Ultrafast and coupled: Atomic vibrations in the quantum material boron
nitride

Date:
October 12, 2021
Source:
Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
(MBI)
Summary:
Materials consisting of a few atomic layers display properties
determined by quantum physics. In a stack of such layers, vibrations
of the atoms can be triggered by infrared light. New experimental
and theoretical work shows that atomic vibrations within the
layers of hexagonal boron nitride, the so-called transverse
optical phonons, couple directly to motions of the layers against
each other. For a period of some 20 ps, the coupling results in
a frequency down-shift of the optical phonons and their optical
resonance. This behavior is a genuine property of the quantum
material and of interest for applications in high-frequency
optoelectronics.



FULL STORY ========================================================================== Materials consisting of a few atomic layers display properties determined
by quantum physics. In a stack of such layers, vibrations of the atoms
can be triggered by infrared light. New experimental and theoretical work
shows that atomic vibrations within the layers of hexagonal boron nitride,
the so-called transverse optical phonons, couple directly to motions of
the layers against each other. For a period of some 20 ps, the coupling
results in a frequency down-shift of the optical phonons and their optical resonance. This behavior is a genuine property of the quantum material
and of interest for applications in high-frequency optoelectronics.


========================================================================== Hexagonal boron nitride consist of layers in which covalently bonded boron
and nitrogen atoms form a regular array of six-rings. Neighboring layers
are coupled via the much weaker van der Waals interaction. Vibrations of
boron and nitrogen atoms in the layer, the so-called transverse optical
(TO) phonons, show an oscillation frequency on the order of 40 Terahertz
(THz, 4x1013vibrations per second) which is ten to hundred times higher
than that of shear and breathing motions of the layers relative to each
other. So far, there was nearly no insight into the lifetime of such
motions after optical excitation and into their coupling.

An international collaboration of scientists from Berlin, Montpellier,
Nantes, Paris and Ithaca (USA) now presents detailed experimental and theoretical results on ultrafast dynamics of coupled phonons in few-layer hexagonal boron nitride. Transverse optical (TO) phonons in a stack of
8 to 9 boron nitride layers display a lifetime of 1.2 ps (1 ps = 10-12
s), while shear and breathing modes show a decay time of 22 ps. Such
lifetimes were directly measured in femtosecond pump-probe experiments
and are in very good agreement with values derived from a theoretical
analysis of the phonon decay channels.

Excitations of shear and breathing modes induce a characteristic
spectral down- shift of the TO phonon resonance in the optical spectra
. Theoretical calculations give the coupling energy between the different
modes of the layer stack and show that the corresponding coupling is
negligibly small in a bulk boron nitride crystal consisting of many
layers. Thus, the observed coupled vibrational dynamics represent a
genuine property of the quantum material.

The spectral shift of the TO phonon resonance in the optical spectra
is a nonlinear optical effect which can be induced by light of moderate
power. This is of interest for applications in optoelectronics and holds potential for optical modulators and switches in the giga- to terahertz frequency range.

========================================================================== 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.


========================================================================== Journal Reference:
1. Taehee Kang, Jia Zhang, Achintya Kundu, Klaus Reimann, Michael
Woerner,
Thomas Elsaesser, Bernard Gil, Guillaume Cassabois, Christos
Flytzanis, Giorgia Fugallo, Michele Lazzeri, Ryan Page,
Debdeep Jena. Ultrafast nonlinear phonon response of few-layer
hexagonal boron nitride. Physical Review B, 2021; 104 (14) DOI:
10.1103/PhysRevB.104.L140302 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211012095021.htm

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