Detection of high-speed voltage waveforms in GaN devices using electric-field-induced second-harmonic generation

We use electric-field-induced second-harmonic generation (EFISHG) to measure high-speed voltage waveforms in commercial GaN light-emitting diodes (LEDs). The output of an ultrafast passively mode-locked Ti : sapphire laser is reflected from the surface of a GaN UV LED. Copropagating with the specularly reflected Ti : sapphire fundamental is second-harmonic light that is proportional to and synchronous with a high-speed voltage pulse that reverse biases the LED. The measured amplitude of the detected pulse was found to vary with dc bias across the p-n junction of the LED indicating that the detected voltage waveform was localized to the p-n junction. Because of synchronous detection, the detection bandwidth (<10 THz) is limited only by the pulsewidth of the probe laser pulse (<100 fs)