Atomically phase-matched second-harmonic generation in a 2D crystal

• Published in: Light, science & applications science & applications, 2016-08, Vol.5 (8), p.e16131-e16131
• Main Authors: Zhao, Mervin, Ye, Ziliang, Suzuki, Ryuji, Ye, Yu, Zhu, Hanyu, Xiao, Jun, Wang, Yuan, Iwasa, Yoshihiro, Zhang, Xiang
• Format: Article
• Language: English
• Subjects: 140/125; 140/133; 3R-MoS2; 639/624/399; 639/766/400/385; Applied and Technical Physics; Atomic; atomically thin; Classical and Continuum Physics; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Crystals; Harmonic analysis; Lasers; Light; Molecular; Optical and Plasma Physics; Optical Devices; Optics; Original; original-article; phase-matching; Photonics; Physics; Physics and Astronomy; second-harmonic generation
• ISSN: 2047-7538; 2095-5545; 2047-7538
• DOI: 10.1038/lsa.2016.131

Second-harmonic generation (SHG) has found extensive applications from hand-held laser pointers to spectroscopic and microscopic techniques. Recently, some cleavable van der Waals (vdW) crystals have shown SHG arising from a single atomic layer, where the SH light elucidated important information such as the grain boundaries and electronic structure in these ultra-thin materials. However, despite the inversion asymmetry of the single layer, the typical crystal stacking restores inversion symmetry for even numbers of layers leading to an oscillatory SH response, drastically reducing the applicability of vdW crystals such as molybdenum disulfide (MoS 2 ). Here, we probe the SHG generated from the noncentrosymmetric 3R crystal phase of MoS 2 . We experimentally observed quadratic dependence of second-harmonic intensity on layer number as a result of atomically phase-matched nonlinear dipoles in layers of the 3R crystal that constructively interfere. By studying the layer evolution of the A and B excitonic transitions in 3R-MoS 2 using SHG spectroscopy, we also found distinct electronic structure differences arising from the crystal structure and the dramatic effect of symmetry and layer stacking on the nonlinear properties of these atomic crystals. The constructive nature of the SHG in this 2D crystal provides a platform to reliably develop atomically flat and controllably thin nonlinear media. Second-harmonic generation: ultrathin nonlinear crystal Few-atom-thick slivers of the 3R phase of molybdenum disulfide (MoS 2 ) are promising for ultrathin second-harmonic generation (SHG). Thinner nonlinear media are desired for use in next-generation optoelectronic devices. Two-dimensional van-der-Waals crystals are promising for realizing this, but symmetry considerations mean that SHG is forbidden for crystals containing more than one atomic layer. Now, by measuring atomically phase-matched SHG in few-atomic-thick 3R MoS 2 crystals, Xiang Zhang of the University of California, Berkeley, and coworkers have found that it generates second harmonic light in all of its layers. In particular, the SHG intensity scales as the square of the number of atomic layers. The researchers note that this finding demonstrates the potential of the 3R phase of group-VI transition metal dichalcogenides such as MoS 2 for use in ultrathin nonlinear optical devices.