Compact ring resonators of silicon nanorods for strong optomechanical interaction.

Optomechanical interaction in microstructures plays a more and more important role in the fields of quantum technology, information processing, and sensing, among others. It is still a challenge to obtain a strong optomechanical interaction in a compact device. Here, we propose and demonstrate that compact ring resonators consisting of silicon nanorods can realize strong optomechanical interaction even surpassing that of most optical microcavities. The proposed ring resonators can well confine infrared optical waves by the quasi-bound states in the continuum. Meanwhile, each nanorod in the resonator acts as a mechanical resonator of GHz resonating frequency, thus realizing an optomechanical coupling rate of up to 1.8 MHz. We have found that the interaction area can be extended by increasing the number of nanorods while maintaining the optomechanical interaction strength. Finally, we have studied the influence of supporting structures for suspended nanorods on the optomechanical interaction properties. The proposed ring resonators of silicon nanorods offer a promising platform for the study of optomechanical interaction.

Defect-assisted, spray-printed colloidal quantum dot microlasers for biosensing.

This study successfully implements spectrally distinguishable CdSe-ZnS core-shell colloidal quantum dot (CQD) microlasers by a simple, efficient spray printing technique and demonstrates its potential in biosensing. We have systematically characterized the optical properties of printed microring lasers with diameters less than 60 µm. The smallest structure that can be excited has a diameter as small as 30 µm, which is much smaller than the counterparts prepared by piezoelectric ink-jet printing. The detection sensitivity of 4.54 nm/min/refractive index unit is verified in glucose sensing using a printed CQD microlaser. Biosensing of diverse glucose and bovine serum albumin solutions using printed microlasers with the assistance of defects demonstrates a new, to the best of our knowledge, prototype for the development of high-performance, low-cost on-chip microcavity sensors.

Efficient acousto-optic modulation using a microring resonator on a thin-film lithium niobate-chalcogenide hybrid platform.

This Letter presents the first, to the best of our knowledge, thin-film lithium niobate-chalcogenide based microring acousto-optic modulator where an interdigital transducer and a chalcogenide strip waveguide are integrated on X-cut thin-film lithium niobate. The microring resonator exhibits a high loaded quality factor of 5 × 105. The developed hybrid acousto-optic modulator with an interaction length of 120 µm demonstrates an effective half-wave voltage of only 1.74 V, which corresponds to a voltage-length product of 0.02 V•cm. The performance of the acousto-optic modulator demonstrated on the unsuspended thin-film lithium niobate-chalcogenide waveguide platform is on par with that obtained from an acoustic cavity assisted homogeneous lithium niobate counterpart.

Highly efficient acousto-optic modulation using nonsuspended thin-film lithium niobate-chalcogenide hybrid waveguides.

A highly efficient on-chip acousto-optic modulator is as a key component and occupies an exceptional position in microwave-to-optical conversion. Homogeneous thin-film lithium niobate is preferentially employed to build the suspended configuration for the acoustic resonant cavity, with the aim of improving the modulation efficiency of the device. However, the limited cavity length and complex fabrication recipe of the suspended prototype restrain further breakthroughs in modulation efficiency and impose challenges for waveguide fabrication. In this work, based on a nonsuspended thin-film lithium niobate-chalcogenide glass hybrid Mach-Zehnder interferometer waveguide platform, we propose and demonstrate a built-in push-pull acousto-optic modulator with a half-wave-voltage-length product VπL as low as 0.03 V cm that presents a modulation efficiency comparable to that of a state-of-the-art suspended counterpart. A microwave modulation link is demonstrated using our developed built-in push-pull acousto-optic modulator, which has the advantage of low power consumption. The nontrivial acousto-optic modulation performance benefits from the superior photoelastic property of the chalcogenide membrane and the completely bidirectional participation of the antisymmetric Rayleigh surface acoustic wave mode excited by the impedance-matched interdigital transducer, overcoming the issue of low modulation efficiency induced by the incoordinate energy attenuation of acoustic waves applied to the Mach-Zehnder interferometer with two arms in traditional push-pull acousto-optic modulators.

Strong optomechanical coupling in chain-like waveguides of silicon nanoparticles with quasi-bound states in the continuum.

We propose and demonstrate that strong optomechanical coupling can be achieved in a chain-like waveguide consisting of silicon nanorods. By employing quasi-bound states in the continuum and mechanical resonances at a frequency around 10 GHz, the optomechanical coupling rate can be above 2 MHz and surpass most microcavities. We have also studied cases with different optical wave numbers and size parameters of silicon, and a robust coupling rate has been verified, benefiting the experimental measurements and practical applications. The proposed silicon chain-like waveguide of strong optomechanical coupling may pave new ways for research on photon-phonon interaction with microstructures.

A review of dielectric optical metasurfaces for spatial differentiation and edge detection.

Dielectric metasurfaces-based planar optical spatial differentiator and edge detection have recently been proposed to play an important role in the parallel and fast image processing technology. With the development of dielectric metasurfaces of different geometries and resonance mechanisms, diverse on-chip spatial differentiators have been proposed by tailoring the dispersion characteristics of subwavelength structures. This review focuses on the basic principles and characteristic parameters of dielectric metasurfaces as first- and second-order spatial differentiators realized via the Green's function approach. The spatial bandwidth and polarization dependence are emphasized as key properties by comparing the optical transfer functions of metasurfaces for different incident wavevectors and polarizations. To present the operational capabilities of a two-dimensional spatial differentiator in image information acquisition, edge detection is described to illustrate the practicability of the device. As an application example, experimental demonstrations of edge detection for different biological cells and a flower mold are discussed, in which a spatial differentiator and objective lens or camera are integrated in three optical pathway configurations. The realization of spatial differentiators and edge detection with dielectric metasurfaces provides new opportunities for ultrafast information identification in biological imaging and machine vision.

Effects of selective calcium channel blockers on ions' permeation through the human Cav1.2 ion channel: A computational study.

Selective calcium channel antagonists are widely used in the treatment of cardiovascular disorders. They are mainly classified into 1,4-dihydropyridine (1,4-DHPs) and non-DHPs. The non-DHPs class is further classified into phenylalkylamines (PAAs) and benzothiazepines (BZTs) derivatives. These blockers are used for the treatment of hypertension, angina pectoris, and cardiac arrhythmias. Despite their well-established efficiency, the structural basis behind their activity is not very clear. Here we report the use of a near-open confirmation (NOC) model of the Cav1.2 cardiac ion channel to examine the mode of binding of these antagonists within the pore domain as well as the fenestration of the pore-forming domains. Effects of calcium ion permeation in the presence of drug molecules were assessed using steered molecular dynamics (SMD) simulations. These studies reveal that nicardipine, a DHP derivative, shows a strong Cav1.2 blocking activity, requiring more 2500 pN force to pull calcium ion towards the channel's pore in the presence of the compound. Similar blocking activity was observed for verapamil, a PAA derivative, requiring almost 2300 pN of force. The least blocking activity was observed for Diltiazem, a BZT derivative. Our results explain the structural basis and the binding details of 1,4-DHPs, PAAs and BZTs at their distinct Cav1.2 sites and offer detailed insights into their mechanism of action in modulating the Cav1.2 channel.

Optical analog computing of spatial differentiation and edge detection with dielectric metasurfaces.

We propose and demonstrate that optical analog computing of spatial differentiation and edge detection can be realized with a single layer of dielectric metasurface. The optical transfer function for second-order derivation is obtained by engineering the spatial dispersion of electric dipole resonance supported by the silicon nanodisks in the metasurface. Benefiting from this unique mechanism of electric dipole resonance, spatial differentiation can be performed for two dimensions and arbitrary polarization with a large spatial bandwidth and high efficiency at the visible wavelength. Explicitly, we have numerically validated the application with one-dimensional spatial functions as well as an image, and the results show excellent performance. Our study can facilitate the research of optical computing with artificial nanostructures.

Resonant broadband unidirectional light scattering based on genetic algorithm.

The spectrum overlapping of the radiative power between magnetic and electric dipole moments in nanoparticles can be used to realize unidirectional light scattering, which is promising for various kinds of applications. Nevertheless, it is still challenging to achieve such overlapping in a broadband manner. Herein, we propose that the combination of a genetic algorithm, Maxwell's equations, and electromagnetic multipole expansion can be used to design a nanoparticle that supports resonant broadband forward light scattering. Microwave experiments are performed to demonstrate our numerical results. The proposed method is quite general, and it can be straightforwardly generalized to design functional unidirectional scatters.

Atomistic modeling and molecular dynamics analysis of human CaV1.2 channel using external electric field and ion pulling simulations.

BACKGROUND: Human CaV1.2 (hCav1.2), a calcium selective voltage-gated channel, plays important roles in normal cardiac and neuronal functions. Calcium influx and gating mechanisms leading to the activation of hCaV1.2 are critical for its functionalities. Lack of an experimentally resolved structure of hCaV1.2 remains a significant impediment in molecular-level understanding of this channel. This work focuses on building atomistic hCaV1.2 model and studying calcium influx using computational approaches. METHODS: We employed homology modeling and molecular dynamics (MD) to build the structure of hCaV1.2. Subsequently, we employed steered molecular dynamics (SMD) to understand calcium ion permeation in hCaV1.2. RESULTS: We report a comprehensive three-dimensional model of a closed state hCaV1.2 refined under physiological membrane-bound conditions using MD simulations. Our SMD simulations on the model revealed four important barriers for ion permeation: this includes three calcium binding sites formed by the EEEE- and TTTT- rings within the selectivity filter region and a large barrier rendered by the hydrophobic internal gate. Our results also revealed that the first hydration shell of calcium remained intact throughout the simulations, thus playing an important role in ion permeation in hCaV1.2. CONCLUSIONS: Our results have provided some important mechanistic insights into the structure, dynamics and ion permeation in hCaV1.2. The significant barriers for ion permeation formed by the four phenylalanine residues at the internal gate region suggest that this site is important for channel activation.

Manipulation of transmission by engineered disorder in one-dimensional photonic crystals.

Uncovering how disorder affects light propagation is a key step to manipulating light transportation and localization utilizing engineered disorder in photonics. Herein, we study the disorder-induced localization, delocalization and inter-transition between them in the regions of photonic band gaps (PBGs) and their associated passbands of one-dimensional (1D) Photonic crystals (PhCs) under different types of disorder effect. We introduce two parameters to quantitatively evaluate how disorder effect can be exploited to tailor the localization and delocalization of light in 1D PhCs. Such parameters can clearly indicate the inter-transition between extended states and localized states. It is shown that both nontrivial and trivial disorder-induced transport scenarios exist depending on the types of disorder introduced to the otherwise periodic system. The disorder-induced band tailing, which is correlated with the disorder-induced redistribution of electric energy compared with periodic case, is crucial for observing these interesting disorder-induced light transportation scenarios. Our results might provide positive insight to the manipulation of light transmission in nanophotonics by engineered disorder.

High-Q Quasibound States in the Continuum for Nonlinear Metasurfaces.

Sharp electromagnetic resonances play an essential role in physics in general and optics in particular. The last decades have witnessed the successful developments of high-quality (Q) resonances in microcavities operating below the light line, which however is fundamentally challenging to access from free space. Alternatively, metasurface-based bound states in the continuum (BICs) offer a complementary solution of creating high-Q resonances in devices operating above the light line, yet the experimentally demonstrated Q factors under normal excitations are still limited. Here, we present the realizations of quasi-BIC under normal excitation with a record Q factor up to 18 511 by engineering the symmetry properties and the number of the unit cells in all-dielectric metasurface platforms. The high-Q quasi-BICs exhibit exceptionally high conversion efficiency for the third harmonic generation and even enable the second harmonic generation in Si metasurfaces. Such ultrasharp resonances achieved in this work may immediately boost the performances of BICs in a plethora of fundamental research and device applications, e.g., cavity QED, biosensing, nanolasing, and quantum light generations.

Tunable optical pulling force mediated by resonant electromagnetic coupling.

In terms of the law of momentum conservation, the optical pulling force (OPF) is a counterintuitive phenomenon for optical manipulation. We investigate analytically and numerically the tunable OPF exerted on the low refractive index nanoparticle (NP) in a hybrid dimer system when it is illuminated by a plane wave based on the coupled dipole approximation method and the finite-difference time-domain method, respectively. The underlying physical mechanism relies on the near-field electromagnetic coupling between the low refractive index dielectric NP and the plasmonic NP. We further evaluate the dependence of the OPF on the geometrical parameters of the system. It is also numerically demonstrated that a Gaussian beam can be used to achieve pure OPF with no transverse force component. The proposed OPF offers an additional degree of freedom for optical sorting, transport, and trapping of NPs.

Molecular Dynamics Simulation and Prediction of Druggable Binding Sites.

Binding site identification and druggability evaluation are two essential steps in structure-based drug design. A druggable binding site tends to have high binding affinity to drug-like molecules. Predicting such sites can have a significant impact on a drug design campaign. This chapter focuses on summarizing the different methods that are used to predict druggable binding sites. The chapter also discusses the importance of including protein flexibility in the search process and the use of molecular dynamics simulations to address this aspect. Case studies from the literature are also summarized and discussed. We hope that this chapter would provide an overview on the different methods employed in binding site identification evaluation.

Unidirectional emission in an all-dielectric nanoantenna.

All-dielectric nanoantennas are a promising alternative to plasmonic optical antennas for engineering light emission because of their low-loss nature in the optical spectrum. Nevertheless, it is still challenging to manipulate directional light emission with subwavelength all-dielectric nanoantennas. Here, we propose and numerically demonstrate that a hollow silicon nanodisk can serve as a versatile antenna for directing and enhancing the emission from either an electric or magnetic dipole emitter. When primarily coupled to both electric and magnetic dipole modes of a nanoantenna, broadband nearly-unidirectional emission can be realized by the interference of two modes, which can be spectrally tuned via the geometric parameters in an easy way. More importantly, the emission directions for the magnetic and electric dipole emitters are shown as opposite to each other through control of the phase difference between the induced magnetic and electric dipole modes of the antenna. Meanwhile, the Purcell factors can be enhanced by more than one order of magnitude and high quantum efficiencies can be maintained at the visible spectrum for both kinds of dipole emitters. We further show that these unidirectional emission phenomena can withstand small disorder effects of in-plane dipole orientation and location. Our study provides a simple yet versatile platform that can shape the emission of both magnetic and electric dipole emitters.

Ideal Magnetic Dipole Scattering.

We introduce the concept of tunable ideal magnetic dipole scattering, where a nonmagnetic nanoparticle scatters light as a pure magnetic dipole. High refractive index subwavelength nanoparticles usually support both electric and magnetic dipole responses. Thus, to achieve ideal magnetic dipole scattering one has to suppress the electric dipole response. Such a possibility was recently demonstrated for the so-called anapole mode, which is associated with zero electric dipole scattering. By spectrally overlapping the magnetic dipole resonance with the anapole mode, we achieve ideal magnetic dipole scattering in the far field with tunable strong scattering resonances in the near infrared spectrum. We demonstrate that such a condition can be realized at least for two subwavelength geometries. One of them is a core-shell nanosphere consisting of a Au core and silicon shell. It can be also achieved in other geometries, including nanodisks, which are compatible with current nanofabrication technology.

All-dielectric hollow nanodisk for tailoring magnetic dipole emission.

We propose a silicon hollow nanodisk for enhancing magnetic dipole (MD) emission. The Purcell factor can be more than 300, which is one order of magnitude larger than the silicon nanosphere case. It is demonstrated that the silicon hollow nanodisk resembles the function of an azimuthally polarized beam for tailoring the magnetic and electric dipole (ED) emission. It is shown that MD emission can be significantly enhanced, while ED emission will be suppressed when emitters are located in the hollow of the nanodisk. The dependence of the Purcell factor on the geometry parameters is also studied. Our results might facilitate the on-chip engineering of magnetic light emission.

Space-coiling metamaterials with double negativity and conical dispersion.

Metamaterials are effectively homogeneous materials that display extraordinary dispersion. Negative index metamaterials, zero index metamaterials and extremely anisotropic metamaterials are just a few examples. Instead of using locally resonating elements that may cause undesirable absorption, there are huge efforts to seek alternative routes to obtain these unusual properties. Here, we demonstrate an alternative approach for constructing metamaterials with extreme dispersion by simply coiling up space with curled channels. Such a geometric approach also has an advantage that the ratio between the wavelength and the lattice constant in achieving a negative or zero index can be changed in principle. It allows us to construct for the first time an acoustic metamaterial with conical dispersion, leading to a clear demonstration of negative refraction from an acoustic metamaterial with airborne sound. We also design and realize a double-negative metamaterial for microwaves under the same principle.

Wave front engineering from an array of thin aperture antennas.

We propose an ultra-thin metamaterial constructed by an ensemble of the same type of anisotropic aperture antennas with phase discontinuity for wave front manipulation across the metamaterial. A circularly polarized light is completely converted to the cross-polarized light which can either be bent or focused tightly near the diffraction limit. It depends on a precise control of the optical-axis profile of the antennas on a subwavelength scale, in which the rotation angle of the optical axis has a simple linear relationship to the phase discontinuity. Such an approach enables effective wave front engineering within a subwavelength scale.

Simultaneous enhanced photon capture and carrier generation in Si solar cells using Ge quantum dot photonic nanocrystals.

We propose a novel solar cell structure with photonic nanocrystals coupled to quantum dots (QDs) for advanced management of photons and carriers. The photonic nanocrystals at the surface create an extra interaction between the photons and the QDs, which promotes light trapping. Photo-generated carriers can be efficiently transported by preparing vertically aligned QDs with electronic coupling. Implementation of the proposed structure was realized in crystalline Si solar cells with Ge QDs by development of a simple and practical formation method based on a wet chemical process without any lithography techniques. The wet process utilizes a periodically modulated etching rate induced by self-organized Ge QDs. The effectiveness of the proposed solar cell was demonstrated by the marked increase of the absolute conversion efficiency when compared with the control crystalline Si solar cells. It is found that light trapping by the photonic nanocrystals has a larger contribution to the efficiency improvement than the contributions from the carrier transport of the vertically aligned QDs.