Nina R. Weisse-Bernstein

Metasurface Broadband Solar Absorber

We demonstrate a broadband, polarization independent, wide-angle absorber based on a metallic metasurface architecture, which accomplishes greater than 90% absorptance in the visible and near-infrared range of the solar spectrum, and exhibits low absorptivity (emissivity) at mid- and far-infrared wavelengths. The complex unit cell of the metasurface solar absorber consists of eight pairs of gold nano-resonators that are separated from a gold ground plane by a thin silicon dioxide spacer. Our experimental measurements reveal high-performance absorption over a wide range of incidence angles for both s- and p-polarizations. We also investigate numerically the frequency-dependent field and current distributions to elucidate how the absorption occurs within the metasurface structure.

Characterization of the thin-film NbN superconductor for single-photon detection by transport measurements

Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA(Dated: May 21, 2013)The fabrication of high-quality thin superconducting films is essential for single-photon detectors. Theirdevice performance is crucially a ected by their material parameters, thus requiring reliable and nondestructivecharacterization methods after the fabrication and patterning processes. Important material parameters to knoware the resistivity, superconducting transition temperature, relaxation time of quasiparticles, and uniformity ofpatterned wires. In this work, we characterize micro-patterned thin NbN films by using transport measurementsin magnetic fields. We show that from the instability of vortex motion at high currents in the flux-flow state ofthe IV characteristic, the inelastic life time of quasiparticles can be determined to be about 2 ns. Additionally,from the depinning transition of vortices at low currents, as a function of magnetic field, the size distribution ofgrains can be extracted. This size distribution is found to be in agreement with the film morphology obtainedfrom scanning electron microscopy and high-resolution transmission electron microscopy images.

Electrocaloric refrigerator using electrohydrodynamic flows in dielectric fluids

Heat switches are a key enabling element of efficient refrigerators that are based on the electrocaloric effect. We demonstrate a new concept for a heat switch that is based on micro-scale electrohydrodynamic (EHD) flows in thin layers of dielectric fluids. In this device, convective flow of the fluid is controlled by applying an electric field across the fluid layer. This creates a heat switch that can be cycled between a “closed” state with efficient convective heat transport and an “open” state with less efficient conductive heat transport. Substantial switching of the thermal transport coefficient was achieved in 500 μm thick layers of commercial hydrofluoroethers and bias voltages of typically 390 V. The efficacy of the heat switch varied by almost four orders of magnitude for different biasing schemes. The highest efficacy was achieved by biasing a patterned strip electrode and using a planar ground electrode. A preliminary experiment found a thermal conductivity contrast of 4.7±1.1 for the switch in the closed vs. open state. We also characterize the electrocaloric response of commercial multilayer ceramic chip capacitors and show that they can serve as serve as a useful surrogate material for first-generation electrocaloric refrigerators until higher performing multilayer structures of ferroelectric polymers are available.

Multi-layer planar terahertz electric metamaterials on flexible substrates

Planar electric metamaterials fabricated on thin, flexible substrates are studied using terahertz-time domain spectroscopy. Transmission measurements are performed to analyze dielectric properties on single and multiple stacked samples and reveal strong resonances at 1.2 THz.

Sensor and Method Development for Analysis of Alpha- and Beta-Decaying Radioisotopes Embedded InsideMicrocalorimeter Detectors

We discuss sensor and method development for the analysis of alphaand beta-decaying radioisotopes encapsulated within superconducting transition-edge sensor microcalorimeter absorbers. For alpha-decaying isotopes, e.g., 238Pu, 241Am, and 210Po, this is a measurement of the total nuclear reaction energy (Q) and the spectra consist of sharp, narrow peaks. The primary peak is at the Q value, with secondary peaks corresponding to gamma-ray-escape peaks. In contrast, for beta-decaying isotopes, e.g., 241Pu, the spectrum is a low energy continuum ending at E=Q. We are developing transition edge-sensor (TES) microcalorimeters to measure these spectra simultaneously in a single sample, hence allowing quantitative analysis of all Pu isotopes from 238 to 242. We have developed and used TES microcalorimeter detectors for this purpose, and it represents a new quantitative analysis tool for nuclear forensics and safeguards. Due to the high efficiency of the embedded source geometry, measurement times can be minimized. The high dynamic range of our sensors creates the opportunity to measure the relatively low energy beta-decay spectrum of 241Pu (Q = 20.78 keV) simultaneously with the Q ~ 5-6 MeV of alpha-decaying actinides. Finally, the technique could also be effective for determining the time since chemical purification of Pu using the 241Pu/241Am isotopic ratio via simultaneous measurement of the low-energy 241Pu beta particles and the high-energy 241Am Q-value.

Near-infrared characterization of ENABLE grown superconducting nanowire single photon detectors

We characterize the near-infrared response of superconducting nanowire single photon detectors grown with the Energetic Neutral Atom Beam Lithography&Epitaxy technique. These SNSPDs show single photon sensitivity, MHz count rate, low-timing jitter, and detection efficiency ~10-3.

Optical correction using fourier transform heterodyne

In this paper we briefly present the theory of Fourier Transform Heterodyne (FTH), describe past verification experiments carried out, and discuss the experiment designed to use this new imaging technology to perform optical correction. FTH uses the scalar projection of a reference laser beam and a test laser beam onto a single element detector. The complex current in the detector yields the coefficient of the scalar projection. By projecting a complete orthonormal basis set of reference beams onto the test beam, the amplitude and phase of the test beam can be measured, allowing the reconstruction of the phasefront of the image. Experiments to determine this techniques applicability to optical correction and optical self-correction are continuing. Applications of this technique beyond optical correction include adaptive optics; interferometry; and active, high background, low signal imaging.