R.J.D. Miller

Diffractive optics-based six-wave mixing: Heterodyne detection of the full χ(5) tensor of liquid CS2

This work exploits the passive phase stabilization of diffractive optics to implement heterodyne detection of the complete χ(5) tensor of liquid CS2 as an example of a simple liquid. This approach permits the use of two different colors for the excitation, probe, and detection beam protocols and enables full optimization of the signal with respect to discrimination against lower order cascaded third-order responses. This work extends the previous study of polarization selectivity, in combination with heterodyne detection, to all independent polarization components to provide further insight into the origins of the fifth-order response and its connection to the multitime correlation of the liquid dynamics. The characteristic feature that clearly distinguishes the direct fifth-order response from lower order cascades is the pronounced ridge along the τ4 axis (probe pulse delay) with very rapid decay along the τ2 axis (excitation pulse delay). This observation is in contrast to recent related work using one-...

Myoglobin dynamics: Evidence for a hybrid solid/fluid state of matter

Abstract The dynamics of carboxy-myoglobin (MbCO) in water are studied from photodissociation of the ligand to bimolecular recombination over 12 decades in time to fully characterize the protein functions. Heterodyne-detected transient grating spectroscopy is used to resolve the dynamics of the protein from 10xa0ns to a few ms and provides a direct observation of the ligand escape. The process is well described by a bi-exponential function with decay rates of 50 and 725xa0ns at 20°C, suggesting that ligand escape occurs via a well defined pathway. Transient absorption in the Q-band (550–630xa0nm) also reveals the sensitivity of the electronic transition to the ligand motions. The dynamic range is extended by 10 6 through femtosecond coherence spectroscopy with 7xa0fs pulses to enable the observation of vibrational modes of energies >1500xa0cm −1 . The power spectrum is calculated by singular value decomposition and vibrational modes involved in the photodissociation are directly observed. The picture that is emerging is that proteins couple solid-like domains to fluid regions to facilitate functions and transport of ligands in and out of the protein to the active site.

Femtosecond electron diffraction: an atomic level view of condensed phase dynamics

We use femtosecond electron diffraction to capture the disordering of the solid and the emergence of liquid structure during strongly-driven laser melting. We discuss the impact of femtosecond pulse propagation on bright electron source design.

Femtosecond Laser Effects on Osseous Tissues

We have investigated the effects of femtosecond laser irradiation on living bone samples and demonstrated intact enzymatic activity on the surface of cells immediately adjacent to cells removed by laser irradiation suggesting no thermal damage

Femtosecond liquid dynamics studied by two-dimensional Raman spectroscopy

Lacking the symmetric order of solids or the isolation of gases, liquids are intrinsically more complicated than the other states of matter due to the inherent randomness and the subtle interplay of various many-body interactions. In understanding the dynamics of liquids in chemical processes, it is sensible to look first at dynamics of pure liquids. The timescales, on which interesting phenomena occur, span many orders of magnitude, but the basic interactions are fundamentally determined by vibrational periods, and these are generally sub-picosecond. Liquids motions are characterized by frequencies that are smaller than typical intramolecular bond vibrations, and are generally in the range from 10-500 cm -1 . Though tremendous recent progress has been made in the study of intramolecular vibrations, their couplings and even hydrogen bonded systems, these approaches yield only a local picture of the liquid. To build a picture of a solvent, one needs to focus on the larger scale dynamics involving many solvent molecules, and direct the attention towards collective dynamics. However, this is a conceptually challenging prospect that places significant demands on the state of the art of both theoretical descriptions and experimental techniques.

Diffractive-optics based fifth-order Raman spectroscopy of ultrafast liquid dynamics

Summary form only given. With a single diffractive optic it is possible to create a beam geometry which significantly phase mismatches the cascaded intermediates while maintaining near-perfect phase matching of the direct, fifth-order signal. Additionally, the diffractive optic provides passive phase-locking (/spl sim//spl lambda//50) necessary to implement heterodyne detection by interfering a reference local oscillator with the generated signal field. We present studies of liquid CS/sub 2/ where, in addition to the phase-contrast inherent in a heterodyne measurement, the effects of other variables, such as polarization and pathlength, distinguish the direct fifth-order response from the lower-order cascades.

Diffractive optics-based heterodyne detected four-wave mixing studies of protein dynamics: insights into ligand escape and cooperativity in heme proteins

Summary form only given. The relationship between molecular structure and function is of fundamental importance for understanding biological systems. The heme proteins hemoglobin and myoglobin provide ideal model systems for investigating this relationship because their structure and function are well characterized. In addition, they are amenable to optical probes, allowing their functional processes to be initiated by photodissociation. Previous studies on the femtosecond timescale have characterized the dynamics of myoglobin from femtoseconds to nanoseconds. The current work extends these studies to the millisecond regime to capture the full range of functionally relevant motions. These motions are often small and require a highly sensitive spectroscopy for their study. Diffractive optics-based four-wave mixing provides the sensitivity needed to observe changes in radius of <0.001 /spl Aring/. The use of diffractive optics facilitates the separation of Real and Imaginary parts of the /spl chi//sup 3/ signal by providing the required beam geometry for mixing the signal with a reference beam. In addition it offers passive phase-stabilization. A novel detection method that exploits the symmetry of the four-wave mixing experiment has been implemented to provide automatic isolation of the Real part of the signal. This simplifies the interpretation of the data by obviating the need to identify the Imaginary part of the signal. Further improvement in the signal-to-noise is an added benefit of this method.

Diffractive optics-based nonlinear spectroscopy: application to the study of deterministic protein motion

Summary form only given. Biological systems constantly transduce various forms of chemical energy into functions in which the inherent response of the system operates at the edge of stability. Excursions from the stability region lead to denaturation; whereas small fluctuations about the stability point lead to highly correlated responses that behave in a deterministic fashion with respect to the function of the system. Exactly how is the bond energy directed in such a complex system and how has the system evolved to minimize entropic losses in conversion efficiency? We have used the oxygen binding heme proteins as model systems for studying the coupling of reaction forces to functionally relevant motions; i.e., structural transitions important to the self regulation of oxygen binding and transport. Since the forces involved become spatially distributed over an enormous number of degrees of freedom, the net relative motions can be exceedingly small (<.1 /spl Aring/). A very sensitive method is needed to detect these motions and the time resolution must be sufficient to follow from the very first events of bond breaking to full relaxation. The use of diffractive optics for the implementation of heterodyne detected grating spectroscopy has recently been demonstrated. The diffractive optic also generates tilted phase fronts to provide true femtosecond time resolution in noncollinear geometries. This approach has sufficient time resolution and sensitivity to follow the mass displacement, as connected through changes in the material index of refraction, to address this issue.