G. Vijaya Prakash

Optical studies of Sm3+ ions doped Zinc Alumino Bismuth Borate glasses

Zinc Alumino Bismuth Borate (ZnAlBiB) glasses doped with different concentrations of samarium (Sm(3+)) ions were prepared by using melt quenching technique and characterized for their lasing potentialities in visible region by using the techniques such as optical absorption, emission and emission decay measurements. Radiative properties for various fluorescent levels of Sm(3+) ions were estimated from absorption spectral information using Judd-Ofelt (JO) analysis. The emission spectra and con-focal photoluminescence images obtained by 410 nm laser excitation demonstrates very distinct and intense orange-red emission for all the doped glasses. The suitable concentration of Sm(3+) ions in these glasses to act as an efficient lasing material has been discussed by measuring the emission cross-section and branching ratios for the emission transitions. The quantum efficiencies were also been estimated from emission decay measurements recorded for the (4)G5/2 level of Sm(3+) ions. From the measured emission cross-sections, branching ratios, strong photoluminescence features and CIE chromaticity coordinates, it was found that 1 mol% of Sm(3+) ions doped ZnAlBiB glasses are most suitable for the development of visible orange-red lasers.

Exfoliation of self-assembled 2D organic-inorganic perovskite semiconductors

Ultra-thin flakes of 2D organic-inorganic perovskite (C6H9C2H4NH3)2PbI4 are produced using micromechanical exfoliation. Mono- and few-layer areas are identified using optical and atomic force microscopy, with an interlayer spacing of 1.6 nm. Refractive indices extracted from the optical spectra reveal a sample thickness dependence due to the charge transfer between organic and inorganic layers. These measurements demonstrate a clear difference in the exciton properties between “bulk” (>15 layers) and very thin (<8 layer) regions as a result of the structural rearrangement of organic molecules around the inorganic sheets.

In Situ Intercalation Dynamics in Inorganic–Organic Layered Perovskite Thin Films

The properties of layered inorganic semiconductors can be manipulated by the insertion of foreign molecular species via a process known as intercalation. In the present study, we investigate the phenomenon of organic moiety (R-NH3I) intercalation in layered metal-halide (PbI2)-based inorganic semiconductors, leading to the formation of inorganic–organic (IO) perovskites [(R-NH3)2PbI4]. During this intercalation strong resonant exciton optical transitions are created, enabling study of the dynamics of this process. Simultaneous in situ photoluminescence (PL) and transmission measurements are used to track the structural and exciton evolution. On the basis of the experimental observations, a model is proposed which explains the process of IO perovskite formation during intercalation of the organic moiety through the inorganic semiconductor layers. The interplay between precursor film thickness and organic solution concentration/solvent highlights the role of van der Waals interactions between the layers, as well as the need for maintaining stoichiometry during intercalation. Nucleation and growth occurring during intercalation matches a Johnson–Mehl–Avrami–Kolmogorov model, with results fitting both ideal and nonideal cases.

In situ intercalation strategies for device-quality hybrid inorganic-organic self-assembled quantum wells

Thin films of self-organized quantum wells of inorganic-organic hybrid perovskites of (C6H9C2H4NH3)2PbI4 are formed from a simple intercalation strategy to yield well-ordered uniform films over centimeter-size scales. These films compare favorably with traditional solution-chemistry-synthesized thin films. The hybrid films show strong room-temperature exciton-related absorption and photoluminescence, which shift with fabrication protocol. We demonstrate the potential of this method for electronic and photonic device applications.

KCa4(BO3)3:Ln3+ (Ln = Dy, Eu, Tb) phosphors for near UV excited white–light–emitting diodes

A series of doped KCa4(BO3)3:Ln3+ (Ln: Dy, Eu and Tb) compositions were synthesized by solid–state reaction method and their photoluminescent properties were systematically investigated to ascertain their suitability for application in white light emitting diodes. The X–ray diffraction (XRD) and nuclear magnetic resonance (MAS–NMR) data indicates that Ln3+–ions are successfully occupied the non–centrosymmetric Ca2+ sites, in the orthorhombic crystalline phase of KCa4(BO3)3 having space group Ama2, without affecting the boron chemical environment. The present phosphor systems could be efficiently excitable at the broad UV wavelength region, from 250 to 350 nm, compatible to the most commonly available UV light–emitting diode (LED) chips. Photoluminescence studies revealed optimal near white–light emission for KCa4(BO3)3 with 5 wt.% Dy3+ doping, while warm white–light (CIE; X = 0.353, Y = 0.369) is obtained at 1wt.% Dy3+ ion concentration. The principle of energy transfer between Eu3+ and Tb3+ also demonstr...

Influence of the annealing temperatures on the photoluminescence of KCaBO3:Eu3+ phosphor

Novel red emitting KCaBO3:Eu phosphors have been synthesized by solid-state reaction at various temperatures. Systematic studies on annealing effects and consequent structural evolution and optical properties were investigated by various structural and photoluminescence studies. With an increase in annealing temperature (from 700 °C to 950 °C), these phosphors show a gradual change from a mixed low crystalline phase to a highly crystalline single phase, with minimized volatile impurities. Photoluminescence studies revealed that the low-temperature annealed phosphors showed distinct mixed emission composed of blue and red emissions upon UV excitation. Such dual emission is due to the coexistence of Eu3+ and Eu2+ ions, wherein the reduction of Eu3+ into Eu2+ was attributed to the presence of volatile impurities. Relatively high-temperature annealed phosphors exhibit strong red color photoluminescence due to homogeneously occupied Eu3+ ions in the host crystal charge-compensated (with K+ ions) sites of Ca2+ ions. The dominant red-to-orange emission intensity ratios and Judd–Ofelt parameters of Eu3+ ions support the strong covalent nature and site-occupation of higher asymmetry sites of K+ and Ca2+ ions. High emission life times and efficient and stable photoluminescence at different excitation wavelengths make these phosphors suitable for white LEDs and other display applications.

Pyramidal micromirrors for microsystems and atom chips

Concave pyramids are created in the (100) surface of a silicon wafer by anisotropic etching in potassium hydroxide. High quality micromirrors are then formed by sputtering gold onto the smooth silicon (111) faces of the pyramids. These mirrors show great promise as high quality optical devices suitable for integration into micro-optoelectromechanical systems and atom chips. We have shown that structures of this shape can be used to laser-cool and hold atoms in a magneto-optical trap.

Strong exciton-photon coupling in inorganic-organic multiple quantum wells embedded low-Q microcavity

Optoelectronic-compatible heterostructures are fabricated from layered inorganic-organic multiple quantum wells (IO-MQW) of Cyclohexenyl ethyl ammonium lead iodide, (C(6)H(9)C(2)H(4)NH(3))(2)PbI(4) (CHPI). These hybrids possess strongly-resonant optical features, are thermally stable and compatible with hybrid photonics assembly. Room-temperature strong-coupling is observed when these hybrids are straightforwardly embedded in metal-air (M-A) and metal-metal (M-M) low-Q microcavities, due to the large oscillator strength of these IO-MQWs. The strength of the Rabi splitting is 130 meV for M-A and 160 meV for M-M cavities. These values are significantly higher than for J-aggregates in all-metal microcavities of similar length. These experimental results are in good agreement with transfer matrix simulations based on resonant excitons. Incorporating exciton-switching hybrids allows active control of the strong-coupling parameters by temperature, suggesting new device applications.

Exciton switching and Peierls transitions in hybrid inorganic-organic self-assembled quantum wells

Hybrid organic-inorganic perovskite semiconductors provide significant opportunities as multifunctional materials for many electronic and optoelectronic applications. These include organic-inorganic light emitting diodes, organicinorganic field-effect transistors, and nonlinear optical switches based on strong exciton-photon coupling in microcavity photonic architectures. 1‐4 The basic structure of these lead II halide-based two-dimensional 2D pervoskites takes the general form R-NH32PbI4 where R is organic consisting of layers of corner-sharing lead iodide octahedra with bilayers of organic cations stacked between the inorganic layers. 5‐7 These form ‘natural’ multiple quantum well structures, where wells of the 2D inorganic semiconducting layer are clad by barriers of the wider bandgap organic layers. Typical layer thicknesses of well and barrier are 6 and 10 A, respectively. The low dimensionality of carriers confined within the inorganic layers by quantum confinement combined with the large dielectric mismatch giving dielectric confinement between the layers, enables formation of stable excitons with large binding energy even at room temperatures. 8,9 These hybrids are thermally stable up

Structural and optical studies of local disorder sensitivity in natural organic–inorganic self-assembled semiconductors

The structural and optical spectra of two related lead iodide (PbI) based self-assembled hybrid organic‐inorganic semiconductors are compared. During the synthesis, depending on the bridging of organic moiety intercalated between the PbI two-dimensional planes, different crystal structures are produced. These entirely different networks show different structural and optical features, including excitonic bandgaps. In particular, the modified organic environment of the excitons is sensitive to the local disorder both in single crystal and thin film forms. Such information is vital for incorporating these semiconductors into photonic device architectures. (Some figures in this article are in colour only in the electronic version)