Terahertz emission by diffusion of carriers and metal-mask dipole inhibition of radiation.

Terahertz (THz) radiation can be generated by ultrafast photo-excitation of carriers in a semiconductor partly masked by a gold surface. A simulation of the effect taking into account the diffusion of carriers and the electric field shows that the total net current is approximately zero and cannot account for the THz radiation. Finite element modelling and analytic calculations indicate that the THz emission arises because the metal inhibits the radiation from part of the dipole population, thus creating an asymmetry and therefore a net current. Experimental investigations confirm the simulations and show that metal-mask dipole inhibition can be used to create THz emitters.

Numerical simulation of optical Stark effect saturable absorbers in mode-locked femtosecond VECSELs using a modified two-level atom model.

The interaction of an optical pulse with a quantum well saturable absorber is simulated using a semi-classical two-level-atom model which has been modified to approximate spectral hole burning in the carrier distribution. Saturable absorption behaviour is examined in the limit where pulse duration approaches the carrier-carrier scattering time. For long pulses bleaching dominates the absorber response but as the pulse duration approaches the carrier-carrier scattering timescale an additional pulse shaping mechanism becomes active, allowing the absorber to continue to shorten pulses beyond the limit set by bleaching. Examination of the spectral and temporal absorption profiles suggests that intense pulses experience additional pulse shortening from the optical Stark effect.

Inhibition of actin filament movement by monoclonal antibodies against the motor domain of myosin.

Conformational transitions in defined regions of the motor domain of skeletal muscle myosin involved in ATP hydrolysis, actin binding and motility were probed with monoclonal antibodies. Competition binding assays demonstrate that three different monoclonal antibodies react with spatially related sites on the myosin heavy chain. One recognizes a sequential epitope between residues 65 and 80 and has no effect on actin filament movement in an in vitro motility assay despite tight binding to myosin and acto-S1. The other two monoclonal antibodies react with sequential epitopes between residues 29 and 60 and both inhibit actin filament movement. A fourth monoclonal antibody reacts with the N-terminus of the heavy chain (residues 1-12) at a spatially distinct site on the myosin head and also inhibits actin filament movement. These four monoclonal antibodies have been mapped by immunoelectron microscopy to the large, actin binding region of the myosin head; however, the antibody binding sites remain accessible on rigor complexes of acto-S1. Thus, this group of monoclonal antibodies identify sequential epitopes in a mobile segment of the myosin heavy chain. In addition, two conformation-sensitive monoclonal antibodies are described that are affected by ATP and actin binding to myosin S1, and display distinct and marked inhibitory effects on actin filament movement. In contrast, an anti-light chain monoclonal antibody that binds near the myosin head-rod junction has little effect on the number and velocity of moving actin filaments. These results identify mobile regions on the myosin head that are perturbed by antibody binding and that may be linked to force production and motion.