Photothermal Microscopy Research Articles

Effect of laser irradiation on silica substrate contaminated by aluminum particles

A major issue in the use of high-power lasers, such as the Laser Megajoule (LMJ), is laser-induced damage of optical components. One potential damage initiator is particulate contamination, but its effect is hard to distinguish from that of other damage precursors. To do so, we introduced artificial contaminants typical of metallic pollution likely to be present on the optical components of the LMJ chains. More precisely, aluminum particles of two different sizes were placed on a silica sample. These dots were characterized by optical microscopy and profilometry. Then they were exposed to a laser beam with a pulse length of 6.5 ns at 1064 nm and fluences in the range from 1 to 40 J/cm(2). Each dot was characterized again with the same techniques and also by photothermal microscopy. To complete the experimental results, we performed numerical simulations with a one-dimensional Lagrangian hydrodynamics code. We show that the particle removal by laser irradiation produces a modification of the silica surface that does not evolve into catastrophic damage under subsequent irradiation. However, the effect does depend on the size of the dots. We demonstrate that a procedure exists that removes the dot and leaves the site capable of resisting high fluence.

Complex finite element analysis applied to photothermal and thermoelastic microscopies

Photothermal and thermoelastic microscopes are non-destructive systems that generally work with lock-in detection. Experimental results are typically given for one modulation frequency. In order to understand related magnitude and phase images obtained with complex geometries, we have developed a finite element analysis (FEA) dynamic method. Particularly, it enables to directly obtain both thermal and thermoelastic harmonic fields for one modulation frequency. It has been applied to thermoelastic microscopy and has shown very good agreement with experiments. Numerical results from a 3D complex geometry model are presented and show the influence of both excitation size radius and modulation frequency on thermoelastic normal displacements.

Photothermal microscopy for studying the role of nano-sized absorbing precursors in laser-induced damage of optical materials

Laser-induced damage in optical components has long been acknowledged as a localized phenomenon linked to the presence of defects. Destructive investigations combined to optical characterizations have led to the conclusion that in high quality components the damage initiators are typically a few nanometers in size and have low densities. The understanding of damage phenomena requires the development of non destructive evaluation techniques with both high spatial resolution and sensitivity to detect these defects. In this context, a High Resolution Photothermal Deflection microscope (HRPD) has been developed and coupled with a damage facility at the same wavelength 1.064 μm. The behavior under irradiation of model defects such as gold inclusions (3 to 250 nm) has been studied. We show how HRPD gives determining information about damage mechanisms.

3J2b-1 Development of Ultraviolet-Excitation Photothermal Microscope and Observation of Non-Stained Biological Cells(Imaging methods/Nondestructive testing)

3J2b-1 Development of Ultraviolet-Excitation Photothermal Microscope and Observation of Non-Stained Biological Cells(Imaging methods/Nondestructive testing)

Photopyroelectric Microscopy of Plant Leaves

The use of photothermal microscopy to obtain superficial and in-depth images, by means of the interaction of a thermal wave with the analyzed material, has reached great interest due to its numerous applications. The application of the photopyroelectric microscopy technique to obtain images of plant leaves is presented in this article. In the experimental setup, a pyroelectric sensor and linear micro-positioners were used to obtain the photothermal signal at each point of the sample. Then it is possible to obtain images of plant leaves through their differences in local thermal properties and thickness.

Measurement of thermal diffusivity, elastic anisotropy and crystallographic orientation by interferometric photothermal microscopy

This paper describes the use of interferometric photothermal microscopy for measuring the thermal diffusivity, crystallographic orientation and elastic anisotropy of a microscopic homogeneous volume of matter. This purely optical technique makes use of an intensity-modulated and focused laser beam to periodically heat the surface of the sample tested. An interferometer is used to detect the thermal expansion. After describing the experimental setup, we explain the inversion scheme allowing the determination of the local thermal diffusivity from the periodic sample surface displacement map, as well as the crystallographic orientation and elastic anisotropy in the case of cubic materials. These parameters are measured with a spatial resolution of a few cubic micrometres.

Dynamic photothermal and thermoelastic microscopies described by complex finite element analysis

Photothermal and thermoelastic microscopes are nondestructive apparatus that generally work with lock-in detection. The related magnitude and phase images are obtained for one modulation frequency. These techniques, devoted to nondestructive evaluation, are suitable for inverse problems such as thermoelastic parameter reconstruction. But up to now the thermoelastic models have been limited to simple geometries or have encountered many problems with temporal sampling. In order to overcome these problems we develop a finite element analysis dynamic method, based upon complex analysis, that enables us to directly obtain both thermal and thermoelastic magnitude and phase fields. This method has been applied to thermoelastic microscopy and has shown very good agreement with experiments.

Selective laser nano‐thermolysis of human leukemia cells with microbubbles generated around clusters of gold nanoparticles

Previously reported studies on laser nano-thermolysis of cancerous cells demonstrated insufficient efficacy and specificity of malignant cell damage. Safety, that is, absence of damage to normal cells in the course of the laser thermolysis was also low due to less than optimal protocol of cancer cell targeting with nanoparticles (NP). The objective of this study was two-fold: to optimize NP targeting to real tumor (human) cells and to better understand physical mechanisms of cell damage for improved control of the laser ablation. We have suggested (1) two-stage targeting method to form clusters of light-absorbing gold NPs selectively in target cells, and (2) the cell damage mechanism through laser-induced generation of vapor bubbles around NP clusters. Experimental investigation of laser nano-thermolysis of leukemia cells was performed using 30 nm spherical gold nanoparticles as a light absorbing agent, and photothermal and fluorescent microscopies as well as flow cytometry as methods to monitor microbubble formation and resulting damage of leukemia cells in human bone marrow specimens. NP clusters were formed and visualized using fluorescence microscopy at cell membranes and in cytoplasm of B-lymphoblasts. Laser irradiation of cells (532 nm, 10 nanoseconds, 0.6 J/cm2) induced microbubbles selectively in leukemia cells with large clusters, but not in cells with single NPs or small clusters. Quantitative analysis demonstrated that only 0.1%-1.5% of tumor cells and 77%-84% of normal bone marrow cells survived laser pulse. Two-stage cell targeting method permits formation of NP clusters selectively in diagnosis-specific tumor cells. The clusters serve as effective sources of photothermally-induced microbubbles, which kill individual target cells after a single laser pulse. The laser fluence threshold for generation of microbubbles is inversely proportional to the volume of NP clusters.

High-resolution photothermal microscope: a sensitive tool for the detection of isolated absorbing defects in optical coatings

The photothermal deflection technique allows us to highlight the presence of inhomogeneities of absorption in optical components. This nondestructive tool is of great interest to the study of the role of contaminants, inclusions, and impurities in the laser-induced damage process. We show that the detection of nanometer-sized isolated absorbing defects requires the development of an adapted photothermal setup with high detectivity and high spatial resolution. Thus it is essential to improve the resolving power up to its theoretical limit.

Carrier recombination velocities at the SiO 2/Si interface investigated by a photo-thermal reflection microscopy

Photo-thermal reflection microscopy has been used for investigating semiconductor materials to evaluate carrier diffusivity, lifetime and surface recombination velocity. We developed this technique to obtain carrier recombination velocities at the interface between SiO 2 oxide film and Si substrate. Two samples with different oxide film thickness of 92 and 45 nm were prepared. Since the oxide film layer does not absorb the pump and probe laser light, carrier recombination velocity at the SiO 2/Si interface can be estimated. Curve fitting procedures with the theoretical prediction results in an estimation of the interface recombination velocity of 100 cm/s for thick oxide sample. When the sample was chemically etched, the recombination velocity increases to 2500 cm/s. The chemical etching results in the drastic increase of the recombination velocity. The etching solution may soak through the SiO 2 oxide film layer and attack the Si surface during the chemical etching. Increase of the number of the interface traps induces the increase of the carrier recombination velocity at the interface. We, therefore, found that the present photo-thermal reflection (PTR) microscopy is a useful technique for investigating the carrier dynamics at SiO 2/Si interface.

Photothermal investigation of the laser-induced modification of a single gold nano-particle in a silica film

Absorbing defects are believed to induce thermal effects that lead to laser damage, but the correlation between defect absorption and damage has not been clearly demonstrated. A model system consisting of a thin-film of silica containing gold nano-particles serving as nano-scale absorbing defects is investigated. For this purpose, a photothermal microscope coupled with a pulsed Nd:YAG laser allow us to follow the evolution of gold inclusion absorption before and after laser irradiation, without repositioning the sample. The “pre-damage” threshold had been defined previously as the laser fluence causing localized modification of the defect absorption without any surface modification. Thanks to the high sensitivity of photothermal microscopy, we find this threshold to be sevenfold less than that for the appearance of surface cracks. Numerical simulations are performed to evaluate the thermal evolution of the inclusion as a function of the laser fluence. From the comparison of experimental and theoretical results, we observe that the pre-damage threshold is closely linked to the melting of the metallic inclusion. Furthermore, the effect of repetitive shots on the evolution of gold inclusion absorption is examined experimentally and discussed.

Elastic anisotropy, crystallographic orientation and texture measurement by photothermal interferometric microscopy

Photothermal interferometric microscopy is used to determine the thermal diffusivity, elastic anisotropy and principal axis orientation of nickel superalloy single crystal samples. The inversion method is explained in details.

Thermal and thermoelastic dynamic analysis with finite element analysis, application to photothermal and thermoelastic microscopies

Photothermal and thermoelastic microscopies generally work with lock-in detection. In order to better understand related magnitude and phase images obtained with complex geometries, we develop a finite element analysis (FEA) dynamic method that enables to directly obtain both thermal and thermoelastic fields. It has been applied to thermoelastic microscopy and has shown very good agreement with experiments.

Photothermal image flow cytometry in vivo

The capability of photothermal (PT) microscopy to image moving, unlabeled cells in real time in vivo is demonstrated in a study of circulating red and white blood cells in blood and lymph microvessels of rat mesentery. Potential applications of this optical tool, called PT flow cytometry, are discussed.

Spectral evaluation of laser-induced cell damage with photothermal microscopy

Determining cell photo-damage is important for laser medicine and laser safety standards. This work evaluated the potential of photothermal (PT) technique for studying invasive laser-cell interaction, with a focus on PT evaluation of spectral dependence of laser-induced damage in visible region at single intact cell level. PT is based on irradiation of a single intact cells with a tunable pump laser pulse (420-570 nm, 8 nanoseconds, 0.1-300 microJ) and monitoring of temperature-dependent variations of the refractive index with a second, collinear probe beam in pulse (imaging) mode (639 nm, 13 nanoseconds, 10 nJ), and continuous (integrated PT response) mode (633 nm, 2 mW). The local and the integrated PT responses from the individual living red blood cells, lymphocytes, and cancer cells (K562) in vitro were obtained at different pump laser fluence and wavelength and compared with data obtained by conventional viability tests (Annexin V--propidium iodide). The cell damage with pump pulse lead to specific change in PT response's temporal shape and PT image's structure. The photodamage thresholds varied in the range of 0.5-5 J/cm2 for red blood cells, 4.4-42 J/cm2 for lymphocytes, and 36-90 J/cm2 for blast cells in the pump wavelength range of 417-555 nm. Damage threshold at different wavelength depends on absorption spectra of cells. Spectral evaluation of laser-damage thresholds can be done in two supplements for each PT mode--PT imaging and integrated PT response. The correlation between specific change of PT parameters and cell damage permits using PT technique to rapidly estimate the invasive conditions of the laser-cell interactions.

Monitoring of apoptosis in intact single cells with photothermal microscope.

The methods for detection of apoptosis, cannot as a rule be applied to intact single cells and do not allow monitoring of the cell population. The photothermal (PT) method was evaluated for detection and monitoring of apoptosis in single intact cells--lymphocytes and blasts. PT microscopy is based on optical registration of cell response to the thermal impact that is induced in a cell due to absorption of a short laser pulse (532 nm, 10 ns) by cellular hem-proteins. With no pretreatment of cells, optical detection of cellular response with this method may allow monitoring of apoptosis. The PT method was applied for in vitro studies of lymphocytes and K562 blast cells during drug-induced apoptosis. Dexamethasone in 10 microM concentration was used and cell properties were monitored for 6 h. PT parameters of cells deviated from control after incubation with Dexamethasone. Control measurements with fluorescent microscopy (using the Annexin-V-FLUOS kit) verified the development of apoptosis in drug-treated cells, while there was no development of necrosis. Also necrotic cells had PT properties similar to those of control cells. The PT method allowed the detection of the influence of Dexamethasone at earlier stages (2-hr incubation) than the fluorescent technique (4 h). Results obtained confirmed the applicability of PT microscopy for detection and monitoring of apoptosis in single intact cells. The sensitivity of the suggested method may exceed the sensitivity of fluorescent methods.

Photothermal deflection microscopy for imaging sub-micronic defects in optical materials

Photothermal deflection is widely used to study defects in optical coatings and role of these defects in laser damage. Because defects responsible for laser damage are assumed to be nanometer-sized and lowly absorbing, both high spatial resolution and high sensitivity are required to detect them. In this work we theoretically and experimentally explore the capability of collinear photothermal deflection to give micronic resolution by reduction of the pump beam diameter. Thanks to a model describing temperature distribution and photothermal deflection, we have studied the effects of pump beam focusing on photothermal deflection. Then, we have developed a high resolution, high sensitivity microscope based on the photothermal deflection of a transmitted probe beam. The setup is characterized and the theoretical predictions are checked. We present a test of lateral spatial resolution obtained on specially prepared absorbing resolution targets and show that a lateral spatial resolution of 1 μm is reached on non-isolated defects. In case of single defects, we expect that 10 nm sized defects could be detected.

Observation by photothermal microscopy of increased silica absorption in laser damage induced by gold nanoparticles

In order to understand laser-induced damage in glass, we subjected engineered SiO2 thin films containing sub-micron gold inclusions to high fluences, and observed the results using several means of analysis. We found decoupling in time between the emission of gold and that of silicon with samples containing gold spheres of diameter 3 nm. We have analyzed the changes in the silica optical absorption at 1064 nm, using photothermal deflection microscopy. We find, upon exceeding a sharp fluence threshold, a thousand-fold increase in absorption of the silica matrix around the inclusion. We conclude that ions from the inclusion permeate the surrounding silica, and form a highly absorbent mixture.

Integrated photothermal microscope and laser damage test facility for in-situ investigation of nanodefect induced damage

An integrated setup allowing high resolution photothermal microscopy and laser damage measurements at the same wavelength has been implemented. The microscope is based on photothermal deflection of a transmitted probe beam : the probe beam (633 nm wavelength) and the CW pump beam (1.06 microm wavelength) are collinear and focused through the same objective. In-situ laser irradiation tests are performed thanks to a pulsed beam (1.06 microm wavelength and 6 nanosecond pulse). We describe this new facility and show that it is well adapted to the detection of sub-micronic absorbing defects, that, once located, can be precisely aimed and irradiated. Photothermal mappings are performed before and after shot, on metallic inclusions in dielectric. Results obtained on gold inclusions of about 600 nm in diameter embedded in silica are presented.

Thermo-elastic properties characterization by photothermal microscopy

A photothermal microscope devoted to microscopic thermal and thermo-elastic characterizations is presented. In thermal configuration, the sample is heated by an intensity modulated laser beam and the periodic temperature increase at the sample surface is detected using the photoreflectance technique. In thermo-elastic configuration, the periodic elevation of the sample surface is measured using a Nomarski interferometer. The spatial resolution is micrometric, the sensivity is better than 10 −3 K/ $\surd$ Hz in thermal configuration and 10 pm/ $\surd$ Hz in the interferometric one. Typical photoreflectance thermal properties measurements are performed to evaluate the performance of the instrument. In thermo-elastic configuration, the interferometric signal has to be carefully analyzed to remove spurious photothermal effects. The thermo-elastic response of isotropic and anisotropic homogeneous samples during modulated photothermal experiments are then calculated to demonstrate that interferometric measurements enable quantitative determination of some of the sample thermo-elastic parameters such as thermal diffusivity, elastic anisotropy and crystalline orientation.