Second-Order Nonlinearity of Graphene Quantum Dots Measured by Hyper-Rayleigh Scattering

Area of Science:

• Materials Science
• Nanotechnology
• Nonlinear Optics

Background:

• Graphene quantum dots (GQDs) are nanomaterials with unique optical properties.
• Characterizing nonlinear optical properties of GQDs is crucial for their applications.
• Hyper-Rayleigh scattering (HRS) is a powerful technique for measuring molecular hyperpolarizability.

Purpose of the Study:

• To determine the first hyperpolarizability of graphene quantum dots (GQDs) suspended in water.
• To explore the potential of GQDs for biological microscopy imaging.
• To investigate the origin of nonlinear optical behavior in GQDs.

Main Methods:

• Utilized the hyper-Rayleigh scattering (HRS) technique with a 1064 nm excitation laser.
• Studied two commercial GQDs: Acqua-Cyan and Acqua-Green.
• Performed polarized resolved HRS experiments to analyze nonlinear optical behavior.

Main Results:

• Measured hyperpolarizabilities of (1.0±0.1)×10⁻²⁷ esu for Acqua-Cyan and (0.9±0.1)×10⁻²⁷ esu for Acqua-Green.
• Estimated static hyperpolarizability per nanosheet using a two-level model.
• Observed electric quadrupolar behavior strongly dependent on surface effects.

Conclusions:

• This study presents the first HRS characterization of GQDs.
• The measured hyperpolarizability and non-toxic nature suggest potential for biological imaging.
• Surface effects significantly influence the nonlinear optical properties of GQDs.