by Tim Palucka
Materials Research Society | Published: 07 March 2013
A metamaterial lattice consists of photo-doped silicon islands in the split ring resonator gap. The green pulse is the terahertz wave exciting the metamaterial. The near infrared femtosecond pump laser beam (shown in red) excites the silicon islands in the metamaterial, thus controlling the group velocity of the terahertz pulse transmitting through the metamaterial. Image credit: Jianqiang Gu. Click image to enlarge.
Light moving through a vacuum is the fastest phenomenon we know, but there could be significant technological advantages to applying the brakes occasionally. For applications such as optical computing, sensing, telecommunications and perhaps even quantum computing, controlling the speed of light through various media could be the key to optimum performance. Now, researchers have developed artificially engineered resonant metamaterials that, when illuminated by a femtosecond near-infrared laser light of varying intensity, can actively tune the group velocity of the terahertz light transmitted through the metamaterials. This is achieved by the dynamic tuning of the resonance enhanced dispersion of the effective medium that comprises subwavelength metamaterial unit cells called meta-molecules. The near field coupling between the meta-molecule resonators is exploited to create a resonant transparency window that mimics the quantum phenomena of electromagnetically induced transparency (EIT).
“The biggest benefit is that if you can slow down light it can interact very strongly with matter, resulting in enhanced optical nonlinearities that would play a major role in the progress of on-chip, all-optical signal processing and quantum computation,” says Ranjan Singh of Los Alamos National Laboratory, one of the lead authors, along with Jianqiang Gu of Tianjin University, of the paper recently published in Nature Communications. Quanta of light— photons—are electromagnetic radiation without any mass or charge, whereas material interactions mostly involve electrons, Singh says; enabling photons to interact more strongly with the electrons in materials “can help us understand the interactions of photons with matter in a much more profound way.”
Singh and his colleagues at Tianjin University in China; the University of Birmingham and Imperial College London in the UK; and Oklahoma State University and Los Alamos National Laboratory in the U.S., have designed the meta-molecules on a sapphire substrate. The meta-molecule unit cell consists of two square, split ring resonators (SRRs) and one cut wire made of aluminum, with Si filling the “splits” in the SRRs. Each unit cell behaves like an active molecule, according to Singh. They have arranged 10,400 of these unit cells together on one 10 x 10 mm sapphire chip. The sizes of the SRRs and the cut wire are chosen in such a way that their fundamental resonance modes coincide at one single frequency. It is the destructive interference between the non-radiative inductive-capacitive (LC) resonance of SRRs and the radiative dipole resonance of the cut wires that results in the EIT effect.
When a 2.5-mm diameter terahertz beam is shined perpendicular to the face of the metamaterial chip, the material is strongly transparent at 0.74 THz. No external laser light is used at this point, and the terahertz pulse transmitted through the chip is at its lowest group velocity. By concurrently shining a femtosecond near-infrared laser pulse at a slight angle to this surface, the LC resonance property of the SRRs change due the Si pads, and the electromagnetically induced resonant transparency begins to diminish. As the laser power is increased from 25 mW to 1,000 mW, the transparency peak slowly fades. At 1,350 mW laser power, the EIT peak is gone, and light propagates through the metamaterial as if it were an ordinary medium.
“The femtosecond infrared laser pulse is photo-exciting the Si, changing it from a semiconductor to a quasi-metal,” Singh says. “The LC resonance of the SRRs gradually quenches with increasing Si conductivity, and that actually tunes the group velocity through the metamaterial. Our metamaterial medium is an active slow light device—you can control how much to slow down the light by using a laser pulse.”
One technological area in which Singh envisions applications for slow light is telecommunications. If you are transmitting two light pulses containing different information through a high bit rate telecom router, the information can get smeared by overlap of the light pulses, he says. By letting one light pulse go through the router at its normal speed, and slowing down the other light pulse using this metamaterial chip, you can create a time delay between the two pulses and transmit their information through routers without any significant interference.
Currently, the group is working on improving the performance of a slow light metamaterial device by suppressing the losses and enhancing the operation bandwidth. “If we can do that, I think it’s possible to have a practical slow light metamaterial device that could act as an all-optical tunable delay line for terahertz and microwaves,” Singh says.
Read the abstract in Nature Communications here.
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