Sharp Resonance Research Articles

Experimental Realization of Subwavelength Spoof Plasmonic Bound States in the Continuum and Ultra‐Sensitive Detection

AbstractOptical bound states in the continuum (BICs) lies in side the continuum and coexists with extended waves, but it remains perfectly confined without any radiation. This unique property of BICs has led to numerous applications, such as highly surface‐sensitive and spectrally sharp resonances for photonic biosensors. However, it remains challenging to experimentally realize the BICs in a single‐particle system, especially for subwavelength structures. This study presents the existence of optical BICs in a subwavelength metallic microstructure, and quasi‐BICs are observed experimentally in a waveguide system with only a single optimized aluminum meta‐particle. This plasmonic BICs is resulting from the destructive interference of two localized surface plasmon modes. Benefiting from its strong localized field confinement and substrate‐free merit of BICs, the experimentally measured quality factor (Q‐factor) of this transmission dip reach to 273. Additionally, this meta‐particle is experimentally verified to show a good sensitivity for both solids and liquids through the spectral shift of the BICs caused transmission dip. This finding extends the optical BICs to a subwavelength scale and opens practical application opportunities for ultrasmall‐quantity detection of biochemical substances.

Fine-Tuned "Click" Functionalization of PAMAM Dendrimers with a Linear Fluorinated Guanidino Linker: Synthesis, Characterization, and Applications.

This study presents the synthesis, characterization, and application of multifunctional PAMAM G2 and G4 dendrimers decorated with a linear fluorinated guanidino linker designed to improve gene delivery efficiency while minimizing cytotoxicity. For the first time, we were able to fine-tune the degree of grafting (DG) during the functionalization process through efficient "click" Michael addition, achieving the synthesis of a collection of six PAMAM conjugates that showed a significant enhancement in transfection efficiency (TE), surpassing the performance of traditional nonviral vectors. The incorporation of fluorinated moieties not only facilitated better deoxyribonucleic acid (DNA) condensation and TE but also introduced potential applications in 19F magnetic resonance imaging thanks to the sharp and intense fluorine nuclear magnetic resonance signals and favorable relaxation parameters. The new dendrimer conjugates demonstrated a promising balance between low cytotoxicity and high TE, with the low-generation PAMAM G2 with lower DG being the best-performing conjugate, making them strong candidates for further development in gene therapy. These findings highlight the potential of these multifunctional PAMAM dendrimers as efficient, nontoxic, and trackable gene delivery vectors.

Examining the structural properties of hydrophilic and hydrophobic organic aerosols using 1H NMR: diurnal variations and source apportionment.

The behaviour of the carbonaceous aerosols during the rainy season and the diurnal variations in their structural groups have not been thoroughly examined. The present study aims to understand the structural composition of hydrophilic and hydrophobic organic aerosols (OA) at an urban background location in Mumbai, India. The carbonaceous fractions, i.e., Elemental (EC) and Organic (OC) Carbon, accounted for 14-34% of the total PM10 (Particulate matter with aerodynamic diameter ≤ 10 μm). The PM10 and EC were maximum in the morning, while OC was the highest in the evening. The aliphatic structural groups were more concentrated in the total fraction, contributing 53-62% of the total resonances. The total concentrations of the structural groups in both hydrophilic (29.2±9.8 μmol/m3) and total (197.8±154.3 μmol/m3) fractions were highest in the morning. Traffic emissions impacted the morning and evening aerosols, as suggested by the broad aliphatic and sharp aromatic resonances observed in the total fraction. This is further corroborated by the variability in EC and OC, their significant correlations with Volatile OC and Nitrogen oxides, and their contribution to regression models and principal components. The afternoon aerosols demonstrated characteristics of Secondary OA. Our work extends the present understanding of the diurnal variability and the heterogeneity of the hydrophilic and hydrophobic structural groups in organic aerosols.

All-dielectric structural coloration empowered by bound states in the continuum.

The technological requirements of low-power and high-fidelity color displays have been instrumental in driving research into advanced coloration technologies. At the forefront of these developments is the implementation of dye-free approaches, which overcome previous constraints related to color resolution and fading. Resonant dielectric nanostructures have emerged as a promising paradigm, showing great potential for high efficiency, high color saturation, wide gamut palette, and image reproduction. However, they still face limitations related to color accuracy, purity, and simultaneous brightness tunability. Here, we demonstrate an all-dielectric metasurface empowered by photonic bound states in the continuum (BICs), which supports sharp resonances throughout the visible spectral range, ideally suited for producing a wide range of structural colors. The metasurface design consists of TiO2 ellipses with carefully controlled sizes and geometrical asymmetry, allowing versatile and on-demand variation of the brightness and hue of the output colors, respectively.

Plasmonic coupling effect of annealed gold nanoarrays

Periodic metal nanodisc arrays have the potential to exhibit regularly spaced large local field enhancements, especially when high-Q collective plasmonic grating resonances can be obtained. Here we demonstrate how Laser interference lithography (LIL) as a maskless and high throughput technique can be used to fabricate these on square centimeter areas. The drawback of LIL is the rather fixed ratio of the size of the individual nanostructure (d) to the period of the array (p) of about d/p ∼ 0.5 for the setup used in the current article, thereby, limiting its ability to create resonances with ultra-high quality factors (Q-factors). To improve the Q-factor of the resonances of the arrays, we study the effect of thermal annealing nanodisk arrays fabricated by LIL and a lift off process. The nanodisk arrays with periods of 400 nm and 500 nm exhibited a plasmonic resonance, which was caused by the interaction of the single disk resonance and a (1 0) grating resonance. Annealing for a short duration lowered the d/p ratio from 0.5 to 0.4, and led to smoothening of the disk surfaces and growth of gold grains, resulting in lower ohmic and radiative losses and doubling of the Q-factor of the resonances. Finite element method (FEM) simulations were used to monitor this improvement in material parameters. Annealing for a longer duration disintegrated the nanodisk into several smaller particles while maintaining the overall periodicity of the array. While the plasmonic resonances of the experimentally investigated fragmented disks were basically destroyed, simulation predict that for larger periods fragmented nanodisk arrays (keeping the d/p ∼ 0.4) can exhibit extremely strong and sharp resonances whose Q-factor increases more than 58.4 times compared to the unfragmented discs. In addition, simulations show a massive enhancement of the local electric field promising immense potential for surface enhanced Raman sensing.

Research on Drug Efficacy using a Terahertz Metasurface Microfluidic Biosensor Based on Fano Resonance Effect.

Advanced biosensors must exhibit high sensitivity, reliability, and convenience, making them suitable for detecting trace samples in biological or medical applications. Currently, biometric identification is the predominant method in clinical practice, but it is complex and time-consuming. In this study, we propose an optical metasurface utilizing the Fano resonance effect, which exhibits a sharp resonance with a transmittance of 32% at 0.65 THz. The resonance dip has a narrow bandwidth of 0.07 THz and a high Q-factor of 42. This resonance arises from the coupling of bright and dark modes, underpinned by the electromagnetic mechanism of Fano resonance. We integrated the metasurface into a microfluidic platform and fabricated low-temperature gallium arsenide photoconductive antennas (LT-GaAs-PCAs) on both sides of the microfluidics to efficiently generate and detect THz waves, significantly reducing the system's volume. The biosensor's detection limits for Escherichia coli (E. coli) and cefamandole nafate are 5 × 103 cells/mL and 5 μg/mL, respectively. Experimentally, when E. coli and cefamandole nafate solutions were sequentially injected into the microfluidic chip, a blue shift in the spectrum was observed. The sensor measured a 95.2% killing rate of E. coli by 40 μg/mL cefamandole nafate solution, with only a 3% deviation from biological experiments. Additionally, a timed killing experiment using 40 μg/mL cefamandole nafate on E. coli revealed a 93.7% killing rate within 3 min. This research presents a THz microfluidic biosensor with rapid detection, high sensitivity, and enhanced portability and integration, offering a promising approach for biomedical research, including antibiotic efficacy assessment and bacterial concentration monitoring.

Thin-film dielectric characterization by bound state in the continuum in high contrast grating

Subwavelength high contrast grating (HCG) is renowned for its remarkable ability to produce sharp optical resonance, known as the bound state in the continuum (BIC). Due to the strong surface field enhancement, the resonant wavelength and quality factor (Q factor) are highly sensitive to the dielectric properties of the surrounding medium. We propose utilizing this extraordinary phenomenon for thin-film dielectric characterization based on a film-substrate-grating configuration. By optimizing the geometrical parameters to control the cross-interference between guided modes in the grating and self-interference of propagating wave in the substrate slab, an accidental BIC with a Q factor reaching 104 is excited. Using this BIC, two retrieval methods based on contour mapping of resonant wavelength and Q factor are proposed to extract the complex permittivity (εf) of the film under test. It has been demonstrated that with a film thickness as thin as 10−5 times the grating period, the error in the retrieved Re[εf] is below 2%, and that of Im[εf] is below 10%. The proposed design is a strong candidate for non-destructive dielectric characterization of thin films with thicknesses below one-thousandth of the operating wavelength. This characterization technique can facilitate the development of high-frequency devices for the 6 G high-speed communication.

Dense and sharp resonance peaks in stretched Moiré waveguides

Dense and sharp resonance peaks in stretched Moiré waveguides

Ultrasensitive Terahertz Fingerprint Retrieval with Multiple‐BIC‐Enabled Meta‐Sensors

AbstractBound states in the continuum (BICs) are an excellent platform enabling highly efficient light–matter interaction in applications for lasing, nonlinear generation, and sensing. However, the current focus in implementing BICs has primarily been on single sharp resonances, limiting the extent of electric field enhancement for multiple resonances. In this study, experimental demonstrations are conducted to showcase how metasurfaces can enable the control of symmetry‐broken and Friedrich–Wintgen BICs by leveraging the asymmetry of split resonant rings. This approach allows for the existence of multiple free‐control BIC resonances and tailored enhancement of controlling light–matter interactions. Further experiments are conducted to validate the effectiveness and performance of meta‐sensors for the identification of the distinct fingerprint of α‐lactose with high sensitivity using only one single metasurface. These findings present a novel and efficient platform for the development of miniaturized and chip‐scale photonics devices with intense light–matter interaction.

Resonant tunneling via the corner states in the double-barrier-modulated second-order topological insulator

In this work, we use the Green's function technique and investigate transport properties of the electronic states in the two-dimensional (2D) second-order topological insulator (TI). We calculated the bulk, edge, and corner states corresponding respectively to the infinite plane, one-dimensional nanoribbon, and the square flake geometries of the 2D second-order TI. Existence of the corner states in the square flake is confirmed. In this approach, we considered tunneling via the localized corner states in the double-barrier-modulated 2D second-order TI. Sharp peaks are found in the transmission probability and conductance incurred by the corner states lying within the surrounding energy gap. The relation between transmission peaks and the corner states is confirmed by their energy correspondence. The extremely discrete and sharp resonance might lend insight to experimental observation of the corner states in the 2D second-order TI.

Boosting spatial and energy resolution in STM with a double-functionalized probe.

The scattering of superconducting pairs by magnetic impurities on a superconducting surface leads to pairs of sharp in-gap resonances known as Yu-Shiba-Rusinov (YSR) bound states. Similar to the interference of itinerant electrons scattered by defects in normal metals, these resonances reveal a periodic texture around the magnetic impurity. The wavelength of these resonances is, however, often too short to be resolved even by methods capable of atomic resolution, i.e., scanning tunneling microscopy (STM). We combine a CO molecule with a superconducting cluster pre-attached to an STM tip to maximize both spatial and energy resolution, thus demonstrating the superior properties of such double-functionalized probes by imaging the spatial distribution of YSR states. Our approach reveals rich interference patterns of the hybridized YSR states of two Fe atoms on Nb(110), previously inaccessible with conventional STM probes. This advancement extends the capabilities of STM techniques, providing insights into superconducting phenomena at the atomic scale.

A Reduced-Symmetry Pd2L4 Cage from a Heterotopic Dipyridyl Ligand.

Two dipyridyl ligands, L3,3 and L3,4, have been used in combination with palladium(II) in the construction of metallosupramolecular species that show anion-dependent behavior in solution. A rare example of a low-symmetry (C2h) lantern-type cage is formed in one instance, [Pd2(L3,3)4]4+, while the isomeric ligand yields a larger double-walled square complex, [Pd4(L3,4)8]8+. [Pd2(L3,3)4](NO3)4 was isolated in crystalline form revealing two anions within the interior of the C2h-symmetry cage. The cage itself is held together by hydrogen bonding between "head-to-tail" pairs of ligands that reinforces the symmetry generated by the ditopic ligands. In solution, the cage with NO3- has sharp 1H nuclear magnetic resonance (NMR) signals at room temperature, while the BF4- analogue has broad signals that sharpen at higher temperatures or upon addition of (Bu4N)(NO3), highlighting the importance of the anion in templating or otherwise influencing self-assembly in solution. Altering the substitution position of one of the pyridyl rings yields a more "open" complex, with [Pd4(L3,4)8](NO3)8 being isolated as a crystalline solid. The double-walled square complex has a greater Pd···Pd separation due to the increased angle that the pyridyl groups subtend at the core of the ligand. NMR spectroscopy and mass spectrometry studies suggest a single species in the presence of nitrate but multiple species with tetrafluoroborate.

Design and Simulation of a Three-Channel Plasmonic Demultiplexer in an MIM Waveguide

This study proposes a structure for optical plasmonic demultiplexers based on nanodisk/nanoring resonators in metal-insulator-metal (MIM) plasmonic waveguides. The proposed structure can transmit the desired wavelength spectra, including telecommunication and visible waves. It consists of a bus waveguide, several nanodisks and nanorings, and three drop waveguides, consisting of several filters that can be extended to N×1 channels. The FWHM of the designed structure was calculated to be approximately 18 nm. The transmission spectrum of each demultiplexer channel can be easily adjusted by changing the geometric parameters and refractive index. As the waveguide refractive index increases, the transmission spectrum shifts to higher wavelengths, and its amplitude decreases due to internal structural losses. Therefore, the differences between the waveguide and metal refractive indices gradually decrease, and the light confinement decreases in the waveguide, leading to an increase in FWHM. The outputs of each channel have two sharp resonance modes, resulting from the coupling of two interacting cavities (ring and disk) due to the Fano resonance phenomenon. This study employed a two-dimensional (2D) finite difference time domain (FDTD) numerical method and simulation by FDTD Lumerical software to evaluate the transmission properties.

Optical switching with high-Q Fano resonance of all-dielectric metasurface governed by bound states in the continuum.

The discovery of bound states in the continuum (BIC) of optical nanostructures has garnered significant research interest and found widespread application in the field of optics, leading to an attractive approach to achieve high-Q (Quality factor) Fano resonance. Herein, an all-dielectric metasurface consisting of four gallium phosphide (Gap) cylinders on the MgF2 substrate is designed and analyzed by the finite element method (FEM). By breaking the symmetry of the plane, specifically by moving the two cylinders to one side, it is possible to achieve a transition from the symmetry-protected BIC to quasi-BIC. This transition enables the excitation of sharp dual-band Fano resonance at wavelengths of 1,045.4 nm and 1,139.6 nm, with the maximum Q factors reaching 1.47 × 104 and 1.28 × 104, respectively. The multipole decomposition and near-field distributions show that these two QBICs are dominated by the electric quadrupole (EQ) and magnetic quadrupole (MQ). Furthermore, bidirectional optical switching can be accomplished by changing the polarization direction of the incident light. As a result, the maximum sensitivity and figure of merit (FOM) are 488.9 nm/RIU and 2.51 × 105 RIU-1, respectively. The results enrich our knowledge about BIC and reveal a platform for the development of high-performance photonics devices such as optical switches and sensors.

Terahertz Sensing Based on Floating Bilayer Metasurface with Toroidal Dipole Resonance Toward Ultra‐High Sensitivity

AbstractMetasurface structures have proven to be effective in enhancing terahertz sensing signals and can thus be used as sensors to improve terahertz detection sensitivity. However, the sensitivity is limited by the poor spatial overlap between the analytes and the local electric field of the metasurface. In this work, a novel design of a floating bilayer metasurface structure for terahertz sensing is proposed and investigated. This structure supports a sharp toroidal dipole resonance and can concentrate near‐field energy on the analyte and metal atoms rather than on the substrate surface by floating the metal atoms. Consequently, the sensitivity is significantly improved to as high as 362 GHz RIU−1; theoretically, this is approximately 2.6 times higher than that of the common metasurface. The ability of the floating bilayer metasurface to quantitatively detect chlorothalonil is experimentally demonstrated. The resonance peak shows a significant frequency shift of 7 GHz for a change of 0.0001 mg dL−1 in chlorothalonil concentration, reaching up to 86 GHz when the change in chlorothalonil concentration is 100 mg dL−1; this is approximately 6.6 times higher than that of the common metasurface. This work provides opportunities for metasurface to realize ultrasensitive sensing in the terahertz regime.

Single-modulator, dual comb serrodyne spectroscopy.

Dual optical frequency comb spectroscopy allows for high speed, broadband measurements without any moving parts. Here, we combine differential chirp downconversion to probe large spectral bandwidths and serrodyne modulation to separate the positive and negative sidebands in a single modulator. As an initial demonstration, we apply this approach to measure a sharp cavity resonance to illustrate the system performance. We then measure methane transitions in the near-infrared and compare the resulting spectra to models based upon the current spectroscopic databases. The serrodyne method has lower hardware requirements compared to many existing approaches, and its simplicity enables a high degree of mutual coherence between the two combs. Further, this method is readily amenable to chip-scale photonic integration.

A highly sensitive modified triple split ring metamaterial-based sensor for blood sample detection based on dielectric property alteration

This research paper demonstrates a metamaterial (MTM) based sensing technique to detect various blood samples by analyzing their dielectric properties. The performance of this MTM-based sensor is evaluated with the help of mimicked human blood samples that closely resemble the dielectric properties of actual human blood samples. Moreover, the ISM band frequency of 2.4 GHz is chosen as one of the reference resonance frequencies due to its various industrial and medical applications. The resonating patch is developed on the FR-4 substrate with a dimension of 10 × 20 mm2 that provides sharp reference resonances of 2.4 and 4.72 GHz for the spectra of the transmission coefficient with a good quality factor (Q-factor). The MTM sensor can detect the mimicked blood samples with a maximum frequency deviation of up to 650 MHz at 2.4 GHz and up to 850 MHz at 4.72 GHz, with maximum sensitivity of 0.917 and 0.707, respectively. The measured results using the prototype of the sensor support the simulation result with good agreement, indicating high sensing capability. Due to its high sensitivity, figure of merit (FoM), and frequency shifting with dielectric property changes in blood samples, the developed MTM-based sensor can be implemented effectively for quick sensing of infected blood samples and biomedical applications.

High-Q magnetic toroidal dipole resonance in all-dielectric metasurfaces

High quality (Q) factor toroidal dipole (TD) resonances have played an increasingly important role in enhancing light–matter interactions. Interestingly, TDs share a similar far-field distribution as the conventional electric/magnetic dipoles but have distinct near-field profiles from them. While most reported works focused on the electric TD, magnetic TDs (MTDs), particularly high-Q MTD, have not been fully explored yet. Here, we successfully realized a high-Q MTD by effectively harnessing the ultrahigh Q-factor guided mode resonances supported in an all-dielectric metasurface, that is, changing the interspacing between silicon nanobar dimers. Other salient properties include the stable resonance wavelength but a precisely tailored Q-factor by interspacing distance. A multipole decomposition analysis indicates that this mode is dominated by the MTD, where the electric fields are mainly confined within the dielectric nanostructures, while the induced magnetic dipole loops are connected head-to-tail. Finally, we experimentally demonstrated such high-Q MTD resonance by fabricating a series of silicon metasurfaces and measuring their transmission spectra. The MTD resonance is characterized by a sharp Fano resonance in the transmission spectrum. The maximum measured Q-factor is up to 5079. Our results provide useful guidance for realizing high-Q MTD and may find exciting applications in boosting light–matter interactions.

Spectroscopy and dynamics of isolated anions: Versatile instrumentation for photodetachment and photoelectron spectroscopy.

Molecular anions are appealing targets for study because, compared with their neutral and cationic counterparts, they can be probed with conventional laboratory lasers without the need for multiphoton ionization schemes, and they provide spectroscopic details on the corresponding neutral molecules. Here, we describe a section of a modular instrument designed to perform high-throughput photoelectron and photodetachment spectroscopy of gas-phase anions, with future provision for time-resolved and isomer-selective spectroscopy. The instrument framework allows for the incorporation and adaptation of several ion sources, as demonstrated here with plasma (electric) discharge sources providing variable hard to soft ion generation conditions. The generated anions are separated according to their mass-to-charge ratio through time-of-flight mass spectrometry (m/zΔm/z = 500-600) and are focused into a set of perpendicular velocity-map imaging electrodes (ΔEE≈4%), where mass-selected anions are probed using laser light and the ejected electrons are velocity-map imaged. Instrument performance is demonstrated through the acquisition of photodetachment and photoelectron spectra for CH2CN-, showing sharp resonances in the vicinity of the detachment threshold assigned to rovibrational states of a dipole-bound anion and broader lifetime-limited spectral features at photon energies well above the threshold assigned to prompt autodetachment from a temporary anion resonance. Similar measurements could be performed on any molecular anions generated in the sources.

Hyperbolic Shear Metasurfaces.

Polar dielectrics with low crystal symmetry and sharp phonon resonances can support hyperbolic shear polaritons, which are highly confined surface modes with frequency-dependent optical axes and asymmetric dissipation features. So far, these modes have been observed only in bulk natural materials at midinfrared frequencies, with properties limited by available crystal geometries and phonon resonance strength. Here, we introduce hyperbolic shear metasurfaces, which are ultrathin engineered surfaces supporting hyperbolic surface modes with symmetry-tailored axial dispersion and loss redistribution that can maximally enhance light-matter interactions. By engineering effective shear phenomena in these engineered surfaces, we demonstrate geometry-controlled, ultraconfined, low-loss hyperbolic surface waves with broadband Purcell enhancements applicable across a broad range of the electromagnetic spectrum.