Complementary tandem configuration of nonlinear organic crystals for efficient terahertz spectral filling

Summary form only given. Recently, nonlinear organic crystals have been proposed as promising materials for efficient generation and detection of broadband terahertz (THz) waves delivering high electric fields [1], because they exhibit much larger optical susceptibility and excellent optical-to-THz energy conversion efficiency at room temperature than nonlinear inorganic crystals and controllability of phase matching condition covering broad spectral bandwidth [2]. For growth of organic crystals, simple techniques based on solution and surface roughness of grown crystals below few nanometer scale without polishing are also beneficial for optical and THz photonic applications [3]. However, it is difficult to synthesize a single organic crystal possessing all the requirements for efficient THz wave generation. Especially, a major bottleneck as THz generator is strong re-absorption of generated THz waves caused by phonon modes resonance which is mainly attributed to the intrinsic constituents consisting of the crystal structure. Such self-absorption of THz waves in the crystal leads to drastic decrease in THz electric fields and undesirable modulation of the spectral shape with many dimples. When the generated THz waves exhibit strong absorption gaps with the distorted time trace, the applicability of THz waves is limited by additional parasitic effects and low signal-to-noise ratio at frequencies where the absorption dimples are located. Until now, it has been rarely reported how to effectively suppress the influence of phonon modes without changing intrinsic material properties of nonlinear organic crystals. One possible strategy was previously reported only by the change of chemical structures [4]. Since there is trade-off between suppression of phonon mode intensity and enhancement of macroscopic nonlinearity, this method is also limited for generation of efficient gap-free THz spectrum.In the present work, we propose a complementary tandem configuration by orientational stacking of different nonlinear organic crystals enabling efficient THz spectral filling. Such a tandem configuration drastically decreases spectral modulation of generated THz waves and additionally enhances optical-to-THz conversion efficiency compared to use of only a single organic crystal as THz generator. The influence of phonon modes in the generated THz waves can be controlled without change of intrinsic material properties of crystals and the spectral dimples are efficiently suppressed by adopting tandem configuration. The complementary rear crystal used in this work which is attached to the front crystal acts as a mediator for enhanced THz wave generation with spectral filling of absorption dimples in THz spectrum caused by the front crystal. The fabricated organic tandem THz generator provides a remarkable spectral enhancement near the phonon dimples with an additional 30% enhancement of the generated THz electric-field. The proposed tandem structure suggests significant potential applicability of organic crystals for state-of-the-art advancement in the promising field of THz technology.