Enhanced Chiral Sensing at the Few-Molecule Level Using Negative Index Metamaterial Plasmonic Nanocuvettes.

Chirality is a fundamental property of biological molecules and some pharmaceutical molecules. Chiral molecules have a pair of chiral isomers (enantiomers) with opposite handedness. Although both enantiomers of the same molecule have identical chemical and physical properties, one enantiomer may be toxic to living organisms while the other one is harmless. The detection of these enantiomers is done using their small differential absorption between right and left circularly polarized light, known as circular dichroism (CD). Considering the macroscopic size of these molecules, combined with their small differential absorption, the obtained CD signal is very small, imposing a severe limitation on the minimal concentration that can be detected. Chiral plasmonic and metamaterial structures have been used to enhance the sensitivity of CD measurements by orders of magnitude through chiral density hot spots (super chiral fields). However, the large background signal due to these structures' intrinsic chirality limits the effectiveness of these methods. Contrary to absorption-based chiral sensing measurements (CD), fluorescence detection circular dichroism (FDCD) sensing can greatly improve chiral measurement sensitivity, down to the ultimate limit of a few and even a single chiral molecule. Like differential absorption, differential fluorescence also produces a weak signal at the few-chiral-molecule limit. However, here we demonstrate a negative-index metamaterial (NIM) cavity that acts as a "plasmonic nanocuvette" with globally enhanced volume super chiral fields. Moreover, the achiral structure of the plasmonic nanocuvette allows for completely background-free chiral sensing. We show that with NIM-cavity-enhanced FDCD, we can detect as low as a few tens of chiral molecules, well within the zeptomole range.

Observation of photonic spin-momentum locking due to coupling of achiral metamaterials and quantum dots.

Chiral interfaces provide a new platform to execute quantum control of light-matter interactions. One phenomenon which has emerged from engineering such nanophotonic interfaces is spin-momentum locking akin to similar reports in electronic topological materials and phases. While there are reports of spin-momentum locking with combination of chiral emitters and/or chiral metamaterials with directional far field excitation it is not readily observable with both achiral emitters and metamaterials. Here, we report the observation of photonic spin-momentum locking in the form of directional and chiral emission from achiral quantum dots (QDs) evanescently coupled to achiral hyperbolic metamaterials (HMM). Efficient coupling between QDs and the metamaterial leads to emergence of these photonic topological modes which can be detected in the far field. We provide theoretical explanation for the emergence of spin-momentum locking through rigorous modeling based on photon Green's function where pseudo spin of light arises from coupling of QDs to evanescent modes of HMM.

3D Printing of Micrometer-Sized Transparent Ceramics with On-Demand Optical-Gain Properties.

Transparent ceramics are usually polycrystalline materials, which are wildly used in many optical applications, such as lasers. As of today, the fabrication of transparent ceramic structures is still limited to conventional fabrication methods, which do not enable the formation of complex structures. A new approach for 3D printing of micrometer-size, transparent ceramic structures is presented. By using a solution of metal salts that can undergo a sol-gel process and photopolymerization by two-photon printing, micrometer-sized yttrium aluminum garnet (YAG) structures doped with neodymium (Nd) are fabricated. The resulting structures are not only transparent in the visible spectrum but can also emit light at 1064 nm due to the doping with Nd. By using solution-based precursors, without any particles, the sintering can be performed under air at ambient pressure and at a relatively low temperature, compared to conventional processes for YAG. The crystalline structure is imaged at atomic resolution by ultrahigh-resolution scanning transmission electron microscopy (STEM), indicating that the doped Nd atoms are located at the yttrium positions. Such miniaturized structures can be used for diverse applications, e.g., optical components in high-intensity laser systems, which require heat resistance, or as light sources in optical circuits.

Ultrasmall Mode Volume Hyperbolic Nanocavities for Enhanced Light-Matter Interaction at the Nanoscale.

Cavities are the building blocks for multiple photonic applications from linear to nonlinear optics and from classical optics to quantum electrodynamics. Hyperbolic metamaterial cavities are one class of optical cavities that have recently been realized and shown to possess desirable characteristics such as engineered refractive indices and ultrasmall mode volumes, both beneficial for enhancement of light-matter interactions at the nanoscale. We hereby report the design, fabrication, and experimental characterization of nanoscale hyperbolic metamaterial cavities at the visible frequency. We show experimentally that these nanocavities enhance the light-matter interaction at the nanoscale and demonstrate increased photonic density of states and enhanced free space radiation efficiency of quantum dots coupled to such cavities, thus demonstrating the importance of hyperbolic metamaterial cavities for applications in solid-state light sources, quantum technologies, and cavity quantum electrodynamics.