Penn Physicists Create Hybrid Light-Matter Particles That Switch Optical Signals at 4 Femtojoules, Pointing Toward All-Optical Computing

Overview

Physicists at the University of Pennsylvania have engineered hybrid light-matter particles known as exciton-polaritons inside a nanoscale optical cavity and demonstrated all-optical signal switching at excitation energies as low as 4 femtojoules (4×10⁻¹⁵ joules). The result, published in Physical Review Letters as an Editor’s Suggestion and cover article, sets a new benchmark for switching energy in two-dimensional exciton-polariton systems and offers a potential route to optical processors that handle computation at the speed of light.

What We Know

Excitons, quasiparticles formed when light excites an electron, can combine with photons confined inside an optical cavity to create exciton-polaritons—hybrid quasiparticles that inherit properties of both parents. As co-first author Li He, now an assistant professor at Montana State University, explained in coverage by Interesting Engineering: “Because they are charge-neutral and have zero rest mass, photons can carry information quickly over long distances with minimal loss, dominating communications technology.” The limitation is the reverse: “They barely interact with their environment, making them bad at the sort of signal-switching logic that computers depend on.”

Exciton-polaritons can bypass that limitation because the excitonic component interacts strongly with other particles, while the photonic component preserves high speed and low loss. The challenge has been engineering polaritons in a compact, tunable platform at low enough energy to be practically useful.

The Penn team, led by Bo Zhen—Jin K. Lee Presidential Associate Professor in the Department of Physics and Astronomy at Penn Arts & Sciences—placed a MoSe₂ monolayer inside a silicon nitride photonic crystal nanobeam cavity. The cavity’s mode volume of approximately 1.3(λ/n)³ concentrates light into an extremely small region, boosting the exciton-photon coupling strength to 16.8 meV and producing a clearly resolved Rabi splitting between upper and lower polariton states, according to the primary paper. The bare cavity resonance sits at 1.736 eV, with a cavity linewidth of 1.9 meV.

The researchers report all-optical switching of the cavity spectrum at excitation energies as low as approximately 4 femtojoules—around 10⁴ photons—on picosecond timescales, with the system recovering to equilibrium within hundreds of picoseconds, as described in the paper. The nonlinear mechanism is excitation-induced dephasing (EID): increasing exciton populations cause linewidth broadening and progressively reduce the Rabi splitting, eventually collapsing the strong-coupling regime. The switching is enabled by a WS₂ transparent top gate that allows electrostatic tuning of the exciton density without adding optical loss.

The authors describe the result in their paper as setting “a new benchmark for switching energy in 2D exciton-polariton systems,” adding that “this nonlinear response far exceeds that of conventional nonlinear optical materials, providing a promising pathway toward all-optical computing and photonic quantum information processing.”

ScienceDaily reports that the work was supported by the US Office of Naval Research and the Sloan Foundation. Co-authors Zhi Wang and Bumho Kim are also from the University of Pennsylvania.

What We Don’t Know

The experiments were conducted in a cryogenic laboratory setting; the polariton states in MoSe₂ are typically stable only at low temperatures, and the paper does not report results at room temperature. Whether the platform can be engineered to operate at or near room temperature—a requirement for practical integration into electronic or photonic chips—remains an open question.

The 4-femtojoule benchmark was established in a research device; translating this to manufacturable, repeatable chip-scale components involves engineering challenges around fabrication tolerance, waveguide coupling, and integration with existing photonic foundry processes that the current study does not address.

Analysis

The significance of the result lies partly in its scale. Conventional nonlinear optical materials require large interaction volumes or intense laser pulses to produce switching effects; photonic crystal nanocavities shrink the mode volume to a regime where only thousands of photons are needed. Exciton-polaritons in 2D materials combine that confinement advantage with strong excitonic nonlinearity, producing what the authors characterize as an exceptionally large nonlinear response relative to other 2D polariton platforms.

As phys.org notes, potential applications include direct optical processing for AI inference chips, where the energy cost of matrix operations in electronic hardware is increasingly a bottleneck. The paper authors suggest the system “could accelerate the development of all-optical neural networks for artificial intelligence, where computation occurs entirely in the optical domain—offering unprecedented speed and energy efficiency beyond the reach of electronic architectures,” according to Interesting Engineering.

Those claims remain prospective: a 4-femtojoule single-device switching demonstration is far removed from a working optical neural network. But the Physical Review Letters editor’s designation marks it as a notable step in what has been a long-running effort to make photons useful not just for communication but for computation.