Coherent two-dimensional terahertz-terahertz-Raman spectroscopy

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 (CHBr 3 ), carbon tetrachloride (CCl 4 ), and dibromodichloromethane (CBr 2 Cl 2 ) shows previously unobserved off-diagonal anharmonic coupling between thermally populated vibrational modes.ultrafast dynamics | terahertz | coherent multidimensional spectroscopy D etailed molecular pictures of the structure and dynamics of liquids drive our understanding of chemistry and biology. Nonlinear 2D infrared and NMR spectroscopies have revealed many specifics of liquid behavior, monitoring the coupling, spectral diffusion, and homogeneous linewidths of intramolecular vibrations and nuclear spins (1, 2). However, the motions that directly participate in solvation and chemical reactivity are manifest in the terahertz (THz) region of the spectrum, making 2D THz studies especially valuable. To date, no 2D technique has been demonstrated that incorporates multiple THz interactions with a liquid.Recent advances in pulsed, high-power THz sources with electric fields exceeding 100 kV/cm have enabled a new generation of nonlinear THz spectroscopy, in which THz radiation is used to both manipulate and record the response of matter (3). It is now possible, for example, to control the alignment of gasphase molecules (4) and antiferromagnetic spin waves (5), drive an insulator-to-metal transition in oxides such as VO 2 (6), and break up Cooper pairs in a superconductor with intense THz pulses (7). Nonlinear THz interactions have also enabled the first demonstrations of 2D THz spectroscopy in a double quantum well system and graphene (8, 9).With weak transition dipole moments yet high THz absorptivity, liquids present many challenges with respect to the development of 2D THz spectroscopy. Initial successes with 2D Raman spectroscopy were later shown to suffer from the interference of cascaded processes (10-12), but new schemes using optical pulse shaping have eliminated the cascaded contributions (13). An alternative approach is resonant 2D THz spectroscopy, analogous to 2D IR spectroscopy. However, this method is hindered by a lack of THz directional phase matching, leading to signals that can be easily overwhelmed by a strong linear background. In the last few years, hybrid optical-THz techniques that circumvent these challenges have emerged, including 2D Raman-THz spectroscopy and THz Kerr effect spectroscopy (14-16). Here, we present the complementary 2D TTR spectroscopy, a natural extension of these hybrid te...

2D THz-THz-Raman Photon-Echo Spectroscopy of Molecular Vibrations in Liquid Bromoform

“…From our previous linear data, 1 the ratio of the integrated intensities is I ν3 /I ν6 =1.1(1), in agreement with the literature values 7 I ν3 /I ν6 =1.2. Using our ratio and a temperature of 295 K, the dipole coupling ratio is calculated from…”

“…S2a. The acquisition time step is <1 fs for t 2 and 50 fs for t 1 . Additional data with a t 1 acquisition time step of 25 fs was taken on diamond to determine the instrument response ( for bromoform are -1.5 to 2.2 ps for t 1 and -0.5 to 5.0 ps for t 2 .…”

“…1 This isolates the vibrational coherences on the t 1 and t 2 axes ( Reduced Density Matrix Modeling The simulation fits of the 2D TTR spectrum were carried out with a reduced density matrix (RDM) model described in our previous work. 1 The time evolution of the density matrix ρ is determined by the Liouville-Von Neumann equation…”

Perspective: THz-driven nuclear dynamics from solids to molecules

“…That is, rather than taking the two THz field interactions from one and the same THz pulse, one applies two THz pulses separated by a second controllable time delay in addition to the time delay of the Raman-probe pulse. Both delay times are coherence times and thus may be 2D Fourier-transformed to reveal a 2D spectrum 136,137 . Furthermore, since this method involves both THz and Raman interactions, interchanging the time-ordering of these processes reveals variants of the method, and both the Raman-THz-THz and THz-Raman-THz pulse sequences have been explored as well 138,140 .…”

Nonlinear two-dimensional terahertz photon echo and rotational spectroscopy in the gas phase

“…However, it remains an experimental challenge to extend multidimensional spectroscopies into the terahertz (THz) frequency range where a rich variety of material degrees of freedom, including gas molecular rotations, lattice vibrations in solids, magnetization dynamics in magnetically ordered materials, and many others (6), find fundamental and technological importance. Despite challenges involved, there have been examples of 2D THz and 2D THz−Raman spectroscopies in the studies of electronic nonlinearities in solids (7,8), coupled vibrational modes in liquid molecules (9), and hydrogen bond dynamics in water (10).…”