Nathan G. Woodard

Photon Counting Using a Large Area Avalanche Photodiode Cooled to 100 K

We have detected single 650 nm photons with quantum efficiencies greater than 60% using a large area silicon avalanche photodiode. We cool a 5 mm diam commercially available device to 100 K and operate in the gain‐mode. For most applications—from the near‐infrared to the ultraviolet—this device is the most sensitive photon counting detector that has ever been demonstrated. In typical photon counting applications, this detector should prove to be between two and one hundred times more sensitive than the best currently available devices.

Characterization of cooled large-area silicon avalanche photodiodes

We characterize the operation of large-area high-gain silicon avalanche photodiodes (APDs) at near liquid-nitrogen temperatures. The APDs that we studied have active areas of 64 mm2 and have gains of up to 20 000 at 85 K. We characterized the devices for both the usual, analog mode of operation and for doing single-photon pulse counting. The experimental results were found to be reasonably well described by the McIntyre theory. We independently measured k, the hole/electron ionization ratio—a key parameter in the McIntyre theory—and found it to be ∼6×10−4. Cooled, large-area, high-gain APDs compare favorably to photomultiplier tubes in applications that require high sensitivity at near-infrared wavelengths.

Fabrication and characterization of extremely smooth large area gold surfaces

We describe our work on gold surfaces that are made by peeling gold films off various smooth glass substrates. We find that we are able to routinely and reliably produce extremely smooth, large area gold surfaces: typically, for areas ≤40 μm square we find the residual roughness of our samples is an order of magnitude less than that of surfaces produced using conventional techniques. While we intend to use these to measure the van der Waals force between alkali atoms and gold surfaces, other potential uses include samples for fundamental surface physics investigations, scanning tunneling microscopy substrates, and reflective optics. We provide details as to how we make these extremely smooth samples and discuss our activities to characterize them.

Optical measurement of electron bunching in vacuum

We report the homodyne detection of phase modulation sidebands induced on a laser beam by a coherently bunched electron beam. This provides a sensitive and nonperturbing measurement of complex Fourier time series components of the electron density. A proof‐of‐principle measurement of the microwave frequency component of electron density in a crossed‐field device, which agrees well with a calculation of the same quantity, is reported.

Low-field vortex matter in YBa 2 Cu 3 O 7¿d : An atomic beam magnetic-resonance study

We report measurements of the low-field structure of the magnetic vortex lattice in an untwinned YBCO single-crystal platelet. Measurements were carried out using an atomic beam magnetic-resonance (ABMR) technique. For a 10.7 G field applied parallel to the c axis of the sample, we find a triangular lattice with orientational order extending across the entire sample. We find the triangular lattice to be weakly distorted by the a-b anisotropy of the material and measure a distortion factor. f=1.16. Model-experiment comparisons determine a penetration depth, λ a b = 140(′20) nm. The paper includes a detailed description of the ABMR technique. We discuss both technical details of the experiment and modeling used to interpret the measurements.

Vortex lattice autocorrelation function studied by an atomic beam technique

Abstract We have developed an atomic beam technique for studying the Abrikosov vortex lattice. Atoms skimming the surface of a superconductor in the intermediate state experience a temporally fluctuating magnetic field due to the spatial variation of the local magnetic induction. This field may drive transitions in the atoms. By measuring the transition probability as a function of atom velocity, we study the vortex lattice autocorrelation function along the direction of travel. We also determine the penetration depth. We demonstrate the technique by studying the vortex lattice of a niobium film.