Seminarium Fizyki Ciała Stałego

Transition metal dichalcogenides (TMDs), such as molybdenum diselenide (MoSe₂), are 2D materials, widely recognized for their strong excitonic resonances and nonlinear optical properties. However, they posses another property crucial for photonic applications – an extremely high refractive index. Specifically, the real part of the refractive index of MoSe₂ exceeds 4 in the visible and infrared spectra regions, accompanied by negligibly small absorption in the infrared. So far, the implementation of TMDs in photonics has been hindered by the reliance of mechanical exfoliation, which limits the available surface area and leads to reproducibility problems. We overcome these constraints by using Molecular Beam Epitaxy (MBE) to grow wafer-scale, homogeneous layers of MoSe₂ with thickness of several dozens of nanometers and sub-nanometer thickness control. Utilizing this scalable platform, we designed and fabricated a 42-nm-thick MoSe₂ subwavelength grating that host an optical bound state in the continuum (BIC). At this thickness, our grating is the thinnest optical structure demonstrated to host a BIC to date.I will present both experimental and numerical results confirming the existence of the BIC in our structures. Angle-resolved reflectivity measurements reveal the non-radiative and topological nature of this mode, further verified by the observation of a polarization vortex in the momentum space. Furthermore, we demonstrate that tuning the structure into a quasi-BIC regime significantly amplifies light-matter interactions, resulting in a 1650-fold enhancement of third-harmonic generation efficiency compared to an unstructured MoSe₂ layer.