Query: Development of a deep silicon phase Fresnel lens

title: Development of a deep silicon phase Fresnel lens using Gray scale lithography and deep reactive ion etching
abstract: We report the first fabrication and development of a deep phase Fresnel lens (PFL) in silicon through the use of gray scale lithography and deep reactive ion etching (DRIE) A Gaussian tail approximation is introduced as a method of predicting the height of photoresist gray levels given the relative amount of transmitted light through a gray scale optical mask. Device mask design is accomplished through command line scripting in a CAD tool to precisely define the millions of pixels required to generate the appropriate profile in photoresist. Etch selectivity during DRIE pattern transfer is accurately controlled to produce the desired scaling factor between the photoresist and silicon profiles. As a demonstration of this technology, a 1.6 mm diameter PFL is etched 43 /spl mu/m into silicon with each grating profile designed to focus 8.4 keV photons a distance of 118 m.

title: Development of ground testable phase fresnel lenses in silicon
abstract: Diffractive optics, such as Phase Fresnel Lenses (PFL's) offer the potential to achieve excellent imaging performance in the x ray and gamma ray photon regimes. In principle, the angular resolution obtained with these devices can be diffraction limited. Furthermore, improvements in signal sensitivity can be achieved as virtually the entire flux incident on a lens can be concentrated onto a small detector area. In order to verify experimentally the imaging performance, we have fabricated PFL's in silicon using gray scale lithography to produce the required Fresnel profile. These devices are to be evaluated in the recently constructed 600 meter x ray interferometry testbed at NASA/GSFC. Profile measurements of the Fresnel structures in fabricated PFL's have been performed and have been used to obtain initial characterization of the expected PFL imaging efficiencies.

title: Phase Fresnel Lens Development for X ray Gamma ray Astronomy
abstract: Observations in the hard X ray and gammaray energy regimes are constrained by limited sensitivity and angular resolution. Significant gains could be made if these high energy photons could be concentrated from a large area onto a small detector element and if diffraction limited measurements could be obtained. Furthermore, the angular resolution in the gamma ray band would be superior to that possible at all other wavelengths since the diffraction limited angular resolution improves with increasing energy. Phase Fresnel Lenses (PFL's) have high throughput at hard x ray and gamma ray energies, can achieve, in principle, diffraction limited angular resolution, and have the capability of being scaled to large dimensions. We have successfully fabricated PFL's and measured near diffraction limited performance with high efficiency in focusing 8 keV x rays at the GSFC 600 meter Interferometry Testbed. The results demonstrate the superior imaging potential in the x ray/gamma ray energy band for PFLbased optics in a format that is scalable for astronomical applications.

title: Negative refraction makes a perfect lens
abstract: With a conventional lens sharpness of the image is always limited by the wavelength of light. An unconventional alternative to a lens, a slab of negative refractive index material, has the power to focus all Fourier components of a 2D image, even those that do not propagate in a radiative manner. Such "superlenses" can be realized in the microwave band with current technology. Our simulations show that a version of the lens operating at the frequency of visible light can be realized in the form of a thin slab of silver. This optical version resolves objects only a few nanometers across.

title: Phase Fresnel Lens Development for X ray and Gamma ray Astronomy
abstract: In principle, diffractive optics, particularly Phase Fresnel Lenses (PFLs) offer the ability to construct large, diffraction limited, and highly efficient X ray/\gamma$ ray telescopes, leading to dramatic improvement in angular resolution and photon flux sensitivity. As the diffraction limit improves with increasing photon energy, gamma ray astronomy would offer the best angular resolution over the entire electromagnetic spectrum. A major improvement in source sensitivity would be achieved if meter size PFLs can be constructed, as the entire area of these optics focuses photons. We have fabricated small, prototype PFLs using Micro Electro Mechanical Systems (MEMS) fabrication techniques at the University of Maryland and measured near diffraction limited performance with high efficiency using 8 keV and higher energy X rays at the GSFC 600 meter Interferometry Testbed. A first generation, 8 keV PFL has demonstrated imaging corresponding to an angular resolution of approximately 20 milli arcseconds with an efficiency \sim$70$ of the theoretical expectation. The results demonstrate the superior imaging potential in the X ray/\gamma$ ray energy band for PFL based optics in a format that is scalable for astronomical instrumentation. Based upon this PFL development, we have also fabricated a `proof of principle' refractive diffractive achromat and initial measurements have demonstrated nearly uniform imaging performance over a large energy range. These results indicate that the chromaticity inherent in diffractive optics can be alleviated.

title: Ultralight stretched Fresnel lens solar concentrator for space power applications
abstract: A unique ultra light solar concentrator has recently been developed for space power applications. The concentrator comprises a flexible, 140 micron thick, line focus Fresnel lens, made in a continuous process from space qualified transparent silicone rubber material. For deployment and support in space, end arches are used to tension the lens material in a lengthwise fashion, forming a cylindrical stressed membrane structure. The resultant lens provides high optical efficiency, outstanding tolerance for real world errors and aberrations, and excellent focusing performance. The stretched lens is used to collect and focus sunlight at 8X concentration onto high efficiency multi junction photovoltaic cells, which directly convert the incident solar energy to electricity. The Stretched Lens Array (SLA) has been measured at over 27% net solar to electric conversion efficiency for space sunlight, and over 30% net solar to electric conversion efficiency for terrestrial sunlight. More importantly, the SLA provides over 180 W/kg specific power at a greatly reduced cost compared to conventional planar photovoltaic arrays in space. The cost savings are due to the use of 85% less of the expensive solar cell material per unit of power produced. SLA is a direct descendent of the award winning SCARLET array which performed flawlessly on the NASA/JPL Deep Space 1 spacecraft from 1998 2001.

title: Novel Terahertz Antenna Based on a Silicon Lens Fed by a Leaky Wave Enhanced Waveguide
abstract: We introduce a novel antenna concept suitable for future integrated arrays at terahertz frequencies. The antenna consists of an extended hemispherical lens antenna fed by a leaky wave waveguide feed that can be integrated with sensors and detectors such as Schottky diodes. In this antenna architecture, a couple of TE/TM leaky wave modes are excited in a resonant cavity formed by a waveguide opening ground plane and a silicon lens. Due to these modes, the field radiated by the waveguide inside the lens is a very directive pattern that illuminates the upper part of the lens. Having a directive primary field helps to increase the f number of the lens improving several factors as spill over, off axis distortions and coating layer fabrication. The antenna structure is compatible with modern semiconductor fabrication technology and lends itself nicely for large format imaging arrays. In this contribution, we investigate the important parameters of a single antenna such as reflection coefficient, directivity, Gaussicity, phase center and off axis displacement tolerances, and we validate our simulations by measuring the far field radiation patterns of a 545 GHz prototype.

title: Reconfigurable near infrared optical filter with a micromechanical diffractive Fresnel lens
abstract: We have fabricated and characterized a micromechanical Fresnel lens that serves as a reconfigurable filter for near infrared spectrometry, for the analysis of gas, liquids, or solids. The lens consists of silicon segments that can be individually actuated. Moving the segments vertically controls the spectrum of the light that is focused on a detector. The holographic pattern on top of the segments permits both focusing of the light and spectral control. We have demonstrated experimentally the desired spectral control of the filter.

title: Fabrication of silicon kinoform lenses for hard x ray focusing by electron beam lithography and deep reactive ion etching
abstract: The focusing of subnanometer wavelength x rays is limited by the ability to fabricate high quality optics. In general, the resolution is of the order of the smallest feature of the optic, so nanometer spot sizes are extremely difficult to achieve with lenses made by traditional fabrication methods. In addition, gains in resolution for a given lithography limit are often made at a sacrifice of focusing efficiency. Kinoform lenses offer a compromise position of high resolution and efficiency. The object of this work is to describe the fabrication of kinoform lenses and to show how their unique properties could provide a path toward nanometer scale focusing. Fabrication is made easier by using higher order focusing and larger features. By combining 100keV electron beam lithography and deep reactive ion etching, the authors have fabricated cylindrical kinoform lenses in silicon. These lenses can be used in a crossed pair to produce a two dimensional focus, but to maintain a large aperture and high resolution

title: Achromatic Fresnel optics for wideband extreme ultraviolet and X ray imaging
abstract: Advances in extreme ultraviolet (EUV) and X ray optics are providing powerful new capabilities in high resolution imaging and trace element analysis of microscopic specimens, and the potential for fabricating devices of smaller critical dimensions in next generation integrated circuit lithography. However, achieving the highest resolution with such optics usually requires the illuminating EUV or X ray beam to be highly monochromatic. It would therefore be highly desirable to have large field of view, sub 100 nm resolution optics that are achromatic to a significant degree, allowing more light to be utilized from broader bandwidth sources such as laser produced plasmas. Here we report an achromatic Fresnel optical system for EUV or X ray radiation that combines a Fresnel zone plate with a refractive lens with opposite chromatic aberration. We use the large anomalous dispersion property of the refractive lens material near an absorption edge to make its fabrication practical. The resulting structure can deliver a resolution comparable to that of the Fresnel zone plates that have achieved the highest resolution (25 nm; ref. 3) in the entire electromagnetic spectrum, but with an improvement of two or more orders of magnitude in spectral bandwidth.