  Nonlinear frequency conversion provides an elegant method to detect photons
in a spectral range which differs from the pump wavelength, making it highly
attractive for photons with inherently low energy. Aside from the intensity of
the light, represented by the number of photons, their phase provides important
information and enables a plethora of applications. We present a
phase-sensitive measurement method in the terahertz spectral range by only
detecting visible light. Using the optical interference of frequency-converted
photons and leftover pump photons of the involved ultrashort pulses, fast
determination of layer-thicknesses is demonstrated. The new method enables
phase-resolved detection of terahertz pulses using standard sCMOS equipment
while achieving sample measurement times of less than one second with a
precision error of less than 0.6%.
