CHAPTER 7 SUMMARY OF THE PRESENT WORK AND SUGGESTIONS FOR FUTURE WORK

Transcription

1 161 CHAPTER 7 SUMMARY OF THE PRESENT WORK AND SUGGESTIONS FOR FUTURE WORK 7.1 SUMMARY OF THE PRESENT WORK Nonlinear optical materials are required in a wide range of important applications, such as optical limiting, optical computing and optical communication. Organic materials exhibiting nonlinear absorption are currently of interest because of their large third order nonlinearities, instantaneous response time, high damage thresholds, ease of processing, structural modifications and their applicability over a wide range of wavelengths. Materials, which exhibit efficient optical nonlinearities, fluorescence quantum yield and large thermal diffusivity are potential candidates for efficient photonic materials. Recently, solid state dye lasers, in which an organic dye doped in a suitable solid matrix replaces the conventional liquid dye solution, have been extensively investigated. Solid state dye lasers exhibit well established advantages over the conventional liquid lasers as they avoid the problems of toxicity and flammability posed by use of organic solvents. At the same time it provides, a low cost gain medium which is compact, and easy to operate and maintain. Hence, adequate knowledge of thermal and optical properties of dye doped polymers is important in identifying suitable laser media. Apart

2 162 from its use as active laser media dye doped polymers find many applications in the modern photonic technology. The quantitative study of any nonlinear material is to find the magnitude of nonlinearity such as nonlinear refractive index and nonlinear absorption coefficient. To find these parameters for various materials, Z-scan technique has become a standard tool because of its simplicity and easy data analysis. This thesis reports the results of spectral and nonlinear optical studies carried out on laser dyes in solution and solid matrix. The dyes chosen for the study are given below. Acid Red 27 - Azo family Sudan IV - Azo family Natural Red 25 - Natural family Ethyl Violet - Triarylmethane family Safranin O - Safranin family LD Coumarin family The dye doped polymer rods and films were synthesized using thermal bulk free radical polymerization method. Methylmethacrylate was chosen as monomer; low molecular weight solvent such as methanol, benzene and ethanol were used as additive. The material characterization of synthesized dye doped polymer film was studied by thermal analysis. TGA and DTA studies were done on the dye doped polymer film. Thermogravimetric analysis were carried from 25 0 C to C at a heating rate of 20 0 C /min in nitrogen atmosphere. The thermogram analysis confirms that the material is stable up to C in the

3 163 N 2 atmosphere. The polymerization is carried out between the temperatures 35 o C to 60 o C. The thermograms reveal that at these temperatures no decomposition of either the dye or PMMA takes place as this might affect the spectral and nonlinear studies undertaken. The spectral characteristics of the dyes were studied and the spectral parameters such as absorption peak wavelength, molar extinction coefficient (), bandwidth () 1/2, oscillator strength (f), fluorescence peak wavelength, full width at half maximum (FWHM), Stoke s shift of the dyes, in liquid and solid media were calculated. The peak wavelength of absorption of Sudan IV, Natural Red 25 and Safranin O in the solid matrix (MPMMA) showed a red shift from that of the liquid media. This may be due to the increase in the rigidity of the polymeric matrix. The peak wavelength of absorption of Acid Red 27, Ethyl Violet and LD 490 in the solid matrix showed a blue shift from the peak wavelength of absorption of the dye in the liquid medium. This may be due to the decrease in the dielectric constant of the solid medium when compared to that of the liquid medium. The fluorescence emission peak wavelength of Acid Red 27, Sudan IV, Ethyl Violet 25, Safranin O and LD 490 in the solid matrix showed a blue shift from the peak wavelength of fluorescence of the above dyes in the liquid medium. This may be attributed to the decrease in the dielectric constant of the polymeric medium when compared to that of the liquid medium. The shifts produced in the absorption and fluorescence spectra of the dye molecule are due to the interaction between the solute dipole and the solvent dipole. The fluorescence lifetime of Safranin O and LD 490 are 195ps and 305ps in methanol, respectively. The residuals shown along with the decay are well within the error limits.

4 164 The nonlinear properties of these dyes in liquid and solid medium are studied under diode pumped Nd:YAG laser, using Z-scan technique. The Z-scan technique is performed by passing an axially symmetric beam through a converging lens, and then through the nonlinear optical sample which is placed near the beam waist. The transmittance of the nonlinear medium is measured through a finite aperture placed in the far field as a function of the sample position (z), with respect to the focal plane of the converging lens. The measurements of nonlinear absorption coefficient and nonlinear refractive index n 2 for all dyes in solvent and in solid form (dye doped polymer film) are carried out using open and closed aperture Z-scan techniques. All the dyes (Acid Red 27, Sudan IV, Natural Red 25,Ethyl Violet, Safranin O and LD 490) studied show a negative nonlinear refractive index, characterized by a pre-focal transmittance maximum (peak) followed by post-focal transmittance minimum (valley) for closed Z-scan trace. The defocusing effect observed in the closed aperture Z-scan is attributed to a thermal nonlinearity resulting from absorption of radiation at the excitation wavelength. The localized absorption of a tightly focused beam propagating through an absorbing dye medium produces a spatial distribution of temperature in the dye solution or polymer film and consequently, a spatial variation of the refractive index and that acts as a thermal lens resulting in severe phase distortion of the propagating beam. Z-scan with an open aperture is insensitive to nonlinear refraction (thin sample approximation). Reverse Saturable absorption is observed in the open aperture Z-scan for the dyes Acid Red 27, Sudan IV, Natural Red 25 and Ethyl Violet. The dyes Safranin O and LD 490 show Saturable absorption.

5 165 The value of T p-v has increased for polymer film when compared to the dyes in solvent. This shows that the change in refractive index due to thermal effect in solid media is higher compared to that of liquid media. The rate of heat dissipation in liquid is more than that in film because of poor thermal conductivity of the film. This leads to higher increase in temperature in solid media compared to temperature rise in liquid media. So, the nonlinear refractive index change is more in solid media than in liquid media. The concentration dependent nonlinear refractive index and nonlinear absorption coefficient are observed in these dyes. It is seen that there is an increasing trend in values of n 2, and (3) as the concentration increases. This may be attributed to the fact that as the number of dye molecules increases when the concentration increases, more particles get thermally agitated resulting in an enhanced effect. The Z-scan measurements indicate that these dyes exhibit large nonlinear optical properties. The UV-Visible absorption spectra of the samples recorded before and after the laser irradiation, show that the pattern and intensity of the spectra have no change. This indicates that all the dye samples possess good photo stability. Optical limiting behavior for dyes under low power cw laser excitation in different solvent media and for different dye concentrations and in dye doped polymer films are studied. The mechanism responsible for optical limiting is mainly attributed to the thermally induced nonlinear refraction. The defocusing effect observed in these samples under cw illumination is utilized to demonstrate their optical limiting action. Based on their nonlinear refractive index, the samples behave as good optical limiters even at low powers. These results are quite encouraging for possible applications in nonlinear optical devices.

6 166 At the valley positions, the optical limiter works at very low powers as the self-defocusing effect is enhanced by the thermal effect which is closely related to the absorptive properties of the samples used. Thus it can be suggested that the best position for a sample, when used for optical limiting based on self-defocusing is at the valley of the Z-scan curve of the medium. The optical limiting responses of the low concentration solutions are generally much weaker than those of high concentrated solutions, as high concentrated solution exhibits strong optical limiting. This indicates that the number density of dye molecules in the laser beam is the main factor affecting the clamped level. From the threshold intensity for optical limiting for each sample, it can be seen that the optical power limiting threshold is inversely proportional to the concentration. The data show that as the concentration increases, a reduction in linear transmittance as well as the clamping level is observed. Experimental observation shows that the dye doped polymer films are better optical limiter compared to dyes in solvent. Sudan IV in benzene, Natural Red 25 and Ethyl Violet in ethanol and LD 490 in methanol show a better optical limiting response under Nd:YAG laser excitation. Table 7.1 Comparative Table Dyes solvent Nonlinear parameters of dyes in solvent and film (0.05mM concentration) n 2 x10-8 cm 2 /W x10-3 cm/w (3) x10-6 (e.s.u) solvent film solvent film solvent film (Optical Limiting (0.1mM concentration) Limiting Amplitude (mw) Acid Red 27 Methanol Sudan IV Benzene Natural Red 25 Ethanol Ethyl Violet Ethanol Safranin O Methanol LD 490 Methanol

7 167 Table 7.1 summaries the nonlinear parameter of different dyes in solvent and films. The nonlinear parameters such as change in refractive index(n 2 ), optical limiting, nonlinear absorption co-efficient() and nonlinear susceptibility (3) of different dyes are inter compared. It is found that the values of nonlinear parameters (NLO) in film are higher than that in solvent. This may be due to Anderson localization of photons was less than in the case of liquids, so the localization of strong electromagnetic field inside the solid was responsible for the increase in nonlinearity in films. The nonlinear studies were done on dyes from different families. Some dyes showed a high third order nonlinear susceptibility. The dyes LD 490, Safranin O and Sudan IV showed a high third order nonlinear susceptibility (3) with Nd: YAG laser excitation. This may be due to the structure of the dye. The Azo dyes has highest change in nonlinear refractive index among other dyes. This study helps one to choose the proper dye for nonlinear applications. 7.2 SUGGESTION FOR THE FUTURE WORK The experiments can be carried out with picosecond pulsed laser to study the contribution of electronic polarization. Third order nonlinear optical properties of these organic dyes under picosecond pulse excitation can be studied. The nonlinearity of same dyes can be studied in the offresonance region (800nm 1000nm) using femtosecond laser. The optical limiting studies can be extended to optical logic gates to perform logical functions such as NAND, AND and OR, employing appropriate modes of data deduction. The dyes can be doped in polymer/gelatin for applications like low-power degenerate four wave mixing, data storage, gating applications etc.