Anna E. Fox

Holographically formed polymer dispersed liquid crystal films for transmission mode spectrometer applications

We show proof of concept of a transmission-mode wavelength filtering device consisting of layered holographically formed polymer dispersed liquid crystal (H-PDLC) cells. H-PDLC cells were fabricated from a thiolene based polymer composite to have transmission notches in the near-IR wavelength range. Wavelength filtering was achieved by stacking four H-PDLC cells with transmission notches spaced at 10 nm intervals. Results show a broad transmission notch spanning the spectral width of the constituent cells. With bias applied to an individual cell within the stack, the transmission notch of the cell inverts and the overall transmission envelope changes shape. Using a transmitted energy sensing device and a lineshape mapping algorithm, spectral content can be determined to a resolution of 0.1 nm for narrow banded signals. Applications for this switchable wavelength filtering device include serial detection of spectral content for telecom data signals or chemical and biological sample identification through absorption or emission spectroscopy.

Observation of non-exponential absorption of ultra-fast pulses in water.

We observe non-exponential absorption as a function of path length for pulses propagating in pure water. Two types of pulses with differing time duration, bandwidth, and repetition rate were compared with simulated absorption predictions. Deviations from exponential behaviour occurred when the launched pulse had a temporal width of 60fs and a repetition rate of 1 kHz. Under these conditions we observe more than 2 orders of magnitude less absorption after propagation through 6 m of water compared to Beers law prediction. No significant deviation was observed for launched pulses of varying bandwidth with temporal widths of 900fs and repetition rates of 80MHz.

Faster recovery time of a hot-electron transition-edge sensor by use of normal metal heat-sinks

Transition-edge sensor microcalorimeters with recovery times near 1 μs have become highly desirable in quantum science applications as near-infrared single-photon detectors with photon-number resolving capability. Previously, the recovery times of these devices could be decreased only by changing device material or modifying the superconducting-to-normal transition. We demonstrate a method for improving this speed that uses a normal-metal heat-sink. This demonstration with tungsten devices realizes a factor of 4 decrease in recovery time without significantly affecting energy resolution. Our approach may enable the creation of high-efficiency transition-edge sensors with decay times short enough to operate with 80 MHz pulsed single-photon sources.

Liquid crystal polymer composite films for reconfigurable photomasking applications

We show proof of concept of a real-time reconfigurable photomask fabricated from holographically formed polymer-dispersed liquid crystal (H-PDLC) reflection gratings on etched indium tin oxide patterned electrodes. H-PDLC films were formed using a thiolene based polymer to have a reflection wavelength that modulates 440nm, the peak sensitivity wavelength of Shipley 1800 series positive photoresist. A working prototype of this adaptable photomask device is shown by comparing patterns formed using the H-PDLC mask and similar patterns formed using a static contact photomask showing that H-PDLC films have the ability to modulate 440nm light and control the exposure dose of photoresist.

Application of liquid crystal polymer films for photolithographic fabrication of 3D structures

In this paper, we demonstrate a silicon etching application of a holographically formed polymer dispersed liquid crystal (H-PDLC) photomask. H-PDLC is a periodically nanostructured material consisting of stratified layers of polymer and liquid crystal. Due to the natural random alignment of the liquid crystal axes with respect to the polymer layers, an index of refraction mismatch exists and a reflection occurs. Application of bias across the film aligns the liquid crystals and eliminates the index mismatch causing the film to become transparent. H-PDLC films have been shown to sufficiently attenuate the UV exposure dose in the photolithographic process when in the unbiased state, and can be electrically controlled to modulate the amount of UV transmission when electric field is applied. We show etch depth profiles of patterns masked on a silicon substrate using the H-PDLC photomask device compared with etch profiles of similar structures patterned with more conventional ink jet printed photomasks and chrome on quartz glass photomasks. We investigate reactive ion etching technique and potassium hydroxide wet etch technique.

Reconfigurable Lithographic Applications Using Polymer Liquid Crystal Composite Films

The proposed application of holographically formed polymer dispersed liquid crystal (H-PDLC) thin films is a real-time dynamically reconfigurable mask for the resist exposure step in the photolithographic process. H-PDLC films, or thin periodic nanostructures of alternating layers of polymer and liquid crystal have unique electro-optic properties including the ability to modulate a particular wavelength as a function of bias applied to the film. The H-PDLC photomask device consists of patterned electrodes that form pixels with independent bias control over each segment. This is achieved by etching the optically clear yet electrically conductive indium-tin-oxide electrodes on the glass confining the H-PDLC film. This mask has been used to cure Shipley 1800 series positive photoresist at its peak sensitivity wavelength of 440 nm. Structures formed using the H-PDLC photomask device have been compared to similar structures formed with a static photomask using an optical profilometer. Near vertical walls have been achieved using the H-PDLC photomask for structures with line width of 260 μm, and more narrow structures have been fabricated with resolution nearing 100 μm. Line width between structures formed using the H-PDLC mask and static photomask differ by less than 15%. Additionally, morphology studies have been performed on developed regions of glass and resist formed using a static and an H-PDLC mask to demonstrate that no structural defects exist due to formation under an H-PDLC grating.

Gap Material Study for Holographically Formed Polymer Dispersed Liquid Crystal Composite Film Stacks

We present a comparison of materials for adhering layers of holographically formed polymer dispersed liquid crystal (H-PDLC) films layered in stacks fabricated on common substrates. The results of this study are used in applications that rely on maximization of transmitted light through the layers of H-PDLC. Data is presented by comparing a simulated transmission envelope modeled on the individual H-PDLC cells with the actual stacked transmission spectrum. Additionally, data is presented on the quality of the wavefront of H-PDLC transmission spectrum individually and layered using air and index matching layers.

Superconducting Transition-Edge Sensors for Waveguide Coupled Single Photon Detection

We present the design and important preliminary superconducting properties of an evanescently coupled number-resolving single photon detector operating near 1550 nm that is in development for integration into a silicon-on-insulator waveguide based optical system.

Lineshape and morphology study of structures formed using a reconfigurable photomasking element

The proposed application of holographically formed polymer dispersed liquid-crystal (H-PDLC) thin films is a real-time dynamically reconfigurable mask for the resist exposure step in the photolithographic process. An adaptable photomask has advantages over traditional binary masks that include multiple fabrication patterns using a single mask, reduction in realignment error between different masks, real-time correction and adjustment of fabricated structures, and the ability to alter the mask pattern during exposure. H-PDLC films, or thin periodic nanostructures of alternating layers of polymer and liquid crystal, have unique electro-optic properties, including the ability to modulate a particular wavelength as a function of bias applied to the film. Structures formed using the H-PDLC photomask device have been compared to similar structures formed with a static photomask using an optical profilometer. Features studied were decreasing lines with a maximum width of 121 µm to a minimum width of 84 µm. Edge width between structures formed using the H-PDLC mask and static photomask differ by <5%. Additionally, morphology studies have been performed on developed regions of glass and resist formed using a static and an H-PDLC mask to demonstrate that no structural defects exist due to formation under an H-PDLC grating.

Coherent Pulse Propagation in Water

We show how transmission through 5.7 meters of water varies with pulse energy and temporal width. 3 orders increase in transmission is observed compared with Beer’s law predictions. The experimental results are compared with 0π-pulses.