Anderson Localization & "Photonic Quasicrystals"

Strong Photon Localization in Disordered Photonic Crystal Waveguides

Symposium: 50 Years of Anderson Localization

Fabrication disorder is perceived as impediment to ideal performance of optical resonators, and disorder ultimately limits the performance of any engineered device. However, when superimposed on a photonic band gap material, disorder itself can induce light localization. We show such strong photon localization by disorder in photonic crystal waveguides. In this design concept, disorder and variability does not limit device performance, but instead is the basis for high-Q resonance. This new approach to engineering circumvents limitations posed by disorder, and illustrates a bioinspired design principle found throughout nature.

Periodic high-index-contrast photonic crystal structures such as two-dimensional arrays of air holes in dielectric slabs confine light in defects where the lattice periodicity is broken. Localized optical modes are formed in cavities that are defined by removing, shifting or changing the size of the lattice components. Optimized introduction of such local structural perturbations has produced optical nanocavities with ultra-small modal volumes and record-high quality (Q) factors of over a million. Sensitivity of such photonic crystal nano-cavities is expected to surpass the single molecule level.

We are interested in a conceptually different approach to photon localization in photonic crystal structures. Our design concept introduces structural perturbations uniformly throughout the fabricated crystal by deliberately changing the shape of elements that form the lattice. Such nanometer-scale disorder effectively represents randomly-distributed strong scatterers that affect propagation of Bloch-waves through the otherwise periodic lattice. We show that the guided modes in line-defect waveguides defined in such disordered photonic crystals experience coherent backscattering that leads to Anderson localization. The effect is observed in a narrow frequency band close to the guided mode's cutoff where the light propagates with a slow group velocity (slow light regime). Optical cavities with Qs of ~2 x 10555 and micron-scale modal volumes are observed along disordered waveguides (picture). Preliminary 2D FDTD calculations performed on random structures yield modal volumes that are comparable to photonic-crystal-heterostructure cavities studied by Noda et al. which suggest that we are indeed dealing with nanocavities. We believe our experiments can find various applications e.g. in optical sensing systems and random nano-lasers.

Read the PRL paper

Read the Applied Physics Letters paper

Read the 2008 SPIE Proceedings



Random Microcavities for Active Devices

The experimental observation of enhanced photoluminescence from high-Q silicon-based random photonic crystal microcavities embedded with PbSe colloidal quantum dots is being reported. The emission is optically excited at room temperature by a continuous-wave Ti-Sapphire laser and exhibits randomly-distributed localized modes with a minimum spectral linewidth of 4 nm at 1550 nm wavelength (collaboration with Bhattacharya's Group, Michigan, and Xu's Group, Penn State).

Read the Applied Physics Letters paper