Characteristics of Electrically Driven Two-Dimensional Photonic Crystal Lasers
ABSTRACT We demonstrate room-temperature low-threshold-current lasing action from electrically driven wavelength-scale high-quality photonic crystal lasers having large spontaneous emission factors by solving the theoretical and technical constraints laid upon by the additional requirement of the current injection. The ultrasmall cavity is electrically pulse pumped through a submicron-size semiconductor “wire” at the center of the mode with minimal degradation of the quality factor. In addition, to better utilize the low mobility of the hole, we employ a doping structure that is inverted from the conventional semiconductors. Rich lasing actions and their various characteristics are experimentally measured in the single-cell and three-cell photonic crystal cavities. Several relevant measurements are compared with three-dimensional finite-difference time-domain computations based on the actual fabricated structural parameters. The electrically driven photonic crystal laser, which is a small step toward a “practical” form of the single photon source, represents a meaningful achievement in the field of photonic crystal devices and photonic integrated circuits as well as of great interest to the quantum electrodynamics and quantum information communities.
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ABSTRACT: Optical microcavities are resonators that have at least one dimension on the order of a single optical wavelength. These structures enable one to control the optical emission properties of materials placed inside them. They can, for example, modify the spatial distribution of radiation power, change the spectral width of the emitted light, and enhance or suppress the spontaneous emission rate. In addition to being attractive for studying the fundamental physics of the interaction between materials and vacuum field fluctuations, optical microcavities hold technological promise for constructing novel kinds of light-emitting devices. One of their most dramatic potential features is thresholdless lasing. In this way and others, controlled spontaneous emission is expected to play a key role in a new generation of optical devices.Science 05/1992; 256(5053):66-70. · 31.03 Impact Factor
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ABSTRACT: The rate equations for a microcavity semiconductor laser are solved and the steady-state behavior of the laser and some of its dynamic characteristics are investigated. It is shown that by manipulating the mode density and the spontaneous decay rates of the cavity modes, the threshold gain can be decreased and the modulation speed can be improved. However, in order to fully exploit the possibilities which the modification of the spontaneous decay opens up, the active material volume in the cavity must be smaller than a certain value. Threshold current using different definitions, population inversion factor, L-I curves, linewidth, and modulation response are discussedIEEE Journal of Quantum Electronics 12/1991; · 1.83 Impact Factor
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ABSTRACT: This paper presents a preliminary guide to realize microcavity semiconductor lasers exhibiting spontaneous emission control effects. It includes: 1) theoretical consideration on the effects; 2) processing techniques for semiconductor microcavities; and 3) some demonstrations of photonic crystal and microdisk cavity. It was shown that, even with a spectral broadening of electron transition, thresholdless lasing operation and alternation of spontaneous emission rate are expected in a cavity satisfying the single mode condition that only one mode is allowed in the transition spectrum. An ideal three-dimensional (3-D) photonic crystal has the potentiality for realizing this condition. In two-dimensional (2-D) crystals and microdisk cavities, thresholdless operation is also expected, but the alternation of spontaneous emission rate may be negligible due to the insufficient optical confinement. In the experiment, some processing techniques for GaInAsP-InP system were investigated and methane-based reactive ion beam etching was selected because of the smooth sidewalls and adaptability to arbitrary structures. A GaInAsP-InP 2-D photonic crystal constructed by submicron columns was fabricated using this method. Owing to the slow surface recombination of this material, a polarized photoluminescence and peculiar transmission spectra were observed at room temperature (RT), which can be explained by a photonic band calculation. However, some technical improvement is necessary for clear demonstration of photonic bandgap, which is minimally required for device applications. In contrast to this, a GaInAsP-InP microdisk cavity of 2 μm in diameter, which corresponds to the cavity volume 2.5 times the single-mode condition, has achieved RT lasing with threshold current as low as 0.2 mA. Further reduction of diameter and realization of continuous-wave (CW) operation will provide a significant regime for the observation of spontaneous emission control effectsIEEE Journal of Selected Topics in Quantum Electronics 07/1997; · 4.08 Impact Factor
IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 9, SEPTEMBER 20051131
Characteristics of Electrically Driven
Two-Dimensional Photonic Crystal Lasers
Hong-Gyu Park, Se-Heon Kim, Min-Kyo Seo, Young-Gu Ju, Sung-Bock Kim, and Yong-Hee Lee
Abstract—We demonstrate room-temperature low-threshold-
current lasing action from electrically driven wavelength-scale
high-quality photonic crystal lasers having large spontaneous
emission factors by solving the theoretical and technical con-
straints laid upon by the additional requirement of the current
injection. The ultrasmall cavity is electrically pulse pumped
through a submicron-size semiconductor “wire” at the center
of the mode with minimal degradation of the quality factor. In
addition, to better utilize the low mobility of the hole, we employ a
doping structure that is inverted from the conventional semicon-
ductors. Rich lasing actions and their various characteristics are
experimentally measured in the single-cell and three-cell photonic
crystal cavities. Several relevant measurements are compared
with three-dimensional finite-difference time-domain computa-
tions based on the actual fabricated structural parameters. The
electrically driven photonic crystal laser, which is a small step
toward a “practical” form of the single photon source, represents
a meaningful achievement in the field of photonic crystal devices
and photonic integrated circuits as well as of great interest to the
Index Terms—Current injection, finite-difference time-domain
(FDTD), microcavity, photonic band gap, semiconductor laser,
single photon source, spontaneous emission factor.
and recently gained renewed attention with the aid of the pho-
tonic crystal that enables strong photon confinement –.
The photonic crystal cavity has a lot of merit as an efficient
light emitter compared with the other types of microcavities:
one can easily control distinct lasing properties such as wave-
length, radiation direction, and near/far mode shapes by slight
modification of lattice parameters –. Also, one can ob-
tain a high quality
factor and small modal volume simul-
taneously, which are the two main conditions of low-threshold
lasers –.Sincethissmallcavitysupportsonly a fewres-
onant modes, the ultimate thresholdless laser with a large spon-
taneous emission factor
near unity can be actualized ,
, , . In addition, once the issue of coupling with a
HE ULTIMATE smallest possible laser has long been a
Manuscript received March 17, 2005; revised April 24, 2005. This work
was supported in part by the National Research and Development Project for
Nanoscience and Technology.
H.-G. Park is with the Department of Chemistry and Chemical Biology, Har-
S.-H. Kim, M.-K. Seo, and Y.-H. Lee are with the Department of Physics,
Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea
(e-mail: firstname.lastname@example.org; email@example.com; firstname.lastname@example.org).
Y.-G. Ju and S.-B. Kim are with the Telecommunication Basic Research
Laboratory, Electronics and Telecommunications Research Institute, Daejeon
305–701, Korea (e-mail: email@example.com; firstname.lastname@example.org).
Digital Object Identifier 10.1109/JQE.2005.852800
photoniccrystal waveguideand/ora commercialtapered fiberis
solved, photons extracted from the extremely small cavity can
be efficiently utilized . Finally, as a strong candidate of a
single photon generator on chip, this ultrasmall cavity with a
high Purcell factor has attracted many research groups .
Two-dimensional (2-D) photonic crystal InGaAsP slabs are
widely used as a basic building block for various photonic de-
vices . When a photonic crystal cavity is formed, photons
tend to be localized in the proximity of the photonic crystal res-
onator by the effects of photonic bandgap and total internal re-
flection . So far, most of photonic crystal defect lasers have
been operated by optical pumping –, –.Electri-
cally driven photonic band edge lasers were also reported with
relatively large modal volume and threshold current –.
However, only if one is able to activate the wavelength-scale
alone simple ultrasmall photon sources that operate with min-
imum power budget. Unfortunately, the electrical pumping of
the wavelength-scale photonic crystal laser structure involves
several critical issues that have to be answered. Therefore, the
and exciting issue of the community. As one of candidates of
the electrically pumped structures, we have suggested the in-
troduction of a small post at the center of the cavity , .
In that structure, it is possible to excite only those modes that
have a node at the post position selectively because the other
modes having a central antinode are suppressed by severe op-
tical losses in the vertical direction. In addition, the post plays
a role of a heat sink . Thanks to this simple idea and deli-
cate fabrication, we recently reported a novel electrically driven
photonic crystal laser structure .
We begin this paper by considering several design issues of
our electrical pumping structure (Section II). Then we explain
fabrication methods of a current blocking layer and a small
central post in Section III. In the subsequent sessions, various
characteristics of the photonic crystal lasers are analyzed.
Throughout the paper, several relevant experimental results
are compared with three-dimensional (3-D) finite-difference
time-domain (FDTD) computations which are performed by
the structure obtained from the actually fabricated sample.
II. DESIGN OF ELECTRICAL CONTACT
Several photonic crystal structures suitable for electrical
pumping have been suggested –. Among them, pho-
tonic band edge lasers were realized first, thanks to their
relatively large active volume and the simple current confine-
ment –. On the other hand, the ultrasmall photonic
crystal defect laser was recently demonstrated after solving
0018-9197/$20.00 © 2005 IEEE
1132 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 9, SEPTEMBER 2005
injection. (b) Designed wafer structure.
(a) Schematic diagram of the free-standing slab structure for current
the issue of current injection into the small cavity . Both
electrical and optical confinements inside that cavity should be
effectively actualized. One possible way to inject current into
the slab with a periodic pattern could be found in the microdisk
structure , . Small posts are positioned below and above
the microdisk and act as the current channel into an active
material embedded inside the cavity. In our photonic crystal
structure, we place only one post under the slab as an electrical
contact and substitute the other one with a large metal contact
around the cavity as in conventional semiconductor devices
–. Because the size of the photonic crystal cavity is
much smaller than that of the microdisk, a two-post-structure
cannot guarantee the mechanical stability. The topmost layer
of the semiconductor slab is heavily n-doped over 10
In particular, the formation of the n-i-p heterojunction slab
increases overall injection efficiency of the carriers into the
cavity since the mobility of electrons is larger than that of holes
. In this configuration, holes that are supplied through the
post do not diffuse far from the center and electrons and holes
recombine mostly near the central post. Experimentally, the
post formation in the ultrasmall cavity is nontrivial. We decided
to place the post at the center of the cavity and to select the
optical mode with a central intensity node. For these modes,
the introduction of a small central post does not degrade the
factor of the mode significantly , . On the other hand,
those modes with central anti-nodes are discarded because they
(b) photonic crystal patterns.
Schematic diagrams of fabrication processes of (a) mesa structure and
undergo unavoidable optical losses through the central post. A
schematic diagram of our structure is shown in Fig. 1(a).
Characteristics of the resonant mode having a central node
havebeen already studied in theory and experiment. In par-
ticular, the monopole and hexapole modes attract our attention
because of the merits of the nondegeneracy, high
small modal volumes. These modes can be found in a modified
well-separated from the other resonant modes.
Fig. 1(b) shows our bare wafer structure. Six intrinsic and
strain-compensated InGaAsP quantum wells (QWs) and highly
doped n- and p- layers are employed. In particular, note that the
delta doping layer in the top layer of the thin slab is contained.
In the case of the microdisk, the slab is mechanically sup-
ported by the pedestal structure , . However, for the pho-
tonic crystal cavity, the central post is much smaller and weaker
and therefore mechanically unstable. We need to insert dielec-
tric material under the slab to support the relatively large slab
structure. In the structure shown in Fig. 1(a), the dielectric ma-
terial consisting of photoresist (PR) is acting also as a current
blocking layer . Several complicated fabrication processes
are required to produce this structure [Fig. 2(a)]. First, mesas
with a diameter of
50 m are formed for electrical isolation
by dry etching processes. Thick PR with thickness of
used as a mask for CH
H reactive ion etching (RIE) plasma
etching. Then, a
- m -large p-InP layer post remains
3 m is
PARK et al.: CHARACTERISTICS OF ELECTRICALLY-DRIVEN 2-D PHOTONIC CRYSTAL LASERS 1133
at the center and the other sacrificial InP region is etched by di-
luted HCl solution HCl:H O
7 min. If the thick PR mask is removed before the wet
etching process, some parts of undercut slab would stick to the
faces. But, in our step, the thick PR prevents the collapse of the
pedestal structure. The etched region is filled with a dielectric
material (another PR) for mechanical stability by spin coating.
Now, all surfaces of the mesa including the etched region are
coated by the PR. Subsequent O ashering removes the entire
exposed PR except that is hidden under the slab. If thin Si N
orSiO layershad beencoatedbelowthefirstPRmask,allrem-
nants of the PRs after the ashering process could be peel off by
blocking layer by PR is hardened at temperature over 250 C.
After the ring-shaped AuGe metal contact is formed, fabrica-
tion of the first post surrounded by the current blocking layer is
We have also tried several other methods in order to con-
struct the current blocking layer besides the above trial. First,
we tried the selective ion implantation scheme where only a
specific region has a high electrical resistance and the desired
current path can be designed , . If heavy ions such as Fe
and Cl are injected into the p-InP layer below the active layer,
current is expected to flow only through the nonattacked region.
Although this method looks easy and simple, the exact loca-
tion and range of the bombing region were difficult because the
semiconductor slab isverythin.Actually, inour fabricatedsam-
ples, the current flow was not easily confined and/or QWs em-
bedded in the slab were damaged by the heavy ions. In the other
trial, we tested selective oxidation techniques  using a dif-
ferent wafer structure: instead of the sacrificial InP layer below
the slab, an Al-containing layer is placed and oxidized to a di-
electric layer with low refractive index. If we use InAlAs ma-
terial, the oxidation time gets very slow and QWs are damaged
by the high temperature of the oxidation process . To avoid
this damage, we added a fusion technique by which a GaAlAs
layer is put under the slab , , . Through a standard
oxidation procedure, effective current confinement structure is
expected . However, this method also has serious problems
in the subsequent processes and we failed to obtain the unox-
idized small post of diameter
second oxidation process. Furthermore, the unoxidized region
is not generally formed at the center of the resonator.
In the following steps shown in Fig. 2(b), photonic crystal
patterns and a small central post are fabricated. The lattice pa-
rameters are designed such that the monopole mode is located
in the middle of photonic band gap. Fig. 3 shows a scanning
electron miscroscope (SEM) picture of the fabricated photonic
crystal structure whose lattice parameters are as follows:
, , whereis the lattice constant,
of the air hole in the th region, respectively. The radii of the air
holes in the th region are all same. Note that the radii of the air
holes are continuously changed and the smallest near the cavity.
The main purpose of this specifically modified structure is to
place the post at the center below the cavity during the etching
process. The post is formed by HCl etching (HCl:H O
at room temperature
1m because of the delicate
Fig. 3.SEM picture (top view) of a fabricated photonic crystal structure.
PR produced in the intentional breaking process.
Cross-sectional SEM image. Dusts around the post are remnants of the
at low temperature near 10 C. The lateral etching rate tends to
depend on the size of air holes through which diluted HCl is
supplied, and we achieve better control of the position and size
of the central post by introducing chirped air holes shown in
Fig. 3. These chirped air holes turn out to increase the
and the modal volume of the cavity slightly. This fact is con-
firmed by 3-D FDTD calculations for an ideal structure with
circular air holes and without a post. In our fabrication, the wet
etching time is actually
6 min which is sensitive and change-
able by the previous dry etching time, cleaning of the wafer and
surrounding air temperature. Fig. 4 shows an SEM picture of a
fabricated post where the slab is intentionally broken to expose
the post. In Fig. 5(a) and (b), we also see the clear formation of
a half of the cavity by focused ion beam (FIB) etching. During
this etching process, the end facet of the slab is affected by the
post and the slab edge [Fig. 5(b)]. So the post looks thick com-
pared to its real shape.
1134IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 9, SEPTEMBER 2005
cavity is cut by FIB machine.
(a)–(b) SEM pictures of the post structures. The region around the
IV. SINGLE-CELL LASERS
A. Experimental Setup
The output photons of the electrically pumped single-cell
photonic crystal laser are collected through a 50
microscope objective lens (numerical aperture of 0.42) and fed
into a spectrometer as shown in Fig. 6. In an image captured
by an infrared (IR) camera (Fig. 7), a current-supplying metal
tip touches the top-metal-contact of the laser structure. To
minimize electrical noises, the fabricated sample is attached
on a very small metal plate by silver paste connected by a
bayonet-type connection (BNC) cable. By measuring voltage
from the oscilloscope, peak current
For comparison, electrical and optical properties of the post
structure without photonic crystal pattern are also measured.
The electroluminescence (EL) spectrum with a center peak of
1.5- m wavelength is observed in Fig. 8(a). In this case, it is
operated by input peak-to-peak voltage of 2 V at room temper-
ature. Fig. 8(b), directly captured from the oscilloscope, shows
good electrical characteristics of the fabricated pedestal struc-
ture. In this current–voltage
–curve, we obtain turn-on
Fig. 6. Schematic diagram of the experimental setup for current injection.
Mesa structure and a current-supplying metal tip captured by an IR
0.5 V and small electrical resistance of60,
B. Lasing Properties
From the cavity of Fig. 3, room-temperature single-mode
lasing action is observed at wavelength of 1519.7 nm, as shown
in Fig. 9(a). Due to the large thermal resistance (346 K/mW)
of the small post structure , , the laser is operated in a
pulsed mode with duty cycle of
6 ns and 2.5 s, respectively. Above threshold, the
spectral linewidth of this nondegenerate lasing mode becomes
0.55 nm. In order to identify the
lasing mode, the mode profile and polarization direction are
measured. In Fig. 9(b), the lasing mode with a central intensity
minimum and a donut-shaped intensity distribution is captured
byan IRcamera.Here thewhite barrepresentsa 2- m rulerand
the white hexagon means region (II)/(III) interface in Fig. 3.
Also, no preferred polarization direction is observed from
the top [Fig. 9(c)]. This is one of the unique characteristics
expected from the monopole mode , . The inset of
Fig. 9 shows a below-threshold spectrum measured at 200 A.
In the coarse resolution regime of the spectrometer, we can find
the other nonlasing resonant mode near 1491 nm as well. This
0.24%. The pulsewidth and
PARK et al.: CHARACTERISTICS OF ELECTRICALLY-DRIVEN 2-D PHOTONIC CRYSTAL LASERS 1135
curve of the mesa structure operated by input peak-to-peak voltage of 2 V.
(a) EL spectrum measured from a fabricated mesa structure. (b) ?–?
resonant mode is found when the injected current is 200 ?A and the slit-width
of the spectrometer becomes ?5 times broader. (b) Near-field image of the
monopole mode captured by an IR camera. (c) Measured polarization state of
the monopole-mode laser. The direction is the same as that of the hexagon in
(a) Lasing spectrum measured at 700 ?A. In inset, the other nonlasing
mode (the second quadrupole mode) did not lase because of the
factor of700 .
Experimental data are compared with the results computed
by the 3-D FDTD method. To remove any unnecessary argu-
ment about the validity and accuracy of this comparison, we
use the numerical data derived directly from the SEM image
as the structural input data file of our FDTD computation .
In this case, all the imperfections introduced during the fabri-
cation processes are included faithfully. We can divide inside
and outside the air holes according to the colors shown in SEM
tion. Fig. 10 shows the actual transformation process from the
quency and mode shape (Fig. 11) of the monopole mode agreed
well with the experimental results. In particular, Fig. 11(b)–(e),
Transformation from a SEM picture to the structure employed in the
vertical positions: (b) ? ? ?; (c) ? ? ? ?; (d) ? ? ? ?; and (e) ? ? ? ?, where
the origin is the center of the slab and ? is lattice constant (linear scale). The
white hexagon with the same size as that of Fig. 9(b) is shown as a scale bar.
Maximum intensity values of (b)–(d) are 100, 3, and 1.5 times larger than that
of (e), respectively.
(a) Electric field intensity profile at the center of the slab (log scale).
pared with the experimental picture of Fig. 9(b). The intensity
profile of the monopole mode, which is confined in the slab, is
shown in Fig. 11(a) as a reference.
factor of a cold cavity for the monopole
mode, as estimated from the spectral linewidth associated with
a transparent current of
225 A, is 2500. The transparent
1136IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 9, SEPTEMBER 2005
the actual structural data of Fig. 3.
(a) ? factor, (b) modal volume, and (c) Purcell factor computed by
current is determined by solving the rate equation that will be
discussed later , . To compare this
puted one, the size of the diamond-like post is estimated from
the SEM image of Fig. 3. Let
in diagonal directions) and then
calculated as a function of multiples of
factor at(3480) compares well with the experimental value.
In addition, the
factor degrades rapidly when the post sizebe-
comes larger than
. On the other hand, electrical and thermal
resistances will be increased more with the smaller post size
, . Thus, the post size should be optimized to obtain a
and Purcell factor of the monopole mode as a function of the
post size, respectively.At
, we obtainthesmallmodal volume
and the large Purcell factor of 389, where
a refractive index of the slab. Due to the large Purcell factor of
factor with com-
be the estimated post size
. In Fig. 12(a), the
threshold is observed (inset). (b) Typical electrical characteristics of the laser.
(a) ?–? curve of the monopole-mode laser. Soft turn-on near the
the monopole mode, one could observe cavity quantum electro-
dynamics (CQED) effects in this electrically driven small and
cavity with the unique mode shape , .
The peak output intensity is shown as a function of the peak
input current in Fig. 13(a). In this
the threshold value of
260 A. This low threshold compares
favorably with those estimated from the optical pumping ex-
periment . In Fig. 13(b), electrical characteristics, –
– , curves of this single-cell photonic crystal laser are
measured. From these curves, relatively high electrical resis-
2.2is obtained, which is mainly attributed to
is also observed due to the nonradiative recombination at the
If this non-negligible leakage is reduced, the threshold current
could be smaller. Additionally, note that there is a clear kink
near the threshold in the
For comparison purposes, an electrically driven monopole-
mode laser is also optically pulse-pumped. Under similar pulse
conditions, we confirmed the monopole mode lasing at almost
an identical resonant frequency.
– curve, we can determine
– curve .
PARK et al.: CHARACTERISTICS OF ELECTRICALLY-DRIVEN 2-D PHOTONIC CRYSTAL LASERS 1137
EMPLOYED PARAMETERS IN THE RATE EQUATION
from the rate equations (lines) for the monopole mode.
Comparison of the measured ?–? curves (dots) with those obtained
C. Spontaneous Emission Factor
As shown in inset of Fig. 13(a), a soft turn-on near the
threshold implies a large
value . To quantify the
we employ the laser rate equations where various parameters
are required. Typical material parameters of InGaAsP QWs are
used , , . In addition, several important parameters
are experimentally determined to reduce the ambiguity of the
estimation. Actually, the size of the light emitting region, one of
the critical parameters, is obtained directly from the EL profile
under the transparent condition . All parameters used for
the computation are summarized in Table I. The spectrally
integrated experimental data points (dots) are plotted together
with those obtained from the rate equations (lines), for the
monopole mode, in the log-log graph of Fig. 14. The
0.25 seems to fit the experimental value best. This is the
value among those reported from the semicon-
ductor nanolasers , , , . The large
is ascribed to the effective carrier localization by electrical
pumping together with the nondegeneracy and the small mode
volume , , .
The reliability of this
value estimation is examined as
we vary fitting parameters. First, the dependence of
the Auger coefficient
is considered as a function of the
injected current. In inset, three selected spectra are shown.
Linewidth of the monopole mode is measured as a function of the
wafer temperature. The value of
times when the wafer temperature increases up to
from room temperature . This is the worst possible sce-
nario imaginable from a freestanding cavity by a pulsed
operation with larger duty cycle of 4% . Even under
this harsh condition,
changes only slightly from 0.25 to
0.23. Therefore, we argue that
on the Auger recombination. Next, the
puted as we change the cavity
could increase 34
depends only very modestly
value is com-
value and found
, and . Here, the
considered as an upper bound. Actually, we obtain a
larger than 2500 from a measured linewidth in the estimated
range of transparency. Therefore, the actual
larger than 0.25. In addition, the spatial distribution of carriers
by diffusion is indirectly considered in our simple rate equa-
tions. Slight variations of the parameters other than the surface
recombination are tolerable in the fitting of . Since the critical
parameters including the surface recombination are obtained
from experimental data, our estimation is believed to be not
very far from the real value.
The semiconductor microlaser with a very large
have a linewidth wider than those of typical lasers . Also,
it was reported that lasing linewidth remains nearly constant as
the pumping level increases if the
above 0.05 . In order to confirm this property in our laser,
the linewidth of the monopole mode is measured as a function
of injection current as shown in Fig. 15. Note that linewidth
drastically changes near the threshold
proached the resolution-limit of the spectrometer
Since the measured linewidth is immediately saturated above
the threshold, we cannot insist that this changing behavior of
the linewidth has the same origin as that of . However, from
Properties of another monopole-mode laser fabricated with
different lattice parameters are measured and shown in Fig. 16.
value of 3480 by the 3-D FDTD method can be
value can be
value of a laser mode is
260A and ap-
1138IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 9, SEPTEMBER 2005
of the lasing peak is ?30 dB larger than that of the background.
?–? curve of another monopole-mode laser with different lattice
the other parameters are unchanged. Compared to the previous
monopole-mode laser, the threshold current of this laser is al-
most identical and the
0.15) is somewhat smaller.
V. THREE-CELL LASERS
The three-cell photonic crystal cavity allows a central post
somewhat larger than that of the single-cell cavity, and the
fabrication of a larger post by the wet etching process becomes
less delicate. In fact, we fabricated a three-cell laser cavity first
to observe lasing action . Using the same wafer structure
of Fig. 1(b), various photonic crystal cavities are fabricated
and tried. We begin measurement with the three-cell cavity of
Fig. 17(a) where only the nearest air holes around the cavity
are modified. At the center of the cavity, the central post (white
shadow region) is shown. Structural parameters are as follows:
is455 nm and the radii of the each of
the air holes are
characteristics of this structure are shown in Fig. 17(b). In
the inset of this figure, several nonlasing resonance peaks are
observed owing to its large cavity size. Also, the peak output
intensity is plotted as a function of the peak input current. Here
the threshold current is 600
than that of the single-cell laser. Since the wider area around
the cavity needs to be pumped, the threshold value increases
naturally. The lasing mode is linearly polarized.
Interestingly, even from the three-cell cavity with an off-cen-
tered post, lasing action is observed as shown in Fig. 18. The
threshold current is
650 A, and the off-centered post leads
to a linear polarization state whose direction is determined by
the position of the post.
The effect of the size of the central post is also studied. From
Fig. 19(a)–(d), etching times are increased and post sizes are
reduced gradually. The etching time of each figure differs by
several tens of seconds. We cannot obtain lasing action in the
case of (a) due to the serious optical loss through a large central
post. Ontheotherhand, asthesizedecreases [(b)and (c)],a few
nonlasing additional resonant modes begin to show up in the
, respectively. Experimental
A, which is somewhat larger
above-threshold spectrum and mode profile (inset).
(a) Top view of a three-cell laser cavity. (b) Measured ?–? curve,
Fig. 18.SEM image of the three-cell cavity with an off-centered post.
gain spectrum. Finally, lasing action is observed in the opti-
mized post size of (d). All measurements are performed under
the same conditions.
PARK et al.: CHARACTERISTICS OF ELECTRICALLY-DRIVEN 2-D PHOTONIC CRYSTAL LASERS1139
several tens of seconds.
(a)–(d) EL spectra obtained by various wet etching times in a three-cell cavity. The etching time of (a) is ?6 min and each one of (a)–(d) differs by
We successfully fabricated an electrically driven, low-
threshold, single-cell photonic bandgap laser at room tem-
perature. This laser shows extremely small mode volume
5.8710m , low threshold current
large spontaneous emission factor (
lasing spectrum, near-field image and polarization state, we
confirm that our lasing mode is the high- , nondegenerate
monopole mode with central node. Additionally, we inves-
tigated the other photonic crystal laser structures and their
The threshold of our laser is slightly larger than our expec-
tation. From the rate equations, the single-cell photonic crystal
laser with very high-
factor and small modal volume can have
ultralow threshold less than 100
lasers . The main origin of this large threshold is the nonra-
diative recombination at surfaces of lots of air holes. To reduce
this effect, some passivation process can be added in the fabri-
cation  or quantum dots (QDs) can be employed as an ac-
tive material . In particular, injection of a single QD into the
small cavity will be a meaningful step to observe CQED effects
by a large Purcell factor and to realize the thresholdless laser.
260 A , and
0.25). By measuring its
A, as reported in microdisk
The authors would like to thank Prof. Koyama of Tokyo
Institute of Technology for offering SEM pictures of samples
processed by the FIB machine, and Prof. J. Shim, Hanyang
University, for helpful discussions.
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presented at the PECS-V, Kyoto, Japan, 2004.
New York: Academic, 1978.
Hong-Gyu Park was born in Seoul, Korea, in 1976.
He received the B.S., M.S., and Ph.D. degrees from
the Departmentof Physics, Korea AdvancedInstitute
of Science and Technology (KAIST), Daejeon, in
1998, 2000, and 2004, respectively. During his
Ph.D. work, he studied the design, fabrication, and
characterization of photonic crystal light-emitting
He is currently with the Department of Chem-
istry and Chemical Biology, Harvard University,
Se-Heon Kim was born in Seoul, Korea, in 1978.
He received the B.S. and M.S. degrees from the
Department of Physics, Korea Advanced Institute of
Science and Technology (KAIST), Daejeon, Korea,
in 2000 and 2002, respectively, where he is currently
working toward the Ph.D. degree in photonic crystal
PARK et al.: CHARACTERISTICS OF ELECTRICALLY-DRIVEN 2-D PHOTONIC CRYSTAL LASERS 1141
Min-Kyo Seo was born in Seoul, Korea, in 1981.
He received the B.S. and M.S. degrees from the
Department of Physics, Korea Advanced Institute
of Science and Technology (KAIST), Daejeon, in
2002 and 2004, respectively. He is currently working
toward the Ph.D. degree at Department of Physics of
His main interest lies on dynamics of semi-
Young-Gu Ju received the B.S., M.S., and Ph.D.
degrees from the Department of Physics, Korea
Advanced Institute of Science and Technology,
Daejeon, in 1992, 1994, and 1998, respectively.
During his Ph.D. work, he studied the design,
fabrication, and characterization of vertical-cavity
surface-emitting laser (VCSEL).
He worked as a Visiting Engineer at University
of California at Santa Barbara and developed coarse
wavelength-division multiplexing VCSEL array. He
continued his research at Electronics and Telecom-
munication Research Institute (ETRI), Daejeon. During his stay at ETRI, he
worked on long-wavelength VCSELs. He is currently working at Department
of Physics Education, KyungPook National University, Daegu, Korea.
Sung-Bock Kim was born in Daejeon, Korea, in
1965. He received the B.S., M.S., and Ph.D. degrees
in physics from Yonsei University, Seoul, Korea, in
1990, 1992, and 2005, respectively.
In 1993, he joined the Electronics and Telecom-
munication Research Institute (ETRI), Daejeon,
Korea, as a Member of Research Staff in the Basic
Research Laboratory. With a research background
in compound semiconductor epitaxy growth, he is
currently studying the optoelectronic materials and
photonics devices grown by MOCVD.
Yong-Hee Lee received the M.S. degree in applied
physics from the Korea Advanced Institute of
Science and Technology (KAIST), Daejon, and the
Ph.D. degree in optical sciences from the University
of Arizona, Tucson, in 1979 and 1986, respectively.
In 1987, he joined AT&T Bell Laborato-
ries, Holmdel, NJ. There, he pioneered the first
proton-implanted vertical-cavity surface-emitting
laser (VCSEL) in 1990. He has continued his re-
search on VCSELs after he joined the Department
of Physics, KAIST, in 1991. Recently, his main
interest lies in photonic crystal laser structures and photonic integrated optical
circuits. He has coauthored more than 100 international journal papers related
to VCSELs and photonic bandgap laser structures.
Dr. Lee received the National Academy of Sciences Award (Natural Science)
in 2002 and the IEEE LEOS Distinguished Lecturer Award in 2003.