Tuning electron-phonon interaction could enable custom opto parts
1 min read
Using a newly developed type of spectroscopy, a research team has shown that electrons in a semiconductor are best described as a nanometre sized cloud. The team also determined that cloud size is a function of the interaction between the electrons and vibrations in the crystal lattice.
Electrons move with high velocity through a semiconductor's crystal lattice and, in so doing, lose part of their kinetic energy by causing vibration in the lattice.
The crystal lattice in semiconductors such as gallium arsenide vibrates with a period of 100fs and such vibrations are quantised. This means the vibrational energy can only be an integer multiple of a vibrational quantum, also known as a phonon. When an electron interacts with the crystal lattice – electron-phonon interaction – energy is transferred from the electron to the lattice.
The research team – from Forschungsverbund Berlin – found that the strength of the electron-phonon interaction depends on electron size, or the spatial extent of its charge cloud. Experiments showed that reducing the electron size increases interaction by up to a factor of 50, producing a strong coupling of the movements of electrons and ions. Electrons and phonons are then believed to form a new quasi particle, a polaron.
To visualise this phenomenon, the researchers used a nanostructure made from GaAs and GaAlAs, in which the energies of electrons and ions were tuned to each other. The coupling of both movements was shown by a new optical technique in which ultrashort light pulses in the infrared excited the system. The subsequent emission of light by the moving charge carriers is measured in real time.
This allows 2D nonlinear spectra to be generated and the determination of the electron-phonon coupling strength. From this, the research team found the size of the electron cloud was between 3 and 4nm. The research also shows the importance of electron-phonon coupling in the optical spectra of semiconductors, something that will enable the development of optoelectronic devices with custom properties.
