The Transient Current Technique (TCT) has been instrumental in the characterization of silicon radiation detectors over the last 20 years. Using visible or infrared lasers, excess carriers can be produced continuously along the beam propagation direction, the penetration depth of the light determining the length of the trail of charges. No spatial resolution is therefore obtained along this direction. However, a new technique called Two Photon Absorption-TCT (TPA-TCT) allows to produce charge carriers in a confined three dimensional volume.
TPA-TCT was successfully tested in non-irradiated diodes. For this conference we present results on diodes irradiated to various fluences between 1e13 and 1e16 neq/cm2. We show that radiation induced crystal defects lead to a linear increasing single photon absorption (SPA) of the 1.3 µm photons used for TPA, thus compromising the method for highly irradiated sensors. However this contamination can be suppressed because the SPA process is independent of the position of the focus of the laser inside the detector. Thus, the resolution along beam propagation direction can be retrieved back by taking a single measurement with the focus outside of the detector and then subtracting this contribution from the total induced current. TPA-TCT is therefore a powerful three dimensional technique that can also be successfully applied to irradiated detectors. TPA-TCT shows for instance the formation of double junction in diodes and LGADs.