Thursday,September 13, 2012 9:00 AM to 11:00 AM
WNLO Room A101
David Hagan received his PhD degree in Physics at Heriot-Watt University, Edinburgh, Scotland in 1985. From 1985 -87 he was a research scientist at the Center for Applied Quantum Electronics and the University of North Texas. He moved to UCF in 1987 as a founding member of the CREOL faculty, where he is currently Professor of Optics and Physics and also serves as Associate Dean for Academic Programs. He is currently Editor in Chief of the OSA journal, Optical Materials Express and a Fellow of OSA. His current research interests include nonlinear optical materials, especially semiconductors and organics, and techniques for nonlinear optical characterization and spectroscopy.
Two-photon absorption (2PA) in semiconductors has long been known to scale as the inverse third power of the energy gap, i.e., which limits the 2PA coefficients available in large gap semiconductors. However it is also known that in the highly nondegenerate case, where the input wavelengths are very different, the 2PA rate can be greatly enhanced over the degenerate case. We have recently verified this for several direct gap semiconductors in pump-probe transmission experiments with femtosecond and picosecond pulses, where we showed that, in many direct-gap semiconductors, nondegenerate 2PA coefficients are enhanced approximately 100-fold over the degenerate case. In GaAs, we observed 2PA coefficients around 1 cm/MW. Coefficients this large were previously only observed in narrow-gap semiconductors such as InSb.
Based on this effect, one may obtain sensitive gated detection using conventional semiconductor photodiodes. We have demonstrated this with standard GaN and GaAs photodiodes using extreme non-degenerate photon pairs with up to 14:1 energy ratio. We can also detect weak IR radiation by employing intense UV gating pulses. The minimum detected IR pulse energy in GaN is as low as 20 pJ energy while for a standard cooled MCT detector, the minimum detectable energy is 200 pJ. We have also now demonstrated cw detection using this method. It is worth noting that this process does not use IR crystals or phase-matching, as employed by �(2) upconversion detection. In this talk, will show how this detection scales to other semiconductors and how one may optimize device geometries for practical detection.