In situ heterodyne detection in femtosecond time resolved four wave mixing Andrey Shalit * , Yuri Paskover, Yehiam Prior Department of Chemical Physics, Weizmann Institute of Science, Rehovot, Israel Received 25 June 2007; in final form 13 November 2007 Available online 19 November 2007 Abstract A novel heterodyne detection technique is introduced for femtosecond time resolved four wave mixing (TRFWM). A ‘local oscillator’ field is generated in situ either by the self alignment of the molecules in the ultrashort field, or by the rotational alignment signal from a small amount of anisotropic molecules added to the sample. Like other heterodyne detection schemes, the method enables linearization of the third-order nonlinear signal and clear identification of fundamental vibrational modes and their separation from vibrational beat frequencies. However, unlike others, the method is easy to implement and does not require interferometric stability of the optical setup. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction Advances in the development of femtosecond lasers pro- moted spectroscopic methods for the real-time observation of dynamical processes occurring in molecules on these ultrashort time scales [1–4]. Among the many techniques used, time resolved four wave mixing (TRFWM), and its well-known variant of time resolved coherent anti-stokes Raman scattering (CARS) are the most prominent [5–7]. In CARS experiments, three input beams generate, via the third-order nonlinear susceptibility v (3) , a fourth coher- ent beam, which carries information about the intra-molec- ular dynamics. In general, FWM measurements may be performed either in the time or in the frequency domains, and with femtosecond (fs) lasers, time resolved measure- ments are more common [8–10]. Moreover, time resolved experiments are often performed with all input pulses hav- ing similar spectral contents, and with the relevant transi- tions covered by the inherently broad bandwidth of the ultrashort pulse. As such, the measurement is often treated as nominally degenerate four wave mixing (DFWM). For the measurement of low-frequency modes, optical heterodyned-detected Raman-induced Kerr effect (OHD- RIKE) has proven useful. OHD-RIKE is a pump–probe polarization technique that was first implemented in the frequency domain by Eesley and Levinson [11] and adopted to the time-domain by Green [12] and Kenney- Wallace [13]. A coherent superposition of ground vibra- tional states is excited, and the resulting change of polari- zation of the probe pulse is measured to monitor the dynamics of the vibrational wavepacket. In fact, in OHD-RIKE experiments one detects, in a heterodyne man- ner, the changes to the transmitted probe pulse. Thus, the method is sensitive to the phase as well as to the amplitude of these pump-induced changes. The method is very sensi- tive and powerful, but as a pump–probe technique one monitors small changes on a nonzero background, and thus it is somewhat limited in its ability to probe certain components of the molecular polarizability tensor [14]. In comparison to OHD-RIKE, TRFWM experiments may be performed in the forward propagating three-dimen- sional folded Boxcars geometry [15]. Then the newly gener- ated signal beam will be emitted in a new direction, making it a very sensitive, null detection method. However, in TRFWM the measured quantity is the intensity of the sig- nal, depending quadratically on the third-order nonlinear 0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.11.027 * Corresponding author. E-mail addresses: andrey.shalit@weizmann.ac.il (A. Shalit), Yehiam. prior@weizmann.ac.il (Y. Prior). www.elsevier.com/locate/cplett Available online at www.sciencedirect.com Chemical Physics Letters 450 (2008) 408–416