Marat Khafizov

Multiple Exciton Generation in Single-Walled Carbon Nanotubes

Upon absorption of single photons, multiple excitons were generated and detected in semiconducting single-walled carbon nanotubes (SWNTs) using transient absorption spectroscopy. For (6,5) SWNTs, absorption of single photons with energies corresponding to three times the SWNT energy gap results in an exciton generation efficiency of 130% per photon. Our results suggest that the multiple exciton generation threshold in SWNTs can be close to the limit defined by energy conservation.

Measurement of the Kapitza resistance across a bicrystal interface

The Kapitza resistance across a Si bicrystal interface was measured using a pump probe optical technique. This approach, termed time resolved thermal wave microscopy (TRTWM), uses ultrafast laser pulses to image lateral thermal transport in bare semiconductors. The sample geometry is that of a Si bicrystal with the vertically oriented boundary intersecting the sample surface. High resolution transmission electron microscopy of the boundary region revealed a thin SiO2 layer at the interface. By comparing experimental results with a continuum thermal transport model the Kapitza resistance between the Si and SiO2 was estimated to be 2.3 × 10−9 m2K/W.

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 Tc. The amplitude of the superconducting component versus temperature follows the superposition of the isotropic dirty gap and the 3-dimensional gap dependences, closing at two different Tc values. The time constant of the few-ps relaxation exhibits a double-divergence at temperatures corresponding to the Tcs of the two gaps.

Kapitza resistance of Si/SiO2 interface

A phonon wave packet dynamics method is used to characterize the Kapitza resistance of a Si/SiO2 interface in a Si/SiO2/Si heterostructure. By varying the thickness of SiO2 layer sandwiched between two Si layers, we determine the Kapitza resistance for the Si/SiO2 interface from both wave packet dynamics and a direct, non-equilibrium molecular dynamics approach. The good agreement between the two methods indicates that they have each captured the anharmonic phonon scatterings at the interface. Moreover, detailed analysis provides insights as to how individual phonon mode scatters at the interface and their contribution to the Kapitza resistance.

Photomixers fabricated on nitrogen-ion-implanted GaAs

We report on fabrication and measurement of photomixers based on nitrogen-ion-implanted GaAs. We used energies of 500keV, 700keV, and 880keV to implant N+ ions into GaAs substrates with an ion concentration of ∼3×1012cm−2. The resulting material exhibited 110fs carrier lifetime due to implantation-induced defects. Our photomixers were fabricated as metal-semiconductor-metal devices, placed at the feed point of a broadband antenna. Optoelectronic measurements were performed in the wavelength range between 350nm and 950nm. In comparison to their counterparts (photomixers fabricated on low-temperature-grown GaAs) the N+-implanted GaAs photomixers exhibit improvements on both the output power and responsivity. A maximal responsivity of above 100mA∕W was achieved and we did not observe any dependence of the mixer cut-off frequency on the bias voltage. These characteristics make N+-implanted GaAs the material of choice for efficient optoelectronic photomixers.

Measurement of thermal transport using time-resolved thermal wave microscopy

A theoretical and experimental analysis of a time resolved thermal wave microscopy (TRTWM) technique used for thermal transport measurements is presented. TRTWM utilizes elements of frequency and time domain laser based thermoreflectance techniques and is well suited to measure both lateral and cross plane thermal transport. A primary advantage of this method is that the pump and probe spot sizes do not have to be known accurately. Implementation of TRTWM to measure thermal transport in oxide substrates coated with thin metal films is demonstrated.

Spatially localized measurement of thermal conductivity using a hybrid photothermal technique

A photothermal technique capable of measuring thermal conductivity with micrometer lateral resolution is presented. This technique involves measuring separately the thermal diffusivity, D, and thermal effusivity, e, to extract the thermal conductivity, k = (e2/D)1/2. To generalize this approach, sensitivity analysis is conducted for materials having a range of thermal conductivities. Application to nuclear fuel is consider by performing experimental validation using two materials (CaF2 and SiO2) having thermal properties representative of fresh and high burnup nuclear fuel. The measured conductivities compare favorably with literature values.

Parametric Study of the Frequency-Domain Thermoreflectance Technique

Without requiring regression for parameter determination, one-dimensional (1D) analytical models are used by many research groups to extract the thermal properties in frequency-domain thermoreflectance measurements. Experimentally, this approach involves heating the sample with a pump laser and probing the temperature response with spatially coincident probe laser. Micron order lateral resolution can be obtained by tightly focusing the pump and probe lasers. However, small laser beam spot sizes necessarily bring into question the assumptions associated with 1D analytical models. In this study, we analyzed the applicability of 1D analytical models by comparing to 2D analytical and fully numerical models. Specifically, we considered a generic n-layer two-dimensional (2D), axisymmetric analytical model including effects of volumetric heat absorption, contact resistance, and anisotropic properties. In addition, a finite element numerical model was employed to consider nonlinear effects caused by temperature d...

Local measurement of thermal conductivity and diffusivity

Simultaneous measurement of local thermal diffusivity and conductivity is demonstrated on a range of ceramic samples. This was accomplished by measuring the temperature field spatial profile of samples excited by an amplitude modulated continuous wave laser beam. A thin gold film is applied to the samples to ensure strong optical absorption and to establish a second boundary condition that introduces an expression containing the substrate thermal conductivity. The diffusivity and conductivity are obtained by comparing the measured phase profile of the temperature field to a continuum based model. A sensitivity analysis is used to identify the optimal film thickness for extracting the both substrate conductivity and diffusivity. Proof of principle studies were conducted on a range of samples having thermal properties that are representatives of current and advanced accident tolerant nuclear fuels. It is shown that by including the Kapitza resistance as an additional fitting parameter, the measured conductivity and diffusivity of all the samples considered agreed closely with the literature values. A distinguishing feature of this technique is that it does not require a priori knowledge of the optical spot size which greatly increases measurement reliability and reproducibility.

Ultrafast Photoresponse of Superconductor/Ferromagnet Nano-Layered Hybrids

Interactions of superconducting nanostructures with external optical radiation represent a very interesting topic in both the framework of nonequilibrium physics and photodetector applications. Heterogeneous nano-layers, such as proximized superconductor/normal metal and superconductor/ferromagnet (S/F) bilayers, are very promising since they exhibit the ultrafast Cooper-pair and quasiparticle dynamics. We have characterized Nb/NiCu, NbN/NiCu, YBaCuO/Au/NiCu, and YBaCuO/LaSrMnO proximized S/F nano-bilayers, using time-resolved, all-optical, femtosecond pump-probe spectroscopy measurements down to 4 K. Our experiments demonstrate that these bilayers are very promising for novel, ultrafast photodetection and spintronics device applications. Moreover, our time-resolved studies of the carrier dynamics in oxide-based S/F structures open the way to novel basic-physics investigations of nonequilibrium effects in correlated systems.