Balanced terahertz wave air-biased-coherent-detection

A balanced heterodyne air-biased-coherent-detection scheme for broadband terahertz waves is achieved using third-order nonlinear susceptibility tensor elements to produce field-induced optical second harmonic photons, which have two orthogonal polarizations. The differential signal between two orthogonally polarized second harmonic photons reduces the common noise in a balanced detection geometry and improves the signal-to-noise ratio by a factor of 2.