Prediction of Fluorooxoborates with Colossal Second Harmonic Generation (SHG) Coefficients and Extremely Wide Band Gaps: Towards Modulating Properties by Tuning the BO₃/BO₃F Ratio in Layers

Abstract

Fluorooxoborates have inspired investigations of deep-ultraviolet (DUV) nonlinear optical (NLO) materials that can meet the multiple criteria. Herein, five stable structures with the composition of BaB 2 O 3 F 2 (I-V) are discoveredusing the ab initio evolutionary algorithm. Among them, BaB 2 O 3 F 2 -I has been synthesized experimentally and confirms the reliability of the method. All of the predicted structures possess extremely wide band gaps (8.1-9.0 eV). Moreover,f our new structures exhibit giant second harmonic generation (SHG) coefficients (> 3 KDP,d 36 = 0.39 pm V À1 ). Anovel type of the [BOF] layer with BO 3 :BO 3 Fr atio of [1:1] is found F 2 -V are solely composed of the BO 3 Fg roup and have colossal SHG coefficients (ca. 4 KDP). It gives the direct evidence that the BO 3 Fg roup could generate strong SHG effect. Most importantly,t he influences of BO 3 :BO 3 F ratio and their number density on band gap,birefringence and SHG effects are investigated.Deep-ultraviolet (DUV, l 200 nm) nonlinear optical (NLO) crystals are unique materials that are capable of generating DUV coherent light through frequency conversion in all-solid-state lasers,c onsequently they are in demand. [1][2][3][4][5][6][7][8][9] However,t he DUV NLO materials are still scarce because they have strict property criteria:w ide transparencyw indow in the UV region (< 200 nm);l arge second harmonic generation (SHG) coefficient (> 1 d 36 (KDP) = 0.39 pm V À1 ); sufficient birefringence to ensure the phase-matching (PM) to DUV region. [10,11] More challenges are that some of these properties are contradictory.S pecifically,S HG coefficients are inversely proportional to the band gap as shown in the sum-over-states formula. [12] Birefringence is also inversely proportional to the band gap.T he multiple criteria and inherent contradictions make exploring DUV NLO materials asevere challenge. [13][14][15][16][17][18][19] To address the challenge,KBe 2 BO 3 F 2 (KBBF) [20] and Sr 2 Be 2 B 2 O 7 (SBBO) [21] are two outstanding available DUV NLO crystal, but the high toxicity and difficult in growing crystals seriously hinder their commercialization. Therefore,itisstill agreat challenge to discover other borates for the DUV region. [11] Recently,w eh ave proposed ab eryllium-free material design and synthesis strategy for DUV NLO materials that introducing (BO x F 4Àx ) (x+1)À (x = 1, 2, 3) (referred to hereafter simply as [BOF]) functional building units (FBUs) into borates could obtain short UV cutoff edge with large birefringence and simultaneously extraordinary SHG effects. [22] The[ BOF] groups are identified as superiority FBUs for bottom-up approaches to design DUV NLO material. Thes trategy has enkindled further investigations of fluorooxoborates as NLO materials that can meet multiple criteria and are promising for applications in DUV region. Under the guidance of this strategy,aseries of new fluorooxoborates with layered structures,s uch as NH 4 B 4 O 6 F, [23] CsB 4 O 6 F, [24] RbB 4 O 6 F, [25] NaB 4 O 6 F [26] and alkaline-earth f...

Li2PbB2O5: A Pyroborate with Large Birefringence Induced by the Synergistic Effect of Stereochemical Active Lone Pair Cations and π-Conjugated [B2O5] Groups

“…Results show that the birefringence of Li 2 PbB 2 O 5 is higher than that of β-Li 4 B 2 O 5 , although the B–O density of Li 2 PbB 2 O 5 is much lower. It is well-known that the parallel alignment and large B–O density of the B–O groups are both beneficial for the material to produce large birefringence . After introducing stereochemically active lone pair cations Pb 2+ , the birefringence of Li 2 PbB 2 O 5 with a lower B–O density is still higher than that of β-Li 4 B 2 O 5 .…”

Structure and Characterization of K2Na3B2P3O13, a New Nonlinear Optical Borophosphate with One-Dimensional Chain Structure and Short Ultraviolet Cutoff Edge

“…The calculated and the experimental SHG coefficients are almost identical when compared to the d 36 (0.39 pm/V) value of KDP. 66 With calculated birefringence being ∼0.011@1064 nm, the value is not in a typical suitable range (0.05−0.1) for implementing phase matching. The calculated refractive index dispersion curve indicates that n z > n x > n y (Figure 9), and it is possible to obtain the shortest n y (ω) = n z (2ω) at 514 nm, demonstrating that the shortest phase-matching wavelength of KNBPO is 514 nm.…”

A Framework for Biosensors Assisted by Multiphoton Effects and Machine Learning

“…Deep-ultraviolet NLO crystals for current and upcoming basic research and technology requirements, a succinct SHG output wavelength, and a frequency conversion ratio are crucial [ 196 ]. SHG coefficients are shown to be inversely related to the band gap via the sum-over-states formula [ 197 ].…”