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Slowing terahertz promises all-optical processing

14 Jul 2008

On-chip integration of optical circuits, optical data storage and ultrafast optical communications are the ultimate goal of researchers developing a metallic grating to slow down terahertz waves.

A metal grating that stops different terahertz frequencies at different positions has been developed by a team of scientists in the US. The researchers believe that their grating could enable compact terahertz photonic devices and hope to extend their approach to the visible spectrum, opening up applications in optical communications and all-optical computing (Physical Review Letters 100 256803).

While promising steps have been taken towards slowing light in solid-state media and semiconductor nanostructures operating at room temperature, these efforts have been constrained by narrow bandwidths, limited working wavelengths and strong temperature dependence.

"Our graded grating reduces the speed of light over an ultra-wide spectral band and operates at ambient temperature," Filbert Bartoli, a researcher at Lehigh University, US, told optics.org. "The physical separation between the stopped waves can be tuned by changing the grade of the grating depths. The grating can slow down electromagnetic waves at various locations depending on frequency."

Bartoli's approach uses plasmons - collective oscillations of free electron gas (plasma) found in metals. Plasmons can couple with photons to create a surface plasma polariton (SPP), which can propagate along the surface of metal.

"What makes SPPs interesting are their potential for spatial confinement of electromagnetic energy within subwavelength dimensions over a wide spectral range," commented Bartoli. "This makes them an attractive candidate to break the size limitation that diffraction imposes on conventional photonic structures."

The group used a graded metallic grating of varying depth to enable SPPs to interact with terahertz light. Terahertz waves with different frequencies are stopped at different positions along the grating structure thanks to their coupling with SPPs.

"We couple the terahertz wave with the SPP by matching their momentums," explained Bartoli. "We used a graded grating structure to gradually achieve the required momentum matching."

While the group's initial focus has been on the terahertz regime, it hopes to extend its method to operate in the infrared and visible domains. "It is easier to fabricate a structure for terahertz waves because feature sizes are in the order of tens of microns," concluded Bartoli. "For a structure to operate in the near infrared and visible regions feature sizes must be in the order of hundreds of nanometres. Any errors at this scale could introduce scattering effects and weaken the slow light."

•Recent work by Ortwin Hess and colleagues at the University of Surrey proposed trapping a range of frequencies of light using a negatively-refracting waveguide, sandwiched between two positively-refracting materials. While the function of Bartoli's structure is similar to that of Hess', Bartoli believes that his work moves beyond the abstract theory of Hess's approach.

"Hess' work does not specify what material would be capable of providing such a negative refractive index, whereas we present a very specific structure with known material properties," said Bartoli. "What's more, we believe that our structure should be practical to fabricate."

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