B N Shivakiran Bhaktha

Synthesis, photophysical and concentration-dependent tunable lasing behavior of 2,6-diacetylenyl-functionalized BODIPY dyes

2,6-Diacetylenyl- and 2,6-bis(arylacetylenyl)-functionalized pentamethyl-difluoroborondipyrromethene (BODIPY) derivatives, namely, PBDP1 and PBDP2–4 (aryl = phenyl, 4-methoxyphenyl, or 4-cyanophenyl), respectively, which exhibit extended π-conjugation, were synthesized and characterized by various spectroscopic methods. Significant bathochromic shifts in both absorption and emission were observed upon modifying the structure of the BODIPY core via the strategy of extending its π-conjugation. The derivatives displayed efficient emission in the yellow-to-red spectral region, with a high fluorescence quantum yield and a relatively large Stokes shift. Under conditions of transverse pumping in a cuvette, PBDP1 and PBDP2 exhibited highly efficient and stable laser activity for up to 180 and 110 minutes of continuous irradiation, respectively. Amplified spontaneous emission (FWHM of ca. 2.5 nm) with an efficiency of 41% and 36% was achieved for PBDP1 and PBDP2, respectively, in toluene, which had tunable ranges of 561 to 580 nm and 602 to 617 nm, respectively, on irradiation with a Q-switched Nd:YAG laser at 532 nm. The lasing properties of PBDP3 and PBDP4, which contain electron-donating (–OMe) and electron-withdrawing (–CN) arylacetylenyl moieties, respectively, were also investigated. A corresponding digold(I) diacetylide organometallic complex, namely, (PPh3)Au–CC–BODIPY–CC–Au(PPh3) (PBDP5) was also synthesized and characterized to study the effect of Au(I). PBDP5 exhibited phosphorescence in the vis-NIR region centered at 751 nm at 77 K owing to heavy-atom-induced intersystem crossing.

Sol-gel fabrication and characterization of ZnO and Zn 2 SiO 4 nanoparticles embedded silica glass-ceramic waveguides

ZnO and Zn2SiO4 nanoparticles embedded SiO2 waveguides, a new candidate for fabrication of low-loss glass-ceramic active-waveguides for integrated optic applications, were fabricated by the sol-gel technique using dip-coating process. The waveguides fabricated from the sol-gel solution composed of (100-x) SiO2–x ZnO (x = 25, 30, and 35 mol %) exhibited uniform thickness (1.5 ± 0.1 µm), and refractive index of 1.529 ± 0.005 (for x = 35 mol %) at 632.8 nm. Propagation loss of 1.4 ± 0.2 dB/cm at 632.8 nm was observed in the transparent glass-ceramic waveguides. The as-prepared waveguides contained nanoparticles of average size ~15 nm uniformly dispersed in the SiO2 matrix. Formation of Zn2SiO4 along with ZnO nanoparticles in the waveguides is confirmed from the X-ray diffraction patterns and photoluminescence spectra. The tuning of the optical and spectroscopic properties by controlled heat-treatment of the as-prepared active-waveguides has been demonstrated.

Plasmonic enhanced optical characteristics of Ag nanostructured ZnO thin films

We have demonstrated the enhanced photoluminescence and photoconducting characteristics of plasmonic Ag–ZnO films due to the light scattering effect from Ag nanoislands. Ag nanoislands have been prepared on ITO-coated glass substrates by thermal evaporation followed by annealing. Plasmonic Ag–ZnO films have been fabricated by depositing ZnO over Ag nanoislands by sol–gel process. The band-edge emission of ZnO is enhanced for 170 nm sized Ag nanoislands in ZnO as compared to pure ZnO. The defect emission is also found to be quenched simultaneously for plasmonic Ag–ZnO films. The enhancement and quenching of photoluminescence at different wavelengths for Ag–ZnO films can be well understood from the localized surface plasmon resonance of Ag nanoislands. The Ag–ZnO M–S–M photoconductor device showed a tenfold increment in photocurrent and faster photoresponse as compared to the control ZnO device. The enhancement in photoresponse of the device is due to the increased photon absorption in ZnO films via scattering of the incident illumination.

Optofluidic two-dimensional grating volume refractive index sensor

We present an optofluidic reservoir with a two-dimensional grating for a lab-on-a-chip volume refractive index sensor. The observed diffraction pattern from the device resembles the analytically obtained fringe pattern. The change in the diffraction pattern has been monitored in the far-field for fluids with different refractive indices. Reliable measurements of refractive index variations, with an accuracy of 6×10-3 refractive index units, for different fluids establishes the optofluidic device as a potential on-chip tool for monitoring dynamic refractive index changes.

Increased photon density of states at defect-mode frequencies led enhancement of tunability of spontaneous emission from Eu2+, 3+ doped SiO2/SnO2 one-dimensional photonic crystals

One-dimensional photonic crystal (1DPhC) micro-cavity resonators, constituted of Eu2+, 3+ doped silica layer inserted between two 5-bilayered SiO2/SnO2 Bragg mirrors, are fabricated by sol?gel process. The 1DPhC exhibits a ?180 nm wide stop-band, with peak-reflectivity of more than 96%. Spectral modifications of the photonic stop-band by varying the angle of detection with respect to the sample surface normal are studied. With an increase in the detection angle from 0? to 45? the defect-mode continuously sweeps from 598.6 nm to 532.0 nm, selectively enhancing the coinciding Eu2+, 3+ emission. To quantify the increase in spontaneous emission at defect-mode frequencies, a relative enhancement factor is devised. The increased photon density of states at defect-mode frequencies is found to enhance the weak emission tail of Eu2+, 3+ ions by a factor of ?9.

Effect of photonic stop-band on the modes of a weakly scattering DCM-PVA waveguide random laser

We present an experimental study on the effect of the photonic stop-band (PSB) on the random laser (RL) emission characteristics of a 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) doped polyvinyl alcohol (PVA) film (DCM-PVA). The film, having its refractive index greater than the substrate and density variations at the microscopic scale, acts as a disordered active planar waveguide. The propagation losses for the transverse magnetic (TM) and transverse electric (TE) modes of the waveguide are observed to be 0.50 and 0.74 dB/cm, respectively, at λ = 632.8 nm. The waveguiding DCM-PVA film is then sandwiched between two silica 3-D photonic crystals (opals). The overlap of the DCM-PVA photoluminescence with the PSB of the opals is controlled by the choice of the particle size used for opal fabrication. The random lasing threshold studies have been carried out for both TM and TE polarizations for opals with different particle sizes. A reduction in the threshold of RL emission, with res...

Ultrafast time-resolved investigations of excitons and biexcitons at room temperature in layered WS 2

Strong light-matter interactions in layered transition metal dichalcogenides (TMDs) open up vivid possibilities for novel exciton-based devices. The optical properties of TMDs are dominated mostly by the tightly bound excitons and more complex quasiparticles, the biexcitons. Instead of physically exfoliated monolayers, the solvent-mediated chemical exfoliation of these 2D crystals is a cost-effective, large-scale production method suitable for real device applications. We explore the ultrafast excitonic processes in WS2 dispersion using broadband femtosecond pump-probe spectroscopy at room temperature. We detect the biexcitons experimentally and calculate their binding energies, in excellent agreement with earlier theoretical predictions. Using many-body physics, we show that the excitons act like Weiner-Mott excitons and explain the origin of excitons via firstprinciples calculations. Our detailed time-resolved investigation provides ultrafast radiative and non-radiative lifetimes of excitons and biexcitons in WS2. Indeed, our results demonstrate the potential for excitonic quasiparticle-controlled TMDs-based devices operating at room temperature.

Angular Distribution of the Emission of a 2-D Optofluidic Random Laser

We observe the regular diffraction pattern and angularly resolved random lasing spikes within a single diffraction line from a 2-D structured optofluidic random laser due to the randomness in the size distribution of the scatterers.

Experimental Investigations of the Emission from a 2D Optofluidic Random Laser

We present a 2D optofluidic random laser based on scatterers of varying sizes distributed in a dye-filled circular microfluidic chamber. The dependence of the emission spectra on spatial distribution of the pump energy is presented.

Plasmonic Ag-ZnO nanostructure thin films for optoelectronic devices

Plasmonic Ag nanoislands have been prepared on ITO-coated glass substrates by thermal evaporation followed by annealing. Enhanced optical properties of ZnO have been observed by depositing ZnO thin films on Ag nanoislands using spin-coating process.