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We present enhanced truncated-correlation phototothermal coherence tomography (eTC-PCT) for non-invasive three-dimensional imaging of small animals. Tumor detection is reported in a mouse thigh by injecting cancerous cells in the thigh followed by eTC-PCT imaging. Detection of the tumor 3 days after injection may lead to potential for using the eTC-PCT method for cancer treatment studies. eTC-PCT was also applied successfully to non-invasive in-vivo mouse brain structural imaging. A unique spatial-gradient-gate adaptive filter was introduced in a scanned mode along the (x,y) coordinates of camera images from different sub-cranial depths, revealing absorber true spatial extent from diffusive photothermal images and restoring pre-diffusion lateral image resolution beyond the Rayleigh criterion limit in diffusion-wave imaging science. The spatial resolution and contrast enhancement demonstrated in photothermal in-vivo and ex-vivo images of the mouse brain revealed not only vascular structures but also other brain structures, such as the brain hemispheres, cerebellum, and olfactory lobes.

Since the mathematical foundations of photothermal effects in condensed phase materials were established in the middle 1970s, new photothermal methodologies have emerged based on the detection of heat generated from the absorption and non-radiative conversion of modulated radiation. Among them, photopyroelectric techniques have become very popular and widely used for a variety of studies ranging from calorimetry (photothermal calorimetry), phase transitions and the measurement of thermophysical (thermal diffusivity, effusivity and conductivity) and optical properties (photopyroelectric spectroscopy). More recently photopyroelectric detection has been used for the direct quantification of the optical absorption coefficient of condensed phase materials and has demonstrated its potential for quantitative analysis. In this work, a survey of various applications of photopyroelectric detection for the analysis of fluids and liquid mixtures is presented with special focus on theoretical and experimental progress in quantitative analytical measurements as new developments in this field.

Air pollutants have severe impact on the global environment and the health of human beings. There is an urgent need for cost-effective devices for trace gas monitoring in ambient conditions. However, water vapor in ambient air is still an obstacle in the trace gas absorption detection field due to its complex and strong infrared absorbing characteristics. In this work, a step-scan Fourier-transform infrared differential photoacoustic spectroscopy (FTIR-DPAS) methodology developed in our laboratory through the introduction of two identical T-resonators for enhancing and resolving the DPA signal from two potentially pollutant gases is extended to multiple ambient gas components: carbon dioxide (CO2) and acetylene (C2H2). A key feature of this technique is the ability to resolve hidden target spectral components in ambient air: Despite the fact that the acetylene absorption peaks lie within the strong water vapor absorption band, the infrared PA absorption spectra of acetylene and carbon dioxide are detected with high sensitivity and selectivity in the presence of significant interference of water vapor in the laboratory ambient air, thereby confirming the superiority and capability of step-scan FTIR-DPAS configuration to effectively and totally suppress often dominant background water signals and simultaneously detect multiple trace gases.

Determining and keeping track of a material’s mechanical performance is very important for safety in the aerospace industry. The mechanical strength of alloy materials is precisely quantified in terms of its stress–strain relation. It has been proven that frequency-domain photothermoacoustic (FD-PTA) techniques are effective methods for characterizing the stress–strain relation of metallic alloys. PTA methodologies include photothermal (PT) diffusion and laser thermoelastic photoacoustic ultrasound (PAUS) generation which must be separately discussed because the relevant frequency ranges and signal detection principles are widely different. In this paper, a detailed theoretical analysis of the connection between thermoelastic parameters and stress/strain tensor is presented with respect to FD-PTA nondestructive testing. Based on the theoretical model, a finite element method (FEM) was further implemented to simulate the PT and PAUS signals at very different frequency ranges as an important analysis tool of experimental data. The change in the stress–strain relation has an impact on both thermal and elastic properties, verified by FEM and results/signals from both PT and PAUS experiments.

Colloidal quantum dots (CQDs) have attracted significant interest for applications in electronic and optoelectronic devices such as photodetectors, light emitting diodes, and solar cells. However, a poor understanding of charge transport in these nanocrystalline films hinders their practical applications. The photocarrier radiometry (PCR) technique, a frequency-domain photoluminescence method spectrally gated for radiative recombination photon emissions and exclusion of thermal infrared photons, has been applied to a coupled PbS CQD thin film with inter-dot spacing of 0.5 nm to 1 nm for the analysis of charge transport properties. As the nanoparticle bandgap depends on the size of the quantum dots, polydispersity of the CQD thin film causes bandgap variability leading to photoexcited carrier (exciton) decay lifetime broadening and temperature dependence. The carrier transport mechanisms of QDs are quite different from bulk semiconductors, so the conventional carrier-diffusion wave-based PCR theory was modified into a non-diffusive limit model. A developed variational discrete lifetime reconstruction approach was used to analyze PCR frequency scans under two optical excitation modes: a modulated laser source without, and with, an additional continuous laser source. Using this model, the CQD mean lifetime values were found and variational discrete lifetime spectra were reconstructed.

Photocarrier radiometry (PCR) with 355 nm laser excitation was used for the study of c-Si covered with intrinsic thin hydrogenated amorphous Si (i-a-Si:H) on one, or both, sides, with thicknesses ranging from 10 nm to 90 nm. Short wave- length excitation allows one to resolve the contribution of the upper i-a-S layer to the PCR signal due to the very small absorption depth (tens of nm) of the excitation beam. As a result, fundamental transport parameters of the composite structure can be evaluated from the PCR frequency dependence. A theoretical model has been devel- oped to describe the diffuse carrier density wave (CDW) in this two-layer system. The model of the one-dimensional CDW fields for composite electronic solids involves front, interface, and back surface recombination velocities, the diffusion coefficient, recombinationlifetimesintheupper andlowerlayers,andtheunoccupied trapdensity at the interface. Simulations of the transport parameter influence on the PCR signal were performed, and the theoretical model was able to describe the experimental data accurately, therefore, making it possible to evaluate the transport parameters of i-a- Si:H and c-Si as well as to elucidate the role of interface electronic traps in the PCR frequency dependence under short wavelength excitation.

The evaluation of the stress–strain state of metallic materials is an important problem in the field of non-destructive testing (NDT). Prolonged cyclic loading or overloading will lead to permanent changes of material strength in an inconspicuous manner that poses threat to the safety of structures, components and products. This research focuses on gauging the mechanical strength of metallic alloys through the application of frequency-domain laser ultrasound (FDLU) based on a continuous-wave diode laser source. The goal is to develop industrial NDT procedures for fatigue monitoring in metallic substrates and coatings so that the technique can be used for mechanical strength assessment. A small-scale, non-commercial rig was fabricated to hold the sample and conduct tensile FDLU testing in parallel with an adhesive strain gauge affixed on the tested sample for independent measurement of the applied stress. Harmonic modulation and lock-in detection were used to investigate the LU signal sensitivity to the stress–strain state of ordinary aluminum alloy samples. A 1 MHz focused piezoelectric transducer was used to detect the LU signal. During the tensile procedure, both amplitude and phase signals exhibited good repeatability and sensitivity to the increasing stress–strain within the elastic regime. Signals beyond the elastic limit also revealed significant change patterns.

A combined theoretical and experimental approach is developed for determining the local efficiency of multicrystalline silicon solar cells by use of quantitative lock-in carrierographic (LIC, also described as frequency-domain photoluminescence, LIP) and lock-in thermographic (LIT) imaging. The characteristic distribution of the saturation current density $$J_{0}$$ , open-circuit voltage $$V_{\mathrm{OC}}$$ , fill factor FF, and efficiency of solar cell $$\eta _{\mathrm{s}}$$ are obtained, and the local inhomogeneities of a solar cell are readily determined by LIT and LIC imaging. The combination of a quantitative LIT and LIC imaging approach is a valuable tool for judging which local defects are especially harmful for degrading the fill factor or the open-circuit voltage.

The purpose of this paper is to show and discuss the effects of the gold nanoparticle (Au-NPs) concentration inside a tissue phantom using a combined system of photoacoustics (PA) and optical beam deflection and their applications particularly to photoacoustic imaging. It was found that the PA signal from aggregated Au nanoparticles is significantly enhanced. The stock concentration of 100 nm Au-NPs was $$3.8\times 10^{9}$$ particles/mL from which three samples with 30 %, 70 %, and 90 % concentration were prepared using polyvinyl chloride-plastisol. Each sample was then irradiated across a line scan using a 10 ns pulsed Q-switched Nd:YAG laser at a 1 Hz repetition rate and $$5~\hbox {W}\cdot \hbox {cm}^{-2}$$ so that no physical ablation was observed. The corresponding photoacoustic pressure was found to approximately cover a range between 10 kPa and 51 kPa. This corresponds to approximately 130 pJ to 315 pJ of acoustic energy radiated by Au-NPs into the tissue. The maximal efficacy of the transformation of optical energy into thermal energy was $${\sim }29~\%$$ . Time-resolved photoacoustic deflection was also used to monitor the laser-interaction process. The results clearly indicated that (i) the photoacoustic signal amplitude varies in a given sample as a result of the non-uniform concentration distribution of embedded Au-NPs; (ii) an increase of the concentration increased the signal amplitude linearly; and (iii) at higher nanoparticle concentrations, the probe deflection was found to increase due to a steeper thermoelastic gradient as a result of a higher absorption by particle agglomerates and particle size-dependent dispersions.

A depth profiling theory for electronic transport properties (carrier diffusivity and lifetime) in ion-implanted semiconductor wafers using infrared photocarrier radiometry (PCR) is proposed. The ion-implanted inhomogeneous sample was sliced into many virtual sub-layers along the depth direction so that the continuously variable electronic properties across the whole thickness can be considered as uniform in each incremental slice. A recursion relationship among the slices was obtained, and the overall PCR signal was built based on contributions from each slice. Experimental lifetime and electronic diffusivity reconstructions of depth profiles at two ion doses (\(3\times 10^{14 }\,\mathrm{cm}^{-2 }{\text { and }}3\times 10^{15}\,\mathrm{cm}^{-2})\) and several implantation energies (from 0.75 MeV to 2.0 MeV) have been demonstrated.

In the field of medical diagnostics, biomedical photoacoustics (PA) is a non-invasive hybrid optical-ultrasonic imaging modality. Due to the unique hybrid capability of optical and acoustic imaging, PA imaging has risen to the frontiers of medical diagnostic procedures such as human breast cancer detection. While conventional PA imaging has been mainly carried out by a high-power pulsed laser, an alternative technology, the frequency domain biophotoacoustic radar (FD-PAR) is under intensive development. It utilizes a continuous wave optical source with the laser intensity modulated by a frequency-swept waveform for acoustic wave generation. The small amplitude of the generated acoustic wave is significantly compensated by increased signal-to-noise ratio (several orders of magnitude) using matched-filter and pulse compression correlation processing in a manner similar to radar systems. The current study introduces the theory of a novel FD-PAR modality for ultra-sensitive characterization of functional information for breast cancer imaging. The newly developed theory of wavelength-modulated differential PA spectroscopy (WM-DPAS) detection has been introduced to address angiogenesis and hypoxia monitoring, two well-known benchmarks of breast tumor formation. Based on the WM-DPAS theory, this modality efficiently suppresses background absorptions and is expected to detect very small changes in total hemoglobin concentration and oxygenation levels, thereby identifying pre-malignant tumors before they are anatomically apparent. An experimental system design for the WM-DPAS is presented and preliminary single-ended laser experimental results were obtained and compared to a limiting case of the developed theoretical formalism.

Ultrasound (US) backscatter from bones depends on the mechanical properties and the microstructure of the interrogated bone. On the other hand, photoacoustics (PA) is sensitive to optical properties of tissue and can detect composition variation. Therefore, PA can provide complementary information about bone health and integrity. In this work, a comparative study of US backscattering and PA back-propagating signals from animal trabecular bones was performed. Both methods were applied using a linear frequency modulation chirp and matched filtering. A 2.2 MHz ultrasonic transducer was employed to detect both signals. The use of the frequency domain facilitates spectral analysis. The variation of signals shows that in addition to sensitivity to mineral changes, PA exhibits sensitivity to changes in the organic part of the bone. It is, therefore, concluded that the combination of both modalities can provide complementary detailed information on bone health than either method separately. In addition, comparison of PA and US depthwise images shows the higher penetration of US. Surface scan images exhibit very weak correlation between US and PA which could be caused by the different signal generation origins in mechanical versus optical properties, respectively.

Carrierographic (spectrally gated photoluminescence) imaging of a crystalline silicon wafer using an InGaAs camera and two spread super-bandgap illumination laser beams is introduced in both low-frequency lock-in and high-frequency heterodyne modes. Lock-in carrierographic images of the wafer up to 400 Hz modulation frequency are presented. To overcome the frame rate and exposure time limitations of the camera, a heterodyne method is employed for high-frequency carrierographic imaging which results in high-resolution near-subsurface information. The feasibility of the method is guaranteed by the typical superlinearity behavior of photoluminescence, which allows one to construct a slow enough beat frequency component from nonlinear mixing of two high frequencies. Intensity-scan measurements were carried out with a conventional single-element InGaAs detector photocarrier radiometry system, and the nonlinearity exponent of the wafer was found to be around 1.7. Heterodyne images of the wafer up to 4 kHz have been obtained and qualitatively analyzed. With the help of the complementary lock-in and heterodyne modes, camera-based carrierographic imaging in a wide frequency range has been realized for fundamental research and industrial applications toward in-line nondestructive testing of semiconductor materials and devices.

A theoretical model for characterizing two-layered semi-cylindrical solid samples illuminated with a modulated incident beam on a curved surface using the Green function method is developed. The analytical expression for the thermal-wave (TW) field in such a semi-cylindrical sample is presented. Based on the model, characteristics of the TW field are simulated with different parameters, and comparisons with the whole cylinder model are conducted. It is found that TW properties of semi- and full cylinders show significant differences, especially in the low frequency range. Preliminary experimental validation is also presented with semi-cylindrical samples. The model provides a theoretical basis to quantitatively characterize semi-cylindrical samples using the TW method in a noncontact and nondestructive way.

This article presents an instrumentation and modality-based study of the salient differences between pulsed and frequency-domain photoacoustic (PA) signals with respect to signal processing and imaging properties. The parameters of both modalities were adjusted to achieve the maximum signal-to-noise ratio (SNR). Experiments were performed by employing a dual-mode PA system and under the condition of maximum permissible exposure for both modalities. In addition, proper filtering has been applied post-processing to enhance the image quality for both methods. Theoretical estimates versus practical issues are discussed. In conclusion, it has been shown that parameters of these two methods can be adjusted to provide a competitive SNR and resolution.

Equivalence of the normalized thermal-wave fields between curved and flat surfaces under certain conditions is investigated based on theoretical models of cylindrical, spherical, and flat solids with multilayer structures. The principle and the physical mechanism of the elimination of the surface curvature effect from the overall photothermal signal of the curvilinear solid are suggested. The effects of the relative values of radii of curvature of the curvilinear solid, the thickness of the inhomogeneous surface layer, and the measurement azimuthal angle on the validity of the equivalence principle are discussed. Consistent experimental reconstructions of thermophysical depth profiles of hardened cylindrical steel rods of various diameters are performed and obtained based on both the curvilinear theory and the equivalent flat surface theory.

The constraints imposed by the laser safety (maximum permissible exposure) ceiling on pump laser energy and the strong attenuation of thermal-wave signals in tissues significantly limit the photothermally active depth in most biological specimens to a level which is normally insufficient for practical applications (a few mm below the skin surface). A theoretical approach for improvement of the signal-to-noise ratio (SNR), minimizing the static (dc) component of the photothermal (PT) signal and making use of the PT radiometric nonlinearity has been introduced. At low frequencies fixed-pulse-width chirps of large peak power were found to be superior to all other equal energy modalities, with an SNR improvement by up to two orders of magnitude. Compared to radar peak delay and amplitude, the long-delayed radar output amplitude is found to be more sensitive to subsurface conditions. Two-dimensional spatial plots of this parameter depicting the back-surface conditions of bones with and without fat tissue overlayers are presented. Pulsed-chirp radar thermography has been demonstrated to monitor the degree of demineralization in goat rib bone with a substantial SNR and spatial resolution that is not practicable with harmonic radars of the same energy density.

In this study, a theoretical model is established for a wedge-like solid with an open sector surrounded by walls of radius $$R$$ of a cylindrical rod illuminated by a modulated circular Gaussian incident beam by means of the Green’s function method in cylindrical coordinates. An analytical expression for the thermal-wave field in such a sample is presented. The theory is validated by reducing the arbitrary geometrical structure of the wedge to simpler geometries. It is shown that the frequency dependence of the thermal-wave field near the edge exhibits a large phase lag compared with that at a location far from the edge. The theory provides a foundation for quantitatively characterizing wedge-shaped industrial samples, such as metals with sintered edges, using photothermal methods in a non-contact and non-destructive manner.

Solution-processed colloidal quantum dots (CQDs) are promising materials for realizing low-cost, large-area, and flexible photovoltaic devices. The study of charge carrier transport in quantum dot solids is essential for understanding energy conversion mechanisms. Recently, solution-processed two-layer oleic-acid-capped PbS CQD solar cells with one layer treated with tetrabutylammonium iodide (TBAI) serving as the main light-absorbing layer and the other treated with 1,2-ethanedithiol (EDT) acting as an electron-blocking/hole-extraction layer were reported. These solar cells demonstrated a significant improvement in power conversion efficiency of 8.55% and long-term air stability. Coupled with photocarrier radiometry measurements, this work used a new trap-state mediated exciton hopping transport model, specifically for CQD thin films, to unveil and quantify exciton transport mechanisms through the extraction of hopping transport parameters including exciton lifetimes, hopping diffusivity, exciton detrapping time, and trap-state density. It is shown that PbS-TBAI has higher trap-state density than PbS-EDT that results in higher PbS-EDT exciton lifetimes. Hopping diffusivities of both CQD thin film types show similar temperature dependence, particularly higher temperatures yield higher hopping diffusivity. The higher diffusivity of PbS-TBAI compared with PbS-EDT indicates that PbS-TBAI is a much better photovoltaic material than PbS-EDT. Furthermore, PCR temperature spectra and deep-level photothermal spectroscopy provided additional insights to CQD surface trap states: PbS-TBAI thin films exhibit a single dominant trap level, while PbS-EDT films with lower trap-state densities show multiple trap levels.

A step-scan differential Fourier transform infrared photoacoustic spectroscopy (DFTIR-PAS) using a commercial FTIR spectrometer was developed theoretically and experimentally for air contaminant monitoring. The configuration comprises two identical, small-size and low-resonance-frequency T cells satisfying the conflicting requirements of low chopping frequency and limited space in the sample compartment. Carbon dioxide (CO2) IR absorption spectra were used to demonstrate the capability of the DFTIR-PAS method to detect ambient pollutants. A linear amplitude response to CO2 concentrations from 100 to 10,000 ppmv was observed, leading to a theoretical detection limit of 2 ppmv. The differential mode was able to suppress the coherent noise, thereby imparting the DFTIR-PAS method with a better signal-to-noise ratio and lower theoretical detection limit than the single mode. The results indicate that it is possible to use step-scan DFTIR-PAS with T cells as a quantitative method for high sensitivity analysis of ambient contaminants.

This paper presents two methods for photoacoustic signal enhancement in biological tissues. One such method is based on the fact that temperature can affect the signals of the photoacoustic radar. Therefore, thermally assisted methods have been used for photoacoustic imaging contrast improvement. Another method is based on harmonic wavelength modulation which results in a differential PA radar signal to strengthen early cancer detection. Two chirped waveforms modulated out-of-phase between 680 nm and 800 nm can effectively suppress the background noise, greatly enhance the SNR and detect small variations in hemoglobin oxygenation levels, thereby distinguishing pre-malignant tumors. Experimental results demonstrate the accuracy of the frequency-modulated differential measurement with sheep blood at different hemoglobin oxygenation (S\(_\mathrm{t}\)O\(_{2})\) levels.

Air pollutants have adverse effects on the Earth’s climate system. There is an urgent need for cost-effective devices capable of recognizing and detecting various ambient pollutants. An FTIR photoacoustic spectroscopy (FTIR-PAS) method based on a commercial FTIR spectrometer developed for air contamination monitoring will be presented. A resonant T-cell was determined to be the most appropriate resonator in view of the low-frequency requirement and space limitations in the sample compartment. Step-scan FTIR-PAS theory for regular cylinder resonator has been described as a reference for prediction of T-cell vibration principles. Both simulated amplitude and phase responses of the T-cell show good agreement with measurement data Carbon dioxide IR absorption spectra were used to demonstrate the capacity of the FTIR-PAS method to detect ambient pollutants. The theoretical detection limit for carbon dioxide was found to be 4 ppmv. A linear response to carbon dioxide concentration was found in the range from 2500 ppmv to 5000 ppmv. The results indicate that it is possible to use step-scan FTIR-PAS with a T-cell as a quantitative method for analysis of ambient contaminants.

Colloidal quantum dots (CQDs) have attracted significant interest for applications in electronic and optoelectronic devices such as photodetectors, light-emitting diodes, and solar cells. However, a poor understanding of charge transport in these nanocrystalline films hinders their practical applications. The photocarrier radiometry (PCR) technique, a frequency-domain photoluminescence method spectrally gated for monitoring radiative recombination photon emissions while excluding thermal infrared photons due to non-radiative recombination, has been applied to PbS CQD thin films for the analysis of charge transport properties. Linear excitation intensity responses of PCR signals were found in the reported experimental conditions. The type and influence of trap states in the coupled PbS CQD thin film were analyzed with PCR temperature- and time-dependent results.

InGaAs camera-based low-frequency homodyne and high-frequency heterodyne lock-in carrierographies (LIC) are introduced for spatially resolved imaging of optoelectronic properties of Si solar cells. Based on the full theory of solar cell photocarrier radiometry (PCR), several simplification steps were performed aiming at the open circuit case, and a concise expression of the base minority carrier density depth profile was obtained. The model shows that solar cell PCR/LIC signals are mainly sensitive to the base minority carrier lifetime. Both homodyne and heterodyne frequency response data at selected locations on a mc-Si solar cell were used to extract the local base minority carrier lifetimes by best fitting local experimental data to theory.

Frequency-domain photoacoustic radar (FD-PAR) imaging of absorbers in turbid media and their comparison and/or validation as well as co-registration with their corresponding ultrasound (US) images are demonstrated in this paper. Also presented are the FD-PAR tomography and the effects of reducing the number of scan lines (or angles) on image quality, resolution, and contrast. The FD-PAR modality uses intensity-modulated (coded) continuous wave laser sources driven by frequency-swept (chirp) waveforms. The spatial cross-correlation function between the PA response and the reference signal used for laser source modulation produces the reconstructed image. Live animal testing is demonstrated, and images of comparable signal-to-noise ratio, contrast, and spatial resolution were obtained. Various image improvement techniques to further reduce absorber spread and artifacts in the images such as normalization, filtering, and amplification were also investigated. The co-registered image produced from the combined US and PA images provides more information than both images independently. The significance of this work lies in the fact that achieving PA imaging functionality on a commercial ultrasound instrument could accelerate its clinical acceptance and use. This work is aimed at functional PA imaging of small animals in vivo.

A novel non-destructive, non-contact laser-induced near-infrared imaging technique using an InGaAs camera and based on dynamic carrierography (a spectrally gated modulated photoluminescence modality) was used in direct lock-in (LICG) and heterodyne (HDCG) modes to characterize industrial multicrystalline silicon solar cells. The image amplitude depends on the free-photocarrier diffusion-wave density, the spatial resolution is a function of modulation frequency, and the contrast is due to the spatial distribution of important parameters influencing the efficiency of the solar cell, such as base recombination lifetime. High-spatial-resolution and high-frequency carrierographic images have been obtained using a HDCG method. The relationship between the LICG amplitude and the solar cell terminal photovoltage under an external load was investigated. The influence of surface recombination velocities and damage of the solar cell p–n junction was studied. A correlation between the solar cell efficiency and the surface-integrated LICG amplitude from the entire solar cell is presented.

The ability of frequency-domain photothermal radiometry (PTR) and modulated luminescence (LUM) to detect secondary caries is presented. Signal behavior upon sequential demineralization and remineralization of a spot (diameter ~1 mm) on a vertical wall of sectioned tooth samples was investigated experimentally. From these studies, it was found that PTR–LUM signals change, showing a certain pattern upon progressive demineralization and remineralization. PTR amplitudes slightly decreased upon progressive demineralization and slightly increased upon subsequent remineralization. The PTR phase increased during both demineralization and remineralization. LUM amplitudes exhibit a decreasing trend at excitation/probe distances larger than 200 μm away from the edge for both demineralization and remineralization; however, at locations close to the edge (up to ~200 μm), LUM signals slightly decrease upon demineralization and slightly increase during subsequent remineralization.

A theoretical model for evaluating solid multilayered spherical solids heated by a frequency-modulated light beam using the Green function method is presented. The specific thermal-wave Green function corresponding to the composite structure has been derived. The characteristics of the thermal-wave field with respect to the thermophysical, geometrical, and measurement parameters are presented. Unlike the quadrupole method, the Green function method is capable of evaluating thermal-wave fields at any point of multilayered structures with arbitrary intensity distributions of the incident laser beams. This study establishes applications of thermal-wave fields in both cylindrical and spherical samples using the Green function method and is of importance in characterizing radially inhomogeneous spherical solids.

Energy transport in diffusion-wave fields is gradient driven and therefore diffuse, yielding depth-integrated responses with poor axial resolution. Using matched filter principles, a methodology is proposed enabling these parabolic diffusion-wave energy fields to exhibit energy localization akin to propagating hyperbolic wave fields. This not only improves the axial resolution, but also allows for deconvolution of individual responses of superposed axially discrete sources, opening a new field of depth-resolved subsurface thermal coherence tomography using diffusion waves. The depth-resolved nature of the developed methodology is verified through experiments carried out on phantoms and biological samples. The results suggest that thermal coherence tomography can resolve deep structural changes in hard dental and bone tissues, allowing for remote detection of early dental caries and potentially early osteoporosis.

In this study, the imaging capability of our wide-spectrum frequency-domain photoacoustic (FD-PA) imaging alias “photoacoustic radar” methodology for imaging of soft tissues is explored. A practical application of the mathematical correlation processing method with relatively long (1 ms) frequency-modulated optical excitation is demonstrated for reconstruction of the spatial location of the PA sources. Image comparison with ultrasound (US) modality was investigated to see the complementarity between the two techniques. The obtained results with a phased array probe on tissue phantoms and their comparison to US images demonstrated that the FD-PA technique has strong potential for deep subsurface imaging with excellent contrast and high signal-to-noise ratio. FD-PA images of blood vessels in a human wrist and an in vivo subcutaneous tumor in a rat model are presented. As in other imaging modalities, the employment of contrast agents is desirable to improve the capability of medical diagnostics. Therefore, this study also evaluated and characterized the use of Food and Drug Administration (FDA)-approved superparamagnetic iron oxide nanoparticles (SPION) as PA contrast agents.

Industrial n-type Si wafers were H+-ion-implanted and the electronic transport properties were studied using photo-carrier radiometry (PCR). A fitting procedure was introduced using a relatively simple 2-layer PCR model in lieu of the more realistic but substantially more complicated 3-layer model. It was found that the 2-layer model provides an optimal tool for characterizing H+ ion implants, balancing accuracy, complexity and validity as compared to the simpler, but inaccurate, 1-layer model.

We extend the applications of photothermal diagnostics to solid spheres, in which both theoretical and experimental PTR studies on spherical geometries and thermal diffusivity of the sample are discussed. Based on the Green function method, a full thermal-wave field distribution of a spherical solid is obtained. Experimental results with steel spheres of different diameters exhibit good agreement between the theory and the experiments.

The subject of this research was the dependence of the infrared photocarrier radiometric (PCR) signal on the intensity of the exciting super-bandgap laser beam. It has been shown that the amplitude of the PCR signal is proportional to the intensity to a power β, such that 1≤β≤2. The power dependence of the amplitude is an important indicator of the photoexcited carrier recombination physics, specifically in semiconductors ranging between monopolar (β = 1) and bipolar (β = 2) limits. The study was made with laser beams of varying power and spotsize and wafers with different transport parameters. It has been found that the conventional approach using β = 1 is inadequate and inconsistent with experimental slopes of amplitude vs. power.

Non-contact, non-intrusive Photo-Carrier Radiometry (PCR) was used for monitoring nano-depth junctions in industrial-grade silicon wafers. The silicon wafers were implanted with arsenic to the dose of 5E10 14 cm -2 . The junction depth was in the 30 nm to 100 nm range. Quantitative results for PCR sensitivity to the junction depth and implantation energies are presented. This laser-based carrier-wave technique monitors harmonically photo-excited and recombining carriers and shows great potential advantages for the characterization of multiple semiconductor processes such as ion implantation, ultra shallow junction (USJ) depth determination and other Si wafer process steps.

The evaluation of the hardness case depth and the measurement of the thermophysical properties of cylindrical C1018 heat-treated steel rods has been demonstrated. By using a cylindrical two-layer model and a multi-parameter fitting algorithm, the effective case depth and the corresponding thermal conductivity and thermal diffusivity of the hardened layer are obtained for a nominal 0.04′′ case depth hardening. The measurement results from different samples in the same batch of heat treated cylinders are consistent with each other and are also consistent with mechanical indentater test results in the sense of yielding an effective case depth within the discrete two-layer thermal-wave approximation.

Photoacoustic (or more precisely, photothermoacoustic) signals are generated by the absorption of photons, expressed as acoustic waves, and can be related to the incident laser fluence rate. Here a calibration procedure for frequency domain (FD) photoacoustics is presented which utilizes the relationship between photoacoustic (PA) signal amplitude and laser fluence rate. The sample is irradiated with a continuous wave laser which is amplitude modulated at varying frequencies. Applying the calibration procedure to the PA detected signals, a linear relationship is obtained between the fluence rate and the signal amplitude.

The dimensionality of the photopyroelectric signal and the importance of radiation heat transfer mechanisms in a thermal-wave cavity (TWC) are closely investigated in the current study. Radiation components were added to the purely conduction model by linearizing the radiation heat transfer at the brass film-air boundary. A 3-D theoretical approach based on the Hankel integral was developed and implemented. Experimental results indicate that a 3-D model is necessary to describe the PPE signal, especially at low frequencies. Radiation is found to be the major contributor of the photopyroelectric (PPE) signal when high frequencies and large cavity lengths are used.

Recent trends in biothermophotonics of teeth are presented. The presentation is centered on the development of clinical-level frequency-domain photothermal radiometry and modulated luminescence to address issues associated with the early diagnosis of demineralization caries in human teeth. Biothermophotonic principles and applications to the detection of the carious state in human teeth as embodied by laser photothermal radiometry are presented and further supported by modulated luminescence. The emphasis is on recent developments with regard to abilities of these techniques to diagnose interproximal lesions between teeth, etching with phosphoric acid and with an artificial demineralization gel in order to simulate early demineralization, as well as demineralization and remineralization of dental crown enamel and root dentin. These are lesions which normally go undetected by X-ray radiographs. Comparisons with X rays, Micro-Computed Tomography (μ-CT) and Transverse Micro-Radiography (TMR) are discussed. A theoretical model involving coupled diffuse photon density and thermal-wave fields is developed and applied to frequency scans from demineralized artificial lesions to produce quantitative values for optical and thermophysical parameters of teeth as well as the thickness of the induced lesion.

The effect of the laser source spotsize on the measurement sensitivity of layered system using photothermal radiometry (PTR) technique is presented. The beam size effect on the PTR measurement sensitivity is theoretically examined and experimentally demonstrated using a carbo-nitrided C1018 steel sample. The experimental results using a focused beam and an expanded beam for a carbo-nitrided C1018 sample validated the theoretical prediction, according to which the expanded beam showed a much larger magnitude change (contrast) in both amplitude and phase than results with a focused beam.

A quantitative calibrated methodology based on photothermal radiometric (PTR) depth-profilometry for non-contact, non-intrusive determination of effective case depth in heat treated case-hardened steel products was developed. Several types of heat-treated C1018 industrial steel screw products (with hexa- gonal, cylindrical and spherical heads) were statistically evaluated using the case-depth – induced interferometric thermal-wave phase minima. Calibration curves for each type of sample were established with the help of conventional destructive indenter measurements. It is shown that PTR thermal-wave interferometric phase minima can be used as a fast, on-line inspection methodology of industrial steel products for non-destructive quality control of heat treating processes.

Development of a novel photothermoacoustic (PTA) imaging technique utilizing a frequency-modulated (chirped) optical excitation and Fourier-domain methodology for depth-selective imaging of tissue chromophores is presented. Use of frequency-domain signal detection rather than short-pulse time-resolved measurements of pressure transients give an advantage of higher SNR typical to coherent detection techniques. Additionally, we introduce chirped optical excitation to generate linear frequency modulated PTA response which enables unambiguous and precise depth measurements of tissue chromophores. In order to obtain depth profilometric information from the frequency-domain PTA (FD-PTA) measurements, we describe implementation of two signal processing algorithms: matched filter compression and heterodyne mixing with coherent detection. We show that direct relationship between chromophore depth and spectrum of photothermo acoustic signals can be established to enable depth-selective tissue imaging. Application of amplitude and phase FD-PTA imaging is demonstrated in experiments with light-scattering phantoms and chicken breast tissues containing subsurface inclusions. The potential of the FD-PTA method for noninvasive tissue tomography and molecular imaging is discussed.

A novel optical thermometer based on the principle of laser thermoreflectance has been introduced to monitor the surface temperature of thin coatings on steel parts undergoing an industrial titanium nitride (TiN) alloy deposition process. To study the feasibility of the optical thermometer, various thermo-optical parameters of TiN affected by the deposition process have been investigated; namely, the reflectance-temperature relation, the thermoreflectance coefficient, and the coating thickness dependence of thermoreflectance and of total reflectance. A theory of interferometric thermoreflectance has been introduced to model the total reflectance variations during the coating process. An inverse reflectance-temperature relation for the TiN-D2 steel substrate system has been found and a first-order Taylor series expansion used to model thermoreflectance has been shown to yield a thermoreflectance coefficient which is independent of temperature. Both results are in quantitative agreement with the Drude-Zener theory of conductors and semiconductors. An empirical formula has been derived to effectively model the experimental thermoreflectance coefficient dependence of the TiN-D2 steel system on TiN coating thickness, in qualitative agreement with scattering mechanisms of the Boltzmann transport theory in conductors and semi-conductors.

In this work we introduced two beam photocarrier cross-modulation for creation of an optically driven photonic laser beam modulator using a semiconductor wafer as the active medium. Unlike other laser beam modulators, the process of modulation of an unmodulated sub-bandgap laser beam was made possible by generating a spatially- and free-carrier density-wave-dependent infrared absorption coefficient in the bulk of the semiconductor, following absorption of a collinear super-bandgap modulated laser beam. The experimental results showed that the modulation efficiency strongly depends on the transport parameters of the semiconductor material and on the power of the super-bandgap laser beam.