Light trapping in gravity-like potentials and expansion of supercontinuum spectra in photonic-crystal fibres

Femtosecond pulses of light propagating along photonic-crystal fibres can generate a broad optical supercontinuum(1,2). This striking discovery has applications ranging from spectroscopy and metrology(3) to telecommunication(4) and medicine(5,6). Among the physical principles underlying supercontinuum generation are soliton emission(7), a variety of four-wave mixing processes(8-11), Raman-induced soliton self-frequency shift(12,13), and dispersive wave generation mediated by solitons(7,13,14). Although all of the above effects contribute to supercontinuum generation, none of them can explain the generation of blue and violet light from infrared femtosecond pump pulses. In this work we argue that the most profound role in the shaping of the short-wavelength edge of the continuum is played by the effect of radiation trapping in a gravity-like potential created by accelerating solitons. The underlying physics of this effect has a straightforward analogy with the inertial forces acting on an observer moving with a constant acceleration.