1. Photonics Institute, Vienna University of Technology, Gusshausstrasse 25-29, A-1040 Vienna, Austria
2. Matériaux et Phénomènes Quantiques, Université Paris 7, 75251 Paris Cedex 05, France
3. Ecole Normale Superieure, 75231 Paris Cedex 05, France
4. Alcatel-Thales III-V Lab, Route Départementale 128, 91767 Palaiseau Cedex, France

Correspondence to: Karl Unterrainer¹ Correspondence and requests for materials should be addressed to K.U. (Email: karl.unterrainer@tuwien.ac.at).

Lasers are usually described by their output frequency and intensity. However, laser operation is an inherently nonlinear process. Knowledge about the dynamic behaviour of lasers is thus of great importance for detailed understanding of laser operation and for improvement in performance for applications. Of particular interest is the time domain within the coherence time of the optical transition. This time is determined by the oscillation period of the laser radiation and thus is very short. Rigorous quantum mechanical models^{1, 2} predict interesting effects like quantum beats, lasing without inversion, and photon echo processes. As these models are based on quantum coherence and interference, knowledge of the phase within the optical cycle is of particular interest. Laser radiation has so far been measured using intensity detectors, which are sensitive to the square of the electric field. Therefore information about the sign and phase of the laser radiation is lost. Here we use an electro-optic detection scheme to measure the amplitude and phase of stimulated radiation, and correlate this radiation directly with an input probing pulse. We have applied this technique to semiconductor quantum cascade lasers, which are coherent sources operating at frequencies between the optical (>100 THz) and electronic (<0.5 THz) ranges³. In addition to the phase information, we can also determine the spectral gain, the bias dependence of this gain, and obtain an insight into the evolution of the laser field.

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