Photothermal Lens Research Articles

Ultraviolet-Laser Excitation Microscopic Photothermal Lens Imaging for Observing Biological Cells

An ultraviolet-laser excitation microscopic photothermal lens imaging system for observing chemical compounds in biological cells has been designed and constructed, in which a reflection objective lens is used for focusing an excitation laser beam at 261 nm and a probe laser beam at 784 nm. Photothermal amplitude and phase images of yeast fungus cells are successfully obtained; together with a transmitted light image at 784 nm. The amplitude image represents photo-absorption at 261 nm by the cells. The phase image seems to represent local thermal properties around the cells.

Simultaneous determination of nonlinear optical and thermo-optic parameters of liquid samples

The authors apply the theory of photothermal lens formation and also that of pure optical nonlinearity to account for the phase modulation in a beam as it traverses a nonlinear medium. It is used to simultaneously determine the nonlinear optical refraction and the thermo-optic coefficient. They demonstrate this technique using some metal phthalocyanines dissolved in dimethyl sulfoxide, irradiated by a Q-switched Nd:YAG laser with 10Hz repetition rate and a pulse width of 8ns. The mechanism for reverse saturable absorption in these materials is also discussed.

Three Dimensional Photothermal Deflection and Thermal Lensing in Solids: The Effect of Modulation Frequency

In this paper, a detailed theoretical treatment of the three dimensional photothermal deflection, under modulated cw excitation, is presented for a three layer system (backing-solid sample-fluid). By using a technique based on Green's function and integral transformations we find the explicit expressions for laser induced temperature distribution function and the photothermal deflection of the probe beam. Numerical analysis of those expressions for certain solid samples shows that the laser induced temperature of the sample surface, the effective thermal length and the deflection signal and its width decrease with increasing modulation frequency. Furthermore, increasing the diffusivity/conductivity of solid sample results in a decrease in the deflection signal intensity and slight increase in the signal width. Finally, we find that the focal length of the photothermal lens in normal direction is much greater than that of in tangential direction.

Steady-state absorption rate models for use in relaxation rate studies with continuous laser excited photothermal lens spectrometry.

This paper examines the solutions of kinetic rate equations for prediction of the photothermal lens signals under irradiance conditions that can lead to optical saturation or bleaching. The relaxation kinetics resulting from forcing excited state populations in multiple levels by high excitation irradiance continuous lasers is examined and irradiance-dependent photothermal lens signals are predicted. The analyses described in this paper are based on simple kinetic models for optical excitation and subsequent excited state relaxation. Dark-state relaxation is assumed to be extremely fast compared to limiting kinetics resulting in simplified excited state models. Kinetic models are derived for two, four and five active level molecular systems. Gaussian laser beam profiles are assumed and time dependent photothermal lens signals are calculated. Models account for excitation laser profile, thermal relaxation of the spatially and temporally distorted heating rate distribution resulting from nonlinear absorption, and metastable state relaxation. This heating rate is used to calculate the temperature change distribution and subsequently the optical elements needed to model the experimental photothermal signals.

Optical Bleaching in Continuous Laser-Excited Photothermal Lens Spectrometry

This paper presents methods for measuring and modeling optical bleaching that occurs, by using continuous laser sources for solution-phase organic dye molecule spectrometry. Photothermal lens experiments are used to measure the nonlinear optical absorption coefficients of eosin Y, erythrosin B, and pseudo-isocyanine iodide dyes in ethanol as a function of excitation irradiance. Excitation irradiance-dependent photothermal lens data are subsequently interpreted in terms of the photophysics and excited-state relaxation dynamics of the condensed-phase dye molecules under study. The model uses first-order kinetics for excitation and subsequent metastable-state relaxation back to the ground state and accounts for both ground- and metastable-state absorption. The measurement model accounts for thermal lens formation and apparatus configuration. Nonlinear absorption measurements are fit to the model with nonlinear regression. The resulting parameters allow calculation of triplet-state absorption coefficients and triplet to ground singlet state-relaxation rate constants.

Laser‐Excited Photothermal Lens Spectrometry in a Low‐Volume Cylindrical Sample Cell

AbstractA new apparatus for performing laser‐excited photothermal lens spectroscopy is described. The apparatus uses a sub‐microliter cylindrical sample cell, chopped laser excitation, and a continuous probe laser to perform photothermal lens spectrometry. The sample is irradiated with a constant irradiance beam, and the photothermal lens forms due to thermal diffusion through the sample cell walls. Solutions for the thermal diffusion equations predict a parabolic temperature change under continuous irradiation. Experimental results are obtained for organic and inorganic complexes in ethanol solution and compared to those obtained using a standard sample cell. Results for dicyclopentadienyl iron solutions excited with 10mW 514.5 nm Ar+ laser radiation show that the cylindrical sample cell signal is 32% of that obtained using the standard sample cell. This ratio is in reasonable agreement with that expected based on thermal diffusion models. Results obtained for ruthenium tris‐bipyridyl chloride and perchlorate salts are not stable with respect to irradiation time. The latter results suggest that a significant amount of ruthenium tris‐bipyridyl complex is photolyzed with the mild irradiation conditions used in these experiments.

Diffraction effects in single- and two-laser photothermal lens spectroscopy

A simple method for calculating the effects of optical geometry on photothermal lens signals is shown. This method is based on calculating cumulative electric-field phase shifts produced by a series of Gaussian refractive-index perturbations produced by the photothermal effect. Theoretical results are found for both pulsed-laser and continuous Gaussian laser excitation sources and both single- and two-laser apparatuses commonly employed in photothermal lens spectroscopy. The effects of apparatus geometry on the resulting signal are shown. Analytical time-dependent signal results are found for small signals. Analytical pump-probe focus geometry results allow direct optimization for certain conditions. The calculations indicate that the photothermal lens signal is, in general, optimized for near-field detection-plane geometries.

Photothermal lens spectrometry of homogeneous fluids with incoherent white-light excitation using a cylindrical sample cell

A model for photothermal lens signal generation in a cylindri- cal sample cell under constant irradiance excitation is described and tested. The model is developed with and without the assumption that the sample cell does not change temperature over the irradiation time. In both cases, the photothermal lens is predicted to be parabolic in form with a strength that is independent of sample cell radius. The predicted irradiance independence suggests that incoherent illumination can be used to perform photothermal lens spectroscopy in low-volume cells. Experimental evidence is obtained using a Xe arc lamp to perform pho- tothermal lens spectroscopy in a 6 mL cylindrical spectrophotometric cell. Optical filters are used to reduce the power at IR and UV wavelengths of the Xe lamp emission spectrum. This pseudo-white-light source enables indirect optical absorbance measurement independent of the absorption spectrum of the analyte. The preliminary data reported show that photo- thermal lens signals can be obtained using wide-spectral-bandwidth, in- coherent excitation sources. Although the theoretical enhancement fac- tor is found to be only ;0.01 for these experiments, limits of detection of the order of 30 to 300 pM pseudoisocyanine dye in ethanol solution are found. This corresponds to spectral integrated absorption detection limit from 10 24 to 10 26 au in the centimeter path length cell. These low de- tection limits are found even with low enhancement factors because the factors that affect the noise in the photothermal lens and conventional transmission spectroscopy signals are not the same in these experi- ments. The major sources of uncertainty in these detection limit esti- mates are knowledge of the excitation source spectrum and periodic chaotic behavior of the diode laser used as a probe of the photothermal lens. Examination of the response time of the signal reveals that thermal conductivity of the sample cell influences the characteristic signal rise and decay time constants. The radiative heat transfer model is applied to interpret measured time constants in terms of the cell thermal conduc- tivity and thickness of the sample cell walls. The sample cell thermal conductivity determined by this method is consistent with ferrous mate- rials. © 1997 Society of Photo-Optical Instrumentation Engineers.

Time-resolved photothermal lens calorimetry for investigating mixed-order photoinitiated reaction kinetics in liquids

The pulsed thermal lens experiment is useful for investigating the kinetics and energetics of intermediate species in photoinitiated reactions. We have adapted this technique to allow decay rates to be estimated under conditions where second-order processes make a significant contribution to the observed kinetics. The excited triplet state photophysics of benzophenone in acetonitrile are investigated using this method. Using a mixed-order kinetic model, the rate constant of triplet-triplet annihilation was found to be (1.9±0.2)×10 10 M -1 s -1 , indistinguibhable from a diffusion-controlled value. A finite rate of self-quenching of the excited triplet state was also found. From the amplitudes of the photothermal transients, the triplet energy of the benzophenone triplet was found to be 67±4 kcal/mol in acetonitrile

Accounting for absorption saturation effects in pulsed infrared laser-excited photothermal spectroscopy

Equations that relate photothermal lens focal lengths and photothermal deflection angles to saturation absorption coefficients are derived. These equations are derived for two-level absorbers with both homogeneously and inhomogeneously broadened transitions. Initial and time-dependent photothermal lens signals are calculated. Equations describing the zero-time signals are exact to within the simplifying assumptions of the derivation, while the time-dependent signals are approximate. The approximation is performed by the use of a finite series of Gaussian functions to model the temperature change profile distorted by nonlinear absorption. The excitation irradiance-dependent signal behavior for rectangular and exponential excitation pulse time profiles for homogeneously and inhomogeneously broadened transitions are compared. Absorbed energies are used to calculate effective absorbances obtained by the use of conventional and photothermal lensing spectrometry. The conclusions drawn from these comparisons are tha pulsed laser photothermal spectroscopy is sensitive to the excitationlaser's pulse temporal profile and the transition broadening mechanism.

Temperature effect on photothermal lens phenomena in water: Photothermal defocusing and focusing

Depending on whether the measurements are performed at temperatures lower or higher than −0.01°C, the photoinduced thermal lens in water can have either positive (converging) or negative (diverging) focal length. At precisely −0.01±0.04°C, no thermal lens signal could be observed. This is because at this temperature, the refractive index of water is maximum of the temperature coefficient of the refractive index, d n/d T, is zero.

Photothermal lens aberration effects in two laser thermal lens spectrophotometry

A comparison of theories describing two laser photothermal lens signals is given. The aberrant nature of this lens is accounted for in a theory which treats the propagation of a monitor laser in terms of a phase shift in this laser beam wave front. The difference between theories are discussed in terms of the predicted signal strengths and temporal behavior. The aberrant theory results in smaller theoretical signal strengths and different functional relationships between signal and analyte level.