We experimentally demonstrate a robust Si-photonic waveguide architecture that realizes dynamically encircling an exceptional point (EP) in the optical domain and broadband asymmetric modal transmission as an essential consequence. The structure consists of a pair of coupled channel waveguides and an adjacent slab-waveguide patch that enable precise lithographic controls on the phase velocities and radiation rates of the guided photonic modes. Complex modal index and inter-mode coupling constant profiles required for the encircling-an-EP parametric control are precisely coded in the geometry of those elements. The device created on this basis induces the symmetry-exchanging adiabatic state flip for one transmission direction and symmetry-preserving anti-adiabatic state-jump for the transmission in the opposite direction. In fabrication, we use a state-of-the-art electron-beam lithography for creating mm-long devices with nm-scale transversal precision. A comprehensive spectral measurement for the intensity and phase distributions of the transmitted optical states is obtained with a specially designed phase-sensitive infrared microscopy integrated with a tunable diode-laser system and spectrum analyzer. On this basis, we confirm in the experiment the highly asymmetric modal transmission persisting over a broad spectral band exceeding 100 nm in the telecommunications window around 1,550 nm. Hence, we establish a substantive experimental step toward broadband non-reciprocal photonic devices based on the unique non-Hermitian dynamics.
Jae Woong Yoon, Youngsun Choi, Choloong Hahn, Ki Yeon Yang, Jeong Yub Lee, Gunpyo Kim, Jong Kyun Hong, Yeonghwa Ryu, Seok Ho Song, and Pierre Berini, "Experimental demonstration of a broadband time-asymmetric waveguide architecture enabled by dynamically encircling an exceptional point in the optical domain (Conference Presentation)," Proc. SPIE 10549, Complex Light and Optical Forces XII, 105490V (Presented at SPIE OPTO: January 31, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2288098.5751542811001.
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