Title: Resonance-Free Light Recycling,
Abstract: The inability to efficiently tune the optical properties of waveguiding
structures has been one of the major hurdles for the future scalability of
integrated photonic systems. In silicon photonics, although dynamic tuning has
been achieved with various mechanisms, even the most effective thermo-optic
effect offers a refractive index change of only $1.86 \times 10^{-4} K^{-1}$.
To enhance this small change, light recycling based on resonators has been
employed in order to realize efficient modulators, phase shifters, and optical
switches. However, the resonant enhancement comes at a great cost of optical
bandwidth, fabrication tolerance and system scalability. Here we demonstrate a
scalable light recycling approach based on spatial-mode multiplexing. Our
approach offers a fabrication tolerance of ${\pm}$ 15 nm, in stark contrast to
the non-scalable subnanometer tolerance in typical silicon resonators. We
experimentally demonstrate light recycling up to 7 passes with an optical
bandwidth greater than 100 nm. We realize power-efficient thermo-optic phase
shifters that require only 1.7 mW per ${\pi}$, representing more than an 8-fold
reduction in the power consumption.