Hammer-on technique for atomic vibrations in a crystal

Date:
July 14, 2020
Source:
Forschungsverbund Berlin
Summary:
Vibrations of atoms in a crystal of the semiconductor gallium
arsenide (GaAs) are impulsively shifted to a higher frequency by
an optically excited electric current. The related change in the
spatial distribution of charge between gallium and arsenic atoms
acts back on their motions via electric interactions.



FULL STORY ========================================================================== Vibrations of atoms in a crystal of the semiconductor gallium arsenide
(GaAs) are impulsively shifted to a higher frequency by an optically
excited electric current. The related change in the spatial distribution
of charge between gallium and arsenic atoms acts back on their motions
via electric interactions.


==========================================================================
To hammer-on a guitar, a technique deployed by many rock guitarists,
means to shorten a vibrating string quickly with a second finger and,
thus, switch to a higher tone. This technique allows for faster playing
and legato, a smoother linking of subsequent tones. Researchers from
Berlin and Paris have now demonstrated a hammer-on analogue in crystals by switching the frequency of atomic motions with an impulsively generated electric current. As they report in the most recent issue of the journal Physical Review Letters, an electric current generated by femtosecond
optical excitation shifts particular lattice vibrations, the transverse
optical (TO) phonons, to a higher frequency.

The crystal lattice of GaAs consists of a regular arrangement of gallium
and arsenic atoms held together by covalent chemical bonds. The atoms
in the lattice can undergo a variety of vibrations, among them the TO
phonon with a frequency of 8 THz = 8,000,000,000,000 vibrations per
second. The electron density on the arsenic atoms is somewhat higher
than on the gallium atoms, leading to a local electric dipole moment
and making the crystal lattice electrically polar. This property makes
the vibrational motion susceptible to electric forces.

In the experiments, a first femtosecond optical pulse launches a TO phonon oscillation, which is disturbed by a second pulse exciting electrons from
the valence to the conduction band of the semiconductor. This excitation
is connected with a shift of local charge, i.e., a so-called electric
shift current. The shift current enhances the electron density on the
gallium atoms.

This change in the crystal's electron distribution leads to a transient electric polarization, which generates an electric force and, thus,
acts back on the TO phonon motion. As a result, the TO phonon frequency
in the excited crystal changes by a small amount.

The measurement of the tiny phonon frequency shift represents a big experimental challenge. In the present study, the TO phonon oscillation
was mapped in real-time via the THz wave radiated from the oscillating
phonon dipole moment. The THz wave was measured in amplitude and phase
with extremely high precision. The radiated THz wave displays a frequency up-shift after the second pulse has interacted with the sample. The
frequency shift is obvious from the slightly shorter oscillation period
of the THz wave compared to the case without the second pulse (black
trace). The up-shift of the TO phonon frequency has a value of 100 GHz
or approximately 1 percent of the initial frequency. An analysis of the experimental results shows that one photo-excited electron in a crystal
volume of 20,000 GaAs unit cells induces the one-percent frequency
up shift.

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


========================================================================== Story Source: Materials provided by Forschungsverbund_Berlin. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Ahmed Ghalgaoui, Klaus Reimann, Michael Woerner, Thomas Elsaesser,
Christos Flytzanis, Klaus Biermann. Frequency Upshift of the
Transverse Optical Phonon Resonance in GaAs by Femtosecond
Electron-Hole Excitation.

Physical Review Letters, 2020; 125 (2) DOI: 10.1103/
PhysRevLett.125.027401 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200714101244.htm

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