Topological phonon transport in an optomechanical system
Hengjiang Ren,
1, 2, 3,
∗
Tirth Shah,
4, 5,
∗
Hannes Pfeifer,
4,
†
Christian
Brendel,
4
Vittorio Peano,
4
Florian Marquardt,
4, 5
and Oskar Painter
1, 2, 3, 6
1
Thomas J. Watson, Sr., Laboratory of Applied Physics,
California Institute of Technology, Pasadena, California 91125, USA
2
Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, USA
3
Institute for Quantum Information and Matter,
California Institute of Technology, Pasadena, California 91125, USA
4
Max Planck Institute for the Science of Light, Staudtstrasse 2, 91058 Erlangen, Germany
5
Department of Physics, Friedrich-Alexander Universit ̈at Erlangen-N ̈urnberg, Staudtstrasse 7, 91058 Erlangen, Germany
6
AWS Center for Quantum Computing, Pasadena, California 91125, USA.
‡
(Dated: September 15, 2020)
Recent advances in cavity-optomechanics [1]
have now made it possible to use light not just
as a passive measuring device of mechanical mo-
tion [2], but also to manipulate the motion of
mechanical objects down to the level of indi-
vidual quanta of vibrations (phonons). At the
same time, microfabrication techniques have en-
abled small-scale optomechanical circuits capa-
ble of on-chip manipulation of mechanical and
optical signals [3–12]. Building on these devel-
opments, theoretical proposals have shown that
larger scale optomechanical arrays can be used
to modify the propagation of phonons, realizing
a form of topologically protected phonon trans-
port [12–16]. Here, we report the observation
of topological phonon transport within a multi-
scale optomechanical crystal structure consisting
of an array of over
800
cavity-optomechanical ele-
ments. Using sensitive, spatially resolved optical
read-out [17, 18] we detect thermal phonons in
a
0
.
325
−
0
.
34
GHz band traveling along a topo-
logical edge channel, with substantial reduction
in backscattering.
This represents an impor-
tant step from the pioneering macroscopic me-
chanical systems work [19–23] towards topologi-
cal phononic systems at the nanoscale, where hy-
personic frequency (
&
GHz) acoustic wave cir-
cuits consisting of robust delay lines [24] and non-
reciprocal elements [25–27] may be implemented.
Owing to the broadband character of the topo-
logical channels, the control of the flow of heat-
carrying phonons, albeit at cryogenic tempera-
tures, may also be envisioned.
Topology deals with features invariant to smooth de-
formations. The band structure for waves in a periodic
medium may display such topological features, and this
can have immediate consequences for transport along
∗
These authors contributed equally to this work.
†
Current Address: Institut f ̈ur Angewandte Physik, Universit ̈at
Bonn, Wegelerstrae 8, 53115 Bonn, Germany
‡
opainter@caltech.edu
boundaries, e.g. producing protected edge states [28].
In recent years, these conceptual insights, first acquired
for electrons, were quickly expanded to cover arbitrary
waves [29]. This includes, in particular, mechanical vi-
brations [13, 19–23, 25, 26, 30, 31], with their poten-
tial for far-reaching applications in signal processing and
other domains when implemented in compact chip-scale
acoustic devices. A very promising approach to lower the
footprint for excitation and read-out, and to boost the
sensitivity to high-frequency vibrations, is to use radia-
tion pressure forces in so-called optomechanical crystals
(OMCs) [32–35]. OMCs are patterned structures that
can be engineered to yield large radiation-pressure cou-
pling between cavity photons and phonons.
Here, we demonstrate the optomechanical detection of
topological phonon transport in a multiscale OMC fabri-
cated into the surface of a silicon microchip. In contrast
to standard single-scale devices, the multiscale OMC con-
sists of a superlattice structure, superimposing two pat-
terns with very different but commensurate lattice spac-
ings. This multiscale approach adds an extra degree of
flexibility, decoupling the engineering of photonic and
phononic modes. In our design, at the larger scale is
a phononic crystal. Embedded within each unit cell of
the phononic crystal is a smaller scale photonic crystal,
which hosts a high-
Q
optical nanocavity for optical read-
out of phonons. Local changes within the OMC lattice
of the phononic crystal unit cell are used to create topo-
logically distinct mechanical domains, the boundary of
which host phononic helical edge states based on the Val-
ley Hall effect [36, 37]. The optomechanical arrays in this
work consist of over 800 phononic unit cells, each with
a corresponding optical mode for single-site resolution of
phonon transport.
Images of a fabricated multiscale OMC structure are
shown in Figs. 1a,b. In our design, a triangular lattice of
snowflake-shaped holes with lattice spacing
a
m
= 16
.
02
μ
m is superimposed onto another triangular lattice of
cylindrical holes with a much smaller spacing
a
o
=
450 nm. This hole pattern has been etched into the thin
(220 nm thickness) silicon device layer of a silicon-on-
insulator (SOI) microchip. After releasing the underlying
buried oxide layer, this produces an array of connected
arXiv:2009.06174v1 [cond-mat.mes-hall] 14 Sep 2020