William C. Sweatt

Optimization of retardance for a complete Stokes polarimeter

We present two figures of merit based on singular value decomposition, which can be used to assess the noise immunity of a complete Stokes polarimeter. These are used to optimize a polarimeter featuring a rotatable retarder and a fixed polarizer. A retardance of 132 degrees (approximately three-eighths wave) and retarder orientation angles of +/-51.7 degrees and +/-15.1 degrees are found to be optimal when four measurements are used. Use of this retardance affords a factor-of-1.5 improvement in signal-to-noise ratio over systems employing a quarter-wave plate. A geometric means of visualizing the optimization process is discussed, and the advantages of the use of additional measurements are investigated. No advantage of using retarder orientation angles spaced uniformly through 360 degrees is found over repeated measurements made at the four retarder orientation angles.

Figures of merit for complete Stokes polarimeter optimization

Figures of merit for optimization of a complete Stokes polarimeter based on its measurement matrix are described which are not limited in their application to cases in which four measurements are used in the determination of a single Stokes vector. Singular value decomposition and probability theory are used to investigate the behavior and significance of these figures of merit. Their use to optimize a system consisting of a rotatable retarder and fixed polarizer indicates that a retardance of 132° (approximately three-eighths wave) and retarder orientation angles of ±51.7° and ±15.1° are favorable when four measurements are used. The performance of this system is demonstrated with experimental data.

Scale-up of a cluster jet laser plasma source for extreme ultraviolet lithography

A high-average-power extreme UV (EUV) source based on a laser plasma cluster jet is being developed for EUV lithography. The source employs a cooled supersonic nozzle expansion to produce a dense beam of Xe clusters as the plasma source target. The cluster beam is irradiated with a pulsed laser to create a high-temperature plasma radiating efficiently in the EUV spectral region. To accommodate drive laser repetition rates of up to 6000 Hz, a continuous jet expansion with full Xe gas recycling is employed, rather than earlier pulsed jet expansions. The continuous jet employs an efficient high-throughput pumping scheme to minimize the ambient pressure highly-attenuating Xe gas. Source power scale-up is achieved by increasing laser repetition rate, keeping laser pulse parameters nominally fixed. In the first phase of EUV power scale-up, the continuous cluster jet source has been integrated with a 200 W laser driver operating at repetition rates up to 500 Hz. With this system, a laser-to-EUV conversion efficiency of 0.69 percent is achieved. In the second phase, the jet is being integrated with a 1700 W diode-pumped solid sate laser driver operating at repetition rates up to 6000 Hz. A brief description of the 1700 W laser system and its integration with the continuous cluster jet are discussed.

Optimal cell connections for improved shading, reliability, and spectral performance of microsystem enabled photovoltaic (MEPV) modules

Microsystems enabled photovoltaics (MEPV) is a recently developed concept that promises benefits in efficiency, functionality, and cost compared to traditional PV approaches. MEPV modules consist of heterogeneously integrated arrays of ultra-thin (∼2 to 20 µm), small (∼100 µm to a few millimeters laterally) cells with either one-sun or micro-optics concentration configurations, flexible electrical configurations of individual cells, and potential integration with electronic circuits. Cells may be heterogeneously stacked and separated by dielectric layers to realize multi-junction designs without the constraints of lattice matching or series connections between different cell types. With cell lateral dimensions of a few millimeters or less, a module has tens to hundreds of thousands of cells, in contrast to todays PV modules with less than 100. Hence, MEPV modules can operate at high voltages without module DC to DC converters, reducing resistive losses, improving shading performance, and improving robustness to individual cell failures.

Leveraging scale effects to create next-generation photovoltaic systems through micro- and nanotechnologies

Current solar power systems using crystalline silicon wafers, thin film semiconductors (i.e., CdTe, amorphous Si, CIGS, etc.), or concentrated photovoltaics have yet to achieve the cost reductions needed to make solar power competitive with current grid power costs. To overcome this cost challenge, we are pursuing a new approach to solar power that utilizes micro-scale solar cells (5 to 20 μm thick and 100 to 500 μm across). These micro-scale PV cells allow beneficial scaling effects that are manifested at the cell, module, and system level. Examples of these benefits include improved cell performance, better thermal management, new module form-factors, improved robustness to partial shading, and many others. To create micro-scale PV cells we are using technologies from the MEMS, IC, LED, and other micro and nanosystem industries. To date, we have demonstrated fully back-contacted crystalline silicon (c-Si), GaAs, and InGaP microscale solar cells. We have demonstrated these cells individually (c-Si, GaAs), in dual junction arrangements (GaAs, InGaP), and in a triple junction cell (c-Si, GaAs, InGaP) using 3D integration techniques. We anticipate two key systems resulting from this work. The first system is a high-efficiency, flexible PV module that can achieve greater than 20% conversion efficiency and bend radii of a few millimeters (both parameters greatly exceeding what currently available flexible PV can achieve). The second system is a utility/commercial scale PV system that cost models indicate should be able to achieve energy costs of less than

Fabrication of high performance microlenses for an integrated capillary channel electrochromatograph with fluorescence detection

0.10/kWh in most locations.

Micro-optics for high-efficiency optical performance and simplified tracking for concentrated photovoltaics (CPV)

We describe the microfabrication of an extremely compact optical system as a key element in an integrated capillary channel electrochromatograph with fluorescence detection. The optical system consists of a vertical cavity surface-emitting laser (VCSEL), two high performance microlenses and a commercial photodetector. The microlenses are multilevel diffractive optics patterned by electron beam lithography and etched by reactive ion etching in fused silica. The design uses substrate-mode propagation within the fused silica substrate. Two generations of optical subsystems are described. The first generation design has a 6 mm optical length and is integrated directly onto the capillary channel-containing substrate. The second generation design separates the optical system onto its own substrate module and the optical path length is further compressed to 3.5 mm. The first generation design has been tested using direct fluorescence detection with a 750 nm VCSEL pumping a 10{sup {minus}4}M solution of CY-7 dye. The observed signal-to-noise ratio of better than 100:1 demonstrates that the background signal from scattered pump light is low despite the compact size of the optical system and is adequate for system sensitivity requirements.

Soft-x-ray projection lithography experiments using Schwarzschild imaging optics

Micro-optical 5mm lenses in 50mm sub-arrays illuminate arrays of photovoltaic cells with 49X concentration. Fine tracking over ±10° FOV in sub-array allows coarse tracking by meter-sized solar panels. Plastic prototype demonstrated for 400nm

Diamond milling of micro-optics

Soft-x-ray projection imaging is demonstrated by the use of 14-nm radiation from a laser plasma source and a single-surface multilayer-coated ellipsoidal condenser. Aberrations in the condenser and the Schwarzschild imaging objective are characterized and correlated with imaging performance. A new Schwarzschild housing, designed for improved alignment stability, is described.

Resonator Design With An Intracavity Time-Varying Index Gradient

A diamond mill (a tiny end mill) can cut aspheric lenses and mirrors with diameters smaller than 0.5 mm. The cutting tool has a two-dimensional shape and is spun about the axis of the surface to be cut. As the spinning tool is plunged into the substrate, it cuts a radially symmetric surface to sub-micron accuracies. Commercially available circular diamond tools can be modified to aspheric shapes using a focused ion beam. Fabrication examples are presented and the optical performance of an array of micro-lenses are described