Brian Justin Keay

Photon assisted transport through semiconductor quantum structures in intense terahertz electric fields

Abstract Quantum transport in resonant tunneling diodes, sequential resonant tunneling superlattices and miniband superlattices in the presence of intense terahertz electric fields is marked by new channels opened by the absorption or emission of one or more terahertz photons. In triple barrier resonant tunneling diodes, new transport channels supported by the absorption or stimulated emission of up to three terahertz photons are observed. In sequential resonant tunneling superlattices, dynamic localization is accompanied by absolute negative resistance. Transport in miniband resonances with controlled by coherent tunneling through several barriers and quantum wells, reveals multi-photon resonances with Bloch oscillation. Photon-assisted transport in these semiconductor quantum structures bears a strong analogy to quasi-particle tunneling and the AC Josephson effect in superconducting junctions and opens the arena of superconducting electronics to semiconductor systems. These experiments are made possible by the UCSB free-electron lasers that deliver kilowatts of tunable radiation from 120 GHz to 4.8 THz.

Virtual states, dynamic localization, absolute negative conductance and stimulated multiphoton emission in semiconductor superlattices

We report the observation of dynamic localization, absolute negative conductance (ANC) and multiphoton stimulated emission in sequential resonant tunnelling semiconductor superlattice bow-tie antenna coupled to intense terahertz electric fields. Perhaps the most remarkable observation is that with increasing terahertz field strength the conductance near zero d.c. bias decreases towards zero and then becomes negative. The results presented here compare favourably with a model in which virtual states, familiar from nonlinear optics, take a role similar to the unperturbed quantum well states.

Multiphoton-assisted tunneling, dynamic localization and absolute negative conductance

We report the observation of Absolute Negative Conductance (ANC), multiphoton stimulated emission and dynamical localization in sequential resonant tunneling semiconductor superlattices driven by intense terahertz electric fields. With increasing terahertz field strength the conductance near zero dc bias decreases towards zero and then becomes negative. This is accompanied by new steps and plateaus that are attributed to multiphoton-assisted resonant tunneling between ground states of neighboring quantum wells accompanied by the stimulated emission of a photon.

Probing terahertz electron dynamics in semiconductor nanostructures with the UC Santa Barbara FELs

Abstract The UCSB free electron lasers radiate quasi-cw tunable radiation from 120 GHz to 4.8 THz at the kilowatt power level. These lasers enable researchers to probe high frequency nonlinear electron transport in state-of-the-art semiconductor nanostructures. The impact of this research could have important consequences in the fields of high frequency semiconductor science and technology. Three experiments are described that demonstrate the application of FELs in exploring terahertz dynamics of semiconductor nanostructures: i) photon-assisted tunneling in a semiconductor superlattice; ii) dynamical response of a resonant tunneling diode; and iii) saturation spectroscopy of a single square quantum well.