Appl. Phys. Lett. 96, 251101 (2010); https://doi.org/10.1063/1.3455316 96, 251101
© 2010 American Institute of Physics.
Metal-cavity surface-emitting microlaser at
room temperature
Cite as: Appl. Phys. Lett. 96, 251101 (2010); https://doi.org/10.1063/1.3455316
Submitted: 14 April 2010 • Accepted: 24 May 2010 • Published Online: 21 June 2010
Chien-Yao Lu, Shu-Wei Chang, Shun Lien Chuang, et al.
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Metal-cavity surface-emitting microlaser at room temperature
Chien-Yao Lu,1Shu-Wei Chang,1Shun Lien Chuang,1,a兲Tim D. Germann,2and
Dieter Bimberg2
1Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign,
1406 West Green Street, Urbana, Illinois 61801, USA
2Institut für Festkörperphysik, Technische Universität Berlin, Sekretariat EW 5-2, Hardenbergstrase 36,
10623 Berlin, Germany
共Received 14 April 2010; accepted 24 May 2010; published online 21 June 2010兲
We propose and realize a substrate-free metal-cavity surface-emitting microlaser with both top and
sidewall metal and a bottom distributed Bragg reflector as the cavity structure. The transfer-matrix
method is used to design the laser structure based on the round-trip resonance condition inside the
cavity. The laser is 2.0
m in diameter and 2.5
m in height, and operates at room temperature
with continuous-wave mode. Flip-bonding the device to a silicon substrate with a conductive metal
provides efficient heat removal. A high characteristic temperature about 425 K is observed from 10
to 27 °C. © 2010 American Institute of Physics.关doi:10.1063/1.3455316兴
Metals, with a negative permittivity at optical frequency,
can be engineered to support a surface-plasmon wave and
confine the optical field within a skinny region near the
metal/dielectric interface.1,2With possible energy localiza-
tion near the metal/dielectric interface, metal cavities become
a promising resonance structure in the subwavelength
regime.3Despite its large ohmic loss at optical frequencies,
lasing in metal cavities is realizable even for a moderate
quality factor.4,5Although these quality factors are not higher
than those of conventional dielectric cavities, the photon life
time becomes shorter, and therefore, a higher modulation
speed is expected. Moreover, an enhanced bandwidth
with the shrinkage of the aperture in oxide-confined
vertical-cavity surface emitting lasers 共VCSELs兲due to the
smaller mode volume has been reported.6The mode volume
of the metal-coated devices can be even smaller and poten-
tially leads to a higher modulation bandwidth. Another supe-
rior property originates from the high thermal conductivity
of the metal, which removes the heat that would degrade the
material and device performance, especially for ultrasmall
devices under high-density current injection, like VCSELs.
Room-temperature operation of a metal-coated laser
with bottom emission has recently been demonstrated at
pulsed current injection.5Several interesting effects have
been discovered, including surface-plasmon-mode lasing and
high group indices due to large metal dispersions. However,
when the device size is comparable or even smaller than the
wavelength, several issues such as the small-aperture effect
and resonance shift due to the change in the effective index
should be taken into consideration.7
In this letter, we propose the design, show our fabrica-
tion, and demonstrate a metal-cavity surface-emitting laser
with metal and distributed Bragg reflector 共DBR兲as the feed-
back structures, surrounded by a metal cavity as the sidewall.
The emission from the top of devices can be more easily
coupled to fibers or lens. Moreover, the metal serves as a
broadband high-reflectivity mirror, as well as the substrate
and heat sink for these devices. From an application view-
point, this configuration eases the device integration to sili-
con or other substrates that could potentially be used as the
platform for optical interconnects in the terabit range. In
terms of fabrication, using top metal as the feedback mirror
prevents the deep etching, which is usually challenging for
submicron or nanodevices. Also, by coating the sidewall
with metal, the diffraction loss from the small aperture and
sidewall roughness can be reduced.
The epistructure is composed of a p-type
GaAs共20 nm兲/Al0.2Ga0.8As共96 nm兲cladding and an n-type
17.5-pair alternating Al0.9Ga0.1As/Al0.15Ga0.85As quarter-
wavelength DBR. The active medium consists of a
GaAs/Al0.2Ga0.8As multiple quantum-well 共MQW兲structure
共14 wells and 15 barriers兲. An InGaP etch-stop layer is in-
serted between DBR and GaAs substrate for substrate re-
moval. The thickness of each layer was determined based on
the transfer-matrix method.8To include the metal effect and
modal confinement properties, the modal effective index of
the fundamental HE11 mode is used instead of bulk semicon-
ductor refractive index. Due to the plasma damping in metals
at optical frequencies, the complex permittivity is introduced
in the layer design.8,9The thickness of the insulator layer
between metal and sidewall was also optimized10 and taken
into account in the modal effective index. The total round-
trip phase shift is8
⌽=2k0nGaAsdGaAs +2k0nAl0.2Ga0.8AsdAl0.2Ga0.8As
+2k0nQWdQW +
metal +
DBR,共1兲
where nGaAs and nAl0.2Ga0.8As are the effective indices of the
HE11 mode in these two materials; dGaAs and dAl0.2Ga0.8As are
thicknesses of top p-type GaAs and Al0.2Ga0.8As layers; nQW
is the effective index of the HE11 mode calculated for the
metal-cavity waveguide using an average refractive index of
the MQW region; dQW is the total thickness of the active
region; k0is the free space propagation constant; and
metal
and
DBR are the phase shifts in the complex reflection co-
efficients of the top metal mirror and bottom DBR, respec-
tively. The thickness of active region was tuned to match the
round-trip resonance condition 共
=2m
,m=0,1,2,...兲of
the bottom DBR and top metal for the formation of standing
wave inside the cavity. The phase shift from top metal
metal
was estimated by Fresnel formula with complex wavelength-
a兲Author to whom correspondence should be addressed. Electronic mail:
APPLIED PHYSICS LETTERS 96, 251101 共2010兲
0003-6951/2010/96共25兲/251101/3/$30.00 © 2010 American Institute of Physics96, 251101-1
dependent permittivities of metal and semiconductor.9,11
Also, with the additional phase shift
DBR from the bottom
DBR, the round-trip resonance condition can be fulfilled in-
side the cavity. In our design, the active region thickness was
chosen to be 426.0 nm, corresponding to a standing-wave
pattern of 3.5 antinodes in the active region. Compared to
designs of conventional VCSELs,12 which consider only the
material dielectric constants without the modal properties,
our design, due to the small device size and high permittivity
contrast between metals and dielectrics, brings about more
flexibility for the resonance tuning by adjusting cavity di-
mensions. Figure 1共a兲shows the device structure and stand-
ing wave pattern 共magnitude of the electric field 兩E兩兲 inside
the cavity. The HE11 mode has the highest Qfactor and clos-
est resonance wavelength to the experimental data. The cor-
responding modal profile at the anitnode is shown in Fig.
1共b兲. The designed resonance is found to be at 869.2 nm with
a quality factor of 505. The threshold material gain gth can be
estimated by the following formula:13
gth =
⌫EvgQ,共2兲
where ⌫Eis the energy confinement factor, Qis the cavity
quality factor,
is the optical angular frequency, and vgis
the material group velocity which can be estimated by the
speed of light divided by the group index of GaAs. The
estimated value of the threshold gain is around 8400 cm−1.
The epistructure was grown by industry standard metal-
organic vapor-phase epitaxy using a commercial AIXTRON
200/4 reactor and partly alternative precursors.14 A silicon
nitride 共SiNx兲layer was deposited and patterned as the mask
for the following dry etching. Reactive ion etching on the
epiwafer was performed with chlorine-based reaction gas. A
thin SiNxlayer was conformally deposited after the removal
of the top silicon-nitride mask. Planarization was used to
expose the top of the post covered by silicon nitride, which
was then removed to facilitate the following contact forma-
tion. E-beam evaporation was used to uniformly coat the
device with 200 nm silver and 50 nm gold layers for oxida-
tion protection. The sample was then flip-bonded to a gold-
coated silicon receptor wafer. The GaAs substrate was com-
pletely removed by a mixture of citric acid and hydrogen
peroxide. N-type contacts 共AuGe/Ni/Au兲were formed and
defined on the InGaP etching-stop layer. Figure 1共c兲shows
the fabricated 2.0-
m-diameter cavity before silver coating.
A full-structure simulation was carried out by finite differ-
ence time domain 共FDTD兲method with the real device size
and geometry. According to the field pattern in Fig. 1共d兲,
most of the energy is stored in the active region with an
exponentially decaying tail into the DBR as a consequence
of the successive reflections from DBR periods. The mode
pattern shows that the mode is indeed bouncing back and
forth between top metal and bottom DBRs and confined by
the metal sidewall.
The measurements were performed at 300 K with a
thermoelectrically-cooled heat sink on which the devices
were mounted. The emission was collected by an objective
lens, dispersed by a 1.25 m monocromator, and detected by a
liquid-nitrogen-cooled germanium detector. The current ver-
sus voltage 共I-V兲and light output power versus current 共L-I兲
curves under CW operation shown in Fig. 2共a兲indicate a
turn-on threshold at around 1.66 mA, beyond which a lasing
peak emerges at 868 nm with a gradual redshift as the driv-
1
m
(c) |E|2
(d)
1
m1
m
(c) |E|2
(d) |E|2
(d)
n-contact
n-contact
InGaP
SiNx
Ag
17.5-pair
DBR
MQWs
p-GaAs/Al0.2Ga0.8As
(a)
Ag ~ 10 nm
(b)
|E|
Ag
SiNx
MQWs
1.0
0.8
0.6
0.4
0.2
0.0
n-contact
n-contact
InGaP
SiNx
Ag
17.5-pair
DBR
MQWs
p-GaAs/Al0.2Ga0.8As
(a)
Ag ~ 10 nm
(b)
|E|
Ag
SiNx
MQWs
1.0
0.8
0.6
0.4
0.2
0.0
n-contact
n-contact
InGaP
SiNx
Ag
17.5-pair
DBR
MQWs
p-GaAs/Al0.2Ga0.8As
(a)
Ag ~ 10 nm
(b)
|E|
Ag
SiNx
MQWs
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1
m
(c) |E|2
(d)
1
m1
m
(c) |E|2
(d) |E|2
(d)
n-contact
n-contact
InGaP
SiNx
Ag
17.5-pair
DBR
MQWs
p-GaAs/Al0.2Ga0.8As
(a)
Ag ~ 10 nm
(b)
|E|
Ag
SiNx
MQWs
1.0
0.8
0.6
0.4
0.2
0.0
n-contact
n-contact
InGaP
SiNx
Ag
17.5-pair
DBR
MQWs
p-GaAs/Al0.2Ga0.8As
(a)
Ag ~ 10 nm
(b)
|E|
Ag
SiNx
MQWs
1.0
0.8
0.6
0.4
0.2
0.0
n-contact
n-contact
InGaP
SiNx
Ag
17.5-pair
DBR
MQWs
p-GaAs/Al0.2Ga0.8As
(a)
Ag ~ 10 nm
(b)
|E|
Ag
SiNx
MQWs
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
FIG. 1. 共Color兲共a兲Schematics of the microlaser. The blue line represents
the magnitude of the optical field 兩E兩. The laser cavity is formed with both
top and sidewall metal and a bottom DBR. The laser is 2.0
m in diameter
and 2.5
m in height with the substrate removed. 共b兲The corresponding
HE11 mode pattern. 共c兲A scanning electron micrograph of the fabricated
cavity with the SiNxsurrounding before metal coating. 共d兲The field pattern
兩E兩2of the device simulated by FDTD.
7750 8000 8250 8500 8750 9000
X5
X10
0.64 mA
1.28 mA
1.54 mA
1.91 mA
2.46 m
A
Wavelen
g
th
(
Å
)
X40
Emission Intensity (a.u.)
01234
0.0
1.5
3.0
4.5
6.0
7.5
0.0
1.5
3.0
4.5
6.0
7.5
Current (mA)
Voltage (V)
Power (
W)
(a)
(b)
CW@300K
CW@300K
7750 8000 8250 8500 8750 9000
X5
X10
0.64 mA
1.28 mA
1.54 mA
1.91 mA
2.46 m
A
Wavelen
g
th
(
Å
)
X40
Emission Intensity (a.u.)
01234
0.0
1.5
3.0
4.5
6.0
7.5
0.0
1.5
3.0
4.5
6.0
7.5
Current (mA)
Voltage (V)
Power (
W)
(a)
(b)
CW@300K
CW@300K
FIG. 2. 共Color online兲共a兲The I-V and L-I curves measured at room tem-
perature 共300 K兲under dc current injection. The L-I curve shows a threshold
of 1.66 mA. 共b兲The corresponding current-dependent spectra. Below thresh-
old, a broad spontaneous emission spectrum was observed. The lasing peak
around 868 nm appeared when the injection current exceeded the threshold
value.
251101-2 Lu et al. Appl. Phys. Lett. 96, 251101 共2010兲
ing current increases 关Fig. 2共b兲兴. The output power increases
from 0.5
W near threshold to over 7
W at 4.5 mA. We
also observed a second mode emerging at a wavelength 0.4
nm shorter than the first lasing peak. This additional mode
comes from the splitting of HE11 mode into two orthogonal
and quasilinearly polarized modes, most likely due to a small
anisotropy of the waveguide structure.15 As shown in Fig.
2共a兲, the kink in the L-I curve around 3.2 mA corresponds to
the onset of the second peak. To identify the origin of the
second peak, we show the polarization-resolved L-I curves
of individual modes in Fig. 3共a兲. From Fig. 3共b兲, the onset of
the second lasing peak reflects the kink in the L-I curve
shown in Fig. 2共a兲. Near threshold, the linewidth of the las-
ing peak is estimated to be around 1.5 nm 共@ 1.45 mA兲,
which gives a Qfactor of 580 and agrees quantitatively with
that of our transfer-matrix estimation for a cold cavity. The
narrowest setup-resolvable linewidth is 67 pm at 2.73 mA,
obtained just below the threshold of the second peak. The
threshold current density was estimated to be 52.8 kA/cm2.
With the lossy metal coating and high radiation loss from the
moderate reflectivity 共about 0.93兲of the silver mirror, the
threshold current density is high. The detuning of the cavity
resonance wavelength from the band-edge wavelength 共845
nm兲of the MQW active region and a nonuniform distribu-
tion of carriers in the 14 quantum wells may be additional
reasons for the high threshold. Further improvements, such
as reducing threshold by tuning the resonance toward the
peak gain wavelength and size reduction to submicron diam-
eter, are in progress.
From the measured L-I curves and the threshold currents
for temperature from 10 to 27 °C, we obtained a character-
istic temperature T0of 425 K. This characteristic temperature
is considerably higher than the reported values16 for semi-
conductor quantum-well lasers with bulk substrates. In con-
trast to conventional top-and-bottom-DBR lasers in which
the resonance shifts due to the effect of temperature on DBR
phase and reflectivity,17 the stability of our laser may be due
to the broadband high-reflectivity window provided by top
metallic mirror, which makes the resonance less sensitive to
temperature. Also, we believe that the surrounding metal and
flip bonding remove heat more efficiently and keep the op-
eration temperature more stable.
In summary, we have demonstrated the design and
implementation of room-temperature CW substrate-free
metal-cavity surface-emitting microlasers with a top and
sidewall metal and a bottom DBR as feedback structures.
The modal properties of the guided modes have been incor-
porated into the design. The devices showed good thermal
stability due to metal coating and flip bonding to silicon.
These metal-cavity microlasers provide a great potential for
next generation massively parallel optical interconnects
without crosstalk among laser elements.
This work at UIUC was supported partially by DARPA
W911NF-07–1–0314. S.L.C. also thanks the support by the
Humboldt Research Award. The work at TUB was supported
by Deutsche Forschungsgemeinschaft in the frame of Grant
No. SFB 787.
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8640 8660 8680 8700 8720 8740 8760
Wavelen
g
th
(
Å
)
Emission Intensity
(
a.u.
)
I(mA
)
3.81
3.54
3.27
3.00
2.73
2.45
2.18
2.00
1.91
1.82
1.73
1.64
01234
90o
Power (a.u.)
Current (mA)
0
o
(
a
)
(b)
CW@300K
CW@300K
8640 8660 8680 8700 8720 8740 8760
Wavelen
g
th
(
Å
)
Emission Intensity
(
a.u.
)
I(mA
)
3.81
3.54
3.27
3.00
2.73
2.45
2.18
2.00
1.91
1.82
1.73
1.64
01234
90o
Power (a.u.)
Current (mA)
0
o
(
a
)
(b)
CW@300K
CW@300K
FIG. 3. 共Color online兲共a兲The polarization-resolved L-I curves indicate two
modes due to the splitting of two degenerate HE11 modes. A clear onset of
the second mode around 3.30 mA was observed. 共b兲The spectral evolution
of the two lasing peaks with increasing current. The second peak emerged
above its threshold. A clear redshift due to heat was observed when the
current increased. All data were taken with CW operation at 300 K.
251101-3 Lu et al. Appl. Phys. Lett. 96, 251101 共2010兲
