Towards the coupling of microsphere resonators and self-assembled semiconductor quantum dots. P. Bianucci, J. W. Robertson, A. Muller, L. N. Prill Sempere, C. K. Shih Department of Physics, The University of Texas at Austin, Austin, Texas, 78712 This work was supported by: NSFC (Grant 10344002 and 10474075), NSF (DMR-0210383 and DMR-0306239), the Texas Advanced Technology program and the W. M. Keck Foundation.

APS March Meeting 2005, Los Angeles

Resonators and two level systems I

The emission properties of a two-level system are influenced by the presence of a coupled electromagnetic resonator.

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There are two main phenomena to look for:

Purcell effect, when the coupling is weak. Rabi splitting, when the coupling is strong.

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Wurtzburg: Interface fluctuation quantum dots in micropillar cavities. Caltech: Self-assembled quantum dots in photonic crystal cavities. ENS: Semiconductor quantum dots and atoms in silica microspheres.

A microresonator menagerie There are several types of resonators well suited for coupling to semiconductor quantum dots.

Photonic crystal cavities.

Micropillars with DBRs.

Silica microspheres.

Achieved strong coupling!

Proposed for Cavity QED

Achieved strong coupling!

[T. Yoshie, et al, Nature

experiments.

[J. P. Reithmaier, et al,

432 200-203 (2004)]

[V.Lef` evre-Seguin, S.

Nature 432 197-200

Haroche, Mat. Sci. and

(2004)]

Engin. B48 53-58 (1997)]

Why microspheres? Silica microspheres show good properties: I

Very low losses (high Quality factor: 107 -1010 , compared to no more than 3 ∗ 104 for micropillars and photonic crystal cavities).

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Good confinement (Vm ≈ 300λ3 ).

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Large free spectral range (ideally ≈ 1 THz, in practice around several GHz).

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The cavity is external to the emitters.

Why do we care about the cavity being external? I

We can look for the best coupled resonant emitter by moving the cavity.

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We can try looking for a limited number of emitters simultaneously coupled to the cavity. → Quantum data bus?

Steps to follow

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Microsphere fabrication.

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Coupling light into the sphere, find resonant modes.

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Characterize quality factor spoling by the light coupler.

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Characterize loading by emitter sample (i.e. quantum dots in a wafer).

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Set up the experiments in cryogenic temperatures.

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Look for coupling between a dot and the sphere.

Whispering gallery modes in spheres Simple picture: I

Total internal reflections in the sphere equator confine light to a round trip trajectory.

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FSR ≈

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Modes can be TE or TM polarized

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ν = νn,l

c πnD

Mie Theory:

(pol)

I I I I [B. E. Little, J.-P. Laine and H. A. Haus, J. Lightwave Technol. 17 704-715 (1999)]

n = Number of radial maxima l = Angular momentum number m = z-component of the angular momentum l − m + 1 = Number of polar maxima between the two poles.

Microsphere fabrication [i.e.: M. L. Gorodetsky, et al Optics Letters 21 453-455 (1996)]

Making the microspheres is a simple, two-step process. 1: Pull an optical fiber using a CO2 laser and a weight.

2: Melt the tip using the focused CO2 laser beam.

Finished sphere

Tapered fiber coupling [i.e.: M. Cai, et al, Phys. Rev. Lett. 85 74-77 (2000)]

To couple light into the cavity, we used the evanescent field from a tapered optical fiber.

Schematic view of the coupling configuration.

Actual view of a microsphere and a taper.

By scanning the wavelength of the excitation, we can detect resonant modes as dips in the transmitted signal.

Scattering on the sphere at resonance. Normalized taper transmission.

Cavity loading [i.e.: M. Cai, et al, Phys. Rev. Lett. 85 74-77 (2000)]

As the taper comes closer, more light couples into the sphere, but the quality factor decreases as the taper adds a new channel for loss.

Transmission curves as the taper moves towards the sphere

Q-factor as a function of taper-sphere coupling

Conclusions and work in progress Work realized: I

Manufacturing of silica microsphere.

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Coupling light into microspheres using a tapered optical fiber.

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Observed the loading effect as the taper and cavity couple more strongly.

Work in progress: I

Measure the loading caused by a semiconductor sample placed in contact with the sphere.

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Move the experiments to cryogenic temperatures.

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Look for coupling between semiconductor quantum dots and a microsphere.

Also in progress: I

Manufacturing and coupling to silica microtoroids.