Terahertz laser pulses amplify optical phonons in solids

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The amplification of phonons in solids represents a further step towards new phononic devices for the next generation of sensors, mobile phones and computing 

The amplification of light through stimulated emission or nonlinear optical interactions has had a transformative impact on modern science and technology. The amplification of other bosonic excitations, like phonons in solids, is likely to open up new physical phenomena.

A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Centre for Free-Electron Laser Science in Hamburg, Germany, has demonstrated the amplification of optical phonons in a solid by intense terahertz laser pulses. These light bursts excite atomic vibrations to very large amplitudes, where their response to the driving electric field becomes nonlinear and conventional description fails to predict their behaviour.

In this new realm, fundamental material properties, usually considered constant, are modulated in time and act as a source for phonon amplification. The paper, Parametric Amplification of Optical Phonons,was published in the Proceedings of the National Academy of Sciences of the United States of America at the end of last year. 

When light excites the material and induces large atomic vibrations at frequency ω (blue wave), fundamental material properties are modulated in time at twice such frequency (red wave), acting a source for phonon amplification. Credit:  Jörg Harms / MPSD

The group, led by Professor Andrea Cavalleri at the MPSD, has pioneered the field of controlling materials by driving atomic vibrations (ie phonons) with intense terahertz laser pulses. If the atoms vibrate strongly enough, their displacement affects material properties. This approach has proven successful in controlling magnetism, as well as inducing superconductivity and insulator-to-metal transitions. In this field, it is then important to understand whether the phonon excitation by light can be amplified, potentially leading to performative improvements of the aforementioned material control mechanisms.

In the recent work, Cartella, Cavalleri and coworkers used intense terahertz pulses to resonantly drive large-amplitude phonon oscillations in silicon carbide and investigated the dynamic response of this phonon by measuring the reflection of weak (also resonant) probe pulses as a function of time delay after the excitation.

‘We discovered for large enough intensities of our driving pulses, the intensity of the reflected probe light was higher than that impinging on the sample,’ said Andrea Cartella. ‘As such, silicon carbide acts as an amplifier for the probe pulses. Because the reflectivity at this frequency is the result of the atomic vibrations, this represents a fingerprint of phonon amplification.’

The scientists were able to rationalise their findings with a theoretical model that allowed them to identify the microscopic mechanism of this phonon amplification.

These findings build on another discovery by the Hamburg group published earlier this year, showing phonons can have a response reminiscent of the high-order harmonic generation of light. These new discoveries suggest the existence of a broader set of analogies between phonons and photons, paving the way for phononic devices. 

The Centre for Free-Electron Laser Science (CFEL) is a joint enterprise of DESY, the Max Planck Society and the University of Hamburg. This collaboration also involved Professor Roberto Merlin, of the University of Michigan.

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