Laser Research:
to develop the laser as a precise measurement
tool.
INTENDED OUTCOME AND
BACKGROUND
Laser capabilities today are used for a wide variety of industrial
and scientific purposes, ranging from providing the length scale for mechanical
measurements to providing a means to directly connect the optical- and
microwave-frequency domains. The Quantum Physics Division continues a
leadership role in these fields, developing new techniques vis-à-vis
precision lasers and their applications.
In laser research, various schemes are explored for stabilizing lasers and for
using them as frequency standards. Recent work of the Division addresses the
creation, utilization, and study of ultrafast laser pulses, which can be
applied both to produce and control wave packets and to study nonlinear
optical-wave interactions. The evanescent-wave property of light has been
exploited to guide atoms through hollow fibers and prevent them from touching
the sides. The Division has a rapidly developing research program in ultrafast
phase-control and frequency measurements, which are applied to control atomic
and molecular dynamics, as well as to access wholly new types of frequency and
length standards.
One of our senior scientists, Jan Hall (one of JILA's original scientists), is
widely acknowledged to be the "father" of much of the laser
stabilization and associated precision-measurement methods that are used today
by NIST, the international standards community, and leading universities
throughout the world. Our recent hires serve to further strengthen this
activity and to assure NIST of a continuing world-leadership position in this
critical standards and measurement domain.
Accomplishments
Precision Spectroscopy of Cold Atoms and Molecules
We have extended the coverage of iodine-based, optical-frequency standards
toward the shorter wavelength region, from 532 nm to 500 nm. Owing to
the attractive properties of newly discovered resonances, namely narrow
linewidth and high signal-to-noise ratio, we are developing a new generation of
cell-based optical-frequency standards with instability less than 10 fHz/Hz
at 1 s averaging time. We have made absolute frequency measurements on
several of these transitions using the JILA-developed, femtosecond,
frequency-comb technology. We have also discovered new and interesting changes
in the hyperfine interactions of the iodine molecules when the transitions
approach the dissociation limit in the excited state.
Broadband, femtosecond lasers and ultra-high-resolution spectroscopy have been
linked in experiments on laser-cooled rubidium (Rb) atoms. A phase-coherent,
wide-bandwidth optical comb was used to induce multi-path quantum interference
for the resonantly enhanced, two-photon transition in cold Rb atoms. We have
also demonstrated the possibility of using cold atoms to control both degrees
of freedom for the femtosecond-laser system, namely the comb spacing and the
carrier-offset frequency.
Working with the trap dynamics of laser-cooled, alkaline earth, 88Sr
atoms, we have established the first rigorous test of Doppler-cooling theory in
a strictly two-level atomic system. We have also, for the first time,
magnetically trapped the 88Sr atoms in their long-lived, metastable
states, paving the way for a possible evaporative-cooling route toward quantum
degeneracy of the alkaline-earth atoms. Furthermore, we demonstrated the
existence of a sub-Doppler cooling mechanism for the fermionic isotope
87Sr using its nuclear magnetic substructure. 87Sr has
been identified as the best candidate for an ultimate optical-frequency
standard based on cold neutral atoms, due to the existence of an
electromagnetic-field-insensitive, narrow, forbidden transition from
1S0 to 3P0.
Our cold molecule "machine" has started to produce results. The goal
of this project is to explore control techniques on molecules similar to those
applied to atoms. We have successfully produced rotation-and vibration-cooled
OH radicals from a pulsed, supersonic beam source. These polar molecules can
then be guided downstream using high-voltage, electrostatic fields that can be
switched to slow down the translation velocity. We will eventually trap the
cold molecules in an electrostatic trap and perform the first OH cold
collisions with a controlled magnetic field. We believe these kinds of
experiments will mark a new era in physical chemistry.
Precision Frequency Metrology and Ultrafast Science
By achieving the first experimental demonstration of tight synchronization and
controlled phase coherence between independent femtosecond (fs) lasers, we have
opened up a new sub-field in ultra-fast science. The following is a list of
novel experiments performed in the laboratory in the past year.
- Sub-femtosecond ultrafast laser synchronization. The ultimate goal
of this research is to develop an arbitrary lightwave generator. To do this,
one must have a timing jitter below the optical-carrier period. For 800 nm,
this is 2.7 fs. We implemented a wide-bandwidth servo loop to synchronize
two independent mode-locked lasers on a sub-femtosecond time scale, with
1 ms averaging. Even with a detection bandwidth above 2 MHz, we still
see a timing jitter of under 2 fs. This is the lowest timing jitter ever
reported using an active-synchronization scheme, and even passive techniques
have not been shown to be any better.
There are many benefits of using active synchronization (as opposed to passive).
For example, we can arbitrarily set the time delay between the two pulse trains
to any value we choose, with femtosecond-level precision. We have already
demonstrated all-electronic, precision switching between two time delays, with
high speed and no hysteresis. In principle, our active-synchronization
technique could lock together as many different lasers with different gain
media as desired.
- Nonlinear imaging for biological applications. We have reported
another significant breakthrough in ultrafast technology that has potentially a
wide range of applications. We apply our synchronization technique to coherent
anti-Stokes Raman scattering (CARS) spectroscopy, one of the most powerful
methods for acquiring chemically-selective maps of biological samples. With the
availability of tight synchronization, we can now utilize two independent
picosecond (ps) lasers, with great flexibility of wavelength tuning and
pump-power manipulation. We have shown that technical noise associated with
timing jitter can be completely eliminated from the CARS signal, and we have
recorded CARS vibrational images of living cells and polymer beads with
significantly improved quality. This new technical capability will be of great
interest to chemists, materials scientists, and biophysicists.
- Arbitrary wavelength/spectrum generation. We have demonstrated a new
experimental approach for flexible femtosecond pulse generation in the mid-IR,
using difference-frequency-generation from independent Ti:sapphire (Ti:S)
lasers. This work represents a significant step towards realizing an ideal
light source for coherent control of molecular vibrations in the mid-IR region.
The resultant mid-IR pulse train can be easily tuned, with an adjustable
repetition frequency up to 100 MHz. Arbitrary, rapid switching of the
generated wavelength and programmable amplitude modulation are achieved via
precision setting of the time delay between two original pulses. The average
power of the pulses exceeds 15 µW, which to our knowledge represents the
highest power in the > 7.5 µm spectral range obtained from direct
difference-frequency-generation of Ti:S pulses.
- Optical frequency metrology. Having demonstrated last year the first
optical atomic clock, we have now improved its simplicity and reliability. The
optical atomic clock offers unprecedented short-term stability and will provide
better long-term stability as well. We note that our progress has been
acknowledged with a large MURI contract from the Office of Naval Research, with
JILA as the center of this multi-university research project.
Comparison and dissemination of the optical-frequency standards and
atomic-clock signals have now become an urgent need. We have carried out work
in transmitting and comparing highly stable rf- and optical-frequency standards
using two dark fibers already installed in the Boulder Research and
Administrative Network (BRAN). The optical link between our labs at JILA and
NIST Boulder is 6.9 km long, roundtrip. A hydrogen-maser-based rf standard
with an instability of  240 fHz/Hz at 1 s is transferred from
NIST to JILA via this link. This maser-based rf reference, about ten times more
stable than our commercial, cesium atomic clock, is used as the time base for
femtosecond-laser-based optical combs used for absolute optical-frequency
measurements. The maser-based signal is also used to make direct comparisons
against the output of an optical clock, i.e., the repetition frequency of the
femtosecond comb stabilized by an optical transition. This optical-frequency
standard is based on an iodine-stabilized Nd:YAG laser at 1064 nm with an
instability of 40 fHz/Hz at 1 s, which is transferred from
JILA to NIST using the second fiber. With independent femtosecond frequency
combs operating in both laboratories, the absolute optical frequency can be
measured simultaneously with reference to the common maser-based rf standard.
A direct optical-frequency comparison enabled by the fiber link was also
carried out, with results that are clearly superior to the rf-based
measurements. We measured the degradation of the optical and rf standards due
to the instability in the transmission channel and developed methods for active
frequency-noise cancellation over the 6.9 km roundtrip distance.
Optical-frequency transfer instability between JILA and NIST through the BRAN
fiber is now about 3 fHz/Hz at 1 s after noise cancellation. A unique
aspect of the optical phase-noise compensation is that the transit time of the
optical link is comparable to the coherence time of the laser, leading to
interesting coherence effects.
Progress In Cell-Based Optical-Frequency Standards
With the recent breakthrough in optical-frequency measurements using fs-laser
comb technology, it is clear that optically based frequency standards will play
a decisive role in future frequency standards and applications. Right now, the
NIST program in ion-trap systems is the world leader. However, gas-cell
approaches appear capable of providing comparable stability at vastly less cost
and in more compact space, although with somewhat less accuracy potential.
Accordingly, we have developed a full theory of the limits achievable with
compact, gas-cell-based, optical-frequency standards. This shows that another
factor-of-ten improvement should be possible on our iodine-based system, which
has already been shown to be more stable than other available frequency
sources - including the NIST H-maser - for times less than a few
hundred seconds. Use of a still more favorable transition should provide
another five-fold gain. Another major advance likely to improve the long-term
stability is in the technology of distortion-free modulation, where we recently
demonstrated that fringing fields in the Electro-Optic Modulator (EOM) could be
controlled.
Phase Control of Ultrashort Optical Pulses
During the last few years, there has been a remarkable convergence between the
technology of ultrafast lasers and that used to stabilize single-frequency cw
lasers. This has led to significant advances in the fields of optical-frequency
metrology and optical atomic clocks. It also promises to have a large impact on
ultrafast science by making it possible to control the carrier-envelope phase
of ultrashort pulses.
Control of the carrier-envelope phase will allow arbitrary electronic waveforms
(as opposed to intensity waveforms) to be synthesized at optical frequencies.
Such electronic-waveform synthesis will be useful in signal-processing
applications, in coherent control of quantum processes, and in extremely
nonlinear optics.
Before these avenues can be explored, a train of optical pulses with identical
carrier-envelope phase must be generated, i.e., the carrier-envelope phase
coherence must be preserved for times longer than it takes to perform an
experiment. The carrier-envelope phase evolution in the pulse train emitted by
a mode-locked laser is manifest as an offset frequency, f0,
in its spectral comb, with carrier-envelope phase fluctuations causing the
spectrum of f0 to broaden.
Recent results show that, with optimization of the stabilization, the linewidth
of f0 can be below 1 MHz, corresponding to a
carrier-envelope phase coherence time of hundreds of seconds.
Spin Coherence in n-Doped Semiconductors.
The possibility of using the spin degree-of-freedom of electrons for encoding
information has recently attracted significant attention. Proposed
implementations include devices analogous to traditional microelectronic
devices and those 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. In addition, optical
preparation and probing is likely to be useful for quantum-information
applications.
The observation at the University of California (Santa Barbara) of very long
spin-coherence times in n-doped, bulk GaAs provided significant impetus
for our research. We use the technique of Faraday rotation to probe the spin
coherence of such systems and to study how it depends on the density of
optically excited electrons, which are initially spin polarized.
We have shown that the spin-coherence time decreases at high excitation density
and that the electron g-factor changes subtly. The decreased
spin-coherence time is attributed to an increase in spin-spin scattering
amongst the electrons. The variation in the g-factor is presumably due
to its k-dependence, coupled with the change in the average k of
the electrons as the excitation density is changed. This is being tested by
examining several samples with different doping densities. Measurement of the
k-dependence of the g-factor is important since it effectively
broadens the electron-spin precession, thereby decreasing the observed
spin-coherence time.
Complete Measurement of Exciton Scattering in Quantum Wells
Many-body interactions among carriers influence the performance of
optoelectronic devices, such as laser diodes and semiconductor optical
amplifiers. Ultrafast measurements of coherent processes in semiconductors,
using techniques such as transient four-wave-mixing (TFWM), have proven to be
very sensitive to many-body interactions. The results of these measurements
test the fundamental theories used to model devices.
We are using a 3-pulse implementation of TFWM to study excitonic scattering
processes in GaAs/AlGaAs multiple quantum wells. By using a 3-pulse
configuration, we can measure decay of the Raman-like coherence between the
heavy-hole and light-hole excitons. In the same experiment, it is also possible
to observe coherences between the heavy-hole exciton and the ground state, and
between the light-hole exciton and the ground state. By studying these
simultaneously, insight is gained into how coherence is lost during scattering
events. The relative dependence of the scattering rates on the excitation
density and sample temperature helps distinguish carrier-carrier scattering and
phonon-carrier scattering.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2002" - Table of Contents |
