Tunable Resonance Research Articles

MIM waveguide structure consisting of a baffle and a ring cavity embedded in a square metal layer

This paper presents a design for a tunable plasmon refractive index nanosensor based on Fano resonance. The proposed structure comprises a metal–insulator–metal waveguide featuring a silver baffle and a ring cavity embedded in a square (RCSQ). This innovative structural configuration effectively excites dual Fano resonances by leveraging the unique properties of its constituent elements and geometry. The transmission spectrum and electric field at the resonance were simulated using the finite element method (FEM) in two-dimensional space, providing an in-depth explanation of the formation mechanism of the Fano resonance, as well as analyzing how various structural parameters influence sensor characteristics. Simulation results indicate that the Fano peak can be readily tuned by adjusting cavity parameters and refractive indices. Common tuning of the Fano resonance is achievable through modifications to the proportionality constant of the RCSQ. Finally, we optimize and analyze the sensor characteristics of this structure, achieving sensitivity levels up to 2750 nm/RIU with a figure of merit (FOM) of 3.48×104. Consequently, this structure holds significant potential for applications in optical nanosensors.

Selective Passive Tuning of Cavity Resonance by Mode Index Engineering of the Partial Length of a Cavity

Selective Passive Tuning of Cavity Resonance by Mode Index Engineering of the Partial Length of a Cavity

Fabrication of nanoparticles with precisely controllable plasmonic properties as tools for biomedical applications.

Metal nanoparticles are established tools for biomedical applications due to their unique optical properties, primarily attributed to localized surface plasmon resonances. They show distinct optical characteristics, such as high extinction cross-sections and resonances at specific wavelengths, which are tunable across the wavelength spectrum by modifying the nanoparticle geometry. These attributes make metal nanoparticles highly valuable for sensing and imaging in biology and medicine. However, their widespread adoption is hindered due to challenges in consistent and accurate nanoparticle fabrication and functionality as well as due to nanotoxicological concerns, including cell damage, DNA damage, and unregulated cell signaling. In this study, we present a fabrication approach using nanoimprint lithography in combination with thin film deposition which yields highly homogenous nanoparticles in size, shape and optical properties with standard deviations of the main geometry parameters of less than 5% batch-to-batch variation. The measured optical properties closely match performed simulations, indicating that pre-experimental modelling can effectively guide the design of nanoparticles with tailored optical properties. Our approach also enables nanoparticle transfer to solution. Particularly, we show that the surface coating with a PEG polymer shell ensures stable dispersions in buffer solutions and complex cell media for at least 7 days. Furthermore, our in vitro experiments demonstrate that these nanoparticles are internalized by cells via endocytosis, exhibit good biocompatibility, and show minor cytotoxicity, as evidenced by high cell viability. In the future, our high-precision nanoparticle fabrication method together with tunable surface plasmon resonance and reduced nanotoxicity will offer the possibility to replace conventional nanomaterials for biomedical applications that make use of an optical response at precise wavelengths. This includes the use of the nanoparticles as contrast agents for imaging, as probes for targeted photothermal cancer therapy, as carriers for controlled drug delivery, or as probes for sensing applications based on optical detection principles.

Intervalence plasmons in boron-doped diamond

Doped semiconductors can exhibit metallic-like properties ranging from superconductivity to tunable localized surface plasmon resonances. Diamond is a wide-bandgap semiconductor that is rendered electronically active by incorporating a hole dopant, boron. While the effects of boron doping on the electronic band structure of diamond are well-studied, any link between charge carriers and plasmons has never been shown. Here, we report intervalence plasmons in boron-doped diamond, defined as collective electronic excitations between the valence subbands, opened up by the presence of holes. Evidence for these low-energy excitations is provided by valence electron energy loss spectroscopy and near-field infrared spectroscopy. The measured spectra are subsequently reproduced by first-principles calculations based on the contribution of intervalence band transitions to the dielectric function. Our calculations also reveal that the real part of the dielectric function exhibits a crossover characteristic of metallicity. These results suggest a new mechanism for inducing plasmon-like behavior in doped semiconductors, and the possibility of attaining such properties in diamond, a key emerging material for quantum information technologies.

Deposition of plasmonic ITO nanoparticles by near-infrared laser ablation

Compound nanoparticles (NPs) attract attention because of their unique electrical, optical, and catalytic properties. Among them, plasmonic tin-doped indium oxide (ITO) NPs are characterized by transparency in the visible wavelength range and tunable localized surface plasmon resonance (LSPR) in the near-infrared (NIR) range due to high electronic conductivity. To date, they have been usually synthesized by a chemical solution process. However, the chemically synthesized ITO NPs are capped with organic protecting agents, which often block exchange of charge carriers and access of chemical species to the NPs, limiting some applications. In the present study, we propose a method for direct deposition of ITO NPs on a glass or plastic substrate using commercially available ITO-coated glass via NIR laser ablation. The LSPR characteristics of the ITO NPs, thus, prepared were controlled by changing the ablation conditions such as laser power. In addition, the potential applications of the ITO NPs were also investigated through measurements of their refractive index sensitivity and magnetic circular dichroism.

Tuning of a Viscous Inerter Damper: How to Achieve Resonant Damping Without a Damper Resonance

Inerter dampers are effectively employed to mitigate and dampen structural vibrations in slender or high-rise buildings. The simple viscous inerter damper, with a viscous dashpot placed in series with an inerter, is designed to create resonant vibration damping, although the damper itself is without an internal resonance. The apparent resonant behavior is instead obtained by increasing the damper inertance until the two lowest modes of the considered building model interact, whereafter the viscous coefficient is adjusted until the desired response mitigation is achieved. The present modal interaction tuning requires that the reduced-order single-mode dynamic model of the building includes both inertia and flexibility from the (other) modes otherwise discarded by the model reduction. While the inertia correction adjusts the modal mass of the inerter damper, the corresponding flexibility introduces the apparent damper stiffness that creates the desired damper resonance. Thus, the accurate representation of other modes is essential for the design and resonant tuning of the simple viscous inerter damper. The resonant damper performance by the non-resonant viscous inerter damper is illustrated by a numerical example with a 20-story building model, for which the desired resonant modal interaction requires an inertance of almost ten times the entire translational building mass.

Chiral exceptional point enhanced active tuning and nonreciprocity in micro-resonators

Exceptional points (EPs) have been extensively explored in mechanical, acoustic, plasmonic, and photonic systems. However, little is known about the role of EPs in tailoring the dynamic tunability of optical devices. A specific type of EPs known as chiral EPs has recently attracted much attention for controlling the flow of light and for building sensors with better responsivity. A recently demonstrated route to chiral EPs via lithographically defined symmetric Mie scatterers on the rim of resonators has not only provided the much-needed mechanical stability for studying chiral EPs, but also helped reduce losses originating from nanofabrication imperfections, facilitating the in-situ study of chiral EPs and their contribution to the dynamics and tunability of resonators. Here, we use asymmetric Mie scatterers to break the rotational symmetry of a microresonator, to demonstrate deterministic thermal tuning across a chiral EP, and to demonstrate EP-mediated chiral optical nonlinear response and efficient electro-optic tuning. Our results indicate asymmetric electro-optic modulation with up to 17 dB contrast at GHz and CMOS-compatible voltage levels. Such wafer-scale nano-manufacturing of chiral electro-optic modulators and the chiral EP-tailored tunning may facilitate new micro-resonator functionalities in quantum information processing, electromagnetic wave control, and optical interconnects.

On-chip optical spectrometer with a tunable micro-ring resonator and a Mach-Zehnder interferometer lattice filter.

We propose and demonstrate a compact on-chip optical spectrometer by integrating a tunable micro-ring resonator (MRR) with a 4-channel wavelength demultiplexer (DEMUX) based on a Mach-Zehnder interferometer (MZI) lattice filter. The MRR with a 3-dB bandwidth of 0.15 nm ensures the high resolution of the spectrometer. The 4-channel DEMUX is designed with channel spacing equal to the free spectral range (FSR) of the MRR, providing effective wavelength separation with crosstalk less than -18 dB. The thermally tuned MRR across its FSR, together with the synchronized tuning MZIs, enables wavelength scanning within the bandwidth of 40 nm. With such a design, a compact on-chip spectrometer with a footprint of about 0.2 mm2 is implemented to demonstrate the ability to retrieve the spectra of two laser lines separated by 0.2 nm. Our studies shed light on the configuration design of chip-scale spectrometers.

Tuning of plasmonic surface lattice resonances: on the crucial impact of the excitation efficiency of grazing diffraction orders.

Metallic nanoparticles exhibit remarkable optical properties through localized surface plasmon (LSP) resonances. When arranged in arrays, nanoparticles form surface lattice resonances (SLRs) affected by inter-particle distance. SLRs can offer narrower bandwidths and stronger electric field enhancements than LSP modes, crucial for efficient optical device development. Among important aspects, grazing diffracted orders crucially impact SLR properties, facilitating long-range nanoparticle interactions. This study explores how these photonic modes propagating at the substrate surface influence SLRs, by using experimental and theoretical approaches, including Finite Difference Time Domain simulations on gold disk arrays. Results show SLR properties strongly rely on diffracted mode efficiency controlled by the grating constant along the incident polarization. Notably, large inter-particle spacing along the incident polarization can strongly reduce the SLR red-shift. Understanding and managing long-range interactions in engineered plasmonic structures are highlighted, providing insights for enhanced performance in advanced photonic and optoelectronic devices.

Tunable Casimir Equilibria in a Dual-Liquid System

The Casimir effect, a macroscopic manifestation of quantum phenomena, arises from zero-point energy and thermal fluctuations. When two objects are brought into close proximity, the Casimir effect manifests as a repulsive force, while at greater separations, it transitions to an attractive force. There exists a specific distance at which the Casimir force vanishes, referred to as the stable Casimir equilibrium. Stable Casimir equilibria arise from the curve minima of the Casimir energy, which can create spatial trapping. The manipulation of stable Casimir equilibria offers promising applications in areas such as tunable optical resonators and self-assembly. This paper presents a scheme for achieving tunable Casimir equilibria in a dual-liquid system. The system comprises a multilayered stratified structure with a gold substrate. Above the gold substrate, a stratified liquid system is formed due to the immiscibility between organic solutions and water. The lower-density solution is on top, while the higher-density one lies beneath. Our results suggest that a stable Casimir equilibrium for a suspended gold nanoplate can be realized, when the suspended gold nanoplate is immersed in organic solutions of toluene or benzene. Moreover, the height of the suspended gold nanoplate, determined by the stable Casimir equilibrium, can be precisely tuned by varying the thickness of the water layer. The effects of finite temperature and ionic concentration on the Casimir equilibria are also analyzed in this work. The results suggest that the separation heights of Casimir equilibria decrease with increasing the temperature. Interestingly, the ionic concentration in water significantly affects the Casimir pressure when the Debye screening length is comparable or smaller than the separation, allowing for a wide range of modulations on Casimir equilibria. This work opens a new avenue for tuning Casimir equilibria, with significant applications for "quantum trapping" of micro-nano particles.

Tunable localized surface plasmon resonance absorption in silver-based nanoplasmonics

Tunable localized surface plasmon resonance absorption in silver-based nanoplasmonics

On-chip power efficient MHz to GHz tunable Brillouin microwave photonic filters

Stimulated Brillouin scattering (SBS) offers optically tunable, narrowband gain and loss resonances, which makes it ideal for optical filtering and signal processing applications. However, broadband filters utilizing Brillouin scattering typically necessitate multiple pump sources or linear frequency modulated waveforms, which necessitate the use of overall high pump powers to achieve broadband and multi-band filter configurations. To overcome the trade-off between bandwidth and power consumption, we introduce an on-chip broadband bandpass filter scheme leveraging the SBS response and the concept of radio frequency interference. We present the MHz to GHz reconfigurable microwave photonic filters on a chip. The bandwidth is tunable from 30 MHz to 1.2 GHz at a total pump power of only 15 dBm, corresponding to a figure of merit (maximum bandwidth/pump power) improvement of a factor of 25 compared to previously reported work. Furthermore, the filter exhibits frequency tuning capabilities ranging from 10 to 40 GHz (X to Ka-band) and can be configured from single- to multi-bandpass filter responses and switched from bandpass to bandstop.

Ultrathin BIC Metasurfaces Based on Ultra-Low-Loss Sb2Se3 Phase-Change Material.

Ultrathin and low-loss phase-change materials (PCMs) are highly valued for their fast and effective phase transitions and applications in reconfigurable photonic chips, metasurfaces, optical modulators, sensors, photonic memories, and neuromorphic computing. However, conventional PCMs mostly suffer from high intrinsic losses in the near-infrared (NIR) region, limiting their potential for high quality factor (Q-factor) resonant metasurfaces. Here we present the design and fabrication of tunable bound states in the continuum (BIC) metasurfaces using the ultra-low-loss PCM Sb2Se3. Our BIC metasurfaces, only 25 nm thick, achieve high modulation depth and broad resonance tuning in the NIR with high Q-factors up to 130. Experimentally, we employ these BIC metasurfaces to modulate photoluminescence in rare earth-doped upconversion nanoparticles, reducing the excitation power for multiphoton photoluminescence and enabling emission polarization manipulation. This work offers a promising platform for developing active resonant metasurfaces, with broad applications including optical modulation, ultrafast switches, color filtering, and optical sensing.

Two-dimensional FePS3 nanoelectromechanical resonators with local-gate electrostatic tuning at room temperature

Nanoelectromechanical systems (NEMS) enabled by two-dimensional (2D) magnetic materials are promising candidates for exploring ultrasensitive detection and magnetostrictive phenomena, thanks to their high mechanical stiffness, high strength, and ultralow mass. The resonance modes of such vibrating membrane NEMS can be probed optically and also manipulated mechanically via electrostatically induced strain. Electrostatic frequency tuning of 2D magnetic NEMS resonators is, thus, an important means of investigating magneto-mechanical coupling mechanisms. Toward realizing magneto-mechanical coupled devices, we build circular drumhead iron phosphorus trisulfide (FePS3) NEMS resonators with different diameters (3–7 μm). Here, we report on experimental demonstration of tunable antiferromagnet FePS3 drumhead resonators with the highest fractional frequency tuning range up to Δf/f0 = 32%. Combining experimental results and analytical modeling of the resonance frequency scaling, we attain quantitative understanding of the elastic behavior of FePS3, including the transition from “membrane” to “plate” regime, with built-in tension (γ) ranging from 0.1 to 2 N/m. This study not only offers methods for investigating mechanical properties of ultrathin membranes of magnetic 2D materials but also provides important guidelines for designing future high-performance magnetic NEMS resonators.

MXenes: Exploiting their Unique Properties for Designing Next-Generation Thermal Catalysts and Photocatalysts

Abstract MXenes possess a range of exceptional properties that can be harnessed in catalyst design for thermal and photocatalytic reactions. Herein, we share our perspective on how these properties can beexploited to address challenges in different catalytic reactions, thereby highlighting the potential of the material in catalysis. For thermal catalysis, key properties include their layered structure, reducibility of their surface, partial oxidation ability, basicity/acidity, and electrophilicity. In photocatalysis, MXenes exhibit beneficial properties such as transparency, electrical conductivity, tunable Fermi level, photo-corrosion resistance, and plasmonic resonance. Two fundamental characteristics that underpin MXene’s superiority over many other traditional catalysts are the tunability of its properties to meet specific application needs and the low dimensionality of its structure, which enables nanoscale phenomena to be harnessed. While reasonable progress has been made in this field, we assert that the potential for MXenes in catalytic applications is vast and largely underexplored. As we mark a decade of MXene research, we anticipate significant advancements in their catalytic applications, positioning them as key materials for future breakthroughs in this field.

Dynamically Tunable Chiroptical Activities of Flexible Chiral Plasmonic Film via Surface Buckling.

Plasmonic nanoparticle-based chiral materials have attracted great interest due to their strong light-matter interaction and tunable resonance frequency. However, challenges remain in dynamically modulating the chiroptical activities while maintain strong signals. Here, chiral assemblies of gold nanospheres(AuNSs) are achieved via mechanical-induced surface buckling of elastic materials, in which linear chain assemblies of AuNSs transform into to 3D "S-liked" morphology along with the formation of unidirectional wrinkles during buckling. This asymmetrical structure leads to strong chiroptical responses, exhibiting circular dichroism (CD) response over vis-NIR range with signal as high as 2.6 degrees and a g-factor up to 0.11. Furthermore, the configuration of "S-liked" assembly is closely associated with the wrinkle shape, which can be tuned through post-stretching. This method facilitates mechanically induced dynamic and reversible modulation of CD magnitude, as well as switching of signal handedness. Taking advantages of the strong and alterable CD signal, the plasmonic chiral structures demonstrate great potential for multi-channel and dynamically switchable information encryption as a prototype. The strategy, based on manipulating the surface patterns of soft materials, opens up a new design principle for constructing chiral structure and modulating chiroptical activities in a continuously adjustable manner, advancing the development of chiral optical materials.

Mixed-Chalcogen 2D Silver Phenylchalcogenides (AgE1-xExPh; E = S, Se, Te).

Alloying is a powerful strategy for tuning the electronic band structure and optical properties of semiconductors. Here, we investigate the thermodynamic stability and excitonic properties of mixed-chalcogen alloys of two-dimensional (2D) hybrid organic-inorganic silver phenylchalcogenides (AgEPh; E = S, Se, Te). Using a variety of structural and optical characterization techniques, we demonstrate that the AgSePh-AgTePh system forms homogeneous alloys (AgSe1-xTexPh, 0 ≤ x ≤ 1) across all compositions, whereas the AgSPh-AgSePh and AgSPh-AgTePh systems exhibit distinct miscibility gaps. Density functional theory calculations reveal that chalcogen mixing is energetically unfavorable in all cases but comparable in magnitude to the ideal entropy of mixing at room temperature. Because AgSePh and AgTePh have the same crystal structure (which is different from AgSPh), alloying is predicted to be thermodynamically preferred over phase separation in the case of AgSePh-AgTePh, whereas phase separation is predicted to be more favorable than alloying for both the AgSPh-AgSePh and AgSPh-AgTePh systems, in agreement with experimental observations. Homogeneous AgSe1-xTexPh alloys exhibit continuously tunable excitonic absorption resonances in the ultraviolet-visible range, while the emission spectrum reveals competition between exciton delocalization (characteristic of AgSePh) and localization behavior (characteristic of AgTePh). Overall, these observations provide insight into the thermodynamics of 2D silver phenylchalcogenides and the effect of lattice composition on electron-phonon interactions in 2D hybrid organic-inorganic semiconductors.

Multiscale Thermal Analysis of Gold Nanostars in 3D Tumor Spheroids: Integrating Cellular-Level Photothermal Effects and Nanothermometry via X-Ray Spectroscopy.

In the pursuit of enhancing cancer treatment efficacy while minimizing side effects, near-infrared (NIR) photothermal therapy (PTT) has emerged as a promising approach. By using photothermally active nanomaterials, PTT enables localized hyperthermia, effectively eliminating cancer cells with minimal invasiveness and toxicity. Among these nanomaterials, gold nanostars (AuNS) stand out due to their tunable plasmon resonance and efficient light absorption. This study addresses the challenge of measuring nanoscale temperatures during AuNS-mediated PTT by employing X-ray absorption spectroscopy (XAS) within 3D tumor spheroids. It also aims to investigate the heat generated at the nanoscale and the resultant biological damage observed at a larger scale, utilizing confocal microscopy to establish connections between AuNS heat generation, tissue damage, and their impacts on cellular structure. These nanoscale and microscale thermal effects have been compared with macroscopic values obtained from infrared thermography, as part of a multiscale thermal analysis. The findings underscore the efficacy of AuNS in enhancing PTT and provide insights into the spatial distribution of thermal effects within tumor tissues. This research advances the understanding of localized hyperthermia in cancer therapy and underscores the potential of AuNS-based PTT for clinical applications.

Frequency Tunable Film Bulk Acoustic Resonator Integrating PMN‐PT/FSMA Multiferroic Heterostructure for Flexible MEMS

AbstractFrequency tunable flexible piezo resonators exhibit significant potential for technological advances in wearable magnetic field sensing, futuristic wireless telecommunication devices, and flexible micro‐electromechanical systems. This study presents a multifunctional and flexible 0.67Pb(Mg1/3Nb2/3)O3−0.33PbTiO3(PMN‐PT)/Ni50Mn35In15 (Ni‐Mn‐In) multiferroic heterostructure‐based bulk acoustic wave (BAW) resonator fabricated over Kapton substrate. The fundamental resonance frequency (fR = 5.31 GHz) of the resonator is tunable with magnetic and electric fields. A significant change in resonance frequency (ΔfR) of 405 MHz has been achieved with 3.37 Hz nT−1 sensitivity using a direct current (DC) magnetic field of 1200 Oe, attributed to the delta‐E effect. The resonator displays a significant magnetic field tunability of 8.83%. Additionally, a substantial ΔfR of 360 MHz, 36 Hz µV−1 sensitivity and 6.78% tunability is attained with 10 V of DC bias voltage. The impact of magnetic field and DC bias voltage on the acoustic characteristics have been studied by fitting the resonance frequency curves with an equivalent modified Butterworth‐Van Dyke model. The fabricated BAW resonator exhibits outstanding flexibility with no discernible change in fR up to 2000 bending cycles. Such PMN‐PT/Ni‐Mn‐In‐based multiferroic BAW resonators displaying magnetic and electric field tunability are propitious for next‐generation flexible electronics and magnetic field sensors.

Investigating SERS performance of controllable fabricated Ag periodic nanostructures with morphologies of nanoclaws and nanocones in detecting SARS-CoV-2 S protein

Surface-enhanced Raman scattering (SERS) spectroscopy plays an important role in enabling high-precision detection and non-invasive diagnostics. Periodic metallic nanostructures with strong electromagnetic fields and tunable localized surface plasmon resonance can achieve high SERS sensitivity, but fabricating such nanostructures remains a challenge. In this work, the anodic aluminum oxide (AAO) nanocavity template-assisted process of fabricating periodic nanostructures with modulating morphologies from Ag nanoclaws (AgNCWs) to Ag nanocones (AgNCEs) is proposed. The morphologies-related SERS performances of nanostructures are analyzed by using rhodamine 6 g (R6G) as a probe molecule and the optimized AgNCEs possess a limit of detection (LOD) of 1 × 10−9 M. A sandwich strategy is applied to detect SARS-CoV-2 S protein in phosphate-buffered saline (PBS) and pharyngeal swab solution (PSS). The ACE2-modified AgNCEs (ACE2/AgNCEs) structure is applied to bind to the SARS-CoV-2 S protein. The 4-MBA and SARS-CoV-2 S antibody functionalized Au nanoparticles (AuNPs) are used as nanotags. By forming the ACE2/AgNCEs-SARS-CoV-2 S protein-nanotags sandwich structure, characteristic peak intensities of 1580 cm−1 are used as the report signal, and the LOD for SARS-CoV-2 S protein in PBS and PSS solution are 10 fg/mL.