Electrostatically tunable optomechanical “zipper” cavity laser
R. Perahia, J. D. Cohen, S. Meenehan, T. P. Mayer Alegre, and O. Painter
a
Thomas J. Watson, Sr., Laboratory of Applied Physics, California Institute of Technology, Pasadena,
California 91125, USA
Received 28 June 2010; accepted 21 October 2010; published online 12 November 2010
A tunable nanoscale “zipper” laser cavity, formed from two doubly clamped photonic crystal
nanobeams, is demonstrated. Pulsed, room temperature, optically pumped lasing action at
= 1.3
m is observed for cavities formed in a thin membrane containing InAsP/GaInAsP
quantum-wells. Metal electrodes are deposited on the ends of the nanobeams to allow for
microelectromechanical actuation. Electrostatic tuning over a range of
= 20 nm for an applied
voltage amplitude of 9 V and modulation at a frequency as high as
m
= 6.7 MHz of the laser
wavelength is demonstrated. ©
2010 American Institute of Physics
.
doi:
10.1063/1.3515296
There has been growing interest in combining micro-
optical resonators with microelectromechanical systems
MEMS
to create tunable photonic elements such as lasers,
couplers, and filters.
1
–
3
In particular, MEMS-tunable lasers
are promising for applications such as on-chip spectroscopy
4
and lightwave communication,
5
where large tunability and
fast tuning speed are desirable. Additionally, as lasers and
other resonant elements shrink below the micrometer scale,
fabrication imperfections at the nanometer level can lead to
substantial variance in the operating wavelength. The devel-
opment of widely tunable optical cavities, at both the micro-
and nanoscales, can benefit from recent advances in the field
of cavity optomechanics in which radiation pressure forces
are used to actuate and detect the motion of the cavity
structure.
6
The strength of the radiation pressure force in
such structures is quantified by an optomechanical coupling
constant equal to the rate of change in the cavity resonance
frequency
with the amplitude of mechanical motion
x
,
g
OM
=
d
/
dx
. In the canonical example of a Fabry–Perot, the
coupling constant is proportional to the inverse of the physi-
cal cavity length. Recent theoretical and experimental work
has shown that large gradient radiation pressure forces can
be obtained in guided-wave nanostructures,
7
–
9
with effective
optomechanical cavity lengths less than or equal to the wave-
length of light.
Of particular interest in this work is the double nano-
beam photonic crystal cavity, dubbed a zipper cavity,
9
,
10
in
which giant radiation pressure effects have been measured.
In previous work,
11
we explored theoretically the optical,
mechanical, and electrostatic properties of an integrated op-
tomechanical and MEMS zipper cavity laser, comparing all-
optical radiation pressure and electrostatic actuation for laser
wavelength tuning and modulation. Due to the small mo-
tional mass
m
eff
pg
and large optomechanical coupling
g
OM
100 GHz
/
nm
of the zipper cavity, wavelength tun-
ing rates approaching
/
x
1nm
/
nm at bandwidths well
over a megahertz were predicted. Here, we demonstrate such
an integrated zipper cavity laser structure, formed in the
InGaAsP semiconductor material system and operating in the
1300 nm wavelength band.
As shown in Fig.
1
, the zipper cavity consists of a pair of
nanobeams which are patterned with a linear array of holes
and placed in the near-field of each other. A small chirp in
the period between holes near the center of the beams is used
to form localized resonant modes.
12
The predominantly
transverse-electric optical modes of the two beams strongly
couple and split into symmetric and antisymmetric pairs
see
Fig.
1
a
, with the symmetric modes containing a large frac-
tion of energy in the gap between the beams. The localized
optical modes are strongly coupled to the in-plane mechani-
cal motion of the beams
Fig.
1
c
, with symmetric modes
tuning red and antisymmetric modes tuning blue with a re-
duction in the interbeam gap.
As detailed in Ref.
11
, the zipper cavity geometry natu-
rally lends itself to integration with capacitive electrome-
a
Electronic mail: opainter@caltech.edu.
(f)
(d)
10 μm
(e)
500 nm
500 nm
Actuator
Actuator
Zipper Cavity
Anti-bonded
a
h
y
h
x
x
g
(a)
Bon
d
e
d
(b)
(c)
FIG. 1.
Color online
a
FEM simulated electric field of the fundamental
symmetric and antisymmetric zipper cavity modes.
b
Schematic of the
zipper cavity. The designed cavity structure has an interbeam gap
x
g
200 nm, beam width
w
= 417 nm, lattice constant
a
= 383 nm, hole
height
h
y
= 240 nm, and hole width
h
x
= 120 nm.
c
FEM simulated funda-
mental in-plane mechanical mode
m
,1
= 1.71 MHz,
m
eff
=6 pg,
k
eff
= 0.67 N
/
m
.
d
SEM micrograph of fabricated zipper cavity with positive
top
and ground
bottom
MEMS electrodes.
e
Top view of the end-mirror
section and
f
angled view of the central cavity region of the zipper cavity.
APPLIED PHYSICS LETTERS
97
, 191112
2010
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/191112/3/$30.00
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, 191112-1
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chanical actuators. The zipper cavity structure studied
here is fabricated from a 252 nm thick InAsP/GaInAsP
multiquantum-well
MQW
layer grown on an InP
substrate.
13
Fabrication of the laser cavity begins with the
deposition of an insulating thin 50 nm layer of silicon nitride
SiN
x
, followed by deposition and lift-off patterning of
Ti/Au
=10
/
190 nm thick
metal electrodes. A second SiN
x
masking layer is subsequently deposited, and an aligned
electron beam lithography step is used to pattern the zipper
cavity nanobeams over the metal electrodes. An inductively-
coupled plasma is used to etch the nanobeam pattern through
the top InAsP/GaInAsP MQW layer. Finally, the zipper cav-
ity nanobeams are released from the substrate using a hydro-
chloric acid wet etch
14
that selectively removes the underly-
ing InP substrate, followed by critical point drying to avoid
collapse and stiction of the beams. Scanning electron micro-
scope
SEM
micrographs of a fabricated device are shown
in Figs.
1
d
–
1
f
.
The devices are tested using a microphotoluminescence
micro-PL
setup shown schematically in Fig.
2
a
. The de-
vices are optically pumped with an 830 nm wavelength
pulsed diode laser
pulse width
T
= 20 ns, pulse period
T
=4
s
. A high numerical aperture
0.4
, long-working dis-
tance
32 mm
objective lens is used to focus the pump beam
on the sample surface with a spot size of approximately
2
m. PL from the sample is collected through the same
objective lens, separated from the pump using a beamsplitter/
interference filter combination, and then dispersed and de-
tected by an imaging spectrometer with an attached cooled
InGaAs detector array. A known fraction of the pump power
is picked off by the beamsplitter and sent to a calibrated
detector to estimate the power incident on the device.
A typical subthreshold PL spectrum from a zipper cavity
is shown in Fig.
3
a
. Several longitudinal orders of symmet-
ric and antisymmetric cavity mode pairs are visible in the
spectrum, with the lower order modes occurring at shorter
wavelengths and the higher order modes at longer wave-
lengths. As indicated by the labeling, for each pair of modes
the shorter wavelength mode is antisymmetric and the longer
wavelength mode is symmetric, as confirmed by their direc-
tion of tuning in Fig.
3
a
. A plot of the peak emitted power
into the fundamental symmetric and antisymmetric modes
versus peak absorbed pump power is shown in Fig.
3
b
.A
clear lasing threshold at an absorbed peak pump power of
7
W is evident for the symmetric
1,+
mode while the
emission into the antisymmetric
1,
mode saturates above
threshold due to gain
carrier
clamping. Spectra of the laser
emission below, at, and above threshold are shown in Fig.
3
c
. The laser linewidth narrows from
0.7 nm well
below threshold to
0.3 nm at threshold, and then
slowly rises to
0.5 nm well above threshold. This
anomalous linewidth behavior is not atypical of micro- and
nanoscale semiconductor lasers;
15
however, it is possible in
these devices that the coupling of spontaneous emission fluc-
tuations to the laser wavelength through radiation pressure
may have an effect on the laser linewidth. Future experi-
ments will seek to explore this possibility.
The wavelength tuning properties of the zipper cavity
laser is tested in two different ways. First, measurements of
the tuning range of the laser wavelength versus applied volt-
age amplitude across the capacitive electrodes is measured.
Figures
4
a
and
4
b
shows the resulting laser spectra
well
above threshold
from the same laser device studied in Fig.
3
. The
1,+
laser mode wavelength is seen to tune red over
an extended range of
= 20 nm for an applied voltage am-
plitude of 9 V
81 V
2
. The
1,
antisymmetric mode
Fig.
4
b
, on the other hand, tunes more weakly to the blue. Both
modes tune in a direction corresponding to the reduction in
interbeam gap with increasing voltage, as expected for the
capacitor geometry. The estimated unperturbed interbeam
gap from the measured symmetric/antisymmetric mode-
splitting of this device is
x
g
= 158 nm
consistent with SEM
measurements
, which from finite-element-method
FEM
simulation yields an optomechanical coupling constant of
g
+
1
/
2
= −40 GHz
/
nm
/
x
g
= 0.24 nm
/
nm
and
g
−
1
/
2
= 26 GHz
/
nm
/
x
g
= −0.15 nm
/
nm
for the
1,
+
and
1,
fundamental modes, respectively. A full nu-
merical FEM model of the optical, mechanical, and electro-
static properties of the laser cavity structure versus applied
PD
Filter
BS
IR Spectrometer
830 nm Diode
CCD
200 μm
(a)
(b)
Device
FIG. 2.
Color online
a
Micro-PL setup with static
dc
and modulation
ac
actuation circuits.
b
SEM
micrograph of a wire-bonded device array. In order to
test a larger number of devices simultaneously, an array
of 80 zipper cavity lasers are connected in parallel to a
common pair of contacts.
Wavelen
g
th (nm)
PL Power (a.u.)
1300 1320 1340 1360
1400
1
0
Wavelength (nm)
1300
1340
1400
1
PL Power (a.u.)
(c)
(1, -)
(1, +)
(2, -)
(2, +)
(3, -)
(b)
10
1
10
0
10
-1
10
-2
10
-3
10
-4
10
1
10
0
10
2
Peak Absorbed Pump Power (
μ
W)
Co
ll
ecte
d
PL Pea
k
Power (nW)
(4, -)
(3, +)
(a)
0
1380
(c)
1320
1360
1380
FIG. 3.
Color online
a
Subthreshold spectrum of the optically pumped
zipper cavity laser under no applied capacitor voltage
bottom curve
and a
small applied capacitor voltage
top curve
.
b
Light-in vs light-out
LL
curve for the fundamental symmetric
and antisymmetric
zipper
cavity modes. The peak absorbed pump power is estimated from the pump
duty cycle
1.1%
, the fraction of the pump beam intercepted by the zipper
cavity
19%
, and the material absorption
10%
.
c
PL spectrum below
bottom curve
,at
middle curve
, and above
top curve
threshold corre-
sponding to the filled circle data points in the LL curve of
b
.
191112-2
Perahia
etal.
Appl. Phys. Lett.
97
, 191112
2010
Downloaded 13 Dec 2010 to 131.215.220.185. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
voltage, taking into account the exponential dependence of
the optomechanical coupling on gap distance, is shown as a
white dashed-dotted line in Fig.
4
a
. From this model, a
shift in the nanobeam gap size of
x
g
= −20.7 nm is inferred
for the largest applied amplitude of 9 V.
A second set of tuning measurements are performed by
applying a small signal voltage modulation
V
ac
to the ca-
pacitive electrodes over a drive frequency range from
drive
= 100 to 4 MHz. The optical response of the zipper cavity to
the modulated voltage input is measured by recording the
time-integrated PL spectrum. The recorded spectra versus
twice the drive frequency
the capacitive force scales as
V
ac
2
resulting in mechanical actuation at 2
drive
of a laser
device with a fundamental symmetric laser mode are shown
in Fig.
4
b
. Two resonances around 2
drive
= 1.69 MHz and
2
drive
= 6.52 MHz can be clearly seen in the modulation
spectrum. A zoom-in around each resonance is shown in
Figs.
4
c
and
4
d
, indicating that each mechanical reso-
nance is split into two frequency peaks, indicative of slight
asymmetries in the two nanobeams resulting in independent
beam motion. FEM mechanical simulations of the fabricated
zipper cavities
nanobeam length
L
=31
m
yield mechani-
cal resonance frequencies of
m
,1
= 1.71 MHz and
m
,3
= 6.53 MHz for the optomechanically coupled first and third
order differential in-plane mechanical modes, respectively, in
good correspondence with the measured resonances. Note
that the second order in-plane mode is decoupled from the
laser field due to its odd symmetry. From the frequency and
linewidth
m
,1
150 kHz, due to squeeze-film damping
16
of the first order mechanical resonance, the 3 dB bandwidth
and settling time for wavelength tuning of this device are
estimated to be 3 MHz and 1
s, respectively, comparable
to recently demonstrated MEMS-tunable vertical-cavity
surface-emitting lasers.
2
The demonstrated wavelength tuning range
20 nm at
9V
and wavelength modulation rate
6.7 MHz
of the zip-
per cavity laser can be significantly improved with a few
modifications to the cavity design. A reduction in the nano-
beam length of the zipper cavities to
L
5
m should be
possible, enabling a fundamental in-plane mechanical reso-
nance frequency approaching 50 MHz, greatly enhancing the
laser frequency modulation speed of the device. Reduction in
the interbeam gap below 50 nm, as has been demonstrated in
passive zipper cavity geometries,
17
increases the optom-
echanical coupling and static wavelength tuning rate of the
zipper cavity modes by more than an order of magnitude. An
increase in
g
OM
improves not only the tunability of the cavity
but also increases the radiation pressure force. Accessing a
regime in which radiation pressure becomes significant, ei-
ther from an external laser source or the internal laser field
itself, opens up several new possibilities for all-optical laser
wavelength tuning and locking.
11
,
16
This work was supported by the DARPA NACHOS pro-
gram
Award No. W911NF-07-1-0277
. The authors would
like to thank Jianxin Chen for growth of the laser material
and the Kavli Nanoscience Institute at Caltech.
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1
2
3
4
5
6
7
8
9
1
32
81
332
1
32
71
32
91
33
11
333
1.
4
1.8
2
.
2
6.
0
6.
4
6.8
7.
2
2
ν
d
r
i
v
e
(
MHz
)
Wav
e
l
en
g
t
h
(nm)
Wav
e
l
en
g
t
h
(nm)
0
.
0
0
.
2
0
.
4
0
.6
0
.8
1.
0
PL P
o
w
e
r
(
a.u.
)
(
a
)
(
c
)
(e)
(d)
Am
pl
it
u
de (
V
2
)
Wav
e
l
engt
h
(nm)
PL P
o
w
e
r
(
a.u.
)
1
330
1
334
1
33
81
342
1
34
61
350
0
.
2
0
.
4
0
.6
0
.8
1.
0
20
1
0
1
30
81
3
1
0
0
(b)
(
1
,
+
)(
1
,-)
1
3
1
2
0
20
40
6
0
8
0
FIG. 4.
Color online
a
PL spectra of
1,+
symmetric laser mode as a
function of the applied voltage amplitude squared
Inset
b
: corresponding
1,
antisymmetric mode tuning
. A fit to the tuning curve based upon
numerical FEM simulations of the laser cavity is shown as a dot-dashed
white line.
c
PL spectra of the zipper cavity laser mode vs MEMS drive
frequency. Zoom in of the
d
third-order and
e
first-order in-plane me-
chanical resonances.
191112-3
Perahia
etal.
Appl. Phys. Lett.
97
, 191112
2010
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