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.

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.

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.

In this paper, a meaningful classification of optical caustic beams in two dimensions is presented. It is demonstrated that the phase symmetry of the beam's angular spectrum governs the optical catastrophe, which describes the wave properties of ray singularities, for cusp (symmetric phase) and fold (antisymmetric phase) caustics. In contrast to the established idea, the caustic classification arises from the phase symmetry rather than from the phase power, thus breaking the commonly accepted concept that fold and cusp caustics are related to the Airy and Pearcey functions, respectively. Nevertheless, the role played by the spectral phase power is to control the degree of caustic curvature. These findings provide straightforward engineering of caustic beams by addressing the spectral phase into a spatial light modulator or glass plate.