1. 1
Polarized stimulated emission from photonic
molecule states in coupled microdisk lasers
Xia Li, B. J. Cooley, N. Samarth
Dept. of Physics, The Penn State University
University Park, PA, 16802
F. M. Mendoza, R. C. Myers, D. D. Awschalom
Center for spintronics and quantum computation, University of
California, Santa Barbara, CA, 93106
NSFNSF
2. 2
Motivation
~2μm ~100-200nm
● Engineering of the spin coherence via photon-spin interactions in
microdisk lasers.
--S. Ghosh et al, Nature Materials, 5 261 (2006)
● Potential demonstration of the spin entanglement states through pairs of
coupled microdisks.
--horizontal coupling through evanescent field of electromagnetic (EM) field
3. 3
1 μm
Sample structure and processing
~500nm Al0.8
Ga0.2
As post layer
~110nm optical disk contains GaAs IFQDs
Define pattern with
ebeam-lithography
Metallization and transfer
the pattern to hard mask
by lift-off (aluminum)
Mesa formed by reactive ion
etching (RIE) Cl2
/Ar
Undercutting and forming
disk with chemical selective
etching (S/(NH4
)2
S)
Passivate and package
device with SiNx
coated
by PECVD
● Samples are grown by MBE on semi-insulating (100) GaAs substrates.
● Devices are processed using the following flow.
4. 4
Coupled microdisks: mode splitting
1.58 1.60 1.62 1.64 1.66
1000
1500
2000
2500
1.58 1.60 1.62 1.64 1.66
0
3000
6000
9000
10K
pump at
2.2 eV
10K
pump at
2.2 eV
Energy (eV)
Intensity(a.u.)
Single circular disk Twin coupled circular disks
Intensity(a.u.)
Energy (eV)
Bonding
mode
Antibonding
mode
~5nm
ee oe
oo eo
Coupled microcavities induce modified
electromagnetic field distribution that
differs significantly from that of a single
microcavity. For coupled circular
microdisks, there are four modes: two
(ee, oe) are called bonding and two (oo, eo)
are anti-bonding according to the field
symmetry along x/y axis.
x
y
5. 5
3.6
2.4
1.2
3.0
1.8
0.6
C E_L E_S
1.60 1.62 1.64
4000
8000
12000
C
1.60 1.62 1.64
2000
3000
4000
E_L
1.60 1.62 1.64
0
30000
60000 E_S
Modesplitting(meV)
Spacing: ~200nm;
b: half short axis, a: half long axis
C: circular coupled disks, b/a=1
E_L: elliptical, b/a=0.75, coupled along long
axis
E_S: elliptical, b/a=0.75, coupled along short
axis
Energy (eV)
C E_L E_S
Mode splittings vs. shape/arrangement of twin disks
6K
pump at
2.2 eV
6K
pump at
2.2 eV
6K
pump at
2.2 eV
6K
pump at
2.2 eV
Energy (eV)Energy (eV)
Intenstiy(a.u.)
6. 6
Imaging of far field emission patterns
Coupled circular
disks
Coupled elliptical
disks along long axis
Coupled elliptical
disks along short axis
7. 7
An FDTD simulation of modified EM field components
bonding
ee
Iy
anti-
bonding
eo
Ix
anti-
bonding
eo
Iy
bonding
ee
Ix
x
y
Ex
Ey
Hz
TE mode:
electrical components
are in disk plane.
Ix/y
~|Hz
||Ex/y
|
Finite-difference time-domain
(FDTD) simulation shows:
bonding modes polarize
along y axis, antibonding
modes polarize along x axis
in coupled circular disks. In
elliptical coupled disks, no such
polarized emission is predicted
by the simulation.
8. 8
A result of modified EM field: polarized lasing from twin disks
0 90 180
Linear polarizer angle (degree)
0.6
0.8
1.0
1.2
Intensityratio
(b)
270
6K, CW
Eexc
=2.2 eV
1.62 1.63 1.64
Energy (eV)
2
4
Intensity(104
counts/sec)
(a)
0 90 270
Linear polarizer angle (degree)
1.5
2.5
3.5
Intensityratio
(d)
1800 90 270
Linear polarizer angle (degree)
1.5
2.5
3.5
Intensityratio
(d)
180
Intensityratio
1.0
2.0
3.0
Linear polarizer angle (degree)
(c)
0 90 270180
Intensityratio
1.0
2.0
3.0
Linear polarizer angle (degree)
(c)
0 90 270180
1
3
Circular
coupled
disks
Elliptical
coupled disks
along long
axis
Elliptical
coupled disks
along short
axis
Intensity ratio:
bonding mode/
antibonding
mode
x
y
bonding anti-
bonding
θ
9. 9
Summary
● We fabricate three types of evanescent field coupled twin microdisk lasers:
twin circular, twin elliptical coupled along the long and short axis. Bonding
and antibonding modes of photonic molecule states are observed.
● We simulate the electromagnetic field component patterns in the three types
of coupled twin microdisks are simulated through the finite-difference
time-domain methods. Polarized emission is predicted in the coupled circular
disks.
● We see polarized stimulated emission in the twin circular PMs, the bonding
mode is polarized perpendicular to the disk coupling axis while the anti-
bonding mode is polarized along it. This is in accordance with the simulation
results. The polarization axis is rotated 45 degrees clockwise relative to the
coupling axis in the twin elliptical PMs coupled along the long axis, which is
not predicted in simulations.
● The polarized lasing, which also accompanies directional lasing, is a result of
the modification of the electromagnetic field induced by the cavity coupling.
