Читать книгу Spectroscopy for Materials Characterization - Группа авторов - Страница 65

Kerr‐based femtosecond fluorescence spectroscopy exploits the optical Kerr effect, through which a material becomes birefringent under the action of the intense electric field provided by a light pulse [48, 49]. As for upconversion method, the sample is pumped by a laser beam and the consequent fluorescence is collected. Unlike FLUC, the emission is focused into an isotropic Kerr medium (glass plate or solvent) arranged between two crossed polarizers intercepting the fluorescence path. In standard conditions, the output after the second polarizer is zero (closed shutter condition). When the gate pulse passes through the Kerr media, it opens the shutter, by inducing a transient change in the refractive indexes of the medium that makes it birefringent. Thereby, the emission traversing the Kerr cell becomes elliptically polarized, a portion of it passes through the second polarizer, and reaches the detector. Changing the delay between the pump and the gate then allows to record fluorescence kinetics. In can be shown that the gate pulse should be polarized at 45^∘ with respect to the excitation in order to obtain maximum output [48]. Considering that a phase matching condition is not required in this scheme, a broadband detection is easy and limited only by the optical absorption of the used components and of the detection device.

The temporal resolution is determined by whatever is shorter between the cross‐correlation between excitation and gate and the cross‐correlation between the excitation and the intrinsic response function of the Kerr medium. In fact, if the Kerr medium reacts slowly to the gate modifications, then the temporal resolution is dominated by the latter. For example, with an excitation of ≈50 fs at 475 nm and a gate of 40 fs, the time resolution is ≈120 fs using a 1 mm‐thick fused silica plate as a Kerr medium [49].