Figure 1

Schematic representation of the geometry of the chirped waveguide (top). The radius of the holes in the PhC lattice is linearly increased from left to right. The waveguide mode (continuous line) in the dispersion relation shifts up as the hole size increases. Light with a specific optical frequency (dotted line) is therefore allowed to propagate in regions I (with a relatively high $v_g$) and II (lower $v_g$), but not in region III. The dispersion relations in regions I and III are calculated by 3D FDTD for hole radii of $0.306a$ and $0.321a$, respectively. For all other radii, they are obtained by linear interpolation (with an error of maximally 0.03% in frequency).Reuse & Permissions

Figure 2

(a) $E$-field amplitude from a near-field measurement on the chirped waveguide at $\omega =0.2961$ (in units $2\pi c/a$). (b) Local wave vector as a function of $x$ position, obtained from the measurement in (a) (solid points) The solid (red) line depicts the expected wave vector calculated with the dispersion relations of Fig. 1. (c) Group velocity corresponding to the calculated data in (b). (d) Summed intensity ($|E{|}^2$) over all $y$ points of the image in (a), yielding the total intensity in the waveguide as a function of position $x$. (e),(f) Measured $E$-field amplitude of the same waveguide and area as in (a), now at $\omega =0.2973$ and 0.2950, respectively. The yellow triangles in (a),(e),(f) indicate local minima. The blue triangles indicate the expected cutoff position.Reuse & Permissions

Figure 3

Intensity detected by the near-field probe as a function of the group index $n_g$, normalized at $n_g=20$. The data are averaged over 25 measurements, ranging between $\omega =0.2936$ and $\omega =0.2982$. The intensity of each individual measurement is normalized to $n_g=20$. The black line shows the expected result without losses. The red line represents the best fit to the measurements for the data points with $n_g<30$ only resulting in $\alpha =2$. The histogram depicts at what $n_g$ local intensity minima were observed, indicative of multiple scattering.Reuse & Permissions