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. (c) 2007 Elsevier B.V. All rights reserved.