The in-line x-ray phase-contrast imaging method relies on changes in index of refraction within a body to
produce image contrast. In soft tissue, index of refraction variations arise from density changes so that phase
contrast imaging provides a map of density gradients within a body. An intense, short pulse laser beam that is
differentially absorbed by an object within a body will produce a thermal wave with an associated density change
that propagates outwardly from the interface between the object and the body. Experiments are described where
a pulsed Nd:YLF laser is synchronized to an image intensifier to record the effects of the energy deposited by a
We show that the radiation pressure exerted by a beam of ultrasound can be used for contrast enhancement in high resolution x-ray imaging of tissue. Interfacial features of objects are highlighted as a result of both the displacement introduced by the ultrasound and the inherent sensitivity of x-ray phase contrast imaging to density variations. The potential of the method is demonstrated by imaging various tumor phantoms and tumors from mice. The directionality of the acoustic radiation force and its localization in space permits the imaging of ultrasound-selected tissue volumes. In a related effort we report progress on development of an imaging technique using and electrokinetic effect known as the ultrasonic vibration potential. The ultrasonic vibration potential refers to the voltage generated when ultrasound traverses a colloidal or ionic fluid. The theory of imaging based on the vibration potential is reviewed, and an expression given that describes the signal from an arbitrary object. The experimental apparatus consists of a pair of parallel plates connected to the irradiated body, a low noise preamplifier, a radio frequency lock-in amplifier, translation stages for the ultrasonic transducer that generates the ultrasound, and a computer for data storage and image formation. Experiments are reported where bursts of ultrasound are directed onto colloidal silica objects placed within inert bodies.
Recording of an ultrasonic vibration potential when a burst of ultrasound traverses a body containing a colloidal object can be used as the basis for an imaging method. The fundamentals of the theory of signal production and experimental demonstration of the imaging method are given. In a second imaging method, the use of ultrasound to modify x-ray phase contrast images where the ultrasound acts as a kind of "phase contrast" agent used to translate objects in space is demonstrated.
We report on the first measurements of a XANES spectrum of solvated Fe(CO)5 using an ultrafast laser driven plasma x-ray source operating at 2 kilohertz repetition rate. The measured spectrum is compared to theoretical XANES spectra based on DFT structure calculations of the solvated complex. The used x-radiation is generated by irradiating a solid metal target with ultrafast high-intensity laser pulses. The subsequently generated high-density plasma emits x-ray pulses with sub-picosecond temporal resolution and an x-ray spectrum extends to energies far higher than the desired spectral range. Since these spectral components potentially falsify the XANES measurements, they were suppressed by the x-ray optical setup.
Ultrafast high-intensity laser pulses incident upon condensed matter targets can generate high-density plasmas that emit x-ray pulses with sub-picosecond temporal structure, significant spatial coherence, and high brightness at kilohertz repetition rates. Such laser-driven plasma x-ray sources based on solid and liquid metal targets have been developed in our laboratory. Essential performance features are discussed along with a feasibility evaluation for future routine application in chemical research. Laser-driven x-ray sources are usable for ultrafast x-ray diffraction and ultrafast x-ray absorption spectroscopy. X-ray absorption near-edge spectra of solvated transition metal complexes are presented.
Ultrafast molecular dynamics depends on the structure of the solvated molecule before photo-excitation. This solvation structure, in turn, depends on the solute's interaction with the solvent molecules. Furthermore, the solute's vibrational modes and its structure are correlated, solvent dependent, and can be measured by mid-infrared and x-ray absorption spectroscopy. Such measured spectra are presented and correlated with quantum calculations in order to elucidate the solvation environment of various transition metal coordination complexes.
Ultrafast high-intensity laser pulses incident upon condensed matter targets can generate solid-density plasmas that emit x-ray pulses with sub-picosecond temporal structure and significant spatial coherence. Such ultrafast laser-driven plasma x-ray sources based on solid and liquid targets are currently under construction in our laboratory. Performance details at several kilohertz laser pulse repetition rates are discussed. As an application of the temporal structure of laser-generated x-ray pulses, ultrafast x-ray absorption fine structure (UXAFS), currently under development, is discussed. It allows, in principle, to measure the structural dynamics of atoms during a chemical process in solution. An overview over UXAFS is presented and properties of our ultrafast x-ray absorption spectrometer are discussed. First calculations of time dependent UXAFS-spectra for ironpentacarbonyl are presented. Ultrafast molecular dynamics depend on the structure of the solvated molecule at the moment of photo-excitation. This structure depends on the solute's interaction with the solvent. Furthermore, the solute's vibrational modes and structure are correlated, solvent dependent, and can be measured by mid-infrared and x-ray absorption spectroscopy. Such measured spectra are presented and correlated with semi-empirical quantum calculations in order to elucidate the solvation environment of transition metal coordination complexes in various solvents.
Using ultrafast x-ray diffraction from a laser-plasma x-ray source, we have observed coherent photon generation and propagation in bulk(111)-GaAs, (111)-Ge, and thin(111)-Ge- on-Si films. At higher optical pump fluences, ultrafast melting of Ge films is observed.
Optical pump, x-ray diffraction probe measurements have been used to study the lattice dynamics of single crystals with picosecond-milliangstrom resolution by employing a table- top, laser-driven x-ray source. The x-ray source, consisting of an approximately 30 fs, 75 mJ/pulse, 20 Hz repetition rate, terawatt laser system and a moving Cu wire target assembly, generates approximately 5 X 1010 photons (4π steradians s)-1 of Cu Kα radiation. Lattice spacing changes of as small as 1 X 10-3 Å in a few picoseconds have been detected, utilizing Bragg diffraction from GaAs single crystals. Enhancement of the diffraction intensity associated with degradation of the crystals during and after the laser irradiation has been observed, likely due to a transition from dynamic to kinematic diffraction.
Regenerative pulse shaping is used to overcome gain narrowing during ultrashort pulse amplification. We have demonstrated multiple spectral filters for broadening the amplified spectrum. We have produced amplified pulses with an energy of approximately 5 mJ and bandwidths of approximately 100 nm, or nearly 3 times wider than the gain narrowing limit of Ti:sapphire.
Techniques for the production of multiterawatt, sub-20-fs, optical pulses via chirped pulse amplification are discussed. Regenerative pulse shaping is used to control gain narrowing during amplification and an optimized, quintic-phase-limited, dispersion compensation scheme is used to control higher order phase distortions over a bandwidth of approximately 100 nm. Transform-limited, 18-fs pulses of 4.4-TW peak power have been produced in a Ti:sapphire- based, chirped pulse amplification system at a repetition rate of 50 Hz. Extensions to shorter durations and peak powers approaching 100 TW are also described.
We use classical trajectory Monte Carlo (CTMC) simulations to study the ionization of small rare gas clusters in short pulse, high intensity laser fields. We calculate, for a cluster of 25 neon atoms, the ionization stage reached and the average kinetic energy of the ionized electrons as functions of time and peak laser intensity. The CTMC calculations mimic the results of the much simpler barrier suppression model in the limit of isolated atoms. At solid density our results give much more ionization in the cluster than that predicted by the barrrier suppression model. We find that when the laser intensity reaches a threshold value such that on average one electron is ionized from each atom, the cluster atoms rapidly move to higher ionization stages, approaching NE+8 in a few femtoseconds. This 'ignition' process creates an ultrafast pulse of energetic electrons in the cluster at quite modest laser intensities.
Laser-generated, hard x-rays are produced in a > 1018 W/cm2 focus of an ultrashort-pulse laser system. The application of ultrashort-duration, laser-generated x-rays to diagnostic medical imaging is discussed. Time-gated detection allows removal of scattered radiation, improved image quality and possible reduction of patient exposure. Methods for improvement of x-ray yield, design of appropriate drive lasers, and applications to mammography and angiography are also discussed.
Our goal is to watch the evolution of matter on the atomic length scale and on the time scale on which elementary chemical reactions take place. We present initial experiments made in collaboration between UCSD and the INRS laboratory in Canada, on time-resolved ultrafast, 3 ps temporal resolution, near-edge x-ray absorption of gas phase SF6 at 2.4 keV (4.89 A). We can see both the initial presence of the F atoms around the S and their absence after photodissociation produced by pumping with an intense optical pulse. Simulations of ultrafast EXAFS and diffraction experiments are presented. We are constructing an ultrahigh intensity laser to generate ultrafast x-ray pulses from laser-produced plasmas. This laser is especially designed to achieve high average power, short pulse duration and high intensity to produce very high temperature solid density plasmas and ultrahot electrons for ultrafast hard x-ray production at high x-ray photon flux, which should enable us to perform a variety of ultrafast x-ray absorption and diffraction experiments. Finally, we discuss several means to measure the duration of subpicosecond x-ray pulses.