Fig. 2: Large photovoltaic response with respect to THz field strengths.

a Six THz time-domain waveforms obtained by tuning the THz polarizers for the blue LED detection experiments. b Corresponding ultrafast photovoltaic signals monitored by the oscilloscope (load = 50 Ω). The maximum photovoltaic signal is ~600 mV. c Anisotropic photovoltaic signals obtained under different THz energies of 80.0, 52.8, 24.7 μJ, respectively. The photovoltaic response has strong correlation to the crystal symmetry. However, it has no strong correlation to the THz energy. Two specific THz polarization dependent responses labeled as Perpendicular (1) and Parallel (2) in c are systematically measured by illuminating various THz pump fluences, as exhibited in (d). When the THz pump fluence is <1.2 μJ/mm², both polarization dependent photovoltaic signals obey quasi-quadratic relationship with respect to the THz pump fluence, while a linear behavior is predominant for the higher fluence region. e Photoresponse comparison between the blue LED and the commercial pyroelectric detector (SDX-1152, Gentec). The photoresponse signal and response time from the LED is 11 times larger and 4 orders of magnitudes faster than those obtained in the pyroelectric detector, respectively. The error bars are defined as standard deviations which are calculated automatically via an oscilloscope after 32 samplings.