Two-photon-absorption enhanced terahertz generation from KTP optically pumped in the visible-to-UV range

Area of Science:

• Solid State Physics
• Materials Science
• Nonlinear Optics

Background:

• Potassium titanyl phosphate (KTP) is a widely used nonlinear optical crystal.
• Understanding its optical properties, especially near the bandgap, is crucial for advanced applications.
• Terahertz (THz) pulse generation via optical rectification is a key technique in THz spectroscopy.

Purpose of the Study:

• To investigate the THz generation efficiency in KTP crystals as a function of pump photon energy.
• To elucidate the underlying nonlinear optical mechanisms responsible for THz generation.
• To determine the bandgap nature and phonon modes involved in KTP absorption.

Main Methods:

• Optical rectification of terahertz pulses in KTP using a tunable pump laser.
• Varying pump photon energy across the KTP bandgap.
• Analyzing THz signal amplitude and KTP absorption spectra.

Main Results:

• Observed two distinct peaks in THz signal generation: one at the bandgap energy and a second around half the bandgap energy.
• Attributed the second peak to a two-photon absorption enhanced nonlinearity.
• Confirmed KTP as an indirect bandgap crystal with absorption involving a phonon related to symmetric Ti-O stretching (ν1 (A1g) mode).

Conclusions:

• The nonlinear optical response of KTP for THz generation is significantly influenced by both single-photon and two-photon absorption processes.
• The identification of the specific phonon mode provides insights into the fundamental optical absorption mechanisms in KTP.
• This study enhances the understanding of KTP's optical properties and its potential for THz applications.