Intrinsic Material Loss Research Articles

AlScN-on-SiC microelectromechanical Lamb wave resonators operating at high temperature up to 800 °C

We report on the experimental demonstration of aluminum scandium nitride (AlScN)-on-cubic silicon carbide (3C-SiC) Lamb wave resonators (LWRs) realized via microelectromechanical systems (MEMS) technology, operating at high temperature (T) up to T = 800 °C, while retaining robust electromechanical resonances at ∼27 MHz and good quality factor of Q ≈ 900 even at 800 °C. Measured resonances exhibit clear consistency and stability during heating and cooling processes, validating the AlScN-on-SiC LWRs can operate at high T up to 800 °C without noticeable degradation in moderate vacuum (∼20 mTorr). Even after undergoing four complete thermal cycles (heating from 23 to 800 °C and then cooling down to 23 °C), the devices exhibit robust resonance behavior, suggesting excellent stability and suitability for high-temperature applications. Q starts to decline as the temperature exceeds 400 °C, which can be attributed to energy dissipation mechanisms stemming from thermoelastic damping and intrinsic material loss originating from phonon–phonon interactions.

Nearly Perfect Spin Conversion Based on Topological Singularity in 1D Anisotropic Photonic Crystals

AbstractAlthough the spin‐controlled vortex generation and photonic spin‐Hall effect of spin‐flipped abnormal mode have been widely studied recently, the traditional method based on the metasurface is difficult to fabricate, and the efficiency of the spin‐flipped abnormal mode is rather low due to process errors and intrinsic material loss. Here, a new method is proposed based on the insights into the topological singularity and special Bragger reflections resonant (BRR) mode of one‐dimensional (1D) finite photonic crystals (PhCs) with anisotropic material to realize nearly perfect (100%) spin‐conversion efficiency. For a finite 1D PhC with cell number N, there are 3N complete spin‐conversion (CSC) and complete spin‐maintained (CSM) channels. Two mechanisms of these CSC and CSM channels are revealed. The working bandwidths and the angular ranges of these CSC and CSM are also studied. Based on these theoretical findings, multi‐angles and multi‐frequencies perfect spin‐conversion (‐maintained) devices can be designed. At last, these theoretical results are confirmed by the numerical experiments based on finite‐difference time‐domain (FDTD) methods. This work paves the way to exploring the topological properties and polarization control of PhCs made of anisotropic dielectrics and provides a prospective method for the design of multi‐channels spin optical devices.

Analogue of electromagnetically induced transparency in all-dielectric metasurface induced by quasi-bound state in the continuum for switching

We design and numerically investigate an all-dielectric metasurface composed of ring/disk cavity supporting quasi-bound state in the continuum (BIC) by breaking the in-plane symmetry of the cavity through removing a part from the edge of the central disk that invokes a leakage channel in the form of weakly radiating electromagnetically induced transparency. Far-field radiations and near-field distributions explain the evolution from BIC to quasi-BIC and further point out the dominant magnetic quadrupole component in quasi-BIC. Such quasi-BIC can be switched on and off at orthogonal polarizations and modulated by the intrinsic material loss. Further, we compare the effects of quasi-BICs excited by the offset and removing a part from the edge of the central disk on Q-factors and near-field enhancement, which can be tailored and optimized by selecting the appropriate degree and way of asymmetry.

Investigation of a Possible Material-Saving Approach of Sputtering Techniques for Radiopharmaceutical Target Production

Magnetron sputtering (MS) is a relatively new deposition technique, which is being considered among the cyclotron solid target (CST) manufacturing options now available, aiming at the medical radioisotopes yield for radiopharmaceutical production. However, the intrinsic high material losses during the deposition process do not permit its use with extremely expensive target materials, such as isotopically enriched metals/oxides. In this study, R&D technology for a new recovering shield is instead proposed to assess the dissipation of target material during the sputtering processes and, thus, an estimate of the material recovery that may be feasible and the related amount. The weight-loss analysis method is used to assess the material losses level inside the chamber during processing. In all tests carried out, a high-purity copper (99.99%) was used as a target material. As a result of this study, the material distribution for both magnetron and diode sputtering depositions can be calculated. The feasibility of the ultra-thick coatings growing, devoted to CST production, is demonstrated.

High-efficiency broadband achromatic metalens for near-IR biological imaging window

Over the past years, broadband achromatic metalenses have been intensively studied due to their great potential for applications in consumer and industry products. Even though significant progress has been made, the efficiency of technologically relevant silicon metalenses is limited by the intrinsic material loss above the bandgap. In turn, the recently proposed achromatic metalens utilizing transparent, high-index materials such as titanium dioxide has been restricted by the small thickness and showed relatively low focusing efficiency at longer wavelengths. Consequently, metalens-based optical imaging in the biological transparency window has so far been severely limited. Herein, we experimentally demonstrate a polarization-insensitive, broadband titanium dioxide achromatic metalens for applications in the near-infrared biological imaging. A large-scale fabrication technology has been developed to produce titanium dioxide nanopillars with record-high aspect ratios featuring pillar heights of 1.5 µm and ~90° vertical sidewalls. The demonstrated metalens exhibits dramatically increased group delay range, and the spectral range of achromatism is substantially extended to the wavelength range of 650–1000 nm with an average efficiency of 77.1%–88.5% and a numerical aperture of 0.24–0.1. This research paves a solid step towards practical applications of flat photonics.

Symmetry-protected bound state in the continuum in the metasurface of dielectric nanodisk with the inclined surface

We design and numerically investigate an all-dielectric metasurface with a single nanodisk in a unit cell supporting quasi-bound state in the continuum (BIC) arising from the distortion of symmetry-protected BIC. We demonstrate that by breaking out-of-plane symmetry, in the direction perpendicular to a nanodisk with an inclined surface, the transformation of BIC into quasi-BIC, having Fano line-shape, can be observed with an increasing inclination angle. Such transformation is analysed in detail by calculating charge density distributions, far-field radiations and near-field distributions, demonstrating the dominant electric quadrupole component in the quasi-BIC. In addition, the quasi-BIC can be easily tailored by the intrinsic material loss and the Fermi energy of graphene. By introducing this unique degree of freedom beyond the in-plane symmetry, one can realize access to the multipolar BIC-associated leaky state in simpler metasurface, indicating potential applications in the quasi-BIC-based switching and optical modulators.

Non-Equidistant Arrangement in All-Dielectric Quadrumers and Mirror-Symmetric One-Dimensional Photonic Crystals Hybridstructure for Self-Referenced Sensing Scheme

All-dielectric nanostructures have attracted attention for highly efficient light manipulation. Higher Q-factor Fano resonance can be excited due to the rich phenomenology of optical modes supported by high-index dielectric materials with low intrinsic material loss, including both electric and magnetic multipolar Mie resonances. So far, there have been few studies done on the sensing with Fano-resonant all-dielectric resonator nanostructures. Here, an all-dielectric self-referenced optical sensor is numerically demonstrated based on Si quadrumers and mirror-symmetric 1-D photonic crystals (1DPCs) hybridstructure, operating at the infrared wavelength range with higher sensing performance and much more stable reference signal. Magnetic Fano resonance and Fabry-Perot resonance were combined into such a hybridstructure for the first time. The influences of the gap distance, including Δx and Δy, on the coupling effects between the Si nanodimers along the polarization direction and the 1DPCs are systematically discussed. The sensing performance has been numerically investigated in different analytes with the sensitivity and figure of merit (FOM) of 760 nm/RIU and 1515 RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> respectively, which paves the way to be a much more sensitive detection of small refractive index changes in an unstable environment and suggests potential applications in biological and chemical sensing.

All-dielectric metasurface for high-performance structural color

The achievement of structural color has shown advantages in large-gamut, high-saturation, high-brightness, and high-resolution. While a large number of plasmonic/dielectric nanostructures have been developed for structural color, the previous approaches fail to match all the above criterion simultaneously. Herein we utilize the Si metasurface to demonstrate an all-in-one solution for structural color. Due to the intrinsic material loss, the conventional Si metasurfaces only have a broadband reflection and a small gamut of 78% of sRGB. Once they are combined with a refractive index matching layer, the reflection bandwidth and the background reflection are both reduced, improving the brightness and the color purity significantly. Consequently, the experimentally demonstrated gamut has been increased to around 181.8% of sRGB, 135.6% of Adobe RGB, and 97.2% of Rec.2020. Meanwhile, high refractive index of silicon preserves the distinct color in a pixel with 2 × 2 array of nanodisks, giving a diffraction-limit resolution.

Graphene Oxide Waveguide and Micro‐Ring Resonator Polarizers

AbstractIntegrated waveguide polarizers and polarization‐selective micro‐ring resonators (MRRs) incorporated with graphene oxide (GO) films are experimentally demonstrated. CMOS‐compatible doped silica waveguides and MRRs with both uniformly coated and patterned GO films are fabricated based on a large‐area, transfer‐free, layer‐by‐layer GO coating method that yields precise control of the film thickness. Photolithography and lift‐off processes are used to achieve photolithographic patterning of GO films with precise control of the placement and coating length. Detailed measurements are performed to characterize the performance of the devices versus GO film thickness and coating length as a function of polarization, wavelength and power. A high polarization dependent loss of ≈53.8 dB is achieved for the waveguide coated with 2‐mm‐long patterned GO films. It is found that intrinsic film material loss anisotropy dominates the performance for less than 20 layers whereas polarization‐dependent mode overlap dominates for thicker layers. For the MRRs, the GO coating length is reduced to 50 µm, yielding a ≈8.3 dB polarization extinction ratio between transverse electric (TE) and transverse magnetic (TM) resonances. These results offer interesting physical insights and trends of the layered GO films and demonstrate the effectiveness of introducing GO films into photonic‐integrated devices to realize high‐performance polarization selective components.

Experimental Observation of Superscattering.

Superscattering, induced by degenerate resonances, breaks the fundamental single-channel limit of the scattering cross section of subwavelength structures; in principle, an arbitrarily large total cross section can be achieved via superscattering. It thus provides a unique way to strengthen the light-matter interaction at the subwavelength scale, and has many potential applications in sensing, energy harvesting, bioimaging (such as magnetic resonance imaging), communication, and optoelectronics. However, the experimental demonstration of superscattering remains an open challenge due to its vulnerability to structural imperfections and intrinsic material losses. Here we report the first experimental evidence for superscattering by demonstrating the superscattering simultaneously in two different frequency regimes through both the far-field and near-field measurements. The underlying mechanism for the observed superscattering is the degenerate resonances of confined surface waves, by utilizing a subwavelength metasurface-based multilayer structure. Our work paves the way towards practical applications based on superscattering.

Topologically protected edge states in graphene plasmonic crystals.

A two-dimensional graphene plasmonic crystal composed of periodically arranged graphene nanodisks is proposed. We show that the band topology effect due to inversion symmetry broken in the proposed plasmonic crystals is obtained by tuning the chemical potential of graphene nanodisks. Utilizing this kind of plasmonic crystal, we constructed N-shaped channels and realized topologically edged transmission within the band gap. Furthermore, topologically protected exterior boundary propagation, which is immune to backscattering, was also achieved by modifying the chemical potential of graphene nanodisks. The proposed graphene plasmonic crystals with ultracompact size are subject only to intrinsic material loss, which may find potential applications in the fields of topological plasmonics and high density nanophotonic integrated systems.

Low-loss and energy efficient modulation in silicon photonic waveguides by adiabatic elimination scheme

High-speed Silicon Photonics calls for solutions providing a small footprint, high density, and minimum crosstalk, as exemplified by the recent development of integrated optical modulators. Yet, the performances of such modulators are hindered by intrinsic material losses, which results in low energy efficiency. Using the concept of Adiabatic Elimination, here, we introduce a scheme allowing for the low-loss modulation in densely packed waveguides. Our system is composed of two waveguides, whose coupling is mediated by an intermediate third waveguide. The signal is carried by the two outer modes, while the active control of their coupling is achieved via the intermediate dark mode. The modulation is performed by the manipulation of the central-waveguide mode index, leaving the signal-carrying waveguides unaffected by the loss. We discuss how Adiabatic Elimination provides a solution for mitigating signal losses and designing relatively compact, broadband, and energy-efficient integrated optical modulators.

Approaching the intrinsic quality factor limit for micromechanical bulk acoustic resonators using phononic crystal tethers.

We systematically demonstrate that one-dimensional phononic crystal (1-D PnC) tethers can significantly reduce tether loss in micromechanical resonators to a point where the total energy loss is dominated by intrinsic mechanisms, particularly phonon damping. Multiple silicon resonators are designed, fabricated, and tested to provide comparisons in terms of the number of periods in the PnC and the resonance frequency, as well as a comparison with conventional straight-beam tethers. The product of resonance frequency and measured quality factor (f×Q) is the critical figure of merit, as it is inversely related to the total energy dissipation in a resonator. For a wide range of frequencies, devices with PnC tethers consistently demonstrate higher f×Q values than the best conventional straight-beam tether designs. The f×Q product improves with increasing number of PnC periods, and at a maximum value of 1.2 × 1013 Hz, approaches limiting values set by intrinsic material loss mechanisms.

Infrared Topological Plasmons in Graphene.

We propose a two-dimensional plasmonic platform-periodically patterned monolayer graphene-which hosts topological one-way edge states operable up to infrared frequencies. We classify the band topology of this plasmonic system under time-reversal-symmetry breaking induced by a static magnetic field. At finite doping, the system supports topologically nontrivial band gaps with mid-gap frequencies up to tens of terahertz. By the bulk-edge correspondence, these band gaps host topologically protected one-way edge plasmons, which are immune to backscattering from structural defects and subject only to intrinsic material and radiation loss. Our findings reveal a promising approach to engineer topologically robust chiral plasmonic devices and demonstrate a realistic example of high-frequency topological edge states.

Influence of squeeze film damping on the higher-order modes of clamped–clamped microbeams

This paper presents an experimental study and a finite-element analysis of the effect of squeeze film damping on the resonance frequency and quality factor of the higher-order flexure vibrations modes of clamped–clamped microbeams. Viscoelastic and silicon nitride microbeams are fabricated and are electrostatically actuated by various electrode configurations to trigger the first, second, and third modes. The damping characteristic and the resonance frequency of these modes are examined for a wide range of gas pressure and electrostatic voltage loads.The results of the silicon nitride beams and viscoelastic beams are compared. It is found that the intrinsic material loss is the major dissipation mechanism at low pressure for the viscoelastic microbeams, significantly limiting their quality factor. It is also found that while the silicon nitride beams show higher quality factors at the intrinsic and molecular regimes of pressure, due to their low intrinsic loss, their quality factors near atmospheric pressure are lower than those of the viscoelastic microbeams. Further, the higher-order modes of all the beams show much higher quality factors at atmospheric pressure compared to the first mode, which could be promising for operating such resonators in air. Experimental results and finite element model simulations show good agreement for resonance frequency and quality factor for the three studied modes.

Fundamental limits to optical response in absorptive systems.

At visible and infrared frequencies, metals show tantalizing promise for strong subwavelength resonances, but material loss typically dampens the response. We derive fundamental limits to the optical response of absorptive systems, bounding the largest enhancements possible given intrinsic material losses. Through basic conservation-of-energy principles, we derive geometry-independent limits to per-volume absorption and scattering rates, and to local-density-of-states enhancements that represent the power radiated or expended by a dipole near a material body. We provide examples of structures that approach our absorption and scattering limits at any frequency; by contrast, we find that common "antenna" structures fall far short of our radiative LDOS bounds, suggesting the possibility for significant further improvement. Underlying the limits is a simple metric, |χ|2/Im χ for a material with susceptibility χ, that enables broad technological evaluation of lossy materials across optical frequencies.

Origin of optical losses in gallium arsenide disk whispering gallery resonators.

Whispering gallery modes in GaAs disk resonators reach half a million of optical quality factor. These high Qs remain still well below the ultimate design limit set by bending losses. Here we investigate the origin of residual optical dissipation in these devices. A Transmission Electron Microscope analysis is combined with an improved Volume Current Method to precisely quantify optical scattering losses by roughness and waviness of the structures, and gauge their importance relative to intrinsic material and radiation losses. The analysis also provides a qualitative description of the surface reconstruction layer, whose optical absorption is then revealed by comparing spectroscopy experiments in air and in different liquids. Other linear and nonlinear optical loss channels in the disks are evaluated likewise. Routes are given to further improve the performances of these miniature GaAs cavities.

Extremely Low Loss Phonon-Trapping Cryogenic Acoustic Cavities for Future Physical Experiments

Low loss Bulk Acoustic Wave devices are considered from the point of view of the solid state approach as phonon-confining cavities. We demonstrate effective design of such acoustic cavities with phonon-trapping techniques exhibiting extremely high quality factors for trapped longitudinally-polarized phonons of various wavelengths. Quality factors of observed modes exceed 1 billion, with a maximum Q-factor of 8 billion and Q × f product of 1.6 · 1018 at liquid helium temperatures. Such high sensitivities allow analysis of intrinsic material losses in resonant phonon systems. Various mechanisms of phonon losses are discussed and estimated.

High- Q photonic crystal surface-mode cavities on crystalline SOI structures

High- Q side-coupled surface-mode cavities in two-dimensional (2D) photonic crystals on crystalline silicon-on-insulator (SOI) structures are demonstrated. One of the surface-mode cavities has the measured system Q factor and intrinsic Q factor of 6200 and 13,400, respectively. The experimental results show that the value of system Q factor is ten times and the intrinsic Q factor six times higher than those of similar structures on amorphous SOI structures, respectively, due to the intrinsic material loss is much lower for crystalline silicon. The Q factors of cavities can be further improved by spin-on-glass overlayers, for increasing the structural symmetry. After the spin-on-glass process, the system Q factor and the intrinsic Q factor become nearly twice higher. Meanwhile, the drop wavelengths are largely red-shifted.

Ultralow-dissipation optomechanical resonators on a chip

Cavity-enhanced radiation-pressure coupling of optical and mechanical degrees of freedom gives rise to a range of optomechanical phenomena, in particular providing a route to the quantum regime of mesoscopic mechanical oscillators. A prime challenge in cavity optomechanics has been to realize systems that simultaneously maximize optical finesse and mechanical quality. Here we demonstrate, for the first time, independent control over both mechanical and optical degrees of freedom within the same on-chip resonator. The first direct observation of mechanical normal mode coupling in a micromechanical system allows for a quantitative understanding of mechanical dissipation. Subsequent optimization of the resonator geometry enables intrinsic material loss limited mechanical Q-factors, rivalling the best values reported in the high megahertz frequency range, while simultaneously preserving the resonators' ultrahigh optical finesse. As well as providing a complete understanding of mechanical dissipation in microresonator-based optomechanical systems, our results provide a promising setting for cavity optomechanics.