  Silicon modulators are key components in silicon photonics to support the
dense integration of electro-optic (EO) functional elements on a compact chip
for various applications including high-speed data transmission, signal
processing, and photonic computing. Despite numerous advances in promoting the
operation speed of silicon modulators, a bandwidth ceiling of 67 GHz emerges in
practices and becomes an obstacle to paving silicon photonics toward Tbps level
data throughput on a single chip. Here, we theoretically propose and
experimentally demonstrate a design strategy for silicon modulators by
employing the slow light effect, which shatters the present bandwidth ceiling
of silicon modulators and pushes its limit beyond 110 GHz in a small footprint.
The proposed silicon modulator is built on a coupled-resonator optical
waveguide (CROW) architecture, in which a set of Bragg gratings are
appropriately cascaded to give rise to a slow light effect. By comprehensively
balancing a series of merits including the group index, photon lifetime,
electrical bandwidth, and losses, we found the modulators can benefit from the
slow light for better modulation efficiency and compact size while remaining
their bandwidth sufficiently high to support ultra-high-speed data
transmission. Consequently, we realize a modulator with an EO bandwidth of 110
GHz in a length of 124 {\mu}m, and demonstrate a data rate beyond 110 Gbps by
applying simple on-off keying modulation for a DSP-free operation. Our work
proves that silicon modulators beyond 110 GHz are feasible, thus shedding light
on the potentials of silicon photonics in ultra-high-bandwidth applications
such as data communication, optical interconnection, and photonic machine
learning.
