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Department of Physics & Astronomy

Symmetry breaking in photonic Floquet media

2021-08-30l 조회수 1011
일시 : 2021-12-01 16:00 ~
연사 : 민범기 (KAIST 기계공학과)
담당 : Prof. Bohm Jung Yang, Prof. Dohun Kim
장소 : 온라인
Periodically driven systems, characterized by their inherent non-equilibrium dynamics, are ubiquitously found in both classical and quantum regimes. In the field of photonics, these Floquet systems have begun to provide insight into how time periodicity can extend the concept of spatially periodic photonic crystals and metamaterials to the time domain. However, despite the necessity arising from the presence of non-reciprocal coupling between states in a photonic Floquet medium, a unified non-Hermitian band structure description remains elusive. Here, we experimentally reveal the unique Bloch-Floquet and non-Bloch band structures of a photonic Floquet medium emulated in the microwave regime with a one-dimensional array of time-periodically driven resonators. Specifically, these non-Hermitian band structures are shown to be two measurable distinct subsets of complex eigenfrequency surfaces of the photonic Floquet medium defined in complex momentum space. In the Bloch-Floquet band structure, the driving-induced non-reciprocal coupling between oppositely signed frequency states leads to opening of momentum gaps along the real momentum axis, at the edges of which exceptional phase transitions occur. More interestingly, we show that the non-Bloch band structure defined in the complex Brillouin zone supplements the information on the morphology of complex eigenfrequency surfaces of the photonic Floquet medium. Although one of the most intriguing features of a photonic Floquet medium is the creation of dressed states of negative frequency and their interactions leading to exceptional phase transitions, importantly, the interpretation of experimental observations given here is based on a linearized model. In the strong driving regime, however, ignoring non-linearity along with dissipation and fluctuation becomes increasingly difficult. This implies that a thorough understanding of these effects is required for characterisation of exotic new phases in non-equilibrium photonic matter. In this regard, non-reciprocal phase transitions in active matter seem to share many features in common with the proposed non-equilibrium photonic Floquet medium, which coherently exchanges energy with its environment. Particularly, classical many-body effects can be manifested by non-linearity; whether a subharmonic spectral component arising at the momentum gap is associated with many-body time crystalline behaviour would be an especially interesting question for which further investigation is highly demanded.