Spectral Center Frequency Research Articles

A terahertz broadband and narrowband switchable absorber based on joint modulation of vanadium dioxide and graphene surfaces

Abstract In this paper, a terahertz broadband and narrowband switchable absorber is proposed. The absorption performance tuning for both broadband and narrowband functions is realized based on the joint modulation of vanadium dioxide (VO2) and graphene surfaces. Concretely, while VO2 is in the metallic state, the absorber achieves broadband absorption function. The overall bandwidth of over 90% absorption is 4.04 THz corresponding to a relative bandwidth of 84%. Through regulating the conductivity of VO2, dynamic tuning of the absorption amplitude is obtained and the modulation depth is 96%. By manipulating the graphene Femi energy and VO2 conductivity simultaneously, dynamic tuning of the absorption bandwidth is realized. In particular, the spectral center frequency of broadband absorption remains stable without drifting during the tuning process. While VO2 is in the insulating state, the absorber achieves narrowband absorption function. Calculated results show that two separate perfect absorption peaks are formed, and the absorption amplitudes are 99.6% and 99.2% respectively. Through regulating the Fermi energy of graphene surface, the dynamic tuning of narrowband absorption frequency is realized. Compared with the ones reported in recent years, our absorber has the advantage on function realization, absorption characteristics and performance tuning.

Quantitative analysis of laser-generated ultrasonic wave characteristics and their correlation with grain size in polycrystalline materials

Abstract The quantitative relationship between nanosecond pulsed laser parameters and the characteristics of laser-generated ultrasonic waves in polycrystalline materials was evaluated. The high energy of the pulsed laser with a large irradiation spot simultaneously generated ultrasonic longitudinal and shear waves at the epicenter under the slight ablation regime. An optimized denoising technique based on wavelet thresholding and variational mode decomposition was applied to reduce noise in shear waves with a low signal-to-noise ratio. An approach for characterizing grain size was proposed using spectral center frequency ratio (SCFR) based on time-frequency analysis. The results have demonstrated that the generation regime of ultrasonic waves is not solely determined by the laser power density; even at high power densities, a high energy with a large spot can generate an ultrasonic waveform dominated by the thermoelastic effect. It is ascribed to the intensification of the thermoelastic effect with the proportional increase in laser irradiation spot area for a given laser power density. Furthermore, both longitudinal and shear wave SCFRs are linearly related to grain size in polycrystalline materials; however, the shear wave SCFR is more sensitive to finer-grained materials. This study holds great significance for evaluating metal material properties using laser ultrasound.

Quantitative study on isotope effect of rubidium clusters

Because of the difficulty in measuring the cluster isotope displacement and identifying its cause, the resonance dissociation spectra, the moment shift and Zeeman energy shift of isotope cluster <sup>87,85</sup>Rb<sub><i>n</i></sub> (<i>n</i> = 1, 2, 3, ··· , 13) are obtained by the combination of optical magnetic resonance and thermal dissociation techniques in this study. The quantitative calculation is carried out based on the conceptual model of the giant atom, and the results are in excellent agreement with the measured results, which shows that rubidium clusters can be analyzed as giant atoms. Furthermore, 5s electron shell level structures of the rubidium cluster <sup>87,85</sup>Rb<sub><i>n</i></sub> (<i>n</i> = 1, 2, 3, ··· , 92) are calculated by using Zeeman level interval model. It is found that the main order and step distance of the 5s electron shell structure are similar to those of 3s single electron shell structure of sodium cluster in spherical symmetry. It is confirmed that the structure of the 5s electron shell of the rubidium cluster is determined by the largest energy gap in total Zeeman levels and the characteristic peaks of odd and even alternating and anomalous magnetic moments of special numbers such as <i>n</i> = 2 are caused by the intrinsic properties of electrons and molecular structures. It is also found that <sup>87</sup>Rb<sub><i>n</i></sub> level shell structure and <sup>85</sup>Rb<sub><i>n</i></sub> level shell structure strictly conform to the ratio of 3/2 magnitude relationship, and that there are abnormal differences in spectral center frequency and broadening, which may be directly related to the <sup>85,87</sup>Rb nuclei close to the shell closure.

Spectro-temporal evolution of mode-locked lasing in fiber frequency-shifted feedback laser.

As a type of mode-locked fiber laser, fiber frequency-shifted feedback lasers (FSFLs) have been rarely studied regarding the buildup process of mode locking. We carried out an experimental investigation to show the unique characteristics of the process, such as the direct evolution of the Q switched mode-locked state to the mode-locked state, the unique phenomenon of spectral center frequency oscillation, evolution to a double-pulse state in harmonic mode locking, millisecond stabilization time, and exponentially decreasing change of relative phase per round trip. For the first time, to the best of our knowledge, the double-beam heterodyne method is applied to measure the evolution of laser spectra with pulse width in the nanosecond range. Understanding the unique buildup mechanism may aid in the engineering and application of FSFLs.

Crystal structure, phonon characteristic, and intrinsic properties of Sm(Mg1/2Sn1/2)O3 double perovskite ceramic

Sm(Mg1/2Sn1/2)O3(SMS) microwave dielectric ceramic was prepared by the traditional solid state synthesis method. The samples were tested by X-ray diffraction (XRD), vibrational spectra (Raman scattering and far-infrared reflectance spectroscopy). The Sm(Mg1/2Sn1/2)O3 perovskite is monoclinic with the space group P21/n proved by XRD. Raman vibrational modes were fitted and assigned by Lorentz function. To calculate the intrinsic properties, far-infrared spectra with seven active modes were fitted using a four-parameter semi-quantitative (FPSQ) model. The dielectric constant calculated according to FPSQ is similar to that obtained from the molecular polarization and the Clausius equation. The intrinsic loss has been determined by the relationship between the damping factor and the spectral center frequency, which is slightly larger than the value obtained in the four-parameter fitting, and indicates that A1g(Sm) Raman mode has a enormous effect on the dielectric loss. F3u (3) and F4u (4) have the greatest contribution to the dielectric constant and loss. Finally, the real and imaginary parts of permittivity were analyzed with Kramers-Kronig transformation.

Changes in the power spectrum of electrical signals in maize leaf induced by osmotic stress

To quantify the characteristics of the power spectrum of plant electrical signals, we defined the following concepts: spectral edge frequency (SEF), spectral center frequency (SCF), power index (PI) and power spectral entropy (PSE). These parameters were used to examine and quantify changes in the power spectrum of electrical signals in maize leaves under osmotic stress. In the absence of osmotic stress, the SEF of the electrical signal in maize leaves was approx. 0.2 Hz and the SCF was approx. 0.1 Hz. The electrical signal in maize leaves was mainly a slow wave signal with a frequency of 0–0.1 Hz. After 2 h osmotic stress, the SEF and SCF of the electrical signal increased to higher frequencies. The proportion of the fast wave frequency also increased to 0.1–0.2 Hz, resulting in a dramatic increase in PSE. We also found that the changes in PSE and SCF were significantly correlated during osmotic stress. We propose that the changes in the PSE and SCF in maize leaves can be used as a sensitive signal indicating water deficit in leaf cells under osmotic stress. Thus, measurement of SCF or PSE of electrical signals in maize leaves could be used to develop early warning and rapid diagnosis techniques for the water demands of plants.

Correction for nonstationarity and window broadening in Doppler spectrum estimation

The width of the time-varying spectrum of signals from the pulsed Doppler ultrasonic blood flow detector is overestimated as a result of window and nonstationarity broadening when analyzed using the conventional short-term Fourier transform (STFT). By deriving the broadening function width from a consideration of signal phase variation arising from the deterministic spectral center-frequency variation during each window, the increase In the root-mean-squared (RMS) spectral width can be calculated and the spectral width estimation corrected. Using this procedure on simulated Doppler signals of known characteristics, a significant reduction in spectral width estimation error can be demonstrated.

Processing of diagnostic ultrasound signals

Ultrasound signals reflected from the human liver contain significant information about its condition. Several techniques have been developed to process such signals. In this paper, two approaches are described for estimating the slope of the acoustic attenuation coefficient, denoted by β, which provides important cues about the condition of diffuse liver disease. The first is a nonparametric approach which determines the β value from the slope of log periodogram differences. The second is a parametric approach which requires a Gaussian-shaped spectrum and determines the β value from s shift in the spectral center-frequency. A zero-crossing count procedure allows the parametric approach to be implemented with minimal hardware and in real-time. Simulated and actual signals illustrate the two approaches.

High-resolution spectral analysis with arbitrary spectral centers and arbitrary spectral resolutions

Spectral decomposition via banks of narrowband filters is a classic method of analyzing broadband acoustic signals. FFT algorithms and hardware efficiently perform narrowband analysis but are limited to constant bandwidths at fixed spectral center frequencies. We develop a technique for processing FFT outputs to realize banks of narrowband filters for which spectral band centers and spectral bandwidths may be arbitrarily assigned. In particular we present spectral analyzer configurations with spectral centers uniformily spaced on a logarithmic scale and with bandwidths proportional to center frequencies (constant Q filters) or with bandwidths proportional to the square root of center frequencies (root-proportional filters for linear FM line tracking).