Ultrafast Science is concerned with the applications of ultrafast lasers for time-domain measurements of ultrafast electronic and acoustic processes in various media (mostly condensed matter). We have developed very-high-repetition-rate sources of femtosecond pulses from the visible to the mid-infrared (2-10 microns) and far-infrared (terahertz) regions of the electromagnetic spectrum. We are applying those sources to the study of ultrafast processes in a range of condensed matter systems. One area of long-standing interest is the dynamics of electrons in semiconductors and quantum heterostructures. We have used ultrafast spectroscopy to study ballistic transport, overshoot phenomena, and tunneling in various quantum structures. We have also investigated the dynamics of excitons in semiconductor quantum microcavities, including the first time-resolved linear and nonlinear measurements of cavity polaritons. We have performed comprehensive investigations of electronic relaxation dynamics in self-organized InGaAs quantum dots, including the first observation of the phonon bottleneck in quantum dots, and the role of electron-hole and electron-electron scattering in the dynamics of quantum dot devices such as quantum dot lasers, tunneling injection lasers, and mid-infrared quantum dot infrared photodetectors (QDIP’s). Presently we are using ultrafast spectroscopy to study the dynamics of electrons in quantum cascade lasers and in novel materials such as graphene and nanoplasmonic structures. Our research in semiconductor device physics is carried out in collaboration with the Solid State Electronics Lab here at Michigan, as well as with other groups fabricating state-of-the-art devices around the world including the Georgia Tech Interdisciplinary Research Group (IRG) on Graphene Science and Technology (http:/www.mrsec.gatech.edu).
A second area of interest is the generation of terahertz radiation and its applications to sensing and imaging. We have demonstrated a new technique for generating narrowband (or even arbitrarily shaped) THz waveforms using poled lithium niobate. We have developed GaP waveguides for high-power THz generation using fiber lasers, in collaboration with Prof. Galvanauskas’s group. We have developed a novel technique for THz imaging which we term “time-reversal imaging,” which is leading towards real-time, video rate three-dimensional THz imaging. The method is essentially to measure scattered single-cycle THz fields diffracted from objects, and to reconstruct an image of the objects by time-reversing the measured waveforms and applying the time-domain Huygens-Fresnel diffraction integral. In direct analogy with this method, we are also developing time-reversal imaging using coherent acoustic phonon pulses driven by fs lasers; this enables imaging of embedded nanostructure with 200-nm resolution demonstrated, and the ultimate potential to achieve 15 nm resolution.
A third major theme in our group is the application of short pulses to various biomedical applications and imaging. Further information on this area may be found under the Biomedical Optics link.