Waseda University demonstrates broadband ultrafast optical switching enabled by the transient Pauli blocking effect, paving the way for next-generation optical devices

A research group led by Professor Junjun Jia of the Faculty of Science and Engineering at Waseda University had previously discovered a new optical modulation mechanism that controls the distribution of photoexcited electrons in multivalley semiconductors using femtosecond laser irradiation. Last year, they demonstrated that broadband optical switching spanning the visible to infrared range is achievable. Building on this work, the group has now further demonstrated that in the degenerate semiconductor InN (indium nitride), instantaneous control of electron temperature via femtosecond laser irradiation enables broadband optical switching. These findings were published in Physical Review B, a journal of the American Physical Society.

It is well known that in semiconductor materials, using a high-intensity laser exceeding the bandgap energy, causes a large number of electrons to be excited at high density from the valence band into the conduction band. These electrons then rapidly relax toward the lower part of the conduction band via electron-phonon scattering, increasing the electron occupation at the bottom of the conduction band.

This increase in electron occupation (the Pauli blocking effect) has been observed to temporarily suppress interband absorption and render the material optically transparent. Conventionally, this effect was understood to result primarily from the massive injection of electrons into the conduction band caused by high-intensity optical excitation.

In this study, the researcher group used InN, a "degenerate semiconductor material" in which the carrier (electron or hole) concentration is extremely high due to impurity doping and defects, causing the Fermi level to penetrate into the conduction band (n-type) or the valence band (p-type).

They investigated whether it would be possible to achieve ultrafast switching of the transmission and opacity of multicolor light spanning the near-infrared to visible range by controlling the electron temperature through high-intensity pulsed laser excitation.

The group demonstrated that broadband ultrafast optical switching spanning the visible to near-infrared range is achievable by applying "pump-probe time-resolved transmittance measurements using multicolor probe light"—an experimental technique for observing ultrafast phenomena in materials—to InN thin films.

The demonstration revealed that "transient Pauli blocking"—a phenomenon in which the electron occupation state temporarily changes under high-intensity optical excitation and optical absorption is suppressed—occurs in InN, causing the material to switch instantaneously to an optically transparent state.

The study also revealed that transient Pauli blocking, previously thought to require massive injection of photoexcited carriers, can in fact be induced solely by the restructuring of the electron distribution accompanying a rise in electron temperature.

The research group expects these findings to contribute to the advancement of next-generation ultrafast optical modulators and optical shutters, as well as the development of photonic devices for optical computing and optical communications.

The electron-temperature-driven transient Pauli blocking effect established in this study provides a design guideline for engineering the optical switching wavelength range based on the intrinsic electronic structure of a given material. The group has identified the future extension of this approach to wide-bandgap semiconductor materials as a key challenge.

Furthermore, the group envisions that future challenges of the all-optical nonlinear response operating on sub-picosecond timescales include application in optical neural networks and, ultimately, in photonic AI systems.

Journal Information
Publication: Physical Review B
Title: Transient Pauli blocking in an InN film as a mechanism for broadband ultrafast optical switching
DOI: 10.1103/1cww-zn61