  Intense terahertz (THz) electromagnetic fields have been utilized to reveal a
variety of extremely nonlinear optical effects in many materials through
nonperturbative driving of elementary and collective excitations. However, such
nonlinear photoresponses have not yet been discovered in light-emitting diodes
(LEDs), letting alone employing them as fast, cost effective,compact, and
room-temperature-operating THz detectors and cameras. Here we report
ubiquitously available LEDs exhibited gigantic and fast photovoltaic signals
with excellent signal-to-noise ratios when being illuminated by THz field
strengths >50 kV/cm. We also successfully demonstrated THz-LED detectors and
camera prototypes. These unorthodox THz detectors exhibited high responsivities
(>1 kV/W) with response time shorter than those of pyroelectric detectors by
four orders of magnitude. The detection mechanism was attributed to
THz-field-induced nonlinear impact ionization and Schottky contact. These
findings not only help deepen our understanding of strong THz field-matter
interactions but also greatly contribute to the applications of strong-field
THz diagnosis.
