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.

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.

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