Modular Assembly of Metamaterials Using Light Gradients.

This is a report on a pilot study that tests the feasibility of assembling photonic metamaterials (PMs) using light gradient forces. Following a strategy that works like modular construction, light gradient forces, produced by a tightly focused, 1D standing wave optical trap, time-multiplexed across a 2D lattice are used to assemble voxels consisting of prefabricated, monodispersed nanoparticles (NPs) with radii ranging from 30 to 500 nm into 3D structures on a hydrogel scaffold. Hundreds of NPs can be manipulated concurrently into a complex heterogeneous voxel this way, and then the process can be repeated by stitching together voxels to form a metamaterial of any size, shape, and constituency although imperfectly. Imperfections introduce random phase shifts and amplitude variations that can have an adverse effect on the band structure. Regardless, PMs are created this way using two different dielectric NPs, polystyrene and rutile, and then the near-infrared performance for each is analyzed with angle-, wavelength-, and polarization-dependent reflection spectroscopy. The cross-polarized spectra show evidence of a resonance peak. Interestingly, whereas the line shape from the polystyrene array is symmetric, the rutile array is not, which may be indicative of Fano resonance. So, even with the structural defects, reflection spectroscopy reveals a resonance.

Non-Hermitian ring laser gyroscopes with enhanced Sagnac sensitivity.

Gyroscopes are essential to many diverse applications associated with navigation, positioning and inertial sensing1. In general, most optical gyroscopes rely on the Sagnac effect-a relativistically induced phase shift that scales linearly with the rotational velocity2,3. In ring laser gyroscopes (RLGs), this shift manifests as a resonance splitting in the emission spectrum, which can be detected as a beat frequency4. The need for ever more precise RLGs has fuelled research activities aimed at boosting the sensitivity of RLGs beyond the limits dictated by geometrical constraints, including attempts to use either dispersive or nonlinear effects5-8. Here we establish and experimentally demonstrate a method using non-Hermitian singularities, or exceptional points, to enhance the Sagnac scale factor9-13. By exploiting the increased rotational sensitivity of RLGs in the vicinity of an exceptional point, we enhance the resonance splitting by up to a factor of 20. Our results pave the way towards the next generation of ultrasensitive and compact RLGs and provide a practical approach for the development of other classes of integrated sensor.

Measurements of the size and correlations between ions using an electrolytic point contact.

The size of an ion affects everything from the structure of water to life itself. In this report, to gauge their size, ions dissolved in water are forced electrically through a sub-nanometer-diameter pore spanning a thin membrane and the current is measured. The measurements reveal an ion-selective conductance that vanishes in pores <0.24 nm in diameter-the size of a water molecule-indicating that permeating ions have a grossly distorted hydration shell. Analysis of the current noise power spectral density exposes a threshold, below which the noise is independent of current, and beyond which it increases quadratically. This dependence proves that the spectral density, which is uncorrelated below threshold, becomes correlated above it. The onset of correlations for Li+, Mg2+, Na+ and K+-ions extrapolates to pore diameters of 0.13 ± 0.11 nm, 0.16 ± 0.11 nm, 0.22 ± 0.11 nm and 0.25 ± 0.11 nm, respectively-consonant with diameters at which the conductance vanishes and consistent with ions moving through the sub-nanopore with distorted hydration shells in a correlated way.

Supersymmetric laser arrays.

Scaling up the radiance of coupled laser arrays has been a long-standing challenge in photonics. In this study, we demonstrate that notions from supersymmetry-a theoretical framework developed in high-energy physics-can be strategically used in optics to address this problem. In this regard, a supersymmetric laser array is realized that is capable of emitting exclusively in its fundamental transverse mode in a stable manner. Our results not only pave the way toward devising new schemes for scaling up radiance in integrated lasers, but also, on a more fundamental level, could shed light on the intriguing synergy between non-Hermiticity and supersymmetry.

Unidirectional light emission in PT-symmetric microring lasers.

The synergetic use of gain and loss in parity-time symmetric coupled resonators has been shown to lead to single-mode lasing operation. However, at the corresponding resonance frequency, an ideal ring resonator tends to support two degenerate eigenmodes, traveling along the cavity in opposite directions. Here, we show a unidirectional single-moded parity-time symmetric laser by incorporating active S-bend structures with opposite chirality in the respective ring resonators. Such chiral elements break the rotation symmetry of the ring cavities by providing an asymmetric coupling between the clockwise (CW) and the counterclockwise (CCW) traveling modes, hence creating a new type of exceptional point. This property, consequently, leads to the suppression of one of the counter-propagating modes. In this paper, we first measure the extinction ratio between the CW and CCW modes in a single ring resonator in the presence of an S-bend waveguide. We then experimentally investigate the unidirectional emission in PT-symmetric systems below and above the exceptional point. Finally, unidirectional emission will be shown in systems of two S-bend ring resonators coupled through a link structure.

Method for Dynamically Detecting Secretions from Single Cells Using a Nanopore.

Secreted proteins mediate cell-to-cell communications. Thus, eavesdropping on the secretome could reveal the cellular phenotype, but it is challenging to detect the proteins because they are secreted only in minute amounts and then diluted in blood plasma or contaminated by cell culture medium or the lysate. In this pilot study, it is demonstrated that secretions from single cancer cells can be detected and dynamically analyzed through measurements of blockades in the electrolytic current due to single molecules translocating through a nanopore in a thin inorganic membrane. It is established that the distribution of blockades can be used to differentiate three different cancer cell lines (U937, MDA-MB-231, and MCF-7) in real time and quickly (<20 s). Importantly, the distinctive blockades associated with the chemokine CCL5, a prognostic factor for disease progression in breast cancer, along with other low-mass biomarkers of breast cancer (PI3, TIMP1, and MMP1) were identified in the context of the secretome of these three cell types, tracked with time, and used to provide information on the cellular phenotype.

Discriminating Residue Substitutions in a Single Protein Molecule Using a Sub-nanopore.

It is now possible to create, in a thin inorganic membrane, a single, sub-nanometer-diameter pore (i.e., a sub-nanopore) about the size of an amino acid residue. To explore the prospects for sequencing protein with it, measurements of the force and current were performed as two denatured histones, which differed by four amino acid residue substitutions, were impelled systematically through the sub-nanopore one at a time using an atomic force microscope. The force measurements revealed that once the denatured protein, stabilized by sodium dodecyl sulfate (SDS), translocated through the sub-nanopore, a disproportionately large force was required to pull it back. This was interpreted to mean that the SDS was cleaved from the protein during the translocation. The force measurements also exposed a dichotomy in the translocation kinetics: either the molecule slid nearly frictionlessly through the pore or it slipped-and-stuck. When it slid frictionlessly, regardless of whether the molecule was pulled N-terminus or C-terminus first through the pore, regular patterns were observed intermittently in the force and blockade current fluctuations that corresponded to the distance between stretched residues. Furthermore, the amplitude of the fluctuations in the current blockade were correlated with the occluded volume associated with the amino acid residues in the pore. Finally, a comparison of the patterns in the current fluctuations associated with the two practically identical histones supported the conclusion that a sub-nanopore was sensitive enough to discriminate amino acid substitutions in the sequence of a single protein molecule by measuring volumes of 0.1 nm3 per read.

Plasmon-Induced Transparency by Hybridizing Concentric-Twisted Double Split Ring Resonators.

As a classical analogue of electromagnetically induced transparency, plasmon induced transparency (PIT) has attracted great attention by mitigating otherwise cumbersome experimental implementation constraints. Here, through theoretical design, simulation and experimental validation, we present a novel approach to achieve and control PIT by hybridizing two double split ring resonators (DSRRs) on flexible polyimide substrates. In the design, the large rings in the DSRRs are stationary and mirror images of each other, while the small SRRs rotate about their center axes. Counter-directional rotation (twisting) of the small SRRs is shown to lead to resonance shifts, while co-directional rotation results in splitting of the lower frequency resonance and emergence of a PIT window. We develop an equivalent circuit model and introduce a mutual inductance parameter M whose sign is shown to characterize the existence or absence of PIT response from the structure. This model attempts to provide a quantitative measure of the physical mechanisms underlying the observed PIT phenomenon. As such, our findings can support the design of several applications such as optical buffers, delay lines, and ultra-sensitive sensors.

Spectroscopic Characteristics of Three Dimensional Split-Ring Resonator Arrays at Terahertz Frequencies.

In this work, three-dimensional, stacked arrays of subwavelength, square, dual, concentric split ring resonators exhibiting characteristics at the Terahertz frequencies have been designed, simulated, and fabricated through a photolithographic lift-off process and electron beam evaporation metal deposition. Characterization of the split-ring resonator arrays was performed by transmission mode Terahertz time domain spectroscopy. The effects of the split-ring resonator unit cell spatial dimensions on resonant absorption frequencies and relative absorption strength are investigated as well as effects from the addition of a dielectric spacing layer and additional split-ring resonator layer. The split-ring resonator arrays were seen to develop two distinct and switchable LC resonances as well as a dipole resonance by measuring the arrays at 0° and 90° in-plane rotations about the terahertz propagation vector. The dielectric spacing layer served to effectively lower the resonant frequencies of the split-ring resonantors while the dual split-ring resonator array was seen to have a slight modulating effect on the absorption strength at the resonant frequencies compared to that of the passivated array without the second layer.

Impact of Substrate and Bright Resonances on Group Velocity in Metamaterial without Dark Resonator.

Manipulating the speed of light has never been more exciting since electromagnetic induced transparency and its classical analogs led to slow light. Here, we report the manipulation of light group velocity in a terahertz metamaterial without needing a dark resonator, but utilizing instead two concentric split-ring bright resonators (meta-atoms) exhibiting a bright Fano resonance in close vicinity of a bright Lorentzian resonance to create a narrowband transmittance. Unlike earlier reports, the bright Fano resonance does not stem from an asymmetry of meta-atoms or an interaction between them. Additionally, we develop a method to determine the metamaterial "effective thickness", which quantifies the influence of the substrate on the metamaterial response and has remained challenging to estimate so far. By doing so, very good agreement between simulated and measured group delays and velocities is accomplished. The proposed structure and method will be useful in designing optical buffers, delay lines, and ultra-sensitive sensors.

Design and analysis of perfect terahertz metamaterial absorber by a novel dynamic circuit model.

Metamaterial terahertz absorbers composed of a frequency selective layer followed by a spacer and a metallic backplane have recently attracted great attention as a device to detect terahertz radiation. In this work, we present a quasistatic dynamic circuit model that can decently describe operational principle of metamaterial terahertz absorbers based on interference theory of reflected waves. The model comprises two series LC resonance components, one for resonance in frequency selective surface (FSS) and another for resonance inside the spacer. Absorption frequency is dominantly determined by the LC of FSS while the spacer LC changes slightly the magnitude and frequency of absorption. This model fits perfectly for both simulated and experimental data. By using this model, we study our designed absorber and we analyze the effect of changing in spacer thickness and metal conductivity on absorption spectrum.