Detection of photothermal deflection signals with conjugate masks

A new detection technique for photothermal deflection spectroscopy and photoacoustic deflection spectroscopy is presented. The technique uses a pair of matched multiple slits placed in the path of the probe beam and oriented to block the probe light from the detector in the absence of a deflection signal. Significant improvement in the signal-to-noise ratio and in the frequency bandwidth compared with those available with current techniques is demonstrated.     

Performance of a photothermal detector with turbid liquids

A closed-cell photothermal detector for aqueous analytes has been evaluated at 254 and 678 nm. We used a detector with a water meniscus as a pressure sensor, whose periodic deflection was measured using a low-finesse optical fiber Fabry-Perot interferometer. Performance was compared with a commercial diode array spectrometer and found to be similar for absorption measurements in nonturbid samples, but the results were affected up to 60 times less by scattered light. Finally the photothermal cell was converted into an integrating cavity using ceramic inserts, showing freedom from scattering-related errors at 678 nm but a degradation in performance at 254 nm.     

Trace gas detection by laser intracavity photothermal spectroscopy

A novel laser intracavity photothermal detector is described. In this scheme, sample absorption of the pump laser power takes place within the cavity of a probe He-Ne laser causing modulation in the gain and in turn the output power. Comparison of this intracavity detector with two other photothermal techniques, namely, phase fluctuation optical heterodyne spectroscopy and thermal beam deflection, is made in terms of practicality and sensitivity. For in situ measurements, sensitivity of 0.5 x 10(-7) cm(-1) for a probe length of 3 cm has been achieved.     

Optical saturation in continuous-wave photothermal deflection spectroscopy: quantitative investigation of fundamental and harmonic components

A rigorous theoretical model has been developed that describes the effects of optical saturation on the deflection signals in cw photothermal experiments. In this model the spatial profile of the excitation beam has been taken into account. Numerical calculations have been performed to determine the saturation behavior of the cw photothermal deflection signals at both the fundamental and the second-harmonic frequencies for sine-wave modulation of the excitation beam. Saturation behavior for square-wave modulation has also been investigated.     

Damage threshold determination of bulk polymer samples using pulsed photothermal deflection technique

Photothermal deflection technique was used for determining the laser damage threshold of polymer samples of teflon (PTFE) and nylon. The experiment was conducted using a Q-switched Nd-YAG laser operating at its fundamental wavelength (1-06μm, pulse width 10 nS FWHM) as irradiation source and a He-Ne laser as the probe beam, along with a position sensitive detector. The damage threshold values determined by photothermal deflection method were in good agreement with those determined by other methods.     

Theoretical Modeling of Obliquely Crossed Photothermal Deflection for Thermal Conductivity Measurements of Thin Films

A three-dimensional theoretical model has been developed to calculate the normal probe beam deflection of the obliquely crossed photothermal deflection configuration in samples which consist of thin films deposited on substrates. Utilizing the dependence of the normal component of probe beam deflection on the cross-point position of the excitation and probe beams, the thermal conductivity of the thin film can be extracted from the ratio of the two maxima of the normal deflection amplitude, which occurs when the cross-point is located near both surfaces of the sample. The effects of other parameters, including the intersect angle between the excitation and the probe beams in the sample, the modulation frequency of the excitation beam, the optical absorption and thickness of the thin films, and the thermal properties of substrates on the thermal conductivity measurement of the thin film, are discussed. The obliquely crossed photothermal deflection technique seems to be well suited for thermal conductivity measurements of thin films with a high thermal conductivity but a low optical absorption, such as diamond and diamond-like carbon, deposited on substrates with a relatively low thermal conductivity.     

Development of a conductivity-based photothermal absorbance detector for capillary separations

A contactless conductivity-based absorbance detector has been developed for use with capillary separations. Detection is based on a photothermal process. As analytes pass through the detector they absorb light, producing a thermal perturbation. This thermal event results in a change in the solution conductivity. The measured change in conductivity is directly related to the absorption of light. The major advantage to this type of detector is that the measured absorbance is, to a first approximation, independent of optical path length, allowing small-diameter capillaries to be used. This approach combines the optical simplicity of traditional transmission-based instruments with the path length independence of similar refraction-based photothermal detectors. In addition to the initial development and characterization of the photothermal absorbance detector, multiphysical modeling of the heat transfer within the conductivity cell was performed.     

Effect of beam sizes on the amplitude and phase of photothermal deflection signals for both uniform and nonuniform heating

A detailed theoretical treatment of a one- (1D) and three-dimensional (3D) photothermal deflection (PTD) technique is presented. Important effects of the probe beam size occur in PTD experiments when the radius of this beam is of the order of magnitude of the thermal diffusion length. The calculation of this effect is checked by experiments in paraffin oil at low modulation frequency as well as for 1D and for 3D. In this last case, we have considered two kinds of deflection: normal and transverse, and we have studied their variation for different values of the pump beam radius. The coincidence between theoretical and experimental curves confirms the validity of our theoretical model.     

An approach to the infrared study of materials by photothermal beam deflection spectroscopy

Infrared Fourier transform photothermal beam deflection spectroscopy (PBDS) is briefly outlined. It involves heating a surface by the photothermal effect, using infrared (IR) radiation from a scanning interferometer; energy is transferred to the gas above the surface and the resulting refractive index gradient in the gas is detected by observing the deflection of a laser beam grazing the solid's surface; the deflection is related to the IR radiation which is absorbed, so that the Fourier transform of the deflection signal results in the IR spectrum. PBDS is especially useful for materials which scatter and/or absorb IR radiation so extensively that conventional transmission/absorption techniques are not possible. A variety of examples of applications are given, including spectra of polymers, floor tile, polyester thread and fabric, leather, cellulose, paper, lichen and bone.     

Comparison of BaTiO3 optical novelty filter and photothermal lensing configurations in photothermal experiments.

Photorefractive BaTIO3 is used as an optical novelty filter to highlight the high spatial frequency components of the photothermal signal. A real-time phase grating recorded in BaTIO3 acts as a matched rejection spatial filter for the probe laser. This reduces the stationary background from the optical signals thereby increasing signal contrast ratios. Rejection of the monotonous stationary signal provides a powerful means of improving the photothermal signal. This paper describes the construction of this novel apparatus and the experiments performed in order to compare its performance with photothermal lensing results. A theory that explains photothermal signal filtering with BaTIO3 as an adaptive spatial frequency filter is presented. Results comparing the optical signals obtained in a photothermal lensing experiments and those obtained in the BaTIO3 optical novelty filter experiments are presented. The optical novelty filter signals demonstrate a remarkable improvement in the signal contrasts for moderate photothermal-induced phase shifts.     

Study on the Spatial Resolution of Imaging Technique for Absorption Loss Measurement of Optical Coatings

The uniformity of optical coatings becomes more and more important as large diameter optical devices are widely used. Absorption loss in optical components, particularly in optical coatings, is a limiting factor in high-power laser applications. This article analyzes the main factors, which affect the spatial resolution of three techniques for surface absorption loss measurement, including the photothermal deflection technique, the surface thermal lens technique, and the photothermal detuning technique. The influence of the size of the heating and probe beam on the photothermal detuning technique is studied in detail. Experiments are conducted to study the photothermal signal of the photothermal detuning technique for absorption measurement of the optical coating point by point. The results show that the main factors, which affect the spatial resolution of imaging measurements for absorption loss of coatings, are the heating beam size and the step accuracy of the sample translation stage. The heating and probe beam sizes has a significant impact on the application of the photothermal detuning technique. Experimental result shows that the photothermal detuning technique can be used for imaging of absorption loss measurements of optical coatings. The results provide theoretical and experimental supports for further application of the photothermal detuning technique.     

Experimental Study on GFRP Surface Cracks Detection Using Truncated-Correlation Photothermal Coherence Tomography

In this paper, truncated-correlation photothermal coherence tomography (TC-PCT) was used as a nondestructive inspection technique to evaluate glass-fiber reinforced polymer (GFRP) composite surface cracks. Chirped-pulsed signal that combines linear frequency modulation and pulse excitation was proposed as an excitation signal to detect GFRP composite surface cracks. The basic principle of TC-PCT and extraction algorithm of the thermal wave signal feature was described. The comparison experiments between lock-in thermography, thermal wave radar imaging and chirped-pulsed photothermal radar for detecting GFRP artificial surface cracks were carried out. Experimental results illustrated that chirped-pulsed photothermal radar has the merits of high signal-to-noise ratio in detecting GFRP composite surface cracks. TC-PCT as a depth-resolved photothermal imaging modality was employed to enable three-dimensional visualization of GFRP composite surface cracks. The results showed that TC-PCT can effectively evaluate the cracks depth of GFRP composite.     

Detection of methane with high spatial resolution with photothermal deflection spectroscopy

We demonstrate the use of photothermal deflection spectroscopy for detection and measurement of methane with very high spatial resolution. A high spatial resolution may be important for some applications, and other techniques in current use do not provide this resolution. To the best of our knowledge, this is the first application of photothermal spectroscopy to methane detection. We have succeeded in detecting a signal even from a very weak combination-overtone band of methane in the visible region of the spectrum. If used in conjunction with a strongly absorbing fundamental band, the technique is capable of yielding high sensitivity along with very high spatial and temporal resolutions.     

Development of photoacoustic spectroscopy with a piezofilm

We have developed photoacoustic spectroscopy with a piezofilm. A piezofilm is a piezoelectric element made from plastic polyvinylidene fluoride having piezoelectrical effect. Photoacoustic spectra (375-675 nm) of water, dye aqueous solution, and benzene, are measured with a xenon lamp. The piezofilm is directly immersed in the liquid samples for sensitive detection of the signal. The sensitivity of the method is shown to be as high as for photothermal deflection spectroscopy. Compared with the conventional methods such as photoacoustic spectroscopy with a piezoceramic and photothermal spectroscopy with a double beam configuration, the present method is favorable from its handy and simpler experimental setup.     

光热偏转薄膜热导测试系统及其应用

根据光热偏转光谱（ＰＴＤＳ）原理，研制成功光热偏转法薄膜热导测试系统，对大量试样进行了测量，结果同文献参考值符合较好，测量重复精度优于５０％，为材料科学有关热性质的研究，特别是有关光热偏转薄膜热导率的研究提供了有力的检测手段，为研制高质量金刚石薄膜提供了依据。     

Utilisation de la deflection photothermique pour l'etude de l'effet du chlore sur l'absorption par les etats de la bande interdite dans a-SiHCl

Chlorinated and hydrogenated amorphous silicon (a-SiHCl) have been prepared by glow-discharge of a gas mixture (SiCl₄+H₂). Optical absorption and photothermal deflection spectroscopy have been made on samples with different chlorine contents. Due to its higher sensitivity, the photothermal deflection spectroscopy shows a decrease of the gap states absorption at low energies when the chlorine content is increased.     

Photothermal deflection in multilayer coatings: modeling and experiment

A model of the photothermal deflection signal in multilayer coatings is presented that takes into account optical interference effects and heat flow within the stack. Measurements are then taken of high-reflectivity HfO2/SiO2 ultraviolet mirrors made by plasma ion assisted deposition and compared to calculations. Good agreement is found between the experimental results and the model. Using this model for the calibration and the setup described, one can measure absorption in multilayer coatings accurately down to 10(-7) of the incident power.     

Characterization of optical coatings by photothermal deflection

An overview of photothermal deflection principles and applications is given. The modeling of temperature distribution and the calculation of deflection that is due to both the refractive-index gradient and the thermal deformation of the sample are presented. Three configurations usually employed are compared, and their respective advantages are discussed in relation to their application. The calibration for absolute measurement of absorption is detailed, showing that calibration limits the accuracy of measurement. Some examples of specific information obtained by photothermal mapping of absorption are given.     

Elastomeric diffraction gratings as photothermal detectors

A photothermal detector consisting of a relief grating on the surface of an elastomer was fabricated and characterized. The detector has a sensitivity of the order of microwatts per square millimeter and has rise and fall times of the order of tens of seconds. Numerical and analytical modeling account for the behavior of the detector.     

Measurement of absolute minority species concentration and temperature in a flame by the photothermal deflection spectroscopy technique

It is experimentally demonstrated that absolute concentrations of minority species in flames can be measured by the photothermal deflection spectroscopy (PTDS) technique. In addition, the PTDS signal simultaneously yields the flame temperature the measurement point. Absolute concentration profiles of OH have been measured in a flat-flame burner with methane as fuel. The PTDS measurements agree well with those obtained independently by the absorption technique. The flame temperature measurements by PTDS are also in good agreement with those obtained by the Boltzmann distribution among the rotational levels of OH.