Armen M. Gulian

Thermoelectric cooling at cryogenic temperatures

Experimental results demonstrating Peltier cooling below 10 K are reported, using crystals of the thermoelectric cerium hexaboride (CeB6). Direct measurements of the Peltier cooling showed δT up to ∼0.2 K in magnitude at T∼4–5 K. All three kinetic parameters: resistivity (ρ), heat conductivity (k), and Seebeck coefficient (S), characterizing the thermoelectric figure of merit ZT=S2T/ρk, were measured, giving high-confidence results.

Stabilizing slow light delay in stimulated Brillouin scattering using a Faraday rotator mirror

The delay of the Stokes beam in stimulated Brillouin scattering (SBS) is sensitive to the polarization alignment between the pump and the Stokes. Typically, waveguides exhibit a birefringence that causes beams launched from opposite ends to have fluctuating and suboptimal alignment. We demonstrate in an optical fiber that using a Faraday rotator mirror not only overcomes this problem, but also enhances SBS slow light delay.

X-ray/UV single photon detectors with isotropic Seebeck sensors

Abstract A novel hot-electron microbolometer concept suitable for hyperspectral imaging at high photon counting rates is described. When each photon is absorbed, its energy is thermalized and homogenized within the normal-metal absorber to provide a highly reproducible temperature gradient across a thin film thermoelectric sensor which separates the absorber from a heat sink. The classical Seebeck effect results in an observable signal voltage, proportional to the time-dependent temperature gradient, without any externally applied bias current or voltage. Highly preliminary results on a single pixel device are presented and the projected performance of the fully developed device is analyzed, emphasizing the spectral resolution, noise sources, and suitability for large arrays of detector pixels.

Imaging detectors based on anisotropic thermoelectricity

Abstract Transient voltage pulses spontaneously occur in anisotropic films following the absorption of photons. Experiments on normal state YBCO unambiguously confirm that the pulses sensed between the sample ends arise from the thermoelectric response to the heat flux propagating through the film, perpendicular to the voltage measurement axis. Good agreement is found between the measured voltage characteristics and a macroscopic model based on the Seebeck tensor of the material. The applicability of such materials to single-photon detectors is discussed. A class of materials with sufficiently large low-temperature Seebeck tensor to construct a high resolution detector has been identified but not yet optimized for this use.

Three simple methods to obtain large area thin films by pulsed laser deposition

Three new relatively simple methods of laser deposition of large area thin films are proposed. The thickness uniformity of films obtained with these methods was preserved within the limits of /spl plusmn/3.5% on 100 mm diameter substrates. The first method employs controlled tilting of the target around the axis parallel with the substrate plane, with the respective positions of the laser beam, focal spot, and substrate being kept constant. The peculiarity of the second and third methods is laser deposition of the compound upon a substrate through a mask. The possibilities of different configurations of the slit in the mask are considered. One method uses a mask with a slit in the form of a sector, which is symmetrical with regard to the substrate radius, with various angular dimensions at different distances from the rotation center of the substrate. Another method uses a mask with two slits in the form of a bent sector, the symmetry axis of which coincides with the line of equal velocity of mass transfer of the deposited compound onto the substrate.

True-time delay steering of phased array radars using slow light

Application of slow light linear delay to squint-free (true-time delay) steering of phased array radar antennae is discussed. Theoretical analysis is provided on true-time delay radar requirements, including delay precision, amplitude precision, and bandwidth. We also discuss an improvement to the slow light technique based on stimulated Brillouin scattering by using a Faraday rotator mirror that provides temporally stable, linear (with pump power) delay, applicable to practical implementations. Future directions are considered.

Triplet superconductors from the viewpoint of basic elements for quantum computers

We discuss possibilities of utilizing superconductors with Cooper condensates in triplet pairing states (where the spin of condensate pairs is S=1) for practical realization of quantum computers. Superconductors with triplet pairing condensates have features that are unique and cannot be found in the usual (singlet pairing, S=0) superconductors. The symmetry of the order parameter in some triplet superconductors (e.g., ruthenates) corresponds to doubly-degenerate chiral states. These states can serve as qubit base states for quantum computing.

Alternative non-equilibrium superconducting X-ray detectors

Non-equilibrium X-ray detectors based on isotropic superconductor absorbers offer the hope of energy resolution limited by the statistics of excitation production, i.e., δEE ∼ (ΔE)12, where Δ is the superconducting gap energy. This can be numerically equal to that of the best bolometers at significantly higher operating temperatures. Superconducting Tunnel Junction Detectors (STJDs) count the X-ray event induced excitations (quasiparticles (QP)) as an excursion of the subgap tunnel current. While STJDs are presently the best studied QP readout scheme, they have made slow progress toward the limiting energy resolution. Some of the difficulties are enumerated herein and are inherent in tunnel junctions. One alternative is to measure the conductivity excursion of a superconducting film that follows photon absorption. This excursion obeys exactly the same equation as the effective quasiparticle tunneling conductivity in a Josephson junction. Hence the two schemes should produce the same readout waveforms when the same superconducting absorbers are used. Seven specific approaches to realizing the measurement are presented. All versions of the conductivity readout offer simpler sample fabrication than STJD. Any superconducting material may be used. In commercial applications, the obvious benefit in operating temperature of using the HTSC materials may more than compensate for the penalty in energy resolution.

SBS-based radar true time delay

Stimulated Brillouin scattering (SBS) based slow light is considered for application to squint-free (true time delay) steering of phased array radar antennae. Results are presented on true time delay radar requirements, including delay precision and bandwidth. We experimentally investigated the level of delay precision that exists in actual slow-light systems (based on Brillouin scattering). The practical use of SBS to meet the necessary requirements for radar use is discussed.

Refrigerator with phonon filters: An application of the phonon deficit effect in superconducting tunnel junctions

Thin film devices have the advantage of being extremely compact, operate in a continuous mode, dissipate little power, and can easily be integrated in cryogenic detectors. Motivated by such possibilities, we investigate the phonon deficit effect in thin film