Nonlinear photoacoustic response of s-tetrazine as an important azadiene pigment

Photoacoustic (PA) properties of a familiar and important azadiene pigment, 3,6-dichloro-1,2,4,5-tetrazine (DCT), are studied in toluene as solvent. Different aspects of PA signals of this pigment are compared in various frequencies of the produced sonic waves. Using a focused laser beam with 450-nm wavelength and irradiation of 300 mW, audible sonic waves with 15,500-Hz frequency are created in solvents with high densities of DCT. In low concentrations, where the laser beam transmitted from the cell, dense diffraction rings pattern are observed, which is attributed to the PA properties of this pigment. For the first time, simultaneous study of the diffraction rings pattern, optical PA z-scan, and Fourier transform analyzes results in a more comprehensive understanding of the PA phenomenon. In addition, a new method based on diffraction rings number is presented that could lead to the estimation of non-linear optical phase changes. Moreover, the audible PA signals, as a non-linear phenomenon, could be applied as a density sensor for the solution or intensity sensor for the laser beam.