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The evolution of spectral and temporal profiles of 4.5 optical-cycle pulses propagating near zero-dispersion wavelength (ZDW) in a photonic crystal fiber is investigated experimentally and theoretically beyond the slowly varying-envelope approximation. The excellent agreement between the experimental an theoretical results suggests that the observed gap in the spectral profile, the most distinctive feature, originates from the self-steepening effect. This effect intensifies the spectral component shorter than the ZDW with the decay of higher order solitons and consequently induces the intrapulse four-wave mixing (FWM). As a result, the anti-Stokes and Stokes components produced by the FWM enables us to generate a supercontinuum from 480 to 1020 nm.