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Experimental setup used in this study. Light is emitted by either a pig tailed semiconductor laser diode or a tuneable OPO. The light intensity is controlled thanks to a fibered attenuator. Light is collimated and then focused onto a silicon detector. This detector is placed on a motorized mount allowing two-photon current Z-scans. A thick Si wafer protects the Si detector from any spurious photon with energy higher than the bandgap.
(a) Schematic view of a Z-scan experiment: point A is at the laser focus and point B is far from the focus. (b) Variation of the Si detector current as a function of its position zd for different light intensities. The crosses indicate the experiments while the continuous lines refer to Lorentzian lineshape fits with an adjustable baseline. (c)Variation of the peak current at point A as a function of light intensity. One notes the quadratic behaviour above as well as the onset of linear sub-bandgap photon detection below . (d) Variation of the baseline (determined in point B) as a function of light intensity. One notes a linear behaviour.
(a) I(P) curves measured in the silicon detector at room temperature and in a LN2 Dewar for photon flux at the focal point. One notes that the linear part disappears at low temperature. (b) Arrhenius plots of the dark current and the photonic current, realized out of focus. One notes similar activation energies.
Experimental versus theoretical I(P) curves. The parameters used for the fit are: . The inset represents the different transitions involved in the proposed photo-assisted Shockley-Read mechanism. The experiment is realized at point B, i.e., out of focus, in order to prevent any influence from TPA.
Variation of the responsivity I/P of the Si detector as a function of sub-bandgap photon energies (the additional noise near 1.55 μm is due to an unexplained unstability of our OPO in this spectral range).
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