Nonlinear Phononics

Our group has demonstrated that the selective excitation of infrared-active lattice vibrations to large amplitudes using mid-infrared light pulses is a powerful tool for the control of quantum matter. Crucially, such vibrational stimulation allows the control of solids in their electronic ground state and the reduced dissipation of direct lattice excitation makes it attractive for applications in functional material control. Insulator–metal transitions, magnetic switching and even high-temperature superconductivity have been induced in this way. 

 

In order to describe these light-induced phase transitions, it is crucial to understand the nonlinear lattice dynamics following the direct excitation of infrared active modes. Based on the theoretical framework of Ionic Raman Scattering, we developed a working hypothesis for the structural dynamics underlying these various effects. Comparable to optical rectification in nonlinear photonics, this mechanism rectifies the high-amplitude phonon field to exert a directional force onto the crystal and modifies its atomic structure. 

Left: Dynamical response of the resonantly driven infrared active mode QIR and the nonlinearly coupled Raman mode QR. The oscillatory motion of the IR mode (red) produces a directional force onto the Raman mode, leading to a displacement (blue) proportional to Q2IR .

Through Ionic Raman Scattering, a nonlinear effect of the crystal lattice, high-amplitude coherent excitation of a distinct phonon mode leads to the generation of lower-frequency coherent phonon modes.
Through Ionic Raman Scattering, a nonlinear effect of the crystal lattice, high-amplitude coherent excitation of a distinct phonon mode leads to the generation of lower-frequency coherent phonon modes.

 

First experimental evidence of the nonlinear phonon coupling was provided in an all-optical experiment in the manganite compound La0.7Sr0.3MnO3 [1]. Resonant excitation of an infrared-active Mn-O stretching vibration with mid-infrared femtosecond pulses induced coherent oscillations of a lower-frequency Raman mode involving the rotation of oxygen octahedral in the crystal unit cell.

A second key experiment was carried out at the LCLS free electron laser, exploiting femtosecond hard x-ray diffraction to demonstrate the rectification of the crystal structure along the coordinate of the same, anharmonically coupled, Raman mode [2]. 

This rectification of the vibrational field, which can also be described by density functional theory [3], explains how direct lattice excitation in the nonlinear regime can induce a structural phase transition.


Most recently, by combining ultrafast x-ray diffraction with density functional theory calculations, we determined the transient crystal structure of the high-temperature superconductor YBa2Cu3O6.5, as it is nonlinearly driven into a state that shows superconductivity even at room temperature [4,5]. This transient lattice structure strongly modifies the electronic properties in a way that is likely to favor superconductivity [6].

Related publications

[1]
Nonlinear phononics as a new ultrafast route to lattice control
M. Först, C. Manzoni, S. Kaiser, Y. Tomioka, Y. Tokura, R. Merlin and A. Cavalleri
Nature Physics, 7, 854–856 (2011)
[2]
Displacive lattice excitation through nonlinear phononics viewed by femtosecond X-ray diffraction
M. Först, R. Mankowsky, H. Bromberger, D.M. Fritz, H. Lemke, D. Zhu, M. Chollet, Y. Tomioka, Y. Tokura, R. Merlin, J.P. Hill, S.L. Johnson, A. Cavalleri
Solid State Communications, 169 24–27 (2013)
[3]
Theory of nonlinear phononics for coherent light control of solids
Alaska Subedi, Andrea Cavalleri, Antoine Georges
Physical Review B, 89, 220301(R) (2014)

[4]

Optically induced coherent transport far above Tc in underdoped YBa2Cu3O6+δ
S. Kaiser, C. R. Hunt, D. Nicoletti, W. Hu, I. Gierz, H. Y. Liu, M. Le Tacon, T. Loew, D. Haug, B. Keimer, and A. Cavalleri
Phys. Rev. B 89, 184516 (2014)

[5]

Optically enhanced coherent transport in YBa2Cu3O6.5 by ultrafast redistribution of interlayer coupling
W. Hu, S. Kaiser, D. Nicoletti, C. R. Hunt, I. Gierz, M. C. Hoffmann, M. Le Tacon, T. Loew, B. Keimer & A. Cavalleri
Nature Materials, 13, 705–711 (2014)  
⇒ News & Views by N. Peter Armitage → Press releases

[6]

Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5
R. Mankowsky, A. Subedi, M. Först, S.O. Mariager, M. Chollet, H. Lemke, J. Robinson, J. Glownia, M. Minitti, A. Frano, M. Fechner, N. A. Spaldin, T. Loew, B. Keimer, A. Georges, A. Cavalleri
Nature 516 , 71–73 (2014) ⇒ MPG Press release    ⇒ SLAC Press release
An effective magnetic field from optically driven phonons
T. F. Nova, A. Cartella, A. Cantaluppi, M. Först, D. Bossini, R. V. Mikhaylovskiy, A. V. Kimel, R. Merlin, A. Cavalleri
Nature Physics, 13, 2, 132–136 (2017, Adv. Online 2016)  
→ Press release