Zhihua Yang

Designing a Deep-Ultraviolet Nonlinear Optical Material with a Large Second Harmonic Generation Response

The generation of intense coherent deep-UV light from nonlinear optical materials is crucial to applications ranging from semiconductor photolithography and laser micromachining to photochemical synthesis. However, few materials with large second harmonic generation (SHG) and a short UV-cutoff edge are effective down to 200 nm. A notable exception is KBe2BO3F2, which is obtained from a solid-state reaction of highly toxic beryllium oxide powders. We designed and synthesized a benign polar material, Ba4B11O20F, that satisfies these requirements and exhibits the largest SHG response in known borates containing neither lone-pair-active anions nor second-order Jahn-Teller-active transition metals. We developed a microscopic model to explain the enhancement, which is unexpected on the basis of conventional anionic group theory arguments. Crystal engineering of atomic displacements along the polar axis, which are difficult to attribute to or identify within unique anionic moieties, and greater cation polarizabilities are critical to the design of next-generation SHG materials.

Cs3Zn6B9O21: a chemically benign member of the KBBF family exhibiting the largest second harmonic generation response.

Nonlinear optical (NLO) crystals are essential materials for generation of coherent UV light in solid state lasers. KBBF is the only material that can achieve coherent light below 200 nm by direct second harmonic generation (SHG). However, its strong layer habits and the high toxicity of the beryllium oxide powders required for synthesis limit its application. By substituting Be with Zn and connecting adjacent [Zn2BO3O2]∞ layers by B3O6 groups, a new UV nonlinear optical material, Cs3Zn6B9O21, was synthesized. It overcomes the processing limitations of KBBF and exhibits the largest SHG response in the KBBF family.

Pb2Ba3(BO3)3Cl: A Material with Large SHG Enhancement Activated by Pb-Chelated BO3 Groups

Pb(II) has long been associated with lone pair activity and is often substituted in alkali earth metal borates to create new nonlinear optical (NLO) materials with enhanced second harmonic generation (SHG) capabilities. However, large enhancement in isomorphic Pb-free analogues is rare. Here we report a new NLO material Pb2Ba3(BO3)3Cl with a phase-matching SHG response approximately 3.2× that of KDP and 6× higher than its isomorphic compound Ba5(BO3)3Cl. We show that the enhanced SHG response originates from a unique edge-sharing connection between lead-oxygen polyhedra and boron-oxygen groups, making the dielectric susceptibility more easily affected by the external electric field of an incident photon. This understanding provides a route to identify systems that would benefit from SHG-active cation substitution in isomorphic structures that exhibit weak or null SHG responses.

A new congruent-melting oxyborate, Pb4O(BO3)(2) with optimally aligned BO3 triangles adopting layered-type arrangement

Introduction of the stereo-effect of the lone pair of the Pb2+ cation in borate with layered-type arrangement of BO3 triangles generates a new phase-matching nonlinear optical material, Pb4O(BO3)2. In the structure, the interesting Pb4O10 tetramers are connected to each other by sharing O atoms to form a 3D framework. In the space of the framework, the isolated BO3 and B2O5 groups are arranged parallel to the (100) to balance charge. The title compound melts congruently and exhibits a SHG response of ∼3 times that of KDP and ∼160 times that of a-SiO2. And it has a wide transparent region from the UV to near IR region with a UV cut-off edge of 280 nm, which means that Pb4O(BO3)2 is a very promising NLO material for potential application.

Finding the Next Deep-Ultraviolet Nonlinear Optical Material: NH4B4O6F

Nonlinear optical materials are essential for the development of solid-state lasers. KBe2BO3F2 (KBBF) is a unique nonlinear optical material for generation of deep-ultraviolet coherent light; however, its industrial application is limited. Here, we report a new material NH4B4O6F, which exhibits a wide deep-ultraviolet transparent range and suitable birefringence that enables frequency doubling below 200 nm. NH4B4O6F possesses large nonlinear coefficients about 2.5 times that of KBBF. In addition, it is easy to grow bulk crystals and does not contain toxic elements.

Fluorooxoborates: Beryllium-Free Deep-Ultraviolet Nonlinear Optical Materials without Layered Growth

Deep-ultraviolet nonlinear optical (DUV NLO) crystals are the key materials to extend the output range of solid-state lasers to below 200 nm. The only practical material KBe2 BO3 F2 suffers high toxicity through beryllium and strong layered growth. Herein, we propose a beryllium-free material design and synthesis strategy for DUV NLO materials. Introducing the (BO3 F)4- , (BO2 F2 )3- , and (BOF3 )2- groups in borates could break through the fixed 3D B-O network that would produce a larger birefringence without layering and simultaneously keep a short cutoff edge down to DUV. The theoretical and experimental studies on a series of fluorooxoborates confirm this strategy. Li2 B6 O9 F2 is identified as a DUV NLO material with a large second harmonic generation efficiency (0.9×KDP) and a large predicted birefringence (0.07) without layering. This study provides a feasible way to break down the DUV wall for NLO materials.

Pb17O8Cl18: A Promising IR Nonlinear Optical Material with Large Laser Damage Threshold Synthesized in an Open System

Mid-IR nonlinear optical (NLO) materials are of great importance in modern laser frequency conversion technology and optical parametric oscillator processes. However, the commercially available IR NLO crystals (e.g., AgGaQ2 (Q = S, Se) and ZnGeP2) suffer from two obstacles, low laser damage thresholds (LDTs) and the difficulty of obtaining high-quality crystals, both of which seriously hinder their applications. The introduction of Cl, an element with a large electronegativity, and Pb, a relatively heavy element to promote the optical properties, affords an oxide-based IR NLO material, Pb17O8Cl18 (POC). High-quality POC single crystals with sizes of up to 7 mm × 2 mm × 2 mm have been grown in an open system. Additionally, POC exhibits a large LDT of 408 MW/cm(2), 12.8 times that of AgGaS2. POC also exhibits an excellent second harmonic generation response: 2 times that of AgGaS2, the benchmark IR NLO crystal at 2090 nm, and 4 times that of KDP, the standard UV NLO crystal at 1064 nm. Thus, we believe that POC is a promising IR NLO material.

A New Deep-Ultraviolet Transparent Orthophosphate LiCs2PO4 with Large Second Harmonic Generation Response

LiCs2PO4, a new deep-ultraviolet (UV) transparent material, was synthesized by the flux method. The material contains unusual edge-sharing LiO4-PO4 tetrahedra. It exhibits a very short absorption edge of λ = 174 nm and generates the largest powder second harmonic generation (SHG) response for deep-UV phosphates that do not contain additional anionic groups, i.e., 2.6 times that of KH2PO4 (KDP). First-principles electronic structure analyses confirm the experimental results and suggest that the strong SHG response may originate from the aligned nonbonding O-2p orbitals. The discovery and characterization of LiCs2PO4 provide a new insight into the structure-property relationships of phosphate-based nonlinear optical materials with large SHG responses and short absorption edges.

CsB4O6F: A Congruent‐Melting Deep‐Ultraviolet Nonlinear Optical Material by Combining Superior Functional Units

The discovery of new nonlinear optical (NLO) materials for coherent light generation in the deep-ultraviolet (DUV, wavelength below 200 nm) region is essential for the development of laser technologies. Herein, we report a new material CsB4 O6 F (CBF), which combines the superior structural properties of two well-known NLO materials, β-BaB2 O4 (BBO) and KBe2 BO3 F2 (KBBF). CBF exhibits excellent DUV optical properties including a short cutoff edge (155 nm), a large SHG response (≈1.9×KDP), and a suitable birefringence that enables frequency doubling down to 171.6 nm. Remarkably, CBF melts congruently and shows an improved growth habit. In addition, our rational design strategy will contribute to the discovery of DUV NLO materials.

Simulated pressure-induced blue-shift of phase-matching region and nonlinear optical mechanism for K3B6O10X (X = Cl, Br)

Birefringence plays a great role in phase matching of the nonlinear optical (NLO) crystals. Small birefringence restricts various crystals from achieving deep-ultraviolet laser output although they exhibit short UV cutoff edges and high second-harmonic generation (SHG) intensities. An access to achieve deeper coherent light output through external pressure on NLO crystal, K3B6O10Cl is proposed and demonstrated through computer experiment based on the first principles theory. The “hot spot” in structure that determine the SHG effects and birefringence were highlighted. The shortest achievable phase-matching wavelengths are predicted based on calculated refractive indices. It is found that the quasi-planar (B6O10)2− group is the dominant contributing unit to optical anisotropy. The pressure-induced increase of polarizability anisotropy of (B6O10)2− group can notably enlarge birefringence which extends the shortest achievable wavelength of K3B6O10Cl frequency conversion. The results show that pressure engineering may be a promising scheme to overcome the drawback of small birefringence of some NLO crystals.