Program Results

Introduction to the event

Light plays a central role in our daily life, with different applications requiring photons of different energies. For instance, ultraviolet (UV) light is widely used for sterilization, visible light provides the colorful world we perceive and can be efficiently converted into electricity in solar cells, while infrared light, having lower energy, is often underutilized. When light interacts with matter, it typically loses energy and is converted into lower-energy photons or heat. If these low-energy photons can be converted into higher-energy photons, the overall efficiency of light utilization can be significantly enhanced. This process is known as photonic upconversion.

Using upconversion photoluminescence spectroscopy, Dr. Shao-Yu Chen’s research group discovered that common two-dimensional semiconductors—such as MoS₂, MoSe₂, WS₂, and WSe₂—can exhibit photon upconversion even under ordinary indoor illumination. For example, these atomically thin materials are capable of absorbing low-energy red photons and re-emitting higher-energy photons in the near-UV range.

Furthermore, in collaboration with theoretical physicists from the Department of Electrophysics at National Yang Ming Chiao Tung University and the Department of Physics at Tamkang University, the team identified the key microscopic mechanism: dark exciton–dark exciton annihilation. In this process, two oppositely oriented dark excitons interact and transform into a higher-energy bright exciton, which subsequently releases its energy as upconverted light (see Fig. 1).

These two-dimensional semiconductors also allow spectral tuning of the upconversion process, ranging from green to ultraviolet emission, enabling control of the output wavelength depending on the intended application (see Fig. 2). This finding opens promising opportunities for developing next-generation optoelectronic devices with enhanced efficiency and tunability at the atomic scale.

Reference: Chen, YH., Lo, PY., Boschen, K.W. et al. Efficient light upconversion via resonant exciton-exciton annihilation of dark excitons in few-layer transition metal dichalcogenides. Nat Commun 16, 2935 (2025). https://doi.org/10.1038/s41467-025-57991-4