In an advance for photonic integrated circuits (PICs), Stéphane Kéna-Cohen and his team at Polytechnique Montréal in Canada recently fabricated waveguides for frequency doubling using an organic thin film—based on triphenylamine-dicyanoquinoxaline, a.k.a. TPA-QCN—that strongly interacts with light. This class of materials may help lessen AI’s energy load on data centers because it can be placed onto silicon to process light directly and eliminates converting electric and photonic signals back and forth.
TPA-QCN is deposited via vacuum evaporation to form a layer of molecules that spontaneously adopt a preferred orientation. This alignment provides it with a second-order optical nonlinear response, which means light beams can interact while passing through it.
“We were inspired by beautiful work being done within an entirely different field—organic light-emitting diodes (OLEDs) for displays,” says Kéna-Cohen, a professor of engineering physics, as well as the Canada Research Chair in Light-Matter Photonics. “Researchers realized that certain classes of molecules automatically align during fabrication. In their case, this leads to the buildup of a voltage that typically hurts device performance. This voltage buildup, which requires polar molecules to orient along a preferential direction, was the first sign that we should be able to use similar materials for nonlinear optics.”
Second-order nonlinear photonic devices ahead
Silicon is the dominant platform for integrated photonics today, but it’s not good for fabricating modulators and amplifiers. “One ingredient silicon lacks to make good modulators is the Pockels effect, which allows a direct-current electric field to interact with an electric field at optical frequencies—and it’s a good example of a second-order nonlinear optical effect,” explains Kéna-Cohen. “Parametric amplifiers and oscillators rely on second-order nonlinearities. For these types of effects, engineers need to use other materials such as lithium niobate either as standalone components or go through the complex process of integrating the two.”
Another concept involved in the design of second-order nonlinear photonic components is a phase matching requirement—the phase velocity of the interacting light waves needs to match to avoid destructive interference effects. “Unfortunately, the fact that all materials have dispersion automatically prevents this, so clever tricks need to be used for phase matching. In lithium niobate, a common approach is to use electrodes along the propagation direction to flip the domain orientation—a.k.a. electric field poling.”
Advantages: Deposited on-chip directly, birefringence
The team’s approach brings two key advantages. “First, our organic thin films can be deposited directly onto any chip by using standard dry fabrication processes—without worrying about lattice matching or transfer,” says Kéna-Cohen.
Second, their films show an extremely strong birefringence compared to most common photonic materials. “This birefringence is so strong that it allows us to phase match ‘for free’ if we use interactions between differently polarized modes, because the different polarizations will see different refractive indices,” he says. “It means we can engineer very efficient devices, without needing electrodes for poling or having to use more complicated architectures.”
They used their approach to demonstrate the simplest example of a second-order nonlinear process: Second-harmonic generation, a.k.a. frequency doubling. To do it, the researchers fabricated a waveguide that converts continuous-wave telecom light to visible light (see figure below).