Lalani K. Werake

Ambipolar diffusion of photoexcited carriers in bulk GaAs

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Hot carrier diffusion in graphene

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Femtosecond pump-probe studies of reduced graphene oxide thin films

The dynamics of photocarriers in reduced graphene oxide thin films is studied by using ultrafast pump-probe spectroscopy. Time dependent differential transmissions are measured with sample temperatures ranging from 9 to 300 K. At each sample temperature and probe delay, the sign of the differential transmission remains positive. A fast energy relaxation of hot carriers is observed, and is found to be independent of sample temperature. Our experiments show that the carrier dynamics in reduced graphene oxide is similar to other types of graphene, and that the differential transmission is caused by phase-state filling effects of carriers.

Optical injection and detection of ballistic pure spin currents in Ge

Ballistic pure spin currents are injected into Ge at 295 K using quantum interference between one and two photon absorption processes for 1786 and 893 nm, 200 fs optical pulses. The spin currents are spatially and temporally detected using polarization- and phase-dependent differential transmission techniques with nanometer spatial and femtosecond temporal resolution. We interpret the dynamics in terms of the fast spin relaxation of the holes and intervalley transfer of electrons.

Second-harmonic generation induced by electric currents in GaAs.

We demonstrate a new, nonlinear optical effect of electric currents. First, a steady current is generated by applying a voltage on a doped GaAs crystal. We demonstrate that this current induces second-harmonic generation of a probe laser pulse. Second, we optically inject a transient current in an undoped GaAs crystal by using a pair of ultrafast laser pulses and demonstrate that it induces the same second-harmonic generation. In both cases, the induced second-order nonlinear susceptibility is proportional to the current density. This effect can be used for nondestructive, noninvasive, and ultrafast imaging of currents. These advantages are illustrated by the real-time observations of a coherent plasma oscillation and spatial resolution of current distribution in a device. This new effect also provides a mechanism for electrical control of the optical response of materials.

Observation of second-harmonic generation induced by pure spin currents

We demonstrate second harmonic generation induced by pure spin currents in semiconductors, and show that this effect can be used for the direct, noninvasive, and nondestructive detection of pure spin currents.

Observation of intrinsic inverse spin Hall effect.

We report observation of intrinsic inverse spin Hall effect in undoped GaAs multiple quantum wells with a sample temperature of 10 K. A transient ballistic pure spin current is injected by a pair of laser pulses through quantum interference. By time resolving the dynamics of the pure spin current, the momentum relaxation time is deduced, which sets the lower limit of the scattering time between electrons and holes. The transverse charge current generated by the pure spin current via the inverse spin Hall effect is simultaneously resolved. We find that the charge current is generated well before the first electron-hole scattering event. Generation of the transverse current in the scattering-free ballistic transport regime provides unambiguous evidence for the intrinsic inverse spin Hall effect.

All-optical generation and detection of subpicosecond ac spin-current pulses in GaAs

Permissions were not obtained for sharing the full text of this article. Full text is available at the American Physical Society and at arXiv. See links in this record.

Observation of second harmonic generation induced by pure spin currents in semiconductors

We demonstrate second harmonic generation induced by pure spin currents in semiconductors, and show that this effect can be used for the direct, noninvasive, and nondestructive detection of pure spin currents.

Real-time and real-space observation of plasma oscillation in GaAs

A plasma oscillation of electrons and holes is generated in GaAs by a quantum interference and control technique, and is temporally and spatially resolved by detecting an induced second-harmonic generation process.