Finite-Size and Disorder Effects on Slow-Light Propagation in an Extended Photonic Crystal Coupled-Cavity Waveguides with Group-Index Bandwidth Product Exceeding 0.47

Abstract

Slow light propagation through engineered band dispersion in photonic structures is a highly promising tool for realizing integrated optical delay lines and efficient photonic devices through enhanced optical nonlinearities. A primary goal is to achieve devices over the largest possible bandwidth with large group index and minimal dispersion (i.e. approximately constant group index), flat transmission spectrum, which otherwise would hinders their use for pulse propagation, with setbacks such as pulse distortion and generation of echoes, thus enabling multimode and pulsed operation. We present an experimental proof of record-high group-index bandwidth product (GBP = n g ∆ω ⁄ ω) in genetically optimized coupled-cavity waveguides (CCWs) made of staggered modified L3 photonic crystal cavities. The optimization procedure was applied to the unit cell to achieve maximal GBP combined with low losses. The resulting designs were realized in Si slabs, where CCWs of length ranging between 50 and 800 cavities were fabricated. The samples were characterized by measuring the CCW transmission, the mode dispersion and the group index ng through Fourier-space imaging. Various cavity designs were investigated, with theoretical group index ranging from n g = 37 to n g > 100. Record-high GBP = 0.47 was demonstrated over a bandwidth approaching 20 nm, with n g = 37, a very homogeneous flat-top transmission profile and losses value below 67 dB/ns. On a different design, an average n g = 107 with 15% variation over 7.4 nm was measured. These values range among the best ever demonstrated for a silicon device. Through Fourier-space imaging, slow light properties are directly extracted by reconstructing the dispersion maps, allowing distinguishing finite-size effects from those arising due to structural disorder. We elucidate the influence of the CCW length and design on the adherence of the dispersion to the theoretically predicted periodic-boundary profile. Limitations on slow-light propagation are identified in terms of decay length and the onset of diffusive light transport, considering state-of-the-art fabrication. For such systems where light propagation relies on a resonant tunnelling mechanism, we show that disorder has a cumulative effect on the device response, ultimately capping the achievable slow-down factor. With the aid of Raman spectroscopy, we further explain how the mitigation of stress in the layer is mandatory towards preventing light trapping in the waveguide, in order to retrieve the full operational bandwidth.