Members: Stefan Stoianov, Devon Triplet

We use optical orientation to inject single electon spins and macroscopic spin-polarized currents into nanosized devices fabricated from two-dimensional electron gases (2DEGs). This allows us to investigate the spin physics of devices such as quantum point contacts and single electron transistors in a direct way, with applications in spintronics (spin-based electronics) and quantum computing.

The figure below illustrates the principle of the technique. A layer of silicon dioxide followed by a layer of metal is deposited on top of the device structure. A small (~100-500 nm) aperture is fabricated in the metal at the point where injection of spin polarized electrons is desired. When the aperture is illuminated, electron-hole pairs are created in the semiconductor. The holes are swept away by a nearby gate, leaving the electron behind. The spin polarization of the electrons is controlled directly by the polarization of the incident light, while the amount of charge injected is proportional to the light intensity. Charge and spin currents can therefore be controlled and modulated independently, making it easy to separate out spin-related phenomena.

The same technique can also be used to inject single electrons with a definite spin. The image below shows such a device, which monitors single electrons injected optically into a quantum dot. Metal gates (in yellow) are fabricated on top of an GaAs/AlGaAs heterostructure 2DEG.

Electrons are repelled from the area below the gates when the gates are biased negatively, shaping the 2DEG into the desired pattern. The three right-most together with the central, vertical gate, form a quantum dot (consisting of a puddle of electrons) at the center of the image. The central gate and the left gate form a quantum point contact (QPC) which is sensitive to the number of charges on the quantum dot. An aperture is fabricated above the quantum dot, allowing electron-hole pairs to be injected directly into it.

The graph shows the conductivity of the QPC as a function of time, as 150 micro second pulses impinge on the quantum dot. Occasionally, a photoelectron is captured by the dot, leading to a reduction of the QPC conductivity.