Scheme reverses light pulses

By Eric Smalley, Technology Research News

Getting out exactly what you put in is easier said than done, especially for something as ephemeral as a light pulse. Bouncing and redirecting light pulses with mirrors and lenses inevitably distorts them and diminishes their energy.

Researchers at Stanford University have developed a method for accurately time-reversing electromagnetic pulses, making it possible to receive a light pulse and return a replica of exactly the same size, shape and wavelength. "The outgoing pulse is a perfect time-reversed replica of the original pulse, without any distortion," said Mehmet Fatih Yanik, a researcher at Stanford.

The time reversal scheme could be used to boost optical and microwave communications, said Yanik. "The time-reversal process can be used to compensate for dispersion and to increase the bandwidth of communications systems," he said. "In the microwave domain, it could significantly improve radar systems and signal processing."

The theoretical technique could also lead to room-temperature, on-chip quantum photonics and quantum computing devices, said Yanik. Quantum photonics involves controlling individual photons for communications applications like quantum cryptography. Optical quantum computing involves using properties of individual photons to carry out computations.

The time-reversal method is an extension of the researchers' earlier technique for briefly stopping light pulses using photonic crystal. Photonic crystal is material that blocks or channels specific wavelengths of light using refraction, or the bending of light, at the boundaries of tiny, regularly spaced holes or rods in the material. In the researchers' light-stopping and time-reversing systems, the photonic crystal contains tiny cavities that vibrate at the same frequencies as the light pulses.

The equipment needed is relatively simple, said Yanik. Conventional time reversal schemes require high-power lasers and very special materials, he said. "All we need is tunable resonators, which can be [fabricated] with conventional microprocessing technologies."

The time-reversal photonic crystal system contains two sets of cavities, or resonators. One receives an incoming pulse and the other generates an outgoing pulse. "The light pulse first enters the photonic crystal, which consists of many coupled resonators," said Yanick. "As the photon pulse propagates inside the crystal, we shift the relative resonance frequencies of the cavities to transfer photons from one subsystem of cavities to another," he said.

The second set of cavities is tuned to exactly invert the pulse and send it in the direction the original pulse came from, said Yanik.

Key to the researchers' system is the ability to tune the resonant frequencies of the cavities on-the-fly, which in turn changes the frequency of the trapped light. The challenge was finding a way to change the resonant frequencies very quickly -- as many as 100 billion times the second -- without changing the shapes of the pulses. Light pulses are made up of multiple wavelengths, and pulses tend to spread out when they travel through matter because wavelengths are altered to different degrees. Careful timing of the resonant frequency modulation assures that each wavelength is altered in proportion to the others in order to preserve the pulses.

One hundred cavities is sufficient to time-reverse a 20-gigahertz-wide pulse centered at 200 terahertz, according to Yanik. The terahertz portion of electromagnetic spectrum is below visible light, between microwaves and infrared light.

The researchers are close to building a microwave prototype of their original light-stopping system, said Yanick. Making the light-stopping and time-reversal techniques practical will require high-quality microcavities that can change frequencies without losing much of the light's energy, he said. "Although it might take a while, these challenges are not fundamental and can be overcome with proper engineering."

Microwave applications could be practical in one to three years; optical applications could be practical in five to ten years, said Yanick.

Yanik's research colleague was Shanhui Fan. The work appeared in the October 22, 2004 issue of Physical Review Letters. The research was funded by Stanford University.

Timeline:   1-3 years, 5-10 years
Funding:   University
TRN Categories:  Optical Computing, Optoelectronics and Photonics
Story Type:   News
Related Elements:  Technical paper, "Time Reversal of Light with Linear Optics and Modulators," Physical Review Letters, October 22, 2004


April 6/13, 2005

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