TODA Group

Pump-probe spectroscopy

Optical pulses can be produced by a superposition of light waves with different frequencies and with a well-defined phase relationship between them. Thanks to the development of laser technology, we can now easily use the short pulse laser whose time duration is of the order of subpicosecond (10^-13 second) or less (e.g. mode-locked Ti:sapphire laser). The pump and probe spectroscopy is the most common form of optical spectroscopy using the short pulse laser. In the method, the first "pump" pulse excites electrons to higher energy states and the changes of the electron density distribution are probed by the second pulse ("probe") with a certain delay with respect to the pump. The simplest detection of the changes is achieved by the reflectivity and transmissivity changes of the probe (ΔR).

Figure shows the density plots of the transient reflectivity changes ΔR observed in the quasi-one-dimensional conductor ο-TaS₃. The signal shows decay, which reflects a relaxation of the photoexcited electrons. The sample exhibits a metal to semiconductor phase transition at T_C=220 K by forming a charge density wave (CDW). The temperature dependence of ΔR are investigated to change the equilibrium condition of the electrons (condition without photoexcitation). Owing to the well-defined excited resonance in this sample, pronounced ΔR are observed in both the spectral and temporal domains across the characteristic temperatures including T_C, and can be identified as two types of CDW phases. This is also confirmed by the coherent oscillations whose spectral and temperature dependences highlight the phase transition in each state.