Laser induced x-ray fluorescence were observed against laser polarization ellipticity. While emission from krypton peaks at linear polarization, a signature of recollision, emission from neon shows opposite trend. We attribute it to two competing processes.
We have generated soft X-ray pulses at the wavelength around 30 nm and 10 nm using high-order harmonic generation (HHG) in Ar and Ne gas targets respectively with low repetition rates, hundred-terawatt-level laser system in Shanghai Institute of Optics and Fine Mechanics (SIOM). The shortest wavelength of our generated harmonics is 9.8 nm at the 81th harmonic with the Ne gas target. Around the wavelength of 13 nm, the output energy of the 59th harmonic (13.5 nm) and the 61th harmonic (13.1 nm) reaches 10 nJ per pulse. This highly coherent extreme ultraviolet (XUV) source can be served as a potential seed for the free-electron laser (XFEL) with the method of laser wakefield acceleration (LWFA). Moreover, the 13 nm HHG source can be applied to coherent diffraction imaging (CDI) and XUV lithography.
The phase compensation of the negatively chirp of the attosecond pulse train is demonstrated experimentally. By adding
a weak second harmonic laser field, the phase compensation can be negatively chirp or positively chirp at different time
delay, which support a new way to generate the strong transform-limited attosecond pulse.
We present a new algorithm for camera calibration using two concentric circles, which is a linear approach. In the calibration, a pinhole camera model is used. Different from previous methods, we take the projective equations of 3-D circles, which include the intrinsic parameter matrix of the camera, as the basis of our calibration approach. According to the special structure of the projective equations in algebra, we can get a linear equation system about the intrinsic parameters. After enough equations are constructed, the calibration can be easily finished. With at least three images of the two concentric circles, all five intrinsic parameters can be recovered. Experiments using computer simulated data and real data demonstrate the robustness and accuracy of our method.
We present a camera calibration technique to determine the focal length and the extrinsic parameters of a camera merely by using one perspective view of two coplanar circles with arbitrary radii and topological configuration. We believe our effort is valuable, in that the position of a camera and its focal length are frequently adjusted in many vision applications. In contrast to the iterative optimization technique in previous work, whose convergence relies heavily on proper initialization, we propose a closed-form solution on the basis of simple matrix operations, which turns out to be computationally efficient. It can also be used to initialize existing algorithms for higher accuracy and speed. Our method is based on the projective equation of a circle, first presented here, which naturally encodes the intrinsic and extrinsic camera parameters and the degenerate conic envelope spanned by the image of circular points. Extensive experiments with both synthetic data and real images verify the efficiency and robustness of our technique.
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