A Laser Trick in Semiconductor Film Produces Ultrafast Optical Switching
Speed is the relentless pressure on modern information technology. Electronic transistors switch at frequencies measured in gigahertz - billions of cycles per second. But photonic systems, which route information using light, have long promised something faster. The bottleneck has been finding materials that can modulate light signals quickly enough, across a wide enough range of colors, to make the promise real.
A study published in Physical Review B on January 20, 2026 describes a mechanism in degenerate indium nitride (InN) thin films that may help close that gap. The work, led by Professor Junjun Jia of Waseda University's Global Center for Science and Engineering, shows that a femtosecond laser pulse can transiently block optical absorption across a wide spectral range - from visible to near-infrared - by raising the electron temperature alone, without flooding the material with new charge carriers.
Why Pauli blocking matters in this context
In a conventional semiconductor, optical absorption occurs when a photon promotes an electron from a lower to a higher energy state. If those higher states are already occupied, the absorption is blocked - no room for the incoming photon's energy to be deposited. This is Pauli blocking, named after the exclusion principle that forbids two electrons from occupying the same quantum state.
The standard assumption in the field was that creating broadband Pauli blocking - blocking absorption across many wavelengths simultaneously - required injecting an enormous number of new electrons into the material. The Waseda team overturned that assumption. In their InN films, which already carry a high background electron density, a femtosecond laser pulse merely redistributes the existing electrons into higher energy states by raising electron temperature. The result is transient transparency across multiple spectral windows, simultaneously, without needing to inject carriers at all.
"Our findings enable all-optical switching on femtosecond-picosecond timescales, far exceeding the speed limits of electronic transistors," Jia said. "Such ultrafast switching is particularly relevant for on-chip photonic circuits, where speed and ultra-low latency dominate system performance."
Multicolor modulation from a single material
Most existing optical modulators are narrowband - optimized for a single wavelength. Wavelength-division multiplexing, the technique that lets fiber optic cables carry many data streams simultaneously by using different colors of light, demands components that can handle multiple wavelengths at once. The transient Pauli blocking demonstrated in InN creates multiple spectral switching centers across the same material, a property that aligns naturally with these multiplexing architectures.
The experiments used pump-probe transient transmittance measurements with multicolor probe lasers, allowing the team to track how the material's transparency changed over time across different wavelengths following a pump laser pulse. These measurements were combined with first-principles electronic band-structure calculations, which provided a theoretical framework explaining why the effect appears where it does in the spectrum and how multiple interband transitions contribute to the broadband response.
Applications in photonic neural networks
The paper discusses an additional application domain: optical neural networks. These systems, which process information using light rather than electricity, require nonlinear activation functions - components that transform input signals in a controllable, nonlinear way. Electronic implementations of these functions consume power and introduce latency. The transient Pauli blocking nonlinearity in InN could serve as a physically compact, energy-efficient alternative, enabling the ultrafast, all-optical signal transformations that scalable optical neural networks require.
Several caveats apply to the current results. The experiments were conducted in degenerate InN thin films under controlled laboratory conditions using expensive pulsed laser systems. Translating the effect into practical photonic devices would require integrating the material into chip-scale architectures, developing methods to couple laser pulses efficiently into those structures, and demonstrating reliable operation across environmental conditions. The InN platform itself has been less developed commercially than materials like silicon or gallium arsenide, which adds engineering challenges beyond the optical physics.
The research team included collaborators from Aoyama Gakuin University, the Institute for Molecular Science, and the National Metrology Institute of Japan. Their first-principles calculations were particularly important for identifying the electronic transitions responsible for each switching center, which will inform the design of future materials engineered to place those centers at specific target wavelengths.