of 17
A chip-scale integrated
cavity-electro-optomechanics platform
M. Winger ,
1
T. D. Blasius,
1
T. P. Mayer Alegre,
1
,
2
A. H. Safavi-Naeini,
1
S. Meenehan,
1
J. Cohen,
1
S. Stobbe,
3
,
4
and O. Painter
1
,
1
Thomas J. Watson, Sr., Laboratory of Applied Physics, California Institute of Technology,
Pasadena, CA 91125, USA
2
Current address: Instituto de F
́
ısica “Gleb Wataghin”, Universidade Estadual de Campinas,
UNICAMP 13083-859, Campinas, SP, Brazil
3
DTU Fotonik, Department of Photonics Engineering, Technical University of Denmark,
Ørsteds Plads 343, DK-2800 Kgs. Lyngby, Denmark
4
Current address: Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, DK-2100
Copenhagen, Denmark
opainter@caltech.edu
http://copilot.caltech.edu
Abstract:
We present an integrated optomechanical and electrome-
chanical nanocavity, in which a common mechanical degree of freedom is
coupled to an ultrahigh-
Q
photonic crystal defect cavity and an electrical
circuit. The system allows for wide-range, fast electrical tuning of the
optical nanocavity resonances, and for electrical control of optical radiation
pressure back-action effects such as mechanical amplification (phonon
lasing), cooling, and stiffening. These sort of integrated devices offer a new
means to efficiently interconvert weak microwave and optical signals, and
are expected to pave the way for a new class of micro-sensors utilizing
optomechanical back-action for thermal noise reduction and low-noise
optical read-out.
© 2011 Optical Society of America
OCIS codes:
(230.5298) Photonic crystals; (230.3120) Integrated optics devices;
(230.4110) Modulators; (230.4685) Optical microelectromechanical devices; (220.4880) Op-
tomechanics; (280.4788) Optical sensing and sensors; (350.2460) Filters, interference;
(350.4238) Nanophotonics and photonic crystals.
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1. Introduction
The usually feeble force associated with radiation pressure [1], a manifestation of the mechani-
cal momentum carried by all electromagnetic waves, has recently proven to be quite effective in
manipulating and detecting the motion of micro- and nanomechanical objects embedded within
a resonant cavity [2–4]. The simplest form of a cavity-mechanical system consists of a reso-
nant electromagnetic cavity with its resonance frequency dispersively coupled to the position
of a mechanical object. In such a cavity-based scheme, a narrowband electromagnetic source is
used to pump the cavity. Mechanical motion translates into modulation in the stored intra-cavity
electromagnetic field, and through the filtering properties of the cavity, results in an imprint-
ing of the mechanical motion on the electromagnetic signal. The resonant enhancement of the
pump’s radiation pressure in turn, yields strong back-action effects which modify the dynamic
mechanical and optical properties of the coupled system. Dynamical back-action effects can
include optical stiffening of the mechanical structure [4–8], damping or amplification of the
mechanical motion [6,9–11], or electromagnetically induced transparency [12–14].
Cavity-mechanical systems demonstrating near quantum-limited position read-out and
strong radiation pressure back-action have been realized both in the optical [15,16] and the mi-
crowave frequency domains [17,18]. In the optical domain one has the advantage of shot-noise
limited read-out (even at room temperature) and large radiation pressure coupling due to the rel-
atively large operating frequency, whereas in the microwave domain one has the distinct benefit
of simple electrical interfacing and compact, robust packaging. Here we present a chip-scale
platform for integrating cavity-optomechanics with conventional micro-electromechanical sys-
tems (MEMS) in which the mechanical degree of freedom is strongly coupled via radiation
pressure to both an electrical circuit as well as a high-
Q
optical cavity [19]. Using an integrated
photonic crystal device we demonstrate wide-band (
19 nm) electromechanical tuning of the
optical cavity resonance, near shot-noise-limited optical read-out of mechanical motion, and
electromechanical locking of the optical cavity to a fixed laser source. By combining these de-
vice attributes, a series of key optomechanical back-action effects based on the optical gradient
force [20, 21] are also realized, including optical stiffening, back-action cooling, and phonon
lasing. It is envisioned that these coupled electro- and optomechanical systems, driven by ra-
diation pressure and packaged in a chip-scale form factor, may be used to create sensors of
electrical signals [22], force [15,17], acceleration, or mass [23] operating at the quantum limits
of sensitivity and bandwidth.
2. A tunable slotted-waveguide photonic crystal cavity
2.1. Overview
As discussed above, in this work we seek to develop a common platform for cavity electro- and
optomechanics, in which both electrical and optical signals are coupled to a common mechan-
ical degree of freedom [19]. Planar photonic crystals (PCs) are particularly promising to this
end, since they provide the potential for on-chip integration with well-established microwave
and micro-electromechanical systems (MEMS) technologies, and large radiation pressure cou-
pling due to their nanoscale optical mode volumes [8, 24–26]. Electromechanical control of
microcavities has been shown previously in one-dimensional zipper and double-membrane
cavities [27–29]. These approaches, however, were either limited by low tuning speed, high
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leakage-currents, or the use of low-
Q
cavities, which prohibited the observation of radiation
back-action effects.
The photonic crystal structure studied in this work, along with the conceptually similar tun-
able Fabry-P
́
erot cavity, are sketched in Figs. 1(a) and (b). The PC structure is based on a
silicon membrane slotted cavity, in which the optical cavity field is localized to the air slot be-
tween two PC membranes that are suspended on flexible struts. Two pairs of metal contacts on
each membrane act as capacitive MEMS actuators that provide for electromechanical control
of membrane motion and the cavity air slot width. Localization of light in the optical cavity
is determined by a two-step compression of the PC lattice constant [30] along the length of
the slotted PC waveguide formed from the two PC membrane halves [25]. In this work, the
PC structure was fabricated with a lattice constant of
a
=
470 nm, a relative hole radius of
r
/
a
=
0
.
285, and a slot width of
s
/
a
=
0
.
21 nm so as to produce cavity modes in a wave-
length band around 1500 nm with high-
Q
and large radiation pressure coupling. Theoretical
estimates for the optical cavity mode frequencies and radiation rates were calculated using a
finite-elements method (FEM) solver which is part of the COMSOL Multiphysics [31] soft-
ware package. The cavity is found to support two high-
Q
modes (theoretical
Q
>
10
6
), FEM
simulations of which are shown in Fig. 1(c) and (d).
The strong light confinement in the slot region makes the optical mode frequency (
ω
c
) highly
sensitive to the separation
s
of the two membranes with a theoretical optomechanical coupling
g
OM
=
∂ω
c
/
s
=
2
π
×
152 GHz
/
nm, similar to the value reported in [25]. Expressed in terms of
the length
L
OM
of an equivalent Fabry-P
́
erot cavity, defined by
g
OM
=
ω
c
/
L
OM
, this corresponds
to
L
OM
=
1
.
3
μ
m, significantly smaller than typical cavity sizes realized in other systems based
e.g. on macroscopic free-space cavities [32,33] or micro-toroids [3]. The electrostatic actuators
are formed by pairs of gold contacts that together with the underlying silicon form capacitors
(capacitance
C
) which create an electrostatic force
F
el
=(
1
/
2
)(
d
C
/
d
w
g
)
V
2
a
when applying a
voltage
V
a
across the capacitor gap
w
g
[34].
F
el
leads to contraction of the capacitors, thus
increasing
s
and leading to a blue-shift of the cavity resonances. Figure 1(e) shows a scanning-
electron micrograph of a device fabricated on a microelectronics SOI wafer. The 5
.
77
μ
m
×
19
μ
m large cavity membranes are suspended on
l
=
3
μ
m long struts of width
w
1
=
250 nm
and
w
2
=
80
150 nm, respectively, yielding estimated effective spring constants for in-plane
motion on the order of
k
eff
50 N
/
m. For a metal layer thickness of 200 nm and capacitor gaps
of
w
g
=
200
250 nm we estimate
C
0
.
7 fF and
F
el
1
.
5nN
/
V
2
.
2.2. Fabrication
Samples were fabricated from silicon-on-insulator material from SOITEC. A lift-off mask for
the metal contacts is defined by electron-beam lithography in ZEP-520A positive e-beam re-
sist. We then deposit a 5nm/200nm thick Cr/Au layer in an electron-beam evaporator and strip
the resist with the excess metal on top in Microposit 1165 photoresist remover. A fresh layer
of ZEP-520A is applied, and the etch-pattern for the PC structures, together with the neces-
sary cut-outs for capacitor gaps, membrane suspensions, and strain-relief slices is exposed.
The pattern is transferred into the silicon by a radio-frequency plasma of C
4
F
8
/
SF
6
chemistry.
The excess e-beam resist is removed by cascaded immersion into trichloroethylene, Microposit
1165 remover, and a 10 min etch in Piranha solution (3:1 H
2
SO
4
:H
2
O
2
) at 120
C. The cavity
membranes are released from the underlying SiO
2
layer by immersion into 48% Hydrofluo-
ric acid. Cleaning of the sample surface is finalized by an additional Piranha cleaning step,
followed by a rinse in de-ionized water and a 1 min immersion into 1:10 HF : H
2
O. Finally,
samples are glued to a copper sample holder using GE varnish and electrically contacted with
gold wires by ultrasonic wire-bonding.
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V
DC
w
g
(a)
(b)
(c)
(d)
(e)
w
1
w
2
5
μ
m
Fig. 1. (a) Scanning Fabry-P
́
erot cavity example of an electro-optomechanical system, in
which cavity mirrors are attached to capacitive actuators. (b) Displacement profile of the PC
implementation of an electro-optomechanical cavity. The cavity is formed as a waveguide
defect in between two individual PC membrane halves (region outlined by the black rect-
angle), the distance between which can be adjusted using an electrostatic force generated
between pairs of metal wires. (c) & (d) Electric field distribution
|
E
|
2
of the first ((c)) and
second order (d)) optical cavity modes. (e) Scanning-electron micrograph of a processed
device in a double-capacitor configuration. The PC membrane is suspended on struts with
w
1
=
250 nm and
w
2
=
80 nm.
3. Optical and mechanical characterization
3.1. Optical spectroscopy
The PC cavities are optically investigated by resonant transmission spectroscopy using a near-
field technique based on a dimpled tapered optical fiber [35]. The fiber taper is oriented parallel
to the cavity slot and hovers 1
2
μ
m above the sample surface, such that its evanescent field
is in optical contact with the cavity mode. In order to reduce mechanical drift of the taper, it
can also be mechanically anchored on one of the cavity membranes, usually
1
μ
m laterally
displaced from the slot region (see Appendix C). A swept-wavelength narrow-band telecom-
munications test laser allows for obtaining cavity transmission spectra. Figure 2(a) shows the
transmission spectrum of a device with
w
2
=
150 nm. The two resonances at 1545
.
63 nm and
1554
.
45 nm correspond to the cavity modes depicted in Fig. 1(c) and (d), respectively. When
increasing the applied voltage
V
a
, these resonances blue-shift, as can be seen in Fig. 2(b). For
a maximum applied voltage of
V
a
=
19 V the fundamental (second order) mode reaches a total
shift of
18
.
3nm(
19
.
1 nm) or
+
2
.
32 THz (
+
2
.
4 THz) without a noticeable reduction of the
optical
Q
-factor (
200000 – see section 4). This corresponds to tuning of the cavity frequency
over
900 linewidths. As expected, cavity tuning follows a quadratic voltage dependence.
Defining the tunability
α
by
Δλ
c
=
α
·
V
2
a
, this corresponds to a measured
α
=
0
.
051 nm
/
V
2
,
in good correspondence with the FEM electromechanical simulations of the structure. For de-
vices with
w
2
=
80 nm, we were able to achieve tunabilities up to
α
=
0
.
088 nm
/
V
2
(see
Appendix B). The accessible tuning range of a given device is limited by electrical arching
between the contacts, which occurs around
V
max
20 V in a Nitrogen atmosphere at ambient
pressure. For the device shown in Fig. 2(a) this tuning range is comparable to similar demonstra-
tions of electromechanical tuning [27–29]. Also, due to the large parallel resistance in excess
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