Helico-conical optical beams self-heal

Abstract

An optical beam is said to be self-healing when, distorted by an obstacle, the beam corrects itself upon propagation. In this Letter we show, through experiments supported by numerical simulations, that Helico-conical optical beams self-heal. We observe the strong resilience of these beams with different types of obstructions, and relate this to the characteristics of their transverse energy flow. © 2013 Optical Society of America OCIS codes: 140.3300, 260.6042, 260.0260, 070.2580. The study of the self-healing properties of beams is of great interest in optics [1][2][3][4][5][6]. An optical beam is said to be self-healing when, after propagation, its transverse intensity profile is hardly affected by a small perturbation-a block-that has been placed in its path [1][2][3][4][5]. The surge of interest in self-healing beams is bouyed mainly by its range of applications; self-healing can be advantageous, for instance, in beam propagation through scattering and turbulent media, and in optical manipulation [7,8].Optical beams that exhibit self-healing include Bessel beams (BBs) [6,7,9] [10], and some forms of LaguerreGaussian (LG) beams [3]. In the case of the BBs and Airy beams, self-healing happens at a relatively small propagation distance, while LG beams self-heal at a distance of the order of the Rayleigh length [2,3]. Self-healing is independent of the diffracting nature of the beams, as shown by caustic [4] and LG beams [3].In this Letter, we present another set of beams that self-heal: the Helico-conical optical beams (HCOBs). The main difference between these beams and other self-healing beams is the nonseparability of their radial and azimuthal phases [11]. HCOBs posses a phase ψ that is the product of a helical phase and a conical phase: ψr; θ ℓθK − r∕r 0 ; where ℓ is the winding number around the azimuth angle θ, r 0 normalizes the radial coordinate r, and K takes either the value 0 or 1. At the far field, the intensity profile of these beams resembles a spiral, with K 1 HCOBs having a more pronounced head near the center of the beam axis compared with the K 0 HCOBs. Recently, a K 0 HCOB was reported to cause a spiral motion to a particle along its path [12,13], a three dimensional motion that combines phase gradient with intensity gradient forces [14].It could be argued that, since HCOBs have conical phases, they should behave similar to BBs. In fact, the HCOBs are more likely to be compared with fractional higher-order BBs because of their similar intensity distributions [15]. Joint to this is the fact that HCOBs consist of strings of optical vortices upon propagation [16].However, the far-field intensity pattern of experimentally generated BBs, or any superposition of it, resembles a circle or a δ-ring [17], while HCOBs are spirals in the far field, and not rings when compared to BBs [11]. An important question then arises: Can HCOBs self-heal?Here, we provide evidence that an HCOB reconstructs its intensity profile at a relatively short propagation distance after a small perturbation is placed in ...

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