Self-trapping of incoherent white light

Optical pulses—wave-packets—propagating in a linear medium have a natural tendency to broaden in time (dispersion) and space (diffraction). Such broadening can be eliminated in a nonlinear medium that modifies its refractive index in the presence of light in such a way that dispersion or diffraction effects are counteracted by light-induced lensing1,2. This can allow short pulses to propagate without changing their shape2,3, and the ‘self-trapping’ of narrow optical beams1 whereby a beam of light induces a waveguide in the host medium and guides itself in this waveguide, thus propagating without diffraction4. Self-trapped pulses in space and time have been investigated extensively in many physical systems and, as a consequence of their particle-like behaviour, are known as ‘solitons’ (ref. 5). Previous studies of this phenomenon in various nonlinear media6,7,8,9,10,11,12 have involved coherent light, the one exception being our demonstration13 of self-trapping of an optical beam that exhibited partial spatial incoherence. Here we report the observation of self-trapping of a white-light beam from an incandescent source. Self-trapping occurs in both dimensions transverse to the beam when diffraction effects are balanced exactly by self-focusing in the host photorefractive medium. To the best of our knowledge, this is the first observation of self-trapping for any wave-packet that is both temporally and spatially incoherent.