报告题目：Optical control of spin transport in solids

报 告 人：Ben Murdin and Dr. Juerong Li，University of Surrey, UK

报告时间：4月23日（星期一）上午11:00

报告地点：物理系三楼报告厅

报告摘要：Part I: Spin and Optical Galvanic Effects

In unbiased crystals, symmetry can allow the absorption of polarized light to give rise to a dc electromotive force, known as a Galvanic Effect. There are several different classes of these effects, including the Circular Photo-Galvanic Effect and the Spin Galvanic Effect. They are all equivalent to the mechanical ratchet effect. In mechanics, a ratchet is a device that is used to restrict motion in one direction while permitting it in another. In general, a ratchet can equate to any sort of asymmetric potential, or a potential lacking a centre of spatial inversion.

In fact many unbiased non-equilibrium non-centrosymmetric systems can generate transport of particles and this conception has been exploited in different fields of physics, chemistry and biology. In mechanical, electronic, optical and biological systems, a particle, classical or quantum, charged or neutral, propagating in a periodic potential with broken centrosymmetry and subjected to an ac force exhibits a net dc macroscopic flow. Examples are Abrikosov vortices in superconductors and biological motor proteins. Non-centrosymmetric bulk semiconductors and heterostructures such as topological insulators are natural quantum ratchets. The photo galvanic effect serves as a solid bridge between transport and optics and, therefore, reveals both transport and optical properties of the systems under study.

I shall describe the study of circular- and spin- photo galvanic effects in semiconductor quantum wells, and discuss how they might be used to explore topological protection.

Part 2: Optical and magnetic field Manipulation of Spin transport in nano-scale Semiconductor Structures

The realization of efficient semiconductor based spin filters and manipulators is essential for semiconductor spintronics to achieve its promised potential as a route to faster and more energy efficient electronics. One of the challenges is the creation of spin polarized currents within inherently non-magnetic semiconductors. The conventional approach to achieve this has been via the incorporation of magnetic materials. However, it may be possible to produce non-magnetic spin filters with very high efficiency by exploiting the strong spin-orbit interactions present in narrow gap semiconductors such as InSb which has the strongest Rashba coefficient of all the III-Vs making it an ideal candidate for ballistic spintronic devices.

We have investigated spin dependent transport in InSb QW nanoscale structures using spin polarised photocurrents. We have demonstrated transverse magnetically focussing of photocurrent signals in an InSb QW device. Using optical spin orientation by modulated circularly polarized light an electron spin-dependent signal has been observed due to the spin-orbit interaction.