The Optical Heterodyne Detected - Optical Kerr Effect Method

The Optical Heterodyne-Detected Optical Kerr Effect (OHD-OKE) method is a nonresonant ultrafast technique that allows the orientational dynamics of a liquid to be tracked from the subpicosecond range to their completion, up to hundreds of microseconds later. This extraordinary time range is one of the many advantages of this method. The dynamical information provided by this technique can be compared to complementary spectroscopic techniques and atomistic simulations to provide a multifaceted understanding of both the dynamics and the structure of the liquid and their interplay.

Figure 1.

The OHD-OKE technique measures the orientational relaxation of molecules via the time-dependent birefringence induced by an optical pulse. A schematic of the OHD-OKE process is given in Figure 1. In the absence of an electric field, molecules are at their equilibrium orientation. When a strong, linearly polarized pulse (the pump pulse) passes through the sample, it induces a net alignment of the molecules along the electric field. When the pulse passes, molecules begin to return to their initial orientation. This randomization process is tracked by a variably delayed, weaker pulse (the probe pulse) that is polarized 45o relative to the pump. Any alignment remaining in the molecules will cause the depolarization of the probe beam. This depolarization is measured through a crossed polarizer by a lock-in detector. Data points are collected at many different delays to collect a complete OHD-OKE decay curve. This curve can be fit to reveal information about the motion of the molecules as well as macroscopic properties like phase transition temperatures. Sample data for the ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluorosulfonyl)imide (EmimNTf2) is given with a typical quality fit in Figure 2.

Figure 2.

Pulses for the OHD-OKE experiment are created by an 86 MHz Ti:Sapphire oscillator. These pulses are centered at 800 nm with a FWHM of ~55 nm. These pulses are then amplified by a 5.4 kHz regenerative amplifier before being adjusted to an appropriate power and bandwidth for the experiment. The output of the regenerative amplifier is then split into a pump beam and a probe beam using a beam splitter. The pump beam proceeds directly to the sample while the probe beam goes through a delay line. OHD-OKE easily incorporates heterodyne detection by having a slightly elliptical probe pulse act as a local oscillator. This ellipticity means that a component of the probe beam's electric field points in the same direction as the signal's electric field. This local oscillator couples to the OHD-OKE signal and amplifies it to produce a better signal-to-noise ratio.

Due to the non-resonant nature of the technique, a probe does not have to be introduced to study the dynamics, and the molecules of interest can be directly studied. The OHD-OKE signal is related to the anisotropic polarizability of a molecule, so any molecule with sufficient asymmetry can be studied. Our group has used OHD-OKE to investigate many liquids including supercooled liquids, liquid crystals, polyethers, aprotic ionic liquids and protic ionic liquids.

Relevant Publications

330. "The Boson Peak in Supercooled Liquids: Time Domain Observations and Mode Coupling Theory," Hu Cang, Jie Li, Hans C. Andersen, and M. D. Fayer, J. Chem. Phys. 123, 057405(4) (2005).

349. "Dynamics of a Discotic Liquid Crystal in the Isotropic Phase," Jie Li, Kendall Fruchey, and M. D. Fayer, J. Chem. Phys. 125, 194501-(7) (2006).

394. “Orientational and Translational Dynamics of Polyether/Water Solutions,” Adam L. Sturlaugson, Kendall S. Fruchey, Stephen R. Lynch, Sergio R. Aragón, and Michael D. Fayer J. Phys. Chem. B 114, 5350-5358 (2010).

400. “Temperature and Hydration-Dependent Rotational and Translational Dynamics of a Polyether Oligomer,” Adam L. Sturlaugson and M. D. Fayer J. Phys. Chem. B 115, 945-950 (2011).

412. "Orientational Dynamics of Room Temperature Ionic Liquid/Water Mixtures: Evidence for Water-Induced Structure and Anisotropic Cation Solvation,” Adam L. Sturlaugson, Kendall S. Fruchey, and M. D. Fayer J. Phys. Chem. B 116, 1777-1787 (2012).

429. "Orientational Dynamics in a Lyotropic Room Temperature Ionic Liquid," Adam L. Sturlaugson , Aaron Y. Arima , Heather E. Bailey , and Michael D. Fayer J. Phys. Chem. B 117, 14775–14784 (2013).

468. "The Impact of Hydrogen Bonding on the Dynamics and Structure of Protic Ionic Liquid/Water Binary Mixtures," Heather E. Bailey, Yong-Lei Wang and Michael D. Fayer J. Phys. Chem. B DOI: 10.1021/acs.jpcb.7b06376 (2017).