Abstract
During the process of Bessel beam generation in free space, spatiotemporal optical wave-packets with tunable group velocities and accelerations can be created by deforming pulse-fronts of injected pulsed beams. So far, only one determined motion form (superluminal or luminal or subluminal for the case of group velocity; and accelerating or uniform-motion or decelerating for the case of acceleration) could be achieved in a single propagation path. Here we show that deformed pulse-fronts with well-designed axisymmetric distributions (unlike conical and spherical pulse-fronts used in previous studies) allow us to obtain nearly-programmable group velocities with several different motion forms in a single propagation path. Our simulation shows that this unusual optical wave-packet can propagate at alternating superluminal and subluminal group velocities along a straight-line trajectory with corresponding instantaneous accelerations that vary periodically between positive (acceleration) and negative (deceleration) values, almost encompassing all motion forms of the group velocity in a single propagation path. Such unusual optical wave-packets with nearly-programmable group velocities may offer new opportunities for optical and physical applications.
Highlights
During the process of Bessel beam generation in free space, spatiotemporal optical wavepackets with tunable group velocities and accelerations can be created by deforming pulsefronts of injected pulsed beams
A collimated pulsed beam is reflected by a deformable mirror (DM) to shape its pulse- and phase-fronts from a flat surface to a required axisymmetric surface
A beam splitter (BS) samples the shaped pulsed beam into a parabola telescope, which is used for three purposes: first, to image the shaped pulsed beam into the Bessel beam generation region formed by an axicon for suppressing propagation diffraction; second, to enhance the spatial resolution of the phase-front correction that is limited by the pixel size of spatial light modulator (SLM) by beam reduction; and third, to increase the instantaneous pulse-front variation across the beam aperture by beam reduction
Summary
During the process of Bessel beam generation in free space, spatiotemporal optical wavepackets with tunable group velocities and accelerations can be created by deforming pulsefronts of injected pulsed beams. By adjusting the tilt angle of the plane (kz, ω/c), the group velocity of the optical wave-packet in free space or in transmission materials can be controlled, including all motion forms, i.e. superluminal, subluminal, accelerating, decelerating, and backward-propagation[63,64,65,66,67,68] Another spatiotemporal coupling method is to control the group velocity of the intensity peak of a focused ultra-short pulse within the extended Rayleigh length (named as sliding focus or flying focus) by combining temporal chirp and longitudinal chromatism. When periodically distributed axisymmetric pulse-front deformations are introduced, the group velocity and acceleration of optical wavepackets display periodical variations during propagation, showing an alternate appearance of superluminal−subliminal group velocities and accelerating−decelerating accelerations This unusual optical wave-packet presenting different group velocity motion forms in a single propagation path may provide new opportunities for applications
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