The Dynamic Franz-Keldysh Effect
Supervisor: Dr Ben Murdin, Dr
Steve Hughes
Major Aims:
To investigate what happens
to an electron and hole pair are excited by an ultra-short pulse of ultra-high-intensity
light. This project is of fundamental physics interest, but has applications
to high-speed terahertz telecommunications.
Techniques used and source
of expertise:
Infrared spectroscopy of
semiconductors structures. Use of ultrafast lasers, including the Free-Electron
Laser FELIX in the Netherlands.
In a semiconductor, when
a photon of light is absorbed an electron is excited from the valence band
to the conduction band leaving behind a hole. The electron orbits around
the hole in exactly the same way an electron orbits around a proton in
the hydrogen atom, except that the hole is much "lighter" than a proton.
The orbiting electron/hole pair is called an "exciton". A growing field
of interest in atomic physics is the behaviour of Rydberg (hydrogen-like)
atoms under intense short pulses of an a.c. electric field (i.e. light),
and it would be interesting to do the same in the semiconductor. The electric
field will drive the electron and hole to and fro away from and then towards
each other, and it has been predicted that strange excited states of the
exciton will appear. Effectively the attraction of the charges (i.e. the
Coulomb field) will be overcome by the external a.c. field, and rather
than hydrogenic states the system will have "Floquet" states. We would
like to show that such states can exist in semiconductors using a Free-Electron
Laser FELIX as the source of the teraherz driving field.
