Controlling the Infrared: Macro-to-Micro-to-Nano and from Photons to Electron Waves

Seminar
Thursday, November 17, 2016
4:00 AM to 5:00 AM
MRC 1.606M
Free and open to the public

The mid- and far infrared are tantalizing regions of the electromagnetic spectrum, containing great information about thermal and chemical properties of objects and materials; however, semiconductor devices that emit, detect, and manipulate light in this range are often extremely difficult to use, especially at room temperature. Semiconductor devices have high sensitivity but require cryogenic temperatures and are limited by a fixed bandgap. Thermal detectors and emitters are often the best choices for high temperature operation but they have limitations when spectral selectivity is needed.

This presentation discusses both photon and electron states in the infrared. Photons can be manipulated by optical cavities even when the materials they interact with are spectrally uniform. Electron quantum states can be altered in real time using cavity effects of heterostructures on electron waves. Infrared cavities have dramatically different properties depending on their mode volume. The ultimate performance of a detector has been known for decades for large cavities, but as an infrared cavity becomes extremely small, the background noise behaves differently, leading to modified sensitivity. Quantum optical conditions such as the Purcell effect, can also become important. Just as interesting, the thermal phonon properties of devices are altered as we approach the nanoscale as well, affecting even the fundamental definition of what temperature means to such as device.

While resonant energies for photons are very frequently considered as “tunable” in real time, the same is seldom, if ever, said of electron states. However, the same principles that define discrete resonances for light in a cavity also define quantized energies for electrons in quantum wells. Therefore mechanical manipulation of a quantum well barrier should change the energy states of an electron in a way analogous to the resonant frequencies of an optical cavity. The end of the talk describes how heterostructures coupled across a position-dependent gap can tune electron states in much the same manner that optical cavities couple to control photon states. This technique has dramatic implication in the infrared, where both the gain and absorption of materials can be controlled over a wide range, limited only by thermal broadening and band offsets.

x x

Speaker

Talghader

Joey Talghader

Professor
University of Minnesota Department of Electrical and Computer Engineering

Joseph Talghader obtained his B.S. in electrical engineering from Rice University in 1988. He was awarded an NSF Graduate Fellowship and studied at UC-Berkeley to obtain his M.S. and Ph.D. in 1993 and 1995, respectively, specializing in vertical-cavity lasers. During and after graduate school, he developed nonvolatile memory at Texas Instruments and Waferscale Integration. Dr. Talghader then joined the University of Minnesota, where he is now a Professor. His primary research areas are infrared detectors, optical coatings and materials, nanomechanical systems, and microsensors.