Microcavity physics and design will be reviewed. Following an overview of applications in quantum optics, communications and biosensing, recent advances in ultra-high-Q research will be presented.

Optical microcavities

We demonstrate microfabrication of ultra-high-Q microcavities on a chip, exhibiting a novel toroid-shaped geometry. The cavities possess Q-factors in excess of 100 million which constitutes an improvement close to 4 orders-of-magnitude in Q compared to previous work [B. Gayral, et al., 1999].

Ultra-high-Q toroid microcavities on a chip

Electromagnetically induced transparency is a quantum interference effect observed in atoms and molecules, in which the optical response of an atomic medium is controlled by an electromagnetic field. We demonstrated a form of induced transparency enabled by radiation-pressure coupling of an optical and a mechanical mode. A control optical beam tuned to a sideband transition of a micro-optomechanical system leads to destructive interference for the excitation of an intracavity probe field, inducing a tunable transparency window for the probe beam. Optomechanically induced transparency may be used for slowing and on-chip storage of light pulses via microfabricated optomechanical arrays.

http://science.sciencemag.org/content/sci/330/6010/1520.full.pdf

Optomechanically Induced Transparency

The ability to detect and size individual nanoparticles with high resolution is crucial to understanding the behaviour of single particles and effectively using their strong size-dependent properties to develop innovative products. We report realtime, in situ detection and sizing of single nanoparticles, down to 30 nm in radius, using mode splitting in a monolithic ultrahigh-quality-factor (Q) whispering-gallery-mode microresonator. Particle binding splits a whispering-gallery mode into two spectrally shifted resonance modes, forming a self-referenced detection scheme. This technique provides superior noise suppression and enables the extraction of accurate particle size information with a single-shot measurement in a microscale device. Our method requires neither labelling of the particles nor a priori information on their presence in the medium, providing an effective platform to study nanoparticles at single-particle resolution. With the rapid progress in nanotechnology, nanoparticles of different materials and sizes have been synthesized and engineered as key components in various applications ranging from solar cell

On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh- Q microresonator

The ability to confine and store optical energy in small volumes has implications in fields ranging from cavity quantum electrodynamics to photonics. Of all cavity geometries, micrometre-sized dielectric spherical resonators are the best in terms of their ability to store energy for long periods of time within small volumes. In the sphere, light orbits near the surface, where long confinement times (high Q) effectively wrap a large interaction distance into a tiny volume. This characteristic makes such resonators uniquely suited for studies of nonlinear coupling of light with matter. Early work recognized these attributes through Raman excitation in microdroplets-but microdroplets have not been used in practical applications. Here we demonstrate a micrometre-scale, nonlinear Raman source that has a highly efficient pump-signal conversion (higher than 35%) and pump thresholds nearly 1,000 times lower than shown before. This represents a route to compact, ultralow-threshold sources for numerous wavelength bands that are usually difficult to access. Equally important, this system can provide a compact and simple building block for studying nonlinear optical effects and the quantum aspects of light.

/pdf/ultralow-threshold-raman-laser-using-a-spherical-dielectric-2fdzryn5xk.pdf

Ultralow-threshold Raman laser using a spherical dielectric microcavity

We report on the realization of a whispering-gallery-mode laser based on neodymium-doped silica microspheres. Absorbed pump powers at threshold are as low as 200 nW. The linear variation of the threshold with the loss factor of the cavity mode has also been observed. We discuss the potential of this system as a permanent microlaser operating with a few active ions at liquid-helium temperature.

Very low threshold whispering-gallery-mode microsphere laser.

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.

https://iopscience.iop.org/article/10.1209/0295-5075/23/5/005/pdf

Very high-Q whispering-gallery mode resonances observed on fused 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.

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

Strain-tunable high-Q optical microsphere resonator

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 field 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.

Valérie Lefèvre-Seguin

Papers

Very low threshold whispering-gallery-mode microsphere laser.

Very high-Q whispering-gallery mode resonances observed on fused silica microspheres

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

Strain-tunable high-Q optical microsphere resonator

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