Valérie Lefèvre-Seguin

Very High-Q Whispering-Gallery Mode Resonances Observed on Fused Silica Microspheres

We have studied by phase modulation spectroscopy the whispering-gallery modes (Mie resonances) of 60 to 200 μm diameter microspheres obtained by fusing with a CO2 laser the end of a high-transmission silica fibre. An evanescent wave at 780 nm was produced by total internal reflection of a phase-modulated semiconductor diode laser beam in a glass prism. It was coupled into the Mie mode of the microsphere positioned at a fixed distance from the prism face. The spectrum was obtained by measuring the phase-to-amplitude conversion of the laser beam modulation as its carrier frequency was scanned. Record Q-factors ≥ 2 109 were observed, corresponding to photon storage times longer than 1 μs. Applications of these resonances to fundamental and applied projects are discussed.

Splitting of high-Q Mie modes induced by light backscattering in silica microspheres

We have observed that very high-Q Mie resonances in silica microspheres are split into doublets. This splitting is attributed to internal backscattering that couples the two degenerate whispering-gallery modes propagating in opposite directions along the sphere equator. We have studied this doublet structure by high-resolution spectroscopy. Time-decay measurements have also been performed and show a beat note corresponding to the coupling rate between the clockwise and counterclockwise modes. A simple model of coupled oscillators describes our data well, and the backscattering efficiency that we measure is consistent with what is observed in optical fibers.

Strain-tunable high-Q optical microsphere resonator

Abstract We demonstrate the tuning of high- Q optical whispering-gallery modes in a fused silica microsphere by applying mechanical strain. The measured total tunability is over 0.3 nm in the near-infrared band, with the undercoupled quality-factor being Q >1×10 8 . The tunability of mode frequencies in this high- Q solid-state microcavity expands its applicability in quantum optics and photonics.

Mapping whispering-gallery modes in microspheres with a near-field probe

We demonstrate the use of a near-field probe to map the angular dependence of high-Q whispering-gallery modes in fused-silica microspheres. The mapping is performed by placing a micrometer-sized tip formed on the end of a monomode fiber into the evanescent f ield at the microsphere surface, causing light to be coupled from the microsphere resonance into the fiber guided mode. The light output of the fiber is then measured while the tip is moved to different points on the microsphere surface. We have used this method to investigate the lifting of spherical degeneracy in the system.

Tunable whispering gallery modes for spectroscopy and CQED experiments

We have tuned the whispering gallery modes of a fused silica micro-resonator over nearly 1 nm at 800 nm, i.e.?over half of a free spectral range or the equivalent of 106 linewidths of the resonator. This has been achieved by a new method based on the stretching of a two-stem microsphere. The devices described below will permit new cavity-QED experiments with this ultra high finesse optical resonator when it is desirable to optimize its coupling to emitters with given transition frequencies. The tuning capability demonstrated is compatible with both UHV and low temperature operation, which should be useful for future experiments with laser cooled atoms or single quantum dots. A general overview of the current state of the art in microspheres is given as well as a more general introduction.

Evidence for intrinsic Kerr bistability of high-Q microsphere resonators in superfluid helium

Abstract:Quality factors up to 109 have been obtained in the whispering gallery modes (WGMs) of fused silica microspheres immersed in a superfluid helium bath. We have observed a dispersive bistable behaviour of the WGM resonances with a threshold power of 10 μW, due to the intrinsic Kerr nonlinearity of silica. These results open the way to the realization of a thresholdless microlaser and other cavity QED projects with microspheres.

Frequency tuning of the whispering-gallery modes of silica microspheres for cavity quantum electrodynamics and spectroscopy

We have tuned the whispering-gallery modes of a fused-silica microresonator over nearly 1 nm at 800 nm, i.e., over half a free spectral range, or 10(6) linewidths of the resonator. This result has been achieved by use of a new method based on the stretching of a two-stem microsphere. We describe devices that will permit new cavity QED experiments with this high- Q optical resonator when it is desirable to optimize its coupling to emitters with given transition frequencies. The demonstrated tuning capability is compatible with both UHV and low-temperature operation, which should be useful for future experiments with laser-cooled atoms or single quantum dots.

Towards cavity-QED experiments with silica microspheres

We present experiments performed on highly transparent silica microspheres used as very high Q optical resonators for cavity quantum electrodynamics (CQED) studies. By coupling these microspheres to eroded optical fibers and fiber tips, direct mapping of the whispering-gallery modes has been achieved and the mode quantum numbers have been assessed. Low-threshold whispering-gallery modes lasers made with Nd-doped spheres are described and further CQED projects are proposed.

Very low threshold green lasing in microspheres by up-conversion of IR photons

A green up-conversion laser is demonstrated in a 120 µm diameter microsphere of Er 3+-doped ZBLAN at room temperature. Lasing occurs around 540 nm with a 801 nm diode laser pump. The lasing threshold of only 30 µW of absorbed power is over two orders of magnitude lower than the lowest previously observed in IR-visible up-conversion lasing (Funk D S et al 1997 Electron. Lett. 33 1958-60).

Whispering-gallery mode lasers with doped silica microspheres

Abstract Light can be confined efficiently in the high-Q, small-volume whispering-gallery-modes observed in silica microspheres. The properties of these modes have allowed us to obtain laser action with very low thresholds in Nd-doped silica microspheres. After a brief introduction to the physics of the whispering-gallery modes, we describe our recent experiments on the lasing properties of doped microspheres at room temperature and their extension at 2 K. Further projects in the field of non-linear Optics and Cavity Quantum Electrodynamics are discussed in the conclusion.