Spontaneous emission modulation in biaxial hyperbolic van der Waals material

As a natural van der Waals crystal, alpha-MoO3 has excellent in-plane hyperbolic properties and essential nanophotonics applications. However, its tunable properties are generally neglected. Here, we achieve effective modulation of spontaneous emission (SE) from a single-layer flat plate by changing the crystal directions. Numerical results and theoretical analysis show that alpha-MoO3 exhibits good tunability when the crystal directions of alpha-MoO3 are different in y-z or x-y planes. A modulation factor of more than three orders of magnitude is obtained at 634cm(-1). This phenomenon is caused by the excitation of hyperbolic phonon polaritons in alpha-MoO3 at specific bands. However, when the crystal directions of alpha-MoO3 are different in the x-z plane, the SE of the material exhibits strong angle independence. Additionally, for the semi-infinite alpha-MoO3 flat structure, we determine the distribution of the modulation factor of SE using the wavenumber and rotation angle. Finally, we extend the calculation results from semi-infinite media to finite thickness films. We obtain the general evolution law of the peak angle of the modulation factor with thickness, increasing the modulation factor to approximately 2000, which exceeds the maximum modulation factor observed in previous works by 48 times. We believe this work could guide the SE modulation of anisotropic materials and benefit the field of micro-/nano-lasers and quantum computing.