Sumit Singhal

Femtosecond Laser Pulse Train Induced Photothermal Lensing in Nonlinear Optical Medium

The central idea of this thesis is to manipulate and assess the lensing effect of the nonlinear optical medium to increase the potential of routine techniques and devices relying on it. The underlying physical mechanism and analysis of the lensing phenomena differ drastically with sample under investigation. Still in general, mechanism leading to Kerr lensing is much faster than the photothermal lensing. So, distinct mechanisms can be responsible for lensing of an NLO material at different time scale. Response time of the material to the incident light is of below sub picosecond for Kerr lensing effect, whereas, it is around millisecond for photothermal lensing effect. Ultrafast response enabled Kerr lensing to play a crucial role in the emerging field of laser and photonics technology used in broad range of applications such as mode locked lasers, ultrafast switches, optical signal processing, quantum computing, optical sensors, and many more. Photothermal lensing, however, are used generally to investigate thermodynamic and optical properties of the materials. For experimental studies, I employ femtosecond laser pulse train to induce photothermal lens and Kerr lens in the nonlinear optical (NLO) medium. This thesis has been arranged into six chapters which are listed below Chapter 1 contains a brief introduction of the various nonlinear optical properties of the materials. However, focus is on the third order nonlinear properties, namely, nonlinear refraction and two-photon absorption as only nonlinear optical properties relevant to this thesis. It starts with the definition and importance of nonlinear optical medium, and proceeds with description of the formalism required to understand these phenomena. Two-photon absorption process and lensing phenomena occurrences due to Kerr effect as well as photothermal effect is discussed in detail. In Chapter 2, principle of measurement techniques used in this thesis are described in detail. A well-known single beam Z-scan technique is used for the characterization of lensing effect in the steady state. A time resolved technique is employed for the study of photothermal lensing phenomena especially. The study of nonlinear optical phenomena requires high intensity laser source, and for the use of the mentioned techniques, laser must be well characterized. A brief overview of all the laser systems employed in the distinct experimental setups constructed in this thesis is given here, however other fine details of the particular experiment are given in concerned chapters. Standard autocorrelation techniques used to measure pulse duration and knife edge method used for spatial characteristics of laser beam like beam radius are also presented. In chapter 3, Nonlinear absorption and Kerr type nonlinear properties of pure CS2 are investigated by standard Z-scan method using high repetition rate (HRR) femtosecond laser coming directly from oscillator without amplification. High repetition of laser pulses causes substantial cumulative heating of the sample. Consequently, sample also exhibits strong photothermal lensing effect along with Kerr lensing effect. Single beam Zscan method is sensitive to both effects and cannot distinguish through steady state measurement. Here, I demonstrate that the photothermal lensing effect can be removed if a liquid sample is flowed at a flow rate above or equal to a sufficient value. It allows us to measure only Kerr lensing accurately with HRR lasers. In Chapter 4, Thermal lens measurement of influence of laser power and molecular sizes on the convective heat transfer in alcohols is performed. Thermal lens spectrometry/spectroscopy is a very sensitive analytical technique. This technique is anticipated as an alternative analytical tool to fluorescence or transmittance based methods. It has been practiced avoiding the chance of convective heat transfer in the process of thermal lens formation in almost every case. But, here I illustrate that convection can influence the thermal lens signal significantly, and it can be studied by thermal lens technique. In Chapter 5, I focus on isolating molecular effect of convection by novel time resolved photothermal lens spectroscopy, whereas, both laser power and molecular size collectively affected the convection in chapter 4. Beside that, A new analytical model is included rather than using existing semiempirical treatment of thermal lens signal as used in chapter 4. The model follows all basic assumptions that were included in a widely accepted Shen’s model except neglecting the convection. Only important mathematical steps are shown in the main section; however, intermediate steps between crucial mathematical expressions are given in the appendix. I apply the new model to study the convection effect in alkane samples which fits well in approximation regime since effect of convection are more prone to alkane than methanol due to lack of hydrogen bonding, unlike methanol. Chapter 6 contains conclusions and future prospects.