White Light Photothermal Lens Spectrophotometer for the Determination of Absorption in Scattering Samples

Abstract

We developed a pump-probe photothermal lens spectrophotometer that uses a broadband arc-lamp and a set of interference filters to provide tunable, nearly monochromatic radiation between 370 and 730 nm as the pump light source. This light is focused onto an absorbing sample, generating a photothermal lens of millimeter dimensions. A highly collimated monochromatic probe light from a low-power He-Ne laser interrogates the generated lens, yielding a photothermal signal proportional to the absorption of light. We measure the absorption spectra of scattering dye solutions using the device. We show that the spectra are not affected by the presence of scattering, confirming that the method only measures the absorption of light that results in generation of heat. By comparing the photothermal spectra with the usual absorption spectra determined using commercial transmission spectrophotometers, we estimate the quantum yield of scattering of the sample. We discuss applications of the device for spectroscopic characterization of samples such as blood and gold nanoparticles that exhibit a complex behavior upon interaction with light.

Absorption Spectra of Ethanol and Water Using a Photothermal Lens Spectrophotometer

“…In addition, photothermal spectroscopy has been carried out using arc lamps, 7,8 but only for the determination of the absorption spectra of highly absorbing samples due to the weak TL effect generated by these excitation light sources. 7,8 Although the TLS technique found a wide range of important applications, its applicability is still hindered by the high cost and low compactness of the excitation sources. Additionally, tunability and high sensitivity is needed for determining the absorption spectra of semi-transparent samples such as ethanol and water.…”

“…[4][5][6] So far, these measurements have been performed by the use of high-power tunable laser sources and relatively expensive and experimentally complex setups. In addition, photothermal spectroscopy has been carried out using arc lamps, 7,8 but only for the determination of the absorption spectra of highly absorbing samples due to the weak TL effect generated by these excitation light sources. 7,8 Although the TLS technique found a wide range of important applications, its applicability is still hindered by the high cost and low compactness of the excitation sources.…”

Photothermal Determination of Absorption and Scattering Spectra of Silver Nanoparticles

“…We measure the PTL spectra S( λ) using a homemade spectrophotometer which has been previously described. 28 The device uses a Xenon lamp and a set of interference filters to generate quasi-monochromatic pump light with an average power density of 0.13 mW/nm with a spectral resolution of 5 nm in the range of 370–730 nm. Despite the limited values of the spectral power density, we obtain a relatively good signal-to-noise (S/N) ratio thanks to the sensitivity of the PTL method.…”

“…If we consider that all of the absorbed light energy turns into heat through non-radiation relaxation mechanisms, we conclude that the PTL spectrum corresponds to the absorption spectrum of the NP. As a consequence, the absorption cross-section can be written as: 28,37 where Po(λ) is the power density of the pump light, K is a proportionality factor independent from λ, but dependent on the Rayleigh ranges of the pump and probe fields, their waist position, and other experimental parameters, and S(λ) is the PTL signal defined as the relative change of probe light’s transmission through a small aperture located at the far field. The coefficient K can be determined using samples of negligible scattering.…”

Improvement of an optical fiber sensor for the detection of low concentrations of solutes using the photothermal effect

“…The absorption of light generates local gradients of temperature, which can substantially affect the propagation of the reflected light. The use of TL effects in analytical chemistry is becoming rapidly widespread, particularly for the determination of trace amounts of chemicals in solvents [4] and for light scattering free optical spectroscopy [5]. Other applications can be found in condensed matter physics, for example for thermal diffusivity determination [6].…”