Dipole Resonance Research Articles

Infrared Resonance Tailoring of Individual Split‐Ring Resonators with Phase‐Change Materials by Locally Changing the Dielectric Surrounding of the Antenna Hotspots

AbstractFor miniaturized active metasurfaces, resonance tuning of nanoantennas is a key ingredient. Phase‐change materials (PCMs) have been established as prime candidates for non‐volatile resonance tuning enabled by a change in the refractive index around nanoantennas. Conventionally, this tuning is induced by annealing the entire sample equally and does not allow changes on a meta‐atom level. Recently, it is demonstrated that individual rodantenna resonances can be adjusted by addressing each meta‐atom locally with precise laser pulses and switching the PCM there. However, simultaneously controlling several different modes remains elusive. In this work, PCM‐covered aluminum split‐ring resonators (SRRs) are switched locally to tune both the electric dipole resonances as well as the magnetic dipole resonances. By selectively switching the PCM at different hotspots of the SRRs, both resonances can be tuned individually. Finally, the field enhancement in the magnetic resonance allows continuous tuning of surface‐enhanced infrared absorption of native SiO2. This work serves as a proof of principle for sophisticated resonance tuning via changes in the refractive index at the hotspots of the selected antennas enabling fine‐tuning functionalities on a meta‐atom level and allows for post‐fabrication adjustments of metasurfaces.

High-Q transmission characteristics in terahertz guided-mode magnetic resonance system

Introduction: Guided mode resonance is generated by coupling wave diffractions with the waveguided mode. The guided mode resonances provide narrow-linewidth and resonance intensity for high quality factor (Q-factor) optical resonators.Methods: we demonstrate the high-Q guided mode resonances propagating on a low-loss, terahertz guided-mode magnetic resonance system, which are periodic square lattices of U-shaped split ring resonators (SRRs) on quartz substrates.Results: By choosing a judicious array period, two distinct frequency guided mode resonances and a magnetic dipole resonance with high Q-factor are observed. The interaction of the two resonances at similar frequencies produces a total transmission peak.Discussion: The dependences of the magnetic dipole resonance on the lattice period and structural parameters are investigated and discussed. The frequency difference between these two guided mode resonances widens with increasing Lattice period. The sharp spectral feature of each resonance results in the abrupt degradation of the spectral edge transmission. The proposed scheme is promising for efficient THz sensing, THz switching, and slow-light devices.

Design of embedded metamaterial solar absorber based on genetic algorithm

To effectively collect the solar energy, a metamaterial solar absorber operating in the UV to NIR wavelength range is designed, which consists of an embedded structure stacked with Ti and SiO2. The embedded structure can enhance the magnetic and electric fields to improve the absorption rate. However, the parameter space of the embedded structure is very huge to increase the optimization search time. Therefore, the absorption rate is calculated by the finite difference time domain method and the parameters of the absorber are optimized by genetic algorithm. The results show that the absorption rates of the optimized absorber are all over 95% in the 300–2500 nm and the average absorption rate is 98.7%, and the absorber is of polarization independence and the average absorption rate remains above 95% at an oblique incidence angle of 60°. By analyzing the electromagnetic field distribution of the absorber at the resonance wavelength, it can be concluded that the high absorption characteristics in the wideband are mainly achieved by the combined effects of local surface plasmon resonance, electric dipole resonance, propagating surface plasmon resonance and Fabry-Perot cavity resonance. Perfect absorber in the solar spectral range have significant value in fields such as solar energy harvesting and information detection.

Germanium metasurface assisted broadband detectors

Abstract The demand on broadband near-infrared photodetections with high responsivity is becoming increasingly eminent; however its realization remains a significant technological challenge. Here we design, fabricate, and characterize a broadband Ge photodetector (1000–1600 nm), composed of densely packed 230-nm-thick Ge disks of different diameters (255 nm, 320 nm, and 500 nm), placed on top of a 105-nm-thin Ge layer. Using experimentally measured and calculated transmission and absorption spectra, we demonstrate that the absorption and detector responsivity are increased by nearly 2 orders of magnitude, compared to the unstructured Ge photodetector, due to the excitation of magnetic dipole resonances in Ge disks, while preserving a relatively low dark current. Our approach is simple and can be easily adapted to other semiconductor material platforms and operation wavelengths to enable performance improvements of broadband photodetector devices.

Dynamic selection of visible wavelengths using resonant TiO2 nanostructures

Abstract All-dielectric nanoarrays have strong electromagnetic resonances with various interesting applications and are tuned by adjusting their geometrical parameters. However, their optical properties are permanently encoded during fabrication. This study presents robust dynamically tunable all-dielectric nanoresonators for controllable, reversible, and reproducible color filtering. Our design uses an array of TiO2 nanodiscs embedded in a transparent, stretchable polydimethylsiloxane (PDMS) membrane and exhibits a narrow spectral response due to Mie magnetic and electric dipole resonances hybridized with the TiO2 nanodiscs lattice modes. By mechanically stretching the PDMS membrane, the pitch of the TiO2 nanodiscs was increased and the spectral location of the resonances was altered. Additionally, an optically asymmetric structure was fabricated by partially embedding TiO2 nanodiscs in PDMS. Thus, the magnitude of the Rayleigh anomaly diffraction, which could interrupt the dipole resonances, was reduced. Our design has sharp, frequency-tunable resonances in the visible spectrum, and we demonstrated dynamic tunability by stretching the metasurfaces.

Broadband and high-efficient reflective linear–circular polarization convertor based on three-dimensional all-metal anisotropic metamaterial at terahertz frequencies

In this paper, we proposed a broadband and high-efficient reflective linear–circular​ polarization convertor (LCPC) based on three-dimensional (3D) all-metal anisotropic metamaterial (AMM) in terahertz (THz) region. The LCPC unit-cell is composed of the stand-up inverted U-shaped resonator (USR) deposited on a ground plane. This LCPC design can convert both incident x- and y-polarized (y-pol) waves from linear to circular polarization after reflection in a broadband frequency range of 1.98-4.12THz with an axial ratio (AR) below 3 dB and polarization conversion efficiency (PCE) over 98% on average. The numerical simulation results show that the broadband polarization conversion of the designed LCPC is caused by the overlaps of stronger electric and magnetic dipolar coupling resonances. The polarization conversion properties of the designed LCPC can be adjusted by varying the geometric parameters of the USR. The proposed LCPC design has potential applications in many areas such as spectroscopy, detection, imaging, and communications in the THz region.

Giant and reversible circular dichroism based on phase change materials for near-field image display

Switchable circular dichroism (CD) is in high demand in wireless communication, real-time imaging, etc. At present, the active switching range of CD magnitudes in a nanostructure is relatively narrow. Herein, the giant and reversible CD with large-range modulation is realized by controlling the phase transition of GST (Ge2Sb2Te5) in a double Z-shape array. The demonstrated CD undergoes the large range from −0.85 to 0.85, which is a major advance. The giant CD arises from the polarization-dependent excitations of electric and magnetic dipole resonances. Utilizing the giant and reversible CD, the application for dynamic near-field image display is demonstrated. The reconfiguration capability of our metasurface for large-range switchable CD will promote the prosperity of active chiro-optical devices.

Dielectric dual-dimer metasurface for enhanced mid-infrared chiral sensing under both excitation modes

Abstract Chirality (C) is a fundamental symmetry property of objects. Detecting and distinguishing molecular chirality in the infrared spectrum is important in life sciences, biology, and chemistry. In this paper, we demonstrate an achiral metasurface based on a gaped dual-germanium-dimer array for enhanced mid-infrared chiral sensing under both circularly polarized light (CPL) and linearly polarized light (LPL) excitations. With the metasurface, strong electric and magnetic dipole resonances with large field enhancement can be generated, resulting in an accessible superchiral hotspot in the dimer gaps under both excitation modes. The maximum electric and magnetic field enhancements exceed 220 and 100 for the bare metasurface, and exceed 70 and 60 for the metasurface coated with a 50 nm chiral biolayer under both excitations, respectively. Importantly, a high volume-averaged C enhancement C E_ave of 241 (444) and C E_ave_bio of 161 (102) under CPL (LPL) excitation can be achieved for the bare metasurface and it coated with the chiral biolayer, respectively. These results may open up new possibilities for ultrasensitive vibrational circular dichroism (VCD) and rotational optical dispersion (ORD) spectroscopy in the mid-infrared range.

Accessing dual toroidal modes in terahertz plasmonic metasurfaces through polarization-sensitive resonance hybridization

Plasmonic metasurfaces have been quite a fascinating framework to invoke transformation of incident electromagnetic waves for a while now. Oftentimes, the building block of these metasurfaces or the unit cells consists of two or more meta-resonators. As a consequence, near-field coupling amongst these constituents may occur depending upon the spatial and spectral separation of the individual elements (meta-resonators). In such coupled structures resonance mode-hybridization can help in explaining the formation and energy re-distribution among the resonance modes. However, the coupling of these plasmonic modes is extremely sensitive to the polarization of the incident probe beam and offers ample amount of scope to harness newer physics. A qualitative understanding of the same can be attained through mode-hybridization phenomena. In this context, here, we have proposed a multi-element metastructure unit cell consisting of split ring and dipole resonators aiming to explore the intricate effects of the polarization dependency of these hybridized modes. Multi-resonator systems with varied inter-resonator spacing (sp = 3.0, 5.0, and 7.0 μm) are fabricated and characterized in the terahertz domain, showing a decrement in the frequency detuning (δ) by 30% (approx.) for a particular polarization orientation of THz probe beam. However, no such detuning is observed for the other orthogonal polarization configuration. Therefore, modulation of the resonance-hybridization is strongly dependent on the terahertz beam polarization. Further, as an outcome of the strong near-field coupling, the emergence of dual toroidal modes is observed. Excitation of toroidal modes demands thoughtful mode engineering to amplify the response of these otherwise feeble modes. Such modes are capable of strongly confining electromagnetic fields due to higher Quality (Q-) factor. Our experimental studies have shown significant signature of the presence of these modes in the Terahertz (THz) domain, backed up by rigorous numerical investigations along with multipole analysis. The calculated multipole decomposition demonstrates stronger scattering amplitude enhancements (∼7 times) at both the toroidal modes compared to off-resonant values. Such dual toroidal resonances are capable of superior field confinements as compared to single toroidal mode, and therefore, can potentially serve as an ideal testbed in developing next-generation multi-mode bio-sensors as well as realization of high Q-factor lasing cavities, electromagnetically induced transparency, non-radiating anapole modes, novel ultrafast switching, and several other applications.

Bound states in the continuum (BIC) in silicon nanodisk array on mirror structure: Perfect absorption associated with quasi-BIC below the bandgap

A structure composed of a hexagonal array of Si nanodisks having toroidal dipole resonances and a reflecting mirror separated by a SiO2 spacer is proposed as a platform that exhibits narrow-band perfect absorption in the Si sub-bandgap wavelength range for a CMOS compatible Si based photodetector operating below the bandgap range. The numerical simulation reveals that the structure possesses Fabry–Pérot bound states in the continuum at proper spacer thicknesses due to the interference between the toroidal dipole and its image dipole. By slightly detuning the spacer thickness to meet the critical coupling condition, narrow-band perfect absorption appears despite assumption of a very small extinction coefficient (5 × 10−4). The wavelength of the perfect absorption is controlled in a wide range by the structural parameters of a Si nanodisk hexagonal array and is insensitive to the fluctuation of the extinction coefficient and the choice of a metallic mirror. In the structure, over 90% of incident power can be absorbed in the Si region. This suggests that the structure can be used as a narrow-band photodetector operating in the Si sub-bandgap wavelength range. We also evaluate the sensing performance of the proposed structure as an intensity based refractive index sensor operating in the near-infrared range.

Large Optical Modulation of Dielectric Huygens’ Metasurface Absorber (Advanced Optical Materials 10/2023)

Mie-type optical resonators are applicable to all-optical modulation. In article number 2300102, Hao-Yu Cheng, Kung-Hsuan Lin, Kuo-Ping Chen, and co-workers demonstrate both theoretically and experimentally that modulation depth is enhanced by two orders of magnitude in Huygens' metasurface absorber where electric dipole lattice resonance matches the magnetic dipole resonance. Within picosecond scale delay after ultrafast photoexcitation and a few square micrometer spatial confinement, over 100% photothermal modulation depth is experimentally achieved in Huygens' metasurface absorber based on amorphous silicon.

Spin-Orbit and Zeeman Effects on the Electronic Properties of Single Quantum Rings: Applied Magnetic Field and Topological Defects.

Within the framework of effective mass theory, we investigate the effects of spin-orbit interaction (SOI) and Zeeman splitting on the electronic properties of an electron confined in GaAs single quantum rings. Energies and envelope wavefunctions in the system are determined by solving the Schrödinger equation via the finite element method. First, we consider an inversely quadratic model potential to describe electron confining profiles in a single quantum ring. The study also analyzes the influence of applied electric and magnetic fields. Solutions for eigenstates are then used to evaluate the linear inter-state light absorption coefficient through the corresponding resonant transition energies and electric dipole matrix moment elements, assuming circular polarization for the incident radiation. Results show that both SOI effects and Zeeman splitting reduce the absorption intensity for the considered transitions compared to the case when these interactions are absent. In addition, the magnitude and position of the resonant peaks have non-monotonic behavior with external magnetic fields. Secondly, we investigate the electronic and optical properties of the electron confined in the quantum ring with a topological defect in the structure; the results show that the crossings in the energy curves as a function of the magnetic field are eliminated, and, therefore, an improvement in transition energies occurs. In addition, the dipole matrix moments present a non-oscillatory behavior compared to the case when a topological defect is not considered.

Bandwidth-tunable absorption enhancement of visible and near-infrared light in monolayer graphene by localized plasmon resonances and their diffraction coupling

Bandwidth-tunable light absorption enhancement in monolayer graphene is practically important for graphene-based photoelectric devices. Especially, it is still a huge challenge to achieve high-efficiency graphene absorption with extremely narrow sub-nanometer bandwidth much smaller than one nanometer. In this work, both broadband and narrowband absorption peaks of monolayer graphene are numerically achieved in visible and near-infrared wavelength ranges. The broadband absorption peak is ascribed to localized dipolar plasmon resonances in individual silver nanodisks, and the narrowband absorption peak arises from collective first-order diffraction coupling effect in periodic array of silver nanodisks. By changing the array period, the full width at half maximum (FWHM) of the broadband absorption peak can vary from 100 nm to 50 nm. Correspondingly, the FWHM of the narrowband absorption peak is tuned from about 6.4 nm to only 0.25 nm, thus realizing an ultra-narrow sub-nanometer bandwidth.

Development of a linear ion trap mass spectrometer capable of analyzing megadalton MALDI ions

Detecting megadalton matrix-assisted laser desorption/ionization (MALDI) ions in an ion trap mass spectrometer is a technical challenge. In this study, megadalton protein and polymer ions were successfully measured by MALDI linear ion trap mass spectrometer (LIT-MS) for the first time. The LIT-MS is comprised of a Thermo linear ion trap mass analyzer and a highly sensitive charge-sensing particle detector (CSPD). A newly designed radio frequency (rf) scan mode with dipolar resonance ejection techniques is proposed to extend the mass range of LIT-MS up to one million Thomson (Th). We analyze high mass ions with mass-to charge (m/z) ratios ranging from 100 kTh to 1 MTh, including thyroglobulin, alpha-2-macroglobulin, immunoglobulins (e.g., IgG and IgM), and polymer (∼ 940 kTh) ions. Besides, it is also very challenging for ion trap mass spectrometry to detect megadalton ions at low concentrations. By adopting high affinity carboxylated/oxidized detonation nanodiamonds (oxDNDs) to enrich IgM molecules and form antibody-nanodiamond conjugates, we have successfully reached ∼ 5 nM (5 μg/mL) concentration which is better than that by the other techniques.

Effect of Motion of Atoms on the Character of Subradiance of Cold Dilute Ensembles Excited by Resonant Pulsed Radiation

The dynamics of the fluorescence of dilute atomic ensembles excited by resonant pulsed radiation has been analyzed. It has been found that the motion of atoms in ensembles cooled to sub-Doppler temperatures in certain time intervals noticeably enhances subradiance rather than suppressing it. It has also been shown that the time dependence of the spontaneous decay rate can be nonmonotonic. The revealed features occur because the character of the slow decay of the atomic excitation is determined after a certain time mainly by the dynamics of subradiant two-atom quantum states, which are due to the resonant dipole–dipole interaction between atoms.

Large Optical Modulation of Dielectric Huygens’ Metasurface Absorber

AbstractHigh‐index nanostructures are found to exhibit large nonlinearity of spectral responses under photoexcitation, which is applicable to all‐optical modulation. The nonlinearity stems from Mie‐type resonance shift due to photoinduced variation of refractive index. Here, amorphous silicon‐based Huygens’ metasurface absorber and ultrafast excitation are used to demonstrate two orders of magnitude enhancement of modulation depth, both theoretically and experimentally, when electric dipole lattice resonance matches the magnetic dipole resonance. Within a few square micrometer spatial confinement and picosecond scale delay, over 100% photothermal modulation depth is experimentally achieved at matched resonance condition. This concept is not limited to photothermal modulation in the amorphous silicon but is applicable to dielectric materials with various mechanisms to achieve modulation of refractive index. This work opens the avenue toward all‐optical information processing via optimized modulation by careful spatiotemporal and spectral control over a metasurface.

Dipole resonance conditions and field redistributions of charged nanoparticles

For charged nanoparticles, no relation of the near fields to the resonance conditions has yet been documented. Hence, the “singular” near fields induced by the excess charges have not yet been well explained. In this paper, using the charge-amended scattering theory, we study in detail the excess-charge-regulated resonance conditions and the blue shifts of resonances for a 2D charged cylinder made of polar materials. We show that the blue-shifted resonance of the charged nanoparticles may extend to wavelengths below the longitudinal optical (LO) wavelength, breaking the limit of a neutral nanoparticle. We give the theoretical threshold of the surface charge density, which shifts the resonance to the LO wavelength. Under the resonance shifts, the mechanism of the “singular” near-field redistribution due to the excess charges can be understood. Specifically, the on- and off-resonance distributions can swap with each other excited by the surface charges, which leads to the redistribution of the internal and near external intensities.

Independently tunable Fano resonance from quasibound state in the continuum in hybrid graphene–dielectric metasurface for magnetic field tunability

We study an independently tunable Fano resonance from quasibound state in the continuum and polarization-independent toroidal dipole resonance in hybrid graphene–dielectric metasurface consisting of a nanoring and a cross-shaped nanobar. The unique structural properties of the proposed metasurface can support directional toroidal dipole mode, where the asymmetric magnetic field enhancement can be dynamically switched on and off while maintaining polarization-independent Fano spectral response. Through reducing or increasing inner radius of nanoring, an additional quasi-BIC dominated by magnetic dipole moment can be excited and independently tuned by altering the polarization direction of the incident wave. In addition, the quasi-BIC can be effectively modulated via adjusting the Fermi energy and layer numbers of the graphene. Our results can be of practical interest for a variety of applications including optical modulator, filter, switches, and light trapping.

Опис фотопоглинання фотонними силовими функціями із збудженням двох резонансних станів

The results of the use of photon strength function (PSF) of the model of two coupled damped oscillators (Two State Excitations, TSE) for the description of nuclear data for photoabsorption of electric dipole gamma-rays by atomic nuclei are presented. The response function widths dependent on gamma-ray energy were used instead of constant widths in the initial TSE model. It was demonstrated that such a phenomenological approach is a simple method for the description and prediction of the PSF with excitation of both low-energy (pigmy) dipole resonance (PDR) and high-energy giant dipole resonance (GDR). The best description of the experimental photoabsorption cross-sections is obtained at constant width in the PDR range and linear or quadratic energy-dependent width in the GDR range.

Investigation of giant dipole resonance in Mo isotopes within TDHF theory

The isovector giant dipole resonance (IVGDR) in the chain of even-even Mo isotopes is investigated within the time-dependent Hartree-Fock (TDHF) using the Skyrme force Sly6. The GDR calculated in Mo92−108 are presented, and compared with the available experimental data. An overall agreement between them is obtained. Moreover, the dipole strength in Mo102−108 is predicted. Shape phase transition from spherical to oblate as well as shape coexistence (A ∼ 100) in Mo isotopes are also investigated in this work. In addition, the correlation between the deformation splitting ΔE and the quadrupole deformation parameter β2 is studied. The results confirm that ΔE is proportional to the deformation of nucleus. We also discuss the dependence of GDR strength on some nuclear properties of Skyrme forces, such as the asymmetry energy as. We find that as=32 MeV, corresponding to SLy6, is quite consistent with experimental data.