Photoacoustic Phase Research Articles

Photoacoustic study of elastic deformations in silicon membranes

In an optically excited semiconductor micromechanical structure, photogenerated carriers (plasma waves) can produce elastic deformations (local strains and stresses)–plasmaelastic (PE) deformations. On the other hand, the generation of excess carriers will produce heat due to carrier thermalization and recombination processes. The generated heat can produce other elastic deformations–thermoelastic (TE) deformations. For these two components of elastic deformation, it is possible to consider two types of elastic displacements (two mods of elastic vibrations): elastic expanding and elastic bending. A theoretical model of the photoacoustic (PA) signal for optically excited Si membranes is given, which includes thermal diffusion (TD), TE, and PE mechanisms, in order to study elastic expansion and bending. The relations for the PA amplitude and the phase of elastic expanding and bending in the excited membrane are derived. Analysis of the PA signal indicates that the TD component is dominant for all thicknesses and practically the entire range of observed frequencies. The calculated PA amplitude and phase spectra show that the PA elastic expanding component has a significant influence on the total PA signal at low frequencies. On the other side, the calculated PA spectra show that the PA elastic bending component has a significant influence on the total PA signal at high frequencies. Experimental PA signals of Si membranes were measured using the sample-gas-microphone detection technique with a transmission configuration in relation to the modulation frequency of the optical excitation for different membrane thicknesses. The experimental PA spectra were compared with the theoretical ones.

A multilayer heat and mass transfer model for photoacoustics of aerosol particles (MHM-PA)

A diffusion multi-layer model is introduced which allows time-dependent simulations of heat and mass transfer for photoacoustic experiments on aerosol particles (MHM-PA model). It provides spatial and temporal information on the particle temperature, composition and size during photoacoustic modulation cycles, and thus enables accurate simulations of the photoacoustic amplitude and phase at different modulation frequencies. Simulations for micrometer-sized, mixed water-tetraethylene glycol droplets at different relative humidity and at different modulation frequencies between 4 and 100 kHz reveal a substantial loss of the information content of the photoacoustic signal above ∼30 kHz. The photoacoustic amplitude reduces by two orders of magnitude, causing sensitivity issues. The photoacoustic phase converges to the same constant value for all droplet sizes and humidities, meaning that useful information on heat and mass transfer is essentially lost in the phase.

PHOTOACOUSTIC PHASE ANGLE FOR NONINVASIVE MONITORING OF MICROCIRCULATORY CHANGE IN HUMAN SKIN: A PRELIMINARY INVESTIGATION

Measurement using the currently available tissue oxygen monitoring systems, such as pulse oximeter, is unreliable in patients with compromised microcirculation. Others offer high diagnostic accuracy, but are complicated and expensive in their operation. This paper is motivated to study the use of photoacoustic (PA) phase change as a predictor of skin tissue oxygen levels. This work used EPOCH 650 ultrasonic flaw detector with a longitudinal transducer and a red laser light of wavelength 633 nm for measurement of PA signals. This pilot study was conducted on a group of four human subjects. The pressure waves were collected from their anterior left arm under three experimental conditions, namely at rest, venous and arterial blood flow occlusions, for extraction of hemoglobin absorption dependent phase information.

Simultaneous Fit of the Photoacoustic Signal Amplitude and Phase for Aisi 316 with Fractional Analysis

Simultaneous Fit of the Photoacoustic Signal Amplitude and Phase for Aisi 316 with Fractional Analysis

Analysis of the photoacoustic spectral dispersion in dielectric colloids

The foremost advanced photoacoustic (PA) transport theory, dealing with image formation, relates to target volumes of shallow depths. It assumes the medium as homogeneous with negligible PA amplitude attenuation. Causal changes in the velocity distribution spectrum, as described in Debye’s theory, and related to propagation distance and sample's density, are also neglected. However, these are relevant for imaging targets at larger depths and improving image resolution of PA images of thick biological tissues. Here we introduce some concepts for extending the PA transport model. These are theoretical and experimental considerations for analysing PA attenuation and the significance of spectral dispersion; and in consequence, disclose those conditions at which they should be included as part of the PA transport theory. Departing from the PA Heaviside telegraph equation and causality conditions, we obtain analytic expressions for associated attenuation and dispersion coefficients. As part of the analysis, we propose expressions for the PA group and PA phase velocities, and for the group velocity dispersion parameter; those in analogy with optical fields. In this way we get a refined description for the spectral dispersion. As proof of consistency, the introduced expressions are tested with experimental data extracted from homogeneous colloid samples. The observed performance is compared against already known general acoustic dispersion theory.

Fundamental investigation of photoacoustic signal generation from single aerosol particles at varying relative humidity

Photoacoustic (PA) spectroscopy enjoys widespread applications across atmospheric sciences. However, experimental biases and limitations originating from environmental conditions and particle size distributions are not fully understood. Here, we combine single-particle photoacoustics with modulated Mie scattering to unravel the fundamental physical processes occurring during PA measurements on aerosols. We perform measurements on optically trapped droplets of varying sizes at different relative humidity. Our recently developed technique – photothermal single-particle spectroscopy (PSPS) – enables fundamental investigations of the interplay between the heat flux and mass flux from single aerosol particles. We find that the PA phase is more sensitive to water uptake by aerosol particles than the PA amplitude. We present results from a model of the PA phase, which sheds further light onto the dependence of the PA phase on the mass flux phenomena. The presented work provides fundamental insights into photoacoustic signal generation of aerosol particles.

Intravascular tri-modality system: Combined ultrasound, photoacoustic, and elasticity imaging

We have developed a hybrid intravascular imaging system for the early detection of vulnerable plaque by combining three complementary imaging techniques: ultrasound imaging (USI), photoacoustic imaging (PAI), and photoacoustic elasticity imaging (PEI). As a loading-free elasticity detection method, PEI can quantitatively obtain the tissue elasticity through the photoacoustic phase. By sharing the same ultrasound detection system, USI and PAI could be organically combined. Meanwhile, the optical absorption and elasticity images of plaque are simultaneously reconstructed by the same photoacoustic pulses. To prove the imaging capabilities and complementarity of the system, mimicking phantom experiments were conducted. Furthermore, ex-vivo experiments were performed on the rabbit abdominal aorta to achieve an early assessment of atherosclerosis by detecting the presence and the development extent of lipid plaques. The results illustrate that the system can simultaneously detect the structural composition and the mechanical properties of plaque, thereby showing great potential for accurately diagnosing the atherosclerosis.

Thermal Diffusivity of High-Density Polyethylene Samples of Different Crystallinity Evaluated by Indirect Transmission Photoacoustics

In this work, thermal diffusivity of crystalline high-density polyethylene samples of various thickness, and prepared using different procedures, was evaluated by transmission gas-microphone frequency photoacoustics. The samples’ composition analysis and their degree of crystallinity were determined from the wide-angle X-ray diffraction, which confirmed that high-density polyethylene samples, obtained by slow and fast cooling, were equivalent in composition but with different degrees of crystallinity. Structural analysis, performed by differential scanning calorimetry, demonstrated that all of the used samples had different levels of crystallinity, depending not only on the preparing procedure, but also on sample thickness. Therefore, in order to evaluate the samples’ thermal diffusivity, it was necessary to modify standard photoacoustic fitting procedures (based on the normalization of photoacoustic amplitude and phase characteristics on two thickness levels) for the interpretation of photoacoustic measurements. The calculated values of thermal diffusivity were in the range of the expected literature values. Besides that, the obtained results indicate the unexpected correlation between the values of thermal diffusivity and thermal conductivity with the degree of crystallinity of the investigated geometrically thin samples. The results indicate the necessity of additional investigation of energy transport in macromolecular systems, as well as the possible employment of the photoacoustic techniques in order to clarify its mechanism.

Photoacoustic viscoelasticity imaging dedicated to mechanical characterization of biological tissues

Since changes in mechanical properties of biological tissues are often closely related to pathology, the viscoelastic properties are important physical parameters for medical diagnosis. A photoacoustic (PA) phase-resolved method for noninvasively characterizing the biological tissue viscoelasticity has been proposed by Gao et al. [G. Gao, S. Yang, D. Xing, “Viscoelasticity imaging of biological tissues with phase-resolved photoacoustic measurement,” Opt. Lett. 36, 3341–3343 (2011)]. The mathematical relationship between the PA phase delay and the viscosity–elasticity ratio has been theoretically deduced. Moreover, systems of PA viscoelasticity (PAVE) imaging including PAVE microscopy and PAVE endoscopy were developed, and high-PA-phase contrast images reflecting the tissue viscoelasticity information have been successfully achieved. The PAVE method has been developed in tumor detection, atherosclerosis characterization and related vascular endoscopy. We reviewed the development of the PAVE technique and its applications in biomedical fields. It is believed that PAVE imaging is of great potential in both biomedical applications and clinical studies.

The method of distinguishing the drum from the plasma wave effect in the photoacoustic spectra on the example of CdSe and CdMgSe crystals

This paper presents results of experimental and theoretical studies of the plasma wave and the drum effect in the photoacoustic (PA) spectra of CdSe and CdMgSe samples. The PA experimental amplitude and phase frequency characteristics of these samples have been shown. The experimental results have been interpreted in two different PA models: in the model of a drum effect and in the plasma wave model. The obtained results have been discussed and presented in this paper. They indicate that the values of extracted parameters depend on the model of the PA signal. To distinguish between the plasma and the drum model the aluminium foil test is proposed. From the results obtained one can conclude that the plasma model used for interpretation of the PA characteristics yields overestimated values of the thermal diffusivity and poor PA phase fittings. The results obtained using the drum model agree well with other measurements conducted on the investigated samples.

Electro-acustic influence of the measuring system on the photoacoustic signal amplitude and phase in frequency domain

The paper discusses the most common impacts of the measuring system on the amplitude and phase of the photoacoustic signals in the frequency domain using the open-cell experimental set-up. The highest signal distortions are detected at the ends of the observed modulation frequency range from 20 Hz to 20 kHz. The attenuation of the signal is observed at lower frequencies, caused by the electronic filtering of the microphone and sound card, with characteristic frequencies of 15 Hz and 25 Hz. At higher frequencies, the dominant signal distortions are caused by the microphone acoustic filtering, having characteristic frequencies around 9 kHz and 15 kHz. It has been found that the microphone incoherent noise, the so called flicker noise, is negligibly small in comparison to the signal and does not affect the signal shape. However, a coherent noise originating from the power modulation system of the light source significantly affects the shape of the signal in the range greater than 10 kHz. The effects of the coherent noise and measuring system influence are eliminated completely using the relevant signal correction procedure targeting the photoacoustic signal generated by the sample.

Depth Profiles in Maize (Zea mays L.) Seeds Studied by Photoacoustic Spectroscopy

Photoacoustic spectroscopy (PAS) has been used to analyze agricultural seeds and can be applied to the study of seed depth profiles of these complex samples composed of different structures. The sample depth profile can be obtained through the photoacoustic (PA) signal, amplitude, and phase at different light modulation frequencies. The PA signal phase is more sensitive to changes of thermal properties in layered samples than the PA signal amplitude. Hence, the PA signal phase can also be used to characterize layers at different depths. Thus, the objective of the present study was to obtain the optical absorption spectra of maize seeds (Zea mays L.) by means of PAS at different light modulation frequencies (17 Hz, 30 Hz, and 50 Hz) and comparing these spectra with the ones obtained from the phase-resolved method in order to separate the optical absorption spectra of seed pericarp and endosperm. The results suggest the possibility of using the phase-resolved method to obtain optical absorption spectra of different seed structures, at different depths, without damaging the seed. Thus, PAS could be a nondestructive method for characterization of agricultural seeds and thus improve quality control in the food industry.

Determination of Doping Density in GaAs Semiconductor by Wavelength-Dependent Photoacoustic Spectroscopy

The wavelength dependence of the photoacoustic signal for n-type GaAs semiconductors in the region of the band-gap energies was investigated. The significant changes in the phase and amplitude of the photoacoustic signal near the band-gap absorption wavelengths were observed to occur when the Si-doping densities in GaAs were varied. Particularly, the first derivatives of the photoacoustic phase vs. wavelength graphs were evaluated and fitted with single Gaussian functions. The peak centers and the widths of the Gaussian curves clearly showed linear relationships with the log values of the Si-doping densities in n-type GaAs semiconductors. It is proposed that the wavelength-dependent PA spectroscopy can be used as a simple and nondestructive method for measuring the doping densities in bulk semiconductors.

PHOTOACOUSTIC VISCOELASTICITY IMAGING OF BIOLOGICAL TISSUES WITH INTENSITY-MODULATED CONTINUOUS-WAVE LASER

In this paper, a novel photoacoustic viscoelasticity imaging (PAVEI) technique that provides viscoelastic information of biological tissues is presented. We deduced the process of photoacoustic (PA) effect on the basis of thermal viscoelasticity theory and established the relationship between the PA phase delay and the viscoelasticity for soft solids. By detecting the phase delay of PA signal, the viscoelasticity distribution of absorbers can be mapped. Gelatin phantoms with different densities and different absorption coefficients were used to verify the dependence of PAVEI measurements. Moreover, tissue mimicking phantoms mixed with fat and collagen at different concentrations were used to testify the feasibility of this technique with reliable contrast. Finally, the PAVEI was successfully applied to discrimination between biological tissue constituents. Our experimental results demonstrate that this novel technique has the potential for visualizing the anatomical and biomechanical properties of biological tissues.

Study of TiO2 thin films on Si substrate by the photoacoustic elastic bending method

The quantitative photoacoustic investigation of different TiO2 thin films on Si substrates is presented using the elastic bending method. Photoacoustic signal amplitude and phase spectra versus modulation frequency were measured and analyzed using different sample thicknesses. Within the proposed method particular attention is given to the analysis of optical, thermal, elastic and structural sample parameters. Considerable focus is devoted to the fitting procedure of experimental results using the two-layer sample theoretical model. Characteristics of previously developed photoacoustic apparatus are discussed, attempting to search the ideal experimental conditions which can provide a good signal-to-noise ratio and sensitivity.

Investigation of Micromechanical Structures by Photoacoustic Elastic Bending Method

Photoacoustic (PA) and photothermal (PT) science and technology extensively developed new methods for the investigation of micro (nano)-mechanical structures. PA and PT effects can be important also as driven mechanisms for optically excited micromechanical structures. The photoacoustic elastic bending method (PA-EBM) is based on the optical excitation of the micromechanical structure and detection of the acoustic response (PA signal) with a very sensitive PA detection system. The experimental PA elastic bending signals of the whole micromechanical structure were measured by using a special constructed PA cell (the gas-microphone detection technique with transmission configuration). The PA amplitude and phase spectra were measured, as a function of the modulation frequency in a frequency range from 20 Hz to 20 000 Hz, for different samples (Si chip with square membrane). The electronic and thermal elastic PA effects (electronic deformation and thermoelastic mechanisms of elastic wave generation) in a Si simply supported rectangular plate (3D geometry), photogenerated by a uniform and intensity-modulated optical beam, were studied. The theoretical model for the PA elastic bending frequency distribution by using the Green function method was given. The amplitude and phase PA signals were calculated and analyzed, including the thermalization and surface and volume recombination heat sources. The theoretical results were compared with experimental data.

Photoacoustic technique for simultaneous measurements of thermal effusivity and absorptivity of pigments in liquid solution

A photoacoustic (PA) methodology, in the transmission configuration, for simultaneous measurements of thermal effusivity and molar absorption coefficient (absorptivity) for pigments in liquid solution is introduced. The analytical treatment involves a self-normalization procedure for the PA signal, as a function of the modulation frequency, for a strong absorbing material in the thermally thin regime, when the light travels across the sample under study. Two fitted parameters are obtained from the analysis of the self-normalized PA amplitude and phase, one of them proportional to the sample's optical absorption coefficient and from which, taking it for a series of samples at different concentrations, the pigment's absorptivity in liquid solution can be measured, the other one yields the sample's thermal effusivity. Methylene blue's absorptivity in distilled water was measured with this methodology at 658 nm, finding good agreement with the corresponding one reported in the literature.

Viscoelasticity imaging of biological tissues with phase-resolved photoacoustic measurement

A method for noninvasive viscoelasticity imaging of biological tissues using phase-resolved photoacoustic measurement is presented. We deduced the process of photoacoustic effect on the basis of thermal viscoelasticity theory, and established the relationship between the photoacoustic phase delay and the viscosity-elasticity ratio for soft solids. Agar phantoms with different densities and different absorption coefficients were used to verify the dependence of photoacoustic phase-resolved viscoelasticity measurements. Moreover, viscoelasticity imaging of tissues was obtained with a photoacoustic point scanning system. The photoacoustic phase-resolved method provides a basis for viscoelasticity imaging, which can potentially be used for detection of viscoelastic properties and lesions of biological tissues.

최소 부피 광음향 셀의 광음향 신호 재현성 향상에 관한 연구

광음향 분광학 연구에 빈번히 사용되는 최소 부피 개방형 광음향 셀을 이용하여 광음향 신호를 측정할 경우 최소 부피 광음향셀의 밀폐상태는 광음향 신호의 재현성에 큰 영향을 미친다. 따라서 본 논문에서는 최소 부피 광음향 셀의 밀폐 효율을 높일 수 있는 고무오링 밀폐재를 적용한 구조를 설계하고 제작한 후, 제작한 구조체를 이용하여 다양한 재질의 고무 오링에 대한 광음향 신호를 주파수 별로 측정하고 분석하였다. 실험 결과 진공 그리스를 이용하여 광음향 셀을 밀폐하는 기존의 방식과 비교했을 때, 실리콘 고무 오링을 밀폐재로 사용하는 경우 광음향 신호의 진폭과 위상의 재현성이 각각 최대 53.4%와 80.4%까지 향상될 수 있음을 알 수 있었다. Reproducibility obtained by a minimum-volume cell for photoacoustic (PA) signals is strongly dependent upon the sealing materials and sealing efficiency. Therefore, we have proposed, designed, and constructed an apparatus to enhance the sealing efficiency, and we have examined the reproducibility of amplitude and phase of PA signals with a minimum-volume cell. In particular, we have measured the PA signal as a function of modulation frequency for various rubber O-rings by using our apparatus. The results show that a silicon rubber O-ring can improve the reproducibility of the PA signal and phase by up to 53.4% and 80.4%, respectively, compared to that obtained by using the conventional sealing material, vacuum grease.

Differential photoacoustic cell to study the wetting process during porous silicon formation

AbstractThis paper shows the in‐situ study of the wetting process in Silicon during anodization process using an electrochemical Differential photoacoustic Cell (DPC). The Photoacoustic amplitude and phase signals were obtained for samples in air, ethanol, ethanol/HF and finally air. According to these results ethanol is responsible for a mechanical contact reducing the superficial tension and ethanol/HF produce the removing of the SiOx and SiO2species on the Silicon surface. It was found that the DPC is a powerful technique to study the wet surface before the formation of the porous silicon layer (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)