Near-field refractometry of van der Waals crystals

Area of Science:

• Materials Science
• Optics
• Condensed Matter Physics

Background:

• Traditional refractive index measurement techniques (e.g., ellipsometry, goniometry) fail for van der Waals (vdW) crystals due to high anisotropy and small lateral dimensions.
• Accurate optical characterization of vdW materials is crucial for their application in advanced electronic and photonic devices.

Purpose of the Study:

• To develop and apply a novel optical microscopy technique for precise refractive index determination in vdW crystal flakes.
• To overcome the limitations of conventional methods in characterizing anisotropic, micron-scale vdW materials.

Main Methods:

• Utilized near-field optical microscopy (NSOM) to probe guided optical modes within MoS2 flakes.
• Achieved subwavelength spatial resolution at a 1,570 nm wavelength.
• Analyzed the optical modes to extract permittivity components.

Main Results:

• Successfully determined the in-plane permittivity component of MoS2 to be 16.11.
• Determined the out-of-plane permittivity component of MoS2 to be 6.25.
• Achieved a relative uncertainty below 1% for both measurements.

Conclusions:

• Near-field optical microscopy is an effective method for measuring optical properties of anisotropic vdW materials.
• The determined permittivity values provide critical data for MoS2 device design and understanding.
• This technique offers a pathway for characterizing other challenging nanoscale optical materials.