Prem B. Bisht

Enhancement of Raman scattering by two orders of magnitude using photonic nanojet of a microsphere

The enhancement of Raman signal is observed on excitation through a single microsphere. The dependence of the enhancement ratio (ER) on various parameters viz., numerical aperture (NA) of the microscopic objective lens, pump wavelength, size and refractive index of the microsphere has been studied. The enhancement has been explained due to interaction of the increased field of the photonic nanojet emerging from the single microsphere. The photonic nanojet induced ER of Raman peaks of silicon wafer and cadmium ditelluride is reported here. It is observed for the first time that by suitable selection of the experimental parameters, it is possible to enhance the Raman signal by approximately two orders of magnitude.

Optical nonlinearity of organic dyes as studied by Z-scan and transient grating techniques

The excited state absorption cross-section of 5,5′-dichloro-11-diphenyl-amino-3,3′-diethyl-10, 12-ethylinethiatricarbocyanine perchlorate (IR140) have been measured by using a single beam transmission technique. Z-scan experiments have been used to find out a few nonlinear parameters. The excited state relaxation times have also been measured by using laser induced transient grating (LITG) technique.

High-Q whispering gallery modes of doped and coated single microspheres and their effect on radiative rate

Fluorescence spectra of two organic dyes doped in polymer beads as well as coated on single microparticles of silica exhibit whispering gallery modes (WGMs). For doped microspheres, theoretical simulations on WGMs have been carried out based on the Lorentz-Mie theory by varying the refractive index and the diameter of the microparticle. Similarly, for a coated microsphere, an Aden and Kerker model of the Lorentz-Mie theory has been used to simulate WGMs. For diameters ⩾8 μm, low-resolution simulations of scattering efficiency fail to show modes of higher-quality factors (Q~108). A new procedure of identifying these modes is given here that does not require use of high-performance computing. Effects of WGMs on decay rates have also been studied. It has been found that, while doped microparticles exhibit no effect on the radiative rate, coated microparticles show inhibition of the decay rate for smaller sizes. Decay rates of single-coated microspheres are found to be distinctly different from those of randomly shaped single microcrystals.

Effect of adsorbed concentration on the radiative rate enhancement of photoexcited molecules embedded in single microspheres.

The variation of the molecular density in a single microcavity and its influence on the radiative rate enhancement (RRE) are reported here. The quality factors of the observed morphology-dependent resonances (MDRs) of the microcavity remain unchanged in the absence of any absorbing effects. In contrast, the MDRs tend to disappear in the presence of strong absorption even due to the self-absorption by the molecule. Time-resolved fluorescence studies reveal the fact that the value of RRE decreases with an increase in the adsorbed concentration of the molecules. The results have been explained in terms of a detuning parameter, which is a function of the refractive index of the microcavity. The increased dispersing capability of the microsphere upon increasing its molecular density has been found to be responsible for the observed decrease in RRE.

Cavity quantum electrodynamic effects and control of radiative rate of 9-amino acridine hydrochloride hydrate

Morphology dependent resonances (MDR) are observed in the fluorescence spectra of 9-amino acridine hydrochloride hydrate (9AAHH) in polymer spherical microparticles. Decrease in the fluorescence lifetime as a function of the size of the microparticle indicates the enhancement of the radiative rate. The results have been explained on the basis of cavity quantum electrodynamic (QED) effects. It is shown here that the appearance of MDR and the enhancement of the radiative rate are controlled by the size and the shape of the microparticle.

Theoretical and experimental studies on photophysical parameters of N,N′-bis(2,5,-di-tert-butylphenyl)-3,4:9-10-perylenebis(dicarboximide) (DBPI) by using transient grating technique

Abstract Diffraction efficiency of laser induced transient grating (LITG) for N , N ′ -bis(2,5,-di- tert -butylphenyl)-3,4:9-10-perylenebis(dicarboximide) (DBPI) has been experimentally measured in solutions of benzene and dimethyl sulfoxide. Third-order nonlinear susceptibility ( χ (3) ) values have been obtained from these measurements and have been compared to those obtained from theory. Further, the relaxation time of DBPI has been estimated by measuring the decay profiles of the LITG.

Influence of pulsed and cw pumping on optical nonlinear parameters of laser dyes probed by a closed-aperture Z-scan technique

Two dye pairs suitable for energy transfer along with a photostable dye have been studied by using the closed-aperture (CA) Z-scan technique under pulsed and cw pumping. Here we use a theoretical model to elucidate the refractive and absorptive nonlinearity present simultaneously in the CA Z-scan profile. A separate open-aperture (OA) Z-scan study has been carried out to compare the nonlinear absorption parameters obtained from the CA Z-scan technique. At a fixed pumping wavelength, values of optical nonlinear parameters increase with the absorbance of dyes. It is found that the sign of refractive nonlinearity is dependent on the irradiance and the pulse width of the pump beam. In addition to the contribution of the third-order optical nonlinearity, various other mechanisms such as fifth-order nonlinearity, population relaxation to triplet states, and thermal effects are discussed here.

Mixed β-Pyrrole Substituted meso-Tetraphenylporphyrins and Their Metal Complexes: Optical Nonlinearity Using Degenerate Four Wave Mixing Technique

We have investigated the roles of structural modification and polar effects in the optical nonlinearities of a series of selectively mixed β-pyrrole functionalized tetraphenylporphyrin, MTPP(CHO)(R)2 (R = H, Br, 2-thienyl, phenyl (Ph), phenylethynyl (PE) compounds and their metal (Cu(II), Zn(II)) complexes in toluene. In the present study, we have used phase conjugation geometry of the four wave mixing process to measure the third order nonlinear susceptibility (χ(3)) and the second order hyperpolarizabilty ((γ)) with picosecond laser pulse excitation at 532 nm. An increase in the values of χ(3) and (γ) for electron-withdrawing groups was observed whereas an opposite trend was noticed for the electron-donating groups at the β-pyrrole positions. In the Cu(II) and Zn(II) complexes of substituted free base porphyrins, the distortion of the macrocyclic ring may be responsible for the reduction of the values of χ(3) and (γ). From fluorescence measurements, it has been found that the electron-donating and electron-withdrawing substituted groups at β-pyrrole positions and also the macrocyclic ring distortion of the porphyrin lead to increased radiationless transitions.

Influence of localized surface plasmons on Pauli blocking and optical limiting in graphene under femtosecond pumping

The Pauli blocking limit and optical limiting threshold have been found to be modified following silver-nanoparticle decoration of functionalized hydrogen induced exfoliated graphene. Femtosecond Z-scan experiments have been used to measure the Pauli blocking range, optical limiting threshold, and the third order nonlinear susceptibility (χ(3)) values. The observed results have been explained by modified band structure of graphene in the presence of silver nanoparticles and their localized surface plasmon resonances.

Conical emission in β-barium borate under femtosecond pumping with phase matching angles away from second harmonic generation

The broadband parametric upconverted conical emission (CE) has been investigated in a β-barium borate crystal with phase matching angles away from second harmonic generation. A broad range of CE from 340 to 800 nm has been observed by varying the crystal thickness and the incident energy of the pump beam. It has been found that the tunability range can be increased with the thickness of the medium. The CE spectra and their angular positions have been measured and found to match with those calculated from recent theoretical models.