For the generation of DC from alternating currents, the material used must have a preferred direction. This condition is met by ferroelectric crystals in which the spatial separation of positive and negative ions is associated with electrical polarization. Most ferroelectrics are electrical insulators, i. with small externally applied electric fields almost no electric current flows through them. This behavior changes drastically when you apply a very strong electric field in the range of several 100,000 volts per centimeter for a short time. At such field strengths, bound electrons, so-called valence electrons, can be made freely movable for a short time by means of a quantum-mechanical tunneling process and therefore lead to a current through the crystal.

The properties of such a current were first investigated by researchers at the Max Born Institute in Berlin, about which they report in the current issue of the journal Physical Review Letters 112.146602. In the form of ultrashort intense terahertz pulses (1 terahertz = 10¹² Hz, duration of field vibration 1 picosecond = 10^-12 seconds), an AC field was applied to a thin crystal of lithium niobate (LiNbO3), which generates an electrical current in the material , The properties of this current were determined by measuring and analyzing the field it radiated. In addition to an oscillating current that had the frequency of the applied terahertz field (2 THz) and many of them, the researchers also saw the signature of a rectified (DC) current along the ferroelectric preferred direction of the LiNbO3 crystal.

The rectified current along the ferroelectric axis has its origin in the interaction of quantum mechanical tunneling between the valence and various conduction bands of the LiNbO₃ crystal and the deceleration of the electrons by friction processes. The tunneling process generates freely moving electrons that would oscillate spatially and temporally without friction in time with the applied terahertz field. Friction destroys this vibratory motion, a process called decoherence. Since the tunnel barrier of LiNbO₃ is asymmetric along the ferroelectric axis, decoherence results in spatially asymmetric transport, i. The tunnel barrier allows more electrons from right to left than in the opposite direction. This mechanism is within a unit cell of the crystal, i. on a sub-nanometer length scale and causes the rectification of the terahertz field. The effect can be exploited even at even higher frequencies resulting in interesting applications in the highest frequency electronics.