Terahertz Pulses Amplify Optical Phonons

A study led by scientists of the Max Planck Insti­tute for the Structure and Dynamics of Matter MPSD at the Center for Free-Electron Laser Science in Hamburg presents evidence of 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 behavior. In this new realm, funda­mental material proper­ties usually considered constant are modulated in time and act as a source for phonon ampli­fication.

When light excites the material and induces large atomic vibrations, fundamental material properties are modulated in time, acting as a source for phonon amplification. (Source: J. Harms, MPSD)

The ampli­fication of light drama­tically changed science and technology in the 20th century. This path, which began in 1960 with the invention of the laser, still has such a remarkable impact that the 2018 Nobel Prize in Physics was awarded “for ground­breaking inven­tions in the field of laser physics”. Indeed, the ampli­fication of other funda­mental excitations like phonons or magnons is likely to have an equally trans­formative impact on modern condensed matter physics and tech­nology.

The group led by Andrea Cavalleri has pioneered the field of control­ling materials by driving atomic vibrations i.e. 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 present 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 probe pulses as a function of time delay after the exci­tation. “We disco­vered that 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 rationa­lize their findings with a theo­retical model that allowed them to identify the micro­scopic mechanism of this phonon ampli­fication: funda­mental material properties, usually considered constant, are modulated in time and act as a source for ampli­fication. This is the phononic counter­part of the four-wave-mixing. These findings build upon another discovery by the Hamburg group, showing that 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 the reali­zation of phononic devices. (Source: MPSD)

Reference: A. Cartella et al.: Parametric amplification of optical phonons, Proc. Nat. Ac. Sc., online 14 November 2018; DOI: 10.1073/pnas.1809725115

Link: Quantum Condensed Matter Dynamics, Max-Planck-Institute for the Structure and Dynamics of Matter MPSD, Hamburg, Germany

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