Non-diffracting Optical Beams Research Articles

Spatiotemporal Helicon Wavepackets

Propagation-invariant or non-diffracting optical beams have received considerable attention during the last two decades. However, the pulsed nature of light waves and the structured property of optical media like waveguides are often overlooked. We here present a four-dimensional spatiotemporal approach that extends and unifies both concepts of conical waves and helicon beams, mainly studied in bulk media. By taking advantage of tight correlations between the spatial modes, the topological charges, and the frequencies embedded in an optical field, we reveal propagation-invariant (dispersion- and diffraction-free) space-time wavepackets carrying orbital angular momentum (OAM) that evolve on spiraling trajectories in both time and space in bulk media or multimode fibers. Besides their intrinsic linear nature, we show that such wave structures can spontaneously emerge when a rather intense ultrashort pulse propagates nonlinearly in OAM modes. With emerging technologies of pulse/beam shaping, multimode fibers and modal multiplexing, our proposed scheme to create OAM-carrying helicon wavepackets could find a plethora of applications.

Arrays of spatially structured non-diffracting optical beams.

Non-diffracting optical beams and their structured versions have been extensively studied, theoretically and experimentally, over the last two decades, rendering important applications in fields such as imaging, microscopy, remote sensing, optical manipulation, free space optics, etc. In this paper, we theoretically construct arrays of non-coaxial structured non-diffracting beams by using the so-called frozen wave method. We also develop techniques based on polarization allocations and apodizations to mitigate undesirable interferences among neighboring beams. Our results can find interesting applications in all fields that benefit from the use of non-diffracting beams.

Method to define non-diffracting optical beams mimicking the shape of simple plane curves.

We theoretically and experimentally demonstrate a method to define non-diffracting beams with different geometries. Our findings constitute an alternative to current methods for finding non-diffracting beams, which rely on the solution of the wave equation in a given coordinate system that has a limited number of possibilities or uses a complicated and time-consuming optimization algorithm. Therefore, the method is easier to follow, because it does not require optimization and allows one to obtain non-diffracting beams mimicking the geometry of simple plane curves. The method could find applications in manipulation of matter with optical waves, such as colloidal and living particles, and in quantum, nonlinear, and atom optics.

Nondiffracting light beams carrying fractional orbital angular momentum.

We investigate a new class of nondiffracting optical beams carrying fractional orbital angular momentum (FOAM Bessel beams), which can be constructed by modifying the phase step of the spiral phase plate in a setup originally for the generation of traditional Bessel beams. The FOAM Bessel beams have entirely stable vortex structures, and form an infinite number of orthogonal and complete subsets for monochromatic scalar light in free space. With the advantages of structural stability and orthogonality-completeness, the FOAM Bessel beams are expected to be useful for study and application in various fields of modern optics.

Generation and three-dimensional characterization of complex nondiffracting optical beams

Nondiffracting optical beams play an important role in contemporary optics due to their special propagation characteristics, i.e., nondiffracting in a diffraction-free zone, shape recovering behind obstacles or self-healing property. Liquid crystal spatial light modulators (LC-SLM) are widely used for generating nondiffracting optical beams in virtue of programmable and dynamic features. In this paper, we propose a complex amplitude modulation technique that can encode any scalar complex fields for generating the complex nondiffracting beams. Before experiment, the phase modulation curve of the phase-only LC-SLM is optimized into being linear in a range of 0-2πby gamma correction in the way of variable binary phase gratings. Then, we experimentally generate the nonaccelerating beams, e.g., two zero-order Bessel beams with variable intensity distributions, and the nondiffracting petal-like beams generated by interfering with two coaxial Bessel beams. By scanning a reflection mirror near the focal region along the optical axis, a stack of two-dimensional images is acquired, and then a three-dimensional intensity profile of the beam is reconstructed with a software. We also experimentally demonstrate a new kind of multi-main-lobe accelerating beam with parabolic accelerating trajectory by modifying the spatial spectrum of classical Airy beam. Compared with the so-called vectorial accelerating beam with multiple main lobes in spheroidal coordinates, our generated two-main-lobe accelerating beam has a very high energy efficiency. The self-healing property of the two-main-lobe accelerating beam is also demonstrated. The presented technique can generate a variety of complex nondiffracting optical beams rapidly and obtain their three-dimensional intensity distributions accurately, which has potential applications in the fields of optical microscope, optical date storage, optical trapping, optical micromachining, etc.

Modeling the spatial shape of nondiffracting beams: Experimental generation of Frozen Waves via holographic method

In this paper we experimentally implement the spatial shape modeling of nondiffracting optical beams via computer generated holograms reconstructed optically by spatial light modulators. The results reported here are an experimental confirmation of the so-called Frozen Wave method, developed a few years ago. Optical beams of this type have potential applications in optical tweezers, medicine, atom guiding, remote sensing, etc.

Counterpropagating nondiffracting beams through reflection gratings

We investigate the counterpropagation of paraxial non-diffracting optical beams through a medium hosting a bulk reflection grating in the quasi-Bragg matching condition. The impact of the relative magnitude of the Bragg detuning parameter and the grating depth on the plane wave dispersion relation allows us to identify three distinct regimes where counterpropagation and interaction of nondiffracting beams show qualitatively different features, encompassing longitudinally invariant, periodic or exponential intensity profiles. In one of the identified regimes the dispersion relation is not monotonic and the consequent "longitudinal degeneracy" allows the investigation of new class of nondiffracting beams characterized by a double spectral ring profile.