5000 times faster than a computer

5000 times faster than a computer

Atomic rectifier for light generates directed electric current

Fig. 1: (a) Unit cell of gallium arsenide (GaAs) semiconductor. Density of valence electrons on the gray plane in (a) at the ground state (b) (electrons in the valence band) and in the excited state (c) (electrons in the band conduction).

Fig. 2: Measures (explanation see text)

When light is absorbed into a semiconductor crystal without inverting symmetry, electric currents can be generated. Scientists at the Max Born Institute have now generated directional currents at terahertz (THz) frequencies that far exceed the clock frequencies of modern ultra-high frequency electronics. The researchers showed that an electronic charge transfer between adjacent atoms in the crystal lattice represents the underlying physical mechanism.

Solar cells convert the energy of light into a directed electric current, which then provides power to the electrical consumers. The key physical processes are the separation of charges during the absorption of light and the subsequent transport of the electrical charge to the contacts of the solar cell. Electrical currents are carried by negative (electron) and positive (hole) charge carriers, called bandPerform movements in the different electronic bands of the semiconductor. From a physical point of view, the following questions are essential: What is the smallest unit in a crystal that can produce such a directed light-induced current? What are the highest possible frequencies for such electric currents? What mechanisms of the atomic length scale are responsible for this charge transport?

The smallest unit in a crystal is the unit cell, a well-defined arrangement of atoms, which is determined by chemical bonds. The unit cell of the GaAs prototypic semiconductor is shown in Figure 1 (a) and represents a crystal lattice of gallium and arsenic atoms without an inversion center.L ground state The crystal electronics is characterized by a fully filled valence band whose electronic charge density on the bond. The Ga and As atoms are concentrated in FIG. 1 (b). When absorbing infrared or visible light, an electron is lifted from the valence band into the nearest energy conduction band. In this new state, the electronic charge is shifted in the direction of the Ga atom of Figure 1 (c). This charge transfer corresponds to a local electrical current, called interband current or offset current, and basically differs from the electron movements inside the bands. Until recently, theorists have wondered whether the experimentally induced light-induced current is based on intra-band (as in solar cells) or inter-band motion.

Scientists at the Max Born Institute in Berlin experimentally investigated light-induced electrical currents in semiconductor gallium arsenide (GaAs) for the first time at a super-fast speed of up to 50 femtoseconds (1 fs = 10^-15 Seconds). They report their findings in the newspaper Physical Review Letters 121, 266602 (2018)Using intense and ultra-short infrared (λ = 900 nm) light pulses at the visible spectral range (λ = 650 nm, orange light), they generated GaAs displacement currents that oscillate very rapidly. quickly and thus a THz radiation with a bandwidth up to 20 Generate THz (Fig. 2). The properties of these currents and the underlying electron movements could be determined in detail by radiated THz waves whose amplitude and phase were measured directly by the experiment. THz radiation produces ultrashort pulses of rectified light at frequencies 5000 times higher than the clock frequencies of modern computer systems.

The properties of the motion currents observed experimentally do not agree with the physical image of bandelectron movements or compatible holes. On the contrary, model calculations based on a bandThe motions of electrons in a pseudopotential band structure reproduce the experimental results and show that an interatomic transfer of electronic charge of the order of the length of a chemical bond constitutes the key mechanism. This process takes place in every cell unit of the crystal, i. on a scale of subnanometer length, and allows the rectification of light. This effect can also be exploited at even higher frequencies and opens up new interesting applications in the field of ultra-high frequency electronics.

Fig. 1: (a) Unit cell of gallium arsenide (GaAs) semiconductor. The chemical bonds (blue) link each gallium atom to four neighboring arsenic atoms (and vice versa). Density of valence electrons on the gray plane in (a) at the ground state (b) (electrons in the valence band) and in the excited state (c) (electrons in the band conduction). In addition to the valence electrons presented here, there are still strongly bound electrons near the nuclei of the atoms.

Fig. 2:Above, the principle of measurements is explained. A short near-infrared or visible pulse is sent to a thin layer of GaAs. The electric field of the generated THz radiation is calculated as a function of time (1 ps = 10^-12 s). Below is an example of such a measure. It contains oscillations with a period of 0.08 ps, which corresponds to a frequency of 12 000 GHz = 12 THz.