A double-edged sword in two-dimensional material science and technology is optically forbidden dark exciton. On the one hand, it is fascinating for condensed matter physics, quantum information processing, and optoelectronics due to its long lifetime. On the other hand, it is notorious for being optically inaccessible from both excitation and detection standpoints. Here, we provide an efficient and low-loss solution to the dilemma by reintroducing photonics bound states in the continuum (BICs) to manipulate dark excitons in the momentum space. In a monolayer tungsten diselenide under normal incidence, we demonstrated a giant enhancement (~1400) for dark excitons enabled by transverse magnetic BICs with intrinsic out-of-plane electric fields. By further employing widely tunable Friedrich-Wintgen BICs, we demonstrated highly directional emission from the dark excitons with a divergence angle of merely 7°. We found that the directional emission is coherent at room temperature, unambiguously shown in polarization analyses and interference measurements. Therefore, the BICs reintroduced as a momentum-space photonic environment could be an intriguing platform to reshape and redefine light-matter interactions in nearby quantum materials, such as low-dimensional materials, otherwise challenging or even impossible to achieve.
A fascinating photonic platform with a small device scale, fast operating speed, as well as low energy consumption is two-dimensional (2D) materials, thanks to their in-plane crystalline structures and out-of-plane quantum confinement. The key to further advancement in this research field is the ability to modify the optical properties of the 2D materials. The modifications typically come from the materials themselves, for example, altering their chemical compositions. This article reviews a comparably less explored but promising means, through engineering the photonic surroundings. Rather than modifying materials themselves, this means manipulates the dielectric and metallic environments, both uniform and nanostructured, that directly interact with the materials. For 2D materials that are only one or a few atoms thick, the interaction with the environment can be remarkably efficient. This review summarizes the three degrees of freedom of this interaction: weak coupling, strong coupling, and multifunctionality. In addition, it reviews a relatively timing concept of engineering that directly applied to the 2D materials by patterning. Benefiting from the burgeoning development of nanophotonics, the engineering of photonic environments provides a versatile and creative methodology of reshaping light–matter interaction in 2D materials.
Resonance is instrumental in modern optics and photonics. While one can use numerical simulations to sweep geometric and material parameters of optical structures, these simulations usually require considerably long calculation time and substantial computational resources. Such requirements significantly limit their applicability in the inverse design of structures with desired resonances. The recent introduction of artificial intelligence allows for faster spectra predictions of resonance. However, even with relatively large training datasets, current end-to-end deep learning approaches generally fail to predict resonances with high-quality-factors (Q-factor) due to their intrinsic non-linearity and complexity. Here, a resonance informed deep learning (RIDL) strategy for rapid and accurate prediction of the optical response for ultra-high-Q-factor resonances is introduced. By incorporating the resonance information into the deep learning algorithm, the RIDL strategy achieves a high-accuracy prediction of reflection spectra and photonic band structures while using a comparatively small training dataset. Further, the RIDL strategy to develop an inverse design algorithm for designing a bound state in the continuum (BIC) with infinite Q-factor is applied. The predicted and measured angle-resolved band structures of this device show minimal differences. The RIDL strategy is expected to be applied to many other physical phenomena such as Gaussian and Lorentzian resonances.
Advanced photonic nanostructures have enabled the maximization of synthetic chiroptic activities. The unique structuring of these building blocks has empowered chiral selective interactions with electromagnetic waves in plasmonic structures and dielectric media. Given the repertoire of optimized chiral surfaces in the literature and the ubiquity of chirality in the organic realm, the natural direction to consider is the operation of these devices in larger optical systems much like their chiral organic counterparts. Here, we recapitulate advances in active and nonlinear chiral metamaterials. Many of the results, such as the magneto-chiral anisotropy and third-harmonic Rayleigh scattering optical activity, are relatively unknown members of the more conventional family of tuning methodology and nonlinear processes. We believe that they are poised to play an instrumental role in designing advanced chiroptic systems for applications in biochemistry, valleytronics, spintronics, and chiral quantum optics.
Photonic Bound States in the Continuum In article number 2100658, Xuzhi Ma, Yuan Ma, Shoufeng Lan, and colleagues develop a universal deep learning strategy to design high‐finesse resonances with ultrasharp spectral features. Applying to photonic bound states in the continuum (BICs), they decompose a spectrum into a relatively smooth background and multiple spectral extremes with narrow linewidths, followed by an adaptive data acquisition method for high resolution. They further regulate such a nonlinear neural network with the physical meaning of resonances to prevent overfitting and local minima issues.
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