Principal Investigator Nuh Gedik
Pump probe spectroscopy described above is well suited for measuring the lifetime of electronic excitations with femtosecond time resolution. In order to measure propagation of these excitations in real space, we use transient grating spectroscopy. In this technique, a pair of femtosecond pulses is interfered on the sample to generate a sinusoidal intensity modulation, that in turn induces a density grating of photoexcitations. Because the index of refraction depends on the local excitation density, a periodic modulation of the index of refraction is formed. The period of this pattern in real space can be changed either by changing the wavelength of the laser or the angle between the two beams. An incident probe pulse on this pattern is therefore both reflected and diffracted. Measuring the time evolution of both the reflected and diffracted waves enables us to track the propagation of these excitations in real space.