Abstract-Coherent two-dimensional terahertz-terahertz-Raman spectroscopy

1. Ian A. Finnerana, 
2. Ralph Welscha, 
3. Marco A. Allodia,1, 
4. Thomas F. Miller, IIIa, and 
5. Geoffrey A. Blakea,b,2

1. http://www.pnas.org/content/early/2016/06/02/1605631113.abstract

We present 2D terahertz-terahertz-Raman (2D TTR) spectroscopy, the first technique, to our knowledge, to interrogate a liquid with multiple pulses of terahertz (THz) light. This hybrid approach isolates nonlinear signatures in isotropic media, and is sensitive to the coupling and anharmonicity of thermally activated THz modes that play a central role in liquid-phase chemistry. Specifically, by varying the timing between two intense THz pulses, we control the orientational alignment of molecules in a liquid, and nonlinearly excite vibrational coherences. A comparison of experimental and simulated 2D TTR spectra of bromoform (CHBr3), carbon tetrachloride (CCl4), and dibromodichloromethane (CBr2Cl2) shows previously unobserved off-diagonal anharmonic coupling between thermally populated vibrational modes.

Abstract-Nonlinear terahertz coherent excitation of vibrational modes of liquids

                                     

Marco A. Allodi^1,a), Ian A. Finneran¹ and Geoffrey A. Blake^1,2,b)
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a) mallodi@uchicago.edu; Current address: Department of Chemistry, The Institute for 
b) gab@gps.caltech.edu
Biophysical Dynamics, and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637, USA.
J. Chem. Phys. 143, 234204 (2015); http://dx.doi.org/10.1063/1.4938165

http://scitation.aip.org/content/aip/journal/jcp/143/23/10.1063/1.4938165

We report the first coherent excitation of intramolecular vibrational modes via the nonlinear interaction of a TeraHertz (THz) light field with molecular liquids. A terahertz-terahertz-Raman pulse sequence prepares the coherences with a broadband, high-energy, (sub)picosecond terahertz pulse, that are then measured in a terahertz Kerr effect spectrometer via phase-sensitive, heterodyne detection with an optical pulse. The spectrometer reported here has broader terahertz frequency coverage, and an increased sensitivity relative to previously reported terahertz Kerr effect experiments. Vibrational coherences are observed in liquid diiodomethane at 3.66 THz (122 cm⁻¹), and in carbon tetrachloride at 6.50 THz (217 cm⁻¹), in exact agreement with literature values of those intramolecular modes. This work opens the door to 2D spectroscopies, nonlinear in terahertz field, that can study the dynamics of condensed-phase molecular systems, as well as coherent control at terahertz frequencies.

Abstract-Decade-Spanning High-Precision Terahertz Frequency Comb

Ian A. Finneran, Jacob T. Good, Daniel B. Holland, P. Brandon Carroll, Marco A. Allodi, and Geoffrey A. Blake

Phys. Rev. Lett. 114, 163902 – Published 21 April 2015

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.163902

The generation and detection of a decade-spanning terahertz (THz) frequency comb is reported using two Ti:sapphire femtosecond laser oscillators and asynchronous optical sampling THz time-domain spectroscopy. The comb extends from 0.15 to 2.4 THz, with a tooth spacing of 80 MHz, a linewidth of 3.7 kHz, and a fractional precision of 1.8×10−9. With time-domain detection of the comb, we measure three transitions of water vapor at 10 mTorr between 1–2 THz with an average Doppler-limited fractional accuracy of 6.1×10−8. Significant improvements in bandwidth, resolution, and sensitivity are possible with existing technologies.

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