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Ultrafast Nanophotonics Group
Group Leader: Alan D. Bristow
Eberly College
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Coherent Phenomena

Two-dimensional coherent spectroscopy of exciton-polaritons in a semiconductor microcavity. Modes on the dashed diagonal are self-interaction features, the off-diagonal features are interactions features.

Lasers are able to impose their coherent properties into optical excitations. Coherent phenomena enhance light-matter interaction because the oscillators are synchronously, oscillating in phase. Coherence can be inverted and also controlled by the excitation laser pulse or sequence of pulses. For these reasons coherent phenomena are of importance to device physics. However, the coherent oscillators will decohere or dephase from one another in additions to undergoing relaxation and recombination. Knowing the control mechanism and the dephasing time are often limits for device applications and must be explored. Our group does this in semiconductor systems, where excited electrons couple with nasent holes (missing electrons in the group state) to form excitons. In addition,we develop control schemes for exploring condensed matter physics, such as multidimensional coherent spectroscopy and multicolor all-optical injection of photocurrents.

Coherent excitonic properties in nanostructured semiconductor systems can be used as light-emitting devices. They are also model system for many-body Coulombic interactions, quantum electronics and analogs for some quantum biology processes. Hence, their continued study has merit, especially with new spectroscopy tools or in novel material systems. The group has explored the role of strain on excitons in bulk semiconductors using multidimensional coherent spectroscopy, the role of coherence on the exciton-polaritons in semicondcutor microcavities and exciton-trion psudo-spin decoherence in transition metal dichalcogenides. The latter was performed at TU-Dortmund, while the microcavity work was started in the WVU lab and transitioned to the National Institute of Standards and Technology.

Multidimensional coherent spectroscopy techniques for solid-state physics has been pioneered by the group of Steven Cundiff especially with development of the multidimensional optical nonlinear spectrometer (MONSTR). Subsequent work in our group started in 2010 and continues through strong collaboration with the National Institute of Standards and Technology. We have continued to explore use of quickly adjusting phase control optical elements, such as variable retarders (in addition to their role for polarization control). Moreover, we have been working on developing new analysis methods to overcome limitations of the generic FFT approach.

Coherent control of photocurrents has been explored in quantum materials, such as topological insulators, because these materials exhibit properties such as strong spin and orbital angular momentum coupling, topological protection of spin currents, and quantized edge currents without a magnetic field. Spin-polarized photocurrents can be injected all-optically into these systems using quantum interference control schemes (a form of coherent control previously shown in silicon), where an imbalance in exited carriers in momentum space leads to a real space current. This two-color optical process also introduces a shift current or linear photo-galvanic effect, where a real-space shift in the charge carriers upon excitation leads to a local current. Measuring photocurrents in topological insulators, like Bi 2Se 3, is a springboard for observing novel galvanic effects and determining the characteristics of quantum transport. For example, in Bi 2Se 3, the optical pulses can be set to photon energies that link the first and second Dirac points in the topologically protected surface states, hence the injection current excites both surface and bulk contributions, whereas the shift current only excites surface contributions due to the reduced symmetry at the interface of the material. Collaboration with optical theorists at the University of Toronto allowed our group to separate these contributions and relate them to the band structure of the material.