Streak camera

A streak camera is used in time-resolved spectroscopy. The basic principle is to measure temporal profile by converting it in the spatial profile. There are two basic types of streak cameras: Mechanical and Optoelectrical. The mechanical type of streak camera uses a rotating mirror or drum with a slit to image a time-resolved event onto film. This is basically how we make movies. The second type is widely used now in ultrafast optics. Let us think about two photons in a single light pulse, of which one is in the front, the other at the end of the pulse. They are separated by the time delay of $\Delta$t, the light pulse length. The light pulse hits on a photocathode where the photons are convert into electrons via photoelectric effect. The time delay between the two electrons roughly equals to $\Delta$t (the time delay between the two photons, if it's a linear system). The two electrons then pass an electric field that is fast varied with time. Thus they are deflected by a different amount and can be spatially separated on the imaging plane, and normally recorded by CCD (Charge Coupled Device). By knowing the changing rate of electric field and the time of flight of electrons in the streak tube, one can thus calculate the time-delay between the two photons from the spatial separation of the electrons. The time-resolution is good to a few hundred femtosecond ($10^{-15}$ second).