Strong and coherent ultraviolet light emission devices have enormous medical and industrial appli­cation potential, but generating ultraviolet light emission in an effective way has been challenging. Recently, a colla­borative research team co-led by researchers from City University of Hong Kong (CityU) developed a new approach to generate deep-ultra­violet lasing through a domino upcon­version process of nano­particles using near-infrared light, which is commonly used in tele­communication devices. The findings provide a solution for constructing minia­turized high energy lasers for bio-detection and photonic devices.

In the world of nanomaterials, photon upconversion means that when nanomaterial is excited by light or photons with a long wavelength and low energy, it emits light with a shorter wavelength and higher energy, such as ultraviolet light. Photon upcon­version charac­terized by high-energy emission upon excitation of lower-energy photons is of exceptional interest among scientists. This is because it holds potential for cost-effective construction of minia­turised deep-ultraviolet emission devices, which have enormous medical and industrial application potential, such as microbial sterili­sation and biomedical instrumentation. However, the photon upcon­version process has limited flexi­bility, as it occurs mainly in special lanthanide ions comprising fixed sets of energy levels.

A research team co-led by Wang Feng, from Department of Materials Science and Engi­neering, and Chu Sai-tak from Department of Physics at CityU, together with Jin Limin from the Harbin Institute of Techno­logy (Shenzhen), overcame the obstacle by introducing a domino upconversion tactic. Domino upcon­version is like a chain reaction, in which energy amassed in one upconversion course triggers another succeeding upcon­version process. By using a doughnut-shaped micro­resonator, incorporated with specially designed upcon­version nano­particles, the team success­fully generated high-energy, deep-ultraviolet light emission at 290 nanometers by excitation of low-energy infrared photons at 1550 nano­meters. 

“As the excitation wavelength was in the telecommuni­cation wavelength range, the nano­particles can be readily used and integrated into existing fibre-optic communication and photonic circuits without compli­cated modification or adaptation,” said Wang. The idea of constructing domino upcon­version was inspired by a previous study of energy transfer in core-shell nano­particles by Wang and Chu. The core-shell structure design of the nano­particle allows the multi­photon luminescence process in erbium (Er³⁺) ions. By adapting a similar synthetic protocol, the team success­fully constructed core-shell-shell nanoparticles through a wet-chemistry method to explore the energy-transfer mechanism of lanthanide ions, including thulium (Tm³⁺) ions.

Through the careful design of doping composition and concen­tration in different layers or shells of the upcon­version nano­particles, the team successfully achieved a tandem combination of Er³⁺ and Tm³⁺ ions-based upcon­version processes. The Er³⁺ ions contained in the outer shell responded to 1550 nanometers near-infrared photon excitation, a wavelength located in the telecommuni­cation range. In the experiment, the team incorporated the nano­particles into a doughnut-shaped micro­resonator cavity. Then they excited the nanoparticles with the 1550 nanometes near-infrared photons, and success­fully generated a high-quality ultraviolet micro­laser at 289 nanometers.

“The upcon­version nano­particles act as wavelength converters to multiply the energy of incident infrared photons,” explained Wang. He expects the findings to pave the way for the construction of miniaturized short-wavelength lasers and says they may inspire new ideas for designing photonic circuits. He added that the minia­turized ultra­violet laser using this domino upconversion techno­logy can provide a platform for sensitive bio-detection, such as the detection of cancer cell secretion, by monitoring the lasing intensity and threshold, which offers great biomedical appli­cation potential in the future. (Source: CU Hong Kong)

Reference: T. Sun et al.: Ultralarge anti-Stokes lasing through tandem upconversion, Nat. Commun. 13, 1032 (2022); DOI: 10.1038/s41467-022-28701-1

Link: Dept. of Materials Science and Engineering, City University of Hong Kong, Hong Kong SAR, China