INVESTIGATION OF NONLINEAR OPTICAL RESPONSES OF BOVINE SERUM ALBUMIN AND SOME AMINO ACIDS USING Z-SCAN TECHNIQUE

Abstract

The Z-scan technique is a relatively simple but sensitive technique for studying third-order Nonlinear optical effects in materials. In this research work, we have implemented the continuous wave (CW) closed aperture (CA) Z-scan in conjunction with UV-Visible Spectroscopy for the optical characterization of Bovine serum albumin (BSA) and 13 of its constituent amino acids (AA). The Z-scan and UV-Vis spectroscopy are the two complementary optical techniques used for elucidating the interactions of electromagnetic radiations with studied materials at their molecular levels. Here, CW Z-scan has been employed to study third-order nonlinear refraction in terms of nonlinear optical (NLO) phase shifts (𝛥𝛷0) at 655 nm incident wavelength and at an optical power range of 74 mW to 106 mW, whereas the UV-Visible spectroscopy has been done in the 190 nm to 700 nm wavelengths to find the absorption by the molecules within this spectrum. The main objective of this study was to investigate variations in (𝛥𝛷0) of the amino acids with irradiances and at the same time investigate, whether there exists a relationship between the (𝛥𝛷0) of the Amino acids and that of BSA. To do that we have introduced a parameter called ‘Molar Phase shift Per unit power’ (𝜁𝑀) that is akin to the molar extinction coefficient (𝜀𝑚) used in the comparative study of UV- Visible absorbance. Also, these results are compared with a model that we have proposed herein. As a precursor to our experimental study, we have done an extensive review of the Z-scan covering the basics of the experimental method and its applications in the field of materials characterization, particularly in biomaterials research. The literature review provides a comprehensive overview of the CW Z-scan and the associated thermo-optic interaction affecting the refractive properties of materials. Additionally, our review work covers the historical background of research on amino acids, as well as nonlinear optical study using Z- scan technique. It has been observed that the amino acids with conjugated double bonds serve as active elements in both linear and nonlinear optical interactions. The materials and methods section describes instruments, materials, and step-by-step procedures followed in the preparation and handling of samples for UV-Visible spectroscopy and Z-scan experiments followed by the details of each experimental setup and instrumentation.

ii

The UV-Visible spectroscopy results for BSA and the amino acids have agreed well with previously published results. However, some previously unpublished details of methionine and lysine have been made. Furthermore, we have introduced revised model relating the molar extinction coefficient of BSA at 278 nm to the coefficients associated with constituent amino acids. This modification has resulted in significantly improved outcomes compared to previously reported values. Z-scans results of the samples have been compared in terms of their molar phase shifts per unit power (𝜁𝑀). It has been established that the nonlinear refraction in the CW regime is caused by multiphoton absorption. The data analysis and interpretation of results are presented, along with comparisons with previous studies and theoretical predictions. At the end of the analysis, we suggest extending this work to include other materials, wavelengths, and the time domain in the femtosecond regime to validate the observations made in this study. In summary, this doctoral thesis provides a comprehensive examination of the third-order nonlinear response exhibited by proteins and amino acids in the CW regime at 665 nm, which falls under the domain of optical spectroscopy. The findings of this study may contribute significantly to the advancements in the field of nonlinear optical spectroscopy.