Dielectric Tetramer Nanoresonators Supporting Strong Superchiral Fields for Vibrational Circular Dichroism Spectroscopy.

Chirality (C) is a fundamental property of objects, in terms of symmetry. It is extremely important to sense and distinguish chiral molecules in the fields of biochemistry, science, and medicine. Vibrational circular dichroism (VCD) spectroscopy, obtained from the differential absorption of left- and right- circularly polarized light (CPL) in the infrared range, is a promising technique for enantiomeric detection and separation. However, VCD signals are typically very weak for most small molecules. Dielectric metasurfaces are an emerging platform to enhance the sensitivity of VCD spectroscopy of chiral molecules via superchiral field manipulation. Here, we demonstrate a dielectric metasurface consisting of achiral germanium (Ge) tetramer nanoresonators that provide a proper and accessible high C enhancement (CE). We realize a maximum C enhancement (CE_max) with respect to the incident CPL (CE_max = Cmax/CRCP) of more than 750. The volume-averaged C enhancement (CE_ave = Cave/CRCP) is 148 in the 50 nm thick region above the sample surface and 215 in the central region of the structure. Especially, the corresponding CE_ave values are more than 89 and 183 even when a 50 nm thick chiral lossy molecular layer is coated on the metasurface. The metasurface benefits from geometrically achiral nanostructure design to eliminate intrinsic background chiral-optical signal from the substrate, which is useful in chiral sensing, enantioselectivity, and VCD spectroscopy applications in the mid-infrared range.

Imaging-based spectrometer-less optofluidic biosensors based on dielectric metasurfaces for detecting extracellular vesicles.

Biosensors are indispensable tools for public, global, and personalized healthcare as they provide tests that can be used from early disease detection and treatment monitoring to preventing pandemics. We introduce single-wavelength imaging biosensors capable of reconstructing spectral shift information induced by biomarkers dynamically using an advanced data processing technique based on an optimal linear estimator. Our method achieves superior sensitivity without wavelength scanning or spectroscopy instruments. We engineered diatomic dielectric metasurfaces supporting bound states in the continuum that allows high-quality resonances with accessible near-fields by in-plane symmetry breaking. The large-area metasurface chips are configured as microarrays and integrated with microfluidics on an imaging platform for real-time detection of breast cancer extracellular vesicles encompassing exosomes. The optofluidic system has high sensing performance with nearly 70 1/RIU figure-of-merit enabling detection of on average 0.41 nanoparticle/µm2 and real-time measurements of extracellular vesicles binding from down to 204 femtomolar solutions. Our biosensors provide the robustness of spectrometric approaches while substituting complex instrumentation with a single-wavelength light source and a complementary-metal-oxide-semiconductor camera, paving the way toward miniaturized devices for point-of-care diagnostics.

Membrane activity detection in cultured cells using phase-sensitive plasmonics.

Despite the existence of various neural recording and mapping techniques, there is an open territory for the emergence of novel techniques. The current neural imaging and recording techniques suffer from invasiveness, a time-consuming labeling process, poor spatial/ temporal resolution, and noisy signals. Among others, neuroplasmonics is a label-free and nontoxic recording technique with no issue of photo-bleaching or signal-averaging. We introduced an integrated plasmonic-ellipsometry platform for membrane activity detection with cost-effective and high-quality grating extracted from commercial DVDs. With ellipsometry technique, one can measure both amplitude (intensity) and phase difference of reflected light simultaneously with high signal to noise ratio close to surface plasmon resonances, which leads to the enhancement of sensitivity in plasmonic techniques. We cultured three different types of cells (primary hippocampal neurons, neuroblastoma SH-SY5Y cells, and human embryonic kidney 293 (HEK293) cells) on the grating surface. By introducing KCl solution as a chemical stimulus, we can differentiate the neural activity of distinct cell types and observe the signaling event in a label-free, optical recording platform. This method has potential applications in recording neural signal activity without labeling and stimulation artifacts.

Phase-sensitive plasmonic biosensor using a portable and large field-of-view interferometric microarray imager.

Nanophotonics, and more specifically plasmonics, provides a rich toolbox for biomolecular sensing, since the engineered metasurfaces can enhance light-matter interactions to unprecedented levels. So far, biosensing associated with high-quality factor plasmonic resonances has almost exclusively relied on detection of spectral shifts and their associated intensity changes. However, the phase response of the plasmonic resonances have rarely been exploited, mainly because this requires a more sophisticated optical arrangement. Here we present a new phase-sensitive platform for high-throughput and label-free biosensing enhanced by plasmonics. It employs specifically designed Au nanohole arrays and a large field-of-view interferometric lens-free imaging reader operating in a collinear optical path configuration. This unique combination allows the detection of atomically thin (angstrom-level) topographical features over large areas, enabling simultaneous reading of thousands of microarray elements. As the plasmonic chips are fabricated using scalable techniques and the imaging reader is built with low-cost off-the-shelf consumer electronic and optical components, the proposed platform is ideal for point-of-care ultrasensitive biomarker detection from small sample volumes. Our research opens new horizons for on-site disease diagnostics and remote health monitoring.