Antenna on a Chip Zips Through 3-D ‘Free Space’

A new micron-scale spatial light modulator (SLM) works in 3-D “free space” and runs orders of magnitude faster than those used in sensing and imaging devices, and it holds great potential for imaging, display, holographic, measurement and remote sensing applications.

The “antenna on a chip” device, developed at Rice University, looks like a tiny washboard and could make current commercial products used to manipulate infrared light obsolete.

A new antenna on a chip for spatial light modulation enables the manipulation of infrared light at very high speeds for signal processing and other optical applications. From left, graduate students Ciyuan Qiu, Jianbo Chen and Yang Xia, and Qianfan Xu, an assistant professor of electrical and computer engineering. Courtesy of Jeff Fitlow/Rice University.

Light manipulation is central to the information economy, from light-reflecting compact discs and their video variants to lasers used for sensing, security and surgery. Light carries data through optical fibers for telecommunications and signals on the molecular scale as photonics techniques improve. LEDs power TV displays and are even replacing incandescent lightbulbs in homes.

But in the realm of computers, light has been constrained by 2-D circuitry, tied to waveguides that move it from here to there, said Qianfan Xu, an electrical and computer engineering assistant professor at Rice and leader of the research. The investigators believe that 2-D systems fail to take advantage of the massive multiplexing capability of optics made possible by the fact that “multiple light beams can propagate in the same space without affecting each other.”

The SLM chips are nanoscale ribs of crystalline silicon that form a cavity between positively and negatively doped silicon slabs connected to metallic electrodes. The positions of the ribs are subject to nanometer-scale “perturbations” and tune the resonating cavity to couple with incident light outside.

Crystalline silicon sits between two electrodes in a microscopic antenna on a chip. The chip, a spatial modulator, couples with incident light and makes possible the manipulation of infrared light at very high speeds for signal processing and other optical applications. Courtesy of Xu Group/Rice University.

Such coupling pulls incident light into the cavity. Only infrared light passes through silicon, but once captured by the SLM, it can be manipulated as it passes through the chip to the other side. The electric field between the electrodes turns the transmission on and off at very high speeds.

Individual SLMs can be likened to pixels, and Xu sees the potential for manufacturing chips containing millions of them. With conventional integrated photonics, he said, “you have an array of pixels, and you can change the transmission of each pixel at a very high speed. When you put that in the path of an optical beam, you can change either the intensity or the phase of the light that comes out of the other side.

“LED screens are spatial light modulators; so are micromirror arrays in projectors, in which the mirrors rotate. Each pixel changes the intensity of light, and you see an image. So an SLM is one of the basic elements of optical systems, but their switching speed is limited; some get down to microseconds, which is OK for displays and projection.”

But if you want to put data on each pixel for information processing, the speed is not good enough. The device could potentially modulate a signal at more than 10 Gb/s, he said.

The design of the antenna on a chip for spatial light modulation. The chip can process incident infrared light for signal processing at very high speeds. Courtesy of Xu Group/Rice University.

“With this device, we can make very large arrays with high yield,” he said. “Our device is based on silicon and can be fabricated in a commercial CMOS factory, and it can run at very high speed. We think this can basically scale up the capability of optical information processing systems by an order of several magnitudes.”

For example, the device could give the single-pixel camera in development at Rice — which at the beginning took eight hours to process an image — the ability to handle real-time video.

“Or, you could have an array of a million pixels, and essentially have a million channels of data throughput in your system, with all this signal processing in parallel,” he said. “If each pixel only runs at kilohertz speeds, you don’t get much of an advantage compared with microelectronic systems. But if each pixel is working at the gigahertz level, it’s a different story.”

Although these antennas would not be suited for general computing, they could perform optical processing tasks comparable in power to supercomputers.

“Optical information processing is not very hot,” Xu said. “It’s not fast-developing right now like plasmonics, nanophotonics, those areas. But I hope our device can put some excitement back into that field.”

Details of the antenna appear in Scientific Reports (doi:10.1038/srep00855).

For more information, visit: www.rice.edu