Selected Publications
Light-pulse propagation in angularly dispersive systems is explored in the context of a center-of-mass definition of energy arrival time. In this context the time of travel is given by a superposition of group delays weighted by the spectral content of the pulse. with this description the time of travel from one point to the next for a pulse is found to be completely determined by the spectral content, independent of the state of chirp. The effect of sensor orientation on arrival time is also considered. (C) 2001 Optical Society of America.
The arrival time of a light pulse at a point in space is defined using a time expectation integral over the Poynting vector. The delay between pulse arrival times at two distinct points is shown to consist of reshaping via absorption or amplification. The result provides a context wherein group velocity is always meaningful even for broad band pulses and when the group velocity is superluminal or negative. The result imposes luminality on sharply defined pulses.
It has been known for decades that microscopic dust particles can become trapped near the focus of a continuous laser beam when surrounded by ambient gas such as air. In this photophoretic interaction, the laser heats the particle, which interacts with surrounding gas molecules. Trapped particles typically scatter significant laser light as they are suspended in midair and can be easily observed from the side of the beam. We report on the first on-axis images of photophoretically trapped particles together with the laser-beam profile responsible for the trapping. The radial structure of the laser is recorded using 1:1 imaging, where the 1 W beam must be strongly attenuated without introducing distortion. The trapped particle is weakly illuminated using a different wavelength, chosen to transmit through the filters used to attenuate the laser.