Catching photons in a bottle

Jörg Schwartz, j.schwartz@europhotonics.com

Physicists at Johannes Gutenberg University have developed a bottle-shaped, monolithic microresonator that captures light and tunes it to arbitrary optical frequencies. To do this, they stretched a glass fiber until it reached about half the diameter of a human hair and, with the help of a CO₂ laser, created a bulge-shaped structure.

Inside the resonator, photons with a certain frequency are trapped by continuous reflections and cannot escape; this quality is seen as offering numerous possibilities for technology and research applications. Today, microresonators perform filtering and switching functions in optical communications and sensing, and they are used as a stepping stone toward building microscopically small lasers. But researchers also see these devices as a controlled means of enhancing and studying the interaction between light and matter.

That said, there are a few issues with making good microresonators for this purpose, as professor Arno Rauschenbeutel, the lead researcher, explains. First of all, the “mechanical” design has to be small for confined interaction/mode volumes and high field densities. Similarly important is how successfully the light is held hostage, or the quality of the resonator, which requires high reflectivity at the surfaces and good stabilization. This puts monolithic devices with no moving parts in a good position; however, another important requirement is tunability – the stored light frequency’s ability to interact with a specific type of atom.

This is a key shortcoming in what’s been used to date, namely equatorial whispering-gallery-mode microresonators. They capture the light in a narrow ring along the equator of a circular structure via total internal reflection. While these devices offer small mode volumes and large frequency spacing between modes, tuning is difficult. Their size means that they come with a large free spectral range, and neither the temperature nor the strain dependence of the refractive index is large enough to tune across it.

Rauschenbeutel and his team have overcome this limitation by using a clever design that they have now demonstrated (and published in the July 28, 2009, issue of Physical Review Letters). They applied a two-step “heat and pull” process on standard glass fiber to create a microtapered fiber waist by using a focused CO₂ laser, controlled by a microscope and customized image analysis software. The desired shape is referred to as a “bottle microresonator.” Somewhat similar to the motion of a charged particle stored in a magnetic bottle – i.e., a spatially varying magnetic field of similar shape – the light oscillates back and forth inside the structure between two turning points that are defined by a so-called angular momentum barrier. At those points, the particles can’t move farther into the bottle neck, as they must gain energy to be fast enough for smaller radii. As a result, the light is forced into “bottle modes” and oscillates back and forth along the resonator axis between the turning points.

The light in “bottle modes” harmonically oscillates back and forth along the resonator axis between two turning points. Stretching the shape along its axis offers tunability.

Because the standing waves are no longer specified by the diameter of the microresonator but also have an axial component, it turns out that tuning the resonator via stretching has become a viable option. Having access to a high-quality, tunable microresonator should facilitate new quantum electrodynamics experiments – for example, creating a quantum interface between light and atoms.

Why are those needed? “To make quantum computing work,” says Rauschenbeutel. “Since superposition works against us here, photons alone do not interact with each other.”