Ricardo P. S. M. Lobo

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.

Microwave and infrared dielectric properties of Sr1−3x/2CexTiO3 (x = 0.154–0.400) incipient ferroelectrics at cryogenic temperatures

Sr1−3x/2CexTiO3 (x = 0.154–0.400) or Sr2+nCe2Ti5+nO15+3n (n ≤ 8) ceramics were prepared by the mixed oxide route. The microwave (MW) dielectric properties of the compounds were investigated in the temperature range from 8 to 295 K. The permittivity increases for decreasing temperatures and saturates below 30 K, following Barretts equation, demonstrating the incipient ferroelectric nature of the investigated materials. The dielectric loss tangent decreases for decreasing temperatures, reaching a minimum at about 80–120 K, and again increases with further cooling due to the rotations of TiO6 octahedra. Infrared-reflectivity data show that the dielectric response of the system is driven by the lowest-frequency polar (soft) mode, particularly at lower temperatures, where the phonons become practically uncoupled. The results help us to understand why Sr1−3x/2CexTiO3 materials present more appropriate dielectric properties for MW tunable applications, compared with pure SrTiO3.

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.

Investigation of coherent emission from the NSLS VUV ring

Bursts of coherent radiation are observed from the NSLS VUV ring near a wavelength of 7 mm. The bursts occur when the electron beam current (I) exceeds a threshold value (I/sub th/), which itself varies with ring operating conditions. Beyond threshold, the average intensity of the emission is found to increase as (I-I/sub th/)/sup 2/. With other parameters held nearly constant, the threshold current value is found to increase quadratically with synchrotron frequency, indicating a linear dependence on momentum compaction. It is believed that the coherent emission is a consequence of micro-bunching of the electron beam due to the microwave instability.

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.