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 [1,2]. A primary goal is to achieve devices with large, approximately constant group index over the largest possible bandwidth, thus enabling multimode and pulsed operation [2]. We present an experimental proof of record high group-index bandwidth product (GBP = n_g ∆ω∕ω) [2] in genetically optimized coupled-cavity waveguides (CCWs) made of staggered L3 photonic crystal cavities (Fig.1(a) and (b)). The optimization procedure [3] was applied to the unit cell (Fig. 1(a)) to achieve maximal GBP combined with low losses. The resulting designs [4] were realized in Si slabs (Fig. 1(b)), where CCWs of length ranging between 50 and 800 cavities were fabricated. The samples were characterized by measuring the CCW transmission (Fig. 1(c) and (d)), the mode dispersion through Fourier-space imaging, and the group index n_g with Mach-Zehnder interferometry (Fig. 1(e)). Various cavity designs were investigated, with theoretical group index ranging from n_g=37 to n_g>100. Record-high GBP=0.45 was demonstrated over a bandwidth approaching 20nm (Fig. 1(e)), with n_g=37, a very homogeneous flat-top transmission profile (Fig. 1(c) and (d), variations lower than 10 dB) and losses below 67 dB/ns. On a different design [3], an average n_g=107 with 15% variation over 7.4nm was measured. These values range among the best ever demonstrated for a silicon device.

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