Subwavelength metal apertures significantly enhance single molecule fluorescence signaling systems, but require
efficient illumination and collection optics. On-chip micromirror structures offer a way to markedly improve the
coupling efficiency between such subwavelength metal apertures and the external fluorescence illumination and
collection optics, which in turn greatly simplifies several aspects of instrument design including optics,
optomechanics, and thermal control. Modeling and experimental verification of the gains in illumination and
collection efficiency for subwavelength metal apertures leads to a micromirror design that is both highly efficient
yet also manufacturable. A combination of ray-based and finite-difference-time-domain models is used to optimize
conical micromirrors colocated with subwavelength metal apertures for the case where the illumination light
interacts strongly with the micromirror and the collection optics have modest numerical aperture (NA~0.5).
Experimental methods employing either freely diffusing or immobilized dye molecules are used to measure the
illumination and collection efficiencies of fabricated micromirror prototypes. An overall fluorescence gain of
~100x, comprising a 20x improvement with flood illumination efficiency together with a 5x improvement in
collection efficiency, are both predicted and experimentally verified.
We demonstrate a compact optical transducer (~50μm) based on a gold film perforated with a square array of square holes. The lattice constant (separation between nearest holes) is chosen to be a ~1μm to detect refractive index change around (n~1.4) with resonant wavelength (λ~1.5μm). Both reflectance measurement and finite difference time domain (FDTD) simulations are performed to evaluate the performance of the sensors. The responsivity of the resonant wavelength is measured to be Δλ/Δn ~835nm RIU-1 (RIU= refractive index unit). The linewidth and contrast of resonance are compared with different size of holes from experimental measurement and FDTD simulations. Coupled mode theory analysis is also used to understand the change reflectance spectrum as a function of hole width.
While much work has focused on simulation and measurement of plasmon resonances in noble metal nanostructures, usually the simulation tool is used as a confirmation of experimental results. In this work we use a finite difference time domain (FDTD) technique to calculate the plasmon resonance and electric field enhancement of Ag nanoparticles in regular arrays on quartz substrates. Such structures have also been prepared by e-beam lithography, and the plasmon resonance and surface-enhanced Raman scattering strength of arrays with different nanoparticle size and spacing have been investigated. Arrays of cylindrical nanoparticles were fabricated with varying particle size and interparticle spacing. The observed extinction peaks agree very well with the extinction peaks as calculated by FDTD; typically within a few percent. Experimental plasmon peak widths are considerably larger than their ideal values due to inhomogeneous broadening. As expected, the particle array with highest SERS enhancement has its plasmon resonance nearest the laser and Stokes-shifted wavelengths. We believe the FDTD modeling tool is accurate enough to use as a predictive tool for engineering plasmonic nanostructures.
Step and Flash Imprint Lithography (SFIL) is an alternative to photolithography that efficiently generates high aspect-ratio, sub-micron patterns in resist materials. Other imprint lithography techniques based on physical deformation of a polymer to generate surface relief structures have produced features in PMMA as small as 10 nm, but it is very difficult to imprint large depressed features or to imprint a thick films of resist with high aspect-ratio features by these techniques. SFIL overcomes these difficulties by exploiting the selectivity and anisotropy of reactive ion etch (RIE). First, a thick organic 'transfer' layer (0.3 micrometer to 1.1 micrometer) is spin coated to planarize the wafer surface. A low viscosity, liquid organosilicon photopolymer precursor is then applied to the substrate and a quartz template applied at 2 psi. Once the master is in contact with the organosilicon solution, a crosslinking photopolymerization is initiated via backside illumination with broadband UV light. When the layer is cured the template is removed. This process relies on being able to imprint the photopolymer while leaving the minimal residual material in the depressed areas. Any excess material is etched away using a CHF3/He/O2 RIE. The exposed transfer layer is then etched with O2 RIE. The silicon incorporated in the photopolymer allows amplification of the low aspect ratio relief structure in the silylated resist into a high aspect ratio feature in the transfer layer. The aspect ratio is limited only by the mechanical stability of the transfer layer material and the O2 RIE selectivity and anisotropy. This method has produced 60 nm features with 6:1 aspect ratios. This lithography process was also used to fabricate alternating arrays of 100 nm Ti lines on a 200 nm pitch that function as efficient micropolarizers. Several types of optical devices including gratings, polarizers, and sub-wavelength structures can be easily patterned by SFIL.
Optical FET detectors were fabricated in both the MOSIS/Vitesse HGaAs3 process and the AT&T field-effect-transistor-self-electro-optic effect device (FET-SEED) process. Typical responsivity is in the order of 1,000 A/W and response time in the order of 10 to 100 microsecond(s) ec at 50 nW optical input power. Such high gain detectors through commercially available industrial foundries are especially useful for optical neural network applications where high density integration requires very good uniformity and power dissipation limits the available optical power. The mechanism of such optical FET detectors are discussed.
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