The speed of light in a vacuum is an absolute limit, but it can be intentionally reduced in various media. Slowing down light offers numerous advantages, making it easier to manipulate and enhancing interactions with matter. This, in turn, improves the performance of photonic devices in applications such as optical nonlinearities, optical switching, optical sensing, optical storage, and quantum optics. 

Various techniques have been developed over the years to slow down light, including electromagnetically induced transparency (EIT), Bose-Einstein condensate (BEC), photonic crystals, and stimulated Brillouin scattering (SBS). Notably, researchers from Harvard, led by Hau et al., achieved a breakthrough using EIT to reduce light speed to 17 m/s in an ultracold atomic gas. This success sparked interest in exploring EIT analogues in metasurfaces, a transformative platform in optics and photonics. 

Despite the benefits, slow-light structures face a significant challenge: loss, limiting storage time and interaction length. This issue is particularly severe for metasurface analogues of EIT due to scattering loss of nanoparticles and sometimes absorption loss of materials. 

In a recent paper published online in Nano Letters on Jan. 5, Dr. LI Guangyuan and coauthors from the Shenzhen Institute of Advanced Technology (SIAT) at the Chinese Academy of Sciences introduced a novel strategy to realize a metasurface analogue of EIT while effectively suppressing losses. 

Unlike conventional metasurface analogues of EIT, induced by coupling between two localized resonances supported by closely packed meta-atoms, or between localized and collective resonances, the researchers proposed a new type called "collective EIT-like resonance." This is induced by the coupling between two collective resonances: a Mie electric dipole surface lattice resonance (ED-SLR) and an in-plane or out-of-plane electric quadrupole SLR (EQ-SLR). Using silicon metasurfaces with a 100 nm-thick nanodisk array, they demonstrated collective EIT-like resonances with a quality factor exceeding 2750, more than five times the state-of-the-art. In practical terms, light passing through the silicon nanodisks can be slowed down by more than 10,000 times, with a reduction in loss by more than five times compared to existing literature. 

Dr. LI explained the departure from the conventional belief that metasurface performance depends on how closely meta-atoms can be placed. Instead, they explored the extreme regime of zero distance between meta-atoms, essentially merging them into one. Unlike conventional methods, their approach allows the tuning of surface lattice resonances to overlap spectrally, enabling the realization of metasurface analogues of EIT. 

The researchers further demonstrated a BIC-characterized collective EIT-like resonance, utilizing the transition between the in-plane EQ-SLR and the bound state in the continuum (BIC). This unexpected discovery suggests the potential to slow down light by an arbitrarily large factor while maintaining a growing quality factor. 

The innovative design proposed by the researchers holds promise for unprecedented photon manipulation with greater flexibility and potential applications in slow-light photonic chips. 

Localized ED and EQ resonances supported by individual nanodisks cannot spectrally overlap. In contrast, the coupling of ED- and EQ-SLRs induces collective EIT-like resonance in metasurface composed of periodic nanodisks. (Image by SIAT)

Media Contact:
ZHANG Xiaomin
Email:xm.zhang@siat.ac.cn