Hongping Wu

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.

High power mid-infrared interband cascade lasers based on type-II quantum wells

We report a high power mid-infrared interband cascade laser operating at temperatures up to 170 K. The threshold current densities of this laser are considerably lower than the previously reported values in cascade lasers. The structure was grown by molecular beam epitaxy on a GaSb substrate and comprises 23 periods of active regions separated by digitally graded multilayer injection regions. A peak optical output power of ∼0.5 W/facet and a slope of 211 mW/A per facet, corresponding to a differential external quantum efficiency of 131%, are observed at 80 K and at a wavelength of ∼3.9 μm.

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.

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.

Growth, thermal and optical properties of a novel nonlinear optical material K3B6O10Cl

A high quality single crystal of K3B6O10Cl with dimensions up to 25 × 11 × 7 mm3 was successfully grown by the top-seeded solution growth method for the first time. Morphological analysis reveals that the growth of the crystal is anisotropic. By the minimum deviation technique, the refractive indices of the crystal were measured and fitted to the Sellmeier equations. The limit of type-I phase matching second harmonic generation wavelength was calculated to be 255 nm. The transparency spectrum shows that the as-grown crystal is transparent in the wavelength range of 180–3460 nm. The thermal properties and laser damage threshold have been investigated. The two principal coefficients of thermal expansion along a and c axis were measured to be 3.08 × 10−6 and 28.4 × 10−6 K−1.

New Salt-Inclusion Borate, Li3Ca9(BO3)7·2[LiF]: A Promising UV NLO Material with the Coplanar and High Density BO3 Triangles

Introduction of the coplanar BO3 triangles in the alkali metal and alkaline-earth metal mixed fluoborate generates a new UV nonlinear optical material [Li3Ca9(BO3)7]·2[LiF] (LCBF), which possesses a moderate SHG response as large as that of the KDP and a wide transparent region with a UV cutoff edge at 230 nm. The first-principle theoretical calculation suggests that LCBF has a relatively large birefringence (0.07; 532 nm). These properties make it a potential UV nonlinear optical material. And interestingly, LCBF is a salt-inclusion borate with the Li-F chains residing in the one-dimensional open framework, which caters to the development of the salt-inclusion compound in borate.

Electronic, Crystal Chemistry, and Nonlinear Optical Property Relationships in the Dugganite A3B3CD2O14 Family

A family of six nonlinear optical (NLO) materials, A3B3CD2O14 (A = Sr, Ba, or Pb; B = Mg or Zn; C = Te or W; and D = P or V), has been synthesized and characterized. In addition to the synthesis and crystal structures, comprehensive characterization of these compounds includes second harmonic generation (SHG) measurements, theoretical calculations, infrared and diffuse reflectance spectroscopies, and thermogravimetric measurements. We find that all of the reported materials are SHG-active at 1064 nm, with responses ranging from 2.8 to 13.5 × KDP, and exhibit absorption edges in the mid- to deep-ultraviolet regime. By systematically replacing the A, B, C, and D cations, we are able to tune these properties and investigate the role of different NLO-active structural units in producing the SHG responses. Specifically, our electronic structure calculations reveal that the presence of Pb(2+) on the A-site and Te(6+) on the C-site is critical for generating a large SHG response. The synthesis and structure-property relationships described in this family of materials will enable the design and discovery of new NLO materials.

Borate Fluoride and Fluoroborate in Alkali-Metal Borate Prepared by an Open High-Temperature Solution Method

By incorporation of the largest-electronegativity F atoms into borate, two novel halogen-containing borates, Li6RbB2O6F and K3B3O3F6, have been synthesized. Interestingly, Li6RbB2O6F is the first borate fluoride in alkali-metal borate. Meanwhile, K3B3O3F6 appears to be the first confirmed alkali-metal fluoroborate crystal grown by a high-temperature solution in air.

Na11B21O36X2 (X=Cl, Br): Halogen Sodium Borates with a New Graphene‐Like Borate Double Layer

Borate not graphene: The [B6O12]∞(6-) single borate layer is a graphene-like layer (see figure). The weak Na(+)-Br(Cl)(-) ionic connection between the layers leads to the layer cleavage, and difficulty of the block crystal growth.