Cross-correlation technique for single-shot measurements of weak light pulses

We have developed and demonstrated a simple, single-shot method to measure the duration of a weak ultrafast pulse. The density of free electrons in a semiconductor is changed by a pump beam, causing a time-dependent variation of the refractive index, which in turn determines the spatial distribution of the reflected probe-beam intensity. The derivative of the spatial distribution in the transition region is a cross correlation of the pump and probe pulses. This technique is suitable for measuring weak pulses such as low-level synchrotron light in the mid-infrared-to-far-infrared range.