# Singular beams

Nondifracting acoustic beams can be generated under specific conditions. Now I'm working in the generation of zero-th and higher order Bessel beams generated by diffraction gratings and metamaterials. Focusing of the beams is also considered and acoustic vortices can be easily generated by passive devices. Furthermore, nondifracting nonlinear beams have been also obtained by using periodic materials under self-collimation regime.

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# Bessel beams

We report Bessel-like beam formation of acoustic waves by means of an axisymmetric grating of rigid tori. The results show that the generated beam pattern is similar to that of Bessel beams, characterized by elongated non-diffracting focal spots. A multiple foci structure is observed, due to the finite size of the lens. The dependence of the focal distance on the frequency is also discussed, on the basis of an extended grating theory. Experimental validation of acoustic Bessel-like beam formation is also reported for sound waves. The results can be generalized to wave beams of different nature, as optical or matter waves.

Acoustic Bessel-like beam formation by an axisymmetric gratingN. Jiménez, V. Romero-García, R. Picó, A. Cebrecos, V. Sánchez-Morcillo, L. García-Raffi, J. V. Sánchez-Pérez, K. Staliunas,

*Europhysics Letters*,

**106**, pp 24005, (2014)

# Vortex beams

I'm studying the generation of acoustic vortex using the diffraction of plane waves over spiral shaped structures. We find-out that the diffraction of acoustic waves on multiple-arm Arquimedean spirals generate arbitrary order Bessel beams based in spirals. Moreover vortex focusing can eb achieved by combining Fresnel zone-plates with spiral gratings. Using Fresnel-spirals geometrically-optimal focused vortex beams are observed. This technique can be used to generate cheap lenses for acoustic particle manipulation and acoustic radiation force applications.

Formation of high-order acoustic Bessel beams by spiral diffraction gratingsN. Jiménez, R. Picó, V. Sánchez-Morcillo, V. Romero-García, L. García-Raffi, K. Staliunas,

*Phys. Rev. E*,

**94**(5), pp 053004, (2016)

Sharp acoustic vortex focusing by Fresnel-spiral zone plates

N Jiménez, V Romero-García, LM García-Raffi, F Camarena, K Staliunas,

*Applied Physics Letters*,

**112**, pp 204101, (2018)

# Self-collimated beams

We investigate the propagation of high-intensity sound beams in a sonic crystal, under self-collimation or reduced-divergence conditions. The medium is a fluid with elastic quadratic nonlinearity, where the dominating nonlinear effect is harmonic generation. We explored the conditions for the efficient generation of narrow, nondiverging beam of second harmonic. Numerical simulations are in agreement with the analytical predictions made, based on the linear dispersion characteristics in modulated media and the nonlinear interaction in a quadratic medium under phase matching conditions.

Nonlinear self-collimated sound beams in sonic crystalsE. M. Hamham, N. Jiménez, R. Picó, V. Sánchez-Morcillo, L. G. Raffi, K. Staliunas,

*Phys. Rev. B*,

**92**, pp 054302, (2015)