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In an ultrafast nonlinear optical interaction, the electric field of the emitted nonlinear signal provides direct access to the induced nonlinear transient polarization or transient currents and thus carries signatures of ultrafast dynamics in a medium. Measurement of the electric field of such signals offers sensitive observables to track ultrafast electron dynamics in various systems. In this work, we resolve the real-time phase of the electric field of a femtosecond third-order nonlinear optical signal in the molecular frame. The electric field emitted from impulsively pre-aligned gas-phase molecules at room temperature, in a degenerate four-wave mixing scheme, is measured using a spectral interferometry technique. The nonlinear signal is measured around a rotational revival to extract its molecular-frame angle dependence from pump-probe time-delay scans. By comparing these measurements for two linear molecules, carbon dioxide and nitrogen, we show that the measured second-order phase parameter (temporal chirp) of the signal is sensitive to the valence electronic symmetry of the molecules, whereas the amplitude of the signal does not show such sensitivity. We compare measurements to theoretical calculations of the chirp observable in the molecular frame. This work is an important step towards using electric field measurements in nonlinear optical spectroscopy to study ultrafast dynamics of electronically excited molecules in the molecular frame.
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This content will become publicly available on March 21, 2026
A “hump” in the third-order dielectric response of a highly polar liquid: Now you see it, now you don’t
We have measured the linear and nonlinear dielectric responses of S-methoxypropylene carbonate, a highly polar glass-former, for which it has been reported that the “hump,” which is typical of third harmonic susceptibilities, disappears across a 5 K temperature change. To understand this unusual feature, we have measured the responses to high amplitude ac and dc electric fields at the fundamental frequency. The static limits of these results are entered into a model aimed at reproducing nonlinear dielectric susceptibility spectra using the concept of a fictive electric field. This model reproduces the “hump” in the third-harmonic response and its seeming disappearance. It is revealed that the “hump” is predominantly the result of reduced time constants, a consequence of the energy the sample absorbs from the electric field. At elevated temperatures, the “hump” only appears to vanish because its reduced amplitude submerges below the extraordinarily high level of polarization saturation of this liquid.
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- Award ID(s):
- 2153944
- PAR ID:
- 10598982
- Publisher / Repository:
- AIP Publishing
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 162
- Issue:
- 11
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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