1. Carrier Dynamics in Microdisk Photonic Molecules
Center for Spintronics and Quantum Computation, University of California,
Santa Barbara, CA 93106
Felix M. Mendoza, R. C. Myers, G. Calusine, G. D. Fuchs, A.C. Gossard and
D. D. Awschalom
AFOSR
Materials Research Institute, Penn State University, University Park, PA 16802
X. Li, B. J. Cooley and N. Samarth
APS March Meeting, New Orleans, LA,
March 11, 2008
Spatially, spectrally and time-resolved luminescence
measurements of single “Photonic Molecules”
Coupled Microdisk Cavities – “Photonic Molecules”

2. Motivation – Semiconductor Microcavities
 Solid-state system for studying the
interaction between light and matter, e.g.
between photons and electron spins in the
cavity
 Optoelectronic applications (e.g. as a on-chip
laser source)
 Possible platform for quantum information
processing schemes (e.g. A. Imamoglu et.al.,
PRL 1999)

3. Microdisk Resonators
1 µm
 The dominant modes in these planar cavities are the whispering-gallery
modes that are confined to the perimeter of the disk due to total internal
reflection (top right)
 Light emission from the edges and, thus, in the plane of the disk (red
arrows) highly directional emission
 In thin disks, only the lowest-order modes are supported
nmedium
ndisk

4. Active Region of Our Structures
1 μm
GaAs substrate
Al0.7Ga0.3As pedestal
(500 nm)
Single Disk Diameter = 2 µm
Disk thickness = 110 nm Al0.3Ga0.7As cap (40 nm)
Al0.3Ga0.7As buffer (40 nm)
4.2 nm undoped GaAs QWs with
10 nm Al0.3Ga0.7As barriers
(Ga,Al)As
 Grown by Molecular Beam Epitaxy (MBE)
 “Natural” or “Interface-Fluctuation”
Quantum Dots (QDs)
 large ~ 100 nm in diameter
 provide weak confinement of carriers
2 min
Gammon, PRL, 1996
growth interruption

5. Coupled Microdisks – “Photonic Molecules”
Coupling between individual microdisks
 Overlap of evanescent fields (extent ~150 nm from edge of disk)
 Splitting into a “bonding” and an “anti-bonding” mode
“photonic molecule”
~ 185 nm – 225 nm
SEM Image Photoluminescence
 Polarization-resolved photoluminescence and simulations indicate
coupling
Polarization-resolved PL
bonding
anti-bonding
Simulation
y
x
θ

6. Bonding and Anti-bonding Modes
Splitting the Single Mode of Individual Microdisks through Coupling
(analogous to molecules)
 “Bonding” occurs when there occurs overlap between two mode
maxima or minima at the nearest point (left figure)
 “Anti-bonding” occurs when there occurs overlap between a
mode maximum and minima at the nearest point (right figure)
Energies get shifted due to different nature of overlap
between the evanescent fields
Bonding Mode Anti-Bonding Mode
Max
Min

7. Experimental Setup
Pulsed Ti:Sapphire
laser
Probe
Pump
Spectrometer
with CCD
Streak
Camera
Flip Mirrors
Balanced
Photodiode
Bridge
Imaging
CCD
Delay Line
Flow
Cryostat
B
Sample
 High spatial resolution
limited by laser spot (~ 1 μm)
 Small magnetic field (B) up
to 0.25 T
 Photoluminescence
measurements with spectral
and temporal resolution
 Pump-probe path for time-
resolved Kerr Rotation studies
5 K – 300 K
Lock-in
detection

8. 750 755 760 765 770 775 780
Wavelength (nm)
Photoluminescence(a.u.)
750 755 760 765 770 775 780
T = 20 K
λ = 730 nm
Pexc = 1.2 kW/cm2
Wavelength (nm)
Photoluminescence(a.u.)
1 μm
Counts
226
409
Mode at λdet = 767.8 nm
Spatial Imaging of Photoluminescence
1 μm
White Light Image
1 μm
Counts
226
509
Mode at λdet = 766.2 nm
1 μm
Counts
226
509
Quantum Well at λdet = 763.0 nm
Modes sensitive to location of
excitation

9. T = 20 K
λexc = 720 nm
Pexc = 0.82 kW/cm2
Wavelength (nm)
Photoluminescence(a.u.)
750 755 760 765 770 775
Photoluminescence
Time-Resolved Photoluminescence
0 200 400 600 800
Single “Photonic Molecule”
Time (ps)
Intensity(normaized)
T = 20 K
λdet = 759.5 nm
λexc = 720 nm
0.28 kW/cm2
0.78 kW/cm2
Carrier Dynamics
 Timescale similar to previous measurements on single circular
microdisks (right)
 More complex dynamics in stimulated emission regime
Time (ps)
Intensity(normaized)
T = 5 K
λdet = 770 nm
λexc = 730 nm
S. Ghosh et. al., Nature Materials 5, 261 (2006)
Single Circular Microdisk
0 200 400 600 800
0.03 kW/cm2
1.8 kW/cm2

10. Conclusions
“Photonic Molecules” – Splitting of cavity modes into “bonding” and “anti-
bonding” states
Time-resolved luminescence reveals
more complex carrier dynamics than for
individual microdisks
AFOSR
Modes sensitive to location of excitation on the structure
1 μm

11. Back-up Slides

12. Elliptical “Photonic Molecules”
SEM Image
765 766 767 768 769 770
Wavelength (nm)
Polarizerangleθ
(deg)
PL(a.u)
Polarization-resolved PL
0
30
60
90
y
x
θ
1 μm
Elliptical Photonic Molecules
 Influence of anisotropic shape on photonic properties
Photoluminescence polarization of microdisk pair (coupled along
the long axis) shows similar dependence to circular disks (right)