Continuous wave Bessel beams with a strong longitudinal acceleration in free space

A variety of transversely accelerating optical beams, such as Airy, Mathieu, and Weber beams, have been proposed and intensively studied in the past few decades, while longitudinal acceleration of optical beams in free space has been considered much less and mostly for ultrashort optical pulses. In this work, we create two-component continuous wave Bessel beams that exhibit extremely high longitudinal acceleration in free space, with the group velocity changing by a factor of 10 in just a few centimeters of propagation. The beam components are co-propagating interfering optical beams that can have different frequencies and angular spectra. We also demonstrate large-magnitude negative group velocities and zero-group-velocity modes for a two-component beam. The group velocities are measured interferometrically, using a common-path optical interferometer. The measurement results agree well with the theoretical predictions. The presented methods to control and measure the group velocity of light in free space are expected to attract the attention of researchers working on optical interferometry, ultrafast optics, nonlinear optics, and optical tweezers.