temporalcloaking, [21] breaking of Lorentz reciprocity, [22][23][24][25] optical-wavebased machine learning, [26] and temporal supersymmetry. [27][28][29] Though many of the aforementioned phenomena are typical for magnetic, non-linear, or active materials, modulation in the time domain makes them possible to realize with ordinary linear, passive, and non-magnetic materials.The simplest realization is a temporal slab [30][31][32][33][34] which can exhibit nonreciprocal effects. Furthermore, by arranging such temporal slabs in a multilayer architecture, it is possible to build generalized Bragg reflectors or hyperbolic media. [12,[35][36][37][38] Apart from these 1D configurations, novel architectures based on dynamic particles, that is, particles with time-dependent material or geometrical parameters, [39][40][41][42] and periodic or quasiperiodic arrangements of such, offer further possibilities for more sophisticated designs, for example, space-time varying metamaterials [8,11] and metasurfaces, [43][44][45] which expand the boundaries in light control.What is more, optical transitions, which occur in time-varying environments, can give rise to interesting and potentially useful effects, such as frequency conversion and filtering. Photonic transitions, similar to electronic transitions, have been anticipated in time-modulated photonic crystals by Winn et al. [46] using coupled mode theory. Such transitions through a temporal modulation can take place in different photonic configurations, such as ring resonators [47,48] or linear waveguides, [49] while the modulation in time can be achieved, for example, through an electrically driven change of the permittivity, [50] acoustic or spin waves. However, despite the previous studies that assume discrete optical modes, the problem of optical transitions between leaky modes in open systems, due to the temporal variation of the material properties, did not receive much attention so far.It is the purpose of the present paper to study, by means of full-wave dynamic calculations, the optical transitions between finite-Q modes in arrays of dielectric particles with a periodically varying permittivity. Such collective optical modes originate from the interaction between multipolar Mie resonances of the individual spheres. [51][52][53][54][55] Although for interacting dipolar (ℓ = 1) modes in a periodic lattice physically elucidating semi-analytical models can be employed, this is not the case for collective modes of higher multipolar order (ℓ > 1), which manifest themselves in high-finesse (high-Q) optical resonances in the reflection and/or transmission spectra, [54,56,57] where a fully electrodynamic approach must be undertaken. Here, we An extension of the photonic layer multiple scattering methodology to dynamic spherical scatterers, which exhibit a periodic time-varying response, is presented. The applicability of the method is demonstrated on specific examples of single-and bi-layers of periodically modulated high-refractiveindex spherical particles arranged on a squar...