In a HD-OKE experiment a laser pulse induces a transient birefringence in a medium by means of a non resonant and non-linear effect. The induced birefringence can be probed by a second pulse spatially superimposed within the sample with the pump pulse and having different polarization. The measurement of its polarization changes as a function of the delay time between the pulses gives information about the non-linear response of the studied material. The signal is characterized by an instantaneous electronic contribution plus a decaying contribution, which constitutes the more interesting part because it contains information about the relaxation and vibrational response of the molecules in the sample.

The time window probed in a OKE experiment can be very broad, extending from tens of femtoseconds to hundreds of picoseconds, this makes OKE a very powerful technique, capable of revealing very different dynamic regimes.

In the figure we show our optical set-up for the heterodyne-detected optical Kerr effect measurements (HD-OKE). The laser pulses are produced by a Ti:Sapphire Kerr-lens mode locking cavity and their group velocity dispersion is controlled by a prism compression stage. The laser beam is split by a beam splitter (BS) in the pump and probe beams. The probe pulse is delayed respect to the pump pulse by an optical delay line whose translation stage is computer controlled. An half wave plate on the pump fixes its polarization at 45 degree respect to the probe vertical polarization set by polariser P1. The probe polarization is then converted in circular by a quarter wave plate. Probe and pump beams are focused inside the sample S by the achromatic lens AL1. The probe beam is then re-collimated by the achromatic lens AL2 before and send to the Wollaston polariser P2. The two polarization components, horizontal and vertical linear polarizations, present in the probe are selected by P2.

This optical set-up produces twin OKE signals, in vertical and horizontal probe polarizations characterized by opposite sign. The balanced photodiode detector BPD makes an electronic subtraction of the two polarization components extracting the HD-OKE signal from the other background contributions. The analogue output of the photodiode is sent to a lock-in amplifier together with the reference signal of the pump chopper (C). A DAQ board simultaneously acquires the translation stage position from the encoder board and the lock-in output signal. Finally, the signal and position are stored by the computer and matched to build the final signal decay.