Excitation wavelength dependent fluorescence behaviour of nano colloids of ZnO

In this paper, the fluorescence behaviour of nano colloids of ZnO has been studied as a function of the excitation wavelength. We have found that excitation at the tail of the absorption band gives rise to an emission that shifts with the change of the excitation wavelength. The excitation wavelength dependent shift of the fluorescence maximum is measured to be between 60 and 100 nm. This kind of excitation wavelength dependent fluorescence behaviour, which may appear to be in violation of Kasha’s rule of excitation wavelength independence of the emission spectrum, has been observed for nano ZnO colloids prepared by two different chemical routes and different capping agents. It is shown that the existence of a distribution of energetically different molecules in the ground state coupled with a low rate of the excited state relaxation processes, namely, solvation and energy transfer, are responsible for the excitation wavelength dependent fluorescence behaviour of the systems.

Excitation wavelength dependent fluorescence behaviour of nano colloids of ZnO

In this paper, the fluorescence behaviour of nano colloids of ZnO has been studied as a function of the excitation wavelength. We have found that excitation at the tail of the absorption band gives rise to an emission that shifts with the change of the excitation wavelength. The excitation wavelength dependent shift of the fluorescence maximum is measured to be between 60 and 100 nm. This kind of excitation wavelength dependent fluorescence behaviour, which may appear to be in violation of Kasha’s rule of excitation wavelength independence of the emission spectrum, has been observed for nano ZnO colloids prepared by two different chemical routes and different capping agents. It is shown that the existence of a distribution of energetically different molecules in the ground state coupled with a low rate of the excited state relaxation processes, namely, solvation and energy transfer, are responsible for the excitation wavelength dependent fluorescence behaviour of the systems.

Photodiagnosis of Oral Malignancy using Laser-Induced Fluorescence and Diffuse Reflectance Spectroscopy

Biophotonics Laboratory,Centre for Earth Science Studies

Kinetics of Bacterial Colony Growth by Laser Induced Fluorescence

The growth kinetics of an aerial bacterial colony on solid agar media was studied using laser induced fluorescence technique. Fluorescence quenching of Rhodamin B by the bacterial colony was utilized for the study. The lag phase, log phase, and stationary phase of growth curve of bacterial colony was identified by measuring peak fluorescence intensity of dye doped bacterial colony.

Measurement of the absolute fluorescence quantum yield of rhodamine B solution using a dual-beam thermal lens technique

The dual-beam thermal lens technique has been found to be very effective for the measurement of fluorescence quantum yields of dye solutions. The concentration-dependence of the quantum yield of rhodamine B in methanol is studied here using this technique. The observed results are in line with the conclusion that the reduction in the quantum yield in the quenching region is essentially due to the non-radiative relaxation of the absorbed energy. The thermal lens has been found to become abberated above 40 mW of pump laser power. This low value for the upper limit of pump power is due to the fact that the medium is a resonantly absorbing one.

Studies on Fluorescence Efficiency and Photodegradation of Rhodamine 6G Doped PMMA Using a Dual Beam Thermal Lens Technique

In this paper we report the use of the dual beam thermal lens technique as a quantitative method to determine absolute fluorescence quantum efficiency and concentration quenching of fluorescence emission from rhodamine 6G doped Poly(methyl methacrylate) (PMMA), prepared with different concentrations of the dye. A comparison of the present data with that reported in the literature indicates that the observed variation of fluorescence quantum yield with respect to the dye concentration follows a similar profile as in the earlier reported observations on rhodamine 6G in solution. The photodegradation of the dye molecules under cw laser excitation is also studied using the present method.

Solvent Effect on Absolute Fluorescence Quantum Yield of Rhodamine 6G Determined Using Transient Thermal Lens Technique

Dual beam thermal lens tecbnique is successfully employed for the determination of absolute Fluorescence quantum yield of rhodamine 6G lnser dye in different solvents. A 532 nm radiation from a Q-switched Nd:YAG laser was used for the excitation purpose. The fluorescence quantum yield values are found to be strongly influenced by environmental effects. It has been observed that fluorescence yield is greater for rhodamine 6G in ethylene glycol system than in water or in methanol. Our results also indicate that parameters like concentration of the dye solution, aggregate formation and excited state absorption affect the absolute values of fluorescence yield significantly.

Fluorescence quantum yield of rhodamine 6G using pulsed photoacoustic technique

Experimental method for measuring photoacoustic(PA) signals generated by a pulsed laser beam in liquids is described. The pulsed PA technique is found to be a convenient and accurate method for determination of quantum yield in fluorescent dye solutions. Concentration dependence of quantum yield of rhodamine 6G in water is studied using the above method. The results indicate that the quantum yield decreases with increase in concentration in the quenching region in agreement with the existing reports based on radiometric measurements.

Strategies for Inverse Profiling of Two Dimensional Dielectric Scatterers Using Electromagnetic Illumination

Electromagnetic tomography has been applied to problems in nondestructive evolution, ground-penetrating radar, synthetic aperture radar, target identification, electrical well logging, medical imaging etc. The problem of electromagnetic tomography involves the estimation of cross sectional distribution dielectric permittivity, conductivity etc based on measurement of the scattered fields. The inverse scattering problem of electromagnetic imaging is highly non linear and ill posed, and is liable to get trapped in local minima. The iterative solution techniques employed for computing the inverse scattering problem of electromagnetic imaging are highly computation intensive. Thus the solution to electromagnetic imaging problem is beset with convergence and computational issues. The attempt of this thesis is to develop methods suitable for improving the convergence and reduce the total computations for tomographic imaging of two dimensional dielectric cylinders illuminated by TM polarized waves, where the scattering problem is defmed using scalar equations. A multi resolution frequency hopping approach was proposed as opposed to the conventional frequency hopping approach employed to image large inhomogeneous scatterers. The strategy was tested on both synthetic and experimental data and gave results that were better localized and also accelerated the iterative procedure employed for the imaging. A Degree of Symmetry formulation was introduced to locate the scatterer in the investigation domain when the scatterer cross section was circular. The investigation domain could thus be reduced which reduced the degrees of freedom of the inverse scattering process. Thus the entire measured scattered data was available for the optimization of fewer numbers of pixels. This resulted in better and more robust reconstructions of the scatterer cross sectional profile. The Degree of Symmetry formulation could also be applied to the practical problem of limited angle tomography, as in the case of a buried pipeline, where the ill posedness is much larger. The formulation was also tested using experimental data generated from an experimental setup that was designed. The experimental results confirmed the practical applicability of the formulation.

Fluorescence characterization and gain studies on a dye-doped graded index polymer optical-fiber preform

Preparation of an appropriate optical-fiber preform is vital for the fabrication of graded-index polymer optical fibers (GIPOF), which are considered to be a good choice for providing inexpensive high bandwidth data links, for local area networks and telecommunication applications. Recent development of the interfacial gel polymerization technique has caused a dramatic reduction in the total attenuation in GIPOF, and this is one of the potential methods to prepare fiber preforms for the fabrication of dye-doped polymer-fiber amplifiers. In this paper, the preparation of a dye-doped graded-index poly(methyl methacrylate) (PMMA) rod by the interfacial gel polymerization method using a PMMA tube is reported. An organic compound of high-refractive index, viz., diphenyl phthalate (DPP), was used to obtain a graded-index distribution, and Rhodamine B (Rh B), was used to dope the PMMA rod. The refractive index profile of the rod was measured using an interferometric technique and the index exponent was estimated. The single pass gain of the rod was measured at a pump wavelength of 532 nm. The extent of doping of the Rh B in the preform was studied by axially exciting a thin slice of the rod with white light and measuring the spatial variation of the fluorescence intensity across the sample.

Effect of silver nano particles on the fluorescence quantum yield of Rhodamine 6G determined using dual beam thermal lens method

Nano structured noble metals have very important applications in diverse fields as photovoltaics, catalysis, electronic and magnetic devices, etc. Here, we report the application of dual beam thermal lens technique for the determination of the effect of silver sol on the absolute fluorescence quantum yield (FQY) of the laser dye rhodamine 6G. A 532 nm radiation from a diode pumped solid state laser was used as the excitation source. It has been observed that the presence of silver sol decreases the fluorescence quantum efficiency. This is expected to have a very important consequence in enhancing Raman scattering which is an important spectrochemical tool that provides information on molecular structures. We have also observed that the presence of silver sol can enhance the thermal lens signal which makes the detection of the signal easier at any concentration.

Thermal lens technique to evaluate the fluorescence quantum yield of a schiff base

The fluorescence spectrum of the schiff base obtained from salicylaldehyde and 2-aminophenol is studied using an argon-ion laser as the excitation source and its fluorescence quantum yield (Qf) is determined using a thermal lens method. This is a nondestructive technique that gives the absolute value of Qf without the need for a fluorescence standard. The quantum-yield values are calculated for various concentrations of the solution in chloroform and also for various excitation wavelengths. The value of Qf is relatively high, and is concentration dependent. The maximum value of Qf obtained is nearly 0.78. The high value of the fluorescence quantum yield will render the schiff base useful as a fluorescent marker for biological applications. Photostability and gain studies will assess its suitability as a laser dye.

Effect of Ce3+ Ion on the X-Ray Induced Fluorescence Emission from CaS

The effect of Ce3+ on the fluorescence emission from CaS:Ce3+ phosphor is studied using X-ray excitation. Apart from the emission in the visible region, the phosphor also shows fluorescence emission in the ultraviolet region. Variation in wavelengths and intensities of these emissions due to change in dopant concentration is also analysed.

Size dependent fluorescence spectroscopy of nanocolloids of ZnO

In this article we present size dependent spectroscopic observations of nanocolloids of ZnO. ZnO is reported to show two emission bands, an ultraviolet (UV) emission band and another in the green region. Apart from the known band gap 380 nm and impurity 530 nm emissions, we have found some peculiar features in the fluorescence spectra that are consistent with the nanoparticle size distribution. Results show that additional emissions at 420 and 490 nm are developed with particle size. The origin of the visible band emission is discussed. The mechanism of the luminescence suggests that UV luminescence of ZnO colloid is related to the transition from conduction band edge to valence band, and visible luminescence is caused by the transition from deep donor level to valence band due to oxygen vacancies and by the transition from conduction band to deep acceptor level due to impurities and defect states. A correlation analysis between the particle size and spectroscopic observations is also discussed.

Laser induced fluorescence of BaS: Sm phosphor and energy level splitting of Sm3+ ion

Fluorescence of BaS: Sm phosphor has been studied using a pulsed Nitrogen laser (337.1 nm) as the excitation source. The spectrum consists of a broad band in the region 540–660nm superposed by the characteristic Sm3+ lines. Energy level splitting pattern of Sm3+ due to crystal field effects has been calculated and relevent field parameters are evaluated. Analysis shows that Sm3+ takes up Ba2+ substitutional sites.