Transcript Deterministic Coupling of Single Quantum Dots to Single

Deterministic Coupling of Single
Quantum Dots to Single Nanocavity
Modes
Antonio Badolato, kevin Hennessy, Mete Atatüre,
Jan Dreiser, Evelyn Hu, Pierre M. Petroff, Atac Imamoğlu
Richard Younger
Journal Club
Sept. 15, 2005
The Ultimate Goal
• Strong cavity – emitter coupling
– Sensitive to photon number state
• Single photon source
• Quantum information
processing
RDY 9/15/2005
1
Cavity QED: Review
• System consists of two main
parts: an emitter and a cavity,
plus a place for radiation to
escape to (vacuum modes).
• Cavity QED implies quantum
interactions between cavity
and emitter.
• Consequently, we need a
strong coupling, g, between
them.
• The first indicator that we
have some sort of coupling is
a modification of the emitter
spontaneous emission rate,
called the Purcell effect.
RDY 9/15/2005
g
Γµ
2
Cavity QED: Review 2
Solving the quantized oscillator/cavity system for weak excitation1 (i.e. low #
of photons in the cavity) and matched wavelengths, The spontaneous
emmission spectrum is governed by the coupling parameter g:
2
2
2

e
f

e
f
g2 
μ (r1 ) 
4 r 0 m0
4 r 0 m0Vm
And the condition for strong coupling is
2
 γ C - γ X  γ  γ ; γ  EC
g 
 ; C
C
X
Q
4


2
f – Oscillator strength
Vm – Mode Volume
αµ – Norm. mode fcn.
γC – Cavity Linewidth
γX – Exciton Linewidth
Q – Cavity Quality factor
( γC ~ 100µeV, γXintrinsic ~ 1µeV )
Maximizing the inequality implies maximizing
f
Q
Vm
And maximizing the cavity electric
field amplitude at the emitter
RDY 9/15/2005
3
1. L.C. Andreani, G. Panzarini, J. Gerard, Phys Rev B, 60, 13276 (1999)
The Approach
• Photonic Crystal (PC)
microcavity
– Square lattice, 10 periods/side
– Q ~ 5,000 – 10,000
– Vm ?= 0.07µm3
• InGaAs quantum dot emitter
– Sparse self assembled growth
(~5 x 109 /cm2)
– Exciton emission ~940nm
• µ-PL spectroscopic
measurement
Until now, groups made lots of cavities until by
chance they found a matching cavity and emitter.
RDY 9/15/2005
4
Dot Growth
1. InGaAs selfassembled dot
growth on
GaAs layer
(MBE, density
~5 x 109 /cm2)
2. Dot annealed to produce blue shift1. Emission goes
from ~1110nm to 940nm
3. Strain-correlated dot overgrowth (x5)
4. Au Alignment mark deposition
RDY 9/15/2005
1. J. M. Garcia, T. Mankad, P. O. Holtz, P. J. Wellman and P. M. Petroff, "Electronic states tuning of InAs selfassembled
quantum dots," Appl. Phys. Lett. 72, p. 3172 (1998).
5
Photonic Crystal Cavity manufacture
1. Find indicator dot with STM
2. Correlate STM scale marks
with e-beam lithography scale
3. Write precisely placed PC holes
on ZEP
– (lithographic proximity effect
correction1)
– Placement precision is limited to
STM pixel resolution on distance
scale, nominally 11nm
Remember: a major goal is to
maximize the cavity field at the
QD, so exact alignment of QD
and cavity is critical
RDY 9/15/2005
1. K. Hennessy, et. al. J. Vac. Sci. Tech. B 21(6) (2003) 2918
6
Photonic Crystal Cavity manufacture
4. Using chlorinated inductively
coupled plasma etch (ICP),
transfer hole pattern to GaAs
layer
5. HF wet etch to release
membrane
Qcavity ~8000
RDY 9/15/2005
7
Cavity tuning
• To support cavity QED studies, the resonant
cavity wavelength must match the QD emission
wavelength.
• Cavity wavelength is typically a few 10’s of nm
away from the target dot wavelength at
manufacture – the cavity needs to be tunable.
• “Digital” or stepped etching removes <5Å from
all GaAs surfaces, changing crystal geometry,
and tuning the resonant wavelength:
– Allow the sample to form a native surface oxide in
atmosphere
– Oxide removed with 1M Citric acid (15-60 sec)
RDY 9/15/2005
8
1. K. Hennessey et. al. Appl. Phys. Lett. 87, 021108 (2005)
Cavity Tuning 2
• Each oxide-etch cycle
removes <5Å from all
surfaces, and shifts
resonant λ by 3.4±0.1nm /
cycle
• Surface remains clean,
maintaining Q
• Fine tune using
temperature
f
Q
Vm
RDY 9/15/2005
where
f = oscillator strength,
Q = cavity Q
Vm= cavity mode vol.
9
Results
Low temperature µ-PL:
Ti:Sapph 790nm, 0.55NA.
Spot size ~1µm2,
Resolution 40µeV
The bi-exciton (2X)
intensity decreases as Xgoes to reasonance.
Speculate that X- emits
before it has a chance to
capture an additional
hole.
Psat = 0.59µW
g ~ 80µeV
RDY 9/15/2005
10
Results: 2nd device
Low mode overlap,
weaker coupling.
But able to resolve
lifetime reduction using
time-correlated single
photon counting
measurement (i.e.
observed the Purcell
effect)
Red: Off resonance, τ=1ns
Blue: Detuned resonance , τ=0.6ns
Black: On resonance , τ=0.2±0.1ns
RDY 9/15/2005
11
Summary
• Did not explicitly observe strong tuning (Rabi splitting),
but did see very definite Purcell effect
– Other PC geometries have calculated higher Q and lower Vm,
and other groups have seen strong coupling with them1.
– Coupling in with PC waveguide rather than µscope could greatly
improve collection efficiency.
• Developed methods for placing dots, placing and tuning
cavities to greatly increase the determinism when
constructing cavity QED setups,
• Possible enabled future experiments:
– Coupling to both X and 2X lines.
– Multiple cavity or multiple emitter coupling.
– Devices.
RDY 9/15/2005
12
1. T. Yoshie, et al., Nature 432, 200 (2004)