High-field terahertz (THz) spectroscopy is applied to nonlinearly excite the E phonon-polariton vibrational coordinate in LiNbO3. We compare three THz sources to show that by optimizing the THz waveform, we can drive the atomic motion to large amplitudes and observe nonlinear effects. To maximize the atomic displacement, we show that the spectral amplitude at the resonant frequency of the mode (3.8 THz) is more important than the THz peak electric-field strength. Z-scan and simple 2-dimensional (2D) THz pulse-shaping measurements confirm this. In addition to the effects of an anharmonic potential energy surface that can describe the nonlinear behavior of the excited mode, we also consider a 2-photon absorption mechanism that may be a competing nonlinear excitation pathway. We consider the effects of each model on the observed responses in single-pulse power-dependent measurements, z-scan measurements, and simple 2D measurements, providing important guidance for future measurements to experimentally investigate nonlinear vibrational excitation in solid materials.
In the growing field of high-field terahertz (THz) spectroscopy, many ultrafast research labs are equipped to perform single-pulse (one-dimensional) THz measurements. We describe a simple and versatile method to modify one-dimensional THz experimental setups to generate two variably delayed THz pulses. This method optimizes the use of pump power to create two high-field THz pulses. Adding only a beam splitter and delay stage, research groups can access the wealth of information only available through two-dimensional (2D) measurements. We also provide useful guidance to those new to some challenges of 2D THz spectroscopy.
Organic nonlinear optical (NLO) crystals are among the most efficient (>1%) terahertz (THz) radiation generators. However, one of the limitations of using organic NLO crystals is that the unique THz absorptions in each crystal make it difficult to obtain a strong, smooth, and broad emission spectrum. In this work, we combine THz pulses from two complementary crystals, DAST and PNPA, to effectively fill in spectral gaps, creating a smooth spectrum with frequencies out to 5 THz. The combination of pulses also increases the peak-to-peak field strength from 1 MV/cm to 1.9 MV/cm.
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