Communication pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Dependence of the Second-Harmonic Generation Response on the Cell Volume to Band-Gap Ratio Xiyue Cheng,† Myung-Hwan Whangbo,*,†,‡,§ Maochun Hong,† and Shuiquan Deng*,† †
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State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States § State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China S Supporting Information *
ZnGeP2 is a very good candidate for producing a tunable laser in the near-IR/mid-IR region with good optical transparence over the 0.7−12 μm wavelength region, high SHG response (d36 = 68.9 ± 10.3 pm/V), and adequate birefringence (Δn = 0.04) at 10.6 μm.1c,12 However, it requires a pump wavelength near 2 μm to avoid the harmful two-photon absorption and enable the phase-matching. CdGeAs2 has an even larger SHG response (d36 = 217 ± 33 pm/V), a moderate birefringence (Δn = 0.086) at 10.6 μm, and a wide transparency in the midIR region over 2.4−19 μm, which is applied in the CO2 laser.1c,13 Furthermore, CdSiAs2 and ZnSiAs2 are widely used in photovoltaic solar cells and light-emitting devices.11 There are two types of ABC2 chalcopyrites, that is, AIBIIICVI2 (e.g., AgGaS2) and AIIBIVCV2 (e.g., ZnGeP2). The NLO properties of these chalcopyrites have been investigated by several theoretical methods.14 The density functional theory (DFT) calculations on AIIBIVCV2 (A = Zn, Cd; B = Si, Ge; C = P, As) pointed out that a small value of the band gap Eg is favorable for s large SHG response,14m but a later study found that the ability of a material for SHG is far more subtle than the generally accepted inverse correlation with band gaps.14g A recent DFT study on AIIBIVCV2 chalcopyrites, that is, ZnBP2 (B = Si, Ge, Sn) and CdGeC2 (C = P, As), concluded that “those NLO active units can enlarge the cell size and reduce the band gap to enlarge the NLO effect”.14q These studies suggest that for a series of isostructural NLO materials, the SHG response might increase with the primitive cell volume V, but inversely with the band gap Eg. In this Communication, we probe the dependence of the SHG response on V and Eg by studying the NLO properties of 12 chalcopyrites AIIBIVCV2 (A = Zn, Cd; B = Si, Ge, Sn; C = P, As) based on first-principles DFT calculations to find that their symmetry-averaged SHG coefficients dav (see below for details) are well described by the linear relationship dav = 0.431(V/Eg) + 16.926 (R2 = 0.96, where R denotes the correlation coefficient), with the units of dav and V/Eg chosen as pm/V and Å3/eV, respectively. For the convenience of our discussions, the material in the Supporting Information is grouped into six sections S1−S6. In the 12 ABC2 NLO chalcopyrites, crystallizing in the noncentrosymmetric tetragonal space group I4̅2d with the point group 4̅2m, the AC4 and BC4 tetrahedra are corner-
ABSTRACT: The second-harmonic generation (SHG) coefficients of 12 nonlinear optical chalcopyrites, ABC2 (A = Zn, Cd; B = Si, Ge, Sn; C = P, As) were calculated by first-principles methods to find that given the primitive cell volume V and the band gap Eg of ABC2, the SHG coefficients dav of ABC2 increase almost linearly with increasing value of V/Eg. This suggests that a noncentrosymmetric crystal has a large SHG response when it consists of large atoms, leading to a small band gap.
N
onlinear optical (NLO) crystals are the key materials for laser science and technology due to their ability to double the frequency of an incident laser beam through the second-harmonic generation (SHG) process.1 The NLO materials used commercially in the ultraviolet (UV) or visible region include KH2PO4,2 KTiOPO4,3 β-BaB2O4,4 and LiB3O5.5 However, it has been difficult to find good NLO crystals for the mid-IR/far-IR (
