Modal Identification of Ultralow-Frequency Flexible Structures Based on Digital Image Correlation Method

Traditional modal testing has difficult accurately identifying the ultralow-frequency modes of flexible structures. Ultralow-frequency excitation and vibration signal acquisition are two main obstacles. Aiming at ultralow-frequency modal identification of flexible structures, a modal testing method based on Digital Image Correlation method and Eigensystem Realization Algorithm is proposed. Considering impulse and shaker excitation are difficult to make generate ultralow-frequency vibration of structures, the initial displacement is applied to the structure for excitation. The ultralow-frequency accelerometer always has a large mass, which will change the dynamics performance of the flexible structure, so a structural vibration response was obtained through the Digital Image Correlation method. After collecting the free-decay vibration signal, the ultralow-frequency mode of the structure was identified by using the Eigensystem Realization Algorithm. Ground modal tests were conducted to verify the proposed method. Firstly, a solar wing structure was adopted, from which it was concluded that the signal acquisition using Digital Image Correlation method had high feasibility and accuracy. Secondly, an ultralow-frequency flexible cantilever beam structure which had the theoretical solution was employed to verify the proposed method and the theoretical fundamental frequency of the structure was 0.185 Hz. Results show that the Digital Image Correlation method can effectively measure the response signal of the ultralow-frequency flexible structure, and obtain the dynamics characteristics.

Phonon-assisted electronic states modulation of few-layer PdSe2 at terahertz frequencies

Information technology demands high-speed optoelectronic devices, but going beyond the one terahertz (THz) barrier is challenging due to the difficulties associated with generating, detecting, and processing high-frequency signals. Here, we show that femtosecond-laser-driven phonons can be utilized to coherently manipulate the excitonic properties of semiconductors at THz frequencies. The precise control of the pump and subsequent time-delayed broadband probe pulses enables the simultaneous generation and detection processes of both periodic lattice vibrations and their couplings with electronic states. Combining ultralow frequency Raman spectroscopy with first-principles calculations, we identify the unique phonon mode-selective and probe-energy dependent features of electron–phonon interactions in layered PdSe2. Two distinctive types of coherent phonon excitations could couple preferentially to different types of electronic excitations: the intralayer (4.3 THz) mode to carriers and the interlayer (0.35 THz) mode to excitons. This work provides new insights to understand the excited-state phonon interactions of 2D materials and to achieve future applications of optoelectronic devices operating at THz frequencies.

ON OBSERVATION OF ULF ELECTROMAGNETIC SIGNALS FROM REMOTE EARTHQUAKES AND DISTRIBUTION OF THEIR SOURCES ON THE EARTH’S SURFACE

During long-term observations, the Borok and College Geophysical Observatories have registered ultralow-frequency (ULF) electromagnetic signals from remote earthquakes. We have analysed the characteristics of such signals that occur several minutes before a seismic event. Our analysis shows that the dynamic spectra of the signals from earthquakes that occurred in different regions are similar, although the earthquakes differ in magnitude and focal depth. We investigate and discuss daily and seasonal probabilities for the occurrence of ULF electromagnetic pulses. Attention is given to the uneven distribution of their sources (i.e. earthquakes) on the earth’s surface. Our study shows that the ULF electromagnetic signals are clustered in separate zones and cells. When mapped, these clusters mark seismic electromagnetically active regions. In the northern hemisphere, a maximum cluster is found at latitudes 30–45°. In the longitudinal direction, two maximum clusters are located in the western sector. They are considered as the major and additional peaks (latitudes 120–150° and 0–30°, respectively). Examples are given to illustrate earthquake precursors in various regions. Based on the analysis results, we conclude that the occurrence of ULF electromagnetic pulses before earthquakes is universal. These pulses need to be investigated in a more detail to clarify if an upcoming earthquake is detectable from such signals a few minutes before its occurrence, and whether it is possible, in principle, to use this information for safety alerts before seismic shaking arrives.