Joseph Donald Laveigne

Experimental Constraints on the Axion Dark Matter Halo Density

Most of the mass of the Milky Way is contributed by its halo, presumably in the form of noninteracting cold dark matter. The axion is a compelling cold dark matter candidate. We report results from a search that probes the local Galactic halo axion density using the Sikivie radio frequency cavity technique. Candidates over the frequency range 550 MH MHz (2.3 me meV) were investigated. The absence of a signal z ≤ f ≤ 810 V ≤ m ≤ 3.4 a suggests that the axions of Kim and Shifman, Vainshtein, & Zakharov contribute no more than 0.45 GeV cm 3 of mass density to the local dark matter halo over this mass range. Subject headings: dark matter — Galaxy: halo — instrumentation: detectors

Subnanosecond, time-resolved, broadband infrared spectroscopy using synchrotron radiation

A facility for subnanosecond time-resolved (pump-probe) infrared spectroscopy has been developed at the National Synchrotron Light Source of Brookhaven National Laboratory. A mode-locked Ti:sapphire laser produces 2 ps duration, tunable near-infrared pump pulses synchronized to probe pulses from a synchrotron storage ring. The facility is unique on account of the broadband infrared from the synchrotron, which allows the entire spectral range from 2 cm−1 (0.25 meV) to 20 000 cm−1 (2.5 eV) to be probed. A temporal resolution of 100 ps, limited by the infrared synchrotron-pulse duration (full width at half maximum), is achievable. A maximum time delay of 170 ns is available without gating the infrared detector. To illustrate the performance of the facility, a measurement of electron–hole recombination dynamics for an HgCdTe semiconductor film in the far- and mid-infrared range is presented.

Silicon beam splitter for far-infrared and terahertz spectroscopy

Silicon beam splitters several millimeters thick offer numerous advantages over thin freestanding dielectric beam splitters. For routine spectroscopy for which resolutions of better than 1 cm(-1) are not required, a silicon beam splitter can replace several Mylar beam splitters to span the entire far-infrared region. In addition to superior long-wavelength performance that extends well into the terahertz region, the silicon beam splitter has the additional advantage that its efficiency displays little polarization dependence.

Performance of new infrared beamline U12IR at the National Synchrotron Light Source

The instrumentation and performance of the new infrared beamline U12IR at the National Synchrotron Light Source of Brookhaven National Laboratory is described. This beamline utilizes infrared synchrotron radiation from a bending magnet. A combination of beamline design features and spectroscopic instrumentation allows the facility to reach the extremely low frequency limit of ∼2 cm−1 (i.e., 60 GHz or a photon energy of 250 μeV). The infrared light from the synchrotron emission at U12IR is compared to standard thermal sources and reveals substantial benefits for the study of small samples. In particular, the intensity of the synchrotron radiation in the far infrared can be as much as 200 times greater than that from a blackbody when millimeter-sized samples are measured. The effects of diffraction and noise on beamline performance are also discussed.

Photoinduced time-resolved electrodynamics of superconducting metals and alloys

The photoexcited state in superconducting metals and alloys was studied via pump-probe spectroscopy. A pulsed Ti:sapphire laser was used to create the nonequilibrium state and the far-infrared pulses of a synchrotron storage ring, to which the laser is synchronized, measured the changes in the material optical properties. Both the time- and frequency-dependent photoinduced spectra of Pb, Nb, NbN, Nb0.5Ti0.5N, and Pb0.75Bi0.25 superconducting thin films were measured in the low-fluence regime. The time-dependent data establish the regions where the relaxation rate is dominated either by the phonon escape time phonon bottleneck effect or by the intrinsic quasiparticle recombination time. The photoinduced spectra measure directly the reduction of the superconducting gap due to an excess number of quasiparticles created by the short laser pulses. This gap shift allows us to establish the temperature range over which the low fluence approximation is valid.

Time-resolved spectroscopy using synchrotron infrared pulses

Electron synchrotron storage rings, such as the VUV ring at the National Synchrotron Light Source, product short pulses of IR radiation suitable for investigating time-dependent phenomena in a variety of interesting experimental systems. In contrast to other pulsed sources of IR, the synchrotron produces a continuum spectral output over the entire IR (and beyond), though at power levels typically below those obtained from laser systems. The infrared synchrotron radiation source is therefore well-suited as a probe using standard FTIR spectroscopic techniques. Here we describe the pump-probe spectroscopy facility being established at the NSLS and demonstrate the technique by measuring the photocarrier decay in a semiconductor.

Time-resolved infrared spectroscopy on the U12IR beamline at the NSLS

A facility for performing time-resolved infrared spectroscopy has been developed at the NSLS, primarily at beamline U12IR. The pulsed IR light from the synchrotron is used to perform pump-probe spectroscopy. We present here a description of the facility and results for the relaxation of photoexcitations in both a semiconductor and superconductor.

Evidence for millimeter-wave coherent emission from the NSLS VUV ring

Coherent synchrotron radiation from the NSLS VUV ring has been detected and partially characterized. The observations have been performed at the new far infrared beamline U12IR. The coherent radiation is peaked near a wavelength of 7 mm and occurs in short duration bursts. The bursts occur only when the electron beam current (I) exceeds a threshold value (Ith), which itself varies with ring operating conditions. Beyond threshold, the average intensity of the emission is found to increase as (I-Ith)2. The coherent emission implies micro-bunching of the electron beam due to a longitudinal instability.