Optoelectronics is the branch of electronics that deals with the interaction between light and electronic devices. Optoelectronic devices are used in various applications across a wide range of industries. For instance, optoelectronic devices play a critical role in communication systems, including fiber-optic communication, which enables high-speed data transmission over long distances. Moreover, optoelectronic devices can convert light energy into electrical energy, which is useful for a variety of applications. For example, solar cells use photovoltaic materials to convert sunlight into electricity. Furthermore, optoelectronics is widely used in sensing applications, such as in optical sensors, which can detect changes in light intensity, color, or polarization. These sensors can be used in various applications such as biomedical devices, environmental monitoring, and security systems. With the advances in the optoelectronic field, the need to improve the performance of the existing optoelectronic devices has increased, leading to more and more research in this direction.
The design and fabrication of next-generation optoelectronic devices with the desired performance requires attractive materials with fascinating properties like Moire superlattice and quantum spin Hall effect. Among them, two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers, particularly WS2, MoS2, MoSe2 and WSe2, have drawn significant research attention as potential candidates for next-generation optoelectronics. TMDCs are known to exhibit controllable atomic thickness and bandgap and strong Coulomb interactions. To this end, TMDC monolayers are dominated by excitons, resulting in Rydberg-like excitonic states below the free quasiparticle band gap.
The precise knowledge of high-order Rydberg excitonic states is of extreme significance for the theoretical and fundamental understanding of the many-electronic effects as well as for enhanced device applications. Presently, linear optical spectroscopy is widely used to probe the effects of bright excitonic states. In contrast, dark excitonic states whose properties cannot be observed using linear optical spectroscopy require nonlinear optical (NLO) spectroscopy. Due to the excitonic enhancement effect, many NLO techniques have been used to explore the Rydberg series of excitonic states in TMDCs. Coupling NLO with a plasmonic cavity could reduce the excitonic line width to improve the spectral resolution of TMDCs and ensure an adequate signal-to-noise ratio. However, this possibility remains underexplored, especially at room temperature.
Herein, a group of researchers from National University of Singapore led by Professor Qing-Hua Xu designed a hybrid nanostructure of monolayer WS2 integrated with plasmonic cavity. The excitonic Rydberg states and the properties of the monolayer WS2 were probed using NLO spectroscopy at room temperature. Their work is currently published in the research journal, ACS Nano.
The researchers reported narrowed line widths of NLO excitation spectra, stronger excitonic resonance effects and significant enhancement in the NLO responses when WS2 monolayer was placed in the designed plasmonic cavity, and the fundamental excitation states and excitation wavelengths of the WS2 monolayer matched the wavelengths of the plasmonic cavity. In the optimized cavity structure, two-photon photoluminescence, second harmonic generation and third harmonic generation achieved remarkable optimum enhancement 1000, 3000 and 3800, respectively. Second- and third harmonic generations achieved a maximum conversion efficiency of 1.6 × 10-5 and 5.4 × 10-7, respectively, in the cavity.
The main advantage of the plasmon-enhanced NLO responses was also attributed to the effects of enhanced excitonic resonance characterized by the narrowing of the line width of the second harmonic generation NLO excitation spectra. As a result, remarkable improvement in the signal-to-noise ratio and enhanced spectral resolution was reported, allowing the distinguishing between the discrete excitonic states and small energy differences at room temperature. Additionally, combining the three complementary NLO techniques and linear optical spectroscopy made it possible to accurately determine the energies of Rydberg excitonic states A, B, C and D excitons of WS2 monolayer. This further allowed the calculation of the quasiparticle bandgap and the exciton binding energy.
In summary, the study demonstrated the advantages of integrating a plasmonic cavity with nanostructure monolayer WS2, including its benefits in investigating its NLO properties at a single particle level. It allowed precise probing of both dark and bright excitonic states of WS2 in the cavity structure at room temperature. In a statement to Advances in Engineering, Professor Qing-Hua Xu explained that plasmon-enhanced NLO spectroscopy is a promising tool for studying high-order Rydberg excitonic states of 2D materials for their extended applications in optoelectronics.
Shi, J., Lin, Z., Zhu, Z., Zhou, J., Xu, G. Q., & Xu, Q.-H. (2022). Probing Excitonic Rydberg States by Plasmon Enhanced Nonlinear Optical Spectroscopy in Monolayer WS2 at Room Temperature. ACS Nano, 16, 15862–15872.