Quantum dot photonic crystal lasers are demonstrated at room temperature by optical pulse pumping. Coupled cavities were designed based on square lattice PC slabs. Optimized two-dimensional photonic crystal cavities were defined in 200nm slabs with five-stacked InAS QDs layers. The two- and four-coupled cavities showed as incident pump power threshold as 120μW and 370μW, respectively, both from QD ground state emission range. Both clear threshold in pump power-output resonance power and resonance line width narrowing were observed from our membrane samples. The measured wavelengths matched very well with wavelengths predicted by 3D-Finite Difference Time Domain modelling

Chen, Hao

Deppe, Dennis G.

Scherer, Axel

Shchekin, Oleg B.

Yoshie, Tomoyuki

Caltech Authors - Main

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Design and characterization of
quantum dot photonic crystal lasers
Tomoyuki  Yoshie, Oleg B. Schchekin, Hao  Chen, Dennis
G. Deppe, Axel  Scherer
Tomoyuki  Yoshie, Oleg B. Schchekin, Hao  Chen, Dennis G. Deppe, Axel
Scherer, "Design and characterization of quantum dot photonic crystal lasers,"
Proc. SPIE 5000, Photonic Crystal Materials and Devices,  (9 July 2003); doi:
10.1117/12.479515
Event: Integrated Optoelectronics Devices, 2003, San Jose, CA, United
States
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Design and characterization of quantum dot photonic crystal
lasers
Tomoyuki Yoshie
a
, Oleg B. Schchekin
b
, Hao Chen
b
, Dennis G. Deppe
b
,and Axel Scherer
a
a
Department of Electrical Engineering, Caltech, Pasadena CA 91125-9300 U.S.A.
b
Microelectronics Research Center, Department of Electrical and Computer Engineering,
the University of Texas at Austin, Austin TX, 78712-1084 U.S.A.
ABSTRACT
Quantum dot photonic crystal lasers are demonstrated at room temperature by optical pulse pumping. Coupled
cavities were designed based on square lattice PC slabs. Optimized two-dimensional photonic crystal cavities
were dened in 200nm slabs with ve-stacked InAs QDs layers. The two- and four-coupled cavities showed as
low incident pump power threshold as 120W and 370W, respectively, both from QD ground state emission
range. Both clear threshold in pump power-output resonance power and resonance line width narrowing were
observed from our membrane samples. The measured wavelengths matched very well with wavelengths predicted
by 3D-Finite Dierence Time Domain modelling.
Keywords: Quantum dot, Photonic crystal, Laser, Coupled cavity, Low threshold, Membrane
1. INTRODUCTION
Photonic crystal (PC)
1, 2
planar cavities can be compact building blocks, which will be used in future integrated
nanophotonic systems. One of the most notable characters of PC cavities is that the mode prole and the
quality (Q) factors are geometrically controlled and can be designed. Simple defect cavity designs allowed the
rst photonic crystal lasers
3
on InGaAsP multi-quantum well (QW) structures.
Quantum dot (QD)-PC lasers have some advantages as eÆcient light sources over QW-PC lasers. At rst,
QDs generally have smaller surface recombination rate than QWs.
4
Negligible surface recombination is essential
to thresholdless lasers. Secondly, the sharp homogeneous broadening emission of each QD can be t in high-Q
PC resonances.
5, 6
At last, signicant absorption loss by unnecessary active material can be minimized by
putting single or several QDs only in a center of PC cavity. QD-PC lasers, however, have been more diÆcult
devices to construct due to much lower available gain from QDs material. The smaller gain results not only
from a lower total volume of actively emitting material but also from variations in spatial locality of QDs as
well as inhomogeneous emission broadening consisting of narrow homogeneous broadening. These eects result
in a smaller number of QDs contributing to the gain in a resonance as the Q factor is increased.
We approach these issues by using high Q cavities with relatively larger mode volumes. In this report, we
describe simple designs of coupled cavities in square lattice(SQ) photonic crystals and demonstrate laser action
from such quantum dot photonic crystal cavities.
2. COUPLED PHOTONIC CRYSTAL CAVITIES
Single defect square lattice slab cavity is found to have high-Q whispering gallery modes
6
(WGM). We started
designing of QD-PC nano-lasers from the whispering gallery modes. The predicted mode volumes are generally
larger due to the smaller band-gap. In order to increase optical gain () of QDs contributing to one mode, we
employed coupled cavities
7, 8
based on the whispering gallery modes. A three-dimensional nite dierence time
domain (3D-FDTD) modelling was applied to structures shown in gure 1. Figure 1-(a), (b), and (c) show 2D
SQ-PC slabs with single-, two- and four-defects, respectively. The analysis used 15 as lattice constant (a) and
Further author information: (Send correspondence to T. Yoshie)
T.Yoshie: E-mail: yoshie@caltech.edu, Telephone: 1 626 395 2207
Photonic Crystal Materials and Devices, Ali Adibi, Axel Scherer, Shawn Yu Lin, Editors,
Proceedings of SPIE Vol. 5000 (2003) © 2003 SPIE · 0277-786X/03/$15.00
27
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Figure 1.
PC Cavity structures used in this report. (a)single defect SQ cavity, (b)two-defects coupled cavity, and (c)four-
defects coupled cavity. The slab thickness is d. The lattice spacing is a.
3.4 as slab refractive index (n). Identical defects overlapped in optical eld lead to generation of supermodes,
which cause splitting of eigenmode frequencies and symmetrically combined mode proles. New eigenmodes
((r)) of connected cavities can be described in a superposition form of each unperturbed cavity mode 
m
(r)
with translation r
m
: (r) = 
m
A
m

m
(r   r
m
). Our interests are even modes of electric eld about a middle
slice of slab(TM polarization). A good index to describe supermodes is z component of magnetic eld since the
magnetic eld is always perpendicular to the slab on a middle of the slab for even modes. We chose even modes
to obtain large dipole moment (
E
) from InAs QDs.
Figure 2 shows three fourier spectra obtained from 3D-FDTD modelling on single-, two-, and four-defects
coupled SQ-PC slab cavities. Dashed, solid and dotted lines represent single-, two- and four-defects coupled
cavities. It can be noticed that a WGM resonance of single defect cavity is split symmetrically into two
resonances about the WGM frequency on a spectrum of two-defects coupled cavity. In addition, the two
supermode resonances from two-defects coupled cavity are split symmetrically into four supermodes indicated
as arrows on four-defects coupled cavity spectrum.
Figure 3 shows amplitude proles of x- and y-components of electric eld (E
x
and E
y
)and z-component
of magnetic eld (B
z
) in a middle slice of the slab of single-defect SQ photonic crystal. The mode volume is
generally ranged from 0.5 to 1(=n)
3
, depending on the geometric parameters. Two defect coupled cavities
were analyzed to map eld proles. The analyzed two-defects coupled cavities were located two lattice constants
(a) apart from each other to form one coupled cavity in a 2D-square lattice PC slab with 15 by 17 lattice periods
surrounding the cavities as shown in gure 1(b). The modelled photonic crystal slab geometry had a thickness
(d) of 0.73a, hole radii (r) of 0.36a. Figure 4 shows the Bz amplitude proles in the middle slice of the slab.
You can notice symmetric features of even and odd supermodes in the gures. The electric eld is vertical to
the hole cylinder axis in the middle of the slab. The calculated vertical and the lateral Q factors (Q
?
and Q
k
),
the mode volume (V
mode
=
R
((r)E(r)
2
)dr
3
/ max ((r
0
)E(r')
2
)) are shown in table 1.
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Table 1. Performance of a whispering gallery mode from a single defect cavity and two supermodes from two-defects
coupled cavity. 3D-FDTD simulation was used for the analysis. The unit of mode volume (V
mode
) is in (=n)
3
.
SQ-PC cavity mode type Q
k
Q
?
V
mode
a=
Two defects Even supermode of WGMs 57000 28000 1 0.2892
Odd supermode of WGMs 39000 24000 1 0.2861
Single defect WGM 77000 29000 0.7 0.2874
Figure 2.
Fourier spectra obtained from 3D-FDTD modelling on single-, two-, and four-defects SQ-PC slab cavities.
Dashed, solid and dotted lines are analyzed from single, two, and four defects SQ-PC slab cavities. The
analyzed parameters are r = 0:38a, d = 0:47 and n = 3:4. Arrows indicated on the fourier spectrum are four
supermode resonances of four defects coupled cavity.
As shown in the table 1, two-defects coupled cavities therefore increase the mode volume by 50% and we can
generate high-Q supermodes by coupled cavities with slight decrease in the Q factor. By putting the defects
more away from each other, the high-Q factors of single defect mode can be preserved on coupled cavities
with much smaller interaction of modes and the mode volume approaches two times as big as single defect
cavity WGM. The frequency separation of supermodes would be decreased as the interacted elds decrease.
Supermodes can be used for optical switches and lasers with more distributed mode proles.
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Figure 3.
Amplitude proles of x and y components of electric eld (E
x
and E
y
) and z component of magnetic eld (B
z
)
analyzed from single-defect SQ-PC cavity. Black and white regions have intense amplitudes, but have dierent
signs of amplitudes. The analyzed lattice parameters are r = 0:36a, d = 0:73a and n = 3:4. On the other hand,
E
z
, B
x
and B
y
components are negligible on the simulation results, which are predicted by symmetric analysis
for even modes.
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Figure 4.
Z-component of magnetic eld (Bz) on a middle slice of two-defects coupled cavity slab. (a) and (b) show Bz
mappings from odd (left) and even (right) supermodes. Black and white regions have intense amplitudes, but
have dierent signs of amplitudes. The analyzed lattice parameters are r = 0:36a, d = 0:73a and n = 3:4.
Similarly with results of single-defect SQ-PC cavity, E
z
, B
x
and B
y
components are negligible on the simulation
results.
3. QUANTUM DOT PHOTONIC CRYSTAL LASERS
3.1. Sample preparation
Five stacked self-assembled InAs QDs layers were grown by molecular beam epitaxy. The QD density in our
samples is 510
10
/cm
2
in total and the QD layers were separated by 30nm GaAs layers. Al
0:2
Ga
0:8
As is used for
cladding layers to form thin slab waveguides, which are deposited on an 800nm Al
0:94
Ga
0:06
As sacricial layer
on a top of GaAs substrate. In order to dene 200nm-thick 2D-PC slab cavities, electron-beam lithography,
chemically assisted ion-beam etching, oxidation of Al
0:94
Ga
0:06
As layer, and wet etching of AlOx layer were
performed. The detailed fabrication steps can be found elsewhere.
9
Many geometries of single-, two-, and
four-defects coupled cavities were fabricated within photonic crystals with lattice sizes of 31 by 31, 31 by 33,
and 33 by 33, respectively.
3.2. -photoluminescence
Figure 5 shows measurement results of -photoluminescence. Single defect cavity has one intense resonance
from WGM around 1330nm. Two-defects coupled cavity has two intense supermode resonances and one weak
resonance in lower frequency. Four-defects coupled cavity has four intense supermode resonances in addition to
one weak resonance around 1370nm. The weak resonance is located at a dielectric band edge. These spectra
match with simulation results very well.
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Figure 5.
-photoluminescence spectra taken from single-, two- and four-defects SQ PC slab cavities, respectively, from a
bottom of the gure.
3.3. Pulsed-optical pumping of SQ-PC cavities
A pulsed 780nm semiconductor laser diodes was used to measure luminescence spectra and L
pump
(pump power)-
L
out
(collected resonance power) curves. Twenty nanosecond pulses with 2.86% duty cycle were applied to
the pumping laser diodes. The pump light was conrmed to have good pulse shapes in time domain. The
pumping beam diameter was varied up to 4m. Figure 6-(1) shows two L-L curves taken from two- and
four-defects coupled cavities. In an inset of the gure 6 you can nd scanning electron microscope images
of two-defects and four-defects coupled cavities. Each of these cavities exhibits distinct threshold and linear
increase in output power above threshold. Threshold pump powers of 120W and 370W are measured for
two- and four-defects coupled cavities, respectively. Considering lling factors and transmission of pump light
at air/semiconductor interfaces, the pump light power in the membranes would be approximately only 15%
of incident pump power with perfect absorption by cavities. Fifteen percentages of threshold incident pump
powers are 18W and 56W for two defects and four defects coupled cavities, respectively. Actual absorbed
pump power would be smaller than this estimation due to imperfect absorption. These facts allow us to believe
that our nanolasers are operated by very small power and that sub-W threshold lasers could be constructed by
improvement of fabrication. Figure 6-(2) shows luminescence spectra from a four-defects coupled cavity. Below
threshold (Fig.6 (2-a)), the resonance cannot be clearly observed. Close to threshold, nonlinear amplication
of emission is seen (Fig.6 (2-b)). Above threshold (Fig.6 (2-c)), the resonance intensity increases signicantly.
The lasing wavelength of 1328nm matches the QDs ground state emission. The lasing frequency of a==0.316
in this laser also matches the analyzed mode frequency of two-defects coupled cavities. Spectral line widths
decreased from 1-2 nm (below threshold) to less than 0.2nm (above threshold). With moderate CW pumping,
the resonance showed = = 2100. The combination of distinct threshold in the L
pump
-L
out
curves and line
width narrowing both indicate that the cavities are denitely lasing.
10
For two-defects coupled cavities, the
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Figure 6.
(1) L-L characteristics of two-defects and four-defects coupled cavities, (2) Luminescence spectra taken from
four-defects coupled cavity. These measurements were performed by optical pulse pumping (20nsec, 2.86% duty
cycle) with 780nm semiconductor laser diodes at room temperature (23
Æ
C).
resonance was measured at a==0.3. The simulated mode volumes are 0.8(=n)
3
and 1(=n)
3
, respectively,
by using geometric parameters of lased PC samples. Single defect cavities did not lase in our experiments,
probably due to a lack of suÆcient gain from the QDs, but the single defect cavities have strong possibilities of
lasing from our simulation results.
ACKNOWLEDGMENTS
The authors gratefully appreciate valuable discussions from Marko Loncar. This work was supported by the
NSF under Grant ECS-9912039 and DMR-0103134, the AFOSR under F49620-01-1-0497, and by DARPA under
MDA972-00-1-0019.
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Design and characterization of quantum dot photonic crystal lasers

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