1. Ultrafast Terahertz Kerr Effect Spectroscopy: Detection of Intramolecular Vibrational Coherences Marco Allodi, Ian Finneran, Geoffrey Blake California Institute of Technology

2. Nonlinear THz spectroscopy is very difficult! Electronics Lasers (Photonics) Sizov, Fedir F., et al. Optical Engineering 52.3 (2013)

3. Multi-dimensional NMR 13 C 1H1H 15 N 1H1H 2D-IR and optical experiments 2D Spectroscopies are nonlinear

4. Milestones Necessary for 2D THz Spectroscopy Generate Coherences Manipulate Coherences Detect Coherences

5. Terahertz spectra are complicated, and sensitive to intermolecular structure McGuire et al, in preparation. Brett McGuire RI14

6. Teraherz Kerr Effect Hoffmann, M. C. et al. Appl. Phys. Lett., 2009, 95, 231105. n n = n + Δn E The only molecular, ultrafast, nonlinear THz spectroscopy

7. The Optical Kerr Effect: Precursor to the Terahertz Kerr Effect Δn ∝ E 2 n n = n + Δn E Zhong, Qin, and John T. Fourkas. "Optical Kerr Effect Spectroscopy of Simple Liquids†." The Journal of Physical Chemistry B 112.49 (2008): 15529-15539.

8. REGENERATIVE AMPLIFIER (Coherent Legend) Ti:S 800 nm 1 kHz >4 mJ 35 fs fs Ti:Sa OSCILLATOR Ti:S 80 MHz ~ 5 nJ B.S. 85:15 DETECTION DELAY LINE DETECTION PHOTODIODE λ/4 WOLLASTON PRISM OPA Signal 1450 nm 400 μJ SAMPLE OPTICAL BEAM BLOCK = 1450 nm = 800 nm = THz LENS POL DSTMS THz EMITTER THz Kerr Effect - Experimental Setup Allodi, Finneran, & Blake, 2015 submitted.

9. Heterdyne detection improves sensitivity DETECTION PHOTODIODES λ/ 4 WOLLASTON PRISM SAMPLE OPTICAL BEAM BLOCK LENS POL DSTMS THz EMITTER WIRE-GRID POLARIZER Image Credits: Rainbowphotonics.com Allodi, Finneran, & Blake, 2015 submitted. λ/ 2

10. Heterodyne-Detected THz Kerr Effect Signal!

11. Cuvette Response Allodi, Finneran, & Blake, 2015 submitted.

12. Intramolecular Vibrational Oscillations Oscillations in CH 2 I 2 match perfectly the 122 cm -1 ν 4 mode. Allodi, Finneran, & Blake, 2015, submitted.

13. Vibrational excitation is possible at higher frequencies: CCl 4

14. Two field interactions allow excitation above 3 THz (100 microns) CH 2 I 2 CCl 4 Cuvette material is fused silica (Suprasil)

15. The Optical Kerr Effect is Nonresonant 0 THz time 7 THz

16. There are many overdamped intermolecular vibrations at 3 THz! 0 THz 3 THz 6 THz time

17. Milestones Necessary for 2D THz Spectroscopy ✔ Generate Coherences ? Manipulate Coherences ✔ Detect Coherences

18. Separate pulses will differentiate the excitation process 0 THz 3 THz 6 THz

19. Conclusions & Future Directions Heterodyne detection improves the sensitivity and allows phase-sensitive detection. First nonlinear resonant terahertz vibrational coherences in a molecular liquid. Multi-pulse schemes will reveal more information.

20. Acknowledgments The Blake Group NSF