An ultrafast all-optical switch based on nonlinear photonics?

Caltech engineers built an all-optical switch ideal for time-division multiplexing, and it may enable time-multiplexed information processing and computing architectures to tap into the coveted ultrafast speed (femtosecond range) of optics.

Unlike electronics, optics still lack efficiency in components required for computing and signal processing—and it’s a major barrier to achieving the potential of optics for ultrafast and efficient computing methods.

“During the past few decades, substantial efforts were dedicated to developing all-optical switches that could address this challenge but most of the energy-efficient designs suffered from slow switching times, mainly because they either used high-Q resonators or carrier-based nonlinearities,” says Marandi.

His team’s goal was to use the inherent parametric nonlinearity of lithium niobate, which is instantaneous, to design an ultrafast switch in the ultralow energy (femtojoule) regime. Thanks to the way atoms are arranged within lithium niobate, which doesn’t occur in nature, the optical signals it produces as outputs aren’t proportional to the input signals—it’s a nonlinear crystal.

The engineers’ all-optical switch design intentionally avoids resonators, and instead relies on two other main elements for a nonlinear splitter (see Fig. 2)—and it led to the fastest all-optical switch: <50 fs, with only femtojoules of energy.

First, they use the spatiotemporal confinement of light within nanowaveguides to enhance nonlinear interactions, because the strength of parametric nonlinear processes depends on the peak intensity. This spatiotemporal confinement “is possible in nanophotonic lithium niobate because of the nanoscale cross-section of the waveguides and the possibility of dispersion engineering, which allows femtosecond pulses to remain short as they propagate through the nanoscale waveguide,” says Marandi.

The second crucial element of their design is engineering quasi-phase matching for the nonlinear interactions. “We can design and change the crystallographic orientation of lithium niobate along these waveguides,” he says.

Along their nanowaveguide, they use a periodic pattern for the orientation of the crystal with a defect in the middle, which deterministically switches the nonlinear process from second-harmonic generation (SHG) to optical parametric amplification (OPA).

“By adding a wavelength-selective coupler before this defect, since low-energy input pulses don’t lead to efficient SHG in the first half of the waveguide, they will be dropped by the linear coupler,” Marandi explains. “But high-energy pulses lead to efficient SHG before the coupler, so they won’t be dropped by the coupler because the input energy will be stored in the second-harmonic wavelength of the input. After the defect, the OPA process reverts back the signal to the input wavelength.”