Terahertz Phonon Modes of Highly Efficient Electro-optic Phenyltriene OH1 Crystals

Abstract

Understanding the origin of the phonon modes of highly efficient electro-optic crystals is very important for designing materials and for optimizing their photonic applications. Here we investigate the origin of phonon modes in the 0.1−15 THz range of the benchmark electrooptic OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2enylidene)malononitrile) crystal, which is interesting due to its large electro-optic coefficient and high THz-wave generation efficiency. The phonon modes (and vibrational absorption properties) of OH1 crystals are evaluated theoretically by periodic density functional theory and also experimentally by THz absorption spectroscopy. The theoretical calculations are well-matched with experimental results. The THz absorption properties are highly anisotropic; the amplitude of the vibrational absorption is the largest along the polar c-axis compared to the other two crystallographic axes. For comparison, the vibrational absorption modes of the OH1 molecule in the gas phase are also calculated. The calculated vibrational absorption spectrum of OH1 crystalline powder appears similar to that of the OH1 molecule in the gas phase. However, the molecular vibrational motions in the crystalline state are coupled motions of vibrational motions in the gas phase. Interestingly, the vibrational mode of the torsion of the O−H bond with the largest absorption strength in the gas phase is in the crystal inhibited due to the crystal field effect. The origin of the intense phonon modes of OH1 crystals is mainly related to relatively strong distortions of the push−pull π-conjugated system including electron donor and acceptor groups.

Organic Crystals for THz Photonics

“…Figure 11a shows time domain traces achieved with such a system for DSTMS and OH1, where the pump wavelength has been chosen as the optimum for each of the crystals, 1500 nm and 1350 nm, respectively. The spectral shape (see Figure 11b) such as the gaps in the spectrum for OH1, are due to the phonon and vibrational modes in this crystal, which corresponds well to the measured THz properties shown in Figure 8b and the theoretical calculations of phonon modes in OH1 [169]. Intense THz-wave generation using a high-power Ti:Sapphire/optical parametric amplifier system (65 fs pulse duration, 100 Hz repetition rate) in DSTMS (0.44-mm thick, 1500 nm pump wavelength and 3.8 mJ pump energy, blue curves) and in OH1 (0.48-mm thick, 1350 nm pump wavelength and 3.5 mJ pump energy, red curves without and black dotted curves with a 3-THz low-pass filter): (a) THz time-domain signals detected by air-breakdown-coherent-detection and (b) the corresponding amplitude spectra.…”

“…For example, optical properties in the THz range of organic NLO crystals themselves as those presented in Figure 8b and many other crystals have been measured by organic-based THz-TDS systems [53,83,91,109,126,129]. The refractive index and absorption coefficients have been also measured using THz systems based on difference-frequency generation in organic crystals [127,169].…”

Organic σ‐Hole Containing Crystals with Enhanced Nonlinear Optical Response and Efficient Optical‐to‐THz Frequency Conversion

“…In organic π‐conjugated materials, selective control of intermolecular interactions in the solid state is a very crucial issue to obtain the desired physical properties correlated to electronic and vibrational states . In particular, alterations in the intermolecular interactions of organic nonlinear optical materials can result in dramatic changes in the molecular ordering of chromophores, macroscopic nonlinear optical responses, and molecular vibrational and phonon modes, which are important material characteristics in diverse nonlinear optical and THz photonic applications…”

“…These dimples originate from molecular and phonon vibrations in these crystals that partially absorb the generated THz waves. Such molecular and phonon vibrations in nonlinear optical organic crystals can be suppressed by enhancing (or inducing additional) intermolecular interactions . Consequently, molecular engineering approaches should satisfy three basic requirements for the crystalline state: maintain overall supramolecular interactions to achieve perfectly parallel chromophore alignment, reduce specific intermolecular interactions to enhance macroscopic optical nonlinearity, and enhance intermolecular interactions to suppress phonon vibrational modes.…”

High‐Density Organic Electro‐Optic Crystals for Ultra‐Broadband THz Spectroscopy

“…[ 15 ] Also the nonionic benchmark OH1 crystals show several strong absorption peaks with very large absorption coefficient of over 300 cm –1 . [ 55–57 ] The relative flatness and high amplitude of the generated THz spectrum, as shown in Figure 3b and 3c, is very promising for accurate THz spectroscopy, as demonstrated below.…”