76 research outputs found
Formation of dark excitons in monolayer transition metal dichalcogenides by a vortex beam: optical selection rules
Monolayer transition metal dichalcogenides host tightly-bound excitons, which
dominate their optoelectronic response even at room temperatures. Light beams
are often used to study these materials with the polarization - often termed as
the spin angular momentum of the light - providing the mechanism for exciting
excitonic states. Light beams, however, can also carry an orbital angular
momentum by creating helical structures of their phase front. In this work, we
consider a Laguerre-Gaussian beam possessing an orbital angular momentum in
addition to the spin angular momentum to create excitons in monolayer
transition metal dichalcogenides. We derive optical selection rules that govern
the allowed transitions to various exciton series using symmetry arguments. Our
symmetry considerations show that we can create dark excitons using these
high-order optical beams opening up new avenues for creating long-lived dark
excitons with the potential of exploiting them in quantum information
processing and storage
Dynamic response of elastic beam to a moving pulse: finite element analysis of critical velocity
Dynamic behaviour of a semi-infinite elastic beam subjected to a moving single sinusoidal pulse was investigated by using finite element method associated with dimensionless analysis. The typical features of the equivalent stress and beam deflection were presented. It is found that the average value of maximal equivalent stress in the beam reaches its maximum value when the velocity of moving pulse is closed to a critical velocity. The critical velocity decreases as the pulse duration increases. The material, structural and load parameters influencing the critical velocity were analysed. An empirical formula of the critical velocity with respect to the speed of elastic wave, the gyration radius of the cross-section and the pulse duration was obtained
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Imaging leukocyte trafficking in vivo with two-photon-excited endogenous tryptophan fluorescence
We describe a new method for imaging leukocytes in vivo by exciting the endogenous protein fluorescence in the ultraviolet (UV) spectral region where tryptophan is the major fluorophore. Two-photon excitation near 590 nm allows noninvasive optical sectioning through the epidermal cell layers into the dermis of mouse skin, where leukocytes can be observed by video-rate microscopy to interact dynamically with the dermal vascular endothelium. Inflammation significantly enhances leukocyte rolling, adhesion, and tissue infiltration. After exiting the vasculature, leukocytes continue to move actively in tissue as observed by time-lapse microscopy, and are distinguishable from resident autofluorescent cells that are not motile. Because the new method alleviates the need to introduce exogenous labels, it is potentially applicable for tracking leukocytes and monitoring inflammatory cellular reactions in humans
Pressure-induced superconductivity in topological type II Dirac semimetal NiTeâ‚‚
Very recently, NiTeâ‚‚ has been reported to be a type II Dirac semimetal with Dirac nodes near the Fermi surface. Furthermore, it is unveiled that NiTeâ‚‚ presents the Hall Effect, which is ascribed to orbital magnetoresistance. The physical properties behavior of NiTeâ‚‚ under high pressure attracts us. In this paper, we investigate the electrical properties of polycrystalline NiTeâ‚‚ by application of pressure ranging from 3.4GPa to 54.45Gpa. Superconductivity emerges at critical pressure 12GPa with a transition temperature of 3.7K, and Tc reaches its maximum, 6.4 K, at the pressure of 52.8GPa. Comparing with the superconductivity in MoP, we purposed the possibility of topological superconductivity in NiTeâ‚‚. Two superconductivity transitions are observed with pressure increasing in single crystal
Two-Photon-Excited Tryptophan Fluorescence Microscopy for Leukocytes and Cancer Cells Imaging
Cancer screening and early diagnosis is an important yet controversial issue due to the safety and practicality of methods used. Our objective is to study the efficiency of an in vivo two-photon microscope developed in our laboratories to monitor cell inflammation. First, leukocytes were separated by subpopulation. The tryptophan fluorescence intensity level of each type of leukocyte was then quantified with two-photon microscopy, in their naĂŻve and inflamed states, respectively. Finally the tryptophan fluorescence intensity of multiple myeloma cells was quantified and correlated to the resulting images. The cancerous tissue auto-fluorescence from NADH and FAD was also recorded as a control to determine the specificity of the technique. Comparison of the fluorescence of leukocytes and cancer cells has demonstrated the presence of tryptophan in different quantities per cell, thus offering the potential for distinguishing multiple myeloma cells from leukocytes in circulation and record multiple myeloma cell trafficking process. This is a significant advantage over spectroscopy techniques for safe in vivo imaging of cancer screening, since it can be applied without the need for labeling. It is potentially applicable for tracking leukocytes and monitoring inflammatory cellular reactions in humans
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