Time series analysis of duty cycle induced randomness in thermal lens system.

• First report of investigation of molecular dynamics in a thermal lens system by Time series analysis. • Investigation of the effect of duty cycle on molecular dynamics. • The complexity mapping in TL system with changing duty cycle is studied by phase portrait and fractal analysis. • The Hurst exponent reveals the antipersistent nature of the system. • The study correlates sample entropy with thermodynamic entropy. The present work employs time series analysis, a proven powerful mathematical tool, for investigating the complex molecular dynamics of the thermal lens (TL) system induced by the duty cycle (C) variation. For intensity modulation, TL spectroscopy commonly uses optical choppers. The TL formation involves complex molecular dynamics that vary with the input photothermal energy, which is implemented by varying the duty cycle of the chopper. The molecular dynamics is studied from the fractal dimension (D), phase portrait, sample entropy (S), and Hurst exponent (H) for different duty cycles. The increasing value of C is found to increase D and S, indicating that the system is becoming complex and less deterministic, as evidenced by the phase portrait analysis. The value of H less than 0.5 conforms the evolution of the TL system to more antipersistent nature with C. The increasing value of C increases the enthalpy of the system that appears as an increase in full width at half maximum of the refractive index profile. Thus the study establishes that the sample entropy and thermodynamic entropy are directly related. [ABSTRACT FROM AUTHOR]