39 research outputs found
Light-Sound Interaction in Nanoscale Silicon Waveguides
This thesis studies the interaction between near-infrared light and gigahertz
sound in nanoscale silicon waveguides. Chapter 2 introduces photon-phonon
coupling and its theoretical description, describing basic mechanisms and
developing a quantum field theory of the process. Chapter 3 explores the
dynamical effects in both waveguides and cavities. It also proves a connection
between the Brillouin gain coefficient and the vacuum coupling rate. Chapter 4
deals with the observation of Brillouin scattering in nanoscale silicon
waveguides. The waveguides tightly confine light and acoustic vibrations. The acoustic quality factor remains limited to
about because of leakage into silica substrate. These waveguides are
optically transparent in a narrow band of frequencies at a pump power of . Besides this amplification, we translate a
microwave signal across . Chapter 5 extends the experimental
work of chapter 4 by fabricating a cascade of fully suspended nanowires held by
silica anchors. This enhances the mechanical quality factor from to
, enabling the observation of Brillouin amplification exceeding the
propagation losses in silicon. The amount of amplification is mostly limited by
a rapid drop in acoustic quality as the number of suspensions increases. We
propose a mechanism to cancel this inhomogeneous broadening. Chapter 6 looks at
the potential of narrow silicon slot waveguides to enhance the optomechanical
coupling. For certain dimensions, these waveguides support opto-acoustic modes
with an interaction efficiency simulated an order of magnitude above those of
single-nanobeam systems.Comment: PhD thesis defended at Ghent Universit
Unifying Brillouin scattering and cavity optomechanics
So far, Brillouin scattering and cavity optomechanics were mostly
disconnected branches of research -- although both deal with photon-phonon
coupling. This begs for the development of a broader theory that contains both
fields. Here, we derive the dynamics of optomechanical cavities from that of
Brillouin-active waveguides. This explicit transition elucidates the link
between phenomena such as Brillouin amplification and electromagnetically
induced transparency. It proves that effects familiar from cavity optomechanics
all have traveling-wave partners, but not vice versa. We reveal a close
connection between two parameters of central importance in these fields: the
Brillouin gain coefficient and the zero-point optomechanical coupling rate.
This enables comparisons between systems as diverse as ultracold atom clouds,
plasmonic Raman cavities and nanoscale silicon waveguides. In addition,
back-of-the-envelope calculations show that unobserved effects, such as
photon-assisted amplification of traveling phonons, are now accessible in
existing systems. Finally, we formulate both circuit- and cavity-oriented
optomechanics in terms of vacuum coupling rates, cooperativities and gain
coefficients, thus reflecting the similarities in the underlying physics.Comment: published manuscript, minor change
Analysis of enhanced stimulated Brillouin scattering in silicon slot waveguides
Stimulated Brillouin scattering has attracted renewed interest with the
promise of highly tailorable integration into the silicon photonics platform.
However, significant Brillouin amplification in silicon waveguides has yet to
be shown. In an effort to engineer a structure with large photon-phonon
coupling, we analyzed both forward and backward Brillouin scattering in
high-index-contrast silicon slot waveguides. The calculations predict that
gradient forces enhance the Brillouin gain in narrow slots. We estimate a
currently feasible gain of about , which
is an order of magnitude larger than in a stand-alone silicon wire. Such
efficient coupling could enable a host of Brillouin technologies on a
mass-producible silicon chip
Controlling phonons and photons at the wavelength-scale: silicon photonics meets silicon phononics
Radio-frequency communication systems have long used bulk- and
surface-acoustic-wave devices supporting ultrasonic mechanical waves to
manipulate and sense signals. These devices have greatly improved our ability
to process microwaves by interfacing them to orders-of-magnitude slower and
lower loss mechanical fields. In parallel, long-distance communications have
been dominated by low-loss infrared optical photons. As electrical signal
processing and transmission approaches physical limits imposed by energy
dissipation, optical links are now being actively considered for mobile and
cloud technologies. Thus there is a strong driver for wavelength-scale
mechanical wave or "phononic" circuitry fabricated by scalable semiconductor
processes. With the advent of these circuits, new micro- and nanostructures
that combine electrical, optical and mechanical elements have emerged. In these
devices, such as optomechanical waveguides and resonators, optical photons and
gigahertz phonons are ideally matched to one another as both have wavelengths
on the order of micrometers. The development of phononic circuits has thus
emerged as a vibrant field of research pursued for optical signal processing
and sensing applications as well as emerging quantum technologies. In this
review, we discuss the key physics and figures of merit underpinning this
field. We also summarize the state of the art in nanoscale electro- and
optomechanical systems with a focus on scalable platforms such as silicon.
Finally, we give perspectives on what these new systems may bring and what
challenges they face in the coming years. In particular, we believe hybrid
electro- and optomechanical devices incorporating highly coherent and compact
mechanical elements on a chip have significant untapped potential for
electro-optic modulation, quantum microwave-to-optical photon conversion,
sensing and microwave signal processing.Comment: 26 pages, 5 figure
Net on-chip Brillouin gain based on suspended silicon nanowires
The century-old study of photon-phonon coupling has seen a remarkable revival
in the past decade. Driven by early observations of dynamical back-action, the
field progressed to ground-state cooling and the counting of individual
phonons. A recent branch investigates the potential of traveling-wave,
optically broadband photon-phonon interaction in silicon circuits. Here, we
report continuous-wave Brillouin gain exceeding the optical losses in a series
of suspended silicon beams, a step towards selective on-chip amplifiers. We
obtain efficiencies up to , the highest
to date in the phononic gigahertz range. We also find indications that
geometric disorder poses a significant challenge towards nanoscale phonon-based
technologies.Comment: 14 pages, 8 figure
The group of dyadic unitary matrices
We introduce the group DU(m) of m x m dyadic unitary matrices, i.e. unitary matrices with all entries having a real and an imaginary part that are both rational numbers with denominator of the form 2(p) (with p a non-negative integer). We investigate in detail the finite groups DU(1) and DU(2) and the discrete, but infinite groups DU(3) and DU(4). We further introduce the subgroup XDU(m) of DU(m), consisting of those members of DU(m) that have constant line sum 1. The study of XDU(2) and XDU(4) leads to conclusions concerning the synthesis of quantum computers acting on one and two qubits, respectively
Thermal Brillouin noise observed in silicon optomechanical waveguide
Stimulated Brillouin scattering was recently observed in nanoscale silicon
waveguides. Surprisingly, thermally-driven photon-phonon conversion in these
structures had not yet been reported. Here, we inject an optical probe in a
suspended silicon waveguide and measure its phase fluctuations at the output.
We observe mechanical resonances around 8 GHz with a scattering efficiency of
and a signal-to-noise ratio of 2. The observations
are in agreement with a theory of noise in these waveguides as well as with
stimulated measurements. Our scheme may simplify measurements of mechanical
signatures in nanoscale waveguides and is a step towards a better grasp of
thermal noise in these new continuum optomechanical systems.Comment: 11 pages, 4 figure