Nonresonant Background Research Articles

Searching spin-mass interaction using a diamagnetic levitated magnetic-resonance force sensor

Axion-like particles (ALPs) are predicted to mediate exotic interactions between spin and mass. We propose an ALP-searching experiment based on the levitated micromechanical oscillator, which is one of the most sensitive sensors for spin-mass forces at a short distance. The proposed experiment tests the spin-mass resonant interaction between the polarized electron spins and a diamagnetically levitated microsphere. By periodically flipping the electron spins, the contamination from nonresonant background forces can be eliminated. The levitated microoscillator can prospectively enhance the sensitivity by nearly $10^3$ times over current experiments for ALPs with mass in the range 4 meV to 0.4 eV.

Plasmon-enhanced coherent anti-stokes Raman scattering vs plasmon-enhanced stimulated Raman scattering: Comparison of line shape and enhancement factor.

Plasmon-enhanced coherent Raman scattering microscopy has reached single-molecule detection sensitivity. Due to the different driven fields, there are significant differences between a coherent Raman scattering process and its plasmon-enhanced derivative. The commonly accepted line shapes for coherent anti-Stokes Raman scattering and stimulated Raman scattering do not hold for the plasmon-enhanced condition. Here, we present a theoretical model that describes the spectral line shapes in plasmon-enhanced coherent anti-Stokes Raman scattering (PECARS). Experimentally, we measured PECARS and plasmon-enhanced stimulated Raman scattering (PESRS) spectra of 4-mercaptopyridine adsorbed on the self-assembled Au nanoparticle (NP) substrate and aggregated Au NP colloids. The PECARS spectra show a nondispersive line shape, while the PESRS spectra exhibit a dispersive line shape. PECARS shows a higher signal to noise ratio and a larger enhancement factor than PESRS from the same specimen. It is verified that the nonresonant background in PECARS originates from the photoluminescence of nanostructures. The decoupling of background and the vibrational resonance component results in the nondispersive line shape in PECARS. More local electric field enhancements are involved in the PECARS process than in PESRS, which results in a higher enhancement factor in PECARS. The current work provides new insight into the mechanism of plasmon-enhanced coherent Raman scattering and helps to optimize the experimental design for ultrasensitive chemical imaging.

Velocity-intensity asymmetry reversal of solar radial p-modes

The development of space-borne missions has significantly improved the quality of the measured spectra of solar-like oscillators. Their p-mode line profiles can now be resolved, and the asymmetries inferred for a variety of stars other than the Sun. However, it has been known for a long time that the asymmetries of solar p-modes are reversed between the velocity and the intensity spectra. Understanding the origin of this reversal is necessary in order to use asymmetries as a tool for seismic diagnosis. For stars other than the Sun, only the intensity power spectrum is sufficiently resolved to allow for an estimation of mode asymmetries. We recently developed an approach designed to model and predict these asymmetries in the velocity power spectrum of the Sun and to successfully compare them to their observationally derived counterpart. In this paper we expand our model and predict the asymmetries featured in the intensity power spectrum. We find that the shape of the mode line profiles in intensity is largely dependent on how the oscillation-induced variations of the radiative flux are treated, and that modelling it realistically is crucial to understanding asymmetry reversal. Perturbing a solar-calibrated grey atmosphere model, and adopting the quasi-adiabatic framework as a first step, we reproduce the asymmetries observed in the solar intensity spectrum for low-frequency modes. We conclude that, unlike previously thought, it is not necessary to invoke an additional mechanism (e.g. non-adiabatic effects, coherent non-resonant background signal) to explain asymmetry reversal. This additional mechanism is necessary, however, to explain asymmetry reversal for higher-order modes.

Identification of Additional Jets in the $${\rm{t}}{\rm\bar{t}b}{\rm\bar{b}}$$ Events by Using Deep Neural Network

In the top quark pair production in association with the Higgs boson decaying to a b quark pair t-tbar H (b-bbar), the final state has an irreducible nonresonant background from the production of a top quark pair in association with a b quark pair t-tbar b-bbar. Therefore, understanding of the t-tbar b-bbar process precisely in particular differential cross-section as functions of the properties of the additional b jets not from the top quark decay is essential for improving the sensitivity of a search for the t-tbar H b-bbar process. The two additional b jets can be identified by using various approaches. In this paper, the performances are compared quantitatively in the lepton+jets decay channel in terms of the matching efficiency of assigning two additional b jets as a figure of merit. This study provides valuable information towards the precise measurement of differential cross-sections as a function of properties of the additional b jets in the t-tbar b-bbar events. We showed that a matching efficiency of around 40% could be achieved using a deep neural network method. In the events with at least 4 b jets, this performance is 8% better than that achieved using minimum Delta R(b,bbar) method. This is consistent with the boosted decision tree method within its statistical uncertainty.

Polarization-enabled spectral-focusing CARS microscopy

We describe a spectral-focusing-based polarization-resolved coherent anti-Stokes Raman scattering (SFP-CARS) microscopy system developed by making simple and inexpensive modifications to an existing spectral focusing CARS setup. By using the system to study polarization dependent features in the CARS spectrum of benzonitrile, we assess its capabilities and demonstrate its ability to accurately determine Raman depolarization ratios. Ultimately, the detected anti-Stokes signals are more elliptically polarized than expected, hindering a complete suppression of the non-resonant background. Furthermore, the fact that resonant signals polarized in directions similar to that of the non-resonant background are also substantially suppressed when extinguishing the non-resonant background remains a serious limitation. We conclude that non-resonant background suppression using the SFP-CARS system is best suited for studying Raman modes that generate signals polarized in directions far from that of the non-resonant background instead of for obtaining background-free CARS spectra. In all, we find that the SFP-CARS setup is a useful tool for studying polarization dependent features in the CARS spectra of various samples that is worthy of further investigation. This work aims to illuminate several technical aspects of polarization dependent CARS and inform researchers of the benefits and constraints of integrating polarization dependent detection as an add-on to existing CARS microscopy setups.

Partial Photoionization Cross Sections of Chromium from the Ground and Excited States

Partial and total photoionization cross sections of iron-peak elements are important for the determination of abundances in late-type stars and nebular objects. We have investigated photoionization of neutral chromium from the ground and excited states in the low energy region from the first ionization threshold at 6.77 eV to 30 eV. Accurate descriptions of the initial bound states of Cr I and the final residual Cr II ionic states have been obtained in the multiconfiguration Hartree-Fock method together with adjustable configuration expansions and term-dependent non-orthogonal orbitals. The B-spline R-matrix method has been used for the calculation of photoionization cross sections. The 194 LS final ionic states of Cr II 3d44s, 3d34s2, 3d5, 3d44p, and 3d34s4p principal configurations have been included in the close-coupling expansion. The inclusion of all terms of these configurations has significant impact on the near-threshold resonance structures as well as on the nonresonant background cross sections. Total photoionization cross sections from the ground 3d54sa7S and excited 3d54sa5S, 3d44s2a5D, 3d54pz5P, and 3d44s4py5P states of Cr I have been compared with other available R-matrix calculation to estimate the likely uncertainties in photoionization cross sections. We analyzed the partial photoionization cross sections for leaving the residual ion in various states to identify the important scattering channels, and noted that 3d electron ionization channel becomes dominant at higher energies.

CO2 chirped-probe-pulse femtosecond CARS thermometry

The application of CO2 chirped-probe-pulse (CPP) fs coherent anti-Stokes Raman scattering (CARS) for flame thermometry is discussed. A phenomenological model for CO2/O2 CPP fs CARS was developed and the simulations showed the potential for CO2 temperature measurements free of interferences from O2 and with no contribution from the nonresonant background by using a long time delay for the chirped probe pulse. The accuracy and precision for CO2 CPP fs CARS thermometry is compared with N2 CPP fs CARS thermometry for temperatures between 295 K and 1420 K. The measurement accuracy for the CO2 measurements is comparable with that for the N2 measurements, while the precision for the CO2 temperature measurements is significantly better at lower temperatures. We also demonstrate that accurate flame thermometry using a single set of laser parameters can be achieved using the delayed CPP approach.

Mitigating cross‐phase modulation artifacts in femtosecond stimulated Raman scattering

AbstractIn contrast to spontaneous Raman scattering, coherent Raman scattering techniques, such as femtosecond stimulated Raman scattering (FSRS), show advantages for many applications. Besides an enhanced signal strength, FSRS is free of a nonresonant background but is affected by cross‐phase modulation (XPM). The resulting artifacts in FSRS become relevant for high pulse intensities (GW/cm2‐regime) and ultrashort pulses (<2 ps), both necessary for super‐resolution experiments. As the pulse duration is a crucial parameter for XPM as well as for FSRS, we present a setup in which we adjust the pulse duration across the relevant range (0.5–3 ps), in order to investigate the XPM influence on the spectra. Furthermore, we vary the peak intensity, the temporal overlap between the interacting pulses, and the nonlinear refractive index coefficient n2 of the sample, showing that a trade‐off between all these quantities enables the measurement of unaffected FSRS spectra

Origin of dispersive line shapes in plasmon-enhanced stimulated Raman scattering microscopy

Abstract Plasmon-enhanced stimulated Raman scattering (PESRS) microscopy has been recently developed to reach single-molecule detection limit. Unlike conventional stimulated Raman spectra, dispersive-like vibrational line shapes were observed in PESRS. Here, we propose a theoretical model together with a phasor diagram to explain the observed dispersive-like line shapes reported in our previous study. We show that the local enhanced electromagnetic field induced by the plasmonic nanostructure interferes with the molecular dipole-induced field, resulting in the dispersive profiles of PESRS. The exact shape of the profile depends on the phase difference between the plasmonic field and the molecular dipole field. We compared plasmon-enhanced stimulated Raman loss (PESRL) and plasmon-enhanced stimulated Raman gain (PESRG) signals under the same pump and Stokes laser wavelength. The PESRL and PESRG signals exhibit similar signal magnitudes, whereas their spectral line shapes show reversed dispersive profiles, which is in an excellent agreement with our theoretical prediction. Meanwhile, we verify that the nonresonant background in PESRS mainly originates from the photothermal effect. These new insights help the proper use of PESRS for nanoscale bio-imaging and ultrasensitive detection.

Identifying hidden charm pentaquark signal from non-resonant background in electron–proton scattering * * Supported in part by the National Natural Science Foundation of China (11405222, 11975278), the Pioneer Hundred Talents Program of Chinese Academy of Sciences (CAS) and the Key Research Program of CAS (XDPB09)

In this study, we analyze the electroproduction of the LHCb pentaquark states with the assumption that they are resonant states. Our main concern is to investigate the final state distribution in the phase space to extract a feeble pentaquark signal from a large non-resonant background. The results indicate that the signal to background ratio will increase significantly with a proper kinematic cut, which will be beneficial for future experimental analysis.

Removing non-resonant background from CARS spectra via deep learning

Broadband Coherent Anti-Stokes Raman Scattering (B-CARS) is a powerful label-free nonlinear spectroscopy technique allowing one to measure the full vibrational spectrum of molecules and solids. B-CARS spectra, however, suffer from the presence of a spurious signal, called non-resonant background (NRB), which interferes with the resonant vibrational one, distorting the line shapes and degrading the chemical information. While several numerical techniques are available to remove this unwanted contribution and extract the resonant vibrational signal of interest, they all require the user’s intervention and sensitively depend on the spectral shape of the NRB, which needs to be measured independently. In this work, we present a novel approach to remove NRB from B-CARS spectra based on deep learning. Thanks to the high generalization capability offered by the deep architecture of the designed neural network, trained through realistic simulated spectra, our fully automated model is able to process B-CARS spectra in real time and independently of the detailed shape of the NRB spectrum. This results in fast extraction of vibrational spectra without requiring user intervention or the measurement of reference spectra. We expect that this model will significantly simplify and speed-up the analysis of B-CARS spectra in spectroscopy and microscopy.

Background-free imaging of chemical bonds by a simple and robust frequency-modulated stimulated Raman scattering microscopy.

Being able to image chemical bonds with high sensitivity and speed, stimulated Raman scattering (SRS) microscopy has made a major impact in biomedical optics. However, it is well known that the standard SRS microscopy suffers from various backgrounds, limiting the achievable contrast, quantification and sensitivity. While many frequency-modulation (FM) SRS schemes have been demonstrated to retrieve the sharp vibrational contrast, they often require customized laser systems and/or complicated laser pulse shaping or introduce additional noise, thereby hindering wide adoption. Herein we report a simple but robust strategy for FM-SRS microscopy based on a popular commercial laser system and regular optics. Harnessing self-phase modulation induced self-balanced spectral splitting of picosecond Stokes beam propagating in standard single-mode silica fibers, a high-performance FM-SRS system is constructed without introducing any additional signal noise. Our strategy enables adaptive spectral resolution for background-free SRS imaging of Raman modes with different linewidths. The generality of our method is demonstrated on a variety of Raman modes with effective suppressing of backgrounds including non-resonant cross phase modulation and electronic background from two-photon absorption or pump-probe process. As such, our method is promising to be adopted by the SRS microscopy community for background-free chemical imaging.

Structure Determination of Hen Egg-White Lysozyme Aggregates Adsorbed to Lipid/Water and Air/Water Interfaces.

We use vibrational sum-frequency generation (VSFG) spectroscopy to study the structure of hen egg-white lysozyme (HEWL) aggregates adsorbed to DOPG/D2O and air/D2O interfaces. We find that aggregates with a parallel and antiparallel β-sheet structure together with smaller unordered aggregates and a denaturated protein are adsorbed to both interfaces. We demonstrate that to retrieve this information, fitting of the VSFG spectra is essential. The number of bands contributing to the VSFG spectrum might be misinterpreted, due to interference between peaks with opposite orientation and a nonresonant background. Our study identified hydrophobicity as the main driving force for adsorption to the air/D2O interface. Adsorption to the DOPG/D2O interface is also influenced by hydrophobic interaction; however, electrostatic interaction between the charged protein's groups and the lipid's headgroups has the most significant effect on the adsorption. We find that the intensity of the VSFG spectrum at the DOPG/D2O interface is strongly enhanced by varying the pH of the solution. We show that this change is not due to a change of lysozyme's and its aggregates' charge but due to dipole reorientation at the DOPG/D2O interface. This finding suggests that extra care must be taken when interpreting the VSFG spectrum of proteins adsorbed at the lipid/water interface.

Optical resonance scattering of a dielectric sphere of arbitrary size illuminated by polarized Airy beams

The optical resonance scattering of a dielectric sphere of arbitrary size illuminated by a polarized Airy beam is studied using the generalized Lorenz–Mie theory (GLMT). The electric field is derived using the vector angular spectrum decomposition method (VASDM). The beam shape coefficients (BSCs) are obtained using the multipole expansion method involving vector spherical wave functions (VSWFs). The resonance scattering characteristics are calculated using the GLMT by subtracting a non-resonant background of a perfectly conducting impenetrable sphere. The effects of beam polarization (i.e., linear, circular, radial, azimuthal, and mixed), the attenuation parameter γ and the dimensionless transverse scale kω0 of the incident Airy beam on the scattering and extinction, “resonance” and “interference” efficiencies are examined. Moreover, the scattering efficiency for a perfectly conducting sphere as well as the three-dimensional (3D) scattering directivity patterns of the normalized dimensionless far-field scattering intensity and its resonance counterpart are calculated and plotted. The analysis of the resonance scattering shows that the energy efficiency is not the mere sum of resonance and perfectly conducting background terms; there is an interference between the two denoted by Qscainterf, which strongly affects the resonance extinction and scattering efficiencies. The results presented here are of some importance in studying the optical resonance scattering of polarized Airy beams in applications involving imaging, particle sizing, radiation forces and torques to name a few areas of research.

From Experimental Data to Pole Parameters in a Model Independent Way (Fixed-t Single-Energy Partial Wave Analysis (SE PWA) and Laurent + Pietarinen Model (L+P) for Pole Extraction)

As it has been shown in the invited talk at the NSTAR-2017 conference [1] it is in principle possible to extract pole parameters directly from experiment with minimal model dependence, and in this contribution one way to achieve it in practice by fixed-t analyticity has been demonstrated. Namely, unconstrained partial- wave amplitudes obtained at discrete energies from fits even to complete sets of independent observables which are required to uniquely reconstruct reaction amplitudes do not vary smoothly with energy, and are in prin ciple non-unique. We have demonstrated that this behavior can be ascribed to the continuum ambiguity. We have applied continuum ambiguity invariance to pseudo-scalar meson photoproduction and showed that for a complete set of pseudo-data the non-uniqueness effect can be removed through a phase rotation generating “up-to-a-phase” unique set of single energy partial wave amplitudes. Now we show that for real data this method does not work, but another approach - fixed-t analyticity solves the problem. We present the final results for theηandπ0-photoproduction [2, 3]. Extracting pole positions from partial wave amplitudes is the next step, and we summarize the essence of the new, Laurent + Pietarinen expansion method applicable for continuous and discrete data. It is based on applying the Laurent decomposition of partial wave amplitude, and expanding the non-resonant background into a power series of a conformal-mapping-generated, quickly converging power series obtaining the simplest analytic function with well-defined partial wave analytic properties which fits the input. Unifying both methods in succession, one constructs a model independent procedure to extract pole parameters directly from experimental data without referring to any theoretical model.

Higher-order coherent anti-Stokes Raman scattering microscopy realizes label-free super-resolution vibrational imaging

Coherent Raman scattering (for example, coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering) microscopy has emerged as a powerful tool for label-free biomolecular imaging in biological and biomedical systems, but its spatial resolution is diffraction limited. Here, we report a higher-order coherent anti-Stokes Raman scattering (HO-CARS) microscopy to break the diffraction limit for label-free, super-resolution vibrational imaging. The resolution enhancement of HO-CARS microscopy has been analysed and demonstrated in biological samples (for example, live HeLa and buccal cells). The HO-CARS technique provides an inherent high resonant to non-resonant background ratio compared with conventional CARS microscopy. We affirm that under a tight focusing, the HO-CARS signal originating from the higher-order nonlinear process (χ(5), χ(7)) dominates over the cascaded lower-order nonlinear process (χ(3)), yielding much richer spectroscopic information. This study illustrates that HO-CARS microscopy can be an appealing tool for label-free, super-resolution imaging in biological and biomedical systems with high image contrast. Higher-order (fifth and seventh order) coherent anti-Stokes Raman scattering microscopy is demonstrated to break the diffraction limit for label-free super-resolution vibrational imaging for live cells such as HeLa and buccal cells.

Ultrafast background-free ro-vibrational fs/ps-CARS thermometry using an Yb:YAG crystal-fiber amplified probe.

A novel laser system for ro-vibrational spectroscopy using coherent anti-Stokes Raman Scattering in hybrid fs/ps regime is presented. A single Yb:KGW laser source is used as a master laser to generate the three CARS laser beams, namely the pump and Stokes femtosecond pulses and a 58 ps probe pulse. Master oscillator power amplifier (MOPA) architecture is implemented to increase the probe output power using a custom two stage free space linear amplifier. The probe is 0.37 cm-1 in width and 100 µJ in energy to allow resolving the Q-branch ro-vibrational lines of N2 and recording single shot CARS spectra at kHz repetition rate in flames. An original and simple technique based on the study of the influence of probe delay and polarization has been setup to optimize nonresonant background rejection, with no loss in resonant contribution. CARS performances are reported for N2 thermometry between 300 K and 3000 K, demonstrating state of the art precision.

Squeezing-enhanced Raman spectroscopy

The sensitivity of classical Raman spectroscopy methods, such as coherent anti-stokes Raman spectroscopy (CARS) or stimulated Raman spectroscopy (SRS), is ultimately limited by shot-noise from the stimulating fields. We present the complete theoretical analysis of a squeezing-enhanced version of Raman spectroscopy that overcomes the shot-noise limit of sensitivity with enhancement of the Raman signal and inherent background suppression, while remaining fully compatible with standard Raman spectroscopy methods. By incorporating the Raman sample between two phase-sensitive parametric amplifiers that squeeze the light along orthogonal quadrature axes, the typical intensity measurement of the Raman response is converted into a quantum-limited, super-sensitive estimation of phase. The resonant Raman response in the sample induces a phase shift to signal-idler frequency-pairs within the fingerprint spectrum of the molecule, resulting in amplification of the resonant Raman signal by the squeezing factor of the parametric amplifiers, whereas the non-resonant background is annihilated by destructive interference. Seeding the interferometer with classical coherent light stimulates the Raman signal further without increasing the background, effectively forming squeezing-enhanced versions of CARS and SRS, where the quantum enhancement is achieved on top of the classical stimulation.

Coherent anti-Stokes Raman spectroscopy of single and multi-layer graphene

Spontaneous Raman spectroscopy is a powerful characterization tool for graphene research. Its extension to the coherent regime, despite the large nonlinear third-order susceptibility of graphene, has so far proven challenging. Due to its gapless nature, several interfering electronic and phononic transitions concur to generate its optical response, preventing to retrieve spectral profiles analogous to those of spontaneous Raman. Here we report stimulated Raman spectroscopy of the G-phonon in single and multi-layer graphene, through coherent anti-Stokes Raman Scattering. The nonlinear signal is dominated by a vibrationally non-resonant background, obscuring the Raman lineshape. We demonstrate that the vibrationally resonant coherent anti-Stokes Raman Scattering peak can be measured by reducing the temporal overlap of the laser excitation pulses, suppressing the vibrationally non-resonant background. We model the spectra, taking into account the electronically resonant nature of both. We show how coherent anti-Stokes Raman Scattering can be used for graphene imaging with vibrational sensitivity.

Verification of R-matrix calculations for charged-particle reactions in the resolved resonance region for the 7Be system

R-matrix theory is used to describe nuclear reactions in the resolved resonance region. It uses information on bound states and low energy resonances to accurately parametrize cross sections on the resonances as well as the non-resonant background. Since the seminal work of Lane and Thomas (1958), the approach has been widely used to analyze experimental cross-section data in a broad range of fields spanning nuclear reaction dynamics, nuclear astrophysics, ion beam analysis and their applications. Different R-matrix codes have been developed and used in these different applications with very little communication among the developers or practitioners on the capabilities, achievements or limitations of the codes. A limited comparison among three R-matrix codes on neutron-induced reactions was performed by the International Atomic Energy Agency (IAEA) International Evaluation of Neutron Cross Section Standards project (2007). Since then, significant progress has been made in their implementation of the R-matrix algorithms, and R-matrix codes have enhanced capabilities. In this paper we present, for the first time, the results of a comprehensive effort to verify the most widely used R-matrix codes in the various fields of nuclear science and applications: AMUR, AZURE2, CONRAD, EDA, FRESCO, GECCCOS, and SAMMY. In addition to the description of the capabilities of the codes and their specifications, we discuss the results of a joint exercise which was coordinated by the International Atomic Energy Agency. The aim of the exercise was to compare calculations of charged-particle reaction cross sections for the light composite system 7Be. The calculations were performed by the codes using identical input R-matrix parameters and other specifications and were limited to charged-particle channels.