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(a) Infrared quantum counting by ND-TPA. A pump photon from a high intensity beam promotes a valence band electron to a virtual state in the semiconductor gap. A second photon (signal) completes the transition to a real conduction band state. The electron is then accelerated in the space charge region, emitted above the vacuum energy level and induces a photocurrent. (b) Experimental setup: two nearly collimated beams are combined using a BS. Parasitic high energy (short wavelength) photons are filtered out by a high pass filter. Beams are focused through an aspheric lens (Asph. L) on a GaAs photocathode. The pump beam area on the GaAs detector is chosen far larger than the signal one in order to (i) minimize nonlinear absorption effects due to the pump beam and to (ii) stabilize the overlap between the two beams against vibrations.
(a) D pump TPA count rate as a function of the pump power (subtracted on the following graphs). (b) Measured D- and ND-TPA photocounts due to signal photons at various fixed pump powers as a function of the signal power: the black star (solid red line) represents experimental photocounts (extrapolated fitted) when no pump is used (i.e., D-TPA). The inset shows the amplification due to ND-TPA effect compared to the D-TPA signal alone.
(a) ND-TPA photocounts as a function of the signal power for different pump intensities. The two solid lines represent a linear (slope 1) and a quadratic (slope 2) evolution profile. The inset displays the ratio at fixed pump and signal power for different signal wavelengths. (b) ND-TPA normalized efficiency for different pump and signal intensities.
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