Negative refraction mediated by bound states in the continuum

Developing new ways to manipulate the transportation of light is very important from both the fundamental science and the technological points of view in photonics. In general, the incident and the refracted beams are on either side of the interface normal. Envisioned by Veselago in the 1960s, negative refraction has received intense research efforts from the photonics community with long-term quests for new scopes and functionality.


In particular, the experimental demonstration of negative refraction in the photonic crystal (PhC) slab provides on-chip solutions for molding the flow of light for various applications, such as couplers and micro-spectrometer, just to name a few. Total internal reflection (TIR) represents a fundamental guiding mechanism to channel lightwave in an optical chip which is generally considered as a cornerstone for guide wave optics. Such mechanism is essentially below the light cone of the cladding and substrate media.


To data, the canonical way to realize on-chip negative refraction strictly relies on total internal refraction, which inevitably imposes a fundamental limitation that negative refraction can only be achieved below the light cone of surrounding media.


To overcome this limitation, Dr. Yi Xu now in Prof. Yuwen Qin's group from the Advanced Institute of Photonics Technology, School of Information Engineering at Guangdong University of Technology, together with his Master student Zhanyuan Zhang and colleagues, propose and experimentally demonstrate a new mechanism which achieves planar negative refraction beyond total internal refraction. Related research results are published in Photonics Research, Vol. 9, Issue 8, 2021 (Zhanyuan Zhang, Feifei Qin, Yi Xu, Songnian Fu, Yuncai Wang, Yuwen Qin. Negative refraction mediated by bound states in the continuum[J]. Photonics Research, 2021, 9(8): 08001592).


They apply the concept of Bound state in the continuum (BIC) in which electromagnetic modes possess theoretical unbound quality factors to realize negative refraction within the radiative continuum. The proposed mechanism relies on the synchronous engineering of the spatial dispersion and radiative lifetime of the Bloch modes in momentum space for a PhC slab.


Therefore, the out-of-plane guiding is enabled by the quasi-BIC while the in-plane propagation direction is tailored by the elaborated spatial dispersion of PhC slab, simultaneously. The physical mechanism of negative refraction mediated by quasi-BIC is general, and it can be applied over a wide spectral range of electromagnetic waves.


To further verify the theoretical results, microwave experiments are conducted to characterize the negative refraction phenomenon mediated by quasi-BIC. Their experimental measured near-field profiles confirm that the refracted beam in the PhC slab is on the same side of the interface normal as the incident beam, and the acquired phase fronts of the incident and outgoing beam are parallel to each other.


By taking Fourier transform of the measured near-field results, it is validated that the observed negative refraction is indeed beyond the light cone. Compared with the traditional negative refraction phenomenon based on total internal reflection, the proposed mechanism breaks the limitation that the negative refraction can only be achieved in the two-dimensional plane for a PhC slab and provides the possibility to realize stereo-coupling between free-space and the quasi-BIC mode featured with negative refraction (Fig. 1).


Fig 1. Coupling of a Gaussian beam with a stereo incident angle to the planar negative refraction mode mediated by the quasi-BICs.


Although the bandwidth of negative refraction mediated by BIC is limited, which would hamper subsequent applications of the demonstrated concept, merging multiple BICs in momentum space proposed recently can be used to expand the bandwidth. More promisingly, the proposed mechanism is fully compatible with the guiding mechanisms below the light cone, manifested itself as a promising platform for multiplexing of dense optical signals.


The team members think that this work provides an example that the physical concept of BIC can harness new functionalities and possibility for manipulating the flow of light in photonics, which might open up an avenue for exploring anormal refraction within the radiative continuum and find applications in optical couplers, multiplexers, and mode sorting.


Prof. Cheng-Wei Qiu, Editor of Photonics Research from Department of Electrical and Computer Engineering in National University of Singapore, believes that this highly innovative work provides a distinct BIC perspective revisiting the physics of negative refraction and guided wave optics beyond the light line. It thus paves the way toward advanced multiplexing of optical signal beyond and below the light cone simultaneously. Though their proof-of-concept experiment is performed in microwave region, the optical demonstration in silicon platform is readily expected to accomplish in the near future because of the scalability of PhCs.