Frequency-domain Absorption Research Articles

Non-destructive detection for foreign bodies of tea stalks in finished tea products using terahertz spectroscopy and imaging

Foreign bodies e.g. tea stalks are easily to mix into tea leaves during picking up tea leaves with machine harvester. Tea stems have become a major physical pollutant for finishing tea products, and they cannot be effectively detected with machine vision in the visible region. In the current study, terahertz time domain spectroscopy (THz-TDS) and imaging was employed to rapidly and non-destructively detect tea stalk foreign bodies in tea leaves. With the input variables of THz time-domain signal and frequency-domain absorption coefficients, the K nearest neighbor (KNN) qualitative discriminant models were developed with combination of baseline correction algorithms of adaptive iteratively reweighted penalized least squares (AirPLS), asymmetric least squares (AsLS), background correction (Backcor) and baseline estimation and denoising with sparsity (BEADS). Results showed that the AirPLS-KNN model with the input vector of THz time-domain signals presented the best performance, with the accuracy rate of prediction and recall rate reaching 97.3% and 0.96, respectively. With 0.2 mm step length of both the X and Y directions, the THz transmission imaging was scanned to obtain the THz image of 150X130 pixel2 whose resolution and imaging duration were 1.09 mm and 4.4 min respectively, so the outline of tea stalks could be well identified. In conclusion, the THz-TDS and imaging technology serves as a new means for non-destructive detection of tea stalk foreign bodies in tea leaves.

Transient evolution of quasifree electrons of plasma in liquid water revealed by optical-pump terahertz-probe spectroscopy

The fundamental properties of laser-induced plasma in liquid water, such as the ultrafast electron migration and solvation, have not yet been clarified. We use 1650-nm femtosecond laser pulses to induce the plasma in a stable free-flowing water film under the strong field ionization mechanism. Moreover, we adopt intense terahertz (THz) pulses to probe the ultrafast temporal evolution of quasifree electrons of the laser-induced plasma in water on the subpicosecond scale. For the first time, the THz wave absorption signal with a unique two-step decay characteristic in time domain is demonstrated, indicating the significance of electron solvation in water. We employ the Drude model combined with the multilevel intermediate model and particle-in-a-box model to simulate and analyze the key information of quasifree electrons, such as the frequency-domain absorption characteristics and solvation ratio. In particular, we observe that the solvation capacity of liquid water decreases with the increase of pumping energy. Up to ∼50 % of quasifree electrons cannot be captured by traps associated with the bound states as the pumping energy increases to 90 μJ / pulse. The ultrafast electron evolution in liquid water revealed by the optical-pump/THz-probe experiment provides further insights into the formation and evolution mechanisms of liquid plasma.

Vibrational spectral and structural characterization of multicomponent ternary co-crystal formation within acetazolamide, nicotinamide and 2-pyridone.

Ternary co-crystal, as a novel co-crystal design strategy developed on the basis of binary co-crystal, could be used to improve the physicochemical properties of active pharmaceutical ingredients (APIs) efficiently. However, it is difficult to obtain specific ternary co-crystals since such ternary one involves complex assembly of three different molecules. There are few reports on the micro-molecular structure respect of specific ternary co-crystal systems. In present work, 1:1:1 ternary co-crystal between acetazolamide (ACZ), nicotinamide (NAM) and 2-pyridone (2HP) has been synthesized successfully by mechanical grinding approach, and their structures are investigated by terahertz time-domain spectroscopy (THz-TDS) and Raman spectroscopy combined with theoretical calculation at the molecular level. The experimental THz spectral results showed that ACZ-NAM-2HP ternary co-crystal and the starting parent materials exhibited a few distinct spectral features in frequency-domain absorption spectra. Likewise, the Raman spectral result also shows some difference between the co-crystal and starting raw materials. Through density functional theory (DFT) calculations, the theoretical THz/Raman spectra and vibrational modes of two kind of possible ternary co-crystal theoretical forms (form I and II) between ACZ, NAM and 2HP were obtained. By comparing experimental and theoretical spectral results, the most suitable structure and vibrational modes of ACZ-NAM-2HP ternary co-crystal were determined. These results provide a wealth of information and unique method for studying molecular assembly and also inter-molecular interactions in specific ternary co-crystals at the molecular level in the emerging pharmaceutical co-crystal fields.

Force-detected nanoscale absorption spectroscopy in water at room temperature using an optical trap.

Measuring absorption spectra of single molecules presents a fundamental challenge for standard transmission-based instruments because of the inherently low signal relative to the large background of the excitation source. Here we demonstrate a new approach for performing absorption spectroscopy in solution using a force measurement to read out optical excitation at the nanoscale. The photoinduced force between model chromophores and an optically trapped gold nanoshell has been measured in water at room temperature. This photoinduced force is characterized as a function of wavelength to yield the force spectrum, which is shown to be correlated to the absorption spectrum for four model systems. The instrument constructed for these measurements combines an optical tweezer with frequency domain absorption spectroscopy over the 400-800 nm range. These measurements provide proof-of-principle experiments for force-detected nanoscale spectroscopies that operate under ambient chemical conditions.

Analysis of a systematic error appearing as a periodic fluctuation in the frequency-domain absorption spectra of metamaterial absorbers.

A metamaterial is a periodic structure which can be considered as an effective medium that exhibits an atypical response for a corresponding range of electromagnetic (EM) waves. For metamaterial absorbers (MMA) working in the GHz regime, the frequency-domain-measurement setup consisting of two horn antennas connected to a network analyzer is widely used. The absorption spectra obtained with this setup usually exhibits some periodic fluctuation (PF) across the entire range of measurement. Typically, this PF in the frequency-domain spectrum is ignored because it is distinct from major features and has a small amplitude. We have examined the PF through a finite-difference time-domain (FDTD) simulation in order to verify the validity of such assumption. We concluded that the PF is caused by a part of EM wave radiated by an antenna (source), and directly propagating to another (detector), resulting in an interference between this leaked EM wave and the one reflected from the sample. We have successfully reproduced the periodic fluctuation of the experimental spectra by FDTD simulation. We examined the angle of incidence dependence. The amplitude increases because the amount of leaked EM waves increases, while the periodicity increases because the distance between the two antennas increases, resulting in a shortened path difference. We conclude that the PF is a systematic error which can be safely ignored.

Review on Multi-scale Structural Design of Submarine Stealth Composite

Research on the acoustic stealth structures of submarines primarily strives for low frequency, pressure resistance, and broad frequency domain absorption. The high-quality acoustic stealth coating on submarines is a complicated Multi-scale and multi-component structure, which consists of numerous macro-structures, such as acoustic air-filled cavities, laminated structures, filling components, and micro-phase structures. However, more information regarding how these components with different anechoic mechanisms obtain good results through synergistic work and how their absorption frequencies superpose effectively to realize acoustic stealth in a broad frequency domain is necessary. Understanding these mechanisms is essential to design a novel anechoic structure in a broad frequency domain. Thus, in this paper, research concerning these mechanisms was reviewed. In addition, the design methods and anechoic mechanism of anechoic structures were discussed for different levels, and the primary concerns in this field were summarized. Several suggestions were made for further research.

An iterative re-weighted least-squares algorithm for the design of active abso- rbing wavemaker controller

In physical model tests for highly reflective structures, one often encounters a problem of multiple reflections between the reflective structures and the wavemaker. Absorbing wavemakers can cancel the re-reflective waves by adjusting the paddle motion. In this paper, we propose a method to design the controller of the 2-D absorbing wavemaker system in the wave flume. Based on the first-order wavemaker theory, a frequency domain absorption transfer function is derived. Its time realization can be obtained by de- signing an infinite impulse response (IIR) digital filter, which is expected to approximate the absorption transfer function in the least- squares sense. A commonly used approach to determine the parameters of the IIR filter is applying the Taylor expansion to linearize the filter formulation and solving the linear least-squares problem. However, the result is not optimal because the linearization cha- nges the original objective function. To improve the approximation performance, we propose an iterative reweighted least-squares (IRLS) algorithm and demonstrate that with the filters designed by this algorithm, the approximation errors can be reduced. Physical experiments are carried out with the designed controller. The results show that the system performs well for both regular and irregu- lar waves.

Attosecond transient absorption in dense gases: Exploring the interplay between resonant pulse propagation and laser-induced line-shape control

We investigate the evolution of extreme ultraviolet (XUV) spectral lineshapes in an optically-thick helium gas under near-infrared (IR) perturbation. In our experimental and theoretical work, we systematically vary the IR intensity, time-delay, gas density and IR polarization parameters to study lineshape modifications induced by collective interactions, in a regime beyond the single atom response of a thin, dilute gas. In both experiment and theory, we find that specific features in the frequency-domain absorption profile, and their evolution with propagation distance, can be attributed to the interplay between resonant attosecond pulse propagation and IR induced phase shifts. Our calculations show that this interplay also manifests itself in the time domain, with the IR pulse influencing the reshaping of the XUV pulse propagating in the resonant medium.

A piston-type active absorbing wavemaker system with delay compensation

Physical model tests with highly reflective structures often encounter a problem of multiple reflections between the structures and the wavemaker. This paper presents a piston-type active absorbing wavemaker system which can absorb most of the reflections. Based on the first-order wavemaker theory, a frequency domain absorption transfer function is modeled. Its time realization can be achieved by designing an IIR digital filter, which is used to control the absorbing wavemaker system. In a real system, time delays often exist in the wave making process. Thus a delay compensation term to the transfer function is proposed. Experimental results show that the system performs well for both regular and irregular waves with periods from 0.6 s to 2.0 s, and the absorption capability is larger than 96.5% at target wave fields.

Quantum focusing and coherent control of nonresonant two-photon absorption in frequency domain.

We theoretically investigate the nonresonant two-photon absorption (TPA) process in a two-level atom induced by a weak chirped pulse in the frequency domain. According to the extremum condition of the two-photon transition probability (TPTP) at the transition center frequency, we propose a Fresnel-inspired pulse tailoring scheme for TPA that is significantly different from that of Broers etal. [Phys. Rev. A46, 2749 (1992)]. Using this scheme, the TPTP can be focused or eliminated completely by constructively or destructively modulating various pathways of the quantum interference. Our results are a significant improvement on those obtained by Broers etal. and will have potential applications in selective two-photon microscopy and spectroscopy.

Nonperturbative vibrational energy relaxation effects on vibrational line shapes

A general formulation of nonperturbative quantum dynamics of solutes in a condensed phase is proposed to calculate linear and nonlinear vibrational line shapes. In the weak solute-solvent interaction limit, the temporal absorption profile can be approximately factorized into the population relaxation profile from the off-diagonal coupling and the pure-dephasing profile from the diagonal coupling. The strength of dissipation and the anharmonicity-induced dephasing rate are derived in Appendix A. The vibrational energy relaxation (VER) rate is negligible for slow solvent fluctuations, yet it does not justify the Markovian treatment of off-diagonal contributions to vibrational line shapes. Non-Markovian VER effects are manifested as asymmetric envelops in the temporal absorption profile, or equivalently as side bands in the frequency domain absorption spectrum. The side bands are solvent-induced multiple-photon effects which are absent in the Markovian VER treatment. Exact path integral calculations yield non-Lorentzian central peaks in absorption spectrum resulting from couplings between population relaxations of different vibrational states. These predictions cannot be reproduced by the perturbative or the Markovian approximations. For anharmonic potentials, the absorption spectrum shows asymmetric central peaks and the asymmetry increases with anharmonicity. At large anharmonicities, all the approximation schemes break down and a full nonperturbative path integral calculation that explicitly accounts for the exact VER effects is needed. A numerical analysis of the O-H stretch of HOD in D(2)O solvent reveals that the non-Markovian VER effects generate a small recurrence of the echo peak shift around 200 fs, which cannot be reproduced with a Markovian VER rate. In general, the nonperturbative and non-Markovian VER contributions have a stronger effect on nonlinear vibrational line shapes than on linear absorption.

Absorption spectra for a model light-harvesting system using non-Markovian theories

Different theories for the determination of the linear absorption are compared. As a test system we used a simplified B850 ring of bacteriochlorophylls within a light-harvesting complex of purple bacteria. Starting with a quantum master equation, the frequency-domain absorption spectra can be calculated within linear response. In these quantum master equations one often decouples the population dynamics from that of the coherences (secular approximation) and neglects memory effects (Markov approximation). The absorption spectra of the ring aggregate resulting from the Redfield theory with and without secular approximation are compared to those results from a time-local and a time-non-local non-Markovian theory.

Theory of linear absorption spectra of biological and non-biological chromophore complexes

Linear frequency-domain absorption spectra of chromophore complexes are studied within a Frenkel-exciton model including static and dynamic disorder. While static disorder is accounted for by explicite numerical ensemble averaging, dynamic disorder is described via the coupling of excitons to low-frequency vibrational modes (solvent or protein modes) and to high-frequency (intra-molecular) modes. Using a time-dependent formulation of the absorption density matrix theory can be applied. It is shown that the non-Markovian version of the Quantum Master Equation offers a certain approximation for the absorption based on a partial summation with respect to the exciton–vibrational coupling. In a first application the approach is used to describe absorption spectra of phenylacetylene dendrimers where static disorder is introduced to account for solvent induced deviations from the ideal structure. A detailed analysis of the long-wavelength tail underlines the dominance of structure fluctuations in the outer part of the dendrimer. While a concrete classification of the low-frequency modes remains impossible (according to the presence of static disorder) a qualitative estimate of the high-frequency modes becomes feasible. In a second application linear absorption of the photosynthetic antenna complex LHC-II of higher plants is analyzed and different structural parameters (Huang Rhys factor, inhomogeneous width of site energies, exciton state life times) are deduced. In particular the measured temperature dependence of the absorption spectrum can be well reproduced. A simultaneous fit of the circular dichroism spectrum at 77 K is used to discriminate between two different structural models.

Improvement of measurement precision in absorption spectra by apodization.

In in vivo MR spectroscopy studies, it is desirable to determine metabolite peak parameters with good precision and to estimate the uncertainties. How to achieve optimal precision for an apodized MR spectrum by using a weighted least squares estimator is discussed and demonstrated. Analytic methods for calculating the uncertainties of all parameters obtained from least squares estimators are described. The time domain apodization before Fourier transformation can substantially decrease the minimum variance bound in parameters obtained from the frequency domain absorption spectrum. A weighted least squares estimation is needed to reach the Cramer-Rao lower bound. When an unweighted least squares estimation is performed, the time domain apodization also leads to improvement in precision.

Dispersion versus absorption (DISPA) plots as an index of static magnetic field inhomogeneity. Use for adjustment of spinning shims in a superconducting magnet

A new method for evaluating static magnetic field inhomogeneity in an NMR sample is described. Deviation from circularity of a plot of dispersion vs absorption (DISPA) provides a measure of the deviation of an experimental NMR lineshape from the ideal Lorentz shape. From DISPA plots constructed from spectra simulated from specified z-gradient (“spinning shim”) inhomogeneity terms, it is clear that the magnitude and direction of displacement of an experimental DISPA curve from its reference circle can serve to discriminate and quantitate the inhomogeneity in each of z, z 2, z 3, z 4, and z 5. As an index for monitoring field inhomogeneity, DISPA is more sensitive than monitoring the absolute area under a free induction decay (particularly for higher order spinning shims), and much faster and more discriminating (between different z n terms) than iterative fitting of the frequency-domain absorption spectrum. Although each of the experimental spinning shim coils does not produce a z gradient corresponding exactly to its labeled z n function, we find that experimental DISPA plots nevertheless correlate well with their theoretical counterparts for z 1 and z 2. DISPA has already proved useful for automated phasing of FT NMR spectra; the present work suggests that DISPA may offer a simple, rapid, sensitive method for testing shim coil designs and ultimately for manual or automated shimming of the static magnetic field as well.

High efficiency up-conversion and real-time storage by two-fold degenerate four-wave mixing in BDN dye

Infrared to visible frequency up-conversion with efficiency as large as 377 percent and real-time storage of nanosecond pulses for a few tenths of a millisecond have been simultaneously achieved in a BDN organic dye solution in toluene. Two remarkable effects influencing the four-wave mixing (FWM) efficiency are found: (1)transient energy transfer between writing beams, and (2) the time matching between reading beam and the molecular relaxation time of medium. The full frequency-domain absorption characteristic and full time-domain storage characteristic of BDN have been studied in detail. The experimental results are identical to preliminary theoretical analysis.