Reflection Peak Research Articles

Reflections of stress: Ozone damage in broadleaf saplings can be identified from hyperspectral leaf reflectance

Tropospheric ozone (O3) causes widespread damage to vegetation; however, monitoring of O3 induced damage is often reliant on manual leaf inspection. Reflectance spectroscopy of vegetation can identify and detect unique spectral signatures of different abiotic and biotic stressors. In this study, we tested the use of hyperspectral leaf reflectance to detect O3 stress in alder, beech, birch, crab apple, and oak saplings exposed to five long-term O3 regimes (ranging from daily target maxima of 30 ppb O3 to 110 ppb). Hyperspectral reflectance varied significantly between O3 treatments, both in whole spectra analysis and when simplified to representative components. O3 damage had a multivariate impact on leaf reflectance, underpinned by changes in pigment balance, water content and structural composition. Vegetation indices derived from reflectance which characterised the visible green peak were able to differentiate between O3 treatments. Iterative normalised difference spectral indices across the hyperspectral wavelength range were correlated to visual damage scores to identify significant wavelengths for O3 damage detection. We propose a new Ozone Damage Index (OzDI), which characterises the reflectance peak in the shortwave infrared region and outperformed existing vegetation indices in terms of correlation to O3 treatment. These results demonstrate the potential application of hyperspectral reflectance as a high throughput method of O3 damage detection in a range of common broadleaf.species.

Bioinspired Low-Angle-Dependent Photonic Crystal Elastomer for Highly Sensitive Visual Strain Sensor.

Photonic crystals (PCs) possess unique photonic band gap properties that can be used in the field of sensors and smart displays if modulated on the micronano structure. Both nonclose-packed (NCP) structure and high refractive index (RI) contrast of PC play important roles in response sensitivity during stretching. Herein, we constructed an NCP-structured PC strain sensor with high RI by a novel coating-etching strategy. Stretch-induced changes in structural color correspond to the strength of the force, enabling the detection of the strength of the acting force by the naked eye. The flexible 3D cross-linked network constructed by poly(ethylene glycol) phenyl ether acrylate and pentaerythritol tetrakis(3-mercaptopropionate) endows the sensor with excellent elasticity and robustness. The designed PC strain sensor achieves high mechanochromic sensitivity (∼8.3 nm/%, 0.02 to 4.21 MPa) and a substantial reflection peak shift (Δλ = 249 nm). More importantly, the high RI contrast (Δn = 0.43) between CdS and polymers imparts isotropic optical properties, ensuring a broad viewing angle while avoiding misleading signals. The research provides a novel fabrication strategy to construct sensitive PC strain sensors, expanding the prospective applicability to human movement monitoring and secure message encryption.

Synthesis of Tetrahedrite Zincian Nanocomposites via solvothermal process at low temperature

This study presents a novel hydrothermal method for synthesizing Cu11Zn1Sb4S13 (tetrahedrite, zincian) nanoparticles at a low temperature of 185 °C for a 6 h annealing at 400 °C for 2 h. CuCl2, ZnCl2, SbNO3, and CH4N2S, and NaBH4 were used for the innovative synthesis of Cu11Zn1Sb4S13 (Tetrahedrite, Zincian) Copper Zinc Antimony Sulfide nanoparticles ink. Structural, morphological and optical properties (optical energy gap) were studied for thin-film Cu11Zn1Sb4S13 deposited the nanoparticles via drop-casting method onto glass substrates are investigated. The structure and characterization of Cu11Zn1Sb4S13 by XRD powder diffraction provide evidence for a cubic structure. The optical calculations indicate that the absorption coefficient (α > 104 cm−1) and optical energy gap Eg are equivalent to 1.68 eV. Comparative work of Crystalline diameters of synthesis was obtained from XRD diffraction powder. The Debye-Scherrer procedure, modified Scherrer procedure and Williamson–Hall inspection are used to examine the individual participatory of the crystalline diameters, lattice stress and dislocation to isotropic line expansion widening reflection peaks of Cu11Zn1Sb4S13 nanoparticles. The modified Scherrer approach is appropriate, with all points along the fitting line. The crystallite size estimated from XRD was 5.798 nm and the particle size calculated from the FESEM method 8–9 nm. This slight difference reflects a spherical shape of the nanoparticles.

Microstructure and reflectance of porous GaN distributed Bragg reflectors on silicon substrates

Distributed Bragg reflectors (DBRs) based on alternating layers of porous and non-porous GaN have previously been fabricated at the wafer-scale in heteroepitaxial GaN layers grown on sapphire substrates. Porosification is achieved via the electrochemical etching of highly Si-doped layers, and the etchant accesses the n+-GaN layers through nanoscale channels arising at threading dislocations that are ubiquitous in the heteroepitaxial growth process. Here, we show that the same process applies to GaN multilayer structures grown on silicon substrates. The reflectance of the resulting DBRs depends on the voltage at which the porosification process is carried out. Etching at higher voltages yields higher porosities. However, while an increase in porosity is theoretically expected to lead to peak reflectance, in practice, the highest reflectance is achieved at a moderate etching voltage because etching at higher voltages leads to pore formation in the nominally non-porous layers, pore coarsening in the porous layers, and in the worst cases layer collapse. We also find that at the high threading dislocation densities present in these samples, not all dislocations participate in the etching process at low and moderate etching voltages. However, the number of dislocations involved in the process increases with etching voltage.

The electronic, magnetic, and optical behaviors of graphyne modulated by metal atoms adsorption: a first-principles study

The electronic, magnetic, and optical behaviors of graphyne modulated by various adsorbed metal atoms (Li, Na, K, Mg, Ca, Al, and Zn) from typical metal-ion batteries are studied by first-principles calculation. Notably, Mg and Zn adsorption systems are deemed unstable. In contrast, Li, Na, K, Ca, and Al systems exhibit two preferential adsorption sites, with the optimal position being the hollow center site within the large acetylenic ring. Upon the adsorption of these metal atoms, except for Ca adsorption systems exhibit semi-metallic behavior, while the other metal adsorption systems induced a transition from p-type to n-type semiconductors with decreased band gaps. Intriguingly, the inherent magnetism of the metal atoms vanished, resulting in a total magnetic moment of 0 μ B for the adsorption systems. Furthermore, the optical absorption and reflectivity peak positions for Ca adsorption systems show a significant redshift from violet to green and blue light regions. Conversely, other adsorption systems exhibit new absorption and reflection peaks in the infrared range, accompanied by an increase in both absorption coefficient and reflectivity across various spectral regions. These findings are conducive to the application in the field of novel optoelectronics and optical films.

A wideband polarization diversified MIMO antenna with improved isolation using machine learning optimized metasurface absorber

This communication presents a model and optimization of a dual-sense circularly polarized (RHCP/LHCP) 2-port MIMO antenna with increased isolation. The antenna operates in a broad frequency range of 1.76–4.0 GHz and is loaded with a metasurface. It has dimensions of 98.5 × 39 mm2 and an edge-to-edge distance of 55.5 mm. The MIMO antenna is created by replicating a single unit and placing it in a mirrored configuration around the y-axis. The dimensions of the unit antenna are 40 × 39 × 0.8 mm3 and it is printed on a substrate made of FR4 with a thickness of 0.8 mm. The substrate has a relative permittivity (ε r) of 4.4 and a loss tangent (tanδ) of 0.02. The isolation between the ports may be enhanced by including a metasurface absorber, increasing from 12 dB to 23 dB. To verify the wideband and CP properties, we examine the MIMO antenna unit element for its reflection coefficient, peak gain (4.2 dB), radiation efficiency (about 90%), axial ratio, and surface current distribution. Machine learning is used to improve the design of metasurface unit cells to achieve optimum absorption. The Envelope correlation coefficient (ECC), Diversity gain (DG), and Channel capacity loss (CCL) of the proposed metasurface absorber-enabled antenna are simulated and calculated to prove its MIMO identity. The HFSS simulations are consistent with the experimental results obtained from the developed model.

Observation of coupling interaction between surface plasmons and Tamm plasmons.

The optical effect analogous to electromagnetically induced transparency (EIT) in atomic systems has attracted broad attention in the field of photonics due to its promising applications in optical storage and integrated devices. Herein, we firstly report the experimental observation of the EIT-like effect generated from the coupling between surface plasmons (SPs) and Tamm plasmons (TPs) in a hybrid multilayer system at the near-infrared band. This multilayer system is composed of a nanofabricated silver grating on a silver/Bragg mirror with a SiO2 spacer. The experimental results show that a narrow reflection peak can appear in the wide reflection spectral dip due to the coupling between the SPs in the silver grating and TPs in the silver/Bragg mirror, which agree well with the finite-difference time-domain (FDTD) simulations. It is also found that the dip position of the EIT-like spectrum presents a redshift with the increase of the silver grating width. These results will provide a new way, to the best of our knowledge, for the generation of the EIT-like effect and light spectral manipulation in multilayer structures.

Quantum Size Effect of Bloch Wave Functions of Ultra-High Energy Electrons in a Thin Single-Crystal Film

The reflection coefficient of ultra-high-energy electrons (~1 MeV) at their normal incidence on a thin single-crystal film is calculated. It is shown that even at such high particle energies, the quantum size effect of the Bloch waves formed in the film is noticeably manifested. Narrow Bragg reflection peaks are found to appear at certain electron energies. A formula is given that determines their position and intensity on the reflection curve. A comparison is made of reflection coefficients at medium, high and ultra-high particle energies.

Influence of mono‐ and bi‐vacancy doped alkaline‐earth metals on the optoelectronic properties of monolayer defective state MoTe2: A first‐principles study

AbstractThe effects of single‐vacancy and double‐vacancy doping with alkaline‐earth metals on the stability, electronic structure, charge transfer, and optical properties of molybdenum ditelluride in the monolayer defective state (MoTe2) have been investigated using first‐principles methods. It is found that the structure of the molybdenum ditelluride system is more stable after double Te vacancies, with direct band‐gap to indirect band‐gap transitions occurring in single Te vacancies and Mo vacancies, and semiconductor to quasi‐metal transitions occurring in double Mo vacancies. The binding energy in the defect state system of molybdenum ditelluride after vacancy defects is always negative, and the structure remains stable. In this paper, the most stable double Te vacancy state was selected for substitutional doping with alkaline‐earth metals, and the Be atoms were doped with the largest amount of morphology, and Ca atoms were doped with the most pronounced decrease in band gap value. The direct band‐gap semiconductor is maintained after doping with Be atoms. Doping with Mg, Sr and Ba changes the band‐gap from direct to indirect. The double Te defect state MoTe2 doped with Ca atoms has a tendency to transition to a quasi‐metal. Regarding photoelectric properties, the system is blue‐shifted on both the absorption and reflection peaks.

Effect of strain on the photoelectric properties of molybdenum ditelluride under vacancy defects: a DFT investigation.

This study explores the impact of deformation on the electrical and optical characteristics of monolayer cadmium telluride (MoTe2) with vacancies, using the foundational principles of density functional theory. It was discovered that both strain and imperfections alter the electrical characteristics of monolayer MoTe2. Under VTe-MoTe2, a direct-to-indirect band-gap transition occurs. In DTe-MoTe2, the band-gap value reduces dramatically, the conduction band changes downward, and the carrier concentration rises. The DVTe-induced band gap state is closer to the Fermi energy level than the VTe-induced band gap state. In this paper, DTe-MoTe2 is chosen for tensile deformation. The results show that the band-gap value tends to decrease by increasing tensile deformation. When the stretching value reaches 10%, the lower bound of the conduction band and the top of the valence band overlap, and the system is converted from a semiconductor to a metal. Considering the density of states, the missing state MoTe2 is mainly contributed by the participation of Te-s, Te-p, and Mo-d orbitals. In terms of optical qualities, the absorption and reflection peaks are red-shifted and blue-shifted, respectively. It is hoped that these effects on the optoelectronic properties will be widely applied. In this study, we utilize the generalized gradient approximation plane-wave pseudopotential method, incorporating Perdew-Burke Ernzerhof (PBE) generalized functions and following the fundamental principles of the density functional theory framework. A 3 × 3 × 1 supercell was constructed as an undoped model based on a MoTe2 monolayer, which consists of 9 Mo atoms and 18 Te atoms. The vacuum flat plate was set to 15Å along the z-direction to avoid interactions between the monolayers. For electronic structure calculations, the energy cutoff was set to 450eV. Each model's computational process and structural optimization were carried out using the Monkhorst-Pack specialized K-point sampling approach. Crystal optimization computations used a 3 × 3 × 1 Monkhorst-Pack K-point grid for molybdenum ditelluride monolayers and a 9 × 9 × 1K-point grid for electronic system analysis, analyzing state density and optical characteristics, respectively. For the structural optimization, the convergence requirements for maximum force, maximum atom displacement, maximum stress, and energy change were defined at 0.03eV/Å, 0.001Å, 0.05 Gpa, and 1.0 × 10-5eV/atom, respectively.

Superparamagnetic photonic crystals with DNA probes for rapid visual detection of mercury

A novel superparamagnetic photonic crystal DNA probe (Fe3O4@SiO2@amino@DNA SPC) was developed to enable rapid visual detection of Hg2+. This unique photonic crystal (PC) was synthesized by combining superparamagnetic nanospheres with DNA probes. The DNA probe, rich in thymine (T), detects mercury ions through base mismatch, resulting in the formation of T–Hg2+–T loop hairpin structures. With the binding of Hg2+ to the probe attached to superparamagnetic nanospheres, the PC structure assembled by these nanospheres, formed by the magnetic field, was changed. This change enhanced the reflection intensity; it could be quantified using a fiber optic spectrometer and was visible to the naked eye. The Fe3O4@SiO2@amino@DNA SPC, specific to Hg2+, exhibited a reflection peak at 679 nm, which intensified with increasing Hg2+ concentration. The reflection intensity increased by 132.58 a.u., and the PC color shifted from red to yellow as the Hg2+ concentration increased from 0.1 μg/L to 1 mg/L.

Multi-colour reflective metagrating with neutral transparency for augmented reality

This paper presents the design and experimental validation of an all-dielectric and transparent metagrating-based metalens. Leveraging multiple guided mode resonances simultaneously, the metagrating enables the generation of two or more spectrally narrow reflection peaks. These peaks are achieved through the precise engineering of guided mode resonances, allowing for the reflection of a comb of vibrant and saturated colours. In addition to the investigation of underlying mechanisms, we introduce an analytical numerical method that facilitates rapid engineering of the spectral positions of the reflection peak comb. Experimental validation is provided for a bichromatic sample. Such metagrating can be promising for augmented reality systems, supporting individuals with mild to moderate cognitive impairments by delivering visual and textual stimuli that can improve indoor navigation, home safety, communication, and decision-making.

Synthesis of four-ring unsymmetrical bent-core mesogens and cybotactic cluster formation in their nematic phase

The synthesis and characterization of novel achiral unsymmetrical four-ring bent-core molecules resembling a hockey-stick shape are described. The present compounds bearing a methyl substituent in the central phenyl ring exhibit the skewed cybotactic nematic (NcybC), smectic C (SmC) and additional tilted smectic phase with tilted molecular arrangement. The skewed cybotactic nematic phase is formed by the bent-core molecules with long range orientational ordered clusters possessing local smectic C type order and within a cluster of tilted molecular assembly. The previously reported four-ring compounds without the methyl moiety did not exhibit nematic phase but show polarization modulated B1revtilt phase. The x-ray diffraction pattern clearly indicated the existence of SmC-type cybotactic clusters over a wide N temperature range; the layer reflection peak is weak at a higher temperature region and becomes more conspicuous with decreasing temperature. The lower temperature phase is distinctly the SmC phase, and in addition another tilted smectic phase emerges below SmC.

Multi-mode fiber Bragg grating for simultaneous detection of strain, torsion and temperature

In this study, a fiber Bragg grating sensor utilizing multi-mode optical fiber (MM-FBG) is proposed. This sensor can simultaneously detect both temperature and torsion, or strain and torsion. Unlike single-mode fiber, the multiple modes of multi-mode fiber can generate several reflection peaks within the FBG area, increasing data capacity and enabling multi-parameter detection. By monitoring the center wavelength and reflection intensity of the MM-FBG reflection peaks, the sensor can detect temperature, torsion, and strain simultaneously. This approach simplifies parameter characterization compared to traditional methods, reducing signal interference and enhancing the clarity of experimental results. Experimental findings demonstrate that the MM-FBG sensor structure exhibits maximum sensitivities of 0.571 dBm/°/cm for torsion, 12 pm/°C for temperature, and 0.7411 pm/με for strain. Therefore, the fiber sensor proposed in this study shows great potential for monitoring bridge and tunnel structures and for civil engineering applications.

Comparison of the effect of C doping on the photovoltaic properties of the defect state transition metal sulfur compounds MX2 (M = Mo, W; X = S, Te): a first-principles study

Two-dimensional layered materials are widely used due to their favorable electrical and optical properties. In this paper, the electronic structure, DOS, charge transfer, and optical properties of the defect state C-MX2 system of transition state metal-sulfur compounds are investigated using first-principle calculations. The electronic structure, DOS, charge transfer and optical properties of three systems, MoS2, MoTe2 and WS2, are systematically compared and analyzed. The results show that MoS2, MoTe2, and WS2 are all direct band-gap semiconductors. After the occurrence of vacancy defects, MoTe2 and WS2 are transformed from direct band-gap to indirect band-gap, while MoS2 still maintains the direct band-gap. We chose C atoms to dope the defective state MX2 system. After doping with a low concentration of C atoms, the Fermi energy level decreases, the valence band shifts upward, and the system undergoes a semiconductor-to-metal transition. In terms of density of states, the Mo-d and W-d orbitals as well as the S-p and Te-p orbitals are gradually enhanced under the effect of defect states and C doping, with the contribution of MoTe2 > MoS2 > WS2. In terms of optical properties, the absorption and reflection peaks of all three systems are blue-shifted after the change of defect states and C doping.

Influence of One-Dimensional Photonic Crystal on Raman Signal Enhancement: A Detailed Experimental Study

The enhancement of Raman signals using photonic crystal structures has been the subject of numerous experimental and theoretical studies, leading to a variety of issues and inconsistencies. This paper presents a comprehensive experimental investigation into the impact of alignment between the laser excitation wavelength and the specific position of the photonic band gap on signal enhancement in Raman spectroscopy. By employing one-dimensional (1D) porous silicon photonic crystals, a systematic analysis across a large number of spectra was conducted. The study focused on examining the signal enhancement of both the Raman ∼520 cm–1 silicon band, representing the constituent material of photonic crystal, and the most prominent Raman bands of crystal violet, used as a probe molecule. The probe molecules were both infiltrated into and adsorbed on top of the photonic crystal structure. The obtained experimental results for the contribution of 1D photonic crystals to Raman signal enhancement are much smaller compared to most predictions. The Raman signal of silicon and the signal from the probe molecule are enhanced ≤2.5 times when the laser excitation aligns with the edge of the photonic band gap, strictly defined as the position at the very bottom of the reflectance peak. The results have been discussed within the context of theoretical explanations.

Optimization of Cr/Sc-based multilayer mirrors for water window soft x-rays.

The development of efficient multilayer mirrors for the water window (a spectral region between absorption edges of carbon and oxygen, from 284 to 543 eV) remains a challenge. As the best candidate, the Cr/Sc multilayer provides maximum theoretical reflectivity of about 60% at near-normal incidence around the Sc L2,3 absorption edge (397 eV). However, the maximum measured peak reflectance published so far just slightly exceeds 20%. We report on a new (to the best of our knowledge) approach to design more efficient Cr/Sc-based multilayer coatings using the process of nitridation of chromium during deposition and adding boron carbide as a third material in the multilayer structure. We discuss our strategy of optimization of the CrN/B4C/Sc multilayer system based on experimental studies. The peak reflectance as high as 32% at 396 eV was measured with this type of coating, which is of main interest for various water window applications such as x-ray microscopy.

Ultrasmooth Ti/Al multilayer with a Ti seed layer for EUV applications

Al-base multilayers have attracted much interest in the extreme ultraviolet (EUV) optics field, but high roughness of this multilayer due to the Al film is still a big concern. Here, a strategy of the seed layer was proposed to reduce the surface roughness and intermixing layer thickness of the Al-base multilayer. Ti film is not only a seed layer, but also an absorption layer in this novel multilayer. An optimized Ti/Al multilayer film structure was designed to work at 21.1 nm, while investigating the use of Ti as a seed layer to reduce the roughness and enhance the peak reflectivity. The experimental results showed that the Ti seed layer effectively reduced the surface roughness and intermixing layer thickness and improved the reflectance. At 21.1 nm, the peak reflectance reached 39.6%, with a bandwidth of only 1.0 nm and an RMS roughness of 0.17 nm. Ti/Al multilayer also exhibits good stability. This multilayer has potential application in high-precision optics, such as corona detection, which requires extreme low light scattering of multilayer mirror.

Demonstration of standing cavity Brillouin random fiber lasers using double fiber Bragg grating arrays

Bidirectional feedback by fiber Bragg grating arrays (FBGAs) reduced the loss of the cavity and increased stimulated Brillouin scattering (SBS) gain by bi-directional Stokes wave through FBGA associated Rayleigh feedback of the pump wave. As a result, the Q value of the Brillouin random fiber laser (BRFL) increased significantly, which leads to narrow linewidth. This is different from the ring configuration with unidirectional SBS gain versus dual SBS gain of the same fiber length. Highly efficient use of the SBS gain fiber for coherent SBS amplification suppressed thermal noise associated Stokes wave. Such an efficient SBS laser is realized by a standing cavity BRFL based on double FBGAs. Multiple scattering of light traveling in strong scattering FBGAs enables light localization and the generation of high-Q reflection peaks. Coherent SBS amplification with high Q help to reduce laser relative intensity noise (RIN) and laser linewidth. Experimental results demonstrate that the BRFL supports localized modes by increasing the scattering strength of the FBGA random feedback, resulting in long lifetime and single-frequency emission with 20 dB noise floor reduction. The BRFL with a 1 km Brillouin gain fiber exhibits lower RIN and narrower linewidth than that with a 10 km Brillouin gain fiber due to the stronger gain competition of more modes in the longer cavity length. The optimized standing caivty BRFL with 1 km gain fiber leads to 3.5 kHz linewidth versus 40 kHz from the pump laser. These findings provide experimental evidence that double FBGAs offer a unique setting to control mode dynamics, realizing low-noise single-frequency lasing.

Revisit of electron temperature effect on stimulated Brillouin scattering in homogenous plasma

Stimulated Brillouin scattering (SBS) has complex dependence on the plasma electron temperature via the Landau damping, particle trapping, and consequent nonlinear frequency shift. It is found from our numerical simulation that the SBS reflectivity in its saturation stage tends to increase with the plasma electron temperature within a certain range, although the linear growth rate of SBS normally reduces with the increasing electron temperature. This is because the phase velocity of an ion acoustic wave (IAW) increases with the electron temperature, which tends to reduce the Landau damping of the IAWs and hence reduce ion trapping. In the nonlinear saturation stage, ion trapping will modify the ion distribution function and induce a negative frequency shift in the IAW. This nonlinear frequency shift will break the three-wave coupling, thereby causing saturation of the SBS. With further increase in the electron temperature, however, electron trapping will dominate over ion trapping, which induces a positive frequency shift in the IAW and can lead to the SBS saturation as well. As a result, the SBS reflectivity first increases and then decreases with increase in the electron temperature. At around the peak of the SBS reflectivity, the positive frequency shift of IAW induced by electron trapping roughly offsets the negative frequency shift induced by ion trapping.