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
Femtosecond Enhancement Cavities
The use of femtosecond enhancement cavities continues to push the frontiers on wide-bandwidth molecular detections and VUV frequency comb generation. Optical frequency comb-based cavity-ringdown spectroscopy has recently enabled high sensitivity
absorption detection of molecules over a broad spectral range.
We have demonstrated an improved system based on a mode-locked erbium-doped fiber laser source centered at
1.5 μm, resulting in a spectrometer that is inexpensive, simple, and robust. It provides a very large spectral bandwidth (1.45 μm to 1.65 μm) for investigation of a wide variety of molecular absorption features. Strong molecular absorption at 1.5 μm allows
for detection at sensitivities approaching
the level of 1 nL/L. We have performed measurements of the rovibrational spectra for CO, NH3, H2O, and C2H2 with an absorption sensitivity of 10-8 cm-1Hz-½ per detection channel. Spectral resolution has been dramatically improved to less than 100 MHz, easily useful for isotope identification.
Femtosecond enhancement cavities can also be used to reach high intensities,
which are usually achieved by using low repetition rate, expensive optical amplifiers. High-harmonic generation at 136 MHz r epetition rate has been produced with a cavity enhanced, Yb-fiber frequency-comb laser. The intracavity field peak intensity reaches above 3 × 1014 W/ cm2, corresponding to a record high 3.5 kW av erage power. We have demonstrated
laser-induced plasmas and high-harmonic generation in Xe, Kr, and Ar. High-harmonic signals after an Al filter have been observed, thus it is clear that we are now reaching shorter wavelengths in comb generations than ever before.
Figure 7. Two-dimensional Fourier Transform spectra of exciton resonances in a GaAs heterostructure. (a) is experimental data, (b)
is theory within a Hartree-Fock approximation, and (c) is a full theory including terms beyond Hartree-Fock.
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Optical Two-Dimensional Fourier Transform Spectroscopy
An optical two-dimensional Fourier transform
spectrometer has been developed and is being used to study many-body effects in semiconductors. Multidimensional Fourier-
transform spectroscopy was originally developed in nuclear magnetic resonance using radio frequencies. Recently there has been significant work to bring these techniques
to infrared and optical frequencies. Achieving the necessary phase control and stability is a challenge. We have achieved it by using actively stabilized delay lines.
Many-body effects in semiconductors were extensively studied using traditional ultrafast spectroscopic techniques. While the experimental results proved the nonlinear
signals are dominated by many-body effects, they were unable to disentangle the various terms. Optical two-dimensional Fourier transform spectroscopy excels at separating the various contributions to the signals. The separation is in part due to the spreading of the signals in two dimensions
and also because full phase information
can be obtained. The experimentally obtained spectra have been compared to calculations and show good agreement, but only when terms beyond the standard Hartree-Fock approximation are included.
A second-generation spectrometer is under development. It will allow all excitation pulses to be phase locked, not just two, as is the case for the current spectrometer. The new spectrometer will be used for continuing
studies on semiconductors and for studies of atomic vapors and proteins.
Grism-Based, ScalableRepetition-
Rate Ti:Sapphire Amplifier
Working with academia and industry, Division scientists have developed efficient reflection grisms (gratings in contact with prisms) for pulse compression in femtosecond
Ti:sapphire laser amplifiers. These components simplify and miniaturize the optics required for commonly used femtosecond
amplifier systems. A simple and efficient grism-based femtosecond amplifier system that produces 36 fs, 300 μJ pulses at 5 kHz to 15 kHz repetition rates was demonstrated using the down-chirped pulse amplification technique.
First strategic focus |
Second strategic focus |
Third strategic focus |
Fourth strategic focus
"Technical Activities 2005-2007" - Table of Contents |
