Microscopic Infrared Antenna on a Chip

silicon crystal spatial light modulator diagramA micron-scale spatial light modulator has been produced by research at Rice University. The spatial light modulator (SLM) is similar to ones used in sensing and imaging devices, but has the possibility of running orders of magnitude faster. Unlike other devices in two-dimensional semiconducting chips, these chips work in three-dimensional “free space.”

The device, which looks like a tiny washboard, and could make obsolete current commercial products used to manipulate infrared light, is detailed this week in the open-access online journal Scientific Reports.

Processing and manipulation of light has become a pillar of the information economy. From light-reflecting compact discs and their video variants to lasers used for sensing in security and surgery, and to light carrying data through optical fibers for telecommunications and signals on the molecular scale as photonics techniques improve. Light-emitting diodes power television displays as viewers clutch infrared remotes, and are even beginning to replace incandescent light bulbs in homes.

But in the realm of computers, light technology has been constrained by two-dimensional circuitry, tied to waveguides that move it from here to there, says Rice researcher Qianfan Xu. He and his colleagues point out in their paper 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.”

Free Space Spatial Light Modulators

crystal silicone antennaGreat potential exists for free-space SLMs in imaging, display, holographic, measurement and remote sensing applications.

The Rice team’s microscopic chips are nanoscale ribs of crystalline silicon that form a cavity sitting 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.

That 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; Xu, an assistant professor of electrical and computer engineering, sees the potential of manufacturing chips containing millions of them. With conventional integrated photonics, “You have an array of pixels and you can change the transmission of each pixel at a very high speed,” said Xu.

“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 the other side. LED screens are spatial light modulators; so are micromirror arrays in projectors, in which the mirrors rotate,” he explains. “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 can get down to microseconds, which is okay for displays and projection. But if you really want to do information processing, if you want to put data on each pixel, then that speed is not good enough.”

10 Gigabits Per Second

According to Xu, the Rice team’s device “can potentially modulate a signal at more than 10 gigabits per second. “What we show here is very different from what people have been doing,” he said. “With this device, we can make very large arrays with high yield. 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 wouldn’t be suited for general computing, they could be capable of optical processing tasks that are comparable in power to supercomputers. “Optical information processing is not very hot,” Xu admitted. “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.”

References:

1. Active dielectric antenna on chip for spatial light modulation Ciyuan Qiu, Jianbo Chen, Yang Xia & Qianfan Xu
Scientific Reports 2, Article number: 855 doi:10.1038/srep00855

2. Watts, M. R., Zortman, W. A., Trotter, D. C., Young, R. W. & Lentine, A. L. Vertical junction silicon microdisk modulators and switches. Opt. Express 19 (22), 21989–22003 (2011)

Images: Xu Group/Rice University