Extending the longitudinal range of plasma channels produced by femtosecond annular (ring) Gaussian beams in atmosphere is numerically investigated. We find that the length of a stable plasma channel induced by the annular Gaussian beam, comparing with a Gaussian beam under the same initial condition, has a significant improvement, which can be well understood from two aspects. The dynamic complementation caused by the optical system composing of an axicon and a diverging lens can extend the plasma channel greatly, and the special initial transverse distribution of ring-Gaussian pulse leads to pulse splitting and redistribution of the pulse energy before the formation of optical filament. The ultrashort ring-Gaussian beam perhaps offers a new and an efficient route towards the generation of extended optical filament.

Low-light image enhancement (LLIE) investigates how to improve the brightness of an image captured in illumination-insufficient environments. The majority of existing methods enhance low-light images in a global and uniform manner, without taking into account the semantic information of different regions. Consequently, a network may easily deviate from the original color of local regions. To address this issue, we propose a semantic-aware knowledge-guided framework (SKF) that can assist a low-light enhancement model in learning rich and diverse priors encapsulated in a semantic segmentation model. We concentrate on incorporating semantic knowledge from three key aspects: a semantic-aware embedding module that adaptively integrates semantic priors in feature representation space, a semantic-guided color histogram loss that preserves color consistency of various instances, and a semantic-guided adversarial loss that produces more natural textures by semantic priors. Our SKF is appealing in acting as a general framework in the LLIE task. We further present a refined framework SKF++ with two new techniques: (a) Extra convolutional branch for intra-class illumination and color recovery through extracting local information and (b) Equalization-based histogram transformation for contrast enhancement and high dynamic range adjustment. Extensive experiments on various benchmarks of LLIE task and other image processing tasks show that models equipped with the SKF/SKF++ significantly outperform the baselines and our SKF/SKF++ generalizes to different models and scenes well. Besides, the potential benefits of our method in face detection and semantic segmentation in low-light conditions are discussed.

We numerically investigate the effects of the external focusing and the pulse parameters on the propagation of the ring Gaussian filaments in air. The simulation results indicate that the onset distance of filament, the length and uniformity of the plasma strings, and the energy deposition strongly depend on these optical parameters. The length of optical filament can be extended greatly by adjusting the lens parameters near the maximum energy deposition. In addition, we find that, under the same initial intensity, the length and uniformity of the plasma strings can be tuned by increasing the beam width better than increasing the beam radius.

Lobster-eye optics is widely applied to space x-ray detection missions and x-ray security checks for its wide field of view and low weight. This paper presents a theoretical model to obtain spatial distribution of focusing efficiency based on lobster-eye optics in a soft x-ray wavelength. The calculations reveal the competition mechanism of contributions to the focusing efficiency between the geometrical parameters of lobster-eye optics and the reflectivity of the iridium film. In addition, the focusing efficiency image depending on x-ray wavelengths further explains the influence of different geometrical parameters of lobster-eye optics and different soft x-ray wavelengths on focusing efficiency. These results could be beneficial to optimize parameters of lobster-eye optics in order to realize maximum focusing efficiency.

In the present paper we investigate the orientation dependence of the nonsequential double ionization (NSDI) of a hydrogen molecule (H${}_2$) exposed to a strong laser field analytically within the strong-field approximation. Our calculations demonstrate that the NSDI yields can increase with the increase of the molecular alignment angle; i.e., the alignment dependence of the NSDI yields exhibit a reversed tendency compared to that of the single-ionization yields. This striking phenomenon is identified as a signal of quantum interference, which arises from the two-center structure of the diatomic molecule and can lead to the dramatic suppression of the NSDI rates at small alignment angles. Moreover, the interference effect can be altered by both laser intensity and internuclear distance. The above finding indicates that the two-center interference can affect NSDI yields dramatically in certain cases and therefore suggests a feasible way to observe the interference effect indirectly in NSDI experiments. DOI: http://dx.doi.org/10.1103/PhysRevA.86.033414 ©2012 American Physical Society