Nonlinear THz Generation through Optical Rectification Enhanced by Phonon-Polaritons in Lithium Niobate Thin Films

Area of Science:

• Materials Science
• Photonics
• Solid-State Physics

Background:

• Lithium niobate (LiNbO3) is a key material in nonlinear optics.
• Terahertz (THz) wave generation using optical rectification is an active research area.
• Understanding material properties is crucial for efficient THz emitter development.

Purpose of the Study:

• To investigate nonlinear THz generation in lithium niobate films and crystals.
• To compare experimental measurements with numerical simulations.
• To explore the role of optical phonon modes in THz generation enhancement.

Main Methods:

• Optical rectification of near-infrared femtosecond pulses.
• Fabrication of lithium niobate films and crystals of varying thicknesses.
• Polarization-resolved THz signal measurements.
• Numerical modeling of THz generation.

Main Results:

• A 2-orders-of-magnitude enhancement in nonlinear response for THz generation compared to optical frequencies.
• Identification of optical phonon modes at 4.5 and 7.45 THz as the source of enhancement.
• Maximal enhancement observed in lithium niobate films thinner than 2 μm due to reduced self-absorption.
• Numerical demonstration of THz generation enhancement in sub-wavelength lithium niobate structures via localized surface phonon-polaritons.

Conclusions:

• Thin film lithium niobate platforms offer significant potential for integrated broadband THz emitters and detectors.
• Optical phonon modes play a critical role in enhancing THz generation efficiency.
• Nanophotonic structures on thin film lithium niobate enable advanced control over THz emission properties.