Determination of laser ablation threshold of Teflon at different harmonics of Nd:YAG laser using photothermal deflection technique

We report the measurement of the laser ablation threshold of Teflon using photothermal beam deflection at 1.06 m, 0.532 m, and 0.355 m irradiation in air. The measured ablation threshold at various wavelengths is used to estimate the absorption coefficient. It is observed that the absorption coefficient increases with a decrease in wavelength.     

Polarization-dependent photothermal beam deflection for the determination of the order parameter C(2) in a laser-oriented polymer system

We demonstrate polarization-dependent photothermal beam deflection as a powerful tool for analyzing quasi-two-dimensional molecular orientation. As examples we used two laser-beam-oriented polymer systems: DR1 in poly(methyl methacrylate) (PMMA) and PMMA with covalently bound DR1 and for comparison Phenol Blue in PMMA. Different order parameters C(2), for these systems have been found. It was also possible to orient by laser beam the DR1 chromophores cyclically by changing the polarization direction of the orienting laser beam and following these chromophore reorientations. The long-term stability of the orientation was investigated as well. The irreversible bleaching that is due to this laser treatment could be determined. Angular hole burning could easily be detected in these systems.     

Thermal characterization of gallium arsenic nitride epilayer on gallium arsenide substrate using pulsed photothermal reflectance technique

Thermal conductivity of gallium arsenic nitride (GaAsN) epilayer on gallium arsenide (GaAs) substrate prepared by molecular beam epitaxy technique was measured using pulsed photothermal reflectance technique. Three-layer model incorporated thermal boundary resistance was applied to extract the thermal properties from the sample's photothermal response. Within the thickness ranging from 20 to 80 nm, no thickness dependent relationship with thermal conductivity of GaAsN epilayer was found, and the average thermal conductivity is approximately 27 W/mK at room temperature. The thermal boundary resistance at the Au/GaAsN interface is in the order of 10⁻⁸ m²K/W.     

Depth determination of chromophores in human skin by pulsed photothermal radiometry

We report on the application of pulsed photothermal radiometry (PPTR) to determine the depth of in-vitro and in-vivo subsurface chromophores in biological materials. Measurements provided by PPTR in combination with a nonnegative constrained conjugate-gradient algorithm are used to determine the initial temperature distribution in a biological material immediately following pulsed laser irradiation. Within the experimental error, chromophore depths (50-450 μm) in 55 in-vitro collagen phantoms determined by PPTR and optical low-coherence reflectometry are equivalent. The depths of port-wine-stain blood vessels determined by PPTR correlate very well with their locations found by computer-assisted microscopic observation of histologic sections. The mean blood-vessel depth deduced from PPTR and histologic observation is statistically indistinguishable (p > 0.94).     

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.     

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.     

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.     

Evaluation of threshold color differences using printed samples

The performances of uniform color spaces and color-difference formulae for predicting threshold color differences were investigated based on visual assessments of 893 pairs of printed color patches under a D65 source. The average ΔE(ab,10)* of the pairs was 1.1 units. A threshold psychophysical experiment was repeated three times by a panel of 16 observers with normal color vision. The experimental data were used to evaluate nine color-difference formulae and uniform color spaces using the standardized residual sum of squares (STRESS) measure. The results indicated that all formulae and spaces performed very similarly to each other, and outperformed CIELAB for threshold color differences. The chromaticity-discrimination ellipses were used to compare with previous results from small color differences [Color Res. Appl. (2011), doi:10.1002/col.20689], and they agreed with each other, except for the purple color center.     

Measurements of thermal diffusivity of boron-silicon film-on-glass structure using phase detection method of photothermal deflection spectroscopy

The phase detection method of photothermal deflection spectroscopy in the transverse configuration was used to measure the overall thermal diffusivity of silicon-boron (Si-B) alloy film on Corning 7059 glass substrate. Results were attained by observing the phase of deflection of the probe beam when it scanned above the film surface relative to the pump beam. Measurements were repeated for different modulation frequencies of the pump beam. Furthermore, both bouncing and skimming configurations were used. The effect of varying the distance between the probe beam and film surface was investigated.     

Non-destructive evaluation of carrier transport properties in CuInS2 and CuInSe2 thin films using photothermal deflection technique

Photothermal deflection technique (PTD) is a non-destructive tool for measuring the temperature distribution in and around a sample, due to various non-radiative decay processes occurring within the material. This tool was used to measure the carrier transport properties of CuInS₂ and CuInSe₂ thin films. Films with thickness < 1 μm were prepared with different Cu/In ratios to vary the electrical properties. The surface recombination velocity was least for Cu-rich films (5 × 10⁵ cm/s for CuInS₂, 1 × 10³ cm/s for CuInSe₂), while stoichiometric films exhibited high mobility (0.6 cm²/V s for CuInS_2, 32 cm²/V s for CuInSe₂) and high minority carrier lifetime (0.35 µs for CuInS₂, 12 µs for CuInSe₂).     

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.     

Optical properties of CdTe thin films studied by photothermal deflection spectroscopy

Photothermal deflection spectroscopy (PDS) was used to analyze CdTe thin films which are to be used to construct solar cells. It was found that CdTe thin films grown on a conducting SnO₂ layer deposited on glass show deviations from a stoichiometric composition due to an excess of tellurium. This non-stoichiometry modifies the absorption spectrum obtained from the PDS measurements.     

Determination of thermal diffusivity of a solid surface by photothermal deflection spectroscopy

The Photothermal Deflection Spectroscopy (PDS) method is applied to the measurement of thermal diffusivity in solid surfaces. Thermal diffusivity of Al₂O₃ and γ- LiAlO₂ ceramic samples at different porosities were measured at room temperature. The values so obtained are compared to those found in the literature.