Using two multiple-cycle optical pulses with incommensurate frequencies (e.g., at wavelengths 1400 nm and 800 nm) in a collinear configuration, a cascaded background-free four-wave mixing EUV field can be realized because the EUV pulse produced by phase-matched high order harmonic generation in combination with the other two optical pulses creates a third-order nonlinear polarisation which drives a phase-matched four-wave mixing process along the propagating direction. The four-wave mixing emission can be manipulated by varying the delay of the second optical pulse. The phase-matching of the four-wave mixing process together with the relatively long interaction path combine to produce a strong output signal which leads to an enhanced signal-noise ratio of the Fourier-transformed signal used to obtain the two-dimensional cross correlation spectrum. Some key features of this two-dimensional spectroscopy, such as the on-axis and off-axis peaks, can lead to the determination of interaction pathways of real dipole-allowed transitions and virtual transitions in the EUV.
We report the results of our long-term monitoring study of cirrus clouds using our locally developed lidar sytem located in Hanoi, Vietnam (21.03°N, 105.85°E). Our data, spanning the period from 2011 to 2017, show that the cirrus clouds above Hanoi are generally found between 6.7 and 18.0 km with a mean height of 14.0 km. The cloud thickness is estimated to vary between 0.2 and 7.0 km with an average thickness of 2.3 ± 1.2 km. We also derive an average optical depth of 0.32 ± 0.22 and an average lidar ratio 30 ± 18. We also compare the geometrical and optical properties of cirrus clouds above Hanoi, where until now lidar study is still lacking, with that obtained for other locations in Asia and around the world.
Two side-pumping schemes suitable for the development of an ultraviolet femtosecond amplifier system using a Ce3+:LiCaAlF6 crystal are reported. Firstly, a Bethune-type prismatic cell configuration that uniformly illuminates the four sides of a micro-pulling down method-grown crystal is used to amplify 290 nm, femtosecond pulses with no significant increase in pulse duration and B-integral. The second pumping scheme uses a two-side-pumped large crystal. These two side-pumping schemes can pave new possibilities for achieving high-energy ultraviolet femtosecond pulses.
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