Deterministic Coupling of Single Quantum Dots to Single Nanocavity

Modes

Richard Younger

Journal Club

Sept. 15, 2005

Antonio Badolato, kevin Hennessy, Mete Atatüre, Jan Dreiser, Evelyn Hu, Pierre M. Petroff, Atac Imamoğlu

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The Ultimate Goal• Strong cavity – emitter coupling

– Sensitive to photon number state

• Single photon source• Quantum information

processing

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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.

gΓµ

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Cavity QED: Review 2

;4

γ-γ2

XC2

g

m00r

22

1μ00r

22

4)(

4 Vm

fe

m

feg

r

Q

ECCγ ;γγ XC

( γC ~ 100µeV, γXintrinsic ~ 1µeV )

And the condition for strong coupling is

QV

f

m

Maximizing the inequality implies maximizing

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:

f – Oscillator strengthVm – Mode Volumeαµ – Norm. mode fcn.

γC – Cavity LinewidthγX – Exciton LinewidthQ – Cavity Quality factor

1. L.C. Andreani, G. Panzarini, J. Gerard, Phys Rev B, 60, 13276 (1999)

And maximizing the cavity electric field amplitude at the emitter

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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.

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Dot Growth1. InGaAs self-

assembled 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

1. J. M. Garcia, T. Mankad, P. O. Holtz, P. J. Wellman and P. M. Petroff, "Electronic states tuning of InAs selfassembledquantum dots," Appl. Phys. Lett. 72, p. 3172 (1998).

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Photonic Crystal Cavity manufacture1. Find indicator dot with STM2. Correlate STM scale marks

with e-beam lithography scale3. Write precisely placed PC holes

on ZEP – (lithographic proximity effect

correction1)– Placement precision is limited to

STM pixel resolution on distance scale, nominally 11nm

1. K. Hennessy, et. al. J. Vac. Sci. Tech. B 21(6) (2003) 2918

Remember: a major goal is to maximize the cavity field at the QD, so exact alignment of QD and cavity is critical

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Photonic Crystal Cavity manufacture4. Using chlorinated inductively

coupled plasma etch (ICP), transfer hole pattern to GaAs layer

5. HF wet etch to release membrane

Qcavity ~8000

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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)

1. K. Hennessey et. al. Appl. Phys. Lett. 87, 021108 (2005)

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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

QV

f

m

where f = oscillator strength, Q = cavity QVm= cavity mode vol.

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Results

Psat = 0.59µW

g ~ 80µeV

The bi-exciton (2X) intensity decreases as X-

goes to reasonance. Speculate that X- emits before it has a chance to capture an additional hole.

Low temperature µ-PL: Ti:Sapph 790nm, 0.55NA. Spot size ~1µm2, Resolution 40µeV

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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=1nsBlue: Detuned resonance , τ=0.6nsBlack: On resonance , τ=0.2±0.1ns

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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.

1. T. Yoshie, et al., Nature 432, 200 (2004)