The concepts of Bloch oscillation and Zener breakdown are fundamental to electron motion in periodic potentials and were described in the earliest theoretical developments of electron transport in solids. But only in the past 10 years have experiments clearly demonstrated various aspects of Bloch oscillation in systems as diverse as cold atoms in periodic optical potentials and electrons in semiconductor superlattices. But, how does one make a Bloch oscillator oscillate?
An electrically biased semiconductor superlattice in the Bloch oscillator limit is shown to be a negative resistance device with an intrinsic high frequency cut-off limited only by the Bloch frequency. I will describe efforts by us to measure the terahertz dynamical conductivity of an electrically biased superlattice and fabricate a terahertz oscillator based on this phenomenon.
Experimental approaches have embraced quasi-optical arrays in terahertz cavities and most recently terahertz waveguides defined by photonic bandgap structures and loaded with arrays of superlattices mesas. The latter appear to allow us to measure the cross over from terahertz loss to gain as a function of frequency and electrical bias.
I will speculate on the outcome of this effort and compare it to recent dramatic advances by QC lasers into the terahertz part of the electromagnetic spectrum.
S. James Allen, currently member of the UCSB Physics Department, received a Ph.D. degree in electrical engineering from M.I.T. in 1965. Prior to appointment at UCSB, he had been a member of technical staff at Bell Labs, during which time he took a brief leave at the Royal School of Mines, Imperial College. Later he joined Bellcore, as Research Manager of Solid State Physics Research.
His research has explored transport and terahertz excitations in semiconductor nano-structures, hot-electron dynamics, high temperature superconductivity, magnetism, superionic conductors, metal physics and clathrate inclusion compounds. Current research focuses on terahertz dynamics in semiconductor quantum structures far from equilibrium, Bloch oscillators, novel terahertz detectors and terahertz circular dichroism and dynamics of bio-polymers.
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EE 317, Purdue University, West Lafayette, IN