Scientists discovered the phenomenon of reciprocating propagation of laser pulse in free space

Laser light makes a comeback (literally).

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Optical pulse propagation, including velocity and direction, is a fundamental characteristic for applications like optical information/communication, laser-matter interaction, etc. 

Recently, Spatiotemporal couplings were used to produce light with tunable group velocity, direction, and trajectory in free space. For example, in the method of flying focus, where temporal chirp and longitudinal chromatism were combined, tunable-velocities and even backward-propagation were demonstrated. 

A past study demonstrated that the flying focus propagates along a specific direction, either forward or backward. However, the velocity of the propagating group can be controlled. 

In a new study, scientists from Osaka University discovered the phenomenon of reciprocating propagation of laser pulse intensity in free space. 

Scientists drastically increased the Rayleigh length in space and the temporal chirp in time. This prompted flying focus propagating along a reciprocating straight-line trajectory in free space. 

Corresponding author Zhaoyang Li said, “The newly created flying focus propagates along the longitudinal axis first forward, then backward, and lastly forward again, showing a reciprocating straight-line trajectory in space and time. The forward-propagating velocity is the light speed in the vacuum, while the backward-propagating velocity is subluminal.” 

“This intriguing phenomenon changes the traditional understanding of light propagation and may be applied in both fundamental and applied physics.” 

“For example, in our radiation pressure simulation, it can produce an on-axis reciprocating trapping or pushing force for a small or big sphere, respectively, in the Rayleigh scattering regime.” 

Journal Reference:
  1. Li, Z., Gu, Y. & Kawanaka, J. Reciprocating propagation of laser pulse intensity in free space. Commun Phys 4, 87 (2021). DOI: 10.1038/s42005-021-00590-8