Molecular Orientation and Alignment by Intense Single-Cycle THz Pulses

Intense single-cycle THz pulses resonantly interacting with molecular rotations are shown to induce field-free orientation and alignment under ambient conditions. We calculate and measure the degree of both orientation and alignment induced by the THz field in an OCS gas sample, and correlate between the two observables. The data presents the first observation of THz-induced molecular alignment in the gas phase.

Figure 1

Schematic representation of the two setups used in this work. An optical pulse was used to generate the single-cycle THz pulse that was passed through the gas cell to induce coherent rotational motion. Either the THz electro-optic (EO) readout pulse was generated for the measurement of the periodic FID emission owing to the induced molecular orientation, or the optical probe pulse was generated for measurement of the THz-induced birefringence due to molecular alignment. For the FID (birefringence) measurements the THz beam ($\mathrm{FWHM}\sim 1.5\mathrm{cm}$) was focused by $f=30\mathrm{cm}(f=15\mathrm{cm})$ off axis parabolic reflectors to create the dashed green (solid blue) THz spatial distributions, respectively. G, grating; LN, lithium niobate THz generation crystal; BS, beam splitter; P, polarizer; W, Wollaston prism; PD, photodetector; PBS, Pellicle beam splitter. (In order to switch between the two setups, we replace the last parabolic reflector and remove the first Pellicle, the $-45°$ P and the single PD).

Figure 2

(a) Electro-optic sampling of the FID emission from the OCS gas cell following the THz pulse. (b) Alignment-induced birefringence in 350 torr OCS at 300 K. (c) Numerical simulation results for orientation $⟨⟨\cos \theta ⟩⟩(t)$ induced by a single-cycle THz pulse. Two simulations are shown for THz pulses with the same pulse envelope (inset, black dotted curve) and with fields phase-shifted by 90° (inset, solid red and dashed blue curves). (d) Numerical simulation results for alignment $⟨⟨{cos}^2\theta ⟩⟩(t)$ induced by a single-cycle THz pulse. Simulations are shown for the same phase-shifted THz waveforms as in (c) and for OCS gas at 300 K. (e) Normalized rotational level ($J$ state) contribution to the coherent signal shown in (b). The calculated thermal rotational distribution at 300 K is depicted by the dashed curve.

Figure 3

(a) Time-dependent orientation and alignment induced in OCS at 300 K, calculated for a series of amplitudes of the driving field profile depicted in the inset of panel $b$. (b) By fitting the peak orientation and peak alignment we get their peak amplitude dependences on the driving field, as marked in the figure.