K.R. Wilson

Generation and application of ultrashort X-ray pulses

Relatively small-scale laser-driven sources of short wavelength radiation covering a range from the extreme ultraviolet to the hard X-ray regime are now available. Because the duration of the X-ray pulses is comparable to, or shorter than the laser pulse width, it is possible to carry out X-ray measurements with picosecond or femtosecond time resolution.

Three dimensional, third harmonic microscopy of living systems

Summary form only given. Recently Barad et al. (1997) showed that the third harmonic generated at the interface of a transparent sample can be used to map the surface of optical glass fibers in an index matching fluid. In the paper, we extend the technique and produce three-dimensional, third harmonic volume images of live biological specimens. Third-harmonic generation (THG) imaging has several advantages for biological imaging: (1) it is a background-free imaging technique requiring no additional staining, (2) it is non-fading in nature, and (3), it can be used with low phase contrast specimens. Because the third harmonic production is localized at interfaces, where there is a change in refractive index or nonlinear susceptibility, the technique as applied to microscopy inherently produces optically sectioned data sets. This allows three-dimensional reconstruction as in traditional confocal microscopy. We present a three dimensional image of a spiral algae formation taken using THG in this manner. In the paper, we demonstrate for the first time to our knowledge, dynamic imaging of live specimens, under moderate NA (0.6) and high NA (1.3) conditions. While significant work remains to fully develop THG microscopy for biological systems, this first demonstration is critical in that it establishes the relevance of the technique to living specimens.

3D-microscopy using third-harmonic generation at interfaces in biological and non-biological specimens

3D imaging of the structure of transparent biological and non-biological samples is demonstrated using third harmonic generation at interfaces within the specimen.

Effect of pulse phase and shape on the efficiency of multiphoton processes: Implications for fluorescence microscopy

Summary form only given. To date, the primary laser variables in ultrashort pulse excitation microscopy which are used to optimize efficiency have been wavelength, average power and peak power. In this paper we introduce two more parameters: pulse shape and phase (or chirp). Normally the pulse shape is usually assumed to be a simple analytical form, such as a Gaussian, and the pulse is assumed to be transform limited (no chirp). However, this also means that the peak power, and consequently intensity at focus have also contained certain assumptions. Thus, knowledge of these parameters is not only useful for maximizing the efficiency of the excitation, but it is also necessary for an exact description of the fields generated at focus. The effects of spectral shape on two-photon fluorescence efficiency were investigated using an acousto-optic pulse shaper to modify femtosecond pulses from a Ti:sapphire laser. By using different shapes, we find that the measured two-photon efficiency can vary by a factor of 2 for differently shaped spectra with the same full-width-half-maximum. We find that these effects are well described by a simple model assuming transform-limited pulses. The fact that even small changes in the spectral wings can significantly affect the efficiency of nonlinear processes has implications for biological multiphoton imaging, where it is desirable to minimize sample exposure to radiation and maximize fluorescence efficiency. In the case of single photon excitation of a fluorophore at high energy densities the fluorescence shows a strong chirp or phase dependence. The method is quite robust and applicable to very large molecules in room temperature liquid.

Two-photon imaging with 15-fs pulses and third-harmonic microscopy imaging techniques

Summary form only given. We have shown that it is possible to produce very short pulses at the focus of a high-NA system. These pulses have been used to produce real-time two-photon images, without the aid of an additional amplifier. The third harmonic is shown to be a useful tool for characterizing the pulse within these systems and to be of sufficient strength that it can be used as an imaging tool as well. The characteristics of the third-harmonic imaging are undergoing further investigation so that this method can be exploited as efficiently as possible.

Feedback Quantum Control of Population Transfer Using Shaped Femtosecond Pulses

Quantum control is extended to complex molecular systems by using experimental feedback to control the acousto-optic tailoring of ultrashort pulses.

New Approaches to Solution Reaction Dynamics: Quantum Control and Ultrafast Diffraction

Much progress has been made in recent years in the theoretical and computational treatments of solution reaction dynamics. On the computational side alone, several hundred papers have been published over the past 20 years as described in a recent review article. Our own work on the molecular dynamics of solution reactions (much of which has been in collaboration with Casey Hynes and Raphy Levine) has covered: I2 and ICN photodissociation in rare gas solution,2–6 atom replacement in rare gas solution,7–10 SN211–15 and SNI16 reactions in water, the effect of rare gas solvent on unimolecular dissociation and IVR,17 parallels to gas phase dynamics,7,8 stochastic models of solution reactions,19 and the link between reaction dynamics and solution structure.20

Femtosecond x-ray diffraction of short-pulse irradiated semiconductors

Summary form only given, as follow. Ultrafast optical-pump, X-ray diffraction probe experiments are providing new ways to study transient processes including the direct observation of the atomic motion by which many solid-state processes and chemical and biochemical reactions take place. Current table-top-terawatt femtosecond laser systems provide an attractive source of few-hundred femtosecond duration bursts of angstrom-scale X-ray radiation with fluxes comparable to standard rotating anode sources. Their compact size enables time resolved structural dynamics to be studied in the small laboratory with temporal resolution better than typical molecular vibrational periods. Ultrafast structural dynamics in crystalline samples are readily studied with such systems and experiments to be discussed in this talk include ultrafast non-thermal solid-to-liquid transition in thin single-crystal Ge-111 films grown on Si-111 substrates; ultrafast non-thermal solid-to-solid transitions in bulk vanadium dioxide from a low temperature insulating phase to a high temperature metallic phase; and harmonic and anharmonic coherent acoustic dynamics in layered Ge-111/Si-111 and bulk GaAs-111 samples. Future improvements in high-average power short-pulse lasers will enable the study of a wider class of materials such as amorphous solids or liquid-phase dynamics of simple molecules, while proposed 4th generation light sources based upon single-pass X-ray free-electron lasers will permit singleshot structural determination of complex biomolecules.

Time-resolved x-ray diffraction study of ultrafast acoustic phonon dynamics in Ge/Si-heterostructures

Using time-resolved x-ray diffraction the ultafast strain dynamics in fslaserexcited Ge/Si-heterostructures has been studied. A fluence dependent, anharmonic damping of the impulsively generated acoustic phonons and vibrational transport across the buried Ge/Si-interface are observed.

Propagation of picosecond acoustic pulses in semiconductor heterostructures probed by ultrafast X-ray diffraction

Summary form only given. We present the first optical-pump X-ray probe study on coherent lattice dynamics in semiconductor heterostructures. We have observed propagation of optically generated picosecond acoustic pulses within crystalline Ge films and into the underlying Si substrate, where 20 femtometer lattice compression was observed.