Hammer-on technique for atomic vibrations in a crystal

Fig. 1. Unit cell (a cube with a length of 0.56 nm [one millionth of 0.56 mm]) Is associated with gallium arsenide by covalent bonds (blue) of gallium (black) and arsenic atoms (red). Gallium arsenide crystals contain several billion such cells. Sincerely: MBI

The vibration of atoms in a crystal of semiconductor gallium arsenide (GaAs) is alternately transferred at high frequency by excited electric current. The corresponding change in the spatial distribution of charge between gallium and arsenic atoms works back to their motions through electrical interactions.

To hammer on a guitar, a technique deployed by many rock guitarists means shortening a vibrating string quickly with a second finger and, thus, switching to higher vocals. This technique allows for faster play and legoto, a smooth linking of the latter tones. Researchers in Berlin and Paris have now demonstrated an analog-on analog in a crystal by varying the frequency of atomic motions with an essentially generated electric current. As they report in the most recent issue of the magazine Physical review letter, An electric current that is generated by phytosecond optical excitation, specifically lattice vibrations, transverse optical (TO) phones move at high frequency.

The crystal lattice of GaAs consists of a regular arrangement of gallium and arsenic atoms (Fig. 1) that are held together by covalent chemical bonds. The lattice may undergo a variety of atoms, calling them with a frequency of 8THz = 8000000000000 vibrations per second. The electron density on arsenic atoms is somewhat higher than on gallium atoms, leading to a local electric dipole moment and making the crystal lattice electrically polar. This property makes vibrational motion susceptible to electrical forces.

Hammer-on technique for atomic vibrations in a crystal

Fig. 2. Emission of TOON with (red) and without (black) excitation by a second pulse. The electric field is plotted as a function of time. The second optical pulse shortens the oscillation period, corresponding to an increase in frequency from 8 to 8.1 THz. Sincerely: MBI

In experiments, a first femtosecond optical pulse initiates a TO phonon oscillation, which is disturbed by a second pulse exciting electrons from the conduction band of the semiconductor from the valley. This excitation is associated with a shift of local charge, that is, a so-called electric shift current. Shift current increases the electron density on gallium atoms. This change in the electron distribution of the crystal leads to a transient electrical polarization, which generates an electric force and, thus, acts back to the TO phonon motion. Consequently, the TO phonon frequency in crystal changes stimulated by a small amount.

The measurement of small phonon frequency shifts represents a major experimental challenge. In the present study, the TOON phone oscillation dipole was mapped in real time via a THz wave irradiated from the dipole moment. The THz wave was measured in amplitude and phase with extremely high precision (Fig. 2). The radiated THz wave shows a frequency up-shift after the second pulse interacts with the sample. The frequency shift is evident from the slightly shorter oscillation period of the THz wave (red trace 2 in fig) than in the case without the second pulse. The value of the up-shift of the TO phonon frequency is 100 GHz or about 1 percent of the initial frequency. Analysis of experimental results shows that a photo-excited electron in a crystal volume of 20 000 GaAs unit cells induces a percentage frequency change.

The change in phonon frequency observed here for the first time should occur in a wide range of semiconductors with polar lattices, and also in ferroelectric materials.

Amplifier for vibration of terahertz lattice in semiconductor crystals

more information:
Ahmed Galgoui et al. Frequency of transverse optical phonon resonance in GaAs by femtosecond electron-hole excitation Physical review letter (2020). DOI: 10.1103 / PhysRevLett.125.027401

Provided by Forschungsverbund Berlin eV (FVB)

Quotes: Hammer-on technique for atomic vibrations in a crystal (2020, July 14) from https://phys.org/news/2020-07-hammer-on-technique-atomic-vibrations-crystal.html on 15 July 2020 Retrieved.

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