Line Shape Of Spectra Research Articles

Line shape and the experimental determination of the J/ψ→γηc branching fraction

We study the photon spectrum line-shape of the $J/\psi \to \gamma\eta_c\to \gamma X$ decay process in a nonrelativistic effective field theory (EFT) framework of quantum chromodynamics (QCD). We take into account the finite width of the $\eta_c$ and include $\mathcal{O}(v^2)$ corrections. We observe that the photon spectrum line-shape is divergent at large energies due to polynomially and logarithmically divergent terms, that upon integration over the photon energies in Dimensional Regularization (DR) produce no contribution or can be renormalized. We propose to subtract these divergences at the line-shape level in a manner consistent with the calculation of the width in DR and $\overline{\hbox{MS}}$ scheme. We analyze CLEO's data with the proposed subtracted line-shape and find good agreement between the theoretical prediction and the experimental result.

Mechanically Modulated Sideband and Squeezing Effects of Membrane Resonators.

We investigate the sideband spectra of a driven nonlinear mode with its eigenfrequency being modulated at a low frequency (<1 kHz). This additional parametric modulation leads to prominent antiresonance line shapes in the sideband spectra, which can be controlled through the vibration state of the driven mode. We also establish a direct connection between the antiresonance frequency and the squeezing of thermal fluctuation in the system. Our Letter not only provides a simple and robust method for squeezing characterization, but also opens a new possibility toward sideband applications.

Optical image edge detection by transmissive metal-dielectric-metal structures

The feasibility of an optical image edge detection based on metal-insulator-metal (MIM) resonance transmission structures is experimentally investigated. The structures are fabricated on a glass substrate and consist of thin aluminum layers separated by a quartz layer. The excitation of Fabry-Perot modes by an incident wave produces resonance line shapes in angular and wavelength transmission spectra. Resonance enhancement and suppression of beams using the MIM structures can be implemented for suppressing the illuminating beam and amplifying the field scattered by an object. By using the MIM structure under oblique incidence, we experimentally observe the efficient image edge detection for phase optical elements at a set of wavelengths. The obtained images of edges of the elements exhibit a directionality of image edge detection that depends on the direction of inhomogeneity gradient in the object plane, as suggested by the angular transmission spectra of the MIM structures. The results of the present work can find applications in optical information processing and optical filtering systems.

Anharmonic DFT Study of Near-Infrared Spectra of Caffeine: Vibrational Analysis of the Second Overtones and Ternary Combinations.

Anharmonic quantum chemical calculations were employed to simulate and interpret a near-infrared (NIR) spectrum of caffeine. First and second overtones, as well as binary and ternary combination bands, were obtained, accurately reproducing the lineshape of the experimental spectrum in the region of 10,000–4000 cm−1 (1000–2500 nm). The calculations enabled performing a detailed analysis of NIR spectra of caffeine, including weak bands due to the second overtones and ternary combinations. A highly convoluted nature of NIR spectrum of caffeine was unveiled, with numerous overlapping bands found beneath the observed spectral lineshape. To properly reflect that intrinsic complexity, the band assignments were provided in the form of heat maps presenting the contributions to the NIR spectrum from various kinds of vibrational transitions. These contributions were also quantitatively assessed in terms of the integral intensities. It was found that the combination bands provide the decisively dominant contributions to the NIR spectrum of caffeine. The first overtones gain significant importance between 6500–5500 cm−1, while the second overtones are meaningful in the higher wavenumber regions, particularly in the 10,000–7000 cm−1 region. The obtained detailed band assignments enabled deep interpretation of the absorption regions of caffeine identified in the literature as meaningful for analytical applications of NIR spectroscopy focused on quantitative analysis of caffeine content in drugs and natural products.

Dispersion of Complex Refractive Indices for Intense Vibrational Bands. II. Implication to Sum Frequency Generation Spectroscopy.

Sharp and intense vibrational bands are characterized with conspicuous dispersion of complex refractive indices. Based on the quantitative data of dispersion in the preceding paper, this paper clarifies the influence of the dispersion on the sum frequency generation (SFG) spectroscopy. As a consequence of the large dispersion, the lineshapes of SFG spectra could be influenced by the frequency dependence of the Fresnel factor as well as the nonlinear susceptibility. This paper argues the relative importance of the two factors in general cases and provides a useful criterion to evaluate their importance. The effect of Fresnel dispersion becomes significant when the SFG spectrum involves a sharp and intense vibrational band as well as a large non-resonant background susceptibility, typically in some solid-liquid interfaces. A possible way to correct the effect of Fresnel dispersion is suggested using the heterodyne measurement.

The narrowband full polarization spectra reconstruction for static channeled spectropolarimetry

Channeled spectropolarimetry is a snapshot scheme for the full polarization spectra measurement. It has the advantages of static, compact, lightweight, and easy to implement. However, while the broadband polarization spectra can be precisely reconstructed using the regular methods, such as the method based on the Fourier transform, the reconstruction of narrowband polarization spectra always suffers from errors caused by the crosstalk among channels in the optical path difference domain. We present an efficient narrowband full polarization spectra reconstruction method for channeled spectropolarimetry. The line shape of the intensity spectrum is estimated by the envelop curve of the measured high frequency modulated spectrum. The full polarization spectra can be reconstructed by the least-square method under the assumption that polarization states change slowly with wavelength for narrowband polarization spectra. The feasibility of the reconstruction method has been verified by the simulation and experimental results.

Interference effects on intense XUV pulse induced resonant transitions between core ionized states of helium

In the context of core-resonant ionization of helium atom the inclusion of the He–He+(2p) ionization channel sets off a quantum mechanical interference between two competing pathways, He–He+(1s)–He+(2p) and He–He+(2p) (He–He+(1s) and He–He+(2p)–He+(1s)), thereby significantly modifying the line shape of the photoelectron spectrum. The signature of this interference arises as an asymmetry in the line shape of the multi-peak photoelectron spectra. On account of the discrepancies over the magnitude of the two photon photoionization cross section for the He–He+(2p) ionization channel the calculations have been performed for three different magnitudes of this photoionization cross section and the findings confirm that asymmetry in the line shape is prevalent for all the chosen cross sections. However, the magnitude of asymmetry depends on the magnitude of the photoionization cross section for the He–He+(2p) ionization channel. It is also interesting to note that at the considered laser peak intensities in this work an additional ionization channel from excited ionic state of helium plays a significant role in influencing the line shape of the photoelectron spectrum. The effects of pulse duration on the line shape of the photoelectron spectrum and the possibility to control the interference induced asymmetry in the multi peak line shape has also been investigated.

Analytic and numerical vibronic spectra from quasi-classical trajectory ensembles.

The truncated Wigner approximation to quantum dynamics in phase space is explored in the context of computing vibronic line shapes for monomer linear optical spectra. We consider multiple model potential forms including a shifted harmonic oscillator with both equal and unequal frequencies on the ground and excited state potentials as well as a shifted Morse potential model. For the equal-frequency shifted harmonic oscillator model, we derive an analytic expression for the exact vibronic line shape that emphasizes the importance of using a quantum mechanical distribution of phase space initial conditions. For the unequal-frequency shifted harmonic oscillator model, we are no longer able to obtain an exact expression for the vibronic line shape in terms of independent deterministic classical trajectories. We show how one can rigorously account for corrections to the truncated Wigner approximation through nonlinear responses of the line shape function to momentum fluctuations along a classical trajectory and demonstrate the qualitative improvement in the resulting spectrum when the leading-order quantum correction is included. Finally, we numerically simulate absorption spectra of a highly anharmonic shifted Morse potential model. We find that, while finite quantization and the dissociation limit are captured with reasonable accuracy, there is a qualitative breakdown of the quasi-classical trajectory ensemble's ability to describe the vibronic line shape when the relative shift in Morse potentials becomes large. The work presented here provides clarity on the origin of unphysical negative features known to contaminate absorption spectra computed with quasi-classical trajectory ensembles.

Analysis of the spectral properties of free electron lasers in X-RAY and other bands

Analytical formalism for the description of free electron laser (FEL) spectrum is proposed with account for the electron beam size, divergences, beam energy spread, the off-axis deflection and the periodic harmonic and non-periodic constant magnetic field components in undulators. The exact analytical expressions for the spontaneous undulator radiation (UR) are obtained in terms of generalized Bessel and Airy functions, which describe the spectrum line shape and intensities. The obtained analytical formulae distinguish the effects of each installation and beam parameter on the FEL harmonic radiation. The enhanced phenomenological model is employed for the FEL modeling; it describes the harmonic powers evolution along the undulators and gradual saturation around the saturation length with account for the saturated powers oscillations. With this model, we study the generation of FEL harmonics, including even, in several instances of FEL experiments at LCLS, LEUTL and PAL-XFEL. We analytically demonstrate the even harmonics reasons in the LCLS and LEUTL FEL experiments. The spectrum line split due to the beam size is demonstrated analytically and compared with experimentally measured spectral characteristics. The PAL-XFEL experiment is analyzed and compared with the LCLS experiment; the possibility of high harmonic radiation in it is explored. The modeling results fully agree with the measured values in the studied FELs. The analytical formalism shows physical background for each harmonic in every FEL. It allows the analysis of the radiation and beam alignment in projected, built and working FELs and it can help minimizing losses and correcting magnetic fields as well as projecting experiments with FELs.

Improving the short-term frequency stability of a magnetic-state-selected cesium beam clock with optical detection.

The microwave spectrum line shape and the signal-to-noise ratio of a compact optically detected magnetic-state-selection cesium beam clock are analyzed in this paper. As the noise analysis shows, the performance is related to the atomic utilization ratio and locking parameters when the laser frequency noise is the dominant noise source. Methods are adopted for realizing better short-term frequency stability of the clock, including using a highly efficient state-selection cesium beam tube, optimizing the locking parameters, and stabilizing the microwave power to maximize the error signal. After optimization, the signal-to-noise ratio of the clock reaches 7.0 × 103 in a bandwidth of 1Hz and the clock demonstrates a short-term stability of 4.1 × 10-12 τ-1/2. The five-day Allan standard deviation reaches 7.7 × 10-15.

Ultimate conversion efficiency of second harmonic generation in all-dielectric resonators of quasi-BICs in consideration of nonlinear refraction of dielectrics

We investigate second harmonic generation (SHG) in all-dielectric resonance nanostructures of high-Q factors assisted by quasi-bound states in the continuum (quasi-BICs). The typical resonators, e.g., guided-mode resonance gratings and asymmetric metasurfaces, fabricated by AlGaAs were numerically studied with the consideration of nonlinear refraction of AlGaAs. The resonance peak and line-shape of linear transmission and SHG spectra in the resonators can be dramatically changed under intense pump intensities. The SHG conversion efficiency in the nanostructures working at quasi-BICs is much lower than the traditionally expected values without considering the nonlinear refraction of dielectrics. The ultimate SHG conversion efficiency is finally obtained. The investigation has the significance for the design and understanding of efficient nonlinear metasurfaces of high-Q factors.

Observing Multiexciton Correlations in Colloidal Semiconductor Quantum Dots via Multiple-Quantum Two-Dimensional Fluorescence Spectroscopy.

Correlations between excitons, that is, electron-hole pairs, have a great impact on the optoelectronic properties of semiconductor quantum dots and thus are relevant for applications such as lasers and photovoltaics. Upon multiphoton excitation, these correlations lead to the formation of multiexciton states. It is challenging to observe these states spectroscopically, especially higher multiexciton states, because of their short lifetimes and nonradiative decay. Moreover, solvent contributions in experiments with coherent signal detection may complicate the analysis. Here we employ multiple-quantum two-dimensional (2D) fluorescence spectroscopy on colloidal CdSe1-xSx/ZnS alloyed core/shell quantum dots. We selectively map the electronic structure of multiexcitons and their correlations by using two- and three-quantum 2D spectroscopy, conducted in a simultaneous measurement. Our experiments reveal the characteristics of biexcitons and triexcitons such as transition dipole moments, binding energies, and correlated transition energy fluctuations. We determine the binding energies of the first six biexciton states by simulating the two-quantum 2D spectrum. By analyzing the line shape of the three-quantum 2D spectrum, we find strong correlations between biexciton and triexciton states. Our method contributes to a more comprehensive understanding of multiexcitonic species in quantum dots and other semiconductor nanostructures.

Robust Fano resonance in the photonic valley Hall states

Rapidly developing photonics brings many interesting resonant optical phenomena, in which the Fano resonance (FR) always intrigues researchers because of its applications in optical switching and sensing. However, its sensitive dependence on environmental conditions makes it hard to implement in experiments. We in this work suggest a topologically-protected FR based on the photonic valley Hall insulators, immune to the system impurities. The topologically-protected FR is achieved by coupling the valley-dependent topological edge states (TESs) with one double-degenerate cavity. The $\delta$-type photonic transport theory we build reveals that this topological FR dates from the interference of the two transmissions that are attributed to the parity-odd cavity mode and the parity-even one. We confirm that the induced Fano line shape of the transmission spectra is robust against the bending domain walls and disorders. Our work may provoke exciting frontiers for manipulating the valley transport and pave a way for the topologically protected photonic devices such as optical switches, low-threshold lasers, and ultra-sensitive sensors.

Modeling solvation effects on absorption and fluorescence spectra of indole in aqueous solution.

Modeling the optical spectra of molecules in solution presents a challenge, so it is important to understand which of the solvation effects (i.e., electrostatics, mutual polarization, and hydrogen bonding interactions between solute and solvent molecules) are crucial in reproducing the various features of the absorption and fluorescence spectra and to identify a sufficient theoretical model that accurately captures these effects with minimal computational cost. In this study, we use various implicit and explicit solvation models, such as molecular dynamics coupled with non-polarizable and polarizable force fields, as well as Car-Parrinello molecular dynamics, to model the absorption and fluorescence spectra of indole in aqueous solution. The excited states are computed using the equation of motion coupled cluster with single and double excitations combined with the effective fragment potential to represent water molecules, which we found to be a computationally efficient approach for modeling large solute-solvent clusters at a high level of quantum theory. We find that modeling mutual polarization, compared to other solvation effects, is a dominating factor for accurately reproducing the position of the peaks and spectral line shape of the absorption spectrum of indole in solution. We present an in-depth analysis of the influence that different solvation models have on the electronic excited states responsible for the features of the absorption spectra. Modeling fluorescence is more challenging since it is hard to reproduce even the correct emitting state, and force field parameters need to be re-evaluated.

Hot-electron photocurrent detection of near-infrared light based on ZnO

We demonstrate an unconventional near-infrared photodetector fabricated from a ZnO chip with a metallic subwavelength grating structure as a contact and optical window, which harvests hot electrons generated by plasmonic resonances introduced by incident light. The grating structure has a strong selection of the polarization of incident light, meaning that the detector is naturally polarization-sensitive. In our device, the polarization extinction ratio is as high as 64:1, much higher than that relying on crystal orientations. Since the photoresponse is introduced by plasmonic resonance, a narrow photoresponse spectrum with a linewidth of 32.1 nm at 1.201 μm is obtained. By simply changing the grating period, the spectral position can be tailored freely within the near-infrared region, i.e., wavelength-selective. Such a spectral response is not likely to be realized with conventional semiconductor photodetectors, which depend on the band edge absorption. We propose a modified Fowler's model, which well explains the line shape of photoresponse spectra of such devices.

Hyperfine structure of ro-vibrational transition of HD in magnetic field

The precise measurement of the infrared transition of hydrogen-deuterium (HD) molecule is used to test quantum electrodynamics and determine the proton-to-electron mass ratio. The saturated absorption spectrum of the R(1) line in the first overtone (2–0) band of HD molecule has been measured by the comb locked cavity ring-down spectroscopy (CRDS) method in Hefei [Tao L G, et al. &lt;ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1103/PhysRevLett.120.153001"&gt; 2018 &lt;i&gt;Phys. Rev. Lett.&lt;/i&gt; &lt;b&gt;120&lt;/b&gt; 153001&lt;/ext-link&gt;], and also by the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) method in Amsterdam [Cozijn F M J, et al. &lt;ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1103/PhysRevLett.120.153002"&gt;2018 &lt;i&gt;Phys. Rev. Lett.&lt;/i&gt; &lt;b&gt;120&lt;/b&gt; 153002 &lt;/ext-link&gt;]. However, there is a significant difference between the line center positions obtained in these two studies. Later the discrepancy was found to be due to unexpected asymmetry in the line shape of the saturated absorption spectrum of the HD molecule. A possible reason is the superposition of multiple hyperfine splitting peaks in the saturated spectrum. However, this model strongly depends on the population transfer caused by intermolecular collisions, which is a lack of experimental and theoretical support. In this paper, the hyperfine structures of the ro-vibrational transition of HD are calculated in the coupled and uncoupled representations. The hyperfine structures of the R(0), P(1) and R(1) lines in the (2–0) band of HD molecule under different external magnetic fields are calculated. The corresponding spectral structures at a temperature of 10 K are simulated. The results show that the transition structure of HD molecule changes significantly with the externally applied magnetic field. The frequency shift of each hyperfine transition line also increases with the intensity of external magnetic field increasing. When the intensity of the external magnetic field is sufficiently high, the hyperfine lines are clearly divided into two branches, and they can be completely separated from each other. Because the dynamic effect of intermolecular collision and the energy level population transfer are very sensitive to the energy level structure, the comparison between experiment and theory will help us to analyze the mechanism of the observed special profiles. It will allow us to obtain accurate frequencies of these transitions, which can be used for testing the fundamental physics.

光纤空气隙法布里-珀罗标准具透射谱线型研究

光纤空气隙法布里-珀罗标准具透射谱线型研究

Аналитический учет внеосевых эффектов в рентгеновском излучении гармоник лазеров на свободных электронах

We give analytical description of generation of harmonics of the undulator radiation (UR) with account for the finite electron beam size, emittance, off-axis beam deviation and electron energy spread, as well as for the constant magnetic components and field harmonics effects. We give exact analytical expressions for the generalized Bessel and Airy functions, which describe the spectrum line shape and intensities in the two-frequency bi-harmonic undulator with account for the above factors. The obtained analytical formulae distinguish contributions of each field component and every undulator and beam parameter on the harmonic radiation in free electron lasers (FEL). The effect of the field on the harmonic radiation is analyzed with account for the beam finite size and its off-axis deviation. The phenomenological model is employed for the FEL modeling; with its help we study the harmonic generation, including even ones, in the experiments LCLS and LEUTL. We demonstrate analytically that strong second FEL harmonic in X-ray FEL at the wavelengths λ = 0.75nm in the LCLS experiment is caused by the deviation of the electron trajectories off the axis in 15 μm on the gain length 1.6 m, which is comparable with the beam size; the strong second FEL harmonic in the LEUTL experiment at the wavelength λ = 192nm can be attributed to interaction of the electrons in wide, ~ 0.2 mm, beam with the photon radiation at the gain length 0.87 m. The modeling results fully agree with the measurements. The developed formalism allows the analysis of projected and built FELs and their radiation, helps minimizing losses and correcting magnetic fields; it also shows physical background and reasons for each harmonic radiated power in the FEL.

Characterization of SBS Gain Spectrum Lineshape in Fiber

Characterization of SBS Gain Spectrum Lineshape in Fiber

Two-Dimensional Electronic-Vibrational Spectroscopy Reveals Cross-Correlation between Solvation Dynamics and Vibrational Spectral Diffusion.

Two-dimensional electronic-vibrational spectroscopy (2DEVS) is a useful technique for studying the structure and dynamics of photoexcited molecules via monitoring of the vibrational spectrum in real-time. However, quantitative modeling or prediction of experimental spectra has been hampered by the lack of a firm theoretical basis for this quantity. Here, we develop a useful theory of 2DEVS and show that the time-dependent line shape of the 2DEVS spectrum provides invaluable information on the cross-correlation function of the solvation dynamics and vibrational spectral diffusion. The center and nodal line slopes of each 2DEVS peak are determined by the associated cross-electronic-vibrational frequency-frequency correlation function, which is shown to be related to the intermolecular interactions and vibrational anharmonicities. The present theory of 2DEVS would thus be of use for a refined understanding of the 2DEVS spectra of reactive chemical and biological systems. We anticipate that further development of the expressions developed here will illuminate the application of 2DEVS studies of vibronically induced energy and electron transfer in functional materials.