The group velocity of an optical beam in free space is usually considered to be equal to the speed of light in vacuum. However, it has been recently realized that, by structuring the beam's angular and temporal spectra, one can achieve well pronounced and controlled subluminal and superluminal propagation. In this work, we consider multifrequency Bessel beams that are known to propagate without divergence and show a variety of possibilities to adjust the group velocity of the beam by means of designed angular dispersion. We present several examples of multifrequency Bessel beams with negative and arbitrary positive group velocities, as well as longitudinally accelerating beams and beams with periodically oscillating local group velocities. The results of these studies can be of interest to scientists working in the fields of optical beam engineering, light amplitude and intensity interferometry, ultrafast optics, and optical tweezers.
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.
A common drawback of high-resolution optical imaging systems is a short depth of field. In this work, we address this problem by considering a 4f-type imaging system with a ring-shaped aperture in the front focal plane of the second lens. The aperture makes the image consist of nearly non-diverging Bessel-like beams and considerably extends the depth of field. We consider both spatially coherent and incoherent systems and show that only incoherent light is able to form sharp and non-distorted images with extraordinarily long depth of field.
Light sheets are optical beam-like fields with one-dimensional intensity localization. Ideally, the field intensity should be independent of the longitudinal and one of the transverse coordinates, which is difficult to achieve even for truncated light sheets. In this work, we present a general theoretical framework for intensity-interferometric continuous wave (cw) light sheets formed by overlapping the interference fringe patterns of mutually uncorrelated frequency components of the field. We show that the key parameters of the light sheets can be calculated using simple analytical expressions. We propose a practical way to generate such light sheets with the help of prisms and demonstrate numerically the abilities of the method. Both bright and dark light sheets with an exceptionally small thickness and long divergence-free propagation distance are possible to generate. We also show that the transverse profile of the generated light sheets can be shaped by modifying the spectrum of the light. We believe our findings advance the beam-engineering technology and its applications.
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