Spectral Resolution Enhancement Research Articles

Insight into Resolution Enhancement in Generalized Two-Dimensional Correlation Spectroscopy

Generalized two-dimensional correlation spectroscopy (2D-COS) can be used to enhance spectral resolution in order to help differentiate highly overlapped spectral bands. Despite the numerous extensive 2D-COS investigations, the origin of the 2D spectral resolution enhancement mechanism(s) is not completely understood. In the work here, we studied the 2D-COS of simulated spectra in order to develop new insights into the dependence of 2D-COS spectral features on the overlapping band separations, their intensities and bandwidths, and their band intensity change rates. We found that the features in the 2D-COS maps that are derived from overlapping bands were determined by the spectral normalized half-intensities and the total intensity changes of the correlated bands. We identified the conditions required to resolve overlapping bands. In particular, 2D-COS peak resolution requires that the normalized half-intensities of a correlating band have amplitudes between the maxima and minima of the normalized half-intensities of the overlapping bands.

Enhancement of spectral resolution and accuracy in asynchronous-optical-sampling terahertz time-domain spectroscopy for low-pressure gas-phase analysis

The spectral resolution and accuracy of asynchronous-optical-sampling terahertz time-domain spectroscopy (ASOPS-THz-TDS) were evaluated by examining low-pressure gas-phase samples. Use of dual 56-MHz, erbium (Er)-doped, mode-locked femtosecond fiber lasers enhanced the spectral resolution to as low as 50.5 MHz and the spectral accuracy to as low as 6.2 × 10(-6). The results indicate that ASOPS-THz-TDS has the potential to achieve high spectral resolution, high spectral accuracy, and wide spectral coverage at the same time. ASOPS-THz-TDS will open a new door to gas-phase spectroscopy of multiple chemical species in the field of atmospheric gas analysis.

Enhancement of spectral resolution and optical rejection ratio of Brillouin optical spectral analysis using polarization pulling

High-resolution, wide-bandwidth optical spectrum analysis is essential to the measuring and monitoring of advanced optical, millimeter-wave, and terahertz communication systems, sensing applications and device characterization. One category of high-resolution spectrum analyzers reconstructs the power spectral density of a signal under test by scanning a Brillouin gain line across its spectral extent. In this work, we enhance both the resolution and the optical rejection ratio of such Brillouin-based spectrometers using a combination of two techniques. First, two Brillouin loss lines are superimposed upon a central Brillouin gain to reduce its bandwidth. Second, the vector attributes of stimulated Brillouin scattering amplification in standard, weakly birefringent fibers are used to change the signal state of polarization, and a judiciously aligned output polarizer discriminates between amplified and un-amplified spectral contents. A frequency resolution of 3 MHz, or eight orders of magnitude below the central optical frequency, is experimentally demonstrated. In addition, a weak spectral component is resolved in the presence of a strong adjacent signal, which is 30 dB stronger and detuned by only 60 MHz. The measurement method involves low-bandwidth direct detection, and does not require heterodyne beating. The measurement range of the proposed method is scalable to cover the C + L bands, depending on the tunable pump source. The accuracy of the measurements requires that the pump frequencies are well calibrated.

Simultaneous Spectral Resolution and Sensitivity Enhancement in MR spectrum: Maximum Likelihood Deconvolution Reconstruction

Although the use of apodization functions in connection with postprocessing of a 2D NMR spectrum proves improved spectral quality, there is usually a trade-off between resolution enhancement and noise suppression due to a classical uncertainty principle. In this study, therefore, a mathematical deconvolution technique called Maximum Likelihood Deconvolution (MLD) was adopted to achieve the spectral resolution and sensitivity enhancement simultaneously. The MLD technique greatly facilitates visualization and restoration of the genuine spectral information from complex 2D NMR spectra that would be problematic with the conventional apodization/FT processing. In particular, application of the MLD to the 2D-NOE spectrum would be very useful to derive the important proton connectivities, which are essential to achieve elucidating the 3D molecular structure.

Spectral resolution enhancement without increasing the number of grooves in grating-based spectrometers

A simple scheme of an optical setup to enhance the spectral resolution of diffraction gratings without increasing the number of grooves is presented. A plane-parallel mirror pair, which consists of a 100% reflectivity mirror and a 50% reflectivity mirror, is placed upon a grating. For certain values of the mirror gap and the angle between the grating and the mirrors, the effective size of the grating doubles, as does the resolution. Using a grating of 830 grooves/mm and a narrow-band diode laser that is tunable around 773 nm, we experimentally demonstrated an enhancement of the resolution by a factor of 2.

Optimally chirped CARS using fiber stretchers

High spectral resolution coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy are demonstrated with femtosecond laser systems. We perform optimal chirping in glass fibers and demonstrate a spectral resolution enhancement to better than 26 cm−1, which is limited by the bandwidth of the measured resonances. Considering the convolution with the resonance bandwidth this corresponds to a spectral resolution of approximately 2.5 cm−1, which is an enhancement by a factor of 165 with respect to the use of bandwidth-limited pulses. In microscopic imaging, a water background suppression of 81.5% is achieved.

Fast and High‐Resolution Stereochemical Analysis by Nonuniform Sampling and Covariance Processing of Anisotropic Natural Abundance 2D 2H NMR Datasets

Natural abundance deuterium (NAD) 2D NMR spectroscopy using chiral or achiral liquid crystals is an efficient analytical tool for the stereochemical analysis of enantio- or diastereomers by the virtue of proton-to-deuterium substitution. In particular, it allows the measurement of enantiopurity of organic synthetic molecules or the determination of the natural isotopic (1)H/(2)H fractionation in biological molecules, such as fatty acid methyl esters (FAME). So far, the NAD 2D spectra of solutes were acquired by using uniform sampling (US) and processed by conventional 2D Fourier transform (FT), which could result in long measurement times for medium-sized analytes or low solute concentrations. Herein, we demonstrate that this conventional approach can be advantageously replaced by nonuniform sampling (NUS) processed by covariance (Cov) transform. This original spectral reconstruction provides a significant enhancement of spectral resolution, as well as a reduction of measurement times. The application of Cov to NUS data has required the introduction of a regularization procedure in the time domain for the indirect dimension. The analytical potential of combining Cov and NUS is demonstrated by measuring the enantiomeric excess of a scalemic mixture of 2-ethyloxirane and by determining the diastereomeric excess of methyl vernoleate, a natural FAME. These two organic compounds were aligned in a polypeptide (poly(γ-benzyl-L-glutamate)) mesophase. In the case of NAD 2D NMR spectroscopy, we show that Cov and NUS methods allow a decrease in measurement time by a factor of two compared with Cov applied to US data and a factor of four compared with FT applied to US data.

Fluorescent microscopy using localized excitation source with gold nanotriangles: A computational study

A new method for fluorescent microscopy has been proposed. Proposed method uses indirect excitation of fluorophores with nanometer localized illuminating source. Localized source is created at corners of gold nanotriangles which are deposited on glass substrate. Actually the combination of gold nanotriangle (deposited on glass) acts as active substrate (where species will be placed) for our proposed method. The structure will be scanned with a focused beam of laser (or combination of beams). Due to electric field enhancement in corners and edges of nanotringle (because of surface plasmons), third order nonlinear effect will be enhanced accordingly. Enhancement in third order nonlinearity will give the chance to third harmonic generation and four wave mixing at hot spots. Enhancement in nonlinear process will give the chance of localized source creation for excitation of fluorophores in near field region. In order to prove our claim, we have numerically analyzed interaction of light with gold nanotriangle deposited on glass substrate. Our method is based on solving Maxwell's equation in three dimensions, considering dispersion behaviour of permittivity and third order nonlinear susceptibility by nonlinear finite difference time domain method. Simulation results show that generated light is highly localized and its wavelength can be tuned by changing the excitation wavelength. As a result of omitting background noise and space dependency of excitation source, enhancement in spectral and spatial resolution of imaging system will result.

Deconvolution in Acousto-Optical Tunable Filter Spectrometry

Spectrometers and spectral imaging systems based on the acousto-optical tunable filter (AOTF) are becoming commonly used in many different fields in which high spectral resolution is crucial, e.g., laser-induced breakdown spectroscopy, Raman spectroscopy, and absorption spectroscopy of gases. As AOTFs have many advantages over other spectroscopic instruments but lack spectral resolution, a procedure for resolution enhancement, composed of point spread function characterization and spectrum preprocessing and deconvolution, is proposed. Wiener, Fourier-wavelet regularized (ForWaRD), Richardson-Lucy, and Wavelet-Lucy deconvolution methods were tested and their performances assessed with two deconvolution quality measures: resolution enhancement and noise amplification. It was shown that the proposed spectral resolution enhancement is feasible and gives good results for line spectra and highly dynamic spectra.

High-speed off-axis Cavity Ring-Down Spectroscopy with a re-entrant configuration for spectral resolution enhancement

Monitoring of changing samples by Cavity Ring-Down Spectroscopy (CRDS) is possible using fast frequency scans of the laser and/or the cavity resonance. Mode-matched cavity excitation improves performance of fast CRDS but data-points result separated by the cavity Free Spectral Range (FSR): low pressure samples demand long cavities. We demonstrate fast CRDS with off-axis injection of a "re-entrant" resonator yielding FSR/N data-points separation. Our N = 4 short-cavity setup is found to perform well compared with other fast-CRDS implementations. Interestingly, the intrinsic chirped ringing affecting ring-down signals in mode-matched fast-CRDS disappear with off-axis injection. This is due to a fine splitting of the re-entrant-cavity degenerate groups of modes by astigmatism.

Quantitative Classification of Two-Dimensional Correlation Spectra

Two-dimensional (2D) correlation spectroscopy, which takes advantage of the apparent enhancement of spectral resolution, is known to be useful in qualitative discrimination of seemingly similar samples. The possibility of quantitative classification of 2D correlation spectra is even more desirable. Two useful parameters, namely Euclidian distance and correlation coefficient between 2D correlation spectra, are introduced for this purpose. Dry and sweet red wine samples are used to demonstrate the utility of these parameters. The distances between the 2D infrared (IR) spectra of sweet and dry red wines are roughly proportional to the differences of sugar contents in them. The result shows that the two parameters are useful measures for the quantitative evaluation of the similarity among the samples and their corresponding 2D correlation spectra.

Linear prediction of exponential decaying sinusoidal superposition signal

In spectral resolution enhancement the linear prediction of exponentially decaying sinusoidal superposition signal is one of the key technologies. In this study, the direct calculating equations of forward and backward linear prediction coefficients, the relationship between forward and backward linear prediction coefficients, and the forward and backward linear predictability conditions are derived. It is shown that, for exponential decaying sinusoidal superposition signal, there is not simple conjugate symmetry between forward and backward linear prediction coefficients, but they are still related. This research provides a theoretical basis for spectral resolution enhancement about decaying interference signals.

The application of two-dimensional fluorescence correlation spectroscopy on the interaction between bovine serum albumin and prulifloxacin

The interaction between bovine serum albumin (BSA) and prulifloxacin was investigated by ultraviolet spectrophotometer (UV) and fluorescence spectroscopy in this paper. Two-dimensional (2D) correlation spectroscopy was applied to the analysis of fluorescence spectra. The results of spectroscopic measurements suggested that prulifloxacin (PL) have a strong ability to quench the intrinsic fluorescence of bovine serum albumin through static quenching procedure. Thermodynamic parameter enthalpy changes (ΔH) and entropy changes (ΔS) were calculated. Owing to the spectral resolution enhancement in 2D correlation spectroscopy, the structure change of prulifloxacin can be observed.

The application of two-dimensional fluorescence correlation spectroscopy on the interaction between bovine serum albumin and prulifloxacin

The interaction between bovine serum albumin (BSA) and prulifloxacin was investigated by ultraviolet spectropho- tometer (UV) and fluorescence spectroscopy in this paper. Two-dimensional (2D) correlation spectroscopy was applied to the analysis of fluorescence spectra. The results of spectroscopic measurements suggested that prulifloxacin (PL) have a strong ability to quench the intrinsic fluorescence of bovine serum albumin through static quenching procedure. Thermodynamic pa- rameter enthalpy changes (ΔH) and entropy changes (ΔS) were calculated. Owing to the spectral resolution enhancement in 2D correlation spectroscopy, the structure change of prulifloxacin can be observed.

The Application of Two-dimensional Fluorescence Correlation Spectroscopy on the Interaction Between Bovine Serum Albumin and Paeonolum in the Presence of Fe(III)

ABSTRACT The effect of Fe3+ on the interaction between bovine serum albumin (BSA) and paeonolum (PAL), which was extracted from the traditional Chinese herb, Paeonia suffruticosa Andr, was investigated by UV and fluorescence spectroscopy. Two-dimensional correlation spectroscopy was applied to the analysis of fluorescence spectra. The results of spectroscopic measurements suggested that PAL had a strong ability to quench the intrinsic fluorescence of BSA through static quenching procedure in the presence of Fe(III). Thermodynamic parameter enthalpy changes (ΔH) and entropy changes (ΔS) were calculated. The binding parameters including binding constant (K), and the distance (r) between PAL and BSA were evaluated on the basis of the theory of Föster energy transfer. Owing to the spectral resolution enhancement in 2D correlation spectroscopy, the structure change of PAL–Fe3+ can be observed.

Concerning the spectral resolution enhancement of dynamic 2D-IR

A dynamic two-dimensional infrared (2D-IR) spectroscopic study of a hydrogen bonded polymer blend, poly(vinyl phenol) (PVPh)/poly(methyl acrylate) (PMA), in the C O stretching range was conducted to investigate one of the major features of this technique, i.e., spectral resolution enhancement. It was found that peak positions in the in-phase dynamic spectrum are different from the ones in the traditional Fourier transform infrared spectrometry (FT-IR) spectrum, which is a result of the subtraction procedure to obtain the former. In addition, the intensities of quadrature spectra are extremely low, which are easily affected by noise or interference, etc. Dynamic 2D-IR plots constructed from such dynamic spectra may produce new features that are actually artifacts. Thus, new features revealed by dynamic 2D-IR spectroscopy have to be interpreted with extreme care. To improve the quality of dynamic FT-IR and thereby the dynamic 2D-IR spectra, new strategies must be sought.

Spectral resolution enhancement of acousto-optic tunable filter by double-filtering

The development of the large angular aperture noncollinear acousto-optic tunable filter (AOTF) is based on the parallel tangents momentum-matching condition. In this letter, we have introduced a special double-filtering method that leads to an enhancement of the spectral resolution. And the birefringence together with the rotatory property of the interaction material has been considered to ensure the accuracy of designing an AOTF. The principle and availability of double-filtering are discussed in detail. It is confirmed that double-filtering method is effective to enhance the spectral resolution on the condition of keeping the quality of imaging, which is significant in practical applications of imaging AOTF.

Thermal Characterization of Self-Assembled Monolayers of Dialkyl Disulfides Containing the Urea Moiety

Thermal behaviors of urea-containing dialkyl disulfides 4-(3-octadecylureido)phenyl disulfide (ODPD) and 3-(octadecylureido)ethane disulfide (ODED) and octadecyl disulfide (ODDS) self-assembled monolayers (SAMs) were investigated by infrared reflection-absorption spectroscopy (IRRAS). Among the SAMs, the alkyl chain of ODPD is thermally reversible in the temperature region between 30 and 138 degrees C, whereas alkyl chains of ODED and ODDS are irreversible. With regard to the thermal stability of alkyl chains, ODED is superior to ODPD and ODDS. It is considered that the good thermal reversibility of the ODPD SAM is due to the rigid phenyl ring and the good thermal stability of the ODED SAM is due to the flexible ethylene linker. Moreover, 2D correlation analysis provides an enhancement of spectral resolution in amide II and CH2 antisymmetric stretching bands and suggests from the comparison of sequences of spectral events of ODPD and ODED SAMs that the last reorientation of the phenyl ring in the ODPD SAM is responsible for the good thermal reversibility.

Spectral resolution enhancement by chemical shift imaging

Three-dimensional chemical shift imaging (3D CSI) with appropriate data postprocessing can be used as a tool to improve spectral resolution in samples where large susceptibility differences and limited shim capabilities prevent good sample shimming. Data postprocessing is reduced to the realignment of individual 3D voxel spectra. As a result, the line broadening due to the field inhomogeneity over the sample's volume is reduced to the broadening by inhomogeneity within individual voxels. We compared this method with the resolution enhancement by window multiplication. We demonstrated, theoretically and experimentally, that in the presence of large, lower-order gradients, 3D CSI achieves better resolution enhancement with smaller sensitivity losses. An application of the method to a simple biological system is presented as well.

The application of two‐dimensional correlation spectroscopy to surface and interfacial analysis

AbstractGeneralized two‐dimensional correlation spectroscopy (2D COS) is a modern analytical tool that is increasingly being used in the interpretation of complex spectra that have broad, multiply overlapped spectral features. Besides functioning as a spectral resolution–enhancement tool similar to curve‐fitting or deconvolution, 2D COS possesses an additional advantage in that it is capable of discerning temporal relationships between intensity variations within a set of spectra. Infrared reflection‐absorbance spectroscopy (IRRAS) at the air–water interface has been a very useful technique in studying the structure of biologically relevant molecules and the interaction between components of biological membranes in an environment close to what is found in nature. To better understand interfacial bio‐mimetic structure, 2D COS can be applied to the IRRAS spectra to analyze changes in infrared intensities as a function of an environmental perturbation such as a change in temperature or surface pressure. To overcome some of the inherent limitations of the generalized 2D method, model‐based approaches such as βν‐ and kν‐correlation analyses and the global 2D phase angle method were developed. This review discusses some of the recent applications of these novel methods to Langmuir monolayers of phospholipids and proteins at the air–water interface and Langmuir–Blodgett polymer films. Copyright © 2006 John Wiley & Sons, Ltd.