Exciton-Enhanced Spontaneous Parametric Down-Conversion in Two-Dimensional Crystals.

Area of Science:

• Quantum Optics
• Materials Science
• Condensed Matter Physics

Background:

• Spontaneous parametric down-conversion (SPDC) is crucial for generating entangled photon pairs.
• Nonlinear optical responses are key to manipulating light for quantum applications.
• Understanding excitonic effects is vital for optimizing nonlinear optical phenomena.

Purpose of the Study:

• To investigate the role of excitonic resonances and interexciton transitions in enhancing SPDC.
• To demonstrate the relevance of excitons in nonlinear optical responses using experimental data.
• To explore the potential of 2D materials for efficient quantum light sources.

Main Methods:

• Ab initio many-body calculations were performed to model the optical response.
• Experimental polar plots of second harmonic generation in NbOCl$_{2}$ were used for benchmarking.
• Theoretical analysis focused on excitonic contributions to second-order nonlinear susceptibility.

Main Results:

• Excitonic resonances and interexciton transitions were shown to enhance SPDC probability.
• Calculations using NbOCl$_{2}$ validated the significant impact of excitons on nonlinear optical properties.
• A strong double-exciton resonance in 2D NbOCl$_{2}$ resulted in a giant enhancement of the second-order susceptibility.

Conclusions:

• Excitons play a critical role in enhancing nonlinear optical processes like SPDC.
• 2D materials, particularly NbOCl$_{2}$, exhibit strong excitonic effects beneficial for nonlinear optics.
• This research provides a pathway towards developing highly efficient, ultrathin quantum light sources.