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

RDY 9/15/20051 The Ultimate Goal Strong cavity – emitter coupling –Sensitive to photon number state Single photon source Quantum information processing

RDY 9/15/20052 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. ΓµΓµ

RDY 9/15/20053 Cavity QED: Review 2 ( γ C ~ 100µeV, γ Xintrinsic ~ 1µeV ) And the condition for strong coupling is Maximizing the inequality implies maximizing Solving the quantized oscillator/cavity system for weak excitation 1 (i.e. low # of photons in the cavity) and matched wavelengths, The spontaneous emmission spectrum is governed by the coupling parameter g : f – Oscillator strength V m – Mode Volume α µ – Norm. mode fcn. γ C – Cavity Linewidth γ X – Exciton Linewidth Q – Cavity Quality factor 1. L.C. Andreani, G. Panzarini, J. Gerard, Phys Rev B, 60, (1999) And maximizing the cavity electric field amplitude at the emitter

RDY 9/15/20054 The Approach Photonic Crystal (PC) microcavity –Square lattice, 10 periods/side –Q ~ 5,000 – 10,000 –V m ?= 0.07µm 3 InGaAs quantum dot emitter –Sparse self assembled growth (~5 x 10 9 /cm 2 ) –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 9/15/20055 Dot Growth 1.InGaAs self- assembled dot growth on GaAs layer (MBE, density ~5 x 10 9 /cm 2 ) 2.Dot annealed to produce blue shift 1. 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 selfassembled quantum dots," Appl. Phys. Lett. 72, p (1998).

RDY 9/15/20056 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 correction 1 ) –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

RDY 9/15/20057 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 Q cavity ~8000

RDY 9/15/20058 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, (2005)

RDY 9/15/20059 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 where f = oscillator strength, Q = cavity Q V m = cavity mode vol.

RDY 9/15/ Results P sat = 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 ~1µm 2, Resolution 40µeV

RDY 9/15/ Results: 2 nd 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/ 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 V m, and other groups have seen strong coupling with them 1. –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)