Triggered by the rapid research progress on the twisted bilayer graphene, the photonic moir superlattices manifest as an important platform for exploring novel physics and distinct functionalities with the versatile degrees of freedom of light and flexible photonic geometries. Recently, it has been proposed that one can overlap two one-dimensional (1D) sublattices with unequal (mismatching) spatial periodicities to construct a so-called 1D moir superlattice. Such construction requires relatively simple geometric configurations but still can hold the similar moir physics, which has then quickly been demonstrated in several photonic platforms. However, to date, researches on the moir physics are mainly focused on the flat band for the strong light confinement, which exhibits distinct difference from the strongly dispersive band with the corresponding profile of light spreading in the spatial geometry. The physics between the two scenarios, where weakly dispersive band exists, has not brought much attentions yet.

In this work, we explore the intrinsic physics of the weakly dispersive band in a 1D synthetic moir superlattice, which can generate an optimal compact electro-optic (EO) frequency comb with mode spacing reduction. Such synthetic moir superlattice is experimentally constructed along the frequency axis of light by coupling two 1D synthetic sublattices at different frequency periodicities, each of which is built in a single ring resonator under the resonant modulation [see Fig. 1(a)-(c)].

Fig 1 (a) Schematic of the experiment. (b) The system forms a 1D synthetic moir superlattice in the frequency dimension. (c) The generated compact EO frequency comb.

The flat band and strongly dispersive band structures corresponding to the moir physics are studied in the synthetic frequency dimension, where we show the spectral wave packet control and the resulting frequency comb generation compared to their spatial counterparts (see Fig. 2). Mode spacing is reduced due to the mode couplings from the unequal sublattice periods of the synthetic moir superlattice, where the optimal compact EO comb is found in the weakly dispersive band regime combining both uniform power distribution and broad frequency spanning.

Fig. 2 Experimentally observed band structures and output frequency combs with different coupling coefficients.

Our work provides a simple experimental platform for studying the moir band transition regime. It also offers insight in generating more compact frequency comb with the small mode spacing otherwise requiring the ring with the larger scale, highlighting the proof-of-principle potential toward the on-chip compact comb generation with limited footprint size.

The research was published in “Guangzhen Li, Yanyan He, Luojia Wang, Yiwen Yang, Danying Yu, Yuanlin Zheng, Luqi Yuan, and Xianfeng Chen, Weakly Dispersive Band in Synthetic Moir Superlattice Inducing Optimal Compact Comb Generation, Physical Review Letters 134, 083803 (2025) ”.

Link : https://doi.org/10.1103/PhysRevLett.134.083803