Criticality of depth of intensity modulation and simulation of refractive index profile in thermal lens technique

The present paper intends to unveil the criticality of the depth of intensity modulation (D) in getting correct results in optical experiments employing electromechanical choppers. The study elucidates experimentally using a single beam thermal lens setup with an optical chopper with variable D, designed and constructed cost-effectively, and also by simulating the refractive index profile generated within the medium. The thermal diffusivity of water with a trace amount of chlorophyll is determined by varying D for a given period. It is observed that for a D above 10%, photodissociation and Soret effect significantly affect the thermal lens signal and thereby giving erroneous value to the thermal diffusivity. The UV-visible spectroscopic analysis reveals reduced absorption for the leaf pigments − chlorophyll a, chlorophyll b, and lutein as a result of photodissociation. Thus the study demonstrates the criticality of D for obtaining error-free measurements.

Linear and Nonlinear Thermal Lens Signal of the (Δν = 6) C–H Vibrational Overtone of Naphthalene in Liquid Solutions of n-Hexane

The thermal lens technique is applied to vibrational overtone spectroscopy of solutions of naphthalene (C10H8) in liquid hexane. The C–H fifth vibrational (Δν = 6) overtone spectrum of C10H8 is detected at room temperature for mole fractions from 0.08 to 19 × 10−6 using n-C6H14 as solvent. By detecting the absorption band in a 19 ppm (parts per million) solution, the peak absorption of the signal is approximately (2.2 ± 0.3) × 10−7 cm−1. A plot of normalized integrated intensity as a function of the mole fraction of naphthalene in solution reveals a dependence of the magnitude of the signal with the probe laser wavelength. If the wavelength of the probe laser is 568 nm, the thermal lens signal (TLS) is linear as a function of the mole fraction of the solution. When the wavelength of the probe laser is 488 nm, the TLS is nonlinear as a function of the concentration. Three different models of nonlinear absorption are discussed. A two-color absorption model that includes the simultaneous absorption of the pump and probe lasers could explain the enhanced magnitude and the nonlinear behavior of the TLS for solutions of mole fraction < 0.1%.

Continuous-Wave-Laser versus Pulsed-Laser Excitation for Crossed-Beam Photothermal Detection in Small Volume Applications: Comparative Features

Crossed-beam thermal lens spectrometry can be implemented using continuous-wave- (cw) laser or pulsed-laser excitation. In both cases, the signal depends on the position of the sample with respect to the probe beam waist, the size of the excitation beam, the beam-size ratio into the sample, and the power or energy of the excitation beam. However, due to differences in the rate of formation and relaxation of the thermal lens, both methods exhibit distinct key features. Optimization of the experimental setup and understanding the thermal lens signal are more complicated under cw-laser excitation than with pulsed-laser excitation. Unlike that observed under pulsed excitation, the effect of the excitation beam waist, of the sample size, and of the flow rate are closely related to the effective size of the thermal element and depend on the chopping frequency. Although the intrinsic sensitivities are almost the same, the performance can significantly differ depending on the chopping frequency or pulse repetition rate, which should be high enough to allow fast data collection and efficient signal averaging.