Pierre M. Echternach

Plasmonic nanoparticle arrays with nanometer separation for high-performance SERS substrates.

We demonstrate a method for fabricating arrays of plasmonic nanoparticles with separations on the order of 1 nm using an angle evaporation technique. Samples fabricated on thin SiN membranes are imaged with high-resolution transmission electron microscopy (HRTEM) to resolve the small separations achieved between nanoparticles. When irradiated with laser light, these nearly touching metal nanoparticles produce extremely high electric field intensities, which result in surface-enhanced Raman spectroscopy (SERS) signals. We quantify these enhancements by depositing a p-aminothiophenol dye molecule on the nanoparticle arrays and spatially mapping their Raman intensities using confocal micro-Raman spectroscopy. Our results show significant enhancement when the incident laser is polarized parallel to the axis of the nanoparticle pairs, whereas no enhancement is observed for the perpendicular polarization. These results demonstrate proof-of-principle of this fabrication technique. Finite difference time domain simulations based on HRTEM images predict an electric field intensity enhancement of 82400 at the center of the nanoparticle pair and an electromagnetic SERS enhancement factor of 10(9)-10(10).

Demonstration of high contrast in 10% broadband light with the shaped pupil coronagraph

The Shaped Pupil Coronagraph (SPC) is a high-contrast imaging system pioneered at Princeton for detection of extra-solar earthlike planets. It is designed to achieve 10-10 contrast at an inner working angle of 4λ/D in broadband light. A critical requirement in attaining this contrast level in practice is the ability to control wavefront phase and amplitude aberrations to at least λ/104 in rms phase and 1/1000 rms amplitude, respectively. Furthermore, this has to be maintained over a large spectral band. The High Contrast Imaging Testbed (HCIT) at the Jet Propulsion Lab (JPL) is a state-of-the-art facility for studying such high contrast imaging systems and wavefront control methods. It consists of a vacuum chamber containing a configurable coronagraph setup with a Xinetics deformable mirror. Previously, we demonstrated 4x10-8 contrast with the SPC at HCIT in 10% broadband light. The limiting factors were subsequently identified as (1) manufacturing defects due to minimal feature size constraints on our shaped pupil masks and (2) the inefficiency of the wavefront correction algorithm we used (classical speckle nulling) to correct for these defects. In this paper, we demonstrate the solutions to both of these problems. In particular, we present a method to design masks with practical minimal feature sizes and show new manufactured masks with few defects. These masks were installed at HCIT and tested using more sophisticated wavefront control algorithms based on energy minimization of light in the dark zone. We present the results of these experiments, notably a record 2.4×10-9 contrast in 10% broadband light.

Electron-beam lithography for micro and nano-optical applications

Direct-write electron-beam lithography has proven to be a powerful technique for fabricating a variety of micro and nano-optical devices. Binary E-beam lithography is the workhorse technique for fabricating optical devices that require complicated precision nano-scale features. We describe a bi-layer resist system and virtual-mark height measurement for improving the reliability of fabricating binary patterns. Analog E-beam lithography is a newer technique that has found significant application in the fabrication of diffractive optical elements. We describe our techniques for fabricating analog surface-relief profiles in E-beam resist, including a technique for overcoming the problem of resist heating. We also describe a multiple field size exposure scheme for suppression of diffraction orders produced field-stitch errors in blazed diffraction gratings on non-flat substrates.

Characterizing SixNy absorbers and support beams for far-infrared/submillimeter transition-edge sensors

We report on the characterization of SixNy (Si-N) optical absorbers and support beams for transition-edge sensors (TESs). The absorbers and support beams measured are suitable to meet ultra-sensitive noise equivalent power (NEP≤10-19W/√Hz) and effective response time (τ) requirements (τ≤100ms) for space-borne far-infrared( IR)/submillimeter(sub-mm) spectrometers, such as the Background Limited far-Infrared/Sub-mm Spectrograph (BLISS) and the SpicA FAR-infrared Instrument (SAFARI) for the SPace Infrared telescope for Cosmology and Astrophysics (SPICA). The thermal response time (τ0) of an absorber suspended by support beams from a lowtemperature substrate depends on the heat capacity (C) of the absorber and the thermal conductance (G) of the support beams (τ0=C/G). In membrane-isolated TESs for BLISS, the effective response time τ is expected to be a factor of 20 smaller than τ0 because of voltage-biased electrothermal feedback operation and assumption of a reasonable open-loop gain, LI≈20. We present design specifications for the arrays of membrane-isolated ultra-sensitive TESs for BLISS. Additionally, we measured G and τ0 for two Si-N noise thermometry device (NTD) architectures made using different fabrication processes: (1) a solid membrane Si-N absorber suspended by thin and long Si-N support beams and (2) a wire-mesh Si-N absorber suspended by long, and even thinner, Si-N support beams. The measurements of G and τ0 were designed to test suitability of the Si-N thermal performance to meet the demands of the two SPICA instruments. The solid membrane NTD architecture is similar to the TES architecture for SAFARI and the mesh membrane NTD is similar to that of BLISS TESs. We report measured values of G and C for several BLISS and SAFARI NTD devices. We observe that the heat capacity of the solid membrane devices can be reduced to the order of 1fJ/K at 65mK for devices that are wet etched by KOH. However, C for these devices is found to be on the order of 100fJ/K for a dry XeF2 process. The heat capacity is similarly large for the mesh devices produced with a dry XeF2 etch.Next generation cosmic microwave background (CMB) polarization anisotropy measurements will feature focal plane arrays with more than 600 millimeter-wave detectors. We make use of high-resolution photolithography and wafer-scale etch tools to build planar arrays of corrugated platelet feeds in silicon with highly symmetric beams, low cross-polarization and low side lobes. A compact Au-plated corrugated Si feed designed for 150 GHz operation exhibited performance equivalent to that of electroformed feeds: ∼ −0.2 dB insertion loss, < −20 dB return loss from 120 GHz to 170 GHz, < −25 dB side lobes and < −23 dB cross-polarization. We are currently fabricating a 50mm diameter array with 84 horns consisting of 33 Si platelets as a prototype for the SPTpol and ACTpol telescopes. Our fabrication facilities permit arrays up to 150mm in diameter.

Fabrication and characteristics of free-standing shaped pupil masks for TPF-coronagraph

Direct imaging and characterization of exo-solar terrestrial planets require coronagraphic instruments capable of suppressing star light to 10-10. Pupil shaping masks have been proposed and designed1 at Princeton University to accomplish such a goal. Based on Princeton designs, free standing (without a substrate) silicon masks have been fabricated with lithographic and deep etching techniques. In this paper, we discuss the fabrication of such masks and present their physical and optical characteristics in relevance to their performance over the visible to near IR bandwidth.

The JWST/NIRCam Coronagraph: Mask Design and Fabrication

The NIRCam instrument on the James Webb Space Telescope will provide coronagraphic imaging from λ =1-5 μm of high contrast sources such as extrasolar planets and circumstellar disks. A Lyot coronagraph with a variety of circular and wedge-shaped occulting masks and matching Lyot pupil stops will be implemented. The occulters approximate grayscale transmission profiles using halftone binary patterns comprising wavelength-sized metal dots on anti-reflection coated sapphire substrates. The mask patterns are being created in the Micro Devices Laboratory at the Jet Propulsion Laboratory using electron beam lithography. Samples of these occulters have been successfully evaluated in a coronagraphic testbed. In a separate process, the complex apertures that form the Lyot stops will be deposited onto optical wedges. The NIRCam coronagraph flight components are expected to be completed this year.

Broadband suppression and occulter position sensing at the Princeton occulter testbed

The Princeton occulter testbed uses long-distance propagation with a diverging beam and an optimized occulter mask to simulate the performance of external occulters for finding extrasolar planets. We present new results from the testbed in both monochromatic and broadband light. In addition, we examine sensing and control of occulter position using out-of-band spectral leak around the occulter and occulter position tolerancing. These results are validated by numerical simulations of propagation through the system.

Progress on the occulter experiment at Princeton

An occulter is used in conjunction with a separate telescope to suppress the light of a distant star. To demonstrate the performance of this system, we are building an occulter experiment in the laboratory at Princeton. This experiment will use an etched silicon mask as the occulter, with some modifications to try to improve the performance. The occulter is illuminated by a diverging laser beam to reduce the aberrations from the optics before the occulter. We present the progress of this experiment and expectations for future work.

The background-limited infrared-submillimeter spectrograph (BLISS) for SPICA: a design study

We are developing the Background-Limited Infrared-Submillimeter Spectrograph (BLISS) for SPICA to provide a breakthrough capability for far-IR survey spectroscopy. SPICAs large cold aperture allows mid-IR to submm observations which are limited only by the natural backgrounds, and BLISS is designed to operate near this fundamental limit. BLISS-SPICA is 6 orders of magnitude faster than the spectrometers on Herschel and SOFIA in obtaining full-band spectra. It enables spectroscopy of dust-obscured galaxies at all epochs back to the rst billion years after the Big Bang (redshift 6), and study of all stages of planet formation in circumstellar disks. BLISS covers 35 - 433 microns range in ve or six wavelength bands, and couples two 2 sky positions simultaneously. The instrument is cooled to 50 mK for optimal sensitivity with an on-board refrigerators. The detector package is 4224 silicon-nitride micro-mesh leg-isolated bolometers with superconducting transition-edge-sensed (TES) thermistors, read out with a cryogenic time-domain multiplexer. All technical elements of BLISS have heritage in mature scientic instruments, and many have own. We report on our design study in which we are optimizing performance while accommodating SPICAs constraints, including the stringent cryogenic mass budget. In particular, we present our progress in the optical design and waveguide spectrometer prototyping. A companion paper in Conference 7741 (Beyer et al.) discusses in greater detail the progress in the BLISS TES bolometer development.

The JWST/NIRCam Coronagraph Flight Occulters

The NIRCam instrument on the James Webb Space Telescope will have a Lyot coronagraph for high contrast imaging of extrasolar planets and circumstellar disks at λ=2 - 5 μm. Half-tone patterns are used to create graded-transmission image plane masks. These are generated using electron beam lithography and reactive ion etching of a metal layer on an antireflection coated sapphire substrate. We report here on the manufacture and evaluation of the flight occulters.