Pioneering interdisciplinary research drives innovation in optics and photonics

The Hong Kong Polytechnic University (PolyU) scholars harness the University’s multidisciplinary strengths to specialise in research on material synthesis, characterisation, and device fabrication for applications in lasers, photosensors and photothermal technologies. With a focus on synthesizing, processing and characterizing low-dimensional materials, the research led by Prof. Yuen Hong TSANG, Professor of the Department of Applied Physics and his team drives impactful applications across various fields of optics and photonics.

2D Materials for Nonlinear Optics and Ultrafast Photonics
Ultrafast lasers represent a remarkable advancement in photonics, with wide-ranging applications in precision micromachining, medical imaging, and spectroscopy. Their ability to generate extremely short pulses enables high-resolution imaging and advanced material processing, making them invaluable in both research and industrial settings.

Recently, two-dimensional (2D) materials have emerged as key contributors to the development of next-generation photonic devices. In PolyU laboratory, Prof. TSANG’s research team investigates the nonlinear optical (NLO) properties of 2D materials and leverages them to produce ultrashort laser pulses. The team’s recent research has focused on the NLO responses of 2D ternary GeSeTe nanosheets, which they have successfully employed as saturable absorbers. This approach has enabled the generation of ultrashort laser pulses with durations of 1.017 picoseconds and 531 femtoseconds.

By harnessing the unique characteristics of these materials, Prof. TSANG aims to further enhance the performance of ultrafast laser systems, paving the way for innovative applications in telecommunications, biomedical engineering and fundamental research.

Multivariate Optimization of Van der Waals Photodiodes for Multi-functional Optoelectronics
Prof. TSANG’s research involves a multifactorial study of van der Waals (vdW) photodiodes. The team analyses and compares key figures of merit, such as the power exponent (α) and recombination order (β), and investigates their evolution across multiple devices to achieve near-unity values in all vdW (a-vdW) devices. This demonstrates recombination-trapping resilience. In contrast, a similar device patterned using traditional lithography techniques shows significant degradation, with the value of α decreasing to almost half. This suggests that most recombination-trapping and performance degradation occur at the metal-2D interface, supporting our argument for a renewed approach to contact integration strategies for 2D photodiodes.

Additionally, efficiency analysis, along with the measured Fermi-level alignment at the heterojunction of our a-vdW devices, highlights the importance of precisely engineered layer thicknesses to achieve a robust p-n junction. This balance is critical for optimizing photocarrier generation, recombination, separation, transport, and extraction. Furthermore, due to the excellent photovoltaic performance of the photodiode, it has been successfully utilized in demonstrating multi-band imaging applications, serving as both a single-pixel detector and a gate-tunable optoelectronic logic AND gate. This positions the device as a promising candidate for multi-functional optoelectronics.

Photothermal Materials for Sustainable Water and Energy Solutions
Prof. TSANG’s research also focuses on synthesizing and analysing the characteristics of various photothermal materials, including plasmonic, semiconductor-based, and carbon-based materials, to address real-world challenges. These materials have the ability to absorb sunlight and convert it into heat energy. The team fabricate various types of solar evaporators by incorporating photothermal materials into porous substrates with low thermal conductivity. These solar evaporators float on the surface of water, efficiently absorbing a broad spectrum of sunlight and converting it into heat to evaporate seawater or wastewater at the air-water interface. The resulting vapor can then be condensed to produce freshwater.

Unlike conventional systems, this approach does not use thermal energy to heat bulk water from the bottom of the reservoir, significantly reducing heat loss and enhancing system efficiency. The solar-to-vapor generation efficiency of such systems often exceeds 80%. Furthermore, the system operates without the need for electricity, making it highly suitable for addressing the challenges of the water-energy nexus.

In addition, the team has developed a solar evaporator-based system capable of generating high-temperature steam for sterilising medical equipment. These systems are not only cost-effective but also have the potential to contribute to a greener world by reducing carbon emissions.