A broad bandwidth detector to measure intensity information of terahertz (THz) frequency pulses. The detector includes: coupling optics coupled to a coherent optical source; a GaSe substrate aligned such that the probe beam path intersects a first surface at a phase-matching angle; a polarization detector aligned in the probe beam path; and calculation means coupled to the polarization detector. The coupling optics direct the probe optical beam along a beam path that is substantially collinear with the pulse beam path of the THz frequency pulses. The polarization of the probe optical beam is varied based on interactions between the probe optical beam and the THz frequency pulses within the GaSe substrate. The polarization detector detects the varied polarization of the probe optical beam. The calculation means determine the intensity information of the THz frequency pulses based on the detected probe polarization of the probe optical beam.