Subcycle Optical Response Caused by a Terahertz Dressed State with Phase-Locked Wave Functions.

The coherent interaction of light with matter imprints the phase information of the light field on the wave function of the photon-dressed electronic state. A driving electric field, together with a stable phase that is associated with the optical probe pulses, enables the role of the dressed state in the optical response to be investigated. We observed optical absorption strengths modulated on a subcycle time scale in a GaAs quantum well in the presence of a multicycle terahertz driving pulse using a near-infrared probe pulse. The measurements were in good agreement with the analytical formula that accounts for the optical susceptibilities caused by the dressed state of the excitons, which indicates that the output probe intensity was coherently reshaped by the excitonic sideband emissions.

Nonadiabatic generation of coherent phonons.

The time-dependent density functional theory (TDDFT) is the leading computationally feasible theory to treat excitations by strong electromagnetic fields. Here the theory is applied to coherent optical phonon generation produced by intense laser pulses. We examine the process in the crystalline semimetal antimony (Sb), where nonadiabatic coupling is very important. This material is of particular interest because it exhibits strong phonon coupling and optical phonons of different symmetries can be observed. The TDDFT is able to account for a number of qualitative features of the observed coherent phonons, despite its unsatisfactory performance on reproducing the observed dielectric functions of Sb. A simple dielectric model for nonadiabatic coherent phonon generation is also examined and compared with the TDDFT calculations.

First-principles calculations for multiphoton absorption in α-quartz under intense short laser irradiation.

We present a first-principles description for the electron dynamics and multiphoton absorption in crystalline SiO(2) induced by an optical field. We rely upon time-dependent density-functional theory with adiabatic local-density approximation, and a real-space and real-time method is employed to solve the time-dependent Kohn-Sham equation. Computational results show the absorption of photons to be in excess of the minimum required for electron excitation from the valence band to the conduction band. This is similar to the above threshold ionization in atoms and molecules.

First-principles description for coherent phonon generation in diamond.

We report a first-principles description for coherent phonon generation in diamond based on the time-dependent density functional theory. The time-dependent Kohn-Sham equation is solved in real time in order to calculate the electron dynamics in periodic solid exposed to an ultrashort laser pulse. We find that the calculated forces acting on ions are consistent with measurements as regards the selection rule and the dependence on the laser intensity.

Laser tunnel ionization from multiple orbitals in HCl.

Tunneling, one of the most striking manifestations of quantum mechanics, influences the electronic structure of many molecules and solids and is responsible for radioactive decay. Much of the interaction of intense light pulses with matter commences with electrons tunneling from atoms or molecules to the continuum. Until recently, this starting point was assumed to be the highest occupied orbital of a given system. We have now observed tunneling from a lower-lying state in hydrogen chloride (HCl). Analyzing two independent experimental observables allowed us to isolate (via fragment ions), identify (via molecular frame photoelectron angular distributions), and, with the help of ab initio simulations, quantify the contribution of lower-lying orbitals to the total and angle-dependent tunneling current of the molecule. Our results bolster the emerging tenet that the coherent interaction between different orbitals--which can amplify the impact of lower orbitals--must be considered in tunneling processes.

Influence of laser irradiation condition on a femtosecond laser-assisted tomographic atom probe.

Influence of femtosecond laser pulse condition on the performance of an energy-compensated optical tomographic atom probe has been investigated. The unstable oscillator makes the mass peaks significantly broadened. Double 80fs pulse train with 10ns interval makes the mass peaks slightly shifted to the higher mass side. The mass peak shift corresponds to the fight time of ions triggered by laser pulsing. Chirping ratio for the laser pulses ranging from 80fs to 10ps is controlled by the pulse compressor for the fragile specimens such as oxide dispersion strengthen steel or insulator materials. A first-principle calculation for optical dielectric breakdown in diamond has been successfully demonstrated. It is shown that effective conductive increase has appeared at the laser intensity around 10(13)W/cm(2).

First-principles calculation of the electron dynamics in crystalline SiO(2).

We present a first-principles description for electron dynamics in crystalline SiO(2) induced by an optical field in both weak and intense regimes. We rely upon the time-dependent density-functional theory with the adiabatic local-density approximation, and a real-space and real-time method is employed to solve the time-dependent Kohn-Sham equation. The response calculation to a weak field provides us with information on the dielectric function, while the response to an intense field shows the optical dielectric breakdown. We discuss the critical threshold for the dielectric breakdown of crystalline SiO(2), in comparison with the results for diamond.

Subclinical and reversible hepatic effects of occupational exposure to trichloroethylene.

To detect early hepatic effects of chronic exposure to low-level trichloroethylene (TCE), serum total cholesterol (T-C), high-density-lipoprotein cholesterol (HDL-C) and activities of three serum enzymes [aspartate aminotransferase (AST), alanine aminotransferase (ALT), and gamma-glutamyl transpeptidase (GGT)] were determined in 148 workers (a cross-sectional study) and in 13 workers (a 2-year follow-up study) occupationally exposed to TCE in air. In the cross-sectional study, three exposure groups were defined by urinary total trichloro-compounds (U-TTC) levels of the workers [low-exposure group (n = 49): U-TTC < 10 mg/g creatinine; moderate-exposure group (n = 56): U-TTC 10- < 100; high-exposure group (n = 43): U-TTC > or = 100]. With increasing exposure levels, T-C (P = 0.143 by ANOVA) and HDL-C (P = 0.080 by ANOVA) slightly increased. The exposure, however, had no effect on the activities of the three serum enzymes. In the follow-up study, the fluctuations in U-TTC were well reflected in subclinical changes in HDL-C, AST, and GGT, but not in T-C or ALT. These results suggest that exposure to low-level TCE influences hepatic functions, affecting cholesterol metabolism rather than causing hepatic cell damage, and that these influences are subclinical and reversible. The increases in HDL-C caused by exposure to low-level TCE may be an example of "chemical hormesis" in humans.

Sister-chromatid exchanges in lymphocytes from 12 chromium platers: a 5-year follow-up study.

To detect the mutagenic effects of hexavalent chromium (Cr) in humans, sister-chromatid exchange (SCE) frequency in lymphocytes and urinary Cr was determined in 66 blood-urine paired samples from 12 male Cr-platers during 5 years (1984-1989). Multiple regression of SCE frequency on age, urinary Cr and smoking habits was analyzed in all 66 samples. Neither age (P = 0.204) nor urinary Cr (P = 0.056) was a significant predictor for SCE frequency. Although urinalysis revealed obvious exposure to Cr in the platers, exposure was unable to influence SCE frequency. Smoking habits were a highly significant (P less than 0.001) positive predictor for SCE frequency, and smoking of one cigarette per day was associated with an increase of 0.054 SCEs/cell in SCE frequency. SCE frequency significantly fluctuated from year to year in 3 subjects. The smoking habits of these 3 subjects did not change during the follow-up period. The results suggest that there are other unknown factors influencing SCE frequency in addition to smoking habits.