Experimental observation of strong photon localization in disordered photonic crystal waveguides (0706.3040v1)

Abstract: 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 the effective Qs of over 30,000 are found in scattering spectra of disordered waveguides. The resonances are observed in a ~20-nm bandwidth centered at the cutoff of slowly-guided Bloch-modes. Their origin can be explained with 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.

• The paper demonstrates experimental evidence of Anderson localization in photonic crystal waveguides through controlled geometrical disorder.
• It employs electron beam lithography and spectral analysis to reveal narrow (~20 nm) resonant peaks with quality factors exceeding 30,000.
• The findings suggest promising advances for photonic devices, including low-threshold lasing and optical coding via engineered defect scattering.

Experimental Observation of Strong Photon Localization in Disordered Photonic Crystal Waveguides

The research paper titled "Experimental observation of strong photon localization in disordered photonic crystal waveguides" by J. Topolancik, B. Ilic, and F. VoLLMer presents a significant experimental paper investigating photon localization within disordered waveguide structures. Utilizing photonic crystal (PhC) waveguides, the authors provide experimental evidence for Anderson localization, a phenomenon originally described for matter-waves in disordered atomic systems but applicable to classical waves as well.

The paper is carried out on photonic crystal-based waveguides with intrinsic geometrical disorder. These waveguides are integrated within silicon slabs featuring a hexagonal lattice of air holes. The paper demonstrates that even minor deviations from structural periodicity can result in multiple scattering necessary for strong localization without necessitating the presence of absorption. The localization manifests in a narrow bandwidth (~20 nm) centered at the cutoff of slowly-guided Bloch modes, displaying resonant peaks with effective quality factors (Qs) surpassing 30,000.

Key Observations and Numerical Results

1. Geometrical Disorder and Waveguide Design:
   • The waveguides are formed within high-refractive-index silicon slabs, patterned using electron beam lithography. Although the fill factor remains constant, the hole shapes deviate from ideal circles, a disorder that plays a crucial role in scattering.
   • The paper identifies that the disorder is adeptly controlled, such that traditional photonic crystal band structure remains mostly unaltered while promoting scattering necessary for localization.
2. Spectral Analysis and Localization:
   • Spectral data collected from the structures reveal sharp peaks with line widths around 50 pm. The peaks are observed consistently across different configurations of the disordered waveguides.
   • The authors attribute these spectral features to Anderson localization, reinforced by the fact that peaks persist in various configurations both with and without donor-defect cavities.
3. Dispersion and Scattering:
   • Through simulations using the supercell approach and 3D plane-wave expansion, the authors calculate band structures demonstrating the effects of disorder. These simulations are critical for correlating observed spectral features with theoretical frameworks.

Implications and Future Directions

The implications of the findings are both technologically promising and theoretically enriching. The demonstrated photonic localization within disordered PhC waveguides at optical frequencies could pave the way for novel photonic devices with applications in low-threshold lasing, ultra-sensitive detection, and unique optical coding mechanisms. From a theoretical standpoint, the paper substantiates the feasibility of utilizing structural disorder deliberately to induce specific photonic behavior—a concept reminiscent of tailored defect engineering in PhCs.

Further exploration into the interplay of disorder across various scales and configurations offers viable pathways for expanding control over wave guidance and manipulation in photonic circuits. Systematic studies facilitated by altering lithographic parameters could provide deeper insights into the dependence of localization on disorder magnitude and configuration. The potential for achieving high confinement and exceptional Q in disordered systems may stimulate interest in utilizing these mechanisms within broader electromagnetic spectral regimes.

In summation, this research articulates a compelling case for integrating controlled disorder within photonic design, leveraging intrinsic material properties to induce and harness photon localization phenomena for advanced photonic applications.