Bright Modes Research Articles

Laser Control of Specular and Diffuse Reflectance of Thin Aluminum Film-Isolator-Metal Structures for Anti-Counterfeiting and Plasmonic Color Applications

Plasmonic structural color originates from the scattering and absorption of visible light by metallic nanostructures. Stacks consisting of thin, disordered semicontinuous metal films are attractive plasmonic color media, as they can be mass-produced using industry-proven physical vapor deposition techniques. These films are comprised of random nano-island structures of various sizes and shapes resonating at different wavelengths. When irradiated with short-pulse lasers, the nanostructures are locally restructured, and their optical response is altered in a spectrally selective manner. Therefore, various colors are obtained. We demonstrate the generation of structural plasmonic colors through femtosecond laser modification of a thin aluminum film–isolator–metal mirror (TAFIM) structure. Laser-induced structuring of TAFIM’s top aluminum film significantly alters the sample’s specular and diffuse reflectance depending on the fluence value and the number of times a region is scanned. A “negative image” effect is possible, where a dark field observation mode image is a negative of a bright field mode image. This effect is visible using an optical microscope, the naked eye, and a digital camera. The use of self-passivating aluminum results in a long-lasting, non-fading coloration effect. The reported technique could be used in anti-counterfeiting and security applications, as well as in plasmonic color printing and macroscopic and microscopic marking for personalized fine arts and aesthetic products such as jewelry.

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.

Controllable dual resonances of Fano and EIT in a graphene-loaded all-dielectric GaAs metasurface and its sensing and slow-light applications

Abstract Active nanophotonic metasurfaces have attracted considerable attention for their promise to develop compact, tunable optical metadevices with advanced functions. In this work, we theoretically demonstrated the dynamically controllable dual resonances of Fano and electromagnetically induced transparency (EIT) using a graphene-loaded all-dielectric metasurface with U-shaped gallium arsenide (GaAs) nanobars operating in the near-infrared region. The destructive interference between a subradiant mode (i.e., a dark mode) supported by two vertical GaAs bars and two radiative modes (i.e., two bright modes) supported by a horizontal GaAs nanobar gives rise to a Fano resonance and an EIT window with high transmission and a large quality factor (Q-factor) in the transmission spectrum. Importantly, the transmission amplitudes can be flexibly modulated by adjusting the graphene Fermi levels without rebuilding the nanostructures. This modulation results from the controllable light absorption by the loaded graphene monolayer due to its interband losses in the near-infrared spectrum. Furthermore, the peak wavelengths of the Fano resonance and EIT window with high Q-factors are highly sensitive to variations in the refractive index (RI) of the surrounding medium, giving the proposed metasurface a relatively good sensitivity of ~700 nm/RIU and a high figure of merit (FOM) of 280, making it an effective RI sensor. Additionally, the metasurface features an adjustable slow light effect, indicated by the adjusted group delay time ranging from 0.12 ps to 0.38 ps. Therefore, the metasurface system proposed in this work offers a viable platform for advanced multi-band optical sensing, low-loss slow light devices, switches, and potential applications in nonlinear optical fields.

Multiband electromagnetically induced transparency-like on metamaterials

In recent years, the electromagnetically induced transparency-like (EIT-like) of metamaterials has been widely concerned due to obtain high Q, which has potential applications in the field of sensing and slow light. In this paper, an EIT-like metamaterial structure in the microwave band was investigated, which can realize to multi-band EIT-like phenomena. The equivalent circuit method and the electric field distribution are used to study the EIT -like effect’ mechanism, and the multi-band EIT-like phenomenon is discussed by analyzing its structural parameters. The results show that the multi-band EIT in the proposed structure is obtained by coupling multiple bright modes to multiple dark modes. The change of transmission spectrum is discussed by changing the environmental permittivity. The results have potential applications in areas such as refractive index sensing, filters.

Adjustable slow light with high group index in a graphene metasurface based on plasmon-induced transparency

A relatively simple metasurface structure is proposed to achieve a dynamically tunable slow light effect. The metasurface consists of two horizontal graphene strips and two vertical continuous graphene strips. Strong interference between the bright and dark modes enables the metasurface to generate a significant triple plasmon-induced transparency (PIT). Varying the coupling distance between the horizontal strips, double-PIT and triple-PIT can be transformed into each other. During the reduction of the transparent window, electromagnetic waves undergo strong phase changes, resulting in a higher group index. After structural optimization, the maximum time delay and group index can be as high as 2.624 ps and 3934, surpassing comparable slow light devices. The proposed patterned graphene metasurface provides theoretical guidance for designing high-performance slow light devices for optical storage, nonlinear optics and quantum optics.

Metamaterial-induced-transparency engineering through quasi-bound states in the continuum by using dielectric cross-shaped trimers

This study presents a novel approach to activate a narrowband transparency line within a reflecting broadband window in all-dielectric metasurfaces, in analogy to the electromagnetically-induced transparency effect, by means of a quasi-bound state in the continuum (qBIC). We demonstrate that the resonance overlapping of a bright mode and a qBIC-based nearly-dark mode with distinct Q-factor can be fully governed by a silicon trimer-based unit cell with broken-inversion-symmetry cross shape, thus providing the required response under normal incidence of a linearly-polarized light. Our analysis that is derived from the far-field multipolar decomposition and near-field electromagnetic distributions uncovers the main contributions of different multipoles on the qBIC resonance, with governing magnetic dipole and electric quadrupole terms supplied by distinct parts of the dielectric “molecule”. The findings extracted from this research open up new avenues for the development of polarization-dependent technologies, with particular interest in its capabilities for sensing and biosensing.

Full-Stokes polarization mid-wave infrared photodetector based on the polarization-converting metasurface

It is very challenging to achieve direct capture of the circular polarization due to the complex mechanism of the chiral materials. We numerically demonstrate a circular polarization photodetector composed of the silicon (Si) metasurfaces rotated 45° clockwise presenting as the phase delayer and the grating-coupled quantum well infrared photodetectors (QWIP) chip detecting the transverse magnetic (TM) mode. The circular dichroism and extinction ratio of the circular polarization (CP) photodetector are 0.25 and 20 dB at 4 μm operation wavelength, respectively. In addition, the linear dichroism (LD) and extinction ratios (ER) of the linear polarization device are 0.22 and 60 dB, respectively. The excellent LD performance is caused by the Fano resonance of the Si/SiO2 linear grating corresponding to the interference of the bright electric dipole mode and the dark electric quadrupole mode. The full Stokes pixel of the six-image-element technique can almost accurately obtain arbitrary polarized light at 4 μm operation wavelength. The errors of the degree of linear polarizations (Dolp) and the degree of circular polarizations (Docp) are less than −30 dB and −20 dB, respectively.

Plasmonic induced transparency and slow light integrated device for graphene-based metamaterial

The terahertz excitation characteristics of a graphene-based band structure are studied. This structure has different planar band structures that can excite two bright modes and then it can achieve an excellent plasmonic induced transparency effect through the two bright modes. By analyzing the boundary conditions of the electromagnetic field, the dispersion relationship of this structure can be smoothly obtained. Furthermore, the internal loss factor in the matching coupled mode theory can be calculated. Meanwhile, the tuning performance of this simple structure is analyzed through the external voltage. The Fermi level regulated by voltage is also analyzed, and its influence on the entire tuning mechanism is studied. A detailed analysis is conducted on the slow light performance of this structure, and the influence of Fermi level on the slow light performance is explored. This investigation is expected to provide potential theoretical foundations for slow light, optical storage, and other fields.

Multi-frequency and multi-functional optical switch based on dual plasmon-induced transparency

Research into multi-frequency and multi-functional optical switches for complex applications is pioneering territory. By employing a single-layer structure comprising three distinct graphene strips, we successfully created a dual-PIT effect through the destructive interference among two bright modes and a dark mode. The numerical simulations were corroborated by coupled mode theory, reflecting a high degree of consistency between the theory and the simulations. Remarkably, the modulation of the Fermi level in graphene metamaterials through gate voltage enabled the realization of asynchronous optical switches capable of operating at six, five, four, and three frequencies. Notably, the six-frequency switch exhibited an impressive modulation depth of 88.54% and an insertion loss of just 0.15 dB, highlighting its superior performance. This study lays a solid foundation for future multi-frequency and multi-functional optical switch designs, offering significant implications and practical applications.

Symmetry-Engineered Dual Plasmon-Induced Transparency via Triple Bright Modes in Graphene Metasurfaces

Dynamical manipulation of plasmon-induced transparency (PIT) in graphene metasurfaces is promising for optoelectronic devices such as optical switching and modulating; however, previous design approaches are limited within one or two bright/dark modes, and the realization of dual PIT windows through triple bright modes in graphene metasurfaces is seldom mentioned. Here, we demonstrate that dual PIT can be realized through a symmetry-engineered graphene metasurface, which consists of the graphene central cross (GCC) and graphene rectangular ring (GRR) arrays. The GCC supports a bright mode from electric dipole (ED), the GRR supports two nondegenerate bright modes from ED and electric quadrupole (EQ) due to the C2v symmetry breaking, and the resonant coupling of these three bright modes induces the dual PIT windows. A triple coupled-oscillator model (TCM) is proposed to evaluate the transmission performances of the dual PIT phenomenon, and the results are in good agreement with the finite-difference time-domain (FDTD) method. In addition, the dual PIT windows are robust to the variation of the structural parameters of the graphene metasurface except for the y-directioned length of the GRR. By changing the carrier mobility of graphene, the amplitudes of the two PIT windows can be effectively tuned. The alteration of the Fermi level of graphene enables the dynamic modulation of the dual PIT with good performances for both modulation degree (MD) and insertion loss (IL).

Tunable plasmon-induced transparency in anisotropic graphene-black phosphorus photonic device for high-performance sensors and switchers

A novel photonic device composed of graphene and black phosphorus (G-BP) has been proposed, which achieves high-performance plasmon-induced transparency (PIT) effect within the THz range and maintains substantial tunability and anisotropy. The anisotropy of the PIT effect arises from the near-field coupling between two bright modes characterized by distinct effective electron masses of BP, resulting transparency window at 33.35 THz for TE polarization and at 26.92 THz for TM polarization. Through the modulation of Fermi energy in graphene, doping levels of BP and geometric parameters separately, a tunable transparency window is achieved. Notably, the convergence or divergence of the anisotropic transparency windows can be well manipulated with different BP doping levels. Furthermore, the proposed G-BP photonic device exhibits a high sensitivity to changes in the surrounding refractive index and substrates, with a maximum sensitivity of 12.04 THz/RI, rendering it suitable for sensor applications. Overall, the proposed photonic device exhibits notable PIT effects characterized by high anisotropic performance, substantial tunability, great sensitivity, and stability, making it a promising candidate for applications in sensors, polarizers, and switchers.

Bimodal Absorber Frequencies Shift Induced by the Coupling of Bright and Dark Modes.

In this paper, we demonstrate that the absorption frequencies of the bimodal absorber shift with the coupling strength of the bright and dark modes. The coupling between the bright mode and the dark mode can acquire electromagnetically induced transparency, we obtain the analytical relationship between the absorbing frequencies, the resonant frequencies, losses of the bright mode and dark mode, and the coupling strength between two modes by combining the coupled mode theory with the interference theory. As the coupling strength between the bright mode and the dark mode decreases, the two absorption peaks gradually move closer to each other, inversely, they will move away from each other. The simulation employs three distinct metasurface structures with coupling of the bright and dark modes, thereby verifying the generality of the theoretical findings.

Performance of PSG in retrieving exoplanet parameters from JWST data simulated with several tools

Abstract Our research explores different methods of utilizing several known tools (petitRADTRANS, PandExo, and PSG) to simulate spectral data acquired by the James Webb Space Telescope (JWST). We specifically focus on how the choice of method affects the performance of the Planetary Spectrum Generator (PSG) in retrieving exoplanet parameters from such data. To investigate this, we consider the case of exoplanet TOI-1266 c being observed by JWST’s Near Infrared Spectrograph in the Bright Object Time Series mode. Our results show that differences between PSG and PandExo regarding their noise simulation and method of adjusting spectral resolution lead to significant differences in retrieval quality. When utilizing PandExo to simulate noise for non-binned down spectral data, we recommend ensuring that the resolution of the theoretical forward model spectrum input into PandExo closely matches the resolution of the output spectral data, within the same order of magnitude.

Active tunable terahertz multifunctional device switching from multiple narrowband perfect absorption to plasma-induced transparency

This paper puts forward a terahertz multifunctional device based on reversible phase transition property of VO2, which can realize active switching from multiple narrowband perfect absorption to plasma-induced transparency (PIT). By means of impedance matching theory and electric field distributions, we analyze the mechanism behind three absorption formants, and the absorption spectrum is consistent with data obtained from coupled mode theory. Three perfect absorption formants at the frequencies of 4.37 THz, 5.27 THz and 6.1 THz are extremely sensitive to the changes in external environment, which possesses a higher sensitivity up to 1.29 THz/RIU and FOM up to 9.45, indicating prominent sensing performance. By regulating the Fermi energy in a unified manner, three absorption formants can be observed within a specific frequency range. When the Fermi energy of three graphene patterns is individually manipulated, trimodal narrowband perfect absorption will transform into one bimodal and two unimodal ones. While the device is under PIT operation mode, two transmission windows are formed at 5.1 THz and 5.92 THz. It can be concluded that dual PIT arises from the mutual coupling between two bright modes and a dark mode formed by patterned graphene, when combined with the transmission spectra and the corresponding electric field distributions. Similarly, the modulation of graphene Fermi energy can realize dynamic tuning of the transmission performance, and single PIT can be achieved when graphene patterns are regulated separately. In summary, the proposed device can meet various functional requirements for multiple perfect absorption and PIT under different conditions and promote developments of terahertz technology in fields of environmental detection, electromagnetic modulation, multi-function devices and other application.

Multifunctional terahertz device based on plasmon-induced transparency

Enhancing light-matter interaction is crucial in optics for boosting nanophotonic device performance, which can be achieved via plasmon-induced transparency (PIT). In this study, a polarization-insensitive PIT effect at terahertz frequencies is achieved using a novel metasurface composed of a cross-shaped graphene structure surrounded by four graphene strips. The high symmetry of this metasurface ensures its insensitivity to changes in the polarization angle of incident light. The PIT effect, stemming from the coupling of graphene bright modes, was explored through finite difference time domain (FDTD) simulations and coupled mode theory (CMT) analysis. By tuning the Fermi level in graphene, we effectively modulated the PIT transparent window, achieving high-performance optical switching with a modulation depth (88.9% < MD < 98.0%) and insertion losses (0.17 dB < IL < 0.51 dB) at a carrier mobility of 2 m2/(V·s). Furthermore, the impact of graphene carrier mobility on the slow-light effect was examined, revealing that increasing the carrier mobility from 0.5 m2/(V·s) to 3 m2/(V·s) boosts the group index from 126 to 781. These findings highlight the potential for developing versatile terahertz devices, such as optical switches and slow-light apparatus.

Tunable optical devices with multiple plasmon-induced transparency based on graphene-black phosphorus hybrid metasurface

In this research, we propose a simple and novel graphene-black phosphorus composite structure. Through the coupling mechanism of bright–bright mode, four layers of graphene and one layer of black phosphorus produce a quadruple plasmon-induced transparency (PIT) effect. The response of the proposed structure to the angle of polarized light is investigated, and it is found that the sensitivities of graphene and black phosphorus to the polarization angle of light are different. In addition, the dynamic regulation of PIT by the Fermi level of graphene and the carrier concentration of black phosphorus are discussed. The modulation depths of the fivefold optical switching are 96%, 99%, 87%, 93%, and 80%, respectively. The quadruple PIT can also be converted into triple PIT, in which single, dual, or triple optical switching are realized, respectively. Finally, with the change in the refractive index of the environment medium, the sensitivity response of the proposed structure is as high as 4.790 THz⋅RIU−1. We believe that this research will contribute to the development of optical switches, modulators, and sensors.

Smart and Intelligent Weather Adaptive Street Lighting System Using IoT

In this paper, an Internet of Things (IoT)- based smart and intelligent weather-adaptive street lighting system containing an Arduino UNO, an ESP 8266, an LDR, a voltage sensor, a current sensor, a PIR, a relay, and a mobile or web application is designed and implemented. This project's primary goal is to create an intelligent and adaptable street lighting system that will guarantee constant lighting for rural residents. This project is to minimize the electricity losses, reduce the manpower used in manually switching on-off the street lights and &amp; energy cost. There are three primary parts to the system: An Arduino NANO, ESP 8266 is a Wi-Fi module for signal transfer and a smart phone that can operate web page servers. These days, consumers are searching for smart systems that require less maintenance and use less energy. This intelligent street light only turns on when needed, and it continues to run on its own subsequent evenings even in the absence of sunlight. Furthermore, an Arduino NANO was used to include a PIR sensor, which detects motion and turns on the lights to bright mode by turning on more lights. Taking those factors into account, these smart streetlights are equipped with an Internet of Things (IoT) powered adaptive system that can detect malfunctions and send message to parent system.

A methodology for phase characterization in pellet feed using digital microscopy and deep learning

This work proposes a new method for characterizing pellet feed, employing Deep Learning (DL) and Convolutional Neural Networks (CNNs). The main minerals in the composition of the studied pellet feed are hematite, magnetite, goethite, and quartz. Over time, several characterization methodologies have been developed that use Digital Microscopy and Image Analysis tools. The greatest difficulties in this characterization lie in differentiating the textures of hematite particles, the different shapes of their crystals, or discriminating between quartz and resin in reflected light optical microscopy images. This work proposes a mineral characterization methodology based on the Mask R-CNN algorithm. The goal is to perform instance segmentation, that is, to identify, classify, and segment objects in the images. Two DL models were combined: the BF Model performs instance segmentation for the compact, porous, martite, and goethite classes in images obtained in Bright Field mode, and the CPOL Model uses images acquired in Circularly Polarized Light to segment the monocrystalline, polycrystalline, and martite classes. An F1-score of around 80% was obtained for the BF Model and around 90% for the CPOL Model. The results were promising and can be improved as the training image database increases.

Hot deformation behaviour of Swaged Zircaloy-4 at varying strain rate and its microstructural evolution

Zirconium alloy demonstrates a favourable combination of strength, ductility, low neutron absorption cross-section, as well as excellent corrosion and oxidation resistance. In the context of its application as a reactor component, the critical hot deformation behaviour of isothermally swaged Zr-4 alloy has been extensively studied. Different combinations of elevated temperatures (873 K to 1023 K) and strain rates (0.01 s−1 to 10/s) have shown enhancement in flow stress. Analysis of stress flow, microstructural changes, and activation energy behaviour was performed using a state-of-the-art thermomechanical simulator with 50% reduction in sample thickness. The modified Arrhenius equation was employed to correlate theoretical and experimental flow stress behaviour. EBSD was utilized to understand texture evolution in hot deformed samples, revealing a transformation from pure basal to basal plus pyramidal poles concerning the compression direction. Texture evolution played a significant role in microstructural development, further characterized through TEM, which showed influential characteristics like subgrain boundaries, dislocations, and precipitate behaviour. Dynamic recrystallization mechanisms in swaged Zr-4 hot deformed samples were analysed using bright and dark field TEM modes, which were subsequently correlated with other microstructural characteristics such as grain morphology and structural defects.

Characterisation of rare earth element-bearing mineral phases present in South African coal ash using Mineral Liberation analysis

Critical raw materials, including rare earth elements (REE), are the cornerstone of modern technologies. Due to their unique properties, REE are in high demand, leading to the exploration and research of alternative sources. Coal combustion by-products, specifically coal ash, has been identified as a potential alternative source of REE, but more research is required. This study aimed to identify REE’s occurring in power station and laboratory-derived coal ash. Results from Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and Mineral Liberation Analysis are presented herein, to gain insight into rare-earth-bearing minerals occurring in the ash samples.REEs were determined in both sets of ash samples at values higher than 600 parts per million (ppm) using ICP-MS. Critical REE occur in ranges from 203 to 406 ppm; uncritical REE from 179 to 256 ppm; and excessive REE ranging from 197 to 348 ppm in these samples. Laboratory-derived coal ash and power station-generated ash samples were prepared as polished blocks and coated for examination using a Mineral Liberation Analyser (MLA). A bright phase search mode was employed to detect rare earth-bearing mineral phases using the back-scattered electron detector (BSE).Energy dispersive spectroscopy (EDS) was used to determine elemental compositions in these phases.Four rare earth-bearing mineral phases were determined, namely: monazite-REE, xenotime-REE, Ce-REErium rich, and REE-silicate. The laboratory-derived ash samples had a greater proportion of liberated rare earth-bearing minerals, related to the lower proportion of aluminosilicate glassy phase, related to the low temperature ashing procedure.The detailed mineralogy provides insight into the design of REE extraction processes, targeting specific REE’s hosted in the various minerals. This will be documented in a future paper.