Mie scattering enhances chiral control

Spectral degradation of singularities is characteristic of non-Ermi systems and has been used in research areas such as lasers, controlling light transmission, and enhancing sensor response. However, traditional methods of achieving singularities on resonators using two nano-tips can lead to resonant stability issues and additional losses.

Scientists at the University of Delaware have designed a Mie-scattering device based on a definition on silicon insulators that enables chiral light transmission without post-adjustment.

The work was published in the new journal eLight of the High Start of the Excellence Program under the title “Chiral exceptional point and coherent suppression of backscattering in silicon microring with low loss, Mie scatterer.”

The spectral degradation of non-Ermi systems, i.e. singularities, has been used in research areas such as lasers, controlling light transmission, and enhancing sensor response. This can be brought to a singularity point by controlling the coupling between clockwise and counterclockwise modes of frequency degradation in the ring resonator, which is often achieved by introducing two or more nanotips into the mode field volume of the resonator.

While this approach provides an avenue to study singularity physics, it ignores the effect of the shape and size of the nanotip on the non-semination of the system, as well as the additional losses caused by plane and vertical scattering. Limited resonant stability presents a significant challenge for designing switches or modulators using singularity effects, which require stable cavity resonance and fixed laser-cavity imbalance.

Professor Gu Tingyi’s team from the Department of Electrical and Computer Engineering at the University of Delaware used lithography to define asymmetric and symmetrical Michaelis scatterers, realize wave transmission and reflection control at sub-wavelength scales, and avoid additional radiation channels. They show that these predefined components can bring the system to a singularity point without post-hoc adjustments and enable chiral light transmission within the resonator.

Surprisingly, geometric defects in components can improve the quality factor measured on the transmission port by coherently suppressing backscattering of surface roughness. The proposed device platform enables predefined chiral light propagation and backscatter-free resonance for applications such as frequency combs, solitons, sensors, and other nonlinear optical processes such as photon blocking and regenerative oscillators.

This work not only opens up a completely new path for chiral silicon photonics, but also has the following four aspects of significance.

First, it reveals the critical role of spatial asymmetry between nanotips and Michaelis scatterers in bringing the system to a singularity.

Second, the path that drives the non-Ermi system to and away from the singularity by means of scattering geometry control is detailed.

Third, the designed system is mechanically stable. By comparing the transmission and reflection spectra of perturbated microcavities, the contribution of nanotips/scatterers to the diagonal term is revealed. This contrasts with the traditional way of achieving singularities with two nanotips, which has stability flaws.

Fourth, this work demonstrates for the first time an enhancement method for empirical quality factors extracted from transmission spectroscopy. (Source: China Optics)

Figure 1: Singularity achieved by embedding a pair of Michaelis scatterers in a waveguide and resonator. (a) Scanning electron microscope image of a channel waveguide with lithographic defined symmetrical elements on a silicon insulator substrate. (b) Electron microscopic image (top) and parametric design (bottom) of channel waveguides with lithographically defined rectangular symmetric Michaelis scatterers on silicon insulator substrates. (c) Singularities resulting from optical impedance matching.

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