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Ultrafast Nanophotonics Group
Group Leader: Alan D. Bristow
Eberly College
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Ultrafast Dynamics & Transport

Transient absorption of an In/AlAsSb superlattice with type-II alignment cooled to 4 K. The photon energy is varied to observe the onset of a metastability in the early time dynamics (inset). The body shows the various exponential fit components.

Rationally design of solid-state device for optoelectronic, electronic and photonics applications can be aided by knowledge of the charge-carrier and carrier-lattice dynamics. In particular the amplitude and longevity of photo-excited carriers versus excitation and sample conditions can identify scattering/relaxation mechanisms and electron-phonon interactions. Transient absorption spectroscopy is a ubiquitous tool for measuring charge-carrier and carrier-lattice dynamics. The group has expertise in collecting and analyzing experimental data to interpret the mechanisms that govern the dynamics of a range of solid-state systems. The group works with a multidisciplinary assortment of international collaborators, providing group members the opportunity to work in teams and make connections between research topics, such as plasmonic light-harvesting, dielectric photocatalysts, hot-carrier photovoltaics and thin-film magnets.

Plasmonic light-harvesting work began with former student Dr. Scott Cushing, who graduated in December 2015 and is now a Professor at the California Institute of Technology. In particular we discovered a coherent electrodynamic mechanism in hybrid metallodielectric nanoparticles known as plasmon-induced resonance energy transfer (PIRET) to overcome lossy charge transfer mechanisms.

Dielectric-based photocatalysis work has taken various forms from quantum-dot sensitized TiO 2 and core-shell nanoparticles to doping bulk crystals with novel properties. This work has engaged several Ph.D. students and international collaborators. The most recent project is to find all-dielectric materials that exhibit a coherent response similar to PIRET with collaborators from University of South Florida (formerly Oklahoma State University).

Hot-carrier photovoltaics work has explored novel III-V semiconductor heterostructures that offer high carrier mobility, protracted hot-carrier lifetimes and potential methods for hot-carrier extraction. This work is being performed in collaboration with the University of Oklahoma and University at Buffalo.

Thin-film magnets were explored for dynamics and longitudinal wave propagation in collaboration with departmental-colleague Prof. Mikel Holcomb.

Spin transport properties were explored in two-dimensional electron gases (2DEGs) at TU Dortmund. Time-resolved Kerr microscoopy was used to measure the drift and diffusion of spin-polarized carriers after excitation and under the influence of a static magnetic field or in-plane electric fields. Dresselhaus and Rashba spin-orbit coupling were engineered to allow for persistent spin helices to form in the longer-time dynamics of these systems. Among the main achievements, this work demonstrated a traveling persistent spin helix by application of balanced magnetic and electric fields simultaneously.

THz Conductivity work allows for determination of carrier transport and can be applied through THz time-domain spectroscopy to the ground state of a material or after photoexcitation to determine the mobility of photocarriers. We have explored low conductivity in nonlinear crystals and nanographene, the photocarrier transport in superlattices and are investigating group-IV semiconductors for infrared and electro-optic applications.