TECHNICAL ACTIVITIES 1998 - NISTIR 6268

FUTURE DIRECTIONS

• Ultrafast Soft X-Rays. A table top soft x-ray laser operating at 1000 Hz has been developed to study time-resolved x-ray photoelectron spectroscopy of dissociating and reacting systems. An ultrafast laser system operating on Ti:sapphire at 800 nm produces 2.5 mJ at 1000 Hz, with 75 fs pulse duration. The ultrafast laser is focused into a jet of neon rare gas, also pulsed at 1000 Hz. High harmonics are generated up to about 90 eV photons. The harmonics are separated with a grazing incidence grating and approximately 3 billion photons per sec are estimated thus far at 80 eV. In new experiments, these photons will be used to probe the core level x-ray photoelectron spectra of atoms in dissociating molecules and clusters. (S.R. Leone).

• Single Molecule and Single Quantum Dot Confocal Microscopy. In close conjunction with the NSOM efforts, we are developing new capabilities in high sensitivity detection and spectroscopic characterization of single molecules. This is based on laser excitation and fluorescence detection via scanning confocal microscopy, coupled with a high sensitivity, CCD array spectrometer. Most recently, we have been using this new capability to investigate the photobleaching dynamics of individual dye molecules, green fluorescence protein, and CdSe quantum dots. Our long-range interest is in developing methods to optically tag biomolecules at single molecule sensitivity and high spatial resolution for probing physical properties of biopolymers. Particularly interesting is the observation of multiple time scales for fluorescence from individual quantum dots, ranging from the fast radiative and nonradiative recovery on the ns time scale to so called "blinking" fluorescence intermittency due to multiphoton ionization of single quantum dots on the millisecond to second time scale. (For example, see cover figure for this section.) (D.J. Nesbitt).

• Coherent Phenomena in Optically Dense Material. Coherent phenomena in semiconductors have been the subject of significant research during the last decade due to the development of lasers capable of producing pulses with widths below 1 ps. The general understanding of coherent phenomena is derived from extensive work in dilute atomic vapors. Semiconductors display new phenomena and modifications to well known phenomena. Some of these are due to the large optical density; others are unique to the many-body interactions among mobile carriers. Coherent phenomena are not well understood in dense atomic vapors either. Experiments are being set up to explore coherent phenomena in semiconductors and dense atomic vapors. Comparison of the phenomena in the two media should yield interesting insights. (S.T. Cundiff).

• Nonlinear Spectroscopy of Mixed-Valent Materials. Mixed valent materials have a band of strongly localized f-electrons energetically overlapping a conduction band of itinerant d-electrons. The interaction results in strong electron correlation and the opening of an energy gap. Electron correlation is important in many solids of current interest, including high T_c superconductors, colossal magneto-resistance materials, Mott insulators and Kondo insulators. Recent theoretical results predict that electron pairing should occur, leading to ferroelectric behavior and a breaking of the inversion symmetry present in the crystal structure. This will imbue the material with a second order, nonlinear, optical response. The nonlinear optical response will be resonantly enhanced near the gap energy, which is about 10 meV. Experiments are being set up to search for this nonlinearity using intense pulses of THz radiation. The THz pulses are generated using photoconductive emitters excited by ultrashort optical pulses. (S.T. Cundiff).

• Guided-Atom Optics. A new generation of atom-guiding technology is currently in development. Atoms will be guided by the magnetic fields generated by the electrical currents in a pair of lithographically patterned wires. The atoms will "fly" just a few microns above the substrate, much like the cars of a miniature, magnetically levitated train. This work is an offshoot of the ongoing QPD experiments in which atoms are optically guided inside of hollow glass fibers. The lithographic method used to pattern the "train tracks" will be readily extended to create "switches" (i.e., atomic beam splitters) and "loop-the-loops" (i.e., Sagnac-type circular atom interferometers.) Initial experiments will inject laser-cooled atoms into the guide; second generation experiments will couple-in Bose-Einstein condensates. Ultimately the technology will enable the construction of robust, large-area, atom-interferometers with enormous sensitivity to electric, magnetic, and gravitational fields and to various spatial and temporal gradients of those fields. (E.A. Cornell).

• W.M. Keck Optical Measurement Laboratory. JILA has received a grant from the W.M. Keck foundation to establish an Optical Measurement Laboratory. It will support ongoing NIST research in many areas, including that on Bose-Einstein condensation, ultrafast optical processes, optical nano-material science, and near field scanning microscopy. This state-of-the-art facility will encompass three main areas: optical materials preparation, nanofabrication, and laser characterization. A clean area is being constructed and a suite of instrumentation is being assembled to provide the required new capabilities.

Mission  |  Organization  |  Current Directions  |  Technical Highlights  |  Future Directions

TECHNICAL ACTIVITIES 1998 - Contents

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Online: April 1999