Abstract-Terahertz-magnetic-field induced ultrafast Faraday rotation of molecular liquids

Vasileios Balos, Genaro Bierhance, Martin Wolf, and Mohsen Sajadi

https://journals.aps.org/prl/accepted/fa073Y60Xd41a265164477f66ca727873f379d22d

Rotation of the plane of the polarization of light in the presence of a magnetic field, known as the Faraday rotation, is a consequence of the electromagnetic nature of light and has been utilized in many optical devices. Current efforts aim to realize the ultrafast Faraday rotation on a sub-picosecond time scale. To this end, the Faraday medium should allow an ultrafast process by which in the presence of an ultrashort intense magnetic field, the light polarization rotates. We meet the criteria by applying an intense single cycle THz magnetic-field to simple molecular liquids and demonstrate the rotation of the plane of polarization of an optical pulse traversing the liquids on a sub-picosecond time scale. The effect is attributed to the deflection of an optically induced instantaneous electric polarization under the influence the THz magnetic field. The resolved Faraday rotation scales linearly with the THz magnetic field and quadratically with the molecular polarizability.

Abstract-Anharmonic Coupling between Intermolecular Motions of Water Revealed by Terahertz Kerr Effect

Tobias Kampfrath, Martin Wolf, Mohsen Sajadi

(Submitted on 24 Jul 2017)

https://arxiv.org/abs/1707.07622

Formation of local molecular structures in liquid water is believed to have marked effect on the bulk properties of water, however, resolving such structural motives in an experiment is challenging. This challenge might be handled if the relevant low-frequency structural motion of the liquid is directly driven with an intense electromagnetic pulse. Here, we resonantly excite diffusive reorientational motions in water with intense terahertz pulses and measure the resulting transient optical birefringence. The observed response is shown to arise from a particular configuration, namely the restricted trans-lational motion of water molecules whose motions are predominantly orthogonal to the dipole moment of the excited neighboring water molecules. Accordingly, we estimate the strength of the anharmonic coupling between the rotational and the restricted translational degrees of freedom of water.

Abstract-The sign of the polarizability anisotropy of polar molecules is obtained faithfully from terahertz Kerr effect

Tobias Kampfrath, Martin Wolf, Mohsen Sajadi

(Submitted on 29 Jun 2017)

https://arxiv.org/abs/1706.09623

Optically heterodyned detected terahertz Kerr effect of gases of polar molecules is reported. Strikingly, the birefringence signal from fluoroform is found to have opposite polarity compared to water and acetonitrile. This behavior is a hallmark of the opposite sign of the polarizability anisotropy of these molecules. Resonant excitation of the rotational degrees of freedom of the molecules aligns their permanent dipoles along the terahertz electric field. This alignment is translated into an optical birefringence through the polarizability anisotropy of each molecule. Therefore, the resulting net signal scales with the polarizability anisotropy, whose sign is imprinted faithfully onto the transient birefringence signal.

Abstract-Transient birefringence of liquids induced by terahertz electric-field torque on permanent molecular dipoles

Mohsen Sajadi, Martin Wolf, Tobias Kampfrath,

http://www.nature.com/articles/ncomms14963

Collective low-frequency molecular motions have large impact on chemical reactions and structural relaxation in liquids. So far, these modes have mostly been accessed indirectly by off-resonant optical pulses. Here, we provide evidence that intense terahertz (THz) pulses can resonantly excite reorientational-librational modes of aprotic and strongly polar liquids through coupling to the permanent molecular dipole moments. We observe a significantly enhanced response because the transient optical birefringence is up to an order of magnitude higher than obtained with optical excitation. Frequency-dependent measurements and a simple analytical model indicate that the enhancement arises from resonantly driven librations and their coupling to reorientational motion, assisted by the pump field and/or a cage translational mode. Our results open up the path to applications such as efficient molecular alignment, enhanced transient Kerr signals and systematic resonant nonlinear THz spectroscopy of the coupling between intermolecular modes in liquids.

Abstract-Transient birefringence of liquids induced by terahertz electric-field torque on permanent molecular dipoles

Mohsen Sajadi, Martin Wolf, Tobias Kampfrath

(Submitted on 13 Oct 2016)

https://arxiv.org/abs/1610.04024

Microscopic understanding of low-frequency molecular motions in liquids has been a longstanding goal in soft-matter science. So far, such low-frequency motions have mostly been accessed indirectly by off-resonant optical pulses. A more direct approach would be to interrogate the dynamic structure of liquids with terahertz (THz) radiation. Here, we provide evidence that resonant excitation with intense THz pulses is capable of driving reorientational-librational modes of aprotic polar liquids through coupling to the permanent molecular dipole moments. We observe a hallmark of this enhanced coupling: a transient optical birefringence up to an order of magnitude higher than obtained with optical excitation. Our results open up the path to applications such as efficient molecular alignment and systematic study of the coupling of rotational motion to other collective motions in liquids.