Time-resolved Studies on Thermo-optical Nonlinearities in Organic Molecules

Sinha Ambast, Deepak Kumar (2015) Time-resolved Studies on Thermo-optical Nonlinearities in Organic Molecules. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

This thesis work has presented, in general, the experimental investigations of nonlinear optical phenomena in organic molecular materials and polymeric films. In particular, thermo-optical and nonlinear optical properties have been investigated using the techniques of nonlinear refractive index and absorption coeffcient measurement. For this purpose we developed several nonlinear optical setups in our laboratory. We studied both nonlinear refraction and nonlinear absorption phenomena. We discussed through our experimental results that third-order nonlinear optical phenomena, where refractive index depends on the intensity of light, are leading towards the self-defocusing of a Gaussian laser beam propagating through the medium. Using the phenomena of self-defocusing we probe various thermo-optical properties of the medium. We studied the self-defocusing behavior of a propagating Gaussian laser beam through a very dilute solution (10⁻⁵M) of thiophene derived organic molecular material using a milli-second time-resolved z-scan technique. We find that pure chloroform did not show any thermal lens effect, but the addition of a small amount of dye impurities helps in enhancing the thermal lensing effect. This is attributed to a significantly higher absorption coeffcient of dyes in comparison to pure chloroform. We observed that a very small amount of dye impurities added in to chloroform not only affect the optical properties, but also affect the thermooptical properties of the solvent. In addition, solutions of two different dyes show different time-evolution of the thermal lens signal and consequently we measured the different diffusion time-constants of two different dyes added to pure chloroform. A pronounced asymmetry present in the time-evolution of the thermal lens signal about the focus of a lens in the z-scan transmittance measurement shows the non-negligible effects of convective heat transport and indicates that heat diffusivity itself is temperature dependent. Further, we also studied the thermo-optical properties of a photochromic material using the techniques of spatial self-phase modulation. We synthesized a photochromic material BDHP, which has two forms. Both forms of the samples have the same molecular formula, but the one which has the π-electron delocalized over its chain length is called the closed form of the sample and the other where due to breaking of one covalent bond, the π-conjugation breaks is called the open form. The forms of the sample BDHP are changed by exposing the closed form of the BDHP to a visible light to get the open form of the BDHP. We observe that the two forms of the photoisomer shows different strengths of thermo-optical nonlinearity when irradiated in the near infra-red wavelength of spectrum. Our particular finding of this part of the work reveals that thermo-optical nonlinearity and, consequently, many thermo-optical quantities are molecular structure dependent and this facilitate the possibility of tuning the nonlinear thermo-optical coeffcient. Our study also shows the possibility that the technique of spatial self-phase modulation (SSPM) may be used as a noninvasive probe of concentration. The linear absorption characteristics of any material are characterized by the UVVis- NIR absorption spectra. The linear absorption properties give information of the electronic properties of the material. However, intense light may affect the electronic properties of a material in a different way that forbidden transitions may now be possible. For the purpose of looking at the nonlinear absorption behavior due to intergap states between the conduction and valance band of the conjugated polymer, we synthesize a polymer PESeE using an electrochmical method. Further, we oxidized this polymer to have a bipolaron formation. Formation of a bipolaron gives rise to intergap energy states. We studied the nonlinear absorption measurements for these two forms (doped form having bipolaron and undoped form without bipolaron) of conjugated polymer PESeE using the steady state z-scan technique. The steady state z-scan measurement reveals a distinct feature of the doped polymer compared with that of undoped polymer. We further perform the two-beam pump-probe type experiments to find the dynamics of induced nonlinearity. The time-scale of about 100 ps indicates the origin of nonlinearity due to the reverse saturable absorption in case of doped polymer near the focal point of the sample position in the steady state z-scan experiment. We explain that electronic occupation of bipolaron-like defects created in the doped polymer PESeE are sensitive to the pumping intensity of the laser beam in opening a new channel for the observed nonlinear optical effect. We explain this new channel as being responsible for the observation of reverse saturable absorption in the case of a doped polymer.

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