Experimental Observation of Strong Photon Localization in Disordered Photonic Crystal Waveguides

We demonstrate experimentally that structural perturbations imposed on highly dispersive photonic crystal-based waveguides give rise to spectral features that bear signatures of Anderson localization. Sharp resonances with effective $Q$’s of over 30 000 are found in scattering spectra of disordered waveguides. The resonances are observed in a $\sim 20\mathrm{\text{−}}\mathrm{nm}$ bandwidth centered at the cutoff of slowly guided Bloch modes. The origin of the spectral features can be explained by the interference of coherently scattered electromagnetic waves which results in the formation of a narrow impurity (or localization) band populated with spectrally distinct quasistates. Standard photon localization criteria are fulfilled in the localization band.

Figure 1

(a) Schematic of the measurement setup. Vertically scattered light is collected with an objective (O) and imaged with a lens (L1) onto a field-stop (FS) consisting of a variable aperture which selects the field of view. Another lens (L2) focuses light from the selected area into an IR photodiode (PD). A beam splitter redirects a fraction of the collimated beam into an IR charge coupled device (IR-CCD) for imaging. (b) SEM of a typical disordered PhC pattern; (c) the cross section of a freestanding W1 PhC waveguide; (d) top image of a W1 waveguide; and (e) example of a donor-defect cavity.

Figure 2

(a) Spectrum acquired from a single-defect cavity separated from the W1 waveguide by four rows of airholes; (b) 4 nm-wide detailed scan showing the solitary broad feature; (c) IR image of the collected light; and (d) high-resolution scan of a 4 nm-wide section of the band containing sharp spectral peaks.

Figure 3

(a) Calculated band structure of a W1 waveguide ($f\cong 0.28$) with an even-parity guided mode (black circles). The dark-shaded region around the mode’s edge outlines the strong-localization window. The inset shows the supercell used in the plane wave expansion simulations. (b) Calculated dispersion curves for the even mode in waveguides with three different fill factors; and (c) spectra collected from fabricated waveguides with the equivalent fill factors. The insets show the corresponding SEM images of the PhC structures.

Figure 4

Detailed spectrum of the localization band collected from a disordered W1 ($f\cong 0.30$). The inset shows the calculated DOS of a defect-free waveguide. Disorder creates a localization band (gray) around the slow mode’s cutoff.