...at JILA, which is a joint institute of the University of Colorado and the National Institute for Standards and Technology
Spin Dynamics in Semiconductors
The recent interest in the design of spin-based electronics (spintronics) or even quantum computing devices has boosted the research on spins in semiconductors. Recently, surprisingly long spin coherence times – a prerequisite for efficient devices – have been observed in bulk semiconductors. The origin of these long relaxation times and the role of doping carriers vs. optically excited carriers remains unclear.
Spin Lifetime
Faraday rotation signal of n-doped GaAs (n = 2.4 – 5.3 × 1016 cm–3).
We investigate undoped and n-doped GaAs samples with varying doping densities by performing time-resolved Faraday rotation experiments at high optical excitation levels. In these experiments, a magnetic field is applied parallel to the sample surface. 100 fs laser pulses provided by a mode-locked Ti:sapphire laser tuned around the bandgap of the semiconductor create spin-polarized electrons. These electrons precess in the applied magnetic field (Larmor spin precession). By measuring the polarization rotation of a linearly polarized probe beam transmitted through the sample (Faraday rotation) using a polarization bridge one is able to monitor the spin precession of the electrons over time. A typical Faraday rotation signal is shown in the figure on the left. By varying doping via optical pumping and across different samples, the role of many-body effects can be investigated.
Spin Diffusion
Diffracted signal from a spin grating in an n-doped GaAs quantum well (2e10 e/cm2) at a temperature of 4 K and a magnetic field of 2 T. Inset: Spin grating decay rate vs the grating wavevector squared. The black, blue, and green circles represent data taken with pump powers of 0.15, 0.4, and 1.0 mW, respectively. Diffusion constants are given for each of these pump powers.
We are also investigating spin diffusion by measuring transient spin gratings in GaAs quantum wells. In these experiments, two coincident laser pulses (pumps) with orthogonal linear polarizations interfere to form a spin grating, in which the electron spin orientation alternates across the sample. A delayed laser pulse (probe) is diffracted off of this grating, measuring the grating amplitude as a function of time (see typical data at right). The spin grating decays due to both spin relaxation and spin diffusion. In a doped quantum well, the spin grating lasts longer than the optically excited carriers, indicating a spin grating is formed in the electron gas. We are able to obtain the spin diffusion rates of the electron gas under various conditions, providing information relevant to spintronic applications.