Spectral Phenomena Research Articles

Non-self-adjoint quasi-periodic operators with complex spectrum

We give a precise and complete description on the spectrum for a class of non-self-adjoint quasi-periodic operators acting on ℓ2(Zd) which contains the Sarnak's model as a special case. As a consequence, one can see various interesting spectral phenomena including PT symmetric breaking, the non-simply-connected two-dimensional spectrum in this class of operators. Particularly, we provide new examples of non-self-adjoint operator in ℓ2(Z) whose spectra (actually a two-dimensional subset of C) can not be approximated by the spectra of its finite-interval truncations.

Spectral splitting phenomenon caused by strong coupling of surface plasmon polariton mode and epsilon-near-zero mode

The strong coupling effect plays an important role in optimizing the optical properties of nano-photonics and improves the efficiency of manipulating light. However, coupling systems for nano-photonic devices usually have problems with complex structure, large size and low coupling strength. To address these issues, we design a nanograting structure to realize the strong coupling between the surface plasmon polariton mode and the epsilon-near-zero mode. The reflection spectra of the designed nanostructure show obvious Rabi splitting phenomenon. The Rabi splitting phenomenon is further verified by the theoretical result. In addition, the factors influencing the coupling effect strength in the nanostructure are analyzed. The proposed nanostructure can be used in nano-photonic devices such as sensors.

Identifying colours in the absorption spectrum with an app

This study addresses the challenges faced by students with colour blindness during the exploration of light spectra, emphasizing the crucial adaptation of educational materials for effective learning. The proposed solution, characterized by its low-cost and straightforward implementation, involves the use of smartphone apps, notably the ‘ColorADD,’ to assist students who cannot perceive colours in understanding spectral phenomena. This topic is integrated into the curricula of both high school and undergraduate physics courses in Portugal and Brazil. This approach represents a tangible opportunity to promote inclusive scientific education, fostering active participation and ensuring equal opportunities for colour-blind students without compromising their overall visual perception.

Simultaneous intramolecular and intermolecular coupling of polar bond vibrations induced spectral splitting phenomenon

The positions of splitting bands resulting from simultaneous intramolecular (such as Fermi resonance) and intermolecular interactions were calculated by a variational method. Hamiltonian calculation model employed an antiparallel arrangement. Four nondegenerate energy solutions were obtained, which correspond exactly to the four peaks observed in polarized Raman spectroscopy and high resolution infrared spectroscopy: two in the VV component and two in the VH component. The intramolecular coupling constant W, unperturbed intensity ratios R, and unperturbed band difference X0 for vinylene carbonate (VC) were calculated from the derived equations and the experimental data at infinite dilution concentration. Additionally, the intermolecular interaction constants V of VC in different polar solvents (VC/CH3CN, VC/CHCl3 and VC/CH3OH binary mixtures) at varying concentrations were calculated. The curvature of V vs concentration was found to fit the Logan model very well.

Intrinsic Phase-Preserving Networks for Depth Super Resolution

Depth map super-resolution (DSR) plays an indispensable role in 3D vision. We discover an non-trivial spectral phenomenon: the components of high-resolution (HR) and low-resolution (LR) depth maps manifest the same intrinsic phase, and the spectral phase of RGB is a superset of them, which suggests that a phase-aware filter can assist in the precise use of RGB cues. Motivated by this, we propose an intrinsic phase-preserving DSR paradigm, named IPPNet, to fully exploit inter-modality collaboration in a mutually guided way. In a nutshell, a novel Phase-Preserving Filtering Module (PPFM) is developed to generate dynamic phase-aware filters according to the LR depth flow to filter out erroneous noisy components contained in RGB and then conduct depth enhancement via the modulation of the phase-preserved RGB signal. By stacking multiple PPFM blocks, the proposed IPPNet is capable of reaching a highly competitive restoration performance. Extensive experiments on various benchmark datasets, e.g., NYU v2, RGB-D-D, reach SOTA performance and also well demonstrate the validity of the proposed phase-preserving scheme. Code: https://github.com/neuralchen/IPPNet/.

Structure, luminescence properties, and valence-induced spectral behavior of a bismuth-activated garnet phosphor

The increasing demand for full-visible-spectrum lighting has prompted the development of novel phosphors with multicolor emission.

Solving the spectral gap problem with a four-mirror bow-tie cavity in waveguide free-electron laser oscillators

The generation of intense coherent radiation pulses in the far-infrared and terahertz regimes is of considerable interest to the free-electron laser (FEL) radiation user community. At long wavelengths, the diffraction effect can be quite severe, therefore, an optical waveguide is required to confine the radiation field. However, it will also bring about some new phenomena, and the most noteworthy one is the spectral gap phenomenon: at some particular wavelengths, regardless of electron beam adjustments, the coupling efficiency and output power of waveguide FEL oscillators drop significantly. Such spectral gap has an adverse effect on experimental results since numerous experiments require continuous spectral scanning. In this paper, we propose to utilize a bow-tie cavity instead of conventional cavities to the waveguide FEL to solve the spectral gap problem. The simulation was carried out based on the parameters of FELiChEM, a newly built user facility in China. Numerical simulation code OPC combining with modified GENESIS is used to enable the modelling, for the first time, of a bow-tie cavity based FEL in the far-infrared wavelength regime. The simulation results indicate that this novel structure can effectively eliminate the spectral gaps and substantially enhance long-wavelength laser performance.

Self-Phase Modulation-Induced Instability of High-Power Narrow-Linewidth Fiber Amplifiers

In this study, we investigated the effect of self-phase modulation (SPM)-induced modulation instability (MI) on the spectral purity of high-power narrow-linewidth fiber amplifiers and established a spectral evolution model for SPM-induced MI in those amplifiers. The spectral evolution process of MI under different laser powers was simulated and analyzed. The results show that, at low power (100 W), SPM can cause a dynamic change in the spectral sideband secondary peak and the spectral wingspan. An increase in laser power led to the cascade effect of MI, forming a zigzag secondary sideband with a larger spectral width and causing the spectral main peak and spectral broadening to split. Experiments based on the fiber Bragg grating (FBG) of oscillating seed sources were carried out on high-power narrow-linewidth laser amplifiers, and the above spectral evolution phenomenon was observed. The experimental results indicate that the spectral evolution model based on SPM-induced MI can effectively explain the dynamic change in the spectral secondary peak, spectral wingspan and zigzag broadening phenomenon in the power amplification process of narrow-linewidth lasers.

Complex Organics in Space: A Changing View of the Cosmos

Planetary explorations have revealed that complex organics are widely present in the solar system. Astronomical infrared spectroscopic observations have discovered that complex organics are synthesized in large quantities in planetary nebulae and distributed throughout the galaxy. Signatures of organics have been found in distant galaxies, as early as 1.5 billion years after the Big Bang. A number of unsolved spectral phenomena such as diffuse interstellar bands, extended red emissions, 220 nm feature, and unidentified infrared emission bands are likely to originate from organics. In this paper, we discuss the possible chemical structures of the carriers of these unexplained phenomena, and how these organics are synthesized abiotically in the universe. We raise the possibility that the primordial solar system was enriched by complex organics synthesized and ejected by evolved stars. The implications of possible stellar organics in primordial Earth are also discussed.

Complexation of Flavonoids: Spectral Phenomena, Regioselectivity, Interplay with Charge and Proton Transfer.

The review compiles information on the spectral classification of flavonoids, the changes in their electronic structure upon complex formation, and the manifestation of these changes in the absorption and emission spectra. Part of the review is devoted to the regioselectivity of the complex formation process, including types of complexation sites, the structure of chelates and 'open' complexes, and the correlation between the structure of complexes and their spectral properties. The interplay between complex formation and other processes occurring in flavonoids during electronic excitation is also considered, such as intramolecular inter-fragment charge transfer (ICT) and intramolecular proton transfer (ESIPT). The review also contains systematic data on the study of regioselectivity and spectral properties of flavone complexes, obtained by the author and their colleagues over the past decades.

From Black Hole Spectral Instability to Stable Observables.

The quasinormal mode spectrum of black holes is unstable under small perturbation of the potential and has observational consequences in time signals. Such signals might be experimentally difficult to observe and probing this instability will be a technical challenge. Here, we investigate the spectral instability of time-independent data. This leads us to study the Regge poles (RPs), the counterparts to the quasinormal modes in the complex angular momentum plane. We present evidence that the RP spectrum is unstable but that not all overtones are affected equally by this instability. In addition, we reveal that behind this spectral instability lies an underlying structure. The RP spectrum is perturbed in such a way that one can still recover stable scattering quantities using the complex angular momentum approach. Overall, the study proposes a novel and complementary approach on the black hole spectral instability phenomena that allows us to reveal a surprising and unexpected mechanism at play that protects scattering quantities from the instability.

Band Dynamics of Multimode Resonant Nanophotonic Lattices with Adjustable Liquid Interfaces.

Subwavelength resonant lattices offer a wide range of fascinating spectral phenomena under broadside illumination. The resonance mechanism relies on the generation of lateral Bloch modes that are phase matched to evanescent diffraction orders. The spectral properties and the total number of resonance states are governed by the structure of leaky modes and the mode count. This study investigates the effect of interface modifications on the band dynamics and bound-state transitions in guided-mode resonant lattices. We provide photonic lattices comprising rectangular Si3N4 rods with a liquid film with an adjustable boundary. The band structures and band flips are examined through numerical simulations using the rigorous coupled-wave analysis (RCWA) method and analyzing the zero-order spectral reflectance as a function of the incident angle. The band structures and band flips are examined through numerical simulations, and the influences of the refractive index and the thickness of the oil layer on the band dynamics are investigated. The results reveal distinct resonance linewidths corresponding to different refractive indices of the oil layer. Furthermore, the effect of the oil thickness on the band dynamics is explored, demonstrating precise control over the number of propagating modes within the lattice structure. Theoretical simulations and experimental results are presented for a subwavelength silicon-nitride lattice combined with a liquid film featuring an adjustable boundary. The presence of a relatively thick liquid waveguiding region enables the emergence of additional modes, including the first four transverse-electric (TE) leaky modes, which produce observable resonance signatures. Through experimental manipulation of the basic lattice's duty cycle, the four bands undergo quantifiable band transitions and closures. The experimental results obtained within the 1400-1600 nm spectral range exhibit reasonable agreement with the numerical analysis. These findings underscore the significant role played by the interface in shaping the band dynamics of the lattice structure, providing valuable insights into the design and optimization of photonic lattices with adjustable interfaces.

Frequency modes filtering of spectral peaks in optical frequency combs through molecular gas absorption and nonlinear polarization rotation.

Spectral peak generation is a recently reported phenomenon that narrow spectral dips of the optical spectrum turn into sharp peaks as they propagate through nonlinear optical fibers. We demonstrated the nonlinear polarization rotation-based spectral peak mode filtering to increase the signal-to-background ratio (SBR). The spectral peaks with almost constant frequency separation were generated from the femtosecond pulses absorbed by the CH4 gas through the highly nonlinear fiber. The generated spectral peaks were filtered through the polarizing beam splitter by the nonlinear polarization rotation, and the SBR was improved from 9 dB to ∼20 dB. The spectral peak generation phenomenon and the mode filtering were numerically confirmed by solving the coupled nonlinear Schrödinger equations. The demonstrated method can generate strong comb modes with wide frequency spacing which are useful for highly sensitive environmental gas sensing spectroscopy. The wavelengths of the spectral peaks are fixed by the absorption spectra of the used gas cells. Therefore, this method can generate high quality spectral peaks of any wavelengths with wide spectral ranges through proper combinations of gas cells.

Spectral Relaxation Imaging Microscopy II: Complex Dynamics.

The dynamics of condensed matter can be measured by the time-dependent Stokes shift of a suitable fluorescent probe. The time-dependent spectral correlation function is typically described by one or more spectral relaxation correlation times, which, in liquid solvents, characterize the timescales of the dipolar relaxation processes around the excited-state probe. The phasor plot provides a powerful approach to represent and analyze time and frequency-domain data acquired as images, thus providing a spatial map of spectral dynamics in a complex structure such as a living cell. Measurements of the phase and modulation at two emission wavelength channels were shown to be sufficient to extract a single excited-state lifetime and a single spectral relaxation correlation time, supplying estimates of the mean rate of excited-state depopulation and the mean rate of spectral shift. In the present contribution, two more issues were addressed. First, the provision of analytic formulae allowing extraction of the initial generalized polarization and the relaxed generalized polarization, which characterize the fluorescence spectrum of the unrelaxed state and the fully relaxed state. Second, improved methods of model discrimination and model parameter extraction for more complex spectral relaxation phenomena. The analysis workflow was illustrated with examples from the literature.

Effect of Fe doped and capping agent – Structural, optical, luminescence, and antibacterial activity of ZnO nanoparticles

Zinc oxide nanoparticles (ZnO NPs) are indispensable materials for spectral and optical phenomena, prepared by a chemical precipitation method. We studied their properties resulting from different dopants in the nanoscale. The samples' structure, texture, energy band, and photoluminescence were examined using XRD, TEM, FTIR, PL, and UV–VIS spectroscopy. XRD shows the single hexagonal Wurtzite with crystallite sizes from 10.1 to 17.8 nm and the spherical shape particles. The octahedral sites at 470–489 cm−1 and the tetrahedral sites at 616 cm−1 were observed in the FTIR studies. PL showed a wider bandgap, better UV emission, and decreased defect density of the Fe doped than that of the pristine one. Fe-doped ZnO NPs are more active than pristine ZnO NPs for their quenching or absorbing properties, indicating their antibiotic properties.

Spectral pulsations of evolving soliton molecules in an anomalous dispersion fiber laser

We investigate in detail the spectral pulsation of evolving soliton molecules in an anomalous dispersion fiber laser by utilizing the dispersive Fourier transform technique. The spectral pulsations of an evolving soliton molecule with different properties were achieved in our experiments, such as single-period and double-period pulsation. This type pulsation is characterized by the fact that the soliton molecules periodically experience oscillation in spectral profile and peak power due to the periodic vibration evolution of the internal pulses. However, the continuous weak energy exchange between the two solitons inside the soliton molecules results in an almost invariable pulse energy. In addition, analytical fitting models are given to further reveal the spectral pulsation phenomenon of the evolving soliton molecules. These experimental findings can shed some light on the dynamics of soliton molecules in fiber lasers.

Hyperspectral and Multispectral Image Fusion with Automated Extraction of Image-Based Endmember Bundles and Sparsity-Based Unmixing to Deal with Spectral Variability.

The aim of fusing hyperspectral and multispectral images is to overcome the limitation of remote sensing hyperspectral sensors by improving their spatial resolutions. This process, also known as hypersharpening, generates an unobserved high-spatial-resolution hyperspectral image. To this end, several hypersharpening methods have been developed, however most of them do not consider the spectral variability phenomenon; therefore, neglecting this phenomenon may cause errors, which leads to reducing the spatial and spectral quality of the sharpened products. Recently, new approaches have been proposed to tackle this problem, particularly those based on spectral unmixing and using parametric models. Nevertheless, the reported methods need a large number of parameters to address spectral variability, which inevitably yields a higher computation time compared to the standard hypersharpening methods. In this paper, a new hypersharpening method addressing spectral variability by considering the spectra bundles-based method, namely the Automated Extraction of Endmember Bundles (AEEB), and the sparsity-based method called Sparse Unmixing by Variable Splitting and Augmented Lagrangian (SUnSAL), is introduced. This new method called Hyperspectral Super-resolution with Spectra Bundles dealing with Spectral Variability (HSB-SV) was tested on both synthetic and real data. Experimental results showed that HSB-SV provides sharpened products with higher spectral and spatial reconstruction fidelities with a very low computational complexity compared to other methods dealing with spectral variability, which are the main contributions of the designed method.

Spectral splitting of the lasing emission of nitrogen ions pumped by 800-nm femtosecond laser pulses.

We report on a spectral splitting effect of the cavity-less lasing emission of nitrogen ions at 391.4 nm pumped by 800-nm femtosecond laser pulses. It was found that with the increase of the nitrogen gas pressure and pump pulse energy, both R and P branches experience spectral splitting. With an external injected seeding pulse, a similar split spectral line is observed for the amplified emission. In contrast, for the fluorescence radiation, no such spectral splitting phenomenon is observed with much more abundant R branch structures. Our theoretical model considers gas ionization by the pump pulse, the competition of excitation of all relevant electronic and vibrational states, and an amplification of the seeding pulse in the plasma with a population inversion. Our simulation reproduces this spectral splitting effect, which is attributed to the gain saturation resulting in the oscillation of the amplitude of the amplified signal.

First-principle studies of monolayer and bulk InSe1−xSx

S-substitution doped monolayer and bulk InSe exhibit opposite bandgap behaviors. The bandgap in the monolayer InSe1−xSx (x = 0, 0.125, 0,25) decreases with increasing S concentration. However, the bandgap in bulk InSe1−xSx increases with the increase of S concentration. It is mainly because In, and Se(S) at the Fermi level have different strengths of bonding and anti-bonding states. In addition, the first-neighbor interatomic force constant (IFC) of the dopant atom S is higher than that of Se. The enhanced IFC in bulk InSe1−xSx corresponds to the Raman spectral blueshift phenomenon observed in earlier experiments. The electro-acoustic interactions of the material are strongly correlated with dimensionality, which leads to differences in optical properties between monolayer and bulk InSe1−xSx. The optical dielectric functions indicate that monolayer and bulk InSe1−xSx can be applied in different fields. Substitutional doping could modulate pz-like orbital interactions, which provides clues for modulating the electronic orbital structure of InSe materials for optoelectronic and optical applications.

The intermolecular polar bond interaction and coupling induced spectral splitting phenomenon for a binary mixture.

To explain the polarization Raman noncoincidence effect of specific polar bonds and the noncoincidence phenomenon between FT-Raman and FT-IR spectra, aggregation-induced spectral splitting theory was proposed. In this paper, the vibration splitting theory was demonstrated using two strategies: improving the spectral resolution with cryogenic matrix isolation techniques and identifying cases where the coupling splitting is large enough to be distinguishable. The monomer and dimer splitting bands of acetone were detected when cryogenically isolated by the Ar matrix. Additionally, the polarization Raman and two-dimensional infrared spectra of a β-propiolactone (PIL)/CCl4 binary mixture were collected at room temperature, and the spectral splitting phenomenon was clearly observed. The dynamic transformation between the monomer and dimer could be achieved and detected by adjusting the PIL concentration. The observed splitting phenomenon was further confirmed by theoretical DFT calculations based on the monomer and dimer of PIL, as well as the FT-IR and FT-Raman spectra of PIL. Concentration-triggered 2D-COS synchronous and asynchronous spectra also confirmed the splitting phenomenon and the dilution kinetics of PIL/CCl4.