An innovative infrared nonlinear optical crystal CsGeBr3 was synthesized. Ab initio calculations on CsGeBr3 were also carried out in order to analyse the second-order nonlinear susceptibilities. From its powder x-ray diffraction pattern, this crystal was characterized as a rhombohedral structure with an (R3m, No 160) space group symmetry. The reflection powder second-harmonic generation (PSHG) measurement of CGBr showed that its nonlinear optical efficiency is 1.62 times larger than that of rhombohedral CsGeCl3 and is 9.63 times larger than that of KH2PO4 (KDP), and most important of all that CsGeBr3 is phase-matchable. The rescaled deff(2) of CGBr was about 2.45 times larger than that of rhombohedral CsGeCl3, and this trend was coincident with the ab initio calculation results. The infrared transparent spectrum of rhombohedral CsGeBr3 was extended to more than 22.5 µm. The rhombohedral CsGeBr3 shows the potential in the realm of nonlinear optics and can be applied to the infrared region.

We analyse the electronic structure of the perovskite crystal CsGeI3 by means of first-principles calculations and compare our findings to experimental results. Our calculation indicates that CsGeI3 has a direct-transition gap of 0.74 eV at k⃗ = (π/{a})(111). The top of the valence bands was found to mainly comprise the 5p orbitals of iodine, while the bottom of the conduction bands is dominated by the 4p orbital of germanium. Photoluminescence (PL) measurements on a single crystal of CsGeI3 indicate two peaks, one at 0.82 µm (1.51 eV) and the other at 1.15 µm (1.08 eV). The shorter-wavelength PL peak is assigned as arising from an interband transition at k⃗ = (π/{a})(111) and the longer-wavelength PL is presumably ascribable as originating from a transition involving an energy level within the fundamental band gap. Fourier-transformed infrared spectroscopy reveals that the transparent range of CsGeI3 could extend from ~2 µm to >12 µm. The short-wavelength cut-off is mainly limited by the energy band gap, while the long-wavelength limit possibly originates from lattice phonon absorption. Raman spectra of the crystal exhibit two major peaks at 105 cm-1 and 151 cm-1 and the corresponding overtones at 220 cm-1 and 293 cm-1.

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A sensitive SERS platform for the simultaneous detection of S. aureus and E. coli on the basis of dual recognition by vancomycin and aptamers is reported.

Systematic studies based on first-principles calculations of second-order optical susceptibilities as well as the dielectric function of CsGeX 3 (X = Cl, Br, and I; CGX) are presented. The relationship between structural properties and optoelectronic responses are examined. The structural factors Á, and d Ge , d X are proposed to describe the degree of distortion from an ideal perovskite structure. Á and d Ge increase when halide anions are changed from Cl to I; while halide anion displacement, d X , decreases. The structural distortion effect on these rhombohedral CGX crystals is analyzed by first-principles calculations. The dielectric function and the second harmonic generation (SHG) response coefficient also increase with increasing Á and d Ge . The direct band gaps (E G ) of CsGeX 3 all occur at the R-point, ÁE R . The experimental band gaps of CGX crystals become smaller, i.e., E CGC G (¼ 3:67 eV) > E CGB G (¼ 2:32 eV) > E CGI G (¼ 1:53 eV), as Á and d Ge increase, i.e., d CGC Ge < d CGB Ge < d CGI Ge . Partial density of states (PDOS) analysis revealed that the valence band maximum (VBM) and conduction band minimum (CBM) are mainly contributed by the p-orbitals of germanium. The calculated magnitudes of ð2Þ ijk are close to some reported experimental values near the band gap. #

The structural, optical and mechanical properties of orthorhombic ternary nitride crystals have been analysed theoretically with first-principles calculation. Our results indicate that these nitrides possess fairly large optical bandgap (4-6 eV), second-order nonlinear optical susceptibility (13-18 pm V −1), and bulk modulus 170-370 GPa. Therefore these materials could be useful for optical and protective coating applications. Our analysis with the band-by-band and atomic species projection techniques not only yields useful information about material properties, but also provides deep insight into the fundamental understanding of the mechanical and optical properties of orthorhombic ternary nitrides.

Innovative infrared nonlinear optical crystals CsGe(BrxCl(1-x))3 were synthesized. Their powder X-ray diffraction patterns indicated that they had rhombohedral structures with (R3m, No. 160) space group symmetry. Their structural distortion increased with x. The Kurtz powder techniques revealed that the nonlinear optical efficiency of CsGeBr3 is about 9.64 times larger than that of rhombohedral CsGeCl3 and 28.29 times larger than that of KH2PO4 (KDP); most importantly, CsGe(BrxCl(1-x))3 is phase-matchable. The transparent infrared spectrum of rhombohedral CsGe(BrxCl(1-x))3 was extended to over 30 mum and demonstrated its potential in the field of nonlinear optics and applicability in the infrared region.

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