Schematic of active optical control of terahertz waves in electromagnetically induced transparency metamaterials.Wireless communications and optical computing could soon get a significant boost in speed, thanks to “slow light” and specialized metamaterials through which it travels.

Researchers have made the first demonstration of rapidly switching on and off “slow light” in specially designed materials at room temperature. This work opens the possibility to design novel, chip-scale, ultrafast devices for applications in terahertz wireless communications and all-optical computing.

Significance of the research
In slow light, a propagating light pulse is substantially slowed down, compared with the velocity of light in a vacuum. This is accomplished by the light’s interaction with the medium through which it is shining. Slow light has potential applications in telecommunications because it could lead to a more orderly traffic flow in networks.

Like cars slowing down or speeding up to negotiate an intersection, packets of information are better managed if their transmission speed is changeable. Another potential application is the storage of information carried by light pulses, leading to a potential all-optical computing system. Current semiconductor materials used in computing devices are reaching some of their limits, and an all-optical system would potentially enable improvements in size reduction and calculation speeds.

The effects of strong light-matter coupling used in slowing down light might create entangled photon pairs that lead to quantum computing capabilities beyond those of modern computers, the researchers say.

Giving classical optical structures a quantum twist
Electromagnetically induced transparency is a quantum interference effect that produces a sharp resonance with extremely low loss and dispersion. However, implementing electromagnetically induced transparency in chip-scale applications is difficult due to the demands of stable gas lasers and low-temperature environments. The key to success is the use of metamaterials, engineered artificial materials containing structures that are smaller than the wavelength of the waves they affect.

Researchers integrated photoconductive silicon into the metamaterial unit cell. This material enables a switching of the transparency resonance window through the excitation of ultrafast, femtosecond optical pulses. This phenomenon causes an optically tunable group delay of the terahertz light. The “slow light” behavior can be controlled at an ultrafast time scale by integrating appropriate semiconductor materials with conventional metamaterial designs.

In this research, the medium is an active metamaterial that supports a sharp resonance, which leads to a rapid change in the refractive index of the medium over a small range of frequencies. This phenomenon causes a dramatic reduction in the velocity of terahertz light propagation. The resonance can be switched on and off on a time scale of a few picoseconds. When the resonance transparency is on, the system produces slow light. When the resonance is off, the slow light behavior disappears. This on and off process happens on an ultrafast (picosecond) time scale when a femtosecond laser pulse excites the metamaterial. Nature Communications published the research.

Source: Los Alamos National Laboratory