Echo phenomenon is ubiquitous in nature, with the most common example of an acoustic wave reflected by the rocks: the sound echo. Echo effects have been observed in various domains of physics, ranging from the famous nuclear spin echo in NMR spectroscopy and imaging , and its optical analogue of photon echo , and even the electron-beam echo in particle-accelerator and synchrotron radiation . Recently a new kind of echoes was discovered in the rotational motion of molecules: the rotational alignment echoes, with a series of interesting physics and fascinating applications such as fractional, imaginary and rotated echoes, and the echo-enabled rephasing of molecular centrifugal distortions [4-6].
In this report, I will introduce our recent works in utilizing rotational echoes to measure ultrafast collisional dissipation in high-pressure gases . The formation of rotational echoes is first explained by a classical phase-space model with emphasis about the complex dependence of rotational echoes on the two excitation laser pulses, which is in sharp contrast to the spin/photon echoes observed in inhomogeneously broadened two-level systems. By a smart control of the second laser pulse, the rotational echoes are successfully applied to measure the ultrafast collisional dissipation in high-pressure CO2 gases and CO2-He gas mixtures until 50 bar. The extracted decay rates corroborate the dominance of inelastic collisions during the collisional dissipation of molecules, and a natural and tempting extension of the present method would thus be to carry similar investigations in the liquid phase for which any rotational information or coherence imprinted in the system has vanished due to very quick thermalization at time scales of a few ps.
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張海粟，2014年8月博士畢業于中科院上海光機所強光實驗室，隨后于2015-2016年在希臘克里特的先進技術研究院(Foundation for Research and Technology-Hellas) 從事飛秒激光直寫光子晶體的研究，2016年底開始在法國勃艮第大學(Université de Bourgogne)交叉學科實驗室從事博士后工作，主要研究方向為飛秒激光操控分子取向和多模光纖中四波混頻。目前研究興趣包括分子轉動回波現象(rotational echo)及其經典-量子對應，以及強場電離誘導產生空氣激光(Air-lasing)中分子轉動效應的理論模擬。