Biomedical subjects

Antonio Picozzi

Publications and source records attributed to Antonio Picozzi.

9 recordsLinked to original sources

PubMed2006

Velocity locking of incoherent nonlinear wave packets.

We show both theoretically and experimentally in an optical fiber system that a set of incoherent nonlinear waves irreversibly evolves to a specific equilibrium state, in which the individual wave packets propagate with identical group velocities. This intriguing process of velocity locking can be explained in detail by simple thermodynamic arguments based on the kinetic wave theory. Accordingly, the selection of the velocity-locked state is shown to result from the natural tendency of the isolated wave system to approach the state that maximizes the nonequilibrium entropy.

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PubMed2006

Nonequilibrated oscillations of coherence in coupled nonlinear wave systems.

We show that a conservative system of a pair of coupled incoherent nonlinear waves exhibits huge oscillations of coherence, which are characterized by a recurrent transfer of noise fluctuations between the coupled waves. This sustained oscillatory behavior is in contradiction with the expected irreversible evolution towards equilibrium. As a consequence, the process of coherence transfer is characterized by a reduction of nonequilibrium entropy, which violates the H theorem of entropy growth inherent to the kinetic theory.

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PubMed2005

Condensation of classical nonlinear waves.

We study the formation of a large-scale coherent structure (a condensate) in classical wave equations by considering the defocusing nonlinear Schrödinger equation as a representative model. We formulate a thermodynamic description of the classical condensation process by using a wave turbulence theory with ultraviolet cutoff. In three dimensions the equilibrium state undergoes a phase transition for sufficiently low energy density, while no transition occurs in two dimensions, in complete analogy with standard Bose-Einstein condensation in quantum systems. On the basis of a modified wave turbulence theory, we show that the nonlinear interaction makes the transition to condensation subcritical. The theory is in quantitative agreement with the numerical integration of the nonlinear Schrödinger equation.

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PubMed2005

Influence of dispersion on the resonant interaction between three incoherent waves.

We study the influence of group-velocity dispersion (or diffraction) on the coherence properties of the parametric three-wave interaction driven from an incoherent pump wave. We show that, under certain conditions, the incoherent pump may efficiently amplify a signal wave with a high degree of coherence, in contrast with the usual kinetic description of the incoherent three-wave interaction. The group-velocity dispersion is shown to be responsible for a spectral filtering process, in which the coherence of the generated signal increases, as the coherence of the pump wave decreases. As a result, the coherence acquired by the signal in the presence of an incoherent pump, is higher than that acquired in the presence of a fully coherent pump. The mechanism underlying this intriguing result is based on the emergence of a mutual coherence between the incoherent pump and the generated idler wave. We calculate explicitly the degree of mutual coherence between the pump and idler waves and show that the two incoherent waves become completely correlated in the full incoherent regime of interaction. The theory is in quantitative agreement with the numerical simulations. To motivate the experimental confirmation of our theory, we characterize the dispersion properties of an actual quadratic nonlinear optical crystal in which the process of signal coherence enhancement induced by pump incoherence may be studied experimentally.

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PubMed2004

Entropy and degree of polarization for nonlinear optical waves.

I show that the usual definitions of entropy and degree of polarization are not relevant for nonlinear optical waves, because they would lead to the erroneous conclusion that a genuine process of reversible propagation would be characterized by an irreversible process of entropy production. To circumvent this problem, I propose appropriate definitions of entropy and degree of polarization that depend on all the statistical moments of the nonlinear wave, so that they exhibit the satisfactory property to be conserved during a reversible propagation process.

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PubMed2004

Incoherent solitons in instantaneous response nonlinear media.

We show theoretically and experimentally in an optical fiber system that solitons can be spontaneously generated from incoherent light in an instantaneous response nonlinear Kerr medium. The theory reveals that the unexpected existence of these incoherent solitons relies on a phase-locking mechanism, which leads to the emergence of a mutual coherence between the incoherent waves that constitute the soliton.

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PubMed2004

Condensation in Hamiltonian parametric wave interaction.

In the generic Hamiltonian problem of parametric wave interaction, we show theoretically the existence of a sudden transition leading the wave system from completely incoherent states towards highly coherent states. This self-organization process is characterized by a reduction of the nonequilibrium entropy, in contrast with the H theorem of entropy growth inherent to the random phase approximation approach. The mechanism underlying this intriguing condensation process is in essence a reversible nonlinear damping. As a result, the lower the coherence of the initial state, the higher the coherence of the final state.

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PubMed2002

Coherence properties of the parametric three-wave interaction driven from an incoherent pump.

We consider the basic problem of the parametric generation process from an incoherent pump wave. The analysis of the degenerate configuration of the two-wave interaction reveals that the mutual convection (i.e., group-velocity difference) between the incoherent pump and the signal (i.e., the daughter wave) may quench their parametric interaction, so that the gain experienced by the signal may become arbitrarily small. Conversely, in the absence of convection, the incoherent pump efficiently amplifies the signal wave, although this amplification process cannot lead to the generation of a coherent signal. However, in the case of nondegenerate three-wave interaction, we show the existence of a convection-induced phase-locking mechanism in which the incoherence of the pump is absorbed by the idler wave allowing the signal wave to grow efficiently with a high degree of coherence. We calculate explicitly the autocorrelation function of the generated signal in this regime of coherent-incoherent interaction. The analysis reveals that, owing to the convection-induced averaging process that accompanies the phase-locking mechanism, the degree of coherence of the signal increases as the degree of coherence of the pump decreases. We establish the experimental conditions that would allow for the observation of the transition between the incoherent and the coherent regimes of the three-wave parametric interaction.

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PubMed2002

Hidden coherence along space-time trajectories in parametric wave mixing.

We show theoretically that the waves generated through the generic parametric three- or four-wave-mixing processes exhibit, as a general rule, a hidden coherence characterized by skewed coherence lines along specific space-time trajectories. Our study generalizes the concept of coherence in the sense that these previously unrecognized coherent states cannot be described through the standard definitions of spatial and temporal coherence.

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