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Max Born Institute in Berlin: Ultra-short current surges through rectified light

Last Update Time: 2019-05-11 13:47:17

Semiconductor unit cell of gallium arsenide (GaAs)

Scientists at the Max Born Institute have produced directional currents at terahertz frequencies that far exceed the clock frequencies of modern UHF electronics. The mechanism behind it has long been controversial.

 

Solar cells convert light energy into directional currents and then ensure the power supply to the electricity consumers. The key physical process here is the charge separation during light absorption and the subsequent transfer of charge to the contacts of the solar cell. The current is carried by negative (electrons) and positive charge carriers (holes), which perform so-called in-band motion in various electronic bands of the semiconductor.


The smallest unit in a crystal is the so-called unit cell, a well-defined atomic arrangement, determined by chemical bonds. The unit cell of the prototype semiconductor GaAs is the lattice of gallium and arsenic atoms, with no inversion center. The electronic ground state of a crystal is characterized by a fully filled valence band whose electron charge density is concentrated on the bond between the Ga and As atoms.


When absorbing infrared or visible light, electrons rise from the valence band to the nearest conduction band. In this new state, the electron charge moves in the direction of the Ga atoms. This charge transfer corresponds to a local current, which is referred to as an inter-band current or a shift current, and is substantially different from the electron movement within the band. Until recently, theorists have been controversial, that is, the experimentally observed light-induced current is based on in-band (as in solar cells) or between bands.


At the Born Institute in Berlin, scientists experimented with semiconductor gallium arsenide (GaAs), which was used to scale the photoinduced current at ultrafast time to 50 femtoseconds at the first time. By means of infrared (λ = 900 nm) to the visible spectral range (λ = 650 nm, orange light) ultra-short, intense light pulse devices, they generate displacement currents in the fast-oscillation GaAs and have a bandwidth to generate radiation up to 20 hertz,


The characteristics of these currents and the fundamental electron motion can be determined in detail by the radiated THz wave, whose amplitude and phase are directly measured experimentally. Terahertz radiation shows rectified light of ultrashort pulses at a frequency that is 5000 times the clock frequency of modern computer systems.


The experimentally observed displacement current characteristics are incompatible with the physical image of the in-band motion of electrons or holes. In contrast, the model calculation based on the inter-band motion of electrons in the pseudo-potential band structure reproduces the experimental results, and indicates that the interatomic transfer of the chemical bond length of the electron charge is a key mechanism. This process takes place in each unit cell of the crystal, ie in the sub-nanometer length range, and allows rectification of light. This effect can also be exploited at higher frequencies and opens up new and interesting applications in the field of ultra-high frequency electronics.