Nonlinear Optical Benzoindolium-Based Single Crystals: H-Contacts Induced Noncentrosymmetric Alignment Approaching Efficient Terahertz-Wave Generation

Abstract

Ionic-type nonlinear optical (NLO) crystals, which belong to noncentrosymmetric (NCS) space groups, have been the core device materials in the field of laser technology, especially in the field of terahertz-wave generation. In this study, three iodide benzoindolium-based crystals, namely, MBI (C 24 H 24 NOI, Cc), m-OHBI (C 23 H 22 NOI, P2 1 ), and p-OHBI (C 23 H 22 NOI, P2 1 /c), with different substitution sites of oxygen-containing donor groups have been successfully synthesized and grown. Acentric MBI and m-OHBI crystals exhibit transparent edges below 639 nm, good environmental stabilities, and a strong second-harmonic generation (SHG) response close to typical OH1 (d 33 = 120 pm/V at 1.9 μm), and the oxygen-containing donor groups are analyzed to be effective for inducing noncentrosymmetric alignment of the functional building units. Notably, for the first time, a widely tunable terahertz wave has been realized through an MBI prototype device by the difference frequency generation method. This work provides a feasible strategy for a new NLO crystal design to achieve strong SHG intensity and efficient terahertz-wave generation.

Organic Nonlinear Optical Crystals for Highly Efficient Terahertz-Wave Generation

“…The short-wave cutoff edge of MBI is 639 nm. Compared with the reference crystal DAST (680 nm), MBI can exhibit a wide transmission region and cover a wide pump wavelength range [55].…”

Synthesis and characterization of a new isoxazolone-based nonlinear optical crystal: MPMOI

“…In the past few decades, many organic nonlinear optical crystals have been discovered. 20–28 The selection of acceptors in the D–π–A structures is crucial in order to design organic nonlinear optical crystals with large nonlinear optical susceptibilities. Isoxazolone is one of the strong acceptors, with the same electron donor CH 3 S– in chloroform solution, λ max,isoxazolone (408 nm) 29 > λ max,Pyridinium (320 nm), 30 indicating that the simple asymmetric structure of isoxazolone also has strong electron absorption capacity.…”

New Organic Nonlinear Optical Pyridinium-Based Hydrate Crystals: Trimethoxy-Induced Noncentrosymmetric Alignment

“…For a long time, the design and synthesis of crystals with a large NLO response and suitable optical transparency have been ongoing. Generally, in organic NLO crystals, push–pull structures always show a large macroscopic second-order NLO coefficient, such as the typical 4- N , N -dimethylamino-4′- N ′-methyl-stilbazolium tosylate (DAST) and its numerous derivatives that confirm this. ,− However, DAST crystals will form a central symmetric crystal structure after encountering water and lose their value. − Based on this, researchers have explored the derivatives of DAST to seek NLO crystals with a stable structure and performance.…”

“…In the design of the D–π–A-type structure of cation chromophore, different substituents are usually selected as the electron donor (such as –OH, –OCH 3 , –N­(CH 3 ) 2 ...) or acceptor (pyridinium, quinolinium, benzothiazolium, indolinium...) to modify the delocalized π conjugation system to balance the varying performance of the crystals. ,− Particularly, selecting the methoxy electron donor is beneficial to the macro NLO effect, such as the reported 4-methoxybenzaldehyde- N -methyl-4-stilbazolium tosylate (MBST) crystals, which has a high powder second harmonic generation (SHG) efficiency 17 times that of urea. , Subsequently, the 3,4-dimethoxybenzaldehyde- N -methyl-4-stilbazolium tosylate (DOST) crystals with dimethoxy groups were reported to also have a high SHG efficiency of 21 times that of standard KDP …”