Optical control of excited-state vibrational coherences of a molecule in solution: The influence of the excitation pulse spectrum and phase in LD690.

Spectral and phase shaping of femtosecond laser pulses is used to selectively excite vibrational wave packets on the ground (S0) and excited (S1) electronic states in the laser dye LD690. The transient absorption signals observed following excitation near the peak of the ground-state absorption spectrum are characterized by a dominant 586 cm(-1) vibrational mode. This vibration is assigned to a wave packet on the S0 potential energy surface. When the excitation pulse is tuned to the blue wing of the absorption spectrum, a lower frequency 568 cm(-1) vibration dominates the response. This lower frequency mode is assigned to a vibrational wave packet on the S1 electronic state. The spectrum and phase of the excitation pulse also influence both the dephasing of the vibrational wave packet and the amplitude profiles of the oscillations as a function of probe wavelength. Excitation by blue-tuned, positively chirped pulses slows the apparent dephasing of the vibrational coherences compared with a transform-limited pulse having the same spectrum. Blue-tuned negatively chirped excitation pulses suppress the observation of coherent oscillations in the ground state.

Resonance Raman and semiempirical electronic structure studies of an odd-electron dinickel tetraiminoethylenedimacrocycle complex.

Resonance Raman studies of Ni2TIED3+ (TIED = tetraiminoethylenedimacrocycle) reveal that many modes couple to the intense electronic transition centered at 725 nm, a feature that is nominally similar to the intense delocalized intervalence absorption bands observed in the same region for Fe2(TIED)L4(5+) and Ru2(TIED)L4(5+) (L is any of several axial ligands). Time-dependent spectral modeling of the Raman and absorption spectra for the nickel compound was undertaken to understand the electronic transition. We were unable to model the Raman and absorption spectra successfully with a single electronic transition, suggesting that the absorption band is made up of two overlapping transitions. Semiempirical electronic structure calculations corroborate the suggestion. Additionally, these calculations indicate that the transitions are in fact ligand-localized transitions, with little metal involvement and no charge-transfer character. Furthermore, the ground-state electronic structure is best described as an identical pair of NiII centers bridged by a radical anion rather than a three-site mixed-valence assembly. Previous EPR studies (McAuley and Xu, Inorg. Chem. 1992, 31, 5549) had indicated primarily ligand character for the radical. The assignments are consistent with the resonance Raman results where the dominant modes coupled to the transitions are assigned as totally symmetric bridge vibrations.

Stable, intense picosecond pulse generation using intracavity GaAs.

A reliable pulsed Nd:YAG laser system has been designed to produce 150-mJ pulses of 260-ps duration with a shot-to-shot standard deviation of ±5.8% and a full-angle divergence of 0.6 mrad. The stability is due to a new cavity-dumped oscillator design that uses an intracavity GaAs plate to stabilize the pulse development and inhibit large fluctuations. The resulting output pulse from the cavity-dumped oscillator is 103 µJ, with an average shot-to-shot standard deviation of ±1.1% and a full-angle divergence of 2 mrad.

Laser mediated release of dye from liposomes.

Liposomes made from phospholipids and containing sulforhodamine dye (1-50 mM) have been irradiated with nanosecond and picosecond laser pulses. Individual liposomes were locally heated by laser absorption of dye dimers during a single laser pulse, and heating was sufficient to release the liposome contents. The extent of dye release produced by a single laser pulse was shown to be quantitatively dependent on several interdependent variables, including dye concentration, liposome size, laser excitation parameters and initial temperature of the dye-liposome system. Fluorescence lifetime data having three components have been obtained and analyzed in terms of three dye environments. Quantitative estimates support a photo-induced thermal mechanism for liposome lysis and release of its contents. These results may be useful for laser induced delivery of therapeutic agents or other applications of lasers in biological systems.

New refractive method for laser thermal keratoplasty with the Co:MgF2 laser.

We have observed corneal curvature changes from laser thermal keratoplasty with a Co:MgF2 laser. We studied corneal curvature changes in rabbits and have identified specific treatment patterns and laser parameters that can correct myopia and astigmatism. These corneal changes, some as large as 8 diopters, have been stable for at least one year, and slitlamp examination demonstrates clear central corneas with normal appearance.

Chrono-coherent imaging for medicine.

We describe a new method for imaging with visible and near visible light inside media, such as tissues, which have strong light scattering. The chrono-coherent imaging (CCI) method is demonstrated in this paper for a transmission geometry where an absorbing object is completely hidden from normal visual observation by light scattering of the media. The resultant images are most similar to X rays, with cumulative transmission showing absorption features and refractive index differences in the media. We discuss laser coherence properties, coherence measurements, the relation of CCI to light-inflight holography, holographic film properties relevant to CCI, a particular optical setup for CCI, the results of a demonstration experiment imaging an absorbing object hidden by light scattering, and an experiment to estimate the clinical applicability of CCI.

Chrono-coherent imaging for medicine.

Imaging inside the body with optical wavelengths encounters problems due to light scattering of tissues and other optical distortions. A description is given of chrono-coherent imaging (CCI), a new method for imaging with visible and near-infrared light that uses coherent image formation and subpicosecond timing to remove the time-delayed scattered light. With CCI it is possible to form images in the presence of the overwhelming amount of scattered light that blocks conventional image formation. The coherent image formation process allows the use of long-duration, high-total-energy, and modest-peak-power pulses to overcome scattering losses. CCI is an extension of light-in-flight holography, which combines the properties of short-duration pulses and hologaphy for various measurements of objects and optical wavefronts.

Single pulse light-in-flight recording by holography.

We have succeeded we believe for the first time, in making a light-in-flight (LIF) recording using one single pulse of a mode-locked frequency doubled Nd:YAG laser. Many experiments made earlier with multiple picosecond pulses have been repeated and the results were the same. New experiments have become possible such as the contouring of a fast moving object and recording through a nonrigid scattering medium. We demonstrate a LIF transmission method for rejecting scattered light which can be developed into a method for diagnostic imaging inside living tissue.

Subnanosecond time-correlated photon counting with tunable lasers.

We present several laser based methods to improve the technique of time-correlated photon counting. Our Ar(+) laser pumped tunable dye laser can be operated in three timing configurations: acousto-optically mode locked, cavity dumped, and cavity dumped-mode locked. Performance characteristics of the laser system in various operational modes are described along with measurement techniques for both gas and liquid phase. The subnanosecond pulses generated by mode locking are extremely stable and they maintain identical pulse shapes over a 6-h period, as shown via photon counting measurements at a 15-psec channel resolution. Our RCA C31034 photomultiplier with a red sensitive GaAs photocathode provides wavelength-independent response to detected fluorescence in both the visible and ultraviolet. The present limit of our apparatus is controlled by the accuracy of deconvoluting fluorescence decay from the finite response width caused by photomultiplier transit time dispersion (0.8 nsec FWHM). Our system stability is sufficient to accurately determine exponential decays as short as 50 psec. Furthermore, we can successfully analyze dual exponential decays such as those arising from solution reorientation times of 390 psec competing with a fluorescence lifetime of 725 psec. Examples of the laser performance are selected from a variety of measurements in the gas phase and from the fluorescent dye rose bengal in the liquid phase.

Dewar design for temperature tuned SHG.

We report a design for a wide temperature range dewar for SHG applications. This Dewar has been used for ADP, ADA, and RDP crystals. We include a schematic for a simple proportional temperature controller.