Organic Kainate Single Crystals for Second-Harmonic and Broadband THz Generation

Organic crystals with unique nonlinear optical properties
have
been attracting attention owing to their capability to outperform
their conventional nonorganic counterparts. Since nonlinear material
responses are linked to a crystal’s internal microscopic structure,
molecular engineering of maximally unharmonic quantum potentials can
boost macromolecular susceptibilities. Here, large-scale kainic acid
(kainate) single crystals were synthesized, and their linear and nonlinear
optical properties were studied in a broad spectral range, spanning
the visible to THz spectral regions. The non-centrosymmetric zwitterionic
crystallization, molecular structure, and intermolecular arrangement
were found to act as additive donor–acceptor domains, enhancing
the efficiency of the intrinsic second-order optical nonlinearity
of this pure enantiomeric crystal. Molecular simulations and experimental
analysis were performed to retrieve the crystals’ properties.
The crystals were predicted and found to have good transparency in
a broad spectral range from the UV to the infrared (0.2–20
μm). Second-harmonic generation was measured for ultrashort
pumping wavelengths between 800 and 2400 nm, showing an enhanced response
around 600 nm. Broadband THz generation was demonstrated with a detection
limited bandwidth of >8 THz along with emission efficiencies comparable
to and prevailing those of commercial ZnTe crystals. The broadband
nonlinear response and high transparency make kainate crystals extremely
attractive for realizing a range of nonlinear optical devices.