Pushing and Pulling Ponderomotive Forces in Wavepackets and Beat Waves

TL;DR

This paper analyzes how ponderomotive forces act on particles within wave packets and beat waves, revealing mechanisms for pushing or pulling particles, including applications to optical and acoustic tractor beams.

Contribution

It introduces a detailed analysis of ponderomotive forces on particles in various wave packet configurations, including beat waves, highlighting new ways to generate pulling forces.

Findings

Particles can be pushed away or pulled toward the wave source depending on wave characteristics.

Beat waves can emulate periodic pulses that induce either forward or backward particle motion.

A simple mechanism for optical and acoustic tractor beams is demonstrated.

Abstract

We consider ponderomotive forces acting on small particles in propagating wave packets (pulses). Specifically, we analyze simple point particles as well as composite dipole and dumbbell particles in the fields of forward-propagating (parallel phase and group velocities) and backward-propagating (antiparallel phase and group velocities) wave packets. Depending on the characteristics of the wave packet, particles may be pushed away from the wave source or pulled toward it. We also examine particle dynamics in the field of a beat wave generated by two forward-propagating waves with slightly different frequencies. Such a beat wave can emulate a periodic sequence of either forward- or backward-propagating pulses. In particular, this provides a simple mechanism for realizing pulling forces as employed in optical and acoustic `tractor beams'.