Caustic Beam Research Articles

Spin-wave diffraction, caustic beam emission, and Talbot carpets in a yttrium iron garnet film with magnonic Fabry-Perot resonator gratings

Spin-wave diffraction, caustic beam emission, and Talbot carpets in a yttrium iron garnet film with magnonic Fabry-Perot resonator gratings

Comparisons of ray and finite element simulations of ultrasonic wave fields in smoothly inhomogeneous austenitic welds of thick-walled components

This work addresses the crucial challenges of simulating ultrasonic inspections in welded parts. We focus on the simulation of radiated ultrasonic fields in welded materials, incorporating spatially varying anisotropy and damping properties. Using macroscale descriptions to capture variations in weld properties as a function of grain orientation, two simulation methods are evaluated: a ray method and a hybrid finite element solution within the CIVA software. Simple simulations validate both models, but complexities emerge in real-world scenarios, notably caustic phenomena and beam splitting due to specific grain orientations. The proposed hybrid FE model can therefore provide reliable reference solutions, even for 3D configurations, allowing the biases arising from radius approximations to be quantified.

Magnonic notch filter based on spin wave caustic beams

Here, we study a magnonic crystal made of low-damping yttrium iron garnet that utilizes pseudo-caustic spin wave beams generated from subwavelength square well features arranged in a two-dimensional array. The lattice symmetry and the angle between the caustic beam propagation direction and the applied magnetic field were tailored to optimize the interaction of spin waves with the engineered defects. A prominent, narrow 3 MHz feature with large rejection efficiency is observed in the spin wave transmission spectrum that could be useful as a narrowband notch filter, and time- and space-resolved Brillouin light scattering (BLS) measurements suggest that both caustic interference and edge effects may contribute to this notch feature. Furthermore, the BLS measurements show that caustics are generated efficiently at the laser ablated wells, and by tuning the frequency by 30 MHz, the caustic beam angles and, hence, the details of how the spin wave caustics hit the ablated wells change sufficiently to add and remove caustic beams, which can be used to create additional device functionality. The generation and conversion of caustic beams show promise for applications that require directional energy transport and for magnonic devices.

Laser trapping of cavitation bubbles in liquids

Cavitation has a significant impact on pulsed laser ablation of liquids. At high repetition rates of laser pulses, it tends to hinder laser breakdown and associated processes. Here, we report on the particular case of the cavitation process occurring only at the elevated pulse repetition rate — the formation of a laser trapped bubble. The effect was observed in liquid hydrocarbons (n-hexane and ethanol) and water. The cavitation bubbles generated by picosecond pulses were captured in the caustic beam in front of the laser focus and stay there for a long time (several seconds). After growing up to ∼100μm in size, such a bubble completely blocks the laser radiation and stops all processes in the laser waist. The scenario of laser trapped bubble formation is investigated, the effects of pulse energy and pulse repetition rate are analyzed, and a possible mechanism for the observed effect is proposed. The special importance of laser trapped bubbles for the optimization of laser synthesis of new materials, particularly the linear carbon chains in hydrocarbons, is demonstrated.

Manipulation of polarization conversion and dual foci in a twisted caustic vector optical field in free space

Manipulation of polarization states in a complex structured optical field during propagation has become an important topic due to its fundamental interest and potential applications. This work demonstrates the effect of the caustic and twisting phases on the polarization states of a vector beam experimentally and theoretically. The novel properties of polarization evolution, especially the conversions of different states of polarization (SoPs) in a twisted caustic vector beam, occur during propagation in free space because of the modulation of twisting and caustic phases. The orthogonal polarization components tend to appear on the beam centers of two foci, and the two focal distances are closely related to the caustic and twisting phases. The twisting and caustic phases can manipulate the conversions between linear and circular polarization components that occur during propagation. These results provide a new approach to more complex manipulations of a structured optical field, especially in tailoring the evolution of polarization states and two foci. They may find potential applications in the corresponding field.

The Influence of Bottom Sediment on the Propagation of Caustic Beams in Oceanic Waveguides

Numerical modeling with the use of mode theory was used to investigate the regularities of the spatial (depth and horizontal distance) distribution of the acoustic field intensity formed by multiple interaction of a caustic beam with a layered bottom in a shallow oceanic waveguide with an underwater sound channel open to the bottom. It has been established that at the values of the sound velocity at the upper boundary in the sedimentary layer, less than the sound velocity at the bottom in the water layer, the formation of a multi-beam structure of the acoustic field is possible. It was found that, starting from certain distances, newly formed beams can play the main role in the spatial distribution of the acoustic field intensity.

Caustic spin wave beams in soft thin films: Properties and classification

In the context of wave propagation, caustics are usually defined as the envelope of a finite-extent wavefront; folds and cusps in a caustic result in enhanced wave amplitudes. Here, we tackle a related phenomenon, namely, the existence of well-defined beams originating solely from the geometric properties of the corresponding dispersion relation. This directional emission, termed caustic beam, is enabled by a stationary group velocity direction, and has been observed first in the case of phonons. We propose an overview of this ``focusing'' effect in the context of spin waves excited in soft, thin ferromagnetic films. Based on an analytical dispersion relation, we provide tools for a systematic survey of caustic spin wave beams. Our theoretical approach is validated by time-resolved microscopy experiments using the magneto-optical Kerr effect. Then, we identify two cases of particular interest both from fundamental and applicative perspectives. Indeed, both of them enable broadband excitations (in terms of wave vectors) to result in narrow-band beams of low divergence.

Influence of Bottom Irregularities on Caustic Beam Propagation in Oceanic Waveguides

Influence of Bottom Irregularities on Caustic Beam Propagation in Oceanic Waveguides

Self-accelerating beam dynamics in the space fractional Schrödinger equation

Self-accelerating beams are fascinating solutions of the Schr\"odinger equation. Thanks to their particular phase engineering, they can accelerate without the need of external potentials or applied forces. Finite-energy approximations of these beams have led to many applications, spanning from particle manipulation to robust in vivo imaging. The most studied and emblematic beam, the Airy beam, has been recently investigated in the context of the fractional Schr\"odinger equation. It was notably found that the packet acceleration would decrease with the reduction of the fractional order. Here, I study the case of a general nth-order self-accelerating caustic beam in the fractional Schr\"odinger equation. Using a Madelung decomposition combined with the wavelet transform, I derive the analytical expression of the beam's acceleration. I show that the non-accelerating limit is reached for infinite phase order or when the fractional order is reduced to 1. This work provides a quantitative description of self-accelerating caustic beams' properties.

LED焦散光束的传输特性

LED焦散光束的传输特性

部分相干焦散光束的形成及其相干特性

部分相干焦散光束的形成及其相干特性

Caustic beams from unusual powers of the spectral phase.

Caustic optical beams arising from a spectral phase whose power lies in an unusual range of values less than two are presented. Unlike what happens for conventional phase powers greater than two, it is feasible to generate caustic structures having properties that do not follow the established sorting. For instance, an asymptotic cusp caustic beam having a cusp point at infinity is demonstrated. For the sake of completeness, the caustic beam properties are analyzed within the whole real range of the phase power. Accurate behavior rules between the symmetries of the beam spectral phase and its intensity distribution are found. These findings strengthen the fundamentals and engineering on caustic beams in diverse optical and physical branches.

Dual behavior of caustic optical beams facing obstacles

A full propagation analysis on both fold-type and cusp-type caustic optical beams under various setups of obstructions is theoretically and experimentally performed. It is demonstrated that the self-healing property of caustic optical beams that include the famous Airy beam is a quite relative property. In fact, fold-type and cusp-type beams cannot only behave as self-healing beams by blocking the main intensity peak, but also behave as self-breaking ones in a nonintuitive manner: by blocking a lateral side of the beam without touching the central intensity peak. The regeneration and rupture processes of caustic beams follow a nonlocal propagation dynamic unlike the other conventional beams. Moreover, deep differences between fold and cusp caustic beams are pointed out once facing certain obstructions. The cusp-caustic beam can be broken down by the obstacle placed in a dark zone outside the caustic region, while the fold-type one remains unaltered. This beam rupture confirms the key role of a hidden propagating field in the shadow region for cusp beams that coexist with the evanescent one. The obtained results cast down the established idea that the Airy beam is a robust self-healing beam since any caustic beam can behave in a dual manner depending on the obstruction location. These facts open up different perspectives for the applications in which the self-healing properties of the beam are relevant.

Formation of a weakly diverging caustic beam in an underwater sound channel open to the bottom

The paper studies the patterns manifesting themselves in the formation and propagation of caustic and weakly diverging beams using the geometric acoustic approximation and mode theory applied to the simplest model of an underwater sound channel open to the bottom in the form of the bilinear dependence of the square of the refractive index on depth. It is established that when a vertical array emitting a tonal sound signal is located at a certain critical depth, the multimode caustic beam which forms near the reference ray horizontally departing from its center and which predominates in intensity is simultaneously the most weakly diverging beam. It is shown that the reference ray of such a weakly diverging caustic beam corresponds to the smoothest minimum for the dependence of the length of the ray cycle on their angle of departure from the center of the array and to the depth level of revolution at the critical depth characteristic of the given oceanic waveguide.

Intensity-symmetric accelerating caustic beams

We construct and generate symmetric accelerating caustic beams (ACBs) by using 3/2-order phase-only masks with elliptical contour based on optical caustics and diffraction theory. The symmetric ACBs are a type of bimodal accelerating caustic beam with two quasi-constant intensity peaks, very similar to the combination of two face-to-face Airy-like beams judging by appearance. Their fundamental optical morphology and force properties of particles in ACBs are subsequently provided. The unique optical properties of ACBs can be exploited for practical uses, such as accelerating electrons and clearing micrometer-sized particles as a laser micrometer-sized "water pump" instead of a laser micrometer-sized "snowblower" of accelerating Airy beams.

柱透镜聚焦高阶Bessel光束产生焦散光束

柱透镜聚焦高阶Bessel光束产生焦散光束

Formation of caustic beams in a refractive oceanic waveguide

We consider the patterns in the formation of the spatial interference structure of an acoustic field excited in the near-surface channel of a vertical array consisting of point sources emitting a tonal signal inphase. It is established that when the array aperture is increased to a certain optimal size, only one caustic beam forms in the channel. As the array aperture further increases, the sequential formation of a certain number of beams after the first is observed.

Sending femtosecond pulses in circles: highly nonparaxial accelerating beams

We use caustic beam shaping on 100 fs pulses to experimentally generate nonparaxial accelerating beams along a 60° circular arc, moving laterally by 14 µm over a 28 µm propagation length. This is the highest degree of transverse acceleration reported to our knowledge. Using diffraction integral theory and numerical beam propagation simulations, we show that circular acceleration trajectories represent a unique class of nonparaxial diffraction-free beam profile which also preserves the femtosecond temporal structure in the vicinity of the caustic.

Transformation of Bessel beams by means of a cylindrical lens

The diffraction patterns of a conical lens illuminated by plane and spherical waves have been intensively investigated. A practical method is described to transform the Bessel profile of a zero-order Bessel beam into a caustic one through the use of a combination of a conical lens and a cylindrical lens, which in turn transforms the field generated by the conical lens. The cylindrical lens was illuminated by a zero-order Bessel beam, producing a lips caustic beam. It is shown that this type of wave tends to regenerate during propagation, although the waves are severely perturbed.

Self-healing property of a caustic optical beam

It is well known that Bessel beams and the other families of propagation-invariant optical fields have the property of self-healing when obstructed by an opaque object. Here it is shown that there exists another kind of field distribution that can have an analog property. In particular, we demonstrate that a class of caustic wave fields, whose transverse intensity patterns change on propagation, when perturbed by an opaque object can reappear at a further plane as if they had not been obstructed. The physics of the phenomenon is fully explained and shown to be related to that of self-healing propagation invariant optical fields.