The quintessential concept of metamaterials is to obtain material properties via structuring, and that of photonic crystals is to master band structures inspired by condensed matter. The combination of the two gives rise to an exciting optical frontier called reciprocal or k-space metaphotonics, with a prominent example of bound states in the continuum (BICs). Here, we will discuss our recent efforts on artificial intelligence to design BICs, utilizing them for optically forbidden excitons, and investigate the topological nature of BICs for nonlinear high-harmonic generations. The high finesse and relative ease of fabrication could render BIC-based k-space metaphotonics promising for applications.
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.
This Conference Presentation, "Metaphotonics: from backward phase-matching to augmented reality," was recorded at Photonics West 2020 held in San Francisco, California, United States.
One of the most fundamental mysteries is the homochirality of living organisms on the Earth. Scientists have spent endless efforts in understanding the origin of the enantiomeric excess, in which a magneto-chiral effect is believed to play a role. However, this magneto-chiral effect observed so far is very weak in diamagnetic bulk crystals under a strong magnetic field, synthetic chiral molecules with magnetic components in solutions, or thin-film chiral magnets and metamaterials. Recently, atomically flat two-dimensional materials have emerged with intriguing properties such as optical anisotropy, two-dimensional ferromagnetism, and valley pseudospins. Here, we report the observation of giant magneto-chiral dichroism in atomically thin van der Waals crystals. We found such giant magneto-chiral dichroism originated from two unique physical processes. The parity-inversion symmetry breaking induces a large chirality, and time-reversal symmetry breaking results in strong magnetic moments. Such an approach offers rich physics with the interplay of the magnetism, chirality, and valley pseudospins in a unified manner. The observed giant magneto-chiral effect may further our understanding of the enantiomeric excess that is important for photochemical reactions, asymmetric synthesizes, and drug delivery.
Nanostructured metals have utilized the strong spatial confinement of surface plasmon polaritons to harness enormous energy densities on their surfaces, and have demonstrated vast potential for the future of nano-optical systems and devices. While the spectral location of the plasmonic resonance can be tailored with relative ease, the control over the spectral linewidth associated with loss represents a more daunting task. In general, plasmonic resonances typically exhibit a spectral linewidth of ~50 nm, limited largely by the combined damping and radiative loss in nanometallic structures. Here, we present one of the sharpest resonance features demonstrated by any plasmonic system reported to date by introducing dark plasmonic modes in diatomic gratings. Each duty cycle of the diatomic grating consists of two nonequivalent metallic stripes, and the asymmetric design leads to the excitation of a dark plasmonic mode under normal incidence. The dark plasmonic mode in our structure, occurring at a prescribed wavelength of ~840 nm, features an ultra-narrow spectral linewidth of about 5 nm, which represents a small fraction of the value commonly seen in typical plasmonic resonances. We leverage the dark plasmonic mode in the metallic nanostructure and demonstrate a resonance enhanced plasmoelectric effect, where the photon-induced electric potential generated in the grating is shown to follow the resonance behavior in the spectral domain. The light concentrating ability of dark plasmonic modes in conjunction with the ultra-sharp resonance feature at a relatively low loss offers a novel route to enhanced light-matter interactions with high spectral sensitivity for diverse applications.
We describe a novel method based on optical coherence tomography (OCT) for the accurate measurement of the refractive index of in vitro human teeth. We obtain the refractive indices of enamel, dentin, and cementum to be 1.631±0.007, 1.540±0.013, and 1.582±0.010, respectively. The profile of the refractive index is readily obtained via an OCT B scan across a tooth. This method can be used to study the refractive index changes caused by dental decay and therefore has great potential for the clinical diagnosis of early dental caries.
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