Linear Spectrum Research Articles

Improving Spectral, Spatial, and Mechanistic Resolution Using Fourier Transform Nonlinear Optics: A Tutorial Review.

Fourier transform nonlinear optics (FT-NLO) is a powerful experimental physical chemistry tool that provides insightful spectroscopic and imaging data. FT-NLO has revealed key steps in both intramolecular and intermolecular energy flow. Using phase-stabilized pulse sequences, FT-NLO is employed to resolve coherence dynamics in molecules and nanoparticle colloids. Recent advances in time-domain NLO interferometry using collinear beam geometries makes determination of molecular and material linear and nonlinear excitation spectra, homogeneous line width, and nonlinear excitation pathways straightforward. When combined with optical microscopy, rapid acquisition of hyperspectral images with the information content of FT-NLO spectroscopy is possible. With FT-NLO microscopy, molecules and nanoparticles colocated within the optical diffraction limit can be distinguished based on their excitation spectra. The suitability of certain nonlinear signals for statistical localization present exciting prospects for using FT-NLO to visualize energy flow on chemically relevant length scales. In this tutorial review, descriptions of FT-NLO experimental implementations are provided along with theoretical formalisms for obtaining spectral information from time-domain data. Select case studies that illustrate the use of FT-NLO are presented. Finally, strategies for extending super-resolution imaging capabilities based on polarization-selective spectroscopy are offered.

Complexity of the Upper Solar Atmosphere Revealed from Spectropolarimetry during a Solar Eclipse

We analyze linear polarimetric spectrum data of solar emission lines with different formation temperatures in a visible light band from 516.3–532.6 nm, obtained during the 2013 Gabon solar eclipse using the prototype Fiber Arrayed Solar Optical Telescope. Complexities are found from the chromosphere through the transition zone to the corona at the spatial resolution limit of 2″ and temporal resolution of seconds. The observations show irregular spatial and spectral variations in linear polarization amplitudes, directions, and profile shapes. Within the observational band, spectral lines with different formation temperatures can have comparable polarization amplitudes in one spatial volume but one order difference in another, and at the same spatial volume, the amplitudes can differ by one order at different lines. The polarization amplitudes do not consistently increase with elongation in local regions. The variation in the direction of the polarization along the elongation is found from the green coronal line and the transition zone line more frequently than from the chromospheric lines. Such a variation in orientation is not synchronous for the different lines. Finally, Stokes Q/I profiles of the broad lines, such as the magnesium triplet and the green coronal line, show very diverse and complicated patterns. After pixel binning, we show that some of the complexity may be caused by the integration over different polarization sources at subresolution scales and/or along the line of sight in the optically thin layers with complex geometric corrugations.

Phase diagrams and excitations of anisotropic S=1 quantum magnets on the triangular lattice

The $S=1$ bilinear-biquadratic Heisenberg exchange model on the triangular lattice with a single-ion anisotropy has previously been shown to host a number of exotic magnetic and nematic orders [Moreno-Cardoner $\textit{et al.}$, Phys. Rev. B $\textbf{90}$, 144409 (2014)], including an extensive region of "supersolid" order. In this work, we amend the model by an XXZ anisotropy in the exchange interactions. Tuning to the limit of an exactly solvable $S=1$ generalized Ising-/Blume-Capel-type model provides a controlled limit to access phases at finite transverse exchange. Notably, we find an additional macroscopically degenerate region in the phase diagram and study its fate under perturbation theory. We further map out phase diagrams as a function of the XXZ anisotropy parameter, ratio of bilinear and biquadratic interactions and single-ion anisotropy, and compute corrections to the total ordered moment in various phases using systematically constructed linear flavor-wave theory. We also present linear flavor-wave spectra of various states, finding that the lowest-energy band in three-sublattice generalized (i.e. with $S^z=\pm1,0$) Ising/Blume-Capel states, stabilized by strong exchange anisotropies, is remarkably flat, opening up the way to flat-band engineering of magnetic excitation via stabilizing non-trivial Ising-ordered ground states.

Large-scale motions and self-similar structures in compressible turbulent channel flows

The present study investigates the scale characteristics of the log- and outer-region motions and structures in subsonic and supersonic wall turbulence. The energy distribution among the multiscale structures in the outer region is found to be dominated by the semilocal friction-Reynolds-number effects rather than the Mach-number effects. The geometrical characteristics of the self-similar structures populating the logarithmic region are also revealed by adopting a linear coherence spectrum.

Quantum parametric double Raman oscillators with co- and counterpropagating fields: Relative intensity squeezing and spatial photon correlations

We present a comprehensive study of co- and counterpropagating Stokes and anti-Stokes quantum fields in double Raman four-wave mixing parametric oscillators using full quantum Heisenberg-Langevin framework with noise operators. General analytical solutions of the fields operators at any point in the Raman medium are obtained for four cases: two possible copropagating (forward) and two counterpropagating (backward) Stokes and anti-Stokes. We analyze the symmetrical properties of the complex linear and nonlinear susceptibilities spectra of the quantum fields, nonclassicality of two-photon correlation functions, spatial variations of the quantum fields, and the two-mode relative intensity squeezing. We compare the results of forward and backward cases for several limiting double Raman schemes. We find interesting resonant effects of medium length, laser detunings and laser field strengths (Rabi frequencies) for backward-propagating geometries. Analysis of the solutions provide insights on the resonant conditions while computation over multiple variables enables us to identify the values of laser detuning, field strength, and propagation length that give enhanced nonclassical intensity squeezing and persistent correlations. The present work sets the crucial foundations for optimization of the nonclassicality of photons in double Raman systems and would be useful for quantum information storage, quantum nonlinear optics, and quantum spectroscopy.

Fisher matrix for the one-loop galaxy power spectrum: measuring expansion and growth rates without assuming a cosmological model

We introduce a methodology to extend the Fisher matrix forecasts to mildly non-linear scales without the need of selecting a cosmological model. We make use of standard non-linear perturbation theory for biased tracers complemented by counterterms, and assume that the cosmological distances can be measured accurately with standard candles. Instead of choosing a specific model, we parametrize the linear power spectrum and the growth rate in several k and z bins. We show that one can then obtain model-independent constraints of the expansion rate E(z) = E(z)/H 0 and the growth rate f(k,z), besides the bias functions. We apply the technique to both Euclid and DESI public specifications in the range 0.6 ≤ z ≤ 1.8 and show that the gain in precision when going from k max = 0.1 to 0.2 h/Mpc is around two- to threefold, while it reaches four- to ninefold when extending to k max = 0.3 h/Mpc. In absolute terms, with k max = 0.2 h/Mpc, one can reach high precision on E(z) at each z-shell: 8–10% for DESI with Δz = 0.1, 5–6% for Euclid with Δz = 0.2–0.3. This improves to 1–2% if the growth rate f is taken to be k-independent. The growth rate itself has in general much weaker constraints, unless assumed to be k-independent, in which case the gain is similar to the one for E(z) and uncertainties around 5–15% can be reached at each z-bin. We also discuss how neglecting the non-linear corrections can have a large effect on the constraints even for k max = 0.1 h/Mpc, unless one has independent strong prior information on the non-linear parameters.

Dephasing and phase-locking: Dual role of radial electric field in edge MHD dynamics of toroidally confined plasmas

We carry out several numerical simulations to illustrate how the radial electric field (Er) impacts the edge magnetohydrodynamic (MHD) instabilities. The analyses reveal that Er-shear (Er′, here the prime denotes the derivative with respect to the radial direction) tends to stabilize the kink/Peeling–Ballooning modes by dephasing the perturbed radial velocity (ṽr) and displacement (ξ̃r). However, Er-curvature (Er″) tends to destabilize the kink/peeling modes by inducing a phase lock between ṽr and ξ̃r. More specifically, the ratio between them could be measured to quantify their relative competition strength. Consequently, the shape of Er is crucial to the shape of linear growth rate spectrum γ(n) (here n is the toroidal mode number), which further determines the nonlinear dynamics. On the one hand, relatively larger Er-curvature causes narrower γ(n), leading to larger nonlinear energy loss fraction. On the other hand, relatively larger Er-shear has the opposite effect.

Embedded gold nanoparticles in TiO2 crystal for photonic applications

• Plasmonic Au nanoparticles are synthesized in TiO 2 crystal by ion implantation. • Third-order optical nonlinearity tailoring is achieved through charge transfer between Au nanoparticles and TiO 2 . • Enhanced modulation depth and optimized saturation intensity suggest significant potential photonic applications. Gold nanoparticles (Au NPs) are synthesized inside TiO 2 crystal by Au + ion implantation. The incorporation of Au + ions in TiO 2 brings out the modification of optical features of the bulk due to the localized surface plasmon resonance (LSPR) effect. The linear absorption spectra of Au NPs dispersed in TiO 2 crystal possesses an absorption peak at 638 nm. The third order optical nonlinearities of TiO 2 with Au NPs are modulated with reduced saturation intensity and increased modulation depth, compared with pure TiO 2 bulk. Our results pave a way to use TiO 2 with embedded Au NPs for nonlinear photonic applications.

Tailoring the linear viscoelastic response of industrial double dynamics networks through the interplay of associations

We investigate the linear viscoelastic properties of industrial pressure sensitive adhesives comprising double networks with an entangled acrylate-based polymer and two types of intermolecular associations (crosslinking), permanent (epoxide) and reversible (metal-chelate), having different compositions. A combination of shear rheometry and an appropriately modified version of the Time Marching Algorithm (TMA) allows to probe and analyze the behavior of the different double dynamic networks, in particular, the effects of the type and amount of crosslinks on their linear viscoelastic spectra. To this end, the dynamics of the double networks are compared with the respective individual responses of the polymeric component without crosslinks and the single networks (possessing only physical or only chemical crosslinks), in order to quantify their contributions to the relaxation mechanisms, particularly the interplay between disentanglement and bond association/dissociation processes. With the help of the TMA model, we also examine the respective roles of the lifetime of stickers, polydispersity, and molar mass. Triggered by the good comparison between predictions and experimental data, we propose a framework to tune material parameters in order to obtain a desired viscoelastic behavior.

Topological Insulator Films for Terahertz Photonics.

We discuss experimental and theoretical studies of the generation of the third terahertz (THz) frequency harmonic in thin films of Bi2Se3 and Bi2-xSbxTe3-ySey (BSTS) topological insulators (TIs) and the generation of THz radiation in photoconductive antennas based on the TI films. The experimental results, supported by the developed kinetic theory of third harmonic generation, show that the frequency conversion in TIs is highly efficient because of the linear energy spectrum of the surface carriers and fast energy dissipation. In particular, the dependence of the third harmonic field on the pump field remains cubic up to the pump fields of 100 kV/cm. The generation of THz radiation in TI-based antennas is obtained and described for the pump, with the energy of photons corresponding to the electron transitions to higher conduction bands. Our findings open up possibilities for advancing TI-based films into THz photonics as efficient THz wave generators and frequency converters.

5-Halogenation of Uridine Suppresses Protonation-Induced Tautomerization and Enhances Glycosidic Bond Stability of Protonated Uridine: Investigations via IRMPD Action Spectroscopy, ER-CID Experiments, and Theoretical Calculations.

Uridine (Urd), a canonical nucleoside of RNA, is the most commonly modified nucleoside among those that occur naturally. Uridine has also been an important target for the development of modified nucleoside analogues for pharmaceutical applications. In this work, the effects of 5-halogenation of uracil on the structures and glycosidic bond stabilities of protonated uridine nucleoside analogues are examined using tandem mass spectrometry and computational methods. Infrared multiple photon dissociation (IRMPD) action spectroscopy experiments and theoretical calculations are performed to probe the structural influences of these modifications. Energy-resolved collision-induced dissociation experiments along with survival yield analyses are performed to probe glycosidic bond stability. The measured IRMPD spectra are compared to linear IR spectra predicted for the stable low-energy conformations of these species computed at the B3LYP/6-311+G(d,p) level of theory to determine the conformations experimentally populated. Spectral signatures in the IR fingerprint and hydrogen-stretching regions allow the 2,4-dihydroxy protonated tautomers (T) and O4- and O2-protonated conformers to be readily differentiated. Comparisons between the measured and predicted spectra indicate that parallel to findings for uridine, both T and O4-protonated conformers of the 5-halouridine nucleoside analogues are populated, whereas O2-protonated conformers are not. Variations in yields of the spectral signatures characteristic of the T and O4-protonated conformers indicate that the extent of protonation-induced tautomerization is suppressed as the size of the halogen substituent increases. Trends in the energy-dependence of the survival yield curves find that 5-halogenation strengthens the glycosidic bond and that the enhancement in stability increases with the size of the halogen substituent.

Confined Excitons in Spherical-Like Halide Perovskite Quantum Dots.

Quantum dots (QDs) offer unique physical properties and novel application possibilities like single-photon emitters for quantum technologies. While strongly confined III-V and II-VI QDs have been studied extensively, their complex valence band structure often limits clear observations of individual transitions. In recently emerged lead-halide perovskites, band degeneracies are absent around the bandgap reducing the complexity of optical spectra. We show that for spherical-like CsPbBr3 QDs with diameters >6 nm, excitons confine with respect to their center-of-mass motion leading to well-pronounced resonances in their absorption spectra. Optical pumping of the lowest-confined exciton with femtosecond laser pulses not only bleaches all excitons but also reveals a series of distinct induced absorption resonances which we attribute to exciton-to-biexciton transitions and are red-shifted by the biexciton binding energy (∼40 meV). The temporal dynamics of the bleached excitons further support our exciton confinement model. Our study provides the first insight into confined excitons in CsPbBr3 QDs and gives a detailed understanding of their linear and nonlinear optical spectra.

Deciphering the Biochemical Similarities and Differences Among Human Neuroglial Cells and Glioma Cells Using Fourier Transform Infrared Spectroscopy

The use of Fourier transform infrared spectroscopy to identify the peritumoral tissue of gliomas proves the potential of this technique to distinguish normal brain tissues from glioma tissues. However, due to the heterogeneity of gliomas, it is difficult to characterize the representative spectra of normal brain tissues and glioma tissues. The linear spectra of major cellular components, such as microglia, astrocytes, and glioma cells, were obtained to quantify the biochemical changes between healthy cells and tumor cells, and provide supporting data for the final distinction between tumor and normal brain tissue. Fourier transform infrared was used to measure human astrocytes, microglia (HM1900), and glioma cells (U87, BT325), and the cellular components of the 4 types of cells were analyzed by means of average spectra, second-derivative spectra, principal component analysis, hierarchical cluster analysis, and difference spectra. The proteomics, lipidomics, genomics, and metabolic statuses of the cells were different. The amide I, lipid, and nuclear acid regions of the spectra are the most obvious regions to use for distinguishing the 4 types of cells. We conclude that an improved understanding of both similarities and differences in the cellular components of astrocytes, microglia, and glioma cells can help us better understand the heterogeneity of gliomas. We suggest that targeting cellular metabolism (protein, lipid, and nuclear acids) is helpful to distinguish between normal brain tissue and glioma tissue, which has broad application prospects.

Vortex gap solitons in spin–orbit-coupled Bose–Einstein condensates with competing nonlinearities

The formation and dynamics of full vortex gap solitons (FVGSs) are investigated in two-component Bose–Einstein condensates with spin–orbit coupling (SOC), Zeeman splitting (ZS), and competing cubic and quintic nonlinear terms, while the usual kinetic energy is neglected, assuming that it is much smaller than the SOC and ZS terms. Unlike previous SOC system with the cubic-only attractive nonlinearity, in which solely semi-vortices may be stable, with the vorticity carried by a single component, the present system supports stable FVGS states, with the vorticity present in both components (such states are called here “full vortex solitons”, to stress the difference from the half-vortices). They populate the bandgap in the system’s linear spectrum. In the case of the cubic self-attraction and quintic repulsion, stable FVGSs with a positive effective mass exist near the top of the bandgap. On the contrary, the system with cubic self-repulsion and quintic attraction produces stable FVGSs with a negative mass near the bottom of the bandgap. Mobility and collisions of FVGSs with different topological charges are investigated too.

Phase relaxation control in heterostructures featuring charge-transfer excitons

Type-II quantum well heterostructures are considered high-potential candidates for next-generation active semiconductor devices. They promise low fundamental transition energies and suppressed Auger scattering as well as temperature stability in device performance. Understanding their fundamental properties, such as scattering and diffusion, and revealing intricate differences from type-I quantum structures are important steps towards optimized structure design. Using degenerate four-wave mixing spectroscopy, we investigate the phase coherence of excitonic polarizations in a (Ga,In)As/GaAs/Ga(As,Sb) type-II double-quantum-well structure. It is designed to exhibit spectrally isolated resonances in its linear absorption spectrum including a well-resolved charge-transfer exciton resonance. This allows us to study the coherent dynamics of the unperturbed charge-transfer exciton polarization. In addition, the effects of many-body interactions with free charge carriers as well as excitons that are injected by an optical prepulse are revealed. Scattering of charge-transfer excitons with free charge carriers is three times more efficient than scattering of charge-transfer excitons with each other. The comparison with Wannier excitons in a type-I quantum well structure shows that the interaction strength between charge-transfer excitons is about twice as large as for excitons in a type-I quantum well structure.

Rural Acoustic Landscape Analysis Based on Segmentation and Extraction of Spectral Image Feature Information.

Spectrogram is an image that can record voice information, which can be analyzed by analyzing the received image. Spectrograms are used in mechanical fault diagnosis systems to answer questions such as the location, type, and extent of the fault. It is the main tool for analyzing vibration parameters. In actual use, there are three types of spectrograms, namely linear amplitude spectrum, logarithmic amplitude spectrum, and self-power spectrum. The ordinate of the linear amplitude spectrum has a clear physical dimension and is the most commonly used. In this paper, the feature extraction information of rural acoustic landscape is mainly carried out through spectral images, which can effectively improve the segmentation efficiency, ensure the integrity of information, and determine the feasibility of establishing acoustic landscape in rural areas. This article aims to study the analysis of rural acoustic landscape in Guilin, Guangxi, based on the segmentation and extraction of spectral image feature information, through the segmentation and extraction of spectral image feature information, and then analyze the advantages and disadvantages of rural acoustic landscape. In this article, the Gabor wavelet filtering method is proposed to filter and analyze the spectral image. Through the detailed analysis of the insect and bird calls of the forest community near the village of Guilin, Guangxi, finally, the satisfaction and attention of the rural villagers to the acoustic landscape are investigated. The experimental results show that the sound of insects and birds reaches the maximum in spring and the minimum in autumn and winter. Moreover, the attention of rural villagers to acoustic landscape is also very high, with satisfaction of 87.12% and attention of 92.68%.

Spin-Polarized Study of the Structural, Optoelectronic, and Thermoelectric Properties of the Melilite-Type Gd2Be2GeO7 Compound

The present work is a theoretical study of the structural and spin-polarized dependent optoelectronic thermoelectric properties of the melilite-typeGd2Be2GeO7 compound, using the full potential linearized augmented plane wave approach in the framework of density functional theory. The predicted structural parameters are in good accordance with the measured counterparts. It is found that the title compound is more stable in the ferromagnetic order than in the non-magnetic order. The calculated band structure using the modified Becke–Johnson potential reveals that the studied compound has a wide bandgap of 3.78 eV. The frequency-dependent linear optical spectra are studied in an energy range expanding from 0 to 30 eV. Finally, the semi classical Boltzmann theory as incorporated in the Boltztrap code is used to study the spin-polarized dependent transport properties. The obtained results show that Gd2Be2GeO7 is a potential candidate for conversion energy device applications.

Nonlinear optical properties of polyaniline doped with cardanol based dye

We report the third-order nonlinear optical properties of polyaniline doped with an azo dye synthesized using cardanol. Cardanol, which is distilled from CSNL (Cashew Nut Shell Liquid), is a renewable resource since it is a byproduct of the Cashew nut industry. The linear absorption spectrum of the sample was determined in the wavelength range 100 – 800 nm. FTIR spectroscopic data of undoped and azo dye-doped polyaniline samples were compared. The morphology and structure of the azo dye-doped polyaniline are studied using SEM images and XRD. Nonlinear absorption studies were carried out using the single beam z scan technique. A Q-switched Nd: Yag laser operating at 532 nm wavelength with a pulse width of 7 ns was used as the source of light. The nonlinear absorption coefficient and nonlinear susceptibility were determined to be of the order of 10−11 m/W and 10−11 esu respectively. The results indicate that the material is a good candidate for optoelectronic applications.

Linear and non-linear optical properties of boron carbide thin films

Boron carbide films deposited by PECVD are subjected to post deposition vacuum annealing and plasma treatment for investigation of changes in the structure, linear and non-linear optical properties of the films. Deconvolution of core level XPES spectra of B1s, C1s and O1s indicates changes in the surface chemical states of the constituent atoms after post deposition treatments. Particle size estimated from XRD spectrum is found increasing from 135 to 140 nm after annealing. Envelop method is used to estimate refractive index, extinction co-efficient, and thickness from their measured optical transmission spectra in VIS-NIR region (400–1400 nm). The refractive index value is found to be 1.619, highest for the vacuum annealed boron carbide film. Direct and indirect optical bandgap remains same after vacuum annealing (Eg = 2.84), the plasma treatment causes a slight decrease in the bandgap. 5% decrease in film thickness is observed after post deposition annealing and plasma treatment owing to film densification. Energy loss in the medium measured in terms of tanδ and ELF’s reveals that the vacuum annealing of the film reduces the electromagnetic and electron energy loss in the film. Non-linear refractive index estimated using Ticha and Tichy relation exhibits similar trend as the linear refractive index spectra.

Fingerprint and Universal Markovian Closure of Structured Bosonic Environments.

We exploit the properties of chain mapping transformations of bosonic environments to identify a finite collection of modes able to capture the characteristic features, or fingerprint, of the environment. Moreover we show that the countable infinity of residual bath modes can be replaced by a universal Markovian closure, namely, a small collection of damped modes undergoing a Lindblad-type dynamics whose parametrization is independent of the spectral density under consideration. We show that the Markovian closure provides a quadratic speedup with respect to standard chain mapping techniques and makes the memory requirement independent of the simulation time, while preserving all the information on the fingerprint modes. We illustrate the application of the Markovian closure to the computation of linear spectra but also to nonlinear spectral response, a relevant experimentally accessible many body coherence witness for which efficient numerically exact calculations in realistic environments are currently lacking.