Linear propagation at constant speed in free space is a fundamental characteristic of light. In a recent study published in Communications physics, researchers at Osaka University have discovered the phenomenon of reciprocal propagation of the intensity of laser pulses in free space.
Space-time couplings have recently been used to produce light with adjustable group speed, direction, and trajectory in free space. For example, flying focusing (a moving laser pulse intensity in the extended Rayleigh length), where longitudinal chromatism and temporal beep are combined to control the focus-dependent separation of the spectrum in space and l The location of the spectrum-dependent pulses in time, respectively, has an arbitrarily tunable speed and direction of the propagation group in space and time.
However, in the previous result, the flying focus can only propagate along a certain direction either forward or backward, although the group’s propagation speed can be freely controlled.
In this study, by dramatically increasing Rayleigh length in space and temporal chirping over time, the newly created flying focus propagates along an alternating straight line trajectory in free space. Clear reciprocating flight focus with high spatial resolution is also possible by further increasing the time beep.
“The newly created flying focus propagates along the longitudinal axis first forward, then backward, and finally forward again, showing an alternating rectilinear trajectory in space and time. The forward speed of propagation is the speed of light in a vacuum, while the speed of backward movement is the speed of light in a vacuum. – the speed of propagation is subluminal ”, explains the corresponding author Zhaoyang Li.
This intriguing phenomenon alters the traditional understanding of the propagation of light and can be applied in both fundamental and applied physics.
“For example, in our radiation pressure simulation, it can produce a trapping or pushing force in the reciprocating axis for a small or large sphere, respectively, in the Rayleigh scattering regime,” says Zhaoyang Li.
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