Dip In Spectrum Research Articles

Dual symmetry-protected bound states in the continuum enabled by toroidal dipoles in dielectric metamaterials

Bound states in the continuum (BICs) are a special kind of resonant states that remain localized even though they coexist with a continuous spectrum of radiating waves. They have already emerged as an important design principle for creating systems with high-quality ([Formula: see text] factor states to enhance light–matter interaction. Many approaches have been proposed to achieve BICs, but it is still of great challenge to design multiple BICs simultaneously, and especially working at terahertz (THz) frequencies. In this paper, we propose an all-dielectric metamaterial, consisting of four hollow cylinders in each unit cell. We show dual BICs exist in such a simple structure, and as breaking the symmetry via varying the inner radius of two diagonal cylinders, they will turn to quasi-BICs with high yet finite Q-factor. These quasi-BICs are manifested themselves as two new dips in the transmission spectrum, of which the resonance shapes can be well described by the Fano formula with a few fitting parameters. We find the evolution of their Q-factors still follows the simple linear relationship with respect to the inverse square of the asymmetry parameter. Based on the multipole decomposition method, two BICs are further investigated to show different multipole components for them. Interestingly, the higher frequency BIC is closely related to the excitation of toroidal dipole (TD), which will split into two TDs as breaking the symmetry. The proposed metamaterial provides an alternative platform to merge the physics of BICs, Fano, and TD, and to pave the way for potential device applications (since the states of extremely high-Q factor).

Specularly-Reflected Wave Guidance of Terahertz Plasmonic Metamaterial Based on the Metal-Wire-Woven Hole Arrays: Functional Design and Application of Transmission Spectral Dips.

Terahertz (THz) plasmonic metamaterial, based on a metal-wire-woven hole array (MWW-HA), is investigated for the distinct power depletion in the transmittance spectrum of 0.1-2 THz, including the reflected waves from metal holes and woven metal wires. Woven metal wires have four orders of power depletion, which perform sharp dips in a transmittance spectrum. However, only the first-order dip at the metal-hole-reflection band dominates specular reflection with a phase retardation of approximately π. The optical path length and metal surface conductivity are modified to study MWW-HA specular reflection. This experimental modification shows that the first order of MWW-HA power depletion is sustainable and sensitively correlated with a bending angle of the woven metal wire. Specularly reflected THz waves are successfully presented in hollow-core pipe wave guidance specified from MWW-HA pipe wall reflectivity.

Morphology dependent excitation of hybrid Tamm-bandedge state in metal coated opal three-dimensional photonic crystal

The optical Tamm state is a type of electromagnetic mode that exists at the interface between a periodic dielectric structure and a metal layer. While optical Tamm states have recently gained attention as a potential replacement for surface plasmon polaritons in various applications, most of these demonstrations have been limited to one-dimensional periodic dielectric structures, such as Bragg mirrors. In this article, we present a detailed study on the existence of optical Tamm states at the interface between a metal layer and opal-like three-dimensional photonic crystal (3D-PhC) structures, with a focus on two types of devices with different cross-sectional morphologies. One device consisted of a corrugated silver layer on self-assembled PS opals, while the second one was realized by self-assembling PS opals on a thin flat gold film. Our experimental and numerical observations revealed a dip in the reflection spectra for both configurations, but a hybrid Tamm-PhC bandedge state was found to exist only between the corrugated metal layer and 3D-PhC. Importantly, the nature of the hybrid Tamm-PhC bandedge states was found to strongly depend on the thickness of the metal layer. In contrast, no mode hybridization was observed at the interface between the flat metal and 3D-PhC, and the reflection dip in this case corresponded to a PhC edge mode. Our demonstration highlights the potential for realizing cost-effective and integrated hybrid plasmonic-photonic crystal structures for sensitive nanophotonic sensors.

Fiber optic temperature and strain sensor using dual Mach-Zehnder interferometers.

A fiber optic temperature and strain sensor using dual Mach-Zehnder interferometers (MZIs) is proposed. The dual MZIs were fabricated by fusion splicing of two different fibers between two single-mode fibers. The two fibers of thin-core fiber and small-cladding polarization maintaining fiber were fusion spliced with a core offset. As the responses of the two MZIs are different in terms of temperature and strain, simultaneous temperature and strain measurement were experimentally validated by selecting two resonant dips in the transmission spectrum to construct a matrix. Experimental results show that the proposed sensors had the maximum temperature sensitivity of 66.67pm/°C and the maximum strain sensitivity of -2.0p m/µε. The minimum discriminated temperature and strain of the two proposed sensors were 0.20°C and 0.71µε, and 0.33°C and 0.69µε, respectively. The proposed sensor has promising application prospects due to the merits of ease of fabrication, low costs, and good resolution.

Anomalous dips in reflection spectra of optical polymers deposited on plasmonic metals

Abstract We have studied reflection spectra of dye-doped and undoped polymers deposited onto Ag and Au substrates and found anomalous dips in the UV spectral range. On top of Ag substrates, the λ ∼ 375 nm dips were observed in undoped PMMA, PVP, and PS polymers as well as PMMA doped with Rh590 and HITC laser dyes. In silver-based samples, the spectral positions of the observed reflection dips were close to singularities in the refractive indexes of surface plasmon polaritons (SPPs) propagating at the interface between Ag and polymer. The latter singularities can tentatively explain the λ ∼ 375 nm reflection dips, if the scattering of Ag and polymeric films is large enough to launch SPP without any prism or grating. The dips observed in reflection of Rh590:PMMA and HITC:PMMA on top of Au, were more pronounced than those on Ag, broader, shifted to shorter wavelengths, and their spectral positions had large standard deviations. Furthermore, no anomalous dips in gold-based samples were observed in the reflection spectra of undoped PMMA, PVP, and PS polymers, and a modest singularity in the SPP refractive index, predicted theoretically at λ ∼ 500 nm, cannot explain the dips in the UV reflection spectra observed experimentally. It appears likely that the origin of the reflection dips on top of Au substrates is different from that on top of Ag substrates.

Optical Tamm states in a hybrid structure with a holographic polymer-liquid crystal grating

ABSTRACT The reflection spectrum of a hybrid structure formed by a metal film and a holographic grating consisting of a sequence of polymer and liquid crystal (LC) layers is theoretically studied. It is shown that in such a hybrid structure, a narrow dip in the reflection spectrum appears in the region of the grating band gap, which is associated with the formation of an optical Tamm state (OTS). The spectral position of the reflection dip depends on the parameters of the polymer and LC. The type of metal and its thickness affect the value of the OTS reflection dip, but they do not affect its spectral position. The dependence of the OTS wavelength on the LC director orientation makes it possible to control the spectral position of the reflection dip using an external electric or magnetic field.

Evaluating the Use of Surface Plasmon Resonance-Based Sensing for Colorectal Mucosa Cancer Detection

This paper presents an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in silico</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in vitro</i> study concerning the application of a trapezoidal prism-based surface plasmon resonance chip to detect colorectal cancer by analyzing the mucosa tissue. The sensor is a multilayered structure and works in the wavelength interrogation mode. Performance parameters related to the metallic film characteristics, such as thickness and type of metal were determined. For the analyzed noble metals, the silver film of 40 nm has the highest sensitivity of 6,454.12 nm/RIU, and the 60 nm-thick presents the best values for full width at half maximum and χ parameter (20.20 nm and 286.63 RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">–1</sup> ). The palladium nanolayer deposition enhanced the sensitivity by 10.81% for copper film and peaks at 6,497.04 nm/RIU for the silver film. The palladium nanolayer also provokes deeper dips in the reflectance spectrum. Tissue phantoms were used to study the sensor response to solid surrounding medium to show the feasibility and the correctness of the proposed methodology.

Fine structure of 4f–5d absorption spectra of Me[formula omitted]-Yb[formula omitted] in the vacuum ultraviolet region under synchrotron excitation

Narrow dips were found in the excitation spectra of crystals of alkaline earth fluorides doped with ErF3 or/and YbF3. The positions of the dips are in good agreement with the positions of the narrow absorption lines Er3+, Yb3+ measured earlier. The absorption lines of the crystals were calculated from the excitation spectra. The observed dips in the excitation spectra are due to the absorption lines of 4f–5d transitions in lanthanide ions.

Manipulating light scattering and optical confinement in vertically stacked Mie resonators

Abstract High index dielectric nanoresonators have gained prominence in nanophotonics due to lower losses compared to plasmonic systems and their ability to sustain both electric and magnetic resonances. The resonances can be engineered to create new types of optical states, such as bound-states in a continuum (BIC) and anapoles. In this work, we report on the optical properties of vertically stacked AlGaAs nanodisk Mie resonators. The nanodisks are designed to support an anapole state in the visible wavelength region (400–700 nm). The vertically stacked nanodisk resonators are fabricated from AlGaAs/GaAs multilayer samples with a fast and scalable patterning method using charged sphere colloidal lithography. Both measurements and finite difference time domain (FDTD) simulations of two and three stacked resonators show a sharp dip in the reflectance spectra at the anapole wavelength. For the 2 and 3 disk stacks the reflectance dip contrast at the anapole wavelength becomes very pronounced in the specular reflectance and is attributed to increased directional scattering due to an antenna effect. FDTD simulations show there is enhanced field confinement in all the disks at the anapole wavelength and the confined energy within the individual disks in the stack is at least 2–5 times greater compared to an isolated single nanodisk of the same dimension. Furthermore, the field confinement consistently increases with adding more disks in the stack. These vertically stacked AlGaAs nanodisk resonators can be a very exciting platform to engineer light matter interactions for linear and non-linear optical applications. The general principles of the fabrication method can be adapted to other wavelength ranges and can also be adapted for other III–V material combinations as well as for Si/SiO2.

Underwater Performance Analysis of Perovskite Solar Cells

Herein, perovskite solar cells are fabricated and tested in underwater conditions in the laboratory. The fabricated perovskite solar cell (PSC) shows an efficiency of 14.29% outside water. It demonstrates an impressive power density of 10.49 and 11.88 mW cm−2 at a depth of 10 cm in seawater and deionized (DI) water environment, respectively. Compared with commercial silicon solar cells, PSCs are less affected by the water depth level due to their wide bandgap. The quantum efficiency measurement studies show that the perovskite solar cell is quite unaffected over the 400–800 nm wavelength in underwater environment. On the other hand, the spectral response of silicon solar cell is significantly hampered in the infrared region (700–1050 nm) due to light absorption by water. Absorption peaks of water correspond to the observed dips in quantum efficiency spectra of solar cells, indicating that light absorption due to water is primarily responsible for the decrease in photocurrent and electrical performance in underwater conditions.

Multi-Level High-Throughput Screening for Discovery of Ligands That Inhibit Insulin Aggregation.

We have developed a multi-level virtual screening protocol to identify lead molecules from the FDA inactives database that can inhibit insulin aggregation. The method is based on the presence of structural and interaction specificity in non-native aggregation pathway protein-protein interactions. Some key challenges specific to the present problem, when compared with native protein association, include structural heterogeneity of the protein species involved, multiple association pathways, and relatively higher probability of conformational rearrangement of the association complex. In this multi-step method, the inactives database was first screened using the dominant pharmacophore features of previously identified molecules shown to significantly inhibit insulin aggregation nucleation by binding to its aggregation-prone conformers. We then performed ensemble docking of several low-energy ligand conformations on these aggregation-prone conformers followed by molecular dynamics simulations and binding affinity calculations on a subset of docked complexes to identify a final set of five potential lead molecules to inhibit insulin aggregation nucleation. Their effect on aggregation inhibition was extensively investigated by incubating insulin under aggregation-prone aqueous buffer conditions (low pH, high temperature). Aggregation kinetics were characterized using size exclusion chromatography and Thioflavin T fluorescence assay, and the secondary structure was determined using circular dichroism spectroscopy. Riboflavin provided the best aggregation inhibition, with 85% native monomer retention after 48 h incubation under aggregation-prone conditions, whereas the no-ligand formulation showed complete monomer loss after 36 h. Further, insulin incubated with two of the screened inactives (aspartame, riboflavin) had the characteristic α-helical dip in CD spectra, while the no-ligand formulation showed a change to β-sheet rich conformations.

Femtosecond laser-inscribed off-axis high-order mode long-period grating for independent sensing of curvature and temperature.

We propose and demonstrate a novel curvature and temperature sensor based on an off-axis small-period long-period fiber grating (SP-LPG) which is inscribed in a single mode fiber by a femtosecond laser in one step. The total length of the SP-LPG is only 2.1 mm. The period of the SP-LPG is 30 µm, which is smaller than that of conventional long period fiber gratings. Essentially, the SP-LPG is a high-order mode long period fiber grating. Due to the off-axis structure, the SP-LPG can be used for two-dimensional vector bending sensing. The curvature can be demodulated by the intensity variation of the dips in the transmission spectrum. When the incident light is polarized, the instantaneous curvature sensitivity of the SP-LPG can exceed 20 dB/m-1. Meanwhile, a series of Bragg resonant peaks can be observed in the reflection spectrum, which can be used to monitor the fluctuation of temperature. The transmission dip is insensitive to temperature and the reflection peak is insensitive to curvature, which allows the SP-LPG to measure curvature and temperature independently. The characteristics of high curvature sensitivity, two-dimensional bending direction identification, real-time temperature measurement, and compact structure make the device expected to be applied in the field of structural health monitoring and intelligent robotics.

Simultaneous measurement of refractive index and temperature based on tapered no-core fiber cascaded with a fiber Bragg grating

A Mach-Zehnder interference fiber-optic sensor for simultaneous measurement of temperature and refractive index (RI) is proposed and fabricated. In the sensing structure, no-core fiber (NCF) is cascaded with fiber Bragg grating (FBG), two enlarged tapers between NCF and the single-mode fiber serve as the light coupler, and the shrinked taper between two NCFs is used to enhance the interference effect. The different diameter influence of the shrinked taper on the sensing performance of RI and temperature has been evaluated, and the optimum diameter has been obtained. By monitoring the wavelength shift of FBG and the interference dip in the transmission spectra, the simultaneous measurement of temperature and RI is achieved. The maximum sensitivities of the attenuation peak to RI and temperature are 201.7157 nm/RIU and 87.4 pm/°C, respectively. The sensitivities of FBG to RI and temperature are 80.6 pm/RIU and 13.5 pm/°C, respectively. The transformation matrix has also been given. Own to the simple fabrication and high sensitivity, the sensor has the potential application in simultaneous measurement of multiple parameters.

Simultaneous Measurement of Humidity and Temperature Based on ZnO-Coated Hollow Core Bragg Fiber

An optical fiber sensor for simultaneous measurement of relative humidity (RH) and temperature is proposed, which is constructed by sandwiching one section of hollow core Bragg fiber (HCBF) between two sections of single mode fiber (SMF). The ultrathin ZnO film is coated on the outer surface of HCBF as humidity-sensitive material. The dips in the transmission spectrum caused by different anti-resonant (AR) modes of HCBF have different sensitivities to the RH and temperature, and then the simultaneous measurement of above two parameters can be realized by solving a cross matrix. For the two dips monitored in the experiments, the sensitivities of the proposed sensor to RH are 0.7 pm/%RH and 16.95 pm/%RH in the range of 46–73 %RH, and the sensitivities to temperature are 25 pm/°C and 26.8 pm/°C in the range of 20-60 °C, respectively. Taking advantage of compact structure, good stability, easy fabrication and the proposed sensor exhibits great potential application in fields of chemical sensors and biochemical detection.

DV-QKD Coexistence With 1.6 Tbps Classical Channels Over Hollow Core Fibre

The feasibility of coexisting a quantum channel with carrier-grade classical optical channels over Hollow Core Nested Antiresonant Nodeless Fibre (HC-NANF) is experimentally explored for the first time in terms of achievable quantum bit error rate (QBER), secret key rate (SKR) as well as classical signal bit error rates (BER). A coexistence transmission of 1.6 Tbps is achieved for the classical channels simultaneously with a quantum channel over a 2 km-long HC-NANF with a total coexistence power of 0 dBm. To find the best and worst wavelength position for the classical channels, we simulated different classical channels bands with different spacing between the quantum and classical channels considering the crosstalk generated from both Raman scattering and four-wave-mixing (FWM) on the quantum channel. Following our simulation, we numerically estimate the best (Raman spectrum dip) and worst locations (Raman spectrum peak) of the classical channel with respect to its impact on the performance on the quantum channel in terms of SKR and QBER. We further implemented a testbed to experimentally test both single mode fibre (SMF) and HC-NANF in the best and worst-case scenarios. In the best-case scenario, the spacing between quantum and classical is 200 GHz (1.6 nm) with 50 GHz (0.4 nm) spacing between each classical channel. The SKR was preserved without any noticeable changes when coexisting the quantum channel with eight classical channels at 0 dBm total coexistence power in HC-NANF compared to a significant drop of 73% when using SMF at -24 dBm total coexistence power which is 250 times lower than the power used in HC-NANF. In the worst-case scenario using the same powers, and with 1 THz (8 nm) spacing between quantum and classical channels, the SKR dropped 10% using the HC-NANF, whereas in the SMF the SKR plummeted to zero.

Tune the resonance of VO2 joined metamaterial dimers by adjacent cut wires.

Two terahertz metamaterials were joined by a conductivity variable VO2 patch to obtain a metamaterial dimer. By applying voltage or heat to the VO2 patches, active modulation of terahertz wave could be achieved. A cut-wire metamaterial was placed adjacent to the VO2 joined dimer to affect its electromagnetic response. It was found that the cut wire could heavily impact the resonance mode of the VO2 joined dimer, which gives dual resonance dips in transmission spectrum for both insulating and conducting states of VO2 patches. As a result, by tuning the conductivity of VO2, active dual band phase modulation could be achieved with high transmission window by this dimer-cut wire coupling system.

Route to flexible metamaterial terahertz biosensor based on multi-resonance dips.

A flexible terahertz (THz) metamaterial biosensor is theoretically and experimentally investigated. The metamaterial unit cell of the periodic structure array was simply composed of three non-overlapping cut wires with different length parameters on a flexible thin-film (parylene-C) to improve sensitivity. The biosensor sample was fabricated using a lithography process and characterized by a THz time-domain spectroscopy (TDS) system. The metamaterial exhibited multi-resonance dips in transmission spectrum at 0.6-2.0 THz, which can self-correct errors in biosensing. Numerical results show that the Q-factor, figure of merit (FOM) and sensitivity can change in dynamic ranges with the geometric parameters (space and width) of three-cut-wire metamaterial. When space distance was 40 µm and other parameters were default, the sensitivity, FOM and Q-factor reached 710 GHz/RIU (Refractive Index Unit), 9, and 20, respectively. Therefore, through proper design and preparation, the metamaterial can be applied to biochemical detection.

Noisy coherent population trapping: applications to noise estimation and qubit state preparation

Coherent population trapping is a well-known quantum phenomenon in a driven Λ system, with many applications across quantum optics. However, when a stochastic bath is present in addition to vacuum noise, the observed trapping is no longer perfect. Here we derive a time-convolutionless master equation describing the equilibration of the Λ system in the presence of additional temporally correlated classical noise, with an unknown decay parameter. Our simulations show a one-to-one correspondence between the decay parameter and the depth of the characteristic dip in the photoluminescence spectrum, thereby enabling the unknown parameter to be estimated from the observed spectra. We apply our analysis to the problem of qubit state initialization in a Λ system via dark states and show how the stochastic bath affects the fidelity of such initialization as a function of the desired dark-state amplitudes. We show that an optimum choice of Rabi frequencies is possible.

Simultaneous Measurement of Water Pressure and Temperature Based on a Simple Fabry-Pérot Sensor

We proposed a simple and compact Fabry-Perot (FP) sensor for simultaneous measurement of water pressure and temperature, composed of a standard fiber ferrule connector/flat contact (FC/FC) connector and aluminum foil attached to a thin metal plate. Two dips in the interference spectrum are selected to illustrate the response of the FP sensor to the water pressure and temperature, we can obtain the simultaneous measurement via a method of sensitivity coefficient matrix. Experimental results show the maximum sensitivity of −2.948 nm/kPa for water pressure sensing in 0~9 kPa and 1.119nm/°C for temperature sensing in 33~45 °C. In addition, the dual-parameter sensor has the advantages of compactness, stability, simple production, small size, and high sensitivity, making it suitable for a wide range of practical applications.

Micropillared Surface to Enhance the Sensitivity of a Love-Wave Sensor

Surface-acoustic-wave (SAW) sensors have undergone outstanding developments during the last decades. However, for biosensing applications, they struggle to compete with optical sensing technologies. In this work, we present a Love-wave-based sensing platform that leverages the use of the local resonances of micropillars fabricated on the surface of the guiding layer. We demonstrate numerically and experimentally that the Love-wave interaction with the micropillars produces sharp attenuation dips in the transmission spectrum. These dips are associated with the excitations of flexural and torsional resonance modes of the pillars. We utilize these modes to achieve higher detection sensitivity of temperature variation and mass loading using sugar and microbeads concentrations in aqueous solutions. Integrating a lattice of micropillars into SAW-based devices offers a strong potential for detecting low particles concentrations.