Extending the fundamental imaging-depth limit of multi-photon microscopy by imaging with photo-activatable fluorophores

It is highly desirable to be able to optically probe biological activities deep inside live organisms. By employing a spatially confined excitation via a nonlinear transition, multiphoton fluorescence microscopy has become indispensable for imaging scattering samples. However, as the incident laser power drops exponentially with imaging depth due to scattering loss, the out-of-focus fluorescence eventually overwhelms the in-focal signal. The resulting loss of imaging contrast defines a fundamental imaging-depth limit, which cannot be overcome by increasing excitation intensity. Herein we propose to significantly extend this depth limit by multiphoton activation and imaging (MPAI) of photo-activatable fluorophores. The imaging contrast is drastically improved due to the created disparity of bright-dark quantum states in space. We demonstrate this new principle by both analytical theory and experiments on tissue phantoms labeled with synthetic caged fluorescein dye or genetically encodable photoactivatable GFP

Super-nonlinear fluorescence microscopy for high-contrast deep tissue imaging

Two-photon excited fluorescence microscopy (TPFM) offers the highest penetration depth with subcellular resolution in light microscopy, due to its unique advantage of nonlinear excitation. However, a fundamental imaging-depth limit, accompanied by a vanishing signal-to-background contrast, still exists for TPFM when imaging deep into scattering samples. Formally, the focusing depth, at which the in-focus signal and the out-of-focus background are equal to each other, is defined as the fundamental imaging-depth limit. To go beyond this imaging-depth limit of TPFM, we report a new class of super-nonlinear fluorescence microscopy for high-contrast deep tissue imaging, including multiphoton activation and imaging (MPAI) harnessing novel photo-activatable fluorophores, stimulated emission reduced fluorescence (SERF) microscopy by adding a weak laser beam for stimulated emission, and two-photon induced focal saturation imaging with preferential depletion of ground-state fluorophores at focus. The resulting image contrasts all exhibit a higher-order (third- or fourth- order) nonlinear signal dependence on laser intensity than that in the standard TPFM. Both the physical principles and the imaging demonstrations will be provided for each super-nonlinear microscopy. In all these techniques, the created super-nonlinearity significantly enhances the imaging contrast and concurrently extends the imaging depth-limit of TPFM. Conceptually different from conventional multiphoton processes mediated by virtual states, our strategy constitutes a new class of fluorescence microscopy where high-order nonlinearity is mediated by real population transfer

Leptogenesis and light scalar dark matter in a Lμ−LτL_{\mu}-L_{\tau} model

We discuss the possibility of light scalar dark matter in a $L_{\mu}-L_{\tau}$ model, in which the dark matter $\phi_{dm}$ carries $U(1)_{L_{\mu}-L_{\tau}}$ charge but it is a singlet in the Standard Model. We consider the case that the right-handed neutrinos not only generate baryon asymmetry but also are related with dark matter production. We assume that dark matter production mainly comes from scattering associated with a pair of right-handed neutrinos while other related processes are highly suppressed due to the tiny $U(1)_{L_{\mu}-L_{\tau}}$ charge of dark matter, and the dark matter relic density are generated via freeze-in mechanism. A feasible parameter space is considered and we found the correct dark matter relic density can be obtained without influencing the result of leptogenesis, and the allowed dark matter mass region is $[\rm 10^{-5}\ GeV,0.1\ GeV]$

The Relationship of Ultra-Low Permeability Sandstone Aspect Ratio With Porosity, Permeability

The ultra-low permeability sandstone reservoir has large aspect ratio which significantly influences the multi-phase percolation characteristic. The ratio could be accurately measured by rate-controlled mercury porosimetry, but the testing technology is expensive, time-consuming and core-contaminating. There is not a simple effective method to describe the aspect ratio. The pores of the ultra-low permeability sandstone are mainly connected by the very long narrow throats, which could be advantageously simulated by the compound capillary bundles model. The analytical expressions of porosity and permeability about major pore structure parameters are established based on the model for the tight porous media. After solving the two expressions, the relationship between aspect ratio and parameter combination of porosity, permeability is obtained for the ultra-low permeable sandstone. Then the relation is fitted in this article using many previous published rate-controlled mercury data on compact sandstone and the relevance is strong, which proves that aspect ratio of tight rock is able to be calculated with its porosity and permeability.Key words: Ultra-low permeability sandstone; Aspect ratio; Pore; Throat; Porosity; Permeabilit

Application of High Density Resistivity Method in Karst Exploration: A Case Study

During engineering and construction activities, water and mud burst, house and ground collapse, as well as other hazards often occur in places where karst develops, which can seriously threaten the safety of people\u27s life and property and limit the development of local society and economy. Therefore, it is a meaningful work to figure out the locations of karst development so that corresponding prevention measures could be taken in advance. In this study, a case was introduced by using high density resistivity method to study the karst ground collapse. The geological characteristics, distribution law and control factors of karst ground collapse were clarified through high density resistivity method. Based on the results, the development trend was predicted, and the corresponding treatment measures and suggestions were proposed. The results show that 2 karst developing belts and 3 karst seriously-developed centers were delineated in the study region, which indicates that the high density resistivity method can effectively identify abnormal underground areas in the study region. The results shown in this research would provide the whole site for future drilling and the useful experience for underground karst exploration in similar areas