Publications

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The angular sensitivity of guided mode resonant filters (GMRF) is well known. While at times useful for angle tuning of the response, this sensitivity can also be a major detriment as angular changes of tenths of a degree can shift the wavelength response in a narrow bandwidth device by an amount greater than the width of the resonance peak. We identify geometries where the resonance is more angularly stable, demonstrating high reflectivity at the design wavelength for several degrees in both azimuth and inclination angular directions with virtually no change in lineshape of the response. The investigation of GMRFs in both classical and conical mounts through simulation using rigorous coupled wave analysis reveals that there are preferred mounts for greater angular tolerance. We simulate a grating at telecom wavelengths using a design that we have previously fabricated. The identical grating placed in different mounts can exhibit angular tolerances that differ by well over an order of magnitude (60x). The most commonly used classical mount has a much more sensitive angular tolerance than does the conical mount. The lineshape of the resonant response shows only negligible changes across the angular band. The angular band for the sample grating is simulated to be several degrees in the conical mount as opposed to a tenth of a degree in the classical mount. We could thus expand the application space for narrow-band GMRFs into areas where angular tolerance cannot be controlled to the degree that we have believed required in the past. © 2013 SPIE.

We present a broadband, all-dielectric, diffractive optical element (DOE) for spectral beam combining with optimized efficiency. We achieve maximal efficiency and polarization insensitivity for the sum of incident wavelengths by varying grating etch depth and duty cycle of a rectangular profile grating realized with the precision of ebeam mask definition. Design and fabrication considerations that maximize efficiency are quantified, including material options, e-beam defined lithographic parameters such as grating periods and aspect ratios, tailored wavelength dispersion, and polarization independence. These results are compared to published efficiency values of >95% diffraction efficiency for a single polarization and single wavelength and polarization-independent efficiency values of >98% also for a single wavelength. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

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Resonant subwavelength gratings have been designed and fabricated as wavelength-specific reflectors for application as a rotary position encoder utilizing ebeam based photolithography. The first grating design used a two-dimensional layout to provide polarization insensitivity with separate layers for the grating and waveguide. The resulting devices had excellent pattern fidelity and the resonance peaks and widths closely matched the expected results. Unfortunately, the gratings were particularly angle sensitive and etch depth errors led to shifts in the center wavelength of the resonances. A second design iteration resulted in a double grating period to reduce the angle sensitivity as well as different materials and geometry; the grating and waveguide being the same layer. The inclusion of etch stop layers provided more accurate etch depths; however, the tolerance to changes in the grating duty cycle was much tighter. Results from these devices show the effects of small errors in the pattern fidelity. The fabrication process flows for both iterations of devices will be reviewed as well as the performance of the fabricated devices. A discussion of the relative merits of the various design choices provides insight into the importance of fabrication considerations during the design stage. © 2012 SPIE.

We design and fabricate arrays of diffractive optical elements (DOEs) to realize neutral atom micro-traps for quantum computing. We initialize a single atom at each site of an array of optical tweezer traps for a customized spatial configuration. Each optical trapping volume is tailored to ensure only one or zero trapped atoms. Specifically designed DOEs can define an arbitrary optical trap array for initialization and improve collection efficiency in readout by introducing high-numerical aperture, low-profile optical elements into the vacuum environment. We will discuss design and fabrication details of ultra-fast collection DOEs integrated monolithically and coaxially with tailored DOEs that establish an optical array of micro-traps through far-field propagation. DOEs, as mode converters, modify the lateral field at the front focal plane of an optical assembly and transform it to the desired field pattern at the back focal plane of the optical assembly. We manipulate the light employing coherent or incoherent addition with judicious placement of phase and amplitude at the lens plane. This is realized through a series of patterning, etching, and depositing material on the lens substrate. The trap diameter, when this far-field propagation approach is employed, goes as 2.44λF/#, where the F/# is the focal length divided by the diameter of the lens aperture. The 8-level collection lens elements in this presentation are, to our knowledge, the fastest diffractive elements realized; ranging from F/1 down to F/0.025. © 2012 SPIE.

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We demonstrate the effects of integrating a nanoantenna to a midwave infrared (MWIR) focal plane array (FPA). We model an antenna-coupled photodetector with a nanoantenna fabricated in close proximity to the active material of a photodetector. This proximity allows us to take advantage of the concentrated plasmonic fields of the nanoantenna. The role of the nanoantenna is to convert free-space plane waves into surface plasmons bound to a patterned metal surface. These plasmonic fields are concentrated in a small volume near the metal surface. Field concentration allows for a thinner layer of absorbing material to be used in the photodetector design and promises improvements in cutoff wavelength and dark current (higher operating temperature). While the nanoantenna concept may be applied to any active photodetector material, we chose to integrate the nanoantenna with an InAsSb photodiode. The geometry of the nanoantenna-coupled detector is optimized to give maximal carrier generation in the active region of the photodiode, and fabrication processes must be altered to accommodate the nanoantenna structure. The intensity profiles and the carrier generation rates in the photodetector active layers are determined by finite element method simulations, and iteration between optical nanoantenna simulation and detector modeling is used to optimize the device structure. © 2012 SPIE.

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We describe the design of pixelated filter arrays for hyperspectral monitoring of CO2 and H2O absorption in the midwave infrared (centered at 4.25μm and 5.15μm, respectively) using resonant subwavelength gratings (RSGs), also called guided-mode resonant filters (GMRFs). For each gas, a hyperspectral filter array of very narrowband filters is designed that spans the absorption band on a single substrate. A pixelated geometry allows for direct registration of filter pixels to focal plane array (FPA) sensor pixels and for non-scanning data collection. The design process for narrowband, low-sideband reflective and transmissive filters within fabrication limitations will be discussed.

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