Molybdenum disulfide (MoS2) dots have potential applications as optical limiter and beam modulators. In this work, MoS2 quantum dots (QDs) prepared via the solvothermal method are studied for their broadband nonlinear optical (NLO) response. The as-prepared dots are few layers thick with lateral size distribution of 2-8 nm. The photoluminescence spectra show dependence on incident pump wavelength. Tunable femtosecond laser based z-scan technique shows reverse-saturable absorption and self-defocusing behavior. The direct bandgap in visible region achieved via quantum confinement effect enhances the NLO response of QDs. The mechanisms of two photon absorption along with thermal nonlinearity have been found to be operating in the system.
Nitrogen Vacancy (NV) centres in nanodiamonds (NDs) are excellent fluorophores. They find applications in bio-sensing, bio-imaging and as single photon emitters. The photoluminescence (PL) spectra of NV centres span over the visible-near IR region (500-900 nm). The spectra consist of temperature-dependent sharp characteristic peaks of neutral and negatively charged NV centres corresponding to Zero Phonon Line (ZPL) transition. When these NV centres are coupled with microcavities, sharp ripple-like resonant modes known as Whispering Gallery Modes (WGMs) are observed in the emission spectra. The time-resolved fluorescence of NV centres are found to be dependent on excitation and emission energies. The fluorescence decay of NV centres reveals the contribution of different charge states. In addition, jet-like structures (photonic nanojet, PNJs) are observed due to the non-resonant interaction of light and NV centre-coupled microcavity. The PNJs have been used to enhance the weak Raman scattering signal.
MoS2 film shows layer-dependent nonlinear absorption behavior on interaction with ultrafast laser pulses [1]. This makes it a promising materials for use in laser mode-locking. Gold nanoparticles (AuNPs) are known to amplify the near field light owing to the surface plasmon resonances [2]. The change in the nonlinear absorption with the thickness of MoS2 film has been investigated. Moreover, the nonlinear optical behavior of MoS2-AuNPs hybrid systemhas also been compared to pristine MoS2.
Fluorescence spectrum of pyrene molecule in solution form has been of interest in the research community for more than six decades. Sensitivity of these molecules to solvent polarity makes them active probes for sensing impurities. Pyrene molecules when excited form an excimer complex with another molecule in the ground state, typically by the pi - pi orbital interaction. Fluorescence of these excimers are red shifted and results in structureless broad emission band. Pyrene is well-suited for studies on biophysical phenomena like lateral diffusion, inter or trans-bilayer movement of lipid and lateral organization of membranes. Microcrystals of aromatic compounds, such as perylene, coronene, anthracene and pyrene suspended in aqueous solutions, have been found to exhibit anomalous fluorescence spectra different from those of bulk. Our group has previously reported that the fluorescence spectra of single microcrystals are distinctly different from those of excimer emission but are similar to those of the crystalline thin film. In this work, we have used surfactant assisted self-assembly method to recrystallize nano-micro crystals of pyrene. We have undertaken this study in view of the recent investigation that reports excimer formation in nanocrystals. It has been found that while within the absorption band corresponding to the monomer, the excimer emission is dominant, higher aggregates continue to contribute due to excitation into the absorption tail of the nano-micro crystalline pyrene. By doping the synthesized microcrystals on polymer microcavities, whispering gallery modes (WGMs) were observed. WGMs find potential applications in sensing of several physical quantities like size, refractive index, pressure, temperature.
We report the effect of coating of tungsten disulphide (WS2) on fiberized silica microsphere resonator. Transmission measurements of the microsphere resonator have been done with the help of a tunable laser source and a tapered fiber. The quality (Q) factors have been measured for bare and WS2 coated microcavities. Results indicate that although there is a decrease of Q-factor from 107 to 105 after coating, it results in a cleaner spectrum. Cleaning of WGM spectra becomes important for applications such as sensing where observing a single mode resonance is essential in estimating the shift precisely. Asymmetric Fano-type profile has been observed in the clean spectrum due to the interaction of WS2 with the uncoated WGMs at 1550 nm.
Whispering gallery modes (WGMs) occur due to total internal reflection in symmetric dielectric structures. These modes have high quality (Q) factor and low mode volume (Veff). Photoluminescence (PL) of an emitter when coupled with WGMs leads to the modification in the radiative rate of the emitter. This effect on the radiative rate is known as Purcell effect. Purcell factor (FP) is proportional to Q/Veff of the cavity. Localized surface plasmon resonance (LSPR) of metal nanoparticles (NPs) can be used along with the WGMs of the microcavity to have a combined LSPR-WGM hybrid system. LSPR depends on the shape and size of the NPs which can affect FP of the microcavity. In this work, we report the effect of octopod and octahedral shaped gold nanoparticles (AuNPs) on the radiative rate of rhodamine B (RB) doped microspheres. FP values of smaller than unity have been observed indicating the inhibition of the radiative rate in the microcavity.
Distributed feedback dye laser technique gives a small, compact design and reduces the duration of the output pulses significantly compared to that of the pump. In the represent paper, we report our theoretical simulations and experiments on laser action in Rhodamine 6G upon pumping with various input parameters.
Stable ultrashort pulses have applications in ultrafast laser spectroscopy, biology, medicine, and fiber optic communication. Fiber lasers are capable of producing femtosecond duration pulses at 1.55 micrometers . In the present paper we report our simulations on effect of higher order dispersion parameters on pulse generation and propagation in figure of eight laser.
Steady-state and time-resolved laser-fluorescence microscopy has been used to study the fluorescence emission from dye molecules embedded in micrometer-sized spherical polymer particles. Sharp ripple structures are observed in the fluorescence emission spectra of dyes. These structures are found to be consistent with the theoretical calculations of morphology dependent resonances (MDR's). Observed intensities of these MDR's in the fluorescence emission spectra are found to show excitation energy dependence. The MDR's in the beads do not appear to affect the radiative lifetime of the dyes.
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