The electromagnetic Lorentz reciprocity theorem states that in linear, time-independent systems with symmetric constitutive optical tensors the relation between received and transmitted fields are the same for forward and time-reversed processes. While most photonic devices operate under this principle, there are various circumstances in which reciprocity has undesirable effects, e.g., photovoltaic cells re-emitting absorbed solar energy or information back flow due to parasitic scattering in the environment.

Spatio-temporally modulated metasurfaces (STMMs) – dynamic ultrathin arrays of engineered sub-wavelength resonant nanostructures with arbitrary reconfigurability – are versatile photonic systems that can be designed to violate time-reversal symmetry and realize nonreciprocal phenomena. After a brief overview on metasurfaces and nonreciprocity, we will discuss our recent works in space-time metasurfaces for harnessing both classical and quantum light. We introduce a STMMs capable of dynamically tailoring arbitrary wavefronts of free-space electromagnetic waves to demonstrate maximum breakdown of the Lorentz theorem and complete free-space optical isolation by photon-to-surface wave conversion in beam steering and focusing.

We will then extend our discussion to the quantum realm to investigate how space-time quantum metasurfaces (STQMs) allow for shaping the spectral and spatial properties of quantum light. We will show that STQMs may enable disruptive quantum photonic functionalities, including bespoke generation and manipulation of entanglement/hyperentanglement, and the realization of space-time asymmetry at the deepest level of the quantum vacuum via the dynamical Casimir effect.

Bibliography:
[1] A. E. Cardin et al., Nature Communications 11, 1469 (2020).
[2] W. J. M. Kort-Kamp et al., Physical Review Letters 172, 043603 (2021).