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arXiv:0805.1819 · Nano Letters

A Semiconductor Exciton Memory Cell Based on a Single Quantum Nanostructure

Hubert J. Krenner, Craig Pryor, Jun He, Pierre M. Petroff

Published May 24, 2008• 46 citations
arXivHTMLPDF

Abstract

We demonstrate storage of excitons in a single nanostructure, a self-assembled quantum post. After generation, electrons and holes forming the excitons are separated by an electric field toward opposite ends of the quantum post inhibiting their radiative recombination. After a defined time, the spatially indirect excitons are reconverted to optically active direct excitons by switching the electric field. The emitted light of the stored exciton is detected in the limit of a single nanostructure and storage times exceeding 30 msec are demonstrated. We identify a slow tunneling of the electron out of the quantum post as the dominant loss mechanism by comparing the field dependent temporal decay of the storage signal to models for this process and radiative losses.

DOI
10.1021/nl800911n

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Indirect excitons in GaAs coupled quantum well nanostructures are a versatile system for fundamental study of cold neutral bosonic gases and demonstration of novel optoelectronic devices based on excitons -- a bound electron-hole pair -- rather than electrons. Indirect exciton lifetimes range from nanoseconds to microseconds and cool rapidly after photoexcitation to the lattice temperature. Lithographically-patterned electrodes enable design of potential energy landscapes, and both energy and lifetime can be controlled in situ, rapidly, on timescales much shorter than the exciton lifetime. Such intrinsically optoelectronic devices can operate at speeds relevant to optical networks, and later be fabricated in other semiconductors for higher-temperature operation. Two different kinds of devices are demonstrated Photon storage -- an optical memory -- with 250 ps rise time of the readout optical signal and storage time reaching microseconds was implemented with indirect excitons in CQW. The storage and release of photons was controlled by the gate voltage pulse, and the transient processes in the CQW studied by measuring the kinetics of the exciton emission spectra. This control of excitons on timescales much shorter than the exciton lifetime demonstrates the feasibility of studying excitons in in situ controlled electrostatic traps. The Exciton Conveyer is a laterally moving electrostatic lattice potential for actively transporting excitons. Generated by laterally modulated electrodes, the potential velocity and depth are controlled in situ by frequency and voltage. We observed exciton transport characterized by average exciton cloud spatial extension over several tens of microns, and observed dynamical localization-delocalization transitions for the excitons in the conveyer : In the localization regime of deeper potentials and moderate exciton density, excitons are moved by the conveyer; in the delocalized regime of shallower lattice potential or high exciton density, excitons do not follow the conveyer motion. We explore conveyer velocities both slower and faster than phonon velocities. Realizing subnanosecond manipulations of exciton energy and lifetime required versatile control of pulsed and multiple AC RF electrical signals in optical, liquid helium cryogenic systems. Considerable detail is presented of design, construction, and test of flexible experimental apparatus