Terahertz field-induced second harmonic generation (TFISH) is a technique for optical detection of broad-band THz fields. We show that by placing an iris at the interaction volume of the THz and optical fields, the TFISH signal increases by few ten-fold in atmospheric air. The iris-assisted TFISH amplification is characterized at varying air pressures and probe intensities and provides an elegant platform for studying nonlinear phase-matching in the gas phase. _____________________________________________________________________________________________________________________________ Introduction:Terahertz fields with frequencies 10 11 -10 13 Hz have become table-top available in the last two decades and are widely utilized in various fields of research and technology [1][2][3][4]. THz spectroscopy is typically performed in timedomain spectrometers consisting of a THz source (photoconductive antenna [5] / two-color plasmas [6] / rectification in a non-linear crystal [7][8][9] and recently also in surfaces patterned with metamaterials [10,11]) and a time-resolved THz detection module such as photoconductive antenna receiver, electro-optic sampling [12,13] or TFISH [14,15]. The latter, termed and utilized by Cook et.al. for studying THz-induced dynamics of liquid water [14] was explored and developed extensively by Zhang and company to provide optical characterization of broad-band THz fields in ambient air and various gasses [16][17][18][19]. Later on, the same group developed the air-biased-coherentdetection technique (ABCD) [20] for heterodyne detection of broad-band THz fields. The development and optimization of TFISH as a viable method for THz detection yielded various observations such as the effect of Gouy phase of both the THz and the optical probe [21], effects of different gas molecules [22] and gas densities [23], TFISH dependence on the optical probe intensity [17] and many others [24]. The non-linear optical effect that governs TFISH is the mixing of three input fields -a THz field ( ) and two optical fields ( 1 , 2 ) via the 3rd order susceptibility (3) to yield a signal field at the frequency of =

We demonstrate and explore an all-optical technique for direct monitoring of the orientation dynamics in gas-phase molecular ensembles. The technique termed “MOISH” utilizes the transiently lifted inversion symmetry of polar gas media and provides a sensitive and spatially localized probing of the second-harmonic generation signal that is directly correlated with the orientation of the gas. Our experimental results reveal selective electronic and nuclear dynamical contributions to the overall nonlinear optical signal and decipher them apart using the “reporter gas” approach. “MOISH” provides new crucial means for implementing advanced coherent rotational control via concerted excitation by both terahertz and optical fields.

A new and efficient method for orientation echo spectroscopy is presented and realized experimentally. The excitation scheme utilizes concerted rotational excitations by both ultrashort terahertz and near-IR pulses and its all-optical detection is enabled by the molecular orientation-induced second harmonic method [J. Phys. Chem. A 126, 3732 (2022)10.1021/acs.jpca.2c03237]. This method provides practical means for orientation echo spectroscopy of gas phase molecules and highlights the intriguing underlying physics of coherent rotational dynamics induced by judiciously-orchestrated interactions with both resonant (terahertz) and nonresonant (NIR) fields.

light-induced orientation of gas phase molecules is a long-pursued goal in physics and chemistry. Here we experimentally demonstrate a 6-fold increase in the terahertz-induced orientation of iodomethane (CH3I) molecules at room temperature, provided by rotational pre-excitation with a moderately intense near-IR pulse. The paper highlights the underlying interference of multiple coherent transition pathways within the rotational coherence manifold and are analyzed accordingly. Our experimental and theoretical results provide desirable and practical means for all-optical experiments on oriented molecular ensembles.