V. Espinosa

Theoretical prediction of the nondiffractive propagation of sonic waves through periodic acoustic media

We predict theoretically the nondiffractive propagation of sonic waves in periodic acoustic media (sonic crystals), by expansion into a set of plane waves (Bloch mode expansion), and by finite difference time domain calculations of finite beams. We also give analytical evaluations of the parameters for nondiffractive propagation, as well as the minimum size of the nondiffractively propagating acoustic beams.

Self collimation of ultrasound in a three-dimensional sonic crystal

We present the experimental demonstration of self-collimation (subdiffractive propagation) of an ultrasonic beam inside a three-dimensional (3D) sonic crystal. The crystal is formed by two crossed steel cylinders structures in a woodpilelike geometry disposed in water. Measurements of the 3D field distribution show that a narrow beam, which diffractively spreads in the absence of the sonic crystal, is strongly collimated in propagation inside the crystal, demonstrating the 3D self-collimation effect.

Nonlinear dynamics of a class-A two-photon laser with injected signal

Abstract We performed a comprehensive study of the temporal dynamics of a two-photon laser with resonant injected signal in the high-Q cavity limit (class-A laser). The stationary solutions as well as their stability are completely characterized. In particular it is shown that tristable behaviour is possible, in contrast with two-photon optical bistability. It is also shown that there are two independent Hopf bifurcations, the sub- or supercritical character of which is analytically determined. The dynamics of the system far from the bifurcation points is classified through a numerical study.

Types I and II intermittencies in a cascade laser model

Abstract We report on types I and II intermittencies found in a cascade laser model. A continuous transition from one to another type of intermittency, which involves the coexistence of both types of laminar phases within the same time series, is found. Type II intermittency has special characteristics such as its origin at a frequency locked two-torus. When frequency unlocked this torus bifurcates to a three-torus, further giving rise to a type II intermittent like behaviour with new features during the laminar phases.

Strong on-axis focal shift and its nonlinear variation in low-Fresnel-number ultrasound beams

We examine the case of focusing transducers with a strong initial (in linear regime) shift of the main on-axis peak pressure maximum from the geometrical focal point toward the transducer. Such transducers are characterized, using the concepts introduced in this paper, by a low Fresnel number. The displacement of this initially shifted on-axis pressure maximum point toward the geometrical focus, and its backward motion as the driving transducer voltage increases until highly nonlinear regimes, has been experimentally observed. The simultaneous monitoring of the temporal wave-form distortion determines the real roles and interplay between different nonlinear effects (refraction and attenuation) in the observed dynamics of the on-axis pressure maximum. The numerical solution of the corresponding mathematical model confirms the physical interpretation of the observed phenomenon.

R&D studies for the development of a compact transmitter able to mimic the acoustic signature of a UHE neutrino interaction

Abstract Calibration of acoustic neutrino telescopes with neutrino-like signals is essential to evaluate the feasibility of the technique and to know the efficiency of the detectors. However, it is not straightforward to have acoustic transmitters that, on one hand, are able to mimic the signature of a UHE neutrino interaction, that is, a bipolar acoustic pulse with the ‘pancake’ directivity, and on the other hand, fulfil practical issues such as ease of deployment and operation. This is a non-trivial problem since it requires directive transducer with cylindrical symmetry for a broadband frequency range. Classical solutions using linear arrays of acoustic transducers result in long arrays with many elements, which increase the cost and the complexity for deployment and operation. In this paper we present the extension of our previous R&D studies using the parametric acoustic source technique by dealing with the cylindrical symmetry and demonstrating that it is possible to use this technique for having a compact solution that could be much more easily included in neutrino telescope infrastructures or used in specific sea campaigns for calibration.

Simultaneous self-collimation of fundamental and second-harmonic in sonic crystals

Simultaneous self-collimation of sound beams with different frequencies, corresponding to fundamental wave and to its second harmonic, is proposed theoretically and demonstrated experimentally. The result is obtained when the isofrequency contours for each harmonics, corresponding to different propagation bands, develop flat segments along the same direction. The effect can be utilized for managing (enhancement and control) of nonlinear frequency mixing of narrow sound beams.

Nonlinear dynamics of a class-a two-photon laser with injected signal in cascade systems

Abstract The dynamics of a two-photon laser with injected signal (TPLIS) in the limit of high quality cavity (class-A laser) is analysed by explicitly considering the underlying multi-level structure of the amplifying medium. A three-level cascade system is considered which retains the basic physics of the microscopic lasing processes. In this way the usual two-photon laser equations are consistently modified by the appearance of different dynamic frequency shifts dependent on the two-photon population inversion and on the laser intensity (ac-Stark shift). Although the basic instability scenarios are not qualitatively modified with respect to the effective two-level TPLIS, from the quantitative point of view, population-induced shifts heavily affect the threshold of appearance of instabilities.

Self-organization of ultrasound in viscous fluids

We report the theoretical and experimental demonstration of pattern formation in acoustics. The system is an acoustic resonator containing a viscous fluid. When the system is driven by an external periodic force, the ultrasonic field inside the cavity experiences different pattern-forming instabilities leading to the emergence of periodic structures. The system is also shown to possess bistable regimes, in which localized states of the ultrasonic field develop. The thermal nonlinearity in the viscous fluid, together with the far-from-equilibrium conditions, are responsible of the observed effects.

Class-B two-photon Fabry-Pérot laser

Abstract We study the stationary operation and stability properties of a class-B two-photon Fabry–Perot laser. We show that, differently from the one-photon laser, the intensity emitted by the two-photon laser is larger in a Fabry–Perot than in a ring cavity. The lasing solution loses stability through a subcritical Hopf bifurcation, as it occurs in the unidirectional ring laser. The stability domain in the parameter space is larger in the Fabry–Perot than in the ring cavity configuration.