Lens Signal Research Articles

Very low optical absorptions and analyte concentrations in water measured by Optimized Thermal Lens Spectrometry

Thermal Lens Spectrometry has traditionally been carried out in the single-beam and the mode-mismatched dual-beam configurations. Recently, a much more sensitive dual-beam TL setup was developed, where the probe beam is expanded and collimated. This feature optimizes Thermal Lens (TL) signal and allows the use of thicker samples, further improving the sensitivity. In this paper, we have made comparisons between the conventional and optimized TL configurations, and presented applications such as measurements of very low absorptions and concentrations in water and Cr(III) aqueous solution in the UV–vis range. For pure water we found linear absorption coefficients as low as the Raman scattering one due to the stretching vibrational modes of OH group. The detection limit was estimated 1 × 10 −6 cm −1 with a 180-mW excitation power using a 100-mm cell length. This sensitivity is very high, considering that water has a photothermal enhancement factor ∼33 times smaller than CCl 4, for example. For Cr(III) species in aqueous solution, the limit of detection (LOD) was estimated in ∼40 ng mL −1 at 514 nm, or ∼10 ng mL −1 at 405 nm, which is ∼30 times smaller than the LOD achieved with conventional transmission techniques. The more recent TL configuration is very attractive to obtain absorption spectra, since the result does not depend critically on the beam parameters, unlike the other configurations. The main drawbacks of this optimized TL configuration are the longer acquisition time and the need for larger samples.

Pulsed-laser excited thermal lens spectroscopy with sample-fluid heat coupling

Analytical and finite element analysis modeling methods of the pulsed-laser excited photothermal (PT) lens signal of solids samples surrounded by air are presented. The analytical and finite element analysis solutions for the temperatures induced in the sample and in the air were found to agree over the range of conditions in this report. Model results show that the air contribution to the total PT lens signal is significant in many cases. In fact, these solutions open up the possibility of applying the pulsed excited thermal lens method for accurate prediction of the heat transfer to the coupling fluid and subsequently to study the gas surrounding the samples by using a known material solid sample.

Excitation temporal pulse shape and probe beam size effect on pulsed photothermal lens of single particle

We developed the theoretical model of the time-resolved thermal lens spectroscopy of a single particle with various pulse shape optical excitations. To account for the pulse shape optical excitation in the model, a heat diffusion equation of two media (the particle and liquid solvent) is solved using the numerical Laplace transform method. The model also incorporates the propagation of a diffracted Gaussian probe beam due to the thermal lens effect. Numerical results are presented to illustrate the effects of the excitation pulse shape and probe beam size on the evolution of the photothermal lens signal. The developed model is utilized for the thermal diffusivity and size extraction of a red polystyrene particle.

Analytical solution for mode-mismatched thermal lens spectroscopy with sample-fluid heat coupling

This paper presents an improved theoretical description of the mode-mismatched thermal lens effect using models that account for heat transport both within the sample and out to the surrounding coupling medium. Analytical and numerical finite element analysis (FEA) solutions are compared and subsequently used to model the thermal lens effect that would be observed using continuous laser excitation. FEA model results were found to be in excellent agreement with the analytical solutions. The model results show that heat transfer to the air coupling medium introduces only a minor effect when compared with the solution obtained without considering axial air-sample heat flux for practical examples. On the other hand, the thermal lens created in the air coupling fluid has a relatively more significant effect on the time-dependent photothermal lens signals.

Pulsed laser excited photothermal lens spectrometry of cadmium sulfoselenide doped silica glasses

Experimental results for photothermal lens measurements are compared to finite elemental analysis models for commercial colored glass filters. Finite elemental analysis software is used to model the photothermal effect by simulating the coupling of heat both within the sample and out to the surroundings. Modeling shows that heat transfer between the glass surface and the air coupling fluid has a significant effect on the predicted time dependent photothermal lens signals. For comparison with experimental signals, a simple equation based on the finite element analysis result is proposed for accounting for the variance of experimental data where this type of heat coupling situation occurs. The colored glass filters are found to have positive thermo-optical coefficient. Finite element analysis modeling results are also used to correlate experimental measurements of different sample geometries. In particular, the glass samples are compared to ethanol solutions of iron (II) dicyclopentadiene in a sample cuvette even though heat transfer is different for these two samples.

Pulsed Laser Excited Photothermal Lens Spectrometry of CdSxSe1—x Doped Silica Glasses

Experimental results for photothermal lens measurements are compared to finite elemental analysis models for commercial colored glass filters. Finite elemental analysis software is used to model the photothermal effect by simulating the coupling of heat both within the sample and out to the surroundings. Modeling shows that heat transfer between the glass surface and the air coupling fluid has a significant effect on the predicted time-dependent photothermal lens signals. For comparison with experimental signals, a simple equation based on the finite element analysis result is proposed for accounting for the variance of experimental data where this type of heat coupling situation occurs. The colored glass filters are found to have positive thermo-optical coefficients. The net positive dn/dT of CdSxSe(1-x) doped glass filters is considered to be the consequence of counteracting factors: optical nonlinearity, stress-induced birefringence, and the structural network of glass. Finite element analysis modeling results are also used to correlate experimental measurements of different sample geometries. In particular, the glass samples are compared to ethanol solutions of iron (II) dicylopentadiene in a sample cuvette even though heat transfer is different for these two samples.

Electroinduced thermal lens spectrometry

A new technique for the generation of a thermal lens effect is considered. In this technique, the action of focused radiation from an inducing laser is replaced by an analogous action of electric current, which forms a thermal lens at a certain site of a liquid by the generation of a high local current density in a small liquid volume. The main regularities of this technique for thermal lens signal generation are considered, and an expression is derived to relate the detected signal (a relative change in probing radiation intensity at the detector) and the electrolyte concentration. A cell is proposed for electroinduced thermal lens detection. The dependence of the thermal lens signal on the applied voltage and on the concentration of the test electrolyte is determined for a model electrolyte (NaCl), and sensitivity characteristics are found to be consistent with theoretical estimations made in this work.

Investigation of the Soret effect in aqueous and non-aqueous mixtures by the thermal lens technique

In the present work we investigate the thermal diffusion behavior of three different binary mixtures with a thermal lens (TL) setup. In the setup used in this study we avoid the addition of a dye for systems, such as aqueous mixtures, with a weak absorption band at a wavelength of 980 nm. In some aqueous systems with a complex phase behavior the addition of dye significantly affects the apparent measured thermal diffusion properties. The studied systems are dimethylsulfoxide (DMSO) in water, the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate (EMIES) in butanol and a non-ionic surfactant hexaethylene glycol monododecyl ether (C(12)E(6)) in water. The Soret coefficients of the selected systems cover a range of two orders of magnitude. For DMSO in water with a very low Soret coefficient of the order of S(T) approximately 10(-3) K(-1) we find for a low DMSO content (c = 0.33) a reasonable agreement with previous measurements, while the weak thermal lens signal for the DMSO-rich mixture (c = 0.87) leads to 20% too large Soret coefficients with an uncertainty of more than 30%. Secondly we studied a liquid salt 1-ethyl-3-methylimidazolium ethylsulfate (EMIES) in butanol with a roughly ten times higher Soret coefficient of S(T) approximately 10(-2) K(-1). For this system we performed additional measurements with another experimental technique, the classical thermal diffusion forced Rayleigh scattering (TDFRS), which requires the addition of a small amount of dye to increase the absorption. In the entire investigated concentration range the results obtained with the TL and classical TDFRS technique agree within the error bars. As a third system we studied a non-ionic surfactant hexaethylene glycol monododecyl ether (C(12)E(6)) in water with a Soret coefficient of the order of S(T) approximately 10(-1) K(-1). For this system we find good agreement with previous measurements. We conclude that the TL technique is a reliable method for systems with a strong optical contrast and fairly large Soret coefficient of the order of S(T) approximately 10(-2) K(-1).

Effect of the composition of aqueous-organic mixtures on the sensitivity of thermal lens measurements

Thermooptical properties of aqueous solutions of methanol, acetonitrile, dimethyl sulfoxide, ethylene glycol, glycerol, 1,4-dioxane, and sucrose were studied, and it was found that the analytical thermal lens signal depends on the nature of the organic component, most of all, on the polarity and molecular size. The sensitivity coefficient of thermal lens measurements is increased to a maximum extent in methanol solutions (by 7.3 times at the concentration 50 vol %) and acetonitrile (by 8.8 times at 26 vol %). It was found that a small concentration of water slightly affects the thermooptical properties of polar organic solvents.

Thermo-optical properties of gold nanoparticles in colloidal systems

In this work, we report the thermo-optical properties of nanoparticles in colloidal suspensions. Spherical gold nanoparticles obtained by laser ablation in condensed media were characterized using thermal lens spectroscopy pumping at 532 nm with a 10 ns pulse laser-Nd-YAG system. The obtained nanoparticles were stabilized in the time by surfactants (sodium dodecyl sulfate or SDS) in water with different molar concentrations. The morphology and size of the gold nanoparticles were determined by transmission electron microscopy (TEM) and UV–visible techniques. The plasmonic resonance bands in gold nanoparticles are responsible for the light optical absorption, and the positions of the absorption maximum and bandwidth in the UV–visible spectra are given by the morphological characteristics of these systems. The thermo-optical constants such as thermal diffusion, thermal diffusivity, and (dn/dT) are functions of the nanoparticle sizes and the dielectric function of the media. For these reasons, the thermal lens (TL) signal is also dependent on nanoparticle sizes. An analysis of the TL signal of the nanoparticles reveals the existence of an inverse dependence between the thermo-optical functions and the size. This methodology can be used in order to evaluate these systems and characterize nanoparticles in different media. These results are expected to have an impact in bioimaging, biosensors, and other technological applications such as cooling systems.

Ultraviolet Excitation Photothermal Spectroscopy of Non-Labeled Amino Acids and Visible Light-Induced Signal Enhancement

A measurement system for ultraviolet-laser excitation photothermal lens spectroscopy has been designed and constructed for measuring dilute amino acids in liquid solutions. An ultraviolet laser beam is generated as the fourth harmonic of a Ti:sapphire laser in the wavelength range of 212 to 220 nm. Photothermal lens spectra of alanine, phenylalanine, serine, and tryptophan are observed. Photothermal lens spectra of these samples are coincident with their absorption spectra. It is demonstrated that photothermal lens signals of the amino acids can be amplified by simultaneous excitation with a visible laser. This ultraviolet-excitation visible-enhancement system is applicable to photoacoustic detection.

Capturing judgment policy on customers’ creditworthiness: A lens model and SDT approach

Designing an intervention program to train human operator's decision-making process and subsequently to improve their performance requires a thorough understanding of the relationships between the environment, the human operator, and cues on which decisions are based. Understanding these relationships provides opportunities to make better decisions when human operators encounter novel situations. This study investigated the operator's decision-making process from the lens model perspective and signal detection approach, in which operators make judgments on customers’ creditworthiness using three types of information such as (1) days that accounts were unpaid, (2) probability of the customers damaging the institution based on the customers’ credit histories, and (3) case-based reasoning (CBR) scores. Results showed that the operators were conservative, which resulted in extremely low performance while maintaining a good level of domain knowledge. Analyses of the relative cue weights also showed that the aggregated judgment strategy represented by the cue weights was similar to the validity of the cues to the ecology, reflected in the high level of policy matching ( r m). Specifically, the probability and the days cues were more and equally emphasized over the CBR score. Relevance to industry Capturing the human operator's judgment policy can be used to identify and develop customized training needs for individual operator to make better judgments on customers’ creditworthiness or the environment of interest in general.

Analysis of surface thermal lens signal in optical coatings with top-hat beam excitation

An explicit theoretical model based on three-dimensional thermal conduction, thermoelasticity, and Fresnel diffraction integral is developed to describe the surface thermal lens (STL) with a continuous-wave (cw) modulated excitation. The surface displacements and STL amplitudes obtained with both top-hat and Gaussian beam excitations are theoretically computed and compared. A STL experiment is performed with the cw modulated top-hat beam excitation to investigate the dependences of the STL amplitude on the experimental parameters, namely, the detection distance, the modulation frequency, the radius of the excitation beam, etc. Good agreements between the experimental and theoretical results are obtained. The results indicate that high sensitivity of the STL technique can be achieved with the top-hat beam excitation and by optimizing the experimental configuration.

Surface thermal lens technique with an obliquely-incident probe beam

Surface thermal lens (STL) signal produced by a cw modulated top-hat beam excitation is detected by an obliquely-incident probe beam. The dependence of the STL amplitude on the incident angle theta of the probe beam is experimentally investigated. The STL amplitude as a function of the detection distance is measured at different angles of incidence. The results show that the maximum STL amplitude and the optimum detection distance, where the maximum STL amplitude is obtained, decrease by cos theta as the incident angle increases. These experimental dependences are well explained by a simple theoretical model.

Effect of electrolytes on the sensitivity of thermal-lens measurements in aqueous media

The thermooptical properties of aqueous solutions of strong electrolytes (H2SO4, HCl, NaCl, Na2SO4, and NaOH) are considered. The thermal lens signal depends on the nature of the electrolyte. The effect of an electrolyte is not the sum of the effects of the constituent ions. The largest gain in the sensitivity of thermal-lens measurements is achieved with sulfuric acid (sensitivity enhancement factor of 2 for 0.6 M H2SO4 versus water) and sodium chloride (sensitivity enhancement factor of 1.5 for 3 M NaCl), while the effect of hydrochloric acid is negligible.

A pyroelectric infrared biometric system for real-time walker recognition by use of a maximum likelihood principal components estimation (MLPCE) method

This paper presents a novel biometric system for real-time walker recognition using a pyroelectric infrared sensor, a Fresnel lens array and signal processing based on the linear regression of sensor signal spectra. In the model training stage, the maximum likelihood principal components estimation (MLPCE) method is utilized to obtain the regression vector for each registered human subject. Receiver operating characteristic (ROC) curves are also investigated to select a suitable threshold for maximizing subject recognition rate. The experimental results demonstrate the effectiveness of the proposed pyroelectric sensor system in recognizing registered subjects and rejecting unknown subjects.

렌즈모델을 이용한 의사결정자의 Admission Policy 분석 - 과학과 공학분야에서의성차이의 영향을 중심으로

Despite the government promoting women's participation in the engineering field, some statistics show that it has yet to be achieved. Potential reasons for this phenomenon include lower level of applications by women, or inherent gender gap in the professional field. Therefore, this study attempted to find impact of gender on college admission from the Lens Model perspective and Signal Detection Theory. This study consisted of three phases: identifying the necessary cues used in the admission process, analyzing existing data, and conducting two experiments to identify the effect of gender on admission decisions. Although the college application consisted of many cues, only five cues, school ranking, GPA, SAT score, resident status, and gender, were used to capture the officers' judgment policies for engineering admissions. Two experiments were conducted to investigate the impact of the gender factor in college admission. The enrollment officers first were presented with the existing data without the gender and asked to make dichotomous judgments. Secondly, the officers were asked to perform the judgment task with the gender cue present. Results showed that the gender did not play an important role in the judgments as expected. However, ideographical analyses on judgment strategies revealed that there were significant differences between the admission officers. Possible training implications are discussed.

Density dependence of optical Kerr lens signals in neat N 2O and CO 2

Optical Kerr lens spectroscopy has been employed to study the reorientation dynamics of N 2O and CO 2 at the reduced densities 0.8–2.0 and different temperatures in a newly constructed high pressure cell. The transient birefringence induced in the medium by an ultrashort (1 1 0 fs) non-resonant pump beam at 400 nm was monitored by a second, time-delayed non-resonant probe beam at 800 nm. The transient change of the probe beam diameter was detected in the far field using a small-area photodiode. Measurements employing parallel and perpendicular polarization allowed us to extract the nuclear and electronic components of the Kerr effect, where the latter one was by far weaker for both CO 2 and N 2O. Accurate time constants for the reorientation dynamics of the CO 2 and N 2O molecules were extracted from the tail of the nuclear Kerr response. In the case of CO 2 the time constants τ reorientation were between 210 and 280 fs and in good agreement with previous data from NMR and OHD-OKE experiments as well as MD simulations. Reorientation time constants for N 2O were in the range between 200 and 340 fs, and thus very similar to CO 2. In the density range studied, τ reorientation increases with increasing viscosity and decreasing temperature, in agreement with Stokes–Einstein–Debye diffusive models.

Combined laser calorimetry and photothermal technique for absorption measurement of optical coatings

To the best of our knowledge, a combined sensitive technique employing both laser calorimetry and a surface thermal lens scheme for measuring absorption values of optical coatings is presented for the first time. Laser calorimetric and pulsed surface thermal lens signals are simultaneously obtained with a highly reflecting UV coating sample irradiated at 193 nm. The advantages and potential applications of the combined technique and the experimental factors limiting the measurement sensitivity are discussed.

Sensitivity improvement in capillary electrophoresis using organo-aqueous separation buffers and thermal lens detection

It is shown that organo-aqueous separation buffers show much promise when used in capillary electrophoresis separations with photothermal (thermal lens) detection systems. Acetonitrile-water and methanol-water mixtures were selected, as conventionally used in capillary electrophoresis. It is shown that, despite more sophisticated experimental conditions (significant heat outflow from the capillary body) and peak detection, the theoretical ratio of the thermal lens signal for a binary mixture to the thermal lens signal for an aqueous solution (or the corresponding ratio obtained experimentally under bulk batch conditions) can be used to predict the sensitivity of thermal lens detection in capillary electrophoresis. The limits of detection for 2-, 3-, and 4-nitrophenols selected as model compounds in 70% v/v acetonitrile separation buffers are 1 x 10(-6) M, 1 x 10(-6) M and 3 x 10(-7) M, respectively, and are therefore decreased by a factor of six compared to thermal lens detection in aqueous separation buffers. The overall increase in the thermal lens detection sensitivity in a 100% ACN buffer is a factor of 13.