Primary Research Topics

The van Driel group at the University of Toronto harbours interest in several different areas of optics and condensed matter physics:

Coherent Control - Plasmonics - Spintronics

Our research in the optical excitation of solid state materials relies on cutting-edge laser technology. Our laboratory has a strong background in both the development and the application of ultrafast lasers. Today we have state-of-the-art ultrafast, highly tunable optical sources capable of multi-colour coherent-control type experiments on femtosecond (10^-15s) timescales.

We have a strong theoretical collaboration with Prof. J.E.Sipe's group within our own department and experimental collaborations with Prof. A.L.Smirl at the University of Iowa.

Research in Coherent Control

Introduction

Interest in the measurement, manipulation and direct control of electronic charges and spins are of importance in the growing fields of quantum information and spintronics. It is also a vital part of continuing to characterise and engineer, not only the electronic band structure of semiconductor devices, but also the angular momementum states of these crystals. Coherent control is the use of quantum interference between multiple transition pathways linking the same initial and final states. Our group explores coherent control processes in semiconductors via phase-related optical beams, examining both the electrical and spin current dynamics.

Electrical Current Dynamics

We have used interference between one- and two-photon absorption in GaAs to modify the interband optical transitions. Especially, this can be done asymmetrically in momentum space, thus creating a net electric current (see "Observation of Coherently Controlled Photocurrent in Unbiased, Bulk GaAs"). In addition, we can use interference between two one-photon absorption pathways to generate a current associated with orthogonal polarizations to general a current in Wurtzite semiconductors (see "Quantum Interference Control of Currents in CdSe with a Single Optical Beam"). More recently our attention has been to temporally resolve the dynamics and measure the spin of the ballistic currents created in this way.

Also, we can generate the coherent controlled currents, as well as the other photo-currents, with femtosecond pulses. These fast current transients can emit electro-magnetic radiation at terahertz (THz) frequencies. This radiation can be used to examine the dynamics of the various photo-currents in the semiconductors in which they are produced (see "THz radiation from coherently-controlled photocurrents in GaAs" and "Rectification and shift currents in GaAs").

Spin Current Dynamics

Control of electronic spins and spin dynamics in semiconductors is a fledgling field of study. Our main interests lie in optically injecting carrier spins into semiconducting materials and investigating mechanisms of depolarisation and control. Typically, hot-electrons are injected optically into the crystal and, using coherent control techniques, their motion can be controlled and measured by quantum mechanical interference.

In present experiments quantum mechanical interference between two pump beams at different frequencies is used to generate pure spin current in bulk GaAs. Due to the non-collinear geometry of the two pump beams, pure spin current is spatially modulated leading to the generation of a pure spin current grating. The formation of this grating as well as its decay yield information about ballistic and diffusive transport of the carriers. For more information, see our latest publications.

Research in Plasmonics

Introduction

Surface plasmons, i.e collective oscillations of free electrons on a metal-dielectric interface, have recently attracted tremendous interest because of their important role in a broad range of optical phenomena. The intense fields associated with surface plasmons can enhance the optical properties of a material and have been observed in thin metallic films, in artificial nanostructures as well as in clusters and aggregates of nanoparticles. Effects attributed to the field of nanoplasmonics include, e.g., the extraordinary optical transmission of subwavelength metallic nano-hole arrays and the existence of tightly confined surface plasmon polariton modes arising from the coupling of electromagnetic waves to charge density waves on a metal-dielectric interface. The subwavelength confinement of these propagating modes might ultimately serve to bridge the lengthscale gap between optics and modern nanometer scale electronics.

Surface Plasmon Polaritons

Our research interest in the area of plasmonics encompasses both fundamental and applied aspects of metal physics. Recently, we have investigated the nonlinear absorption properties of thin gold films. Especially, the role of nonequilibrium carrier dynamics and transient ultrafast heating on the optical properties has been analyzed in detail. Currently, we are extending this work towards ultrafast studies of surface plasmon polaritons. Specifically, we want to study the length and time scales associated with the propagation of these surface-bound modes as well as the possibility of ultrafast all-optical coupling schemes.

Research in Spintronics

Introduction

Spin-based electronics, or spintronics, is the use of the electron's spin degree of freedom in emergent technology. Our group investigates the fundamental physics behind this broad idea. In recent years, there has been high research interest in the generation and detection of spin polarizations and spin currents in materials. There are a variety of methods which could be utilized to achieve these goals. In our group, we concentrate on all-optical means of injecting spin polarizations and spin currents.

Magnetization from Ultrafast Optical Orientation

Ultrafast optical orientation occurs when ~100 fs circularly-polarized laser pulses are incident upon a nonmagnetic semiconductor, with a photon energy higher than the band gap energy but not high enough to excite carriers from the spin-orbit split-off band. The selection rules for these interband transitions show that a net spin polarization is induced. This means that the number of excited carriers with one spin state is different from those with the opposite spin. In GaAs, a 50% spin polarization can be induced, and along with this spin polarization comes a transient magnetization. Such a time-varying magnetization acts as a source term in Maxwell's equations and emits radiation in the terahertz (THz) range. It is this THz radiation that we observe in order to detect ultrafast optical orientation. We take advantage of beam polarizations and sample geometry to distinguish the THz radiation of the magnetization source from much stronger electric dipole sources in GaAs.