A Chlorinated Indolium‐Based Nonlinear Optical Crystal Achieving Effective Terahertz‐Wave Generation and Highly Sensitive Detection

Abstract Nonlinear optical (NLO) crystals have shown great potential in generating and detecting coherent terahertz waves, which have a wide range of applications. However, designing the microscopic structure of NLO crystals to obtain excellent macroscopic properties has been a challenge. In this work, by the introduction of chloro substitution into the cation part of indolium‐based salt structure, a new acentric OH5CI‐BS crystal, namely, [(C 19 H 19 NOCl) + ∙(C 6 H 5 SO 3 ) − , P 1] is synthesized and grown using a feasible evaporation method. The cut‐off edge of the grown OH5CI‐BS crystal reaches 524 nm, corresponding to a bandgap of 2.37 eV. The largest single crystals of OH5CI‐BS with the size of ≈ 9 × 8 × 0.3 mm 3 are grown. Remarkably, by use of OH5CI‐BS crystals as both the THz‐wave emitter and detector, a widely tunable (from 2.23 to 33.54 THz) and monochromatic THz system is demonstrated for the first time, and the detection depth of the OH5CI‐BS based up‐conversion at room temperature is >4 orders better than a commercial Golay Cell and even 2 orders better than a 4‐K‐Bolometer. This work indicates that OH5CI‐BS is a promising future NLO material for terahertz technology, and also provides a feasible design strategy for other organic optoelectronic functional materials.