A new feature of this year's conference was two special sessions devoted to critical review lectures given by recognized experts on exciting new developments in laser science, including comments on outstanding problems that need further study. For example, Harvard University's Mara Prenstiss reported on light force manipulation of mesoscopic objects.
Paul Barbara of the University of Minnesota described his recent efforts in applying ultrafast spectroscopy to two key problems: the photophysics of the solvated electron in water, and the energy migration dynamics of molecular aggregates. Laser spectroscopy can also be used to diagnose disease and extract histochemical and histopathological information, using optical fiber contact probes, needles, and endoscopic imaging techniques, according to Michael Field of MIT. He summarized recent clinical results and their biophysical basis, as well as prospects for spectral imaging in two and three dimensions.
Duncan Steel of the University of Michigan reported on optically-induced quantum-coherences in semiconductors, states of excitation that arise from the illumination of a resonant electronic transition with coherent optical radiation. They play a critical role in determining the coherent optical response of semiconductors, and are also useful in the general study of materials, since the decay is specific to the type of coherence and reflects on interactions with phonons, defects, and carriers.
Photon Migration Spectroscopy. Scientists at the University of Illinois' Laboratory for Fluorescence Dynamics have developed a portable non-invasive instrument for measuring tissue oxygenation, marking the first time frequency-domain methods have been used in a portable unit. The instrument has eight inexpensive light-emitting diodes arranged in two rows of four each and these light sources are turned on one at a time in rapid succession. The scattering and absorption coefficient are determined separately from the slope of the phase. In addition, several measurements have been performed in vivo, opening the possibility of numerous applications in medical diagnostics.
Diffusing near-infrared-light probes offer new contrast mechanisms for viewing body function and structure, and scientists at the University of Pennsylvania have made significant progress in the development of regional, low- resolution imagers that use diffusive waves. The technique can also be extended to provide low-resolution spatial information about dynamical fluctuations and chromophore lifetimes.
According to Purdue University's Eva Sevick-Muraca, the development of optically active compounds with near-infrared emission and excitation spectra can be used for non-invasive sensing in tissue. Her research team has used photon migration measurements to illustrate metabolic sensing and imaging based upon probe kinetics. "The decay kinetics of some of these compounds can provide local metabolic concentrations, if kinetics can be measured independent of photon 'time-of-flights,'" she said of the experiments.
Spectroscopy for Sensors and Chemical Analysis. A number of new techniques have been developed for sensors and chemical analysis using ultrasensitive fluorescence detection and spectroscopy. At the University of Tennessee Space Institute, subnanosecond photon bursts and micro-capillary injection into a sheath flow are used for detection of single chromophore molecules in aqueous solutions. Researchers at Los Alamos National Laboratory are exploring sensitive imaging of biological molecules such as fluorescently stained DNA strands, single chromosomes, and protein fragments. And a team at the University of California at Berkeley has successfully applied single molecule fluorescence to capillary gel electrophoresis, which opens up new capabilities for high-sensitivity detection of DNA.
Two fiber optic-based sensor systems have also been developed by researchers at the Naval Command, Control and Ocean Surveillance Center for in situ measurement of contaminants in soil. A laser-induced fluorescence (LIF) sensor for measuring petroleum hydrocarbons, and a Raman-based sensor for measuring chlorinated hydrocarbons, have been integrated into instrumented probes, which are pushed into the ground with a truck-mounted hydraulic system to depths of up to 150 feet. Direct spectroscopic measurements of contaminants in the soil are made through a sapphire window in the probe.
Laser-induced breakdown spectroscopy (LIBS) is a rapid method for determining the elemental composition of materials. Using compact lasers and detection instruments, scientists at Los Alamos have developed field portable instruments to analyze metals in soils and contaminants in air and on surfaces. In addition, LIBS sensors in cone penetrometers are being developed for rapid subsurface screening of metal contaminated sites, and scientists at the U.S. Army Research Laboratory are exploring the use of acousto-optical tunable filters as potential detector alternatives to the standard optical multichannel analyzers used for LIBS detection in the cone penetrometer sensors.
A field-rugged wavelength-tunable, pulsed ultraviolet spectrometer for various forms of fiber optic spectroscopy has been developed by Dakota Technologies. "Our approach emphasizes multi-dimensional data acquisition and analysis, with particular emphasis on the time domain," said Gregory Gillispie of the device. Potential applications include detection of petroleum hydrocarbons in groundwater and in soil by time-resolved laser-induced fluorescence, as well as resonance-enhanced Rama spectroscopy of chlorinated solvents and the detection of gas phase species by resonance-enhanced multiphoton ionization.
Lasers and Spectroscopy for Medical Diagnosis. Researchers at Sandia National Laboratories have developed new instrumentation that measures tissue fluorescence from multiple spots on the cervix. Simultaneous fluorescence measurements from multiple fibers quickly yield intensity maps with very high spatial resolution, according to Sandia's D.R. Sandison. Spectral analysis is used to screen for pre-cancers, and initial clinical results show good agreement between fluorescence measurements, histology, and colposcopic examination.
Optical coherence tomography and microscopy are novel techniques for high-resolution sub-surface imaging in biological media, with potential applications in non-invasive early disease diagnosis, according to Joseph A. Izatt of Case Western Reserve University. He reviewed operating principles and characteristics of these techniques, and presented preliminary images in several tissues of clinical interest during a Friday morning session on lasers and spectroscopy for medical diagnosis.
Lasers for Light Force Manipulation and Microsurgery. In the last few years, "optical tweezers" have evolved from a curiosity to an important tool in biological research, often in combination with other techniques. For example, according to AT&T's Karel Svoboda, his group in the Biological Computation Research Department has used optical tweezers and interferometry to explore the mechanics of single kinesin molecules (a molecular motor).
A team of scientists at the University of California, Irvine's Beckman Laser Institute and Medical Clinic has constructed a new laser "workstation" to manipulate cell structure and function, known as a confocal ablation trapping microscopy system (CATS). The system is comprised of two tunable wavelength trapping beams (tweezers), integrated with a tunable wavelength cutting beam (scissors) and confocal fluorescence microscopy. It has been used to generate two photon- induced fluorescences using the focused trapping beam, demonstrating that two photon and thermal events may occur in the tightly focused laser spot.
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