of the element's impedance controls the minimum value of transmission, while scarcely affecting its location. Varying the imaginary part shifts the location of resonance, while only slightly changing the minimum value of transmission. With such fine-tuning, it is possible to make a good fit between theory and experiment near the dipole resonance on any sample. A fixed choice of impedance can provide a reasonable fit to all samples fabricated under the same conditions. The dielectric surroundings change the resonance wavelength of the FSS compared to its value in air. The presence of FSS on the substrate increases the absorptivity/emissivity of the surface in a resonant way. Such enhancement is shown for dipole and cross arrays at several wavelengths.