Boosting Fluorescence-Based Chiral Sensing with Nanophotonics

The handedness of chiral molecules can be detected in their circularly polarized fluorescence, which is typically very weak. Here, we propose dielectric nanophotonics to increase both the fluorescence intensity and polarization contrast. (C) 2021 The Author(s

Nanophotonic Chirality Transfer to Dielectric Mie Resonators: Nano Letters

Nanophotonics can boost the weak circular dichroism of chiral molecules. One mechanism for enhanced chiral sensing relies on using a resonator to create fields with high optical chirality at the molecular position. Here, we elucidate how the reverse interaction between molecules and the resonator, called chirality transfer, can produce stronger circular dichroism. The chiral analyte modifies the electric and magnetic dipole moments of the resonator, imprinting a chiral response on an otherwise achiral resonance. We demonstrate that silicon nanoparticles and metasurfaces tailored for chirality transfer generate chiroptical signals orders of magnitude higher than the contribution from optical chirality alone. We derive closed-form equations for the dependence of chirality transfer on molecular chirality, molecule–resonator distance, and Mie coefficients. We propose a dielectric metasurface for a 900-fold circular dichroism enhancement on the basis of these principles. Finally, we identify a fundamental limit to chirality transfer. Our findings thus establish key concepts for nanophotonic chiral sensing

Role of higher order plasmonic modes in one-dimensional nanogratings

By theoretically investigating the optical behavior of one-dimensional gold nanogratings using Fourier Modal Method, we have shown that both integer and non-integer multiples of surface plasmon polariton wavelengths should be taken into consideration in special optical contrast ratio for highly sensitive sensing. The emergence of higher modes is the key factor for the formation of observed plasmonic band gap. Through considering the significant role of grating period and thickness respectively in horizontal and vertical surface resonances, it was demonstrated that for gold thicknesses below 100nm, the dominant phenomenon is horizontal surface resonances while for increased thicknesses both horizontal and vertical surface resonances mediate. The transmission minima are insensitive to the grating thickness, which confirms that their origins are not vertical surface resonances. This study can open an avenue towards designing highly sensitive sensors with focus not only on the plasmonic resonance wavelength but also on its integer and non-integer multiples whose origins should be investigated in both horizontal and vertical surface resonances

Gain enhancement of circular waveguide antennas using near-zero index metamaterials

In this article, a rigorous analytical methodology is introduced for designing near-zero refractive index metamaterials (NZIMs). Our proposed NZIM media is realized by three stacked layers of perforated metallic surfaces, each layer composed of a fishnet-like periodic array of square holes. By a proper design of such structures, a low refractive index medium is achieved at their corresponding plasma frequency. The low refractive index property is studied by retrieving the effective parameters of NZIM via inversion techniques, which gives an effective near-zero refractive index, at an operating frequency of 1.5 GHz. Then, the designed NZIM is used for gain enhancement of a circular waveguide antenna. The analysis shows that the proposed platform can enhance the directivity of our antenna by 3 dB while maintaining the return loss <-20 dB

Dual Nanoresonators for Ultrasensitive Chiral Detection

The discrimination of enantiomers is crucial in biochemistry. However, chiral sensing faces significant limitations due to inherently weak chiroptical signals. Nanophotonics is a promising solution to enhance sensitivity thanks to increased optical chirality maximized by strong electric and magnetic fields. Metallic and dielectric nanoparticles can separately provide electric and magnetic resonances. Here we propose their synergistic combination in hybrid metal–dielectric nanostructures to exploit their dual character for superchiral fields beyond the limits of single particles. For optimal optical chirality, in addition to maximization of the resonance strength, the resonances must spectrally coincide. Simultaneously, their electric and magnetic fields must be parallel and π/2 out of phase and spatially overlap. We demonstrate that the interplay between the strength of the resonances and these optimal conditions constrains the attainable optical chirality in resonant systems. Starting from a simple symmetric nanodimer, we derive closed-form expressions elucidating its fundamental limits of optical chirality. Building on the trade-offs of different classes of dimers, we then suggest an asymmetric dual dimer based on realistic materials. These dual nanoresonators provide strong and decoupled electric and magnetic resonances together with optimal conditions for chiral fields. Finally, we introduce more complex dual building blocks for a metasurface with a record 300-fold enhancement of local optical chirality in nanoscale gaps, enabling circular dichroism enhancement by a factor of 20. By combining analytical insight and practical designs, our results put forward hybrid resonators to increase chiral sensitivity, particularly for small molecular quantities

Blue-shift ultrasensitivity using rhombus-shaped plasmonic crystal on Si3N4 membrane

Harnessing ultrasensitivity from optical structures to detect tiny changes in the targeted samples is the main goal of scientists in the field of sensor design. In this study, an uncommon rhombus-shape plasmonic structure is proposed for providing blue-shift ultrasensitivity. The physical origin of this optical response relies on multi-faces of gold rhombus and their electromagnetic coupling with their induced images in a high-refractive-index substrate (Si3N4). A characteristic of blue-shift emerges as the Fano resonance in the reflection spectrum. We have experimentally shown that this novel structure has the surface sensitivity to the refractive index difference in the order of 10(-5). These characteristics have been applied for non- and conditioned- cell culture medium with refractive differences in this order.This level of sensitivity is interesting for enhanced fingerprinting of minute quantities of targeted molecules and interfacial ion redistribution. (c) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen

Accessible Superchiral Near-Fields Driven by Tailored Electric and Magnetic Resonances in All-Dielectric Nanostructures

Detection and differentiation of enantiomers in small quantities are crucially important in many scientific fields, including biology, chemistry, and pharmacy. Chiral molecules manifest their handedness in their interaction with the chiral state of light (e.g., circularly polarized light), which is commonly leveraged in circular dichroism (CD) spectroscopy. However, compared to the linear refractive index molecular chirality is extremely weak, resulting in low detection efficiencies. Recently, it has been shown that these weak chiroptical signals can be enhanced by increasing the optical chirality of the electromagnetic fields interacting with chiral samples. Here, we show numerically and analytically that dielectric structures can provide an optimum chiral sensing platform by offering uniform superchiral near-fields. To illustrate this, we first study a simple dielectric dimer and show that circularly polarized light can induce parallel and out of phase electric and magnetic fields, which are spectrally and spatially overlapped, and therefore produce superchiral fields at the midpoint of the dimer. This behavior is in contrast to, for example, a plasmonic dimer, where the optical chirality is limited by the electric dipolar field, which is not completely out of phase with the incident magnetic field. With the insights gained from this analysis, next we develop an approach for overlapping electric and magnetic fields in a single particle, based on Kerker effect. In particular, we introduce a Kerker-inspired metasurface consisting of holey dielectric disks, which offers uniform and accessible superchiral near-fields with CD signal enhancements of nearly 24 times

Duality Symmetry in Hybrid Nanoresonators for Chiral Sensing

Metal nanoparticles support intense electric resonances, while high-index dielectric particles offer strong magnetic resonances. Here, we propose metal-dielectric nanophotonic platforms based on duality symmetry for chiral molecular sensing

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. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen