  The effective Kerr nonlinearity of hollow-core kagome-style photonic crystal
fiber (PCF) filled with argon gas increases over 100 times when the pressure is
increased from 1 to 150 bar, reaching 15 % of that of bulk silica glass, while
the zero dispersion wavelength shifts from 300 to 900 nm. The group velocity
dispersion of the system is uniquely pressure-tunable over a wide range while
avoiding Raman scattering : absent in noble gases and having an extremely high
optical damage threshold. As a result, detailed and well controlled studies of
nonlinear effects can be performed, in both normal and anomalous dispersion
regimes, using only a fixed-frequency pump laser. For example, the absence of
Raman scattering permits clean observation, at high powers, of the interaction
between a modulational instability side-band and a soliton created dispersive
wave. Excellent agreement is obtained between numerical simulations and
experimental results. The system has great potential for the realisation of
reconfigurable supercontinuum sources, wavelength convertors and short-pulse
laser systems.
