Researchers blast past the Q factor

June 27, 2017 // By Nick Flaherty
Researchers at EPFL in Zurich have been able to break past the Q factor to handle over a thousand times the combination of power and bandwidth in today's systems.

The team designed resonant systems that can store electromagnetic waves over a long period of time while maintaining a broad bandwidth.

The study, which has just been published in Science, creates asymmetric resonant or wave-guiding systems using magnetic fields and could boost the design of telecoms systems, optical detection systems and broadband energy harvesting.

Resonant and wave-guiding systems temporarily store energy in the form of electromagnetic waves and then release them.

The idea of the Q factor was first proposed by K. S. Johnson in 1914 at Western Electric Company, the forerunner of Bell Telephone Laboratories. This said that a resonator can either store energy for a long time or have a broad bandwidth, but not both at the same time. Increasing the storage time meant decreasing the bandwidth, and vice versa. A small bandwidth means a limited range of frequencies and therefore a limited amount of data.

The hybrid resonant / wave-guiding system made of a magneto-optic material that, when a magnetic field is applied, is able to stop the wave and store it for a prolonged period, thereby accumulating large amounts of energy. Then when the magnetic field is switched off, the trapped pulse is released.

With such asymmetric and non-reciprocal systems, it was possible to store a wave for a very long period of time while also maintaining a large bandwidth. The conventional time-bandwidth limit was beaten by a factor of 1,000. The team also showed that, theoretically, there is no upper ceiling to this limit at all in these asymmetric (non-reciprocal) systems.

“It was a moment of revelation when we discovered that these new structures did not feature any time-bandwidth restriction at all. These systems are unlike what we have all been accustomed to for decades, and possibly hundreds of years,” said Kosmas Tsakmakidis at EPFL’s Bionanophotonic Systems Laboratory.

One possible application is in the design of extremely quick and efficient all-optical buffers in telecommunication networks. The role of