Figure 1: Phase matching condition and direct observation of dispersive wave generation in silica ridge waveguides.

(a,b) Scanning electron microscope images of an array of silica ridge waveguides on a silicon chip. Silicon pillars support silica layers containing waveguides. The red box in a contains a silica waveguide whose cross section is shown in b; the cross section shows the calculated mode profile of the TM mode at wavelength 830 nm superimposed. Scale bar, 100 μm for (a) and 1 μm for (b). (c) Calculated mode area and zero dispersion wavelength (λ_ZDW) are plotted versus the ridge base width. (d) Calculated group velocity dispersion (GVD, dashed lines) and phase-matching parameter Δβ (solid lines) for dispersive wave generation in TM polarization are plotted versus wavelength. Blue, red and yellow solid and dashed lines correspond to mode areas of 0.83, 1.03, 1.69 μm² respectively. The phase matching condition Δβ=γP_p (830 nm pump) is satisfied at the intersections of the coloured solid lines and the black dashed line. The points with error bars are measured dispersion values obtained from sets of ten scans of spectral fringes measured using a Mach-Zehnder interferometer (see Methods). (e) Ultraviolet–visible dispersive wave generation in a silicon chip containing an array of waveguides with varying mode area (322 nm emission has area of 0.83 μm² and 545 nm emission has area of 2.09 μm²). Image of multiple photographs of scattered light taken from above the 1.5 cm long chip in which infrared pulses are launched at the left side of each waveguide. Pulse energies are set to the threshold pulse energy for dispersive wave generation. The colour emission at the left side of the image is dispersive wave emission that has been reflected at the far right facet of the chip. The initial spectral broadening of the input pulse can be seen as the orange-red emission that gradually shifts diagonally upward right. The visible scattered light shown in the figure is a very small amount of the total generated. The vast majority of the light is forward propagating and collected into a multimode fibre (not shown).