Peter B. Catrysse

Geometries and materials for subwavelength surface plasmon modes

Plasmonic waveguides can guide light along metal-dielectric interfaces with propagating wave vectors of greater magnitude than are available in free space and hence with propagating wavelengths shorter than those in vacuum. This is a necessary, rather than sufficient, condition for subwavelength confinement of the optical mode. By use of the reflection pole method, the two-dimensional modal solutions for single planar waveguides as well as adjacent waveguide systems are solved. We demonstrate that, to achieve subwavelength pitches, a metal-insulator-metal geometry is required with higher confinement factors and smaller spatial extent than conventional insulator-metal-insulator structures. The resulting trade-off between propagation and confinement for surface plasmons is discussed, and optimization by materials selection is described.

Planar lenses based on nanoscale slit arrays in a metallic film.

We experimentally demonstrated planar lenses based on nanoscale slit arrays in a metallic film. Electromagnetic simulations of lens designs and confocal measurements on manufactured structures show excellent agreement, but deviate from simple theory.

Nanopatterned Metallic Films for Use As Transparent Conductive Electrodes in Optoelectronic Devices

We investigate the use of nanopatterned metallic films as transparent conductive electrodes in optoelectronic devices. We find that the physics of nanopatterned electrodes, which are often optically thin metallic films, differs from that of optically thick metallic films. We analyze the optical properties when performing a geometrical transformation that maintains the electrical properties. For one-dimensional patterns of metallic wires, the analysis favors tall and narrow wires. Our design principles remain valid for oblique incidence and readily carry over to two-dimensional patterns.

Curving monolithic silicon for nonplanar focal plane array applications

Despite progress in the performance of image sensors, comparatively little work has focused on overcoming the limitations of planar image sensor arrays. We present a technique to construct curved monolithic silicon structures that can be processed using standard silicon processing prior to curving. The process relies on microstructuring of a monolithic silicon die using a deep reactive ion etch process. This technique can be used to build curved integrated circuits such as image sensors for more compact cameras with improved optical performance.

Integrated color pixels in 0.18-µm complementary metal oxide semiconductor technology

Following the trend of increased integration in complementary metal oxide semiconductor (CMOS) image sensors, we have explored the potential of implementing light filters by using patterned metal layers placed on top of each pixels photodetector. To demonstrate wavelength selectivity, we designed and prototyped integrated color pixels in a standard 0.18-microm CMOS technology. Transmittance of several one-dimensional (1D) and two-dimensional (2D) patterned metal layers was measured under various illumination conditions and found to exhibit wavelength selectivity in the visible range. We performed (a) wave optics simulations to predict the spectral responsivity of an uncovered reference pixel and (b) numerical electromagnetic simulations with a 2D finite-difference time-domain method to predict transmittances through 1D patterned metal layers. We found good agreement in both cases. Finally, we used simulations to predict the transmittance for more elaborate designs.

The optical advantages of curved focal plane arrays.

The design of optical systems for digital cameras is complicated by the requirement that the image surface be planar, which results in complex and expensive optics. We analyze a compact optical system with a curved image surface and compare its performance to systems with planarimage surfaces via optics analysis and image system simulation. Our analysis shows that a curved image surface provides a way to lower the number of optical elements, reduce aberrations including astigmatism and coma, and increase off-axis brightness and sharpness. A method to fabricate curved image focal plane arrays using monolithic silicon is demonstrated.

How small should pixel size be

Pixel design is a key part of image sensor design. After deciding on pixel architecture, a fundamental tradeoff is made to select pixel size. A small pixel size is desirable because it results in a smaller die size and/or higher spatial resolution; a large pixel size is desirable because it results in higher dynamic range and signal-to-noise ratio. Given these two ways to improve image quality and given a set of process and imaging constraints an optimal pixel size exists. It is difficult, however, to analytically determine the optimal pixel size, because the choice depends on many factors, including the sensor parameters, imaging optics and the human perception of image quality. This paper describes a methodology, using a camera simulator and image quality metrics, for determining the optimal pixel size. The methodology is demonstrated for APS implemented in CMOS processes down to 0.18 (mu) technology. For a typical 0.35 (mu) CMOS technology the optimal pixel size is found to be approximately 6.5 micrometers at fill factor of 30%. It is shown that the optimal pixel size scales with technology, btu at slower rate than the technology itself.

Planar metallic nanoscale slit lenses for angle compensation

We demonstrate numerically, using a modified total-field/scattered-field formalism, that metallic lenses, based on arrays of nanoscale slits with varying widths in a planar metallic film, can be used to focus light and compensate for various angles of incidence. These structures could be used as integrated microlenses to improve the efficiency of pixels in solid-state image sensors. Our design guidelines simultaneously accomplish a prism and focusing action. Our results also indicate the importance of the aperture effect for such far-field focusing devices.

Radiative human body cooling by nanoporous polyethylene textile.

Thermal management through personal heating and cooling is a strategy by which to expand indoor temperature setpoint range for large energy saving. We show that nanoporous polyethylene (nanoPE) is transparent to mid-infrared human body radiation but opaque to visible light because of the pore size distribution (50 to 1000 nanometers). We processed the material to develop a textile that promotes effective radiative cooling while still having sufficient air permeability, water-wicking rate, and mechanical strength for wearability. We developed a device to simulate skin temperature that shows temperatures 2.7° and 2.0°C lower when covered with nanoPE cloth and with processed nanoPE cloth, respectively, than when covered with cotton. Our processed nanoPE is an effective and scalable textile for personal thermal management.

Understanding the dispersion of coaxial plasmonic structures through a connection with the planar metal-insulator-metal geometry

We elucidate the dispersion behavior of deep-subwavelength propagating modes in coaxial plasmonic structures by making an explicit connection with the planar metal-insulator-metal geometry. We provide an intuitive picture that allows for a qualitative understanding and a quantitative prediction of the entire dispersion behavior, which includes the number of modes at every frequency, the modal propagation constants, the propagation losses, and the cutoff frequencies of propagating modes supported by these technologically important structures. We validate our analytical approach by comparing its predictions to first-principles finite-difference frequency-domain simulations.