to advance
ultrafast science.
INTENDED OUTCOME AND BACKGROUND
Ultrafast science has traditionally
exploited ultrashort optical pulses with
durations as short as a few femtoseconds.
These pulses provide precise time
resolution and/or high peak powers.
Because they are produced by modelocked
lasers, their frequency comb
spectra can be controlled with the techniques
described in the first strategic
element. In ultrafast science, the control
of the comb spectrum corresponds to
control of the electric field of the pulses
in the time domain. Control of the
electric field of the pulses enables the
observation of unique physical phenomena
and the ability to coherently
synthesize new pulse shapes.
Traditional ultrafast techniques are used
in the Division to study the dynamics of
electron spins in semiconductors and of
biomolecules, providing technologically
important information for both optoelectronics
and biotechnology. We use
phase stabilization to improve measurement
techniques and to develop radical
new ultrafast technologies, such as
"gainless" amplification of femtosecond
pulses in a buildup cavity.
Accomplishments
Passive Optical Cavity
Interactions with a
Femtosecond Comb
We have directly stabilized a mode-locked
laser's optical frequency comb
to a high-finesse passive optical cavity.
We made detailed comparisons of two
stabilization schemes to optimize cavity
stabilization and to develop approaches
for overcoming limitations on transfer
of the cavity's frequency stability to the
microwave domain. We explored the
stability of the frequency comb in both
the optical and the radio frequency
domains.
With an independent, stable CW laser,
we verified that the linewidth and stability
of the frequency comb components
are <300 Hz and 50 fHz/Hz at 1 s averaging
time, respectively, both limited by
the CW laser. Such performance represents
the state of the art in frequency
and phase stabilization of a modelocked
laser. It confirms that a highly
stable, passive optical cavity can directly
stabilize the repetition frequency and
carrier-envelope phase of ultrashort
pulses to a level rivaling that achieved by
CW lasers.
We also demonstrated a technique for
enhancing femtosecond pulses from a
pulse train via coherent buildup in a
high-finesse cavity. By periodically
extracting the intracavity pulse, we
increased the net pulse energy 42-fold
to 70-fold. Pulse energies of >200 nJ
were demonstrated. Greater single-pulse
amplification is achieved using higher
finesse cavities at the expense of a
reduced repetition rate.
We also developed a measurement
protocol for mirror dispersion. The
measurement accuracy is more than ten
times better than the previous state-of-the-art,
white light interferometry technique.
We provided these precise measurements
to the mirror manufacturer,
enabling them to significantly improve
their ability to make large-bandwidth,
low-loss, and low-dispersion mirrors.
Spin Coherence in
Semiconductors
The possibility of using electrons' spin
degree-of-freedom for encoding information
has attracted significant attention.
This technology could lead to
devices analogous to traditional
microelectronics or to others based
on quantum information concepts.
Optical techniques are currently the best
way to prepare and probe spin-coherent
states, as true "spintronic" devices are
still very primitive. Optical preparation
and probing are also likely to be
preferred for quantum information applications.
We are acquiring a basic understanding
of how optical excitations in semiconductors
manifest themselves in traditional
optical techniques that excel at
measuring the g-factor, which determines
how the electron precesses and its
spin coherence time, which limits duration
of any operation. However, interpretation
of the results requires knowledge
of the different optical excitations
created by the incident pulses. In semiconductors,
these include free electron-hole
pairs, excitons (bound electron-hole
pairs), biexcitons (two excitons bound
together, analogous to a hydrogen molecule),
and trions (an exciton bound to
an electron). Trions are particularly
important because they occur in materials
containing excess electrons, which
can have long spin coherence times.
We are exploring ways to control the
number of excess electrons to determine
why their presence increases the spin
coherence time. The most promising
technique is the use of special quantum
wells, where the spatial separation of the
electrons and holes results in a very long
recombination time, which means that
significant densities can be achieved
with modest powers.
Optical Two-Dimensional
Fourier-Transform
Spectroscopy of Excitons
The concept of multidimensional
Fourier-transform spectroscopy was
originally developed in nuclear magnetic
resonance. This powerful technique
excels at elucidating coupling between
resonances.
We have developed an optical two-dimensional
Fourier-transform spectrometer
to study coupling between
optical excitation in semiconductors,
specifically between excitons and electron-
hole pairs. The results are providing
insight into the many-body physics
that underlies the interactions between
the carriers. Specifically, it allows a
direct mapping of the real and imaginary
parts of the self-energy. The self-energy
is a fundamental quantity that
can be directly calculated using manybody
theory, which, in turn, is used to
model optoelectronic devices such as
diode lasers.
First strategic focus |
Second strategic focus |
Third strategic focus |
Fourth strategic focus
"Technical Activities 2004" - Table of Contents |
