Nonlinear 2D layered crystals provide ideal platforms for applications and fundamental studies in ultrathin nonlinear optical (NLO) devices. However, the NLO frequency conversion efficiency constrained by lattice symmetry is still limited by layer numbers of 2D crystals. In this work, 3R MoS with broken inversion symmetry structure are grown and proved to be excellent NLO 2D crystals from monolayer (0.65 nm) toward bulk-like (300 nm) dimension. Thickness and wavelength-dependent second harmonic generation spectra offer the selection rules of appropriate working conditions. A model comprising of bulk nonlinear contribution and interface interaction is proposed to interpret the observed nonlinear behavior. Polarization enhancement with two petals along staggered stacking direction appears in 3R MoS is first observed and the robust polarization of 3R MoS crystal is caused by the retained broken inversion symmetry. The results provide a new arena for realizing ultrathin NLO devices for 2D layered materials.

All-inorganic perovskite micro/nanowire materials hold great promises as nanoscale coherent light source due to their superior optical and electronic properties. The coupling strength between exciton and photon in this system is important for their optical application, however, is rarely studied. In this work, we demonstrated the strong coupling of exciton-photon and polariton lasing in high quality CsPbBr3 micro/nanowires synthesized by a CVD method. By exploring spatial resolved PL spectra of CsPbBr3 cavity, we observed mode volume dependent coupling strength with a vacuum Rabi splitting up to 656 meV, as well as significant increase in group index. Moreover, low threshold polariton lasing was achieved at room temperature within strong coupling regime; the polariton characteristic is confirmed by comparing lasing spectra with waveguided output spectra and the dramatically reduced lasing threshold. Our present results provide new avenues to achieve high coupling strengths potentially enabling application of exciting phenomena such as Bose-Einstein condensation of polaritons, efficient light-emitting diodes and lasers.

approach to realize new functionalities through facile van der Waals coupled 2D layers. [6] Owing to the large dielectric mismatch between the inorganic and organic layers, the quasi-2D RPPs naturally form quantum well structures, in which, the inorganic and organic layers serve as potential wells and barriers, respectively. [7] Moreover, these quantum confined structures impart the appealing characteristics of improved environmental stability and enhanced exciton confinement. [2] These make the quasi-2D RPPs promising for solar cell and light-emitting diode (LED) applications. [5,[8][9][10] Recently, the amplified spontaneous emission (ASE) and lasing behaviors of 2D RPPs have been demonstrated. [11][12][13][14][15][16][17] However, the lasing is mostly obtained from solution-processed spin-coated thin films, in which multiple RPP components inevitably form with different bandgaps that drive cascade carrier transfer and may reshape the build-up of population inversion. [11,14,15,18] Also, the development of continuous-wave or electrically driven RPP lasers central for practical applications is still challenging. The exploitation of homologous RPP lasers is of great importance to gain further insights into the intrinsic lasing mechanisms of these quantum well-like structures as well as the design of low-threshold 2D 2D Ruddlesden-Popper perovskites (RPPs) have aroused growing attention in light harvesting and emission applications owing to their high environmental stability. Recently, coherent light emission of RPPs was reported, however mostly from inhomologous thin films that involve cascade intercompositional energy transfer. Lasing and fundamental understanding of intrinsic laser dynamics in homologous RPPs free from intercompositional energy transfer is still inadequate. Herein, the lasing and loss mechanisms of homologous 2D (BA) 2 (MA) n −1 Pb n I 3n+1 RPP thin flakes mechanically exfoliated from the bulk crystal are reported. Multicolor lasing is achieved from the large-n RPPs (n ≥ 3) in the spectral range of 620-680 nm but not from small-n RPPs (n ≤ 2) even down to 78 K. With decreasing n, the lasing threshold increases significantly and the characteristic temperature decreases as 49, 25, and 20 K for n = 5, 4, and 3, respectively. The n-engineered lasing behaviors are attributed to the stronger Auger recombination and exciton-phonon interaction as a result of the enhanced quantum confinement in the smaller-n perovskites. These results not only advance the fundamental understanding of loss mechanisms in both inhomologous and homologous RPP lasers but also provide insights into developing low-threshold, substrate-free, and multicolor 2D semiconductor microlasers.2D Ruddlesden-Popper perovskites (RPPs), with the general chemical formula of L 2 (MA) n−1 M n X 3n+1 , are composed of welldefined inorganic layers with corner connected [MX 6 ] 4− octahedra and long organic chains (L + ) intercalated between these inorganic fragments. [1][2][3][4][5] This structure promises a viable