Abstract
Space-time (ST) wave packets are pulsed beams in which the spatial frequencies and wavelengths are tightly correlated. Proper design of the functional form of these correlations results in diffraction-free and dispersion-free axial propagation; that is, propagation invariance in free space. To date, observed propagation distances of such ST wave packets has been on the order of a few centimeters. Here we synthesize an ST wave packet in the form of a pulsed optical sheet of transverse spatial width ∼200 μm and spectral bandwidth of ∼2 nm, and observe its diffraction-free propagation for approximately 6 meters. For such ST wave packets, we identify the spectral uncertainty - the precision in associating the spatial and temporal frequencies - as a critical parameter in determining the propagation-invariant distance. We present a design strategy and an experimental methodology that enables further increase in the diffraction-free length.
Highlights
The demonstration of monochromatic quasi-diffraction-free Bessel beams by Durnin et al [1] fueled tremendous interest in devising optical fields that are propagation invariant [2, 3]
We describe the experimental setup used in the synthesis and characterization of the ST wave packets before presenting our measurement results
Design of the ST wave packet The design of a propagation-invariant ST wave packet involves the selection of its critical parameters: (1) the spatial bandwidth ∆kx that determines the transverse spatial beam width; (2) the tilt angle θ of the spectral hyperplane [Fig. 1(a)] that determines the temporal bandwidth ∆λ associated with ∆kx and the nature of the conic section on which the spatio-temporal spectrum lies; and (3) the spectral uncertainty δλ that determines the strength of the spatio-temporal spectral correlations
Summary
The demonstration of monochromatic quasi-diffraction-free Bessel beams by Durnin et al [1] fueled tremendous interest in devising optical fields that are propagation invariant [2, 3]. An experimental strategy for the precise and efficient synthesis of propagation-invariant wave packets in the form of (2 + 1)D pulsed light sheets was devised [19] to produce a variety of wave packets, including hollow beams [19] and non-accelerating Airy wave packets [20] in which one transverse dimension is held uniform. This work has been extended to the synthesis of broadband ST wave packets using refractive phase plates in transmission mode [30], a demonstration of their self-healing properties [31], and a classification of all families of ST wave packets [32] These recent experimental results have been accompanied by several new theoretical investigations [33,34,35,36,37,38,39]
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