Biomedical subjects

Roman Sobolewski

Publications and source records attributed to Roman Sobolewski.

3 recordsLinked to original sources

PubMed2004

Characterization of single and double fiber-coupled diffusing spheres.

A fiber-optic sensor, consisting of an optical fiber with and without a 1.59-mm-diameter spherical ceramic tip, inserted into a 19-mm-diameter Spectralon sphere has been characterized. This sensor is evaluated as a large-area omnidirectional sensor. An optical transport measurement system that rotates the sphere about two axes has been designed. The system measured the UV transport efficiency at 351 nm of light impinging on the sphere with an f-6 cone angle. When a bare fiber was placed at the center of the target sphere, the detection sensitivity was biased in the forward direction. The peak of the sensitivity of the inverse integrating sphere shifted from front to back as the bare fiber was withdrawn from the sphere and the numerical aperture of the fiber viewed more of the scattering volume. The response function with respect to the angle of incidence of the dual sphere was much more uniform than that obtained with a bare fiber in the center of the sphere.

Journal Article↗

PubMed2003

Time-resolved photoexcitation of the superconducting two-gap state in MgB2 thin films.

Femtosecond pump-probe studies show that carrier dynamics in MgB2 films is governed by the sub-ps electron-phonon (e-ph) relaxation present at all temperatures, the few-ps e-ph process well pronounced below 70 K, and the sub-ns superconducting relaxation below T(c). The amplitude of the superconducting component versus temperature follows the superposition of the isotropic dirty gap and the three-dimensional pi gap dependences, closing at two different T(c) values. The time constant of the few-ps relaxation exhibits a double divergence at temperatures corresponding to the T(c)'s of the two gaps.

Journal Article↗

PubMed2003

Femtosecond response of a free-standing LT-GaAs photoconductive switch.

We present a novel, free-standing low-temperature GaAs (LT-GaAs) photoconductive switch and demonstrate its femtosecond performance. A 1-microm-thick layer of a single-crystal LT-GaAs was patterned into 5-10-microm-wide and 15-30-microm-long bars, separated from their GaAs substrate and, subsequently, placed across gold coplanar transmission lines deposited on a Si substrate, forming a photoconductive switch. The switch was excited with 110-fs-wide optical pulses, and its photoresponse was measured with an electro-optic sampling system. Using 810-nm optical radiation, we recorded an electrical transient as short as 360 fs (1.25 THz, 3-dB bandwidth) and established that the photo-carrier lifetime in our LT-GaAs was 150 fs. Our free-standing devices exhibited quantum efficiency of the order of approximately 7%, and their photoresponse amplitude was a linear function of the applied voltage bias, as well as a linear function of the excitation power, below a well-defined saturation threshold.

Journal Article↗