Experiment to observe an optically induced change of the vacuum index
Résumé
Quantum electrodynamics predicts that the vacuum must behave as a nonlinear optical medium: the speed of light should be modified when the vacuum is stressed by intense electromagnetic fields. This optical phenomenon has not yet been observed. The DeLLight (deflection of light by light) experiment aims to observe the optically induced index change of vacuum, a nonlinear effect which has never been explored. The experiment is installed in the LASERIX facility at IJCLab, which delivers ultrashort intense laser pulses (2.5 J per pulse, each of 30 fs duration, with a 10 Hz repetition rate). The proposal is to measure the refraction of a probe laser pulse when crossing a transverse vacuum index gradient, produced by a very intense pump pulse. The refraction induces a transverse shift in the intensity profile of the probe, whose signal is amplified by a Sagnac interferometer. In this article we describe the experimental method and setup, and present the complete theoretical calculations for the expected signal. With a minimum waist at focus of 5μm (corresponding to a maximum intensity of ∼3×1020W/cm2), and with the nonlinear vacuum index derived from QED, the expected refraction angle is 0.13 prad. First results of the interferometer prototype are presented. It is shown that an extinction factor F=0.4×10−5 (corresponding to a signal amplification factor of 250) and a spatial resolution σy=10nm are achievable. The expected signal is then about 15 pm, and could be observed at a 5-sigma confidence level with about one month of collected data.