Phase-sensitive interferometry with ultrashort optical pulses

Abstract

Reliable phase-sensitive time-resolved interferometry with ultrashort pulsed lasers is performed with the use of a closed scanning Michelson interferometer in combination with a fixed MachZehnder interferometer at the front end. The technique is based on measuring the full phase and frequency properties of the pulse distortion of an ultrashort optical pulse introduced by linear or nonlinear interaction with a sample. The necessary stability and reproducibility to perform an interferometric measurement is provided by a commercially available Fourier transform spectrometer enabling time-resolved measurements from the IR well into the visible part of the optical spectrum. The feasibility of the technique is demonstrated by measuring the distortion introduced by an etalon and a surface-plasmon polariton. 0 199.5 American Institute of Physics. We report a method for performing phase-sensitive timeresolved interferometry, using a commercially available Fourier transform spectrometer operating from the IR up to the UV part of the spectrum in combination with a fixed MachZehnder (MZ) at the front end. The technique provides a reliable spectroscopic tool for performing time-resolved interferometry over a wide spectral range. The approach is similar to the method described in Refs. l-5, and is, in principle, identical to the method already used by Michelson.! All these methods are based on measuring the interferometric cross-correlation signal between an undistorted and a distorted light field. After Michelson, the method reported here was again of interest in Fourier tranform infrared spectroscopy.7 Recent interest in the method is in the field of ultrashort pulse generation,le5 due to the role of precise pulse evaluation of both intensity and chirp after they have traveled through optical components. From the cross-correlation signal, both absorption and dispersion in the sample are measured simultaneously, enabling the determination of the group velocity dispersion in materials, for example. In particular, for the distortion of an ultrashort optical laser pulse, the chirp and the sign of the chirp of the pulse distortion can be obtained in a straightforward way without any assumptions. In a pump-probe type of setup, the time resolution is limited by the time duration of the pulses. The shortest reliable time scale in this type of experiment is the time duration of the pulses. However, in our reported method no such limitation occurs, but the shortest reliable time .scale .is in the order of the duration of one cycle of the light field used in the experiments. In the experiment a pulse from a laser is sent through a fixed-length MZ interferometer (Fig. 1) producing two pulses with a separation AL. The sample is placed in one of the arms of the MZ interferometer and this laser pulse is distorted by the sample. The pulse in the other arm remains unaltered and will be used as a reference pulse. The electric field of the reference pulse with light frequency w, may in general be described in the frequency domain by A(o)exp(iwt), with A(o) being a complex slowly varying amplitude function. The complex amplitude function A ( o j describes both the power spectrum of the reference pulse and the relative phase relation between the frequency components. The electric fields of the sum of the distorted pulse and the reference pulse is given by