The interaction between laser light and semiconductor
nanostructures is the basis for current and
future optoelectronics-based information technology.
The objective of the Computational Quantum
Optoelectronics (CQP) project at Ames Research
Center is to explore new optoelectronic devices, to
study speed and size limits imposed by fundamental
physics principles, and to design and optimize the
performance of existing devices to meet NASA's
needs in information technology. During FY99 the
project made significant accomplishments in three
areas: comprehensive semiconductor laser simulation,
ultrafast laser modulation with a terahertz
heating field, and terahertz wave generation in
semiconductor quantum wells.
In terms of comprehensive laser simulation, the
focus in FY99 was on the so-called vertical-cavity
surface-emitting laser (VCSEL). Ames researchers
have developed a comprehensive simulation code
using finite-difference methods in time and two-dimensional
space domain. The model takes into
account the quantum-well structure information and
the material composition of a given VCSEL structure
design. The effects of the detailed Coulomb interactions
of charged carriers are also included. Since
researchers directly solve the resulting partial differential
equations numerically, VCSELs of different
designs are treated with the same ease, such as those
with gain confinement or index confinement, or
devices of different current contact shapes. Also,
time-evolution of VCSEL spatial modes on a picosecond
scale are resolved. This type of space-time-resolved
simulation is especially important when
VCSELs are subject to injection current modulation,
as is the case in VCSEL-based interconnects. Figure 1
shows the output laser intensity patterns at four
different pumping levels for a VCSEL with an annular
current contact.
In the area of ultrafast laser modulation, researchers
have investigated the possibility of increasing the
communication bandwidth by utilizing the much
faster process of heating the electron-hole gas in a
semiconductor with an electrical field. Detailed
investigation has shown the feasibility and limitations
of using such an approach. Researchers have investigated
the underlying physical processes of electron-hole
plasma interacting with semiconductor lattice
vibrations when heated by an applied electrical field
with frequency up to a few terahertz. They developed
a detailed model to study laser modulation under
such a terahertz-heating field. The results show that
this method allows a modulation of semiconductor
lasers at frequencies from tens of gigahertz (10 9 Hz)
to 1 terahertz (10 12 Hz). Even though it is a theoretical
result at this stage, the approach indicates some
fundamental advantages of this new modulation
strategy over existing approaches.
Another closely related area of research in the
overall effort in quantum optoelectronics is terahertz
generation. We have investigated two possibilities
using carefully designed quantum-well structures for
achieving terahertz emission through optical pumping
by another laser. The first possibility is an optically
pumped terahertz laser. The second approach is
based on nonlinear optical wave mixing. Ames
researchers have developed a theoretical model and
computer simulation code that allows them to
optimize the quantum-well structure design to
achieve maximum nonlinear optical coefficients.
Systematic theoretical and numerical simulation has
shown the feasibility of generating radiation at a few
terahertz by using this approach. This frequency is
critical for the Earth Science Enterprise's atmosphere
spectroscopy program and in far-infrared astronomy
for the Space Science Enterprise. Efficient and
compact terahertz sources will also find many
commercial applications.
Point of Contact: C. Ning
(650) 604-3983
cning@nas.nasa.gov
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