November 19, 2012 – Researchers from Rice U. say they have developed a micron-scale spatial light modulator (SLM) built on SOI that runs orders-of-magnitude faster than its siblings used in sensing and imaging devices. The "antenna-on-a-chip for light modulation," developed with backing from the Air Force Office of Scientific Research, is described in Nature‘s Scientific Reports.

While light processing has found use in consumer electronics (CDs and DVDs), communications (fiber optics), of course lighting applications (LEDs) and even industrial materials processing (lasers for cutting, welding, etc.), photonics for computing applications are still being explored, and reliant upon waveguides in 2D space. So-called "free space" spatial light modulators (SLM), however, could tap into "the massive multiplexing capability of optics," in that "multiple light beams can propagate in the same space without affecting each other," explains researcher Qianfan Xu.

To demonstrate, the Rice team built SLM chips with nanoscale ribs of crystalline silicon surrounded by SiO₂ claddings, forming a cavity between positively and negatively dopes Si connected to metallic electrodes. The positions of the ribs are subject to nanoscale "perturbations" and tune the resonating cavity to couple with incident light outside. This coupling pulls incident light into the cavity; infrared light passes through silicon but is captured by the SML and can be manipulated to the chip on the other side, with electrodes’ field switched on/off at very high speeds.

In the paper they go into more detail on the structure of the device:

SLMs are fabricated in a CMOS photonics foundry at the Institute of Microelectronics of Singapore. The fabrication starts on an SOI wafer with a 220nm-thick silicon layer and a 3μm-thick buried oxide layer. To construct the 1D PhC cavities, silicon ribs with the height of 170nm are patterned on a silicon slab with the thickness of 50nm using 248nm deep-UV lithography and inductively-coupled plasma etching. Following the etching, the p-i-n junctions are formed by patterned ion implantations with a dosage of 5 × 10¹⁴ cm^-2 for both the p+ and n+ doping regions. A 2.1μm-thick SiO₂ layer is then deposited onto the wafer using plasma-enhanced chemical vapor deposition (PECVD). Finally, vias are opened on the ion-implanted areas and a 1.5μm-thick aluminum layer is sputtered and etched to form the electric connections. The serial resistance of the diode is measured to be 105 Ω. After the fabrication process, the contact pads connecting to the p-i-n junction are wire-bonded to a SMA connector with a 50-ohms terminal resistor for impedance match.

The 3D FDTD simulations are done with commercial software Lumermical FDTD. A non-uniform grid is used which has a spatial resolution ~30nm around the resonator. Even though perturbation we introduced is much smaller than the grid size, the software is capable of incorporate that in the simulation. When a dielectric interface (Si/SiO₂) lies between two grid points, the program modifies the dielectric constant at the neighboring grid points according to the position of interface. This way, the small shift of the dielectric interface due to the width perturbation is taken into account in the simulation.

Conventional integrated photonics incorporate an array of pixels whose transmission can be manipulated at very high speed, explains Xu; adding an optical beam can change the intensity or phase of the exiting light. In LED screens and micromirror arrays in projectors (both of which are SLMs) where each pixel changes the intensity of light which generates an image, some switching speeds can get down to microseconds, but that’s far too slow for moving data around in a computing application. The new Rice device can "potentially modulate a signal at more than 10 gigabits per second."

Another key to their device is that it is silicon-based and can be fabricated at volume in a CMOS fab, which can scale up the capabilities to build very large arrays with high yield, he adds. For example, Rice researchers are separately creating a single-pixel camera, which initially took eight hours to process an image; this new SLM chip could enable it to handle real-time video. Alternatively, a million-pixel array could mean "a million channels of data throughput in your system, with all this signal processing in parallel" and at gigahertz levels, he said.

Xu is careful to note that the new SLM antenna-on-a-chip is not for general computing, but more for optical processing comparable in power to supercomputers. Optical information processing is " not fast-developing right now like plasmonics, nanophotonics, those areas," he admits, "but I hope our device can put some excitement back into that field."

Left: An illustration showing the design of Rice University researchers’ antenna-on-a-chip for spatial light modulation. The chip couples with incident light and makes possible the manipulation of infrared light at very high speeds for signal processing and other optical applications. Right: Crystalline silicon sits between two electrodes in the antenna-on-a-chip.  (Credit: Xu Group/Rice University)

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