Spectral Peak Position Research Articles

Denoising method for Raman spectra with low signal-to-noise ratio based on feature extraction

Raman spectroscopy is a non-destructive technique utilizing lasers to observe scattered light in order to determine things such as vibrational modes in the molecular system. A major problem inherent to this technique is that due to their short exposure time and the low power of the excitation laser, Raman signals are very weak. They tend to be much weaker than the noise and can even be drowned out. Conventional denoising methods are currently unable to extract Raman peaks with precision so it is necessary to specifically study Raman signal extraction methods that involve a low signal-to-noise ratio (SNR). In this study, a denoising method for Raman spectra with low SNR based on feature extraction was proposed. Based on the Hilbert Vibration Decomposition (HVD) method, the Raman spectra was decomposed into two components. The peaks were located in the first component and compensated by those in the second component. Then based on the position and height of the peaks, their full widths at half maximum (FWHM) are calculated. Finally, based on the position, height and FWHM of the peaks, Gaussian signals are used to reconstruct the Raman peaks from strong noise and baseline. In the data simulation experiment, the denoising method used improved the SNR from 3.5316 to 130.6386 and the mean square error (MSE) was reduced from 213.8635 to 14.0404. In the actual experiment, this method successfully extracted the characteristic peaks of melamine despite the noise from employing a low excitation laser (10 mW). The characteristics such as the amplitude and position of the peaks were identical to those obtained under a high excitation laser (150 mW). The error of the FWHM under different excitation laser powers (10 and 150 mW) was less than the spectral resolution. Using the method proposed in this paper, the Raman signal of biological samples such as rice leaves were extracted from the raw spectrum, and information on the spectral peak position, amplitude and FWHM were obtained with clarity. The characteristic peaks of the carotene molecule, protein amide I, protein phenylalanine, nucleic acid cytosine, cellulose, DNA phosphodiester, RNA phosphodiester, D-glucose, α-D glucose, chlorophyll, lignin and cellulose were all accurate as well. The results from the simulation data and actual experiments show that a method based on feature extraction can effectively extract Raman peaks even when they are submerged in background noise. It should be noted that the practicality of this method lies in the fact that it requires few parameters and is simple to operate and implement.

Investigation of acoustic plasmons in vertically stacked metal/dielectric/graphene heterostructures for multiband coherent perfect absorption.

Coherent absorption, as the time-reversed counterpart to laser, has been widely proposed recently to flexibly modulate light-matter interactions in two-dimensional materials. However, the multiband coherent perfect absorption (CPA) in atomically thin materials still has been elusive. We exploit the multiband CPA in vertically stacked metal/dielectric/graphene heterostructures via ultraconfined acoustic plasmons which can reduce the photon wavelength by a factor of about 70 and thus enable multiple-order resonances on a graphene ribbon of finite width. Under the illumination of two counter-propagating coherent beams, the two-stage coupling scheme is used for exciting multispectral acoustic plasmon resonances on the heterostructure simultaneously, thereby contributing to the ultimate multiband CPA in the mid-infrared region. The strong dependence of the nearly linear dispersion of acoustic plasmons on the chemical potential in graphene and the separation between the metal and the graphene allows the tunability in spectral positions of absorption peaks. Intriguingly, the absorption of each resonant peak is continuously tuned by varying the relative amplitude of two counter-propagating beams, and even their phase difference, respectively. The maximum modulation depth of 4.46*105 is observed. The scattering matrix is employed to demonstrate the principle of CPA and the finite-difference time-domain (FDTD) simulations are used for elucidating the flexible tunability. More importantly, the multiband coherent absorber is robust to the incident angle, and thus undoubtedly benefits extensive applications on optoelectronic and engineering technology areas for modulators and optical switches.

How mobile is the water in the reverse micelles? A 2DIR study with an ultrasmall IR probe

Water molecules in reverse micelle serve as an excellent model for biological water. However, it has been argued previously that water molecules at the centre, even for smaller sized water pools of reverse micelles, behave like bulk water, using ultrafast vibrational lifetime measurements of azide ion. In the present investigation, the vibrational dynamics of an ultrasmall IR probe, azide ion, has been studied in AOT reverse micelle, at various w0 (radius of water pool), using linear IR measurements, vibrational lifetime measurements and 2D-IR measurements. While linear IR measurements show gradual variation in spectral parameters, such as, peak position and width of the vibrational transition, suggesting a gradual change in the micro environment of the probe with variation in radius of the water pool, the vibrational lifetime remains indistinguishable with variation in w0, which has been ascribed earlier to the location of azide ion in the centre of the water pool. Herein, ultrafast 2DIR spectroscopy convincingly demonstrates that, for a smaller sized water pool, the water molecules, even at the centre of the water pool, behave significantly different as compared to bulk water, an inference, which is missed by the previous vibrational lifetime measurements, and suggest an extensive slowdown of the hydrogen bond relaxation dynamics as compared to bulk water.

Light-Controlled Near-Field Energy Transfer in Plasmonic Metasurface Coupled MoS2 Monolayer.

The energy transfer from plasmonic nanostructures to semiconductors has been extensively studied to enhance light-harvesting and tailor light-matter interactions. In this study, the efficient energy transfer from an Au metasurface to monolayered MoS2 within a near-field coupling regime is reported. The metasurface is designed and fabricated to demonstrate strong photoluminescence (PL) and cathodoluminescence (CL) emission spectra. In the coupled heterostructure of MoS2 with a metasurface, both the Raman shift and absorption spectral intensities of monolayered MoS2 are affected. The spectral profile and PL peak position can be tailored owing to the energy transfer between plasmonic nanostructures and semiconductors. This is confirmed by ultrafast lifetime measurement. A theoretical model of two coupled oscillators is proposed, where the expanded general solutions (EGS) of such a model result in a series of eigenvalues that correspond to the renormalization of energy levels in modulated MoS2. The model can predict the peak shift up to tens of nanometers in hybrid structures and hence provides an alternative method to describe energy transfer between metallic structures and two-dimensional (2D) semiconductors. A viable approach for studying light-matter interactions in 2D semiconductors via near-field energy transfer is presented, which may stimulate the applications of functional nanophotonic devices.

Quantification of the As/P content in GaAsP during MOVPE growth

Epitaxial integration of direct-bandgap III–V compound semiconductors with silicon requires overcoming a significant lattice mismatch. To this end, GaAsP step-graded buffer layers are commonly applied. The thickness and composition of the individual layers are decisive for the envisaged strain relaxation. We study GaAsP growth by metalorganic vapor phase epitaxy in situ with reflection anisotropy spectroscopy. We find that the growth surface exhibits optical fingerprints of atomically well-ordered surfaces. These allow for tuning the interface preparation between adjacent layers. The spectral position of the characteristic peaks in the RA spectra, which are related to surface-modified bulk transitions, behaves similarly upon an increased As content as does the E1 interband transition of GaAsP at the growth temperature. The impact of strain on this shift is negligible. We thus monitor a bulk property via the surface reconstruction. An empiric model enables quantification of the As content of individual layers directly in situ without growth interruptions and for various surface reconstructions. Our findings are suitable for a simplified optimization of the GaAsP buffer growth for high-efficiency devices.

The strange attraction phenomenon induced by phonon-mediated off-resonant coupling in a biexciton-cavity system

In this work, we demonstrate that the phonon-mediated off-resonant coupling is the appropriated mechanism that accounts for the phenomenon of cavity-to-exciton spectral shifting experimentally observed for two quantum dots embedded in a photonic crystal cavity. For that aim, we compare theoretical calculations of the emission spectrum as well as the spectral peak positions with available experimental data in the literature. Our findings within the Lindblad master equation approach are in good qualitative agreement with these experimental results, and they provide unequivocal evidence that the cavity mode suffers a blueshift toward the exciton resonances. We also demonstrate that the pure dephasing mechanism merely fails for describing the experimental observations.

Swift Observations of Mrk 421 in Selected Epochs. III. Extreme X-Ray Timing/Spectral Properties and Multiwavelength Lognormality during 2015 December–2018 April

Abstract We present the results from the timing and spectral study of Mrk 421 based mainly on the Swift data in the X-ray energy range obtained during the time interval 2015 December–2018 April. The most extreme X-ray flaring activity on long-term, daily, and intraday timescales was observed during the 2 month period that started in 2017 December, when the 0.3–10 keV flux exceeded a level of 5 × 10−9 erg cm−2 s−1, recorded only twice previously. While the TeV-band and X-ray variabilities were mostly correlated, the source often varied in a complex manner in the MeV–GeV and radio–UV energy ranges, indicating that the multifrequency emission of Mrk 421 could not always be generated in a single zone. The longer-term flares at X-rays and γ-rays showed a lognormal character, possibly indicating a variability imprint of the accretion disk onto the jet. A vast majority of the 0.3–10 keV spectra were consistent with the log-parabolic model, showing relatively low spectral curvature and correlations between the different spectral parameters, predicted in the case of the first- and second-order Fermi processes. The position of the synchrotron spectral energy distribution peak showed an extreme variability on diverse timescales between the energies E p < 0.1 and >15 keV, with 15% of the spectra peaking at the hard X-ray, and was related to the peak height as with α ∼ 0.6, which is expected for the transition from Kraichnan-type turbulence into the “hard sphere” one. The 0.3–300 GeV spectra showed features of the hadronic contribution, jet–star interaction, and upscatter in the Klein–Nishina regime in different time intervals.

Theoretical calculation of the optical absorption property of nanoparticles composed of an Au Core and Si shell embedded in silica

Combining multiple materials in a single nanoparticle has gained much attention in recent years. In this work, the optical absorption property of gold-silicon (Au@Si) core–shell nanoparticles (NPs) embedded in a silica matrix were are theoretically demonstrated in the wavelength range from 400 to 800 nm, based on a discrete-dipole approximation method. For a single core–shell nanoparticle, the study revealed the localized surface plasmon resonance (LSPR) showed a regular redshift with an increase in its Si shell thickness. The observed redshifts in the LSPR peaks were in agreement with the experimental results. The optical absorption property was also observed for two Au@Si core–shell NPs separated, on average, by a distance as small as a few nanometers. The results suggest that the shifts in spectral peak position depend on both the interparticle distance and geometric configuration of the nanoparticles. The obtained results also suggest that this nanomaterial, with a strong wavelength-tuneable absorption property, could be an attractive candidate for applications in biomedicine, nanocatalysis, optical devices, and future functional devices.

Overview on monolithically integrated arrays of microtubular vertical resonators on photonic waveguides for optofluidic applications

A novel platform for optofluidic applications is realized by monolithic integration of an array of ultra-compact three-dimensional (3D) vertically rolled-up microtube ring resonators (VRU-MRRs) with polymer waveguides. The on-chip integrated system is realized by rolling up 2D differentially strained TiO2 nanomembranes into 3D microtubular cavities on a nanophotonic chip. Whispering-gallery modes are observed in the telecom wavelength range, and their spectral peak positions shift significantly when measurements are performed while immersing the tubes or filling their hollow cores with water. The achievement of this work opens up fascinating opportunities to realize massively parallel optofluidic microsystems with exceptional multi-functionality for analysis of biomaterials in lab-in-a-tube systems on a single chip.

Tunable Ultra-Multispectral Metamaterial Perfect Absorbers Based on Out-of-Plane Metal-Insulator-Graphene Heterostructures

The out-of-plane metal-insulator-graphene (MIG) heterostructure separated from metal substrates by a silica spacer has been intensively investigated. Both numerical simulations and theoretical calculations exhibit that due to the dominating effect of adjacent metal layers, multiple strongly confined Fabry-Perot (F-P) resonances are simultaneously excited on the ingenious out-of-plane MIG heterostructure; and thus, are conductive to the realization of a nine-band plasmonic perfect absorber at mid-infrared wavelengths. The spectral positions of absorption peaks are handily tuned by changing the structural parameters of MIG heterostructures and the electrically controlled chemical potential of graphene. More interestingly, the good absorption stability at varying incident angles is observed. Numerical simulations are in excellent agreement with theoretical results. Such tunable angle-independent nine-band perfect absorber undoubtedly finds utility in mid-infrared optical filters and modulators. The proposed out-of-plane MIG heterostructure allows possibilities for multispectral enhancement of light-graphene interactions.

Shaping electron-hole trajectories for solid-state high harmonic generation control.

Solid-state high-harmonic generation (HHG) by an intense infra-red (IR) laser field offers a new route to generate coherent attosecond light pulses in the extreme ultraviolet regime. The propagation of the IR driving field in the dense solid medium is accompanied by non-linear processes which shape the generating waveform. In this work, we introduce a monolithic scheme in which we both exploit the non-linear propagation to manipulate a two color driving field, as well as generate high harmonics within a single crystal. We show that the resulting non-commensurate, bi-chromatic, generating field provides precise control over the periodicity of the HHG process. This control enables us to manipulate the spectral positions of the discrete harmonic peaks. Our method advances solid-state HHG spectroscopy, and offers a simple route towards tunable, robust XUV sources.

Spatially and Spectrally Resolved Absorptivity: New Approach for Degradation Studies in Perovskite and Perovskite/Silicon Tandem Solar Cells

AbstractInstability in perovskite solar cells is the main challenge for the commercialization of this solar technology. Here, a contactless, nondestructive approach is reported to study degradation across perovskite and perovskite/silicon tandem solar cells. The technique employs spectrally and spatially resolved absorptivity at sub‐bandgap wavelengths of perovskite materials, extracted from their luminescence spectra. Parasitic absorption in other layers, carrier diffusion, and photon smearing phenomena are all demonstrated to have negligible effects on the extracted absorptivity. The absorptivity is demonstrated to reflect real degradation in the perovskite film and is much more robust and sensitive than its luminescence spectral peak position, representing its optical bandgap, and intensity. The technique is applied to study various common factors which induce and accelerate degradation in perovskite solar cells including air and heat exposure and light soaking. Finally, the technique is employed to extract the individual absorptivity component from the perovskite layer in a monolithic perovskite/silicon tandem structure. The results demonstrate the value of this approach for monitoring degradation mechanisms in perovskite and perovskite/silicon tandem cells at early stages of degradation and various fabrication stages.

Rare earth doped PDMS elastomeric random lasers

We report a very stable elastomeric random laser (SERL) system composed of two stable materials: inorganic Nd: YAB nanoparticles and polydimethylsiloxane (PDMS). Lasing at a wavelength of 1064.5 nm is observed when the samples are exposed to pulsed nanosecond excitation at 808 nm or 532 nm, with long term (days) operation without degradation. Contrary to other RLs based on polymers, this very stable RL is the first elastomeric system, hence tunable, which allows the systematic investigation of its dynamics over a long time, without the complication of gain reduction or deterioration of the polymer. The RL threshold is estimated to be around 1.0 mJ at 808 nm. The measured linewidth decreases from 3.2 nm to 0.5 nm. The spectral peak position as well as the intensity is conveniently tuned by stretching the elastomer polymer composite. As an application which requires long term operation, Levy-like statistics and replica symmetry breaking in this open cavity random laser were also demonstrated. These lasers have been used over several months without noticeable degradation.

Nanodefects in YAG:Ce-Based Phosphor Microcrystals

The present paper focused on the study of spectral–kinetic characteristics of luminescence in two batches of yttrium aluminum garnet (YAG):Ce-based phosphors synthesized in different years by two manufacturers: NPO “Platan” in Russia and “GrandLux” in the People’s Republic of China (PRC). Upon studying the structural characteristics of the phosphors—elemental composition, morphology, and X-ray diffraction (XRD) patterns—it was concluded that both types of YAG:Ce phosphors are highly imperfect. The presence of heterogeneities of different nature was accompanied by the introduction—to compensate for charges and elastic stresses—of intrinsic lattice defects during synthesis. There is a high probability of creating complex defects during phosphor synthesis. Luminescence properties (full width at half maximum (FWHM), spectral position of the emission peaks, excitation spectra of emission, emission decay time) are affected by the nearest environment of the luminescence center; whereas the degree of correlation of defects (distance between the components of the donor–acceptor pair) does not depend on the concentration of impurities, intrinsic defects, and their ratio. The results do not fit into the framework of existing ideas regarding the processes in phosphors as systems with widely distributed luminescence centers. The patterns obtained in the paper are discussed based on a hypothesis according to which a nanodefect phosphorus crystal phase is formed during the synthesis.

First-principles calculations of the atomic structure and electronic states of LixFeF3

We calculate the atomic and electronic structures of trirutile-type ${\mathrm{Li}}_{x}{\mathrm{FeF}}_{3}\phantom{\rule{4pt}{0ex}}(x=0,0.25,0.5,0.75,\phantom{\rule{4pt}{0ex}}\text{and}\phantom{\rule{4pt}{0ex}}1)$ by first-principles calculations and evaluate the relative stability among the optimized structures by energy analysis. ${\mathrm{Li}}_{0.5}{\mathrm{FeF}}_{3}$ is more stable than the three-phase coexistence of ${\mathrm{FeF}}_{3},{\mathrm{FeF}}_{2}$, and LiF, whereas the other compositions are unstable. The analyses of the local electron density, local atomic volume, and local atomic configurations show that the formal valence of Fe atoms decreases from trivalent (3+) to divalent (2+) after Li insertion. In addition, we calculate Fe $K$-edge x-ray absorption near-edge structure (XANES) spectra in ${\mathrm{Li}}_{x}{\mathrm{FeF}}_{3}$ and compare them with observed spectra. The calculated XANES spectra agree well with the corresponding observed spectra in areas such as the spectral shape and relative position of the main peaks associated with ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$. In particular, partial XANES spectra of ${\mathrm{Fe}}^{3+}$ in ${\mathrm{Li}}_{x}{\mathrm{FeF}}_{3}$, for $x=0.25,0.5$, and 0.75, have a specific peak between the main peaks, associated with ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$. The detailed study reveals that the energy level and intensity ratio of the ${\mathrm{Fe}}^{3+}$ main peaks depend on the adjacent cation site of Fe.

Assembly of Gold Nanoparticles into Chiral Superstructures Driven by Circularly Polarized Light.

Photon-to-matter chirality transfer offers both simplicity and universality to chiral synthesis, but its efficiency is typically low for organic compounds. Besides the fundamental importance of this process relevant for understanding the origin of homochirality on Earth, new pathways for imposing chiral bias during chemical process are essential for a variety of technologies from medicine to informatics. The strong optical activity of inorganic nanoparticles (NPs) affords photosynthetic routes to chiral superstructures using circularly polarized photons. Although plasmonic NPs are promising candidates for such synthetic routes due to the strong rotatory power of highly delocalized plasmonic states (Ma et al. Chem. Rev. 2017, 117 (12), 8041), realization of light-driven synthesis of chiral nanostructures has been more challenging for plasmonic NPs than for the semiconductor due to the short lifetime of the plasmonic states. Here we show that illumination of gold salt solutions with circularly polarized light induces the formation of NPs and their subsequent assembly into chiral nanostructures 10-15 nm in diameter. Despite their seemingly irregular shape, the resulting nanocolloids showed circular dichroism (CD) spectra with opposite polarity after exposure to photons with left and right circular polarization. The sign and spectral position of the CD peaks of illuminated dispersions matched those calculated for nanostructures with complex geometry identified from electron tomography images. Quantification of the complex shapes of NP assemblies using chirality measures revealed a direct correlation with the experimental spectra. The light-driven assembly of chiral nanostructures originates from the asymmetric displacement of NPs in dynamic assemblies by plasmonic fields followed by particle-to-particle attachment. The ability of gold NPs to "lock" the chirality of the incident photons in assembled nanostructures can be used to create a variety of chiral nanomaterials with plasmonic resonances.

Terahertz generation in parallel plate waveguides activated by nonlinear metasurfaces.

We present an extended Maxwell-Hydrodynamic model of free electron dynamics on metal-dielectric interfaces that allows us to study numerically the THz emission from nonlinear metasurfaces. This model is applied on a metasurface consisting of split ring resonators, which has been previously studied and shown to produce broadband terahertz (THz) radiation. Investigations of the emitted THz radiation as function of the duration of the excitation laser reveal a tuning mechanism in terms of both spectral peak position and intensity. We also use the model to propose a new metasurface-activated waveguide platform that efficiently generates THz waveguide modes. Tunability mechanisms of the generated THz are shown. Due to its unique characteristics, we believe that this new platform might play a major role in forthcoming THz applications.

Efficient simulation of overtones and combination bands in resonant Raman spectra.

In this work, we describe how Resonant Raman (RR) spectra can be predicted with good accuracy using the path integral formulation for the harmonic oscillator, including the Herzberg-Teller and Duschinsky rotation effects. In particular, we will focus on approximations to make these calculations more efficient and expand the current methodology to include overtones and combination bands in the final prediction. After outlining the theory, we explain how it can be made into a black-box method and discuss the approximations that rely on its intrinsic parameters. It is shown that the latter can improve the efficiency by orders of magnitude, without substantial loss in accuracy. We will also show that the predicted RR spectra are in good agreement with experiment for both the intensity and position of spectral peaks, even for higher order vibronic excitations. Thus, the method proposed here can be used as a computational aid to experiments to improve the quality of analysis and theoretical understanding of RR.

Cement Pavement Crack Identification Method Based on DFT Transformation and Crack Segment Splicing Method

The groove is the biggest interference factor in the identification of cement pavement cracks. In the process of restraining or eliminating the grooves, it inevitably reduces the contrast between the cracks and the background, at the same time weakens the cracks in the grooves and destroys the connectivity of the cracks. A large number of short crack fragments are mixed with noise, which increases the difficulty of subsequent identification. In order to solve these problems, a slot elimination method based on DFT transform and a splicing method based on grid voting method are proposed. Firstly, the image of cement pavement is transformed from DFT to frequency domain, and the characteristic of groove in frequency domain spectrum is obtained. On this basis, according to the periodicity of the groove in the spatial image, the position of the groove spectral peak in the frequency domain image is deduced, and a series of filters are designed to eliminate these spectral peaks in the frequency domain, and then the slot is eliminated by the inverse DFT transform in the spatial domain. Then, the global spatial image of cement pavement is binarized by Otsu method, and the noise is filtered after the square vote to realize the splicing of cracks so as to achieve the purpose of complete extraction of cracks. The experimental results show that the proposed method can eliminate the interference of the groove and keep the integrity of the crack after splicing.

Full stress tensor measurement using fluorescence spectroscopy

Photoluminescent spectral peak positions are known to shift as a function of mechanical stress state. This has been demonstrated at macroscales to determine mean stress and mesoscales to determine mean stress and a quantity related to shear stress. Here, we propose a method to utilize traction-free surface conditions and knowledge of material orientation to solve for two in-plane displacement fields given two measured spectral peak positions measured at a grid of points. It is then possible to calculate the full stress tensor at each measurement point. This is a significant advancement over the previous ability to measure one or two stress quantities. We validate the proposed method using a simple, two-grain geometry and show that it produces the same mean stress and shear stress measure as the existing direct method. We also demonstrate determination of the full stress field in a polycrystalline alumina specimen.