Schematic of the THz Stark effect (IMAGE)
Caption
Figure 2. (a) Schematic of the THz Stark effect. The local THz field Eloc in the sample induces an energy shift of the electronic ground state S0 and the excited state Sex. As a result, the frequency of the optical transition from S0 to Sex (vertical arrows) is changed. Sign and amount of the frequency shift depend on the projection of the molecular dipole moments µ0 and µex in the S0 and Sex states on the direction of Eloc. The frequency shift is proportional to the product of the projected dipole difference Δµ=µex-µ0 and the local field Eloc. For the sample with retinal dipoles randomly oriented in space, the experiment averages over all dipole directions, resulting in a THz-induced broadening of the absorption spectrum. (b) Absorption change (symbols) induced at the maximum of the THz electric field (time t=0 in Fig. 1d) as a function of optical frequency, and absorption spectrum A0 in absence of the THz pulse (blue line). The THz pulse induces a transient spectral broadening with an absorption decrease in the center of A0 and an absorption increase in its wings. The black solid line originates from a numerical analysis of the data, giving a spectral broadening by ±12 THz and a dipole change Δµ=5 Debye between S0 and Sex.
Credit
MBI/T. Elsaesser
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