image: a, the depiction of a bound vortex of fermions in the gauge field of the cosmic string. b, the schematic of the deformed photonic graphene lattice with a position-dependent coefficient to emulate the cosmic string with a gauge field. The symbol O is the origin center of the photonic graphene lattice where the defect was located. c, the experimental schematic for exciting a series of waveguides in the innermost ring layer around the cosmic string. d, the evolution of light with angular momentum with different propagation distances in the cosmic string. e, the evolution of light without angular momentum with different propagation distances in the cosmic string. view more
Credit: by C. Sheng, Y. Wang, Y. J. Chang, H. M. Wang, Y. H. Lu, Y. Y. Yang, S. N. Zhu, X. M. Jin, and H. Liu
Topology originated from the classification of geometric structures in mathematics, and have prevailed in a variety of branches of physics, ranging from the early electronic topological insulator to the current topological acoustics, topological optics, topological circuits, etc. Akin to topological defects in solids, such as point defects, dislocations and domain walls, theorists predicted that some topological defects may have formed during a symmetry breaking of the vacuum field in the early universe when the topology of the vacuum manifold associated with this symmetry breaking was not simply connected. Among these topological defects, the cosmic string is a one-dimensional topological defect of cosmic space-time. People began to try to find evidence for the existence of topological defects, such as gravitational waves generated by cosmic strings in the early universe. On the other hand, with the flourishment of transformation optics, these underlying theoretical foundations of speculative phenomena in cosmic can be extensively applied to design novel optical applications, for example, the photonic cavity by emulating black holes, the non-diffractive beam inspired by gravitational lensing.
In a new paper published in Light Science & Application, Chong Sheng from Nanjing University, China, and co-workers have experimentally demonstrated bound vortex light on optical chips by simulating gauge fields of cosmic strings through position-dependent coupling coefficients in a deformed photonic graphene. Furthermore, these types of photonic lattices inspired by cosmic strings can simultaneously generate and transport optical vortices, and even can control the orbital angular momentum of photons on integrated optical chips. Specifically, the research team began with the massless Dirac equation with a gauge field in the cosmic string to obtain the equivalent Hamiltonian. After that, with the aid of transformation optics, they exploited the photonic graphene lattice by deliberately tuning the coupling coefficient as a function of the position to map the equivalent Hamiltonian. Owing to the advanced femtosecond laser direct writing technology for fabricating photonic lattices as demand, such a deformed photonic graphene inspired by cosmic strings was constructed in borosilicate glass substrate. It is found that when the single waveguide near the defect is excited, the bound optical vortex near the defect will be generated. At the same time, this type of deformed photonic graphene inspired by cosmic strings has the capability of distinguishing photons with or without orbital angular momentum. When the light with orbital angular momentum was injected into this type of deformed photonic graphene, it exhibited a well-defined local confinement, whereas the light spread into all sites for the case without orbital angular momentum.
Compared to the previous design of transformation optical photonic devices with only considering the simulation of gravitational field, this research work combined both a gravitational field and a gauge field into the design of photonic devices at the same time, which enriches the means of transform optical design of photonic devices. More interestingly, this kind of photonic device inspired by the cosmic string with gauge field can realize the discrimination of orbital angular momentum in integrated photonic devices, and may be used for orbital angular momentum demultiplexing technology in future optical information technology.