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The Semiconductor Industry Association has estimated that at current rates of increase, computers’ energy requirements will exceed the world’s total power output by 2040. Using light rather than electricity to move data would dramatically reduce computer chips’ energy consumption, and the past 20 years have seen remarkable progress in the development of silicon photonics, or optical devices that are made from silicon so they can easily be integrated with electronics on silicon chips.

Existing silicon-photonic devices rely on different physical mechanisms than the high-end optoelectronic components in telecommunications networks do. The telecom devices exploit so-called second-order nonlinearities, which make optical signal processing more efficient and reliable.

MIT researchers have presented a practical way to introduce second-order nonlinearities into silicon photonics. They also report prototypes of two different silicon devices that exploit those nonlinearities: a modulator, which encodes data onto an optical beam, and a frequency doubler, a component vital to the development of lasers that can be precisely tuned to a range of different frequencies. Since in optics, a linear system is one whose outputs are always at the same frequencies as its inputs, a frequency doubler, for instance, is an inherently nonlinear device.

According to the researchers, one can now build a phase modulator that is not dependent on the free-carrier effect in silicon. The benefit is that the free-carrier effect in silicon always has a phase and amplitude coupling. So whenever one changes the carrier concentration, it changes both the phase and the amplitude of the wave that’s passing through it. With second-order nonlinearity, the coupling is broken, to give a pure phase modulator.

If an electromagnetic wave can be thought of as a pattern of regular up-and-down squiggles, a digital modulator perturbs that pattern in fixed ways to represent strings of zeroes and ones. In a silicon modulator, the path that the light wave takes is defined by a waveguide, which is rather like a rail that runs along the top of the modulator. Existing silicon modulators are doped, meaning they have had impurities added to them through a standard process used in transistor manufacturing. Some doping materials yield p-type silicon, where the “p” is for “positive,” and some yield n-type silicon, where the “n” is for “negative.” In the presence of an electric field, free carriers - electrons that are not associated with particular silicon atoms - tend to concentrate in n-type silicon and to dissipate in p-type silicon.

A conventional silicon modulator is half p-type and half n-type silicon; even the waveguide is split right down the middle. On either side of the waveguide are electrodes, and changing the voltage across the modulator alternately concentrates and dissipates free carriers in the waveguide, to modulate an optical signal passing through. The MIT researchers’ device is similar, except that the center of the modulator - including the waveguide that runs along its top - is undoped. When a voltage is applied, the free carriers don’t collect in the center of the device; instead, they build up at the boundary between the n-type silicon and the undoped silicon. A corresponding positive charge builds up at the boundary with the p-type silicon, producing an electric field, which is what modulates the optical signal. Because the free carriers at the center of a conventional silicon modulator can absorb light particles - or photons - traveling through the waveguide, they diminish the strength of the optical signal; modulators that exploit second-order nonlinearities don’t face that problem.

In principle, the modulators can also modulate a signal more rapidly than existing silicon modulators do. That’s because it takes more time to move free carriers into and out of the waveguide than it does to concentrate and release them at the boundaries with the undoped silicon. The current paper simply reports the phenomenon of nonlinear modulation, but the team has since tested prototypes of a modulator whose speeds are competitive with those of the nonlinear modulators found in telecom networks.

The frequency doubler that the researchers demonstrated has a similar design, except that the regions of p- and n-doped silicon that flank the central region of undoped silicon are arranged in regularly spaced bands, perpendicular to the waveguide. The distances between the bands are calibrated to a specific wavelength of light, and when a voltage is applied across them, they double the frequency of the optical signal passing through the waveguide, combining pairs of photons into single photons with twice the energy. They can also be used to build extraordinarily precise on-chip optical clocks, optical amplifiers, and sources of terahertz radiation, which has promising security applications.

To date, efforts to achieve second-order nonlinear effects in silicon have focused on hard material-science problems but the [MIT] team has been extremely clever by reminding the physics community that applying a simple electric field creates the same basic crystal polarization vector that other researchers have worked hard to create by far more complicated means.