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14 Nov 2007
Dynamically confining light in volumes smaller than the wavelength is an important step towards the complete control of light.
A team from Kyoto University has demonstrated a method for dynamically changing the lifetime of a photonic crystal resonator, which in effect can trap a pulse of light inside the system. Susumu Noda and his colleagues showed that the Q factor of their system, a measure of the number of time periods of light after which most of the light has left the system, could be increased by a factor of four from 3,000 to 12,000 within just a few picoseconds (Nature Materials, 6, 862).
"This is a completely new approach in this field, a new concept for dynamically controlling the Q factor that has not been used before," Noda explained to optics.org.
Nanocavities with high Q factors have been demonstrated previously, with Noda's team recently reporting one with a world-beating Q factor of two million, but controlling the Q factor dynamically is the key to developing useful applications for the technology.
"Slowing and/or stopping light could lead to quantum information processing applications, where nanocavities would be integrated on a chip and the transfer, storage and exchange of photons would be possible through integrated waveguides," Noda said. "The important issue is how to deliberately control the storage and release of photons from such a high-Q nanocavity."
The team fabricated its resonator in a two-dimensional photonic crystal. The structure consists of a waveguide with a mirror located at one end and a microresonator coupled to the waveguide.
The microresonator was formed by three missing airholes in the photonic crystal lattice, making a nanocavity in the crystal located five hole-rows away from the waveguide. The mirror was created by a line defect at one end of the waveguide, where the slightly modified lattice constant acted as a perfect reflector.
When a probe pulse of light was fed in through the waveguide, some light waves were reflected back from the cavity and interfered constructively with those reflected back from the mirror, yielding a large coupling of the system to the external light.
The team's breakthrough was to exploit the change in refractive index that can be induced in a silicon-based photonic crystal when it is irradiated with another pulse of light. This change is caused by the non-linear response of the crystal and can be preserved over several nanoseconds, long after the pulse that caused it has faded away.
This change in refractive index affects the interference between the waves reflected directly from the nanocavity and those returning from the mirror, with the result that it can be made either constructive or destructive. When destructive, the interference dramatically reduces the coupling to the external light, and therefore traps the light inside the system for a significantly longer period of time.
"When the cavity Q factor increases, the photon's lifetime is increased and the operational speed becomes slow," Noda explained. "So the Q factor needs to be made small when we introduce the photons, and then increase rapidly before the photons leak out of the nanocavity. And, if necessary, decreased again so that they can be deliberately released quickly when desired. Until now there has been no way to achieve such dynamic control of the nanocavity Q. This work demonstrates a way to do so for the first time."
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