Fig. 2:The experimental concept is shown in the top. A short pulse in the near-infrared or visible spectral range is sent onto a thin GaAs layer. The electric field of the emitted THz radiation is measured as a function of time (1 ps = 10^-12 s). An example of such a THz waveform is shown below. It contains oscillations with a period of 0.08 ps corresponding to a frequency of 12000 GHz=12 THz.

Researchers at the Max-Born-Institute in Berlin, Germany, have investigated optically induced shift currents in the semiconductor gallium arsenide (GaAs) for the first time on ultrafast time scales down to 50 femtoseconds (1 fs = 10^-15 seconds). They report their results in the current issue of the journal Physical Review Letters 121, 266602 (2018) . Using ultrashort, intense light pulses from the near infrared (λ = 900 nm) to the visible (λ = 650 nm, orange color), they generated shift currents in GaAs which oscillate and, thus, emit terahertz radiation with a bandwidth up to 20 THz (Figure 2). The properties of these currents and the underlying electron motions are fully reflected in the emitted THz waves which are detected in amplitude and phase. The THz radiation shows that the ultrashort current bursts of rectified light contain frequencies which are 5000 times higher than the highest clock rate of modern computer technology.

The properties of the observed shift currents definitely exclude an intraband motion of electrons or holes. In contrast, model calculations based on the interband transfer of electrons in a pseudo-potential band structure reproduce the experimental results and show that a real-space transfer of electrons over the distance on the order of a bond length represents the key mechanism. This process is operative within each unit cell of the crystal, i.e., on a sub-nanometer length scale, and causes the rectification of the optical field. The effect can be exploited at even higher frequencies, offering novel interesting applications in high frequency electronics.