Intense terahertz (THz) pulses with MV/cm peak-field amplitudes have become experimentally feasible with the advents in table-top THz generation methodologies. Such fields provide a new control-handle over the rotations of polar molecules in the gas phase, as they primarily interact with molecular rotors via their permanent dipole moments, differing yet complementing ultrashort optical pulses that interact with molecules via their polarizability tensor. When applied to linear molecules, optical pulses induce molecular ALIGNMENT while THz pulses induce ORIENTATION. In linear molecules both the dipole vector and the most polarizable axis coincide at the inter-atomic axis of the molecule, thus serving to rotate the molecules about the same axis. However, for asymmetric tops like SO2, the dipole and the most polarizable axes lie along different molecular frame axes, turning optical and THz fields as two distinct rotational handles. Well-orchestrated application of these pulses can provide complete three-dimensional control over the molecular angular distribution of such molecules - a long standing goal in molecular physics and chemistry.
In the first part of the talk I will present our recent experimental and theoretical results of optical induced alignment and THz induced orientation in gas phase SO2 molecules that highlight the different rotational dynamics induced by these two distinct rotational handles. On the theoretical front, simulating the rotational dynamics of asymmetric molecules like SO2 at room temperature remains a highly demanding computational task that is effectively impossible by exact methods for dynamics. We overcame this issue by employing the Random Phase Wave Functions method [1,2] that was also verified by experimental results .
In the second part I will present a decay phenomenon that was recently unveiled in by a series of time-resolved measurements of the rotational dynamics induced by an optical pulse and a THz-field. We have found that despite their exact same pressure and temperature, transiently oriented molecules decay at a faster rate than aligned molecules . This is attributed a coherent radiative decay mechanism that we believe is general to all resonantly induced dynamics, however has been discarded previously. I will present our recent experimental results and suggest a theoretical model that incorporates a coherent radiative term into the typical Hamiltonian of the problem.
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 S. Kallush, and S. Fleischer, Phys. Rev. A, 91, 063420 (2015).
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Sharly Fleischer, Ran Damari, Shimshon Kallush, and Dina Rosenberg, "Ultrashort optical pulses and single-cycle terahertz fields in the service of molecular rotations: a comparative study (Conference Presentation)," Proc. SPIE 10684, Nonlinear Optics and its Applications 2018, 106840M (Presented at SPIE Photonics Europe: April 24, 2018; Published: 23 May 2018); https://doi.org/10.1117/12.2306799.5788856819001.
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