Bipolar Spintronic Devices with Two-Dimensional Systems

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Semiconductor devices in which the recombination of carriers (electrons and oppositely charged holes) leads to the emission of light (photons) play an increasingly important role. Advances in light emitting diodes (LEDs), recognized by the 2014 Nobel Prize in Physics, demonstrate many advantages over conventional light bulbs, including lower power consumption, longer lifetime, smaller size, and reduced environmental concerns. While in LEDs and lasers the focus is on the emission wavelength and intensity, their utility can be significantly enhanced by also harnessing the polarization of the emitted light. Akin to a preferred hand use in humans (left vs right), the polarization of light can display handedness (left vs right or clockwise vs counterclockwise). This preference is also inherent to many otherwise identical molecules and can signal different biological functions. The polarization-sensitive detection can be used for biomedical diagnosis, including an early detection of cancer. The proposed research investigates polarization control in light emitting diodes and lasers to improve their detection sensitivity as well as enable a much faster operation and lower power consumption. The principle behind this polarization control is the conservation of the total angular momentum when it is converted between different subsystems, such as carriers and photons. In a simple mechanical manifestation, the conservation of angular momentum causes the rotational frequency of ice skaters to increase when their arms are pulled closer to their spinning bodies. In light emitting devices the angular momentum of carriers is associated with their spin that is analogous to a child's top spinning in the clockwise or counterclockwise direction. Through transfer of angular momentum of spin-polarized carriers that preferentially spin in one these directions, the emitted light becomes polarized. The proposed work will provide a closely integrated educational and outreach efforts, as well as develop resources to study spintronics, including Spintronics Handbook: Spin Transport and Magnetism, 2nd Edition, co-edited by the PI. To address a deficiency in the secondary education in which a vast majority of public school students have minimal or no exposure to physical sciences that subsequently deters them from considering careers in science or engineering, the PI will organize Summer Workshops: Light Emitting Diodes and Lasers. The topics will include light diffraction, wavelength measurement, polarization properties of lasers, and modification of laser output. As a part of the annual SPIE: Optics+Photonics Conference, the PI will organize Symposia to bridge the gap between the spintronics and optics communities.The success of practical spintronic devices operating at room temperature is largely limited to unipolar devices where spin-polarized electrons are responsible for magnetoresistive effects implemented in spinvalves. Despite their remarkable success for magnetic storage and sensing, such spin valves are of limited use for advanced signal processing and digital logic. It would therefore be important to assess if there are alternative paths to realize potentially transformative spin-based devices, beyond magnetoresistance. Two paths towards superior light emitting devices are sought: (1) using conventional semiconductors and (2) using novel van der Waals materials that can be made atomically thin. In (1) it is predicted that silicon as the dominant material for conventional electronic, but with poor optical properties, could also lead to the robust light emission and thereby provide unexplored opportunities for silicon optoelectronics. In (2) an atomically-thin active region enables reduction in size and the emission of light is achieved at a fraction of the power consumed in conventional counterparts. In contrast to the common approach for spin-based devices that aims to increase the carrier spin relaxation time, it is predicted that short spin relaxation time supports ultrafast changes in the polarization of the emitted light ( 200 GHz at 300 K) and thereby enables ultrafast optical communication. Theoretical predictions will be closely supported through experimental collaborations to ensure their demonstration.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.