Download Hamza Kurt, who joined Prof

Dae Sin Kim joined Prof. Citrin’s research group in 2002. His research project is on the
physics, simulation, and design of photoconductive sources of transient terahertz pulses.
Such devices at minimum consist of a high-mobility, short carrier lifetime semiconductor
upon which are deposited metalized electrodes that can be electrically biased relative to
each other. When an ultrafast optical pulse whose photon energy exceeds the bandgap of
the semiconductor is incident on the device, the transient current due to the
photogenerated electrons and holes acts as a source term in Maxwell’s equations leading
to the emission of a single- or few-cycle electromagnetic pulse spectrally extending from
dc to a few terahertz. While the use of photoconductive sources of terahertz transients is
the most widely used table-top generation technique, the approach has hitherto been
limited to the nano or microwatt range, with very poor optical-to-terahertz conversion
efficiencies. Together with Prof. S. E. Ralph of the School of Electrical and Computer
Engineering at the Georgia Institute of Technology and Dr. D. Denison of the Signature
Technologies Laboratory of the Georgia Tech Research Institute, in whose groups device
fabrication and characterization are carried out, Mr. Kim is exploring novel device
geometries that we predict will lead both to
higher
optical-to-terahertz
conversion
efficiencies, as well as to higher terahertz
output powers, in the milliwatt range that is
needed for many applications.
Fig. 1: Example of numerical simulations of
terahertz electromagnetic transients for
different electrode geometries and bias
voltages based on the Monte Carlo-Maxwell
solver. The use of structured electrodes
results in a large electric-field enhancement
near the tip leading to larger carrier
acceleration and thus greater peak terahertz
power (blue vs. green curve).
The simulations are based on a self-consistent
spatially resolved, time-domain solution of the
Maxwell and Boltzmann equations for the
electromagnetic field and the electron and hole
distribution functions. The electromagnetic
field is included within the finite-difference
time-domain method while the carrier transport
is computed using the Monte Carlo method
including all relevant scattering mechanisms
and subsidiary valleys in the bandstructure.
Mr. Kim developed the code from scratch. An
example of his calculations is shown in Fig. 1,
where a comparison of the emitted terahertz
transient is shown for two different electrode
geometries. In addition, Mr. Kim is studying
the effects of changing the geometry of the
optical excitation spot as well as the
incorporation of various metalized structures,
including electrodes, lenses, and antennae.