Terahertz phonon mode engineering of highly efficient organic terahertz generators
A recent study reported by KAIST researchers proposes an efficient design strategy for terahertz (THz) phonon mode engineering of organic electrooptic crystals by suppressing THz vibrational modes.
To reduce phonon absorption, a strongly electronegative group acting as strong hydrogen-bond acceptor is incorporated into the molecular anions.
THz photonics, including generation, detection and applications of electromagnetic waves in the frequency range of 0.1 to 10 THz, is attracting considerable interest in basic materials sciences; various applications in biological, pharmaceutical, medical and defense/security technologies are expected to emerge in the future from this work.
Intense table-top THz sources, which have tremendously progressed in the last decade, have become more important for advanced THz science to study light–matter interactions and to pursue subsequent applications.
Ultrafast coherent THz sources are very attractive for THz time-domain spectroscopy and THz nonlinear/pump–probe spectroscopy involving ultrafast magnetization and dynamics.
For expanding research activities based on peculiar properties of THz waves, highly efficient THz sources are essential. Many research groups have suggested different approaches for the generation of efficient broadband THz wave energy, mostly based on plasma, nonlinear optical crystals and large-scale photoconductive semiconductor antennas.
Highly nonlinear organic crystals have also been proposed as very promising materials for the generation of intense broadband THz waves thanks to their large optical nonlinearity and excellent phase-matching characteristics.
In addition, compared to inorganic crystals, organic crystals are regarded as a cost-effective solution: it is easy to fine-tune the nonlinear optical characteristics by modifying its chemical structures.
However, there is strong re-absorption of the generated THz waves caused by optical phonon modes of most nonlinear organic crystals.
Such self-absorption acts as a major bottleneck for efficient THz wave generation, leading to a drastic decrease in THz electric fields and undesirable modulation of the THz spectrum with many absorption dimples at positions of phonon modes.
Although it is well-known that THz phonon modes are playing an important role in THz wave generation characteristics, a particular design strategy in future organic crystals so that they exhibit nonlinear properties and also act to suppress such modes is still not clear.
Recently, researchers at KAIST (Prof. F. Rotermund’s research group) in collaboration with researchers from Ajou University (Korea) have proposed an efficient alternative approach for the generation of intense broadband THz waves based on THz phonon mode engineering of nonlinear organic crystals. This approach is able to overcome certain intrinsic problems.
In this approach, they focused on secondary bonds in the crystalline state for controlling the THz phonon modes.
The absorption of THz modes is proportional to the change of dipole moment by inter- and intra-molecular vibrational motions.
Strong secondary bonds can substantially restrict the inter- and intra-molecular vibrational motions and consequently reduce the change of the dipole moment, which may result in suppressing the absorption of vibrational modes in the THz spectral range.
By introducing influential chemical substituents, the change of dipole moment is reduced and vibrational motion is restricted, which are related to the self-absorption of THz modes in the crystal. Specifically, strongly electronegative trifluoromethyl groups which serve to increase the molecular weight, and acting as strong hydrogen-bond acceptor sites into molecular anion, were introduced.
New HMQ-4TFS crystals exhibit a relatively low absorption coefficient in the THz frequency range of 0.5-4 THz and simultaneously exhibit a large macroscopic nonlinearity, comparable (or even higher) to help benchmark organic electro-optic crystals.
Based on the low intensity of THz phonon modes and the large optical nonlinearity, HMQ-4TFS crystals show superior characteristics for efficient broadband THz wave generation.
They can deliver a 23 times higher peak-to-peak THz electric field than the widely used standard inorganic ZnTe crystal and a broader spectral bandwidth (4×).
Moreover, compared to analogous HMQ-T crystals with the simpler nonpolar methyl group, instead of the highly polar trifluoromethyl group, HMQ-4TFS crystals deliver higher peak-to-peak THz electric field with suppression of many dimples in the THz spectrum.
Conversion efficiency and spectral bandwidth can be further improved by proper combination to suppress various phonon absorptions, as well as optimized phase-matching condition.
As a powerful design tool for suppressing the phonon modes and enhancing the optical-to-THz conversion efficiency, the proposed method will contribute to the development of intense broadband coherent THz sources.
This study was published in the Advanced Functional Materials (cover article) (11 April, 2017).
Website: http://onlinelibrary.wiley.com/doi/10.1002/adfm.201605583/full(paper link)
* lab webpage : http://ulpl.kaist.ac.kr