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Beam profile and spectral characteristic of THz wave in free space

Beam profile and spectral characteristic of terahertz (THz) wave in free space are presented in this paper. The beam width of THz wave at the focus point of parabolic mirror is obtained by means of the narrow-slit measurement. By the spectroscopic imaging, we present the transverse mode distribution of THz beam on the cross-section of focus point. Our experimental results show that the centre wavelength of THz spectra has a distinguished red-shift when it is far away the centre of THz focus point. The spectral range of THz wave far from the focus point is less than that of the centre. This kind of spectral characteristic of THz wave is closely related to the diffraction effect of electromagnetic wave.