Measuring the intersubband relaxation time is important indeed

April 15, 2009--For the first time, scientists have measured and controlled the lifetime of certain quantum states that are of relevance to optoelectronic chips. The breakthrough involved measuring the intersubband relaxation time of charge states in silicon-germanium (SiGe) structures on a picosecond scale.

April 15, 2009--For the first time, scientists have measured and controlled the lifetime of certain quantum states that are of relevance to optoelectronic chips. The breakthrough involved measuring the intersubband relaxation time of charge states in silicon-germanium (SiGe) structures on a picosecond scale.1 Experiments have also shown that it is possible to control and extend these times. The work should boost the development of data processing based on optoelectronic chips.

Transmitting information via photons is both fast and reliable over long distances (via fiber optics, for example) but fails in close quarters. At present, photon-based chip-to-chip communication using silicon chips is not possible in data processing; the problem is the lack of photon sources. Due to its semiconductor structure, silicon does not allow the generation of photons by conventional means. However, unconventional means may provide a solution, which is what the group from the Institute of Semiconductor and Solid State Physics at the University of Linz (Linz, Austria) is working on.

Laser on a chip

One potential solution could be a quantum-cascade laser based on a SiGe heterostructure, which would rely on quantum-physical effects to generate laser light in the IR. "There are currently numerous fundamental issues that need to be clarified in terms of the way that SiGe heterostructures work and how they can be controlled," explains Patrick Rauter, a member of the group led by Thomas Fromherz that is working on the use of these structures for optical applications.

A key parameter is the intersubband relaxation time, which indicates the time frame within which excited charge carriers in the SiGe remain at an elevated energy level before returning to their original state. The duration of this period is a key factor for the quantum-cascade laser, as the length of time that the charge carriers are in a state of excitation is closely linked with their capacity to emit light. Rauter and his colleagues have now succeeded in accurately measuring this timeframe. They were supported in their work by the Foundation for Fundamental Research on Matter (Rijnhuizen, The Netherlands) and its free-electron laser, FELIX. The laser's beam can be pulsed at picosecond time scales, which means it can be used to measure extremely fast processes.

The group determined that the intersubband relaxation time lasts for between 12 and 25 picoseconds. The FELIX beam was split to make the measurements: one beam was used to excite the charge carriers in the SiGe while the other, after a time delay, performed the actual measurement. During this process, a photoelectric current--whose magnitude is determined by the intersubband relaxation time--was measured.

"We were also able to extend the intersubband relaxation lifetime in a controlled manner," says Rauter. "To do this, we applied an external electrical field to the sample. By altering this field, we were able to continuously tune the relaxation time between 12 and 25 picoseconds. In actual fact, we succeeded in doubling the relaxation time--a highly promising result."

The work is part of the Austrian Science Fund's research program IR-ON (InfraRed Optical Nanostructures). A total of ten working groups from Austria and Germany contribute to this program, which focuses on semiconductors, including SiGe compounds, with nanostructures for use in optoelectronic chips.

REFERENCE

1. P. Rauter et al., Phys. Rev. Lett. 102, 147401 (2009) DOI: 10.1103/PhysRevLett.102.147401
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--posted by John Wallace, johnw@pennwell.com

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