Gap soliton formation by nonlinear supratransmission in Bragg media [rapid communication]

A shallow Bragg grating in the nonlinear Kerr regime, submitted to incident continuous wave irradiation at a frequency in a band gap, is shown to switch from total reflection to high transmission for incident energy flux above a threshold. Using the theory of nonlinear supratransmission we prove that: (1) the threshold has an explicit analytic expression in terms of the deviation from the Bragg resonance, (2) the process is not the result of a shift of the gap in the nonlinear dispersion relation, (3) it actually results from a fundamental instability, (4) the transmission does occur by means of gap soliton trains, as experimentally observed [D. Taverner et al., Opt. Lett. 23 (1998) 328], (5) the required energy flux tends to zero close to the band edge. This last property suggests experiments under CW-irradiation at a frequency tuned close to the band edge.