|
Quantum Physics Phase Stabilization of Ultrafast Lasers Ultrafast Spectroscopy of Materials New Absolute Gravity Instrument Single Molecule Confocal Microscopy Slit Jet Discharge Supersonic Expansions |
Division Contact: James Faller The discovery of Bose-Einstein condensation (BEC) by JILA scientists in June 1995 has opened up whole new areas of research. JILA is a joint venture of NIST and the University of Colorado. In BEC, atoms cooled to temperatures as low as 10 nanokelvin undergo a phase transition in which a large percentage of the atoms all take on exactly the same quantum wave function, becoming completely indistinguishable from one another. Preliminary studies on low-lying phonon-like excitations will develop into measurements on vortices and viscosity. Thermal behavior near the critical temperature and the kinetics of condensate formation are also promising topics for study. We also will explore the implications of condensate formation for precision metrology, including theoretically predicted coherent atom beams, dubbed bosers. Contact: Eric A. Cornell JILA scientists first observed Fermi degeneracy in a trapped atom gas in September 1999, introducing a new area of research. In the Fermi gas, atoms are cooled to temperatures as low as 100 nanokelvin where quantum mechanical effects can be observed. By cooling fermions, one of two classes of quantum particles, this work fundamentally extends the research area of quantum gases and introduces a new system for study. Future studies will explore the manifestations of quantum statistics in the Fermi gas of atoms, for example, investigating the theoretically predicted fermionic superfluid state whose underlying physics is analogous to superconductivity. Implications of the ultracold Fermi gas for precision metrology also will be explored. Contact: Deborah
Jin The remarkable price reduction of diode lasers, taken with cost-effective frequency stabilization approaches, leads to consideration of possible future widespread exploration of frequency-stabilized diode lasers in a vast range of new applications. Traditional interferometric control systems profit from the laser system's increased performance along with its decreased cost and generated heat. A new concept in development will allow stabilization to produce a constant laser wavelength for interferometric applications. Advanced low noise tilt and displacement sensors now can be designed usefully and may become widespread when the stabilized laser system cost is decreased by one order of magnitude or more. Contact: John L. Hall return
to top of page Generally, the phase of a coherent optical wave cannot be measured directly and can be compared only to the phase of another coherent wave. However, for a sufficiently short pulse, the pulse envelope serves as a phase reference. Our recent work has resulted in the stabilization of the phase with respect to the envelope on a pulse-to-pulse basis of ultrashort (~ 10 femtoseconds) optical pulses. This is having a dramatic impact on optical frequency metrology, as it provides a comb of absolute optical frequencies. Ongoing work is focusing on measuring and stabilizing the absolute phase with respect to the envelope. This will impact coherent control techniques and extreme non-linear optics such as ultrafast X-ray generation. Contact: Steven Cundiff Ultrafast Spectroscopy of Materials Ultrafast lasers provide an excellent tool for materials studies due to their time resolution and high peak power. We are utilizing these aspects in several related projects: studying coherent carrier dynamics in direct gap semiconductors, probing roughness at the Si/SiO2 interface using second harmonic generation, and searching for non-linear optical susceptibilities in doped semiconductors and mixed valence materials at THz frequencies. Contact: Steven Cundiff The biochemical cycle of mechanoenzymes generates a force and a displacement that can be measured at the single-molecule level. The outstanding question is how motor proteins transduce chemical energy into physical motion. The enabling technology is optical tweezers, a focused laser beam that can manipulate micron-sized beads in solution, allowing measurements of position and force in the nanometer and piconewton ranges respectively. Our research focuses on developing assays and precision instrumentation to measure the properties of single DNA-based molecular motors. Enzymatic motion along the DNA is measured by anchoring the enzyme to a surface and monitoring the position of an optically-trapped bead attached to the DNA's distal end. Steps by enzymes along DNA are currently too small to be resolved. We are building apparatus capable of resolving steps as small as 1 nanometer. In tests with stuck beads, steps as small as 3 angstroms have been observed. These technological improvements are being used to measure biophysical parameters such as step size, processivity, stall force, and velocity. This biophysical information elucidates and constrains the possible mechanisms by which proteins move. We are studying three biological systems: lambda exonuclease; RecBCD; and the T7 replication system. These systems represent a series of increasingly complex protein-DNA interactions. Contact: Thomas
Perkins Images of amorphous silicon films taken by a scanning tunneling microscope at various stages throughout the growth process show particles 3 nanometers to 5 nanometers in size forming in the vapor and bonding to the film surface during growth. If these particles can be prevented from forming or reaching the surface, it should be possible to improve the film's ability to convert light into electrical current. We are developing a laser scattering system to detect the silicon/hydrogen clumps as they are forming. Contact: Alan C. Gallagher In JILA, a new determination of the gravitational constant, G, is under way. This experiment measures the change in the beat-frequency of a laser locked to a Fabry-Perot optical cavity when its independently hung mirrors are deflected in response to 500 Kgm of masses, which are positioned appropriately nearby. Contact: James Faller New Absolute Gravity Instrument A new instrument is being developed at JILA for the measurement of the transfer constant g (the free-fall acceleration of gravity). This small and portable instrument makes three measurements per second using a mechanical cam system to effect the release, the catch, and the return while, throughout all of this necessary activity, keeping the center of mass of the entire instrument fixed. This avoids any recoil-related systematic errors in the measurement. Contact: James
Faller Single Molecule Confocal Microscopy By virtue of the enormous size of Avogadro's number, virtually all traditional studies of chemistry, biochemistry, and biology have been based on highly averaged ensemble measurements of many billions of molecules. The combination of coherent laser excitation, high numerical aperture microscope objectives, and high quantum efficiency photon detectors now makes possible the imaging of single fluorescent molecules (e.g., dye molecules, quantum dots, fluorescent proteins) with high signal-to-noise in extremely dilute environments. In addition to potential advancements in the analysis of materials at trace levels, this capability opens up the novel study of photophysical and photochemical kinetics of molecules at the single molecule level. Contact: David Nesbitt Slit Jet Discharge Supersonic Expansions Molecular radicals are extremely reactive species, responsible for almost all elementary chemical processes ranging from atmospheric pollution to internal combustion to semiconductor etching. Their high reactivity also makes them exceptionally hard to produce in sufficiently high concentrations for detailed laser study, especially under low-temperature conditions that greatly simplify the resulting spectroscopic analysis. We recently have developed new methods that combine high densities of pulsed slit supersonic jet expansions with pulsed electric discharges to produce high densities of hydrocarbon radicals (e.g., methyl, ethyl, allyl, propyl, cyclopropyl). Most importantly, these intense radical sources are subsequently cooled in the supersonic expansion down to 10 kelvin to 20 kelvin in rotational/vibrational degrees of freedom, which now makes possible for the first time detailed high-resolution IR laser spectroscopic characterization of these radicals. Contact: David
Nesbitt Quantum Coherence and Precision Spectroscopy Development of new light sources allows study of novel aspects of light-matter interactions and their application to precision control and measurement of light field, atomic and molecular structure, and matter waves. The research work has led to new experimental techniques in the areas of high resolution, ultrasensitive laser spectroscopy, quantum coherence effects in cold atoms and molecules, and optical frequency metrology. One example is the multipath quantum interference effect in cold Rb atoms probed by a set of phase-stabilized optical combs that span an entire optical octave. The equivalent picture of multipulse interference in the time domain gives an interesting variation and generalization of the two-pulse-based temporal quantum coherent control of the excited state wavepacket. These results will be explored in terms of absolute phase control of a femtosecond laser. Application of these technologies to ultranarrow transitions in cold Sr atoms will be pursued for the next-generation optical frequency standards. Contact: Jun
Ye
Date
created:October
1, 2001 |