Seminars Archive

Scale-free Optics & Diffractionless Waves

Abstract
The propagation features of an optical beam in a medium are rigidly fixed by the scales of the associated physical quantities (wavelength, size, intensity, etc.). In particular this causes diffraction and waves distortion, therefore posing fundamental limits to imaging, microscopy, and communication. Distortion is avoided by employing waveguides or exploiting nonlinearity to produce solitons. Nevertheless in both cases diffraction is only compensated, so that the wavelength still imposes rigid laws on wave shape, size and intensity. In turn, the presence of a nonlinearity in the medium can introduce new spatial scales: in principle, if one were able to identify a nonlinearity term introducing an intensity-independent scale that cancels the wavelength, \scale-free" propagation could occur. In this regime, diffraction ceases, and waveforms will naturally propagate without diffraction nor distortion, forming soliton-like beams of any (in principle arbitrarily low) size and intensity. Recent experiments provided experimental evidences of scale-free optical propagation in supercooled (out-of-equilibrium) copper-doped KTN:Li, a recently developed ferroelectric crystal hosting a glass of disordered dipoles. The phenomenon of the scale-free propagation is supported by diffusive nonlinearity observed in the crystal, and emerges through the enhanced electro-optic response of polar nanoregions (PNRs) that form when the crystal is supercooled close to its room temperature Curie point TC. These findings demonstrate that diffraction can be canceled and not merely compensated, therefore suggesting a completely new paradigm for ultra-resolved imaging and microscopy.