of 3
Surface-plasmon mode hybridization in subwavelength microdisk lasers
R. Perahia,
a

T. P. Mayer Alegre, A. H. Safavi-Naeini, and O. Painter
b

Thomas J. Watson, Sr., Laboratory of Applied Physics, California Institute of Technology, Pasadena,
California 91125, USA

Received 24 August 2009; accepted 31 October 2009; published online 20 November 2009

Hybridization of surface-plasmon and dielectric waveguide whispering-gallery modes are
demonstrated in a semiconductor microdisk laser cavity of subwavelength proportions. A metal
layer is deposited on top of the semiconductor microdisk, the radius of which is systematically
varied to enable mode hybridization between surface-plasmon and dielectric modes. The
anticrossing behavior of the two cavity mode types is experimentally observed via
photoluminescence spectroscopy and optically pumped lasing action at a wavelength of


1.3

m is achieved at room temperature. ©
2009 American Institute of Physics
.

doi:
10.1063/1.3266843

In wavelength-scale lasers, the very small number of
optical modes and small volume of gain material allows
one to probe the subtle and often interesting properties of
lasing action.
1
Semiconductor microdisk lasers, in particular,
have been actively studied due to their simple geometry and
amenability
to
planar
chip-scale
integration
with
microelectronics.
2
4
More recently there has been great inter-
est in using surface-plasmon

SP

modes at a semiconductor-
metal interface for guiding as well as high intensity and sub-
wavelength optical confinement.
5
There has been significant
work on the incorporation of SP waveguides that also act as
electrical contacts in mid-infrared quantum cascade lasers,
6
in increasing SP propagation lengths using SP-dielectric
waveguide mode hybridization,
7
as well as in creating ultr-
asmall laser cavities.
8
,
9
In miniaturizing semiconductor lasers to the nanoscale
one encounters several design challenges that must be ad-
dressed, such as thermal management,
10
,
11
proximity of
metal contacts to the optical cavity, surface states,
12
and de-
manding tolerance levels in fabrication. In this letter, we in-
vestigate the purposeful integration of a metal contact into a
subwavelength whispering-gallery microdisk laser. We show
that whispering-gallery SP and dielectric modes hybridize
into low loss modes. We predict and map out this hybridiza-
tion using finite-element-method

FEM

simulations, and ex-
perimentally measure the properties of fabricated microdisk
laser cavities with varying levels of mode hybridization.
Simulation of the hybrid laser cavities is performed us-
ing fully-three-dimensional FEM simulations with azimuthal
symmetry.
13
A 250 nm thick semiconductor disk with index
n
disk
= 3.4 and radius
R
d
= 0.65

m is simulated with a cen-
tered metal contact of varying radius

R
m

. A schematic of
the microdisk device is shown in Fig.
1

a

. Silver with a
complex refractive index of
n
Ag
= 0.11 −
i
9.5 at

= 1.3

mis
chosen for the metal layer due to its low optical loss.
14
For
this disk size in the 1300 nm wavelength band there occurs
a near degeneracy of the transverse-electric-like

TE-like

whispering-gallery mode

WGM

with dominant electric
field polarization in the plane of the disk and the transverse-
magnetic-like

TM-like

mode with dominant electric field
normal to the disk plane. A plot of the wavelength and
Q
-factor of these two resonances is shown in Fig.
2

a

as a
function of metal radius fraction

R
m
/
R
d

. The resonances
exhibit a clear anticrossing behavior, with the modes hybrid-
izing and picking up significant SP character with increased
metal coverage. At one extreme, where
R
m
/
R
d
is very small,
the upper wavelength branch

mode I

is of TE-like WGM
character and the lower wavelength branch

mode II

is of
TM-like WGM character. At the other extreme,
R
m
/
R
d

1,
the upper branch has taken on a SP mode character whereas
the lower branch is now TE-like. Dominant electric field
components for both cases are plotted in Fig.
2

b

.
Of particular interest would be the lower wavelength
branch

mode II

of the SP-dielectric hybrid modes, as this
mode shows significant robustness in its optical
Q
-factor for
large metal coverage. Further analysis of this regime is per-
formed by studying the effects of an InP pedestal

n
= 3.2

a

Electronic mail: rperahia@caltech.edu.
b

Electronic mail: opainter@caltech.edu.
R
p
R
d
R
m
SiNx
InP
GaInAsP
(5x InAsP QWs)
Ge/Ag/Au
(a)
(b)
(c)
(d)
R
d
R
m
SiNx Cap
D=1.16
μ
m
Ge/Ag/Au
GaInAsP (5x InAsP QWs)
InP
R
p
500nm
500nm
500nm
Disk
FIG. 1.

Color online

a

Schematic of fabricated and simulated microdisks
with a metal contact buried below a SiN
x
cap.

b


d

SEM images of a
diameter
D
= 1.16

m fabricated microdisk with a Ge/Ag/Au 10/80/10 nm
contact buried under a 150 nm conformal layer of SiN
x
. Dashed line delin-
eates GaInAsP – SiN
x
boundary.

c

Top view and

d

cross-sectional view
after SiN
x
has been removed.
APPLIED PHYSICS LETTERS
95
, 201114

2009

0003-6951/2009/95

20

/201114/3/$25.00
© 2009 American Institute of Physics
95
, 201114-1
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used to support the microdisk. Figure
2

c

shows a plot of
cavity
Q
of mode II versus the pedestal radius

R
p

with
R
d
= 0.7

m so the resonant wavelength is


1.3

m. In
this plot the metal-to-disk ratio was set at the optimal value
of
R
m
/
R
d
= 0.7 from Fig.
2

a

. A peak value of
Q
= 4000 is
found for a fractional pedestal radius of
R
p
/
R
d
= 0.8

corre-
sponding mode field plot shown in Fig.
2

d


. From these
simulations it is clear that a microdisk cavity structure with
significant metal coverage and very little undercut can be
designed to have a cavity
Q
-factor sufficient for lasing ac-
tion. Such a structure would facilitate fabrication as well as
good thermal heat sinking.
In order to test the predictions of the FEM-modeling,
microdisks with nominal diameter
D
= 1.2

m were fabri-
cated from 252 nm thick membranes consisting of five
InAsP/GaInAsP compressively strained quantum wells, with
peak spontaneous emission at


1285 nm.
15
First, the
metal layer

Ge
/
Ag
/
Au = 10
/
80
/
10 nm

was deposited and
patterned into round contacts of systematically varying diam-
eter by electron beam lithography

EBL

, electron beam
evaporation, and liftoff. A hard SiN
x
mask was then depos-
ited to protect the patterned metal layer, and the outer disk
shape was patterned by a second, aligned EBL step. An in-
ductively coupled reactive-ion etch was used to transfer the
disk pattern through the hard mask and the 252 nm thick
semiconductor layer. The disks were then undercut, and the
pedestal formed, using HCl : H
2
O solution.
16
A scanning
electron microscope

SEM

micrograph of a final device in-
cluding remaining SiN
x
cap is shown in Fig.
1

b

. After the
device testing described below, the SiN
x
cap layer was re-
moved allowing for SEM imaging

Figs.
1

c

and
1

d


and
measurement of the disk, metal, and pedestal radii

Fig.
3

a


. A systematic variation of 25%–100% metal coverage
was achieved in disks with average diameter
D
= 1.2

m.
Initial resonance mode spectroscopy was performed by
free space optical pumping and collection of the photolumi-
nescence

PL

through an optical fiber taper nanoprobe.
17
The fiber taper provides excellent collection efficiency of the
WGM emission of the microdisk, substantially improving
the sensitivity of the measurement. Using a pulsed external
cavity diode laser at

= 830 nm, the disks were pumped
with pulses of peak power
P
p

1 mW, pulse width

T
= 500 ns, and pulse period
T
=1

s. The fiber-collected
spectra are plotted in Fig.
3

c

for devices with varying
R
m
/
R
d
. Two pairs of modes can be seen at


1.2 and
1.3

m. A normalized PL spectrum taken with continuous
wave pumping from the unprocessed semiconductor material

P
p
= 282

W

is shown in Fig.
3

d

for reference. Despite
the disappearance of the upper wavelength branch mode in
the spectra of Fig.
2

c

for large metal coverage, a result
expected due to the very low-
Q
of the SP mode, the zoom-in
shown in Fig.
3

b

for the longer-wavelength mode pair
shows clear anticrossing behavior indicative of mode hybrid-
ization of the SP and dielectric WGMs. A quantitative com-
parison of the modal
Q
for the different resonant modes is
hindered by the wavelength and pump dependent nature of
the quantum-well active region of these laser devices.
To study laser action in these hybridized cavities the
fiber taper is removed to eliminate external cavity loading
effects, and vertically scattered light emission from the mi-
crodisks is collected via a high numerical aperature lens in-
(
a
)
(b)
Intensit
y
(a.u.)
8
0
4
25
15
5
-5
ModeI(E
z
)
Mode II (E
r
)
r
(
μ
m)
0.2 0.6 1.0
3.4
3.6
z
(
μ
m)
3.8
Ag covered
r
(
μ
m)
0.2 0.6 1.0
12
12
12
8
4
0
12
8
4
0
ModeI(E
r
)
Mode II (E
z
)
r
(
μ
m)
0.2 0.6 1.0
No metal
r
(
μ
m)
0.2 0.6 1.0
r
(
μ
m)
0.2
0.6
1.0
3.4
3.6
z
(
μ
m)
3.8
(d)
disk
pedestal
air
(c)
12
8
4
0
Intensit
y
(a.u.)
λ
0
(
μ
m)
0.4
0.8
0.6
Q
1.300
1.325
1.350
1.400
1.375
R
p
/
R
d
1.0
1.0
2.0
3.0
4.0
x10
3
λ
-Mode I
λ
-Mode II
Q-Mode I
Q-Mode II
0.3
0.4
0.6
0.8
1.0
0.5
0.7
0.9
R
m
/
R
d
0.2
3.0
5.0
7.0
9.0
11
1.0
x10
3
Q
λ
0
(
μ
m)
1.8
1.6
1.4
1.2
1.0
z
(
μ
m
)
3.4
3.6
3.8
FIG. 2.

Color online

A 250 nm thick semiconductor microdisk with radius
R
d
= 0.65

m and a top silver contact is simulated with varying silver cov-
erage.

a

Wavelength


0

and quality factor

Q

for two anticrossing modes

mode I and mode II

are plotted as a function of silver metal radial fraction

R
m
/
R
d

,

b

Azimuthal slices of dominant electric field components of the
two extreme cases: with and without silver. Metal is denoted by white hatch
marks.

c

Wavelength and
Q
of mode II as a function of increasing InP
pedestal radius

R
p

with
R
m
/
R
d
= 0.7.

d

Mode profile of
E
r
field compo-
nent of mode II with maximum
Q

R
p
/
R
d
= 0.8.

0.55
1200
1300
1400
0.60 0.65 0.70
1100
0.50
(d)
1300
5.0
4.0
3.0
2.0
1.0
1260
1340
Normalized Intensity (a.u.)
R
m
/
R
d
= 0.61
0.59
0.57
0.55
0.53
a.u.
(a)
Wavelength (nm)
disk
pedestal
metal
0102030
Device #
0.2
0.3
0.4
0.5
0.6
(b)
0.0
Radius (
μ
m)
Wavelength (nm)
R
m
/
R
d
(c)
FIG. 3.

Color online

a

Measured disk, metal, and pedestal radii of fab-
ricated device array,

b

Zoom-in of anticrossing region as indicated by a
dashed white box in panel

c

. Dotted lines are guides to the eye.

c

Fiber-
taper-collected normalized spectra on a log-scale as a function of
R
m
/
R
d

P
p
= 1 mW,

T
= 500 ns, and
T
=1

s

.

d

Free-space-collected PL of un-
patterned laser material

P
p
= 282

W, cw

.
201114-2
Perahia
etal.
Appl. Phys. Lett.
95
, 201114

2009

Downloaded 07 Dec 2009 to 131.215.193.213. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
stead. The microdisks are also pumped with low-duty-cycle
pulses


T
=20 ns,
T
=4

s

in order to reduce thermal ef-
fects. Threshold curves for a series of microdisks near the
region of strong SP and dielectric mode hybridization are
shown in Fig.
4

a

. The peak absorbed pump power is esti-
mated based on a pump spot size of diameter
D
=2

m,
absorption efficiency of

=10
%
, and accounting for the
pump duty cycle. Disks with
R
m
/
R
d

0.53 exhibit a clear
“S” shaped logarithmic light-in versus light-out curve, indi-
cating lasing action. In each of these cases, the lasing mode
is from the longer wavelength branch of the hybridized
modes. Typical of the laser behavior for these devices is the
laser with
R
m
/
R
d
= 0.42, which has an estimated threshold
peak absorbed pump power of only
P
=5

W. Spectra as a
function of peak absorbed pump power for this laser are
plotted in Fig.
4

b

. A strong blueshift of the laser wave-
length with increased pumping is seen, attributable to free-
carrier dispersion. The inverse linewidth as a function of
integrated output power is also plotted in Fig.
4

c

, and
shows linewidth narrowing typical of a semiconductor laser
with large coupling between carrier density

gain

and refrac-
tive index

cavity frequency

.
18
,
19
In the middle of the anti-
crossing region, for microdisks with 0.55

R
m
/
R
d

0.65, no
lasing was observed. This is likely due to the increased op-
tical loss in this region predicted by simulation

see Fig.
2

a


. For microdisks with
R
m
/
R
d

0.65 lasing action was
also not observed, even for the higher-
Q
shorter wavelength
branch of modes. Although simulations indicate that the op-
tical cavity
Q
-factor for this mode should recover

and in-
crease for an optimal pedestal size

, as can be seen in Figs.
3

c

and
3

d

the mode blueshifts a significant fraction of the
PL bandwidth. As the devices that do lase are pumped quite
hard to reach the lasing condition, any reduction in gain or
Q
could preclude lasing action.
Beyond the initial demonstration of SP-dielectric mode
hybridization and lasing in subwavelength partially metal
coated microdisk cavities presented here, future efforts will
focus on achieving electrically injected lasing action using
the surface-plasmon metal layer as a top metal contact and
thermal heat sink. It is anticipated that further device engi-
neering involving the top metal contact layer should allow
room temperature, continuous-wave

cw

lasing action in
such subwavelength laser cavities. An improved optical qual-
ity factor may also be achieved in the shorter-wavelength
mode branch by engineering the epitaxy thickness. Ulti-
mately, such improved structures should allow for the study
of fundamental as well as practical issues associated with the
scaling laws of deep-subwavelength mode volume semicon-
ductor lasers, where the small-scale system size of the laser
results in significant photon number and carrier number
fluctuations.
20
This work was supported by the DARPA NACHOS pro-
gram

Grant No. W911NF-07–1–0277

. The authors would
like to thank Kartik Srinivasan for helpful discussion regard-
ing the device processing, Jianxin Chen for growth of the
laser material, and the Kavli Nanoscience Institute at
Caltech.
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.
R
m
/R
d
= 0.45
R
m
/R
d
= 0.49
R
m
/R
d
= 0.53
R
m
/R
d
= 0.55
Peak Absorbed Pump Power (
μ
W)
10
0
10
1
10
-2
10
3
10
-3
10
2
10
1
10
0
10
-1
Integrated Laser Power (pW)
(a)
510152025
Wavelength (nm)
1292
1394
1396
1398
1300
Peak Absorbed Power (
μ
W)
160 200
0 40 80 120
1.0
2.0
4.0
3.0
5.0
0.0
(Linewidth)
-1
(nm
-1
)
Integrated Laser Power (pW)
(c)
(b)
1
.
0
0.8
0.6
0.4
0.2
0.0
FIG. 4.

Color online

a

L-L curves of several devices of increasing
R
m
/
R
d
.

b

Normalized spectra vs pump power, and

c

inverse linewidth vs
output laser power for a device with
R
m
/
R
d
= 0.42.
201114-3
Perahia
etal.
Appl. Phys. Lett.
95
, 201114

2009

Downloaded 07 Dec 2009 to 131.215.193.213. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp