RESUMEN

A phase grating that selectively amplifies diffraction orders that are multiples of a determined integer is designed. For the proposed grating, multiples of the fourth order are enhanced. These results are supported by experiments. The structure is inscribed in the volume of a lithium niobate crystal by employing the femtosecond laser pulse micro-machining technique. A model based on the Raman-Nath behavior of the grating predicts a diffraction efficiency enhancement for those selected orders. Moreover, it is observed that by changing the incidence angle allows transferring energy from multiples of fourth orders to multiples of three. These findings have potential applications in optical spectroscopy and optical communications as well as for photonic devices in which a controlled energy exchange between orders is necessary. The basic wave nature of the mentioned effect allows finding a counterpart in different wavelength ranges of the electromagnetic spectrum.

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A current challenge in a caustic beam design is to tailor the intensity distribution along the curved trajectory. To address this matter, we present a robust theoretical framework that relates the propagated complex wave field amplitude with the input spectral signal encoded onto a spatial light modulator which is suitable for fold-type monotonic trajectories as well as for cusp-type nonmonotonic trajectories. Specifically, we derive a general closed-form expression that relates the field amplitude along the beam trajectory with the spectral amplitude and the third derivative of the spectral phase for both monotonic and nonmonotonic curved trajectories. This proposal is suitable for direct experimental implementation in a Fourier transform scheme around the focal region, allowing straightforward beam intensity design by selecting the proper spectral amplitude and phase while preserving the beam trajectory. Experimental results from the famous cubic spectral phase support the theoretical predictions. This research lays the foundation for engineering the intensity of curved beams, which can be useful in applications where a specific modulation of the intensity is required over specific regions of the trajectory such as in optical trapping and laser micromachining.

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We present here a theoretical analysis of the interaction between an ideal two-level quantum system and a super-oscillatory pulse, like the one proposed and successfully synthesized in [J. Opt.23, 075604 (2021)JOOPDB0150-536X10.1088/2040-8986/abfedf; arXiv:2106.09192 (2021)]. As a prominent feature, these pulses present a high efficiency of the central super-oscillatory region in relation to unavoidable sidelobes. Our study shows an increase in the effective bandwidth of the pulse in the super-oscillatory region, and not only the appearance of a local frequency higher than its highest Fourier-frequency component, as in the usual description of the phenomenon of super-oscillations. Beyond introducing the concept of effective super-bandwidth, the presented results could be relevant for experimental applications and opening new perspectives for laser-matter interaction.

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A controllable manipulation of the energy distribution of caustic beams possessing rectangular symmetry is presented. The beams are designed from the spectral phase by adding a linear and/or quadratic perturbation having forced symmetry. This approach breaks the overall caustic structure into branches, allowing a fully controllable displacement of each branch. The caustic breaking leads to peculiar propagation configurations for rectangular beams. Among them, we highlight the abruptly autofocusing beam, which until now was exclusively associated to caustic beams with circular symmetry. Thereby, the abruptly autofocusing effect can be yielded for one-dimensional light sheets, contrary to what happens for circularly symmetric beams. The theoretical predictions are supported by experiments. Besides, the focus width of such rectangular beams can be reduced beyond the standard diffraction limit.

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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.

RESUMEN

In this Letter a new class of light beam arisen from the symmetrization of the spectral cubic phase of an Airy beam is presented. The symmetric Airy beam exhibits peculiar features. It propagates at initial stages with a single central lobe that autofocuses and then collapses immediately behind the autofocus. Then, the beam splits into two specular off-axis parabolic lobes like those corresponding to two Airy beams accelerating in opposite directions. Its features are analyzed and compared to other kinds of autofocusing beams; the superposition of two conventional Airy beams having opposite accelerations (in rectangular coordinates) and also to the recently demonstrated circular Airy beam (in cylindrical coordinates). The generation of a symmetric Airy beam is experimentally demonstrated as well. Besides, based on its main features, some possible applications are also discussed.

RESUMEN

Negative propagation is an unusual effect concerning the local sign change in the Poynting vector components of an optical beam under free propagation. We report this effect for finite-energy Airy beams in a subwavelength nonparaxial regime. This effect is due to a coupling process between propagating and evanescent plane waves forming the beam in the spectral domain and it is demonstrated for a single TE or TM mode. This is contrary to what happens for vector Bessel beams and vector X-waves, for which a complex superposition of TE and TM modes is mandatory. We also show that evanescent waves cannot contribute to the energy flux density by themselves such that a pure evanescent Airy beam is not physically realizable. The break of the shape-preserving and diffraction-free properties of Airy beams in a nonparaxial regime is exclusively caused by the propagating waves. The negative propagation effect in subwavelength nonparaxial Airy beams opens new capabilities in optical traps and tweezers, optical detection of invisibility cloacks and selective on-chip manipulation of nanoparticles.

Asunto(s)

RESUMEN

The limits of the paraxial approximation for a laser beam under ABCD transformations is established through the relationship between a parameter concerning the beam paraxiality, the paraxial estimator, and the beam second-order moments. The applicability of such an estimator is extended to an optical system composed by optical elements as mirrors and lenses and sections of free space, what completes the analysis early performed for free-space propagation solely. As an example, the paraxiality of a system composed by free space and a spherical thin lens under the propagation of Hermite-Gauss and Laguerre-Gauss modes is established. The results show that the the paraxial approximation fails for a certain feasible range of values of main parameters. In this sense, the paraxial estimator is an useful tool to monitor the limits of the paraxial optics theory under ABCD transformations.

Asunto(s)

RESUMEN

A wave propagation analysis carried out in two kinds of q-plates having topological unit charge (1-plates), one plate with a radial orientation of the optical axis and the other with an azimuthal one, reveals that these devices admit nondiffracting Bessel beams as exact solutions of the vector Helmholtz's equation. The phase shifts between the ordinary and the extraordinary waves in both structures were found to be different. The polarization-scrambling term in the Helmholtz equation is responsible for such differences, emphasizing that this term cannot be dropped in radial plates, contrary to the azimuthal case. A phase shift analysis suggests that these plates are relevant for the control of nonconventional polarization states of light. In this way, a novel redistribution of the spin-orbital angular momentum of these nondiffracting beams passing by the 1-plate is demonstrated, which could be useful for applications in classic and quantum regimes.

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A useful relationship among the parameter called paraxial estimator [Opt. Lett. 32, 927 (2007)] and two parameters concerning the beam quality, the M2 factor, and the beam spot size was derived. This relationship allows one to quantify the paraxiality for a monochromatic laser beam from standard measurements extending the applicability of such an estimator for real beams even if the beam profile shape does not have a closed-form representation. Hence, the paraxial estimator might be a suitable tool for deepening the analysis on the quality of a laser beam.

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This Comment shows that the parameter called the degree of paraxiality of a monochromatic beam [Opt. Lett.33, 1360 (2008)] was derived using misleading criteria and that the results in that Letter might lack physical meaning.

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A beam width measure based on Rényi entropy was introduced by Luis [Opt. Lett 31, 3644 (2006)]. That one-dimensional analysis was limited to beam profiles with rectangular symmetry. In this Letter, we derive a general Rényi beam width measure that accounts for the diffraction properties of beams with profiles of arbitrary symmetry. We also show that the square of this measure has a quadratic dependence as a function of the propagation coordinate, so that it can be applied to propagation through arbitrary ABCD paraxial systems. The Rényi beam propagation factor, here introduced, is discussed in examples where the M(2) factor seems to have a limited effectiveness in describing the beam spreading.

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We use a method based on the simultaneous combination of the propagation operator and the Fourier transform with arbitrary index in propagating the transverse component of a nonparaxial beam in free space from an arbitrary initial transverse field structure. Being an iterative method, this approach can easily be implemented computationally. As an example of its efficiency, we derive the closed-form nonparaxial corrections to a Bessel-Gaussian beam, showing that our results differ strongly from those reported previously. The validity of our approach is supported by an analysis of the paraxiality estimator recently introduced in the literature.

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The validity of the paraxial approximation for laser beams in free space is studied via an integral criterion based on the propagation invariants of Helmholtz and paraxial wave equations. This approach allows one to determine the paraxial limit for beams with nondefined spot size and for beams described by more parameters in addition to typical longitudinal wavelength and transverse waist. As examples, the paraxiality of higher-order Hermite, Laguerre, and Bessel-Gaussian beams was completely determined. This method could be extended to nonlinear optics and Bose condensates.

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It is experimentally shown that the competition between the two lasing modes of bichromatic emission in a dye solution with nanoparticle scatterers within the diffuse regime can be externally controlled by varying only the excitation beam diameter and the radiation detector position. It is established that this feature is a consequence of the changes in the reabsorption process strengths between monomer and dimer dye aggregates. The controllable switching of these lasing modes could open the real possibility of implementing previously suggested applications for this effect as optical switches.

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A theoretical analysis on wave propagation and optical properties of slabs with light-induced free charge carriers within a Fabry-Pérot framework is presented. The key of the analysis is to attack the wave propagation problem in terms of the time-averaged Poynting vector modulus within the medium through an alternative approach. This fact allows coupling the microscopic (free charge rate) and macroscopic (electromagnetic field evolution) equations self-consistently by means of the nonlinear permittivity and conductivity, which, in turn, depend on the time-averaged Poynting vector modulus. Thereby, the transmittance, reflectance, and absorptive power are derived as functions of the pump intensity and medium thickness. Bistable behavior is found at relatively high excitation intensity for positive values of the nonlinear permittivity coefficient. The bistability enhances for increasing values of such coefficient and weakens for increasing values of nonlinear photoconductivity coefficient. On the contrary, for negative nonlinear permittivity coefficient, bistability does not appear possessing these media mirrorlike behavior. Some possible applications are suggested.

RESUMEN

This paper presents an approach to wave propagation inside the Fabry-Pérot framework. It states that the time-averaged Poynting vector modulus can be nonequivalent to the squared-field amplitude modulus. This fact permits the introduction of a kind of nonlinear medium whose nonlinearity is proportional to the time-averaged Poynting vector modulus. Its transmittance is calculated and found to differ from that obtained for a Kerr medium, whose nonlinearity is proportional to the squared-field amplitude modulus. The latter emphasizes the nonequivalence of these magnitudes. A space-time symmetry analysis shows that the Poynting nonlinearity should be possible only in noncentrosymmetric materials.