Symmetry breaking by ultrashort light pulses opens new quantum pathways
for coherent phonons

Date:
May 30, 2023
Source:
Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
(MBI)
Summary:
Researchers have demonstrated a novel concept for exciting and
probing coherent phonons in crystals of a transiently broken
symmetry. The key of this concept lies in reducing the symmetry
of a crystal by appropriate optical excitation, as has been shown
with the prototypical crystalline semimetal bismuth (Bi).


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==========================================================================
FULL STORY ========================================================================== Atoms in a crystal form a regular lattice, in which they can move
over small distances from their equilibrium positions. Such phonon
excitations are represented by quantum states. A superposition of phonon
states defines a so- called phonon wavepacket, which is connected with collective coherent oscillations of the atoms in the crystal. Coherent
phonons can be generated by excitation of the crystal with a femtosecond
light pulse and their motions in space and time be followed by scattering
an ultrashort x-ray pulse from the excited material. The pattern of
scattered x-rays gives direct insight in the momentary position of and distances between the atoms. A sequence of such patterns provides a
'movie' of the atomic motions.

The physical properties of coherent phonons are determined by the
symmetry of the crystal, which represents a periodic arrangement of
identical unit cells.

Weak optical excitation does not change the symmetry properties of
the crystal.

In this case, coherent phonons with identical atomic motions in all unit
cells are excited . In contrast, strong optical excitation can break the symmetry of the crystal and make atoms in adjacent unit cells oscillate differently. While this mechanism holds potential for accessing other
phonons, it has not been explored so far.

In the journal Physical Review B, researchers from the Max-Born-Institute
in Berlin in collaboration with researchers from the University of Duisburg-Essen have demonstrated a novel concept for exciting and probing coherent phonons in crystals of a transiently broken symmetry. The key of
this concept lies in reducing the symmetry of a crystal by appropriate
optical excitation, as has been shown with the prototypical crystalline semimetal bismuth (Bi).

Ultrafast mid-infrared excitation of electrons in Bi modifies the
spatial charge distribution and, thus, reduces the crystal symmetry transiently. In the reduced symmetry, new quantum pathways for the
excitation of coherent phonons open up. The symmetry reduction causes a doubling of the unit-cell size from the red framework with two Bi atoms
to the blue framework with four Bi atoms.

In addition to the unidirectional atomic motion, the unit cell with
4 Bi atoms allows for coherent phonon wave packets with bidirectional
atomic motions.

Probing the transient crystal structure directly by femtosecond x-ray diffraction reveals oscillations of diffracted intensity, which persist
on a picosecond time scale. The oscillations arise from coherent wave
packet motions along phonon coordinates in the crystal of reduced
symmetry. Their frequency of 2.6 THz is different from that of phonon oscillations at low excitation level.

Interestingly, this behavior occurs only above a threshold of the optical
pump fluence and reflects the highly nonlinear, so-called non-perturbative character of the optical excitation process.

In summary, optically induced symmetry breaking allows for modifying the excitation spectrum of a crystal on ultrashort time scales. These results
may pave the way for steering material properties transiently and, thus, implementing new functions in optoacoustics and optical switching.

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========================================================================== Story Source: Materials provided by Max_Born_Institute_for_Nonlinear_Optics_and_Short_Pulse
Spectroscopy_(MBI). Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Figures_showing_coherent_phonon_oscillations ========================================================================== Journal Reference:
1. Azize Koc,, Isabel Gonzalez-Vallejo, Matthias Runge, Ahmed
Ghalgaoui,
Klaus Reimann, Laurenz Kremeyer, Fabian Thiemann, Michael Horn-von
Hoegen, Klaus Sokolowski-Tinten, Michael Woerner, Thomas Elsaesser.

Quantum pathways of carrier and coherent phonon excitation in
bismuth.

Physical Review B, 2023; 107 (18) DOI: 10.1103/PhysRevB.107.L180303 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/05/230530125440.htm

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