Prof. Marat Khafizov Profile

Prof. Marat Khafizov

Assistant Professor at Ohio State Univ

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Proceedings Article | 17 September 2018 Presentation

Picosecond ultrasonics and transient thermoreflectance methods for characterization of grain boundary microstructure and thermal transport (Conference Presentation)

Marat Khafizov, David Hurley

Proceedings Volume 10753, 1075307 (2018) https://doi.org/10.1117/12.2321902

KEYWORDS: Ultrasonics, Picosecond phenomena, Reflectivity, Laser beam diagnostics, Interfaces, Ceramics, Nondestructive evaluation, Environmental sensing, Ultrafast laser spectroscopy, Wave propagation

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Utilization of transient reflectivity measurements for nondestructive characterization of material properties are attractive for understanding materials behavior in harsh environments. Two experimental approaches that implement an all optical pump-probe ultrafast spectroscopy will be presented. In picosecond ultrasonics, the pump beam is used to generate ultrasonic wave modes propagating normal to the surface. Transient reflectivity signal exhibits Brillouin oscillations that originate from the interference between two reflections of the probe beam: one from the surface and another from the wavefront of the acoustic wave. The frequency and amplitude of the oscillations depends on the type and velocity of the excited modes and is determined by the crystalline orientation. These features are used to obtain microstructural information of the material. Application of the method to subsurface grain boundary imaging in transparent metal oxides will be presented. The transient thermoreflectance method is used to characterize conductive heat transfer in solid materials, particularly, across interface and grain boundaries. In this approach, the pump beam is used as a heater, and the probe beam measures temperature induced reflectivity changes. Measured reflectivity profiles in the time and spatial domain are analyzed to characterize the heat transfer characteristics of the material with a few micrometer spatial resolution. Examples include measurement of radiation damage induced thermal conductivity degradation in ceramic materials such silicon carbide and cerium dioxide and thermal transport across the interfaces in ceramic matrix composite materials. Presented methods combined with other methods such as Raman spectroscopy offer capability to characterize material properties nondestructively and are attractive methods for in-situ studies of materials behavior in harsh environments.

Proceedings Article | 7 November 2002 Paper

Fabrication and femtosecond photoresponse studies of MgB2 superconducting thin films

Ying Xu, Marat Khafizov, Andrej Plecenik, Peter Kus, Leonid Satrapinsky, Roman Sobolewski

Proceedings Volume 4811, (2002) https://doi.org/10.1117/12.455833

KEYWORDS: Superconductors, Thin films, Femtosecond phenomena, Temperature metrology, Phonons, Crystals, Magnesium, Annealing, Laser beam diagnostics, Argon

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We present fabrication and optical time-resolved photoresponse characterization of MgB₂ superconducting thin films. The films were prepared on crystalline and flexible plastic substrates by vacuum co-deposition of B and Mg precursors and high-temperature annealing in an Ar or vacuum atmosphere. The post-annealed films exhibited very smooth surfaces and amorphous structures with nanocrystal inclusions. The best films exhibited the critical temperature Tc of up to 38 K, the transition width of 1 K, and the current density j_c at 4.2 K of about 10⁶ A/cm². In our pump-probe photoresponse experiments, we used 100-fs-wide optical pulses generated by a Ti:Sapphire laser. The pump and the probe beams had 800-nm wavelength and the measurements were performed in the temperature range from 3.5 K to room temperature. The transient reflectivity change (ΔR/R) signals exhibited around 300-fs (10%-90%) risetime. At room temperature and far above Tc, (ΔR/R) the transient reflectivity change was characterized by a ~160-fs, single-exponential decay, interpreted as the electron-Debye-phonon interaction time. Below 60 K and in the superconducting state, the ΔR/R photoresponse was biexponential, with the initial femtosecond decay followed by a much slower, several-ps-long relaxation. We associate the latter slow relaxation with the electron-phonon interaction related to the Cooper pair recombination dynamics. The existence of this signal above the nominal T_c of our films, we tentatively interpret as the presence of superconducting fluctuations in our MgB₂ films. Our work gives the first insight into the carrier dynamics in MgB₂ by time-resolved experimental studies of the Cooper pair breaking and thermalization mechanisms for the films perturbed by femtosecond optical excitations.

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