On-chip multi-timescale spatiotemporal optical synchronization
Lida Xu,
1, ∗
Mahmoud Jalali Mehrabad*,
1, †
Christopher J. Flower*,
1
Gregory Moille,
2
Alessandro Restelli,
2
Daniel G. Suarez-Forero,
1
Yanne
Chembo,
3
Sunil Mittal,
4
Kartik Srinivasan,
2
and Mohammad Hafezi
1, ‡
1
Joint Quantum Institute and Quantum Technology Center,
University of Maryland, College Park, MD 20742, USA
2
Joint Quantum Institute, University of Maryland and National
Institute of Standards and Technology, College Park, MD 20742, USA
3
Department of Electrical and Computer Engineering,
Institute for Research in Electronics and Applied Physics,
and Joint Quantum Institute, University of Maryland, College Park, MD 20742, USA
4
Department of Electrical and Computer Engineering,
Northeastern University, Boston, MA, USA
ABSTRACT
Mode-locking mechanisms are key resources in nonlinear optical phenomena, such
as micro-ring solitonic states, and have transformed metrology, precision spectroscopy,
and optical communication. However, despite significant efforts, mode-locking has
not been demonstrated in the independently tunable multi-timescale regime. Here, we
vastly expand the nonlinear mode-locking toolbox into multi-timescale synchronization
on a chip. We use topological photonics to engineer a 2D lattice of hundreds of coupled
silicon nitride ring resonators capable of hosting nested mode-locked states with a fast
(≈ 1 THz) single-ring and a slow (≈ 3 GHz) topological super-ring timescales. We
demonstrate signatures of multi-timescale mode-locking including quadratic distribu-
tion of the pump noise with the two-time azimuthal mode dimensions, as expected by
mode-locking theory. Our observations are further corroborated by direct signatures
of the near-transform-limit repetition beats and the formation of the temporal pattern
on the slow timescale. Moreover, we show that these exotic properties of edge-confined
mode-locked states are in sharp contrast to bulk and single-ring counterparts and es-
tablish a clear pathway for their identification. Our unprecedented demonstration of
mode-locking in topological combs unlocks the implementation of lattice-scale synchro-
nization and independently tunable multi-timescale mode-locking phenomena, also the
exploration of the fundamental nonlinearity-topology interplay on a chip.
INTRODUCTION
In the linear regime, a propagating beam of light broadens in space or disperses in time, due
to diffraction or chromatic dispersion, respectively. However, nonlinearities may create opposite
dispersive or diffractive behavior, for which when they exactly counterbalance allows for mode-
locking [1]. The intensity-dependent index of refraction in nonlinear media is an early example that
enables self-focusing mechanisms to counteract the diffraction-led divergence, resulting in spatially
mode-locked optical solitonic solutions [2, 3]. More recently, pivotal breakthroughs were reported
by engineering a delicate balance between the nonlinear Kerr effect and dispersion leading to the
discovery of mode-locked temporal dissipative Kerr solitons (DKS) [4, 5] following the same dy-
namics of their spatial counterpart [6]. DKS have enabled many applications [7] in spectroscopy
[8], frequency synthesis [9], ranging [10], and optical clockwork [11, 12].
