AC/DC for terahertz waves
Researchers at MBI discover an ultrafast rectifier for terahertz radiation
Experiment: The high electric field of the intense terahertz pulse accelerates electrons in a lithium niobate LiNbO<sub>3</sub> crystal. The hexagonal unit cell contains lithium atoms (green spheres), niobium atoms (blue spheres), and oxygen atoms (red spheres) the latter being arranged on the corners of a unit cell. The crystal lacks inversion symmetry and, thus, shows a ferroelectric polarization along the c-axis.
During transport along the c-axis, electrons see alternating different distances between lithium and niobium atoms. Moreover, the niobium atoms are not in the center of the oxygen octahedrons. Such geometry leads to asymmetric barriers the electrons have to pass by quantum mechanical tunneling when moving along the c-axis. The electrons are driven through the barriers by the high terahertz AC field. The barrier asymmetry together with decoherence/friction processes result in a spatially asymmetric transport, i.e., the rectification to a DC current.
In the unit cells of a lithium niobate crystal alternating currents (AC) with a frequency 1000 times higher than that of modern computer systems are transformed into a direct current (DC), thereby generating simultaneously a series of overtones of the terahertz radiation.
When the guitarist Angus Young of the Australian hard rock band AC/DC touches the strings of his electric guitar, a strongly distorted sound rings out from the loudspeaker. The origin of the electronically generated overtones is the rectifying effect in the electronic tubes of the guitar amplifier. In the simplest case an (A)lternating (C)urrent generates a (D)irect (C)urrent, an effect which finds its application in telecommunications at much higher radio or mobile phone frequencies. From a physics point of view the highly interesting question arises: up to which cut-off frequencies can one generate directed currents (DC) and which microscopic mechanisms underlie them?
For the generation of a direct current out of alternating currents the material used must feature a preferred direction. This condition is fulfilled by ferroelectric crystals, in which the spatial separation of positively and negatively charged ions is connected to a static electric polarization. Most ferroelectrics are electric insulators, i.e., low electric fields cannot cause any detectable electric currents in the material. A drastic change of this behavior is observed if one applies for a short period an extremely high electric field in the range of several 100.000 volts per centimeter. At such field strengths, bound electrons, the so called valence electrons can be freed for a short period by means of the quantum mechanical tunneling process leading in turn to a current through the crystal.
Now, researchers at the Max-Born-Institute in Berlin, Germany investigated the properties of such a current for the first time and report their results in the current issue of the journal Physical Review Letters 112.146602 (2014)). Using ultrashort, intense terahertz pulses (1 Terahertz = 1012 Hz, period of a field oscillation 1 picosecond=10-12 seconds) they applied an AC field to a thin lithium niobate (LiNbO3) crystal which causes an electric current in the material. The properties of this current were studied in detail by measuring and analyzing the electric field radiated by the accelerated electrons. Besides an oscillating current with the frequency of the applied terahertz field (2 THz) and several overtones of the latter, the researchers observed the signature of a directed current (DC) along the c-axis the preferred direction of the ferroelectric LiNbO3 crystal.
The rectified current along the ferroelectric c-axis has its origin in the interplay of quantum mechanical tunneling of electrons between the valence and several conduction bands of the LiNbO3 crystal and the deceleration of electrons by friction processes. The tunneling process generates free electrons which in absence of friction would spatially oscillate in time with the applied terahertz field. The friction destroys this oscillatory motion, a mechanism called decoherence. Due to the asymmetry of the tunneling barrier along the ferroelectric c-axis decoherence results in a spatially asymmetric transport, i.e., the tunneling barrier lets pass more electrons from right to left than from left to right. This mechanism is operative within each unit cell of the crystal, i.e., on a sub-nanometer length scale, and causes the rectification of the terahertz field. The effect can be exploited at even higher frequencies, offering novel interesting applications in high frequency electronics.
Movie: www.mbi-berlin.de/de/current/index.html#2014_04_09.
Original article:
C. Somma, K. Reimann, C. Flytzanis, T. Elsaesser, und M. Woerner:
High-Field Terahertz Bulk Photovoltaic Effect in Lithium Niobate
Physical Review Letters 112.146602/2014
Contact
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (MBI)
Dr. Michael Wörner, woerner(at)mbi-berlin.de, Tel.: 030 6392 1470
Carmine Somma, somma(at)mbi-berlin.de, Tel.: 030 6392 1474
Prof. Dr. Thomas Elsässer, elsaesser(at)mbi-berlin.de, Tel.: 030 6392 1400