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EUGENE, Ore., June 2, 2008 -- An attempt to flip the spin of electrons using ultrafast laser bursts may have failed, but in the process researchers found a way to manipulate and control electron spin -- knowledge that may prove useful in a variety of new materials and technologies.

OLearyWang.jpgSpin electronics, or "spintronics," is an emerging science that focuses on harnessing the spin, or magnetic properties of electrons, to encode and process data. The field is expected to significantly broaden the electronics industry by fostering the development of much smaller, faster, energy-saving devices, from medical diagnostic equipment to environmental sensors that can detect nanosized particles much tinier than human cells.

"Spin is another dimension of electrons," said Hailin Wang, a professor of physics at the University of Oregon. "The electronics industry has depended on electron charges for more than 50 years. To make major improvements, we now need to go beyond charges to spin, which has been very important in physics but not used very often in applications."

Wang and his doctoral student Shannon O'Leary theorized that they could flip an electron's spin up to down, or vice versa, by using a nonlinear optical technique called transient differential transmission. They describe their "failure" to flip the spin and their unexpected discovery in Physical Review B.

The overall goal, Wang and O'Leary said, is to be able to force the spin to flip using light. Their studies involved the use of nonlinear optical processes of electron spin coherence in a modulation-doped CdTe quantum well -- semiconductor material formed from cadmium and tellurium, sandwiched in a crystalline compound between two other semiconductor barrier layers. A doped quantum well contains extra embedded electrons in a near two-dimensional state.

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O'Leary initialized a spin in an experiment using a "gyro-like" arrangement with a short pulse of a picosecond laser. At specific times, she hit the spin with another laser pulse with the absorption energy of an exciton (an electron-hole pair) or trion (a charged exciton). Hitting the spin with a third pulse allows them to study what impact the second pulse had on the spin.

"We know that in this particular system, excitons quickly convert into trions by binding to a free electron," O'Leary said. "One surprising aspect is that injecting trions directly does not manipulate the spin. So the manipulation effect has to do with the conversion of the excitons to trions."

The behaviors they discovered were unexpected but intriguing, Wang said. "We were not able to flip the spin, but what we found is something quite puzzling, quite unexpected, that was not supposed to happen. We now want to understand why the system works this way. This will require some more work. We wanted to get from point A to B, but we went to C."

The detour, however, "shows that we can manipulate the spin when we inject excitons at appropriate times in the precession cycle of the spin," O'Leary said. "The result gives scientists a new tool for manipulating spins, and it may prove to be a handy method because it simply requires shining a pulse of light of the appropriate energy at the right time."

The National Science Foundation and Army Research Office funded the research.

For more information, visit: www.uoregon.edu

Published: June 2008
Glossary
electron
A charged elementary particle of an atom; the term is most commonly used in reference to the negatively charged particle called a negatron. Its mass at rest is me = 9.109558 x 10-31 kg, its charge is 1.6021917 x 10-19 C, and its spin quantum number is 1/2. Its positive counterpart is called a positron, and possesses the same characteristics, except for the reversal of the charge.
electronics
That branch of science involved in the study and utilization of the motion, emissions and behaviors of currents of electrical energy flowing through gases, vacuums, semiconductors and conductors, not to be confused with electrics, which deals primarily with the conduction of large currents of electricity through metals.
exciton
An exciton is a quasiparticle that represents the bound state of an electron and a hole in a solid-state material, typically a semiconductor or an insulator. In simpler terms, an exciton is a paired electron and hole created when an electron absorbs a photon and is promoted to a higher energy state, leaving behind an empty state called a hole. Key characteristics of excitons include: Formation: Excitons are formed when an electron in the valence band of a material is excited to the...
light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
nano
An SI prefix meaning one billionth (10-9). Nano can also be used to indicate the study of atoms, molecules and other structures and particles on the nanometer scale. Nano-optics (also referred to as nanophotonics), for example, is the study of how light and light-matter interactions behave on the nanometer scale. See nanophotonics.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
quantum well
A quantum well is a structure in quantum mechanics that confines particles, such as electrons or holes, in one spatial dimension. This confinement leads to quantized energy levels, creating a potential well in which the particles are restricted to move. In semiconductor physics and device engineering, quantum wells are commonly used to create electronic or optical devices. These structures are typically thin layers (often on the order of nanometers) sandwiched between layers of a different...
spin
Acronym for self-aligned polysilicon interconnect N-channel. A metal-gate process that uses aluminum for the metal-oxide semiconductor (MOS) gate electrode as well as for signal and power supply connectors.
BiophotonicsCdTeelectronelectronicsexcitonflipHailin WanglightmedicalmicroelectronicsnanonanoelectronicsNews & FeaturesnonlinearOpticsphotonicsquantum computersquantum wellSensors & DetectorsShannon OLearyspinspintronicstransient differential transmissiontrionsultrafast lasersUniversity of OregonLasers

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