Ricerc@Sapienza

Spontaneous Raman is a well-established tool to probe molecular vibrations. Under resonant conditions, it is a largely used method for characterizing the structure of heme-proteins. In recent years, advances in pulsed laser sources allowed to explore vibrational features with complex techniques based on non-linear optical interactions, among which is stimulated Raman scattering (SRS).

Illuminating a magnetic material with femtosecond laser pulses induces complex ultrafast dynamical processes. The resulting optically detectable response usually contains contributions from both the optical properties and the magnetic degrees of freedom. Disentangling all the different components concurring to the generation of the total signal is a major challenge of contemporary experimental solid-state physics.

tThe control of the transmission of the energy transported by optical waves is of extreme importance forthe realization of those advanced technologies which require high speed of operation and fast switching.Such a task can be approached through the design and preparation of materials that possess modulableoptical properties.

In recent years, nonlinear pulse propagation in multimode fibers (MMFs) is has experienced a dramatic resurgence of interest, because of their potential for opti- cal communications and high-power lasers. However, from the fundamental view- point several aspects still remain to be fully understood. Here we experimentally and theoretically studied the dynamics of high-energy (up to reaching the fiber damage threshold) spatiotemporal solitons in MMFs with a graded-index (GRIN) core profile.

In this work, we reveal that when using femtosecond pump pulses, one has access to a different regime, where the peak power inside the MM fiber may reach and even surpass the critical power Pcr for catastrophic self-focusing (this is about 10 MW in fused- silica). When these peak power levels are reached, the propagating light beam undergoes a so-called filamentation phenomenon, leading to the so-called conical emission (CE)].

In the present study, we reveal the presence of a new type of nonlinear propagation regime in MM fibers, where stable spatiotemporal solitons are created by the fission of the initial pulse. These solitons have different amplitudes and wavelengths, but nearly equal time duration.

Nonlinear optics in crystals with quadratic susceptibility has been largely explored along the last decades, with a particular emphasis on spatial solitons. When in the initial part of the propagation, the nonlinear length is much shorter than the diffraction length, rather than solitons, in these crystals it is possible to observe strong beam reshaping and spectral broadening. This mechanism of nonlinear beam evolution can be induced by combining high laser energies and large input diameters, so to reduce the contribution of diffraction in the initial steps of the propagation.

Multimode fibers received recently a renewed interest because of the ability to control the multimode propagation and to select, at the output, a quasi-single mode supporting the main part of the energy. In this paper we present results on spatial Kerr-beam self-cleaning in multimode optical fiber.
We show how a speckled beam obtained because of multimode propagation can be transformed into a quasi-single-mode emission under the effect of the peak power increase

We analyze the formation of localized structures in cavity-enhanced second-harmonic generation. We focus on the phase-matched limit, and consider that fundamental and generated waves have opposite signs of group velocity dispersion. We show that these states form due to the locking of domain walls connecting two stable homogeneous states of the system, and undergo collapsed snaking. We study the impact of temporal walk-off on the stability and dynamics of these localized states.

Experiments and theory have rapidly progressed on nonlinear optical extreme waves, showing that guided wave nonlinear optics and fiber lasers provide a relatively simple, accessible and controllable test bed for the observations and accurate statistical studies of extreme wave phenomena that obey the same universal rules, which apply to a large ensemble of different physical systems.