We report here two approaches that we have developed recently to help solve the problem of energy crisis and global
warming facing us today. One approach is to use nanoparticles attached to the end of single-walled carbon nanotubes to
catalytically convert CO2 and CH4 into hydrogen and carbon fibers, which can then be used in hydrogen fuel cells and as
the building material in transportation vehicles and many other structures. The second approach is to use silica nanowires
as templates to make nanoscale electrodes to be used in solar cells. The main advantage of this type of solar cells is that
it would be easy to incorporate them directly into glass windows on all the buildings.
A laser driven electron x-ray source (LEXS) emitting tungsten Lα lines has been built based on a high repetition rate, terawatt laser system. The Lα characteristic lines overlap with the absorption edge of Ni near 8333 eV. Using this x-ray source, the x-ray absorption spectra near the absorption edge of different Ni compounds have been measured. We term this new method "ultrafast selected energy x-ray absorption spectroscopy (USEXAS)." It provides an efficient way to study ultrafast reaction dynamics of many metal complexes in solution.
A laser driven electron x-ray source (LEXS) using a high repetition rate, terawatt laser system is described. The laser system has adopted design features that make it more suitable for the generation of hard x-ray pulses. These features include a simplified pumping scheme to reduce cost and complexity, a l/4 broadband regenerative amplifier to support high-energy, short optical pulse generation, and a 50-Hz repetition rate to achieve both desired pulse energy and simple compressor design. Preliminary results on x-ray generation using this system are reported. A new method, ultrafast selected energy x-ray absorption spectroscopy (USEXAS), based on this LEXS is discussed.
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.