Femtosecond laser filaments leave plasma strings at their trail as they propagate through any transparent medium including glasses and polymers. This initial plasma string and the energy deposited from the electrons to the lattice play a fundamental role in the creation of permanent structural modifications in these media.
A critical challenge for the broader implementation of laser cleaning interventions in
modern paintings is the assessment of the extent of any photochemical or structural
modifications induced in the original substrate following laser irradiation. For this reason, we
investigate several polymeric materials (PMMA, Paraloid B72) doped with aromatic
photosensitisers (PhenI, POPOP) of known photochemistry, coated with uniform layers of
acrylics of different thicknesses to simulate real case scenarios. Following laser irradiation, a
variety of spectroscopic techniques LIF, MPEF are employed for the in depth monitoring of
any photochemical and structural modifications induced in the bulk material.
Molecular microsystems, which can operate as actuators, are of high current interest in a variety of diverse micromechanical applications. We present novel polymer-based microsystems, which undergo optomechanical cycles induced by ns laser pulses. It is demonstrated that these opto-actuators can be accurately controlled in a step-by-step manner by proper manipulation of the incident laser pulses. The photoinduced actuation relies on the reversible photochemical changes of photochromic spiropyran molecules incorporated into polymer matrices. In particular, the spiropyran molecule is converted reversibly between its isomeric forms, upon irradiation at appropriate laser wavelengths. These photochromic inter-conversions were found to activate the polymer matrix, resulting in its contraction and lengthening in a highly controllable manner. The reversible optomechancial actuation is demonstrated by laser-induced bending of microcantilevers made of photochromic polymers. The pathways of the optomechanical cycles are being well defined and correlated with stereoisomeric states of the photochromic dopants. The correlation is performed after fluorescence emission measurements of the photochromes in polymers matrices, upon appropriate pulsed laser irradiation.
The study examines chemical and structural modifications effected in the UV ablation of polymers. For the study of the chemical processes, aromatic photosensitive compounds with well-defined photochemistry are employed as dopants and their reactivity is examined as a function of laser parameters (fluence, wavelength and laser pulse width). A 'pump-probe' scheme based on laser-induced fluorescence is employed for monitoring photoproduct formation in the polymeric substrate following UV irradiation. Ablation is shown to result in a change of the photolysis degree of the dopant and in the efficient formation of bi-aryl compounds, indicative of a high species mobility. Furthermore, kinetics of photoproduct formation in the ablative regime is shown to differ distinctly from that in the irradiation at low laser fluences. However, the quantitative extent of these changes is critically affected by the absorptivity of the substrate at the irradiation wavelength. On the other hand, structural modifications induced in polymer films are probed via holographic interferometry. Deformations are shown to be induced at distances far away (approximately 2-3 cm) from the irradiation spot. The implications for UV laser material processing schemes are briefly discussed.
In this paper, investigation of photochemical and photomechanical effects induced in polymer substrates under pulsed UV ablation is presented. The examined laser parameters are the wavelength at 248 nm and 193 nm in the nanosecond regime, and the fluence below and above the ablation threshold. The two polymeric substrates used are PMMA and blends of PMMA and PS.
In this paper, modeling of photochemical and photomechanical effects that may be induced during laser assisted cleaning of painted artworks using pulsed UV ablation is presented. The examined laser parameters are the wavelength at 248 nm and 193 nm in the nanosecond regime, and the fluence below and above the ablation threshold. The two polymeric substrate models used are PMMA and blends of PMMA and PS. A test case study is presented.
The photochemical effects induced by UV irradiation at 248 nm to highly photosensitive organic compounds embedded into polymer matrices are sutdied in a systematic way using laser induced fluorescence. The nature and intensity of the induced photoproducts are monitored below and above the ablation threshold. Host polymers with different absorption coefficients in the employed wavelength are used in order to study their protective role with regard to the photodissociation of the dopants. The employed dopants are the iodo-derivatives of naphthalene and phenanthrene (NapI and PhenI). Photolysis of the dopants, while they are embedded int he weakly absorbing polymer PMMA, at laser fluences below the ablation threshold is found to result in the formation of naphthalene-like and phenanthrene-like photoproducts, whereas above the threshold, additional photoproducts are clearly observed only in the case of the NapI dopant. The photolysis yields for both dopants exhibit very similar behaviour, increasing sharply above the threshold. In contrast, in the case where the dopants are embedded into the strongly absorbing polymer polystyrene, their photolysis yields reach a limiting value closely above the ablation threshold. Therefore, the absorbing polymer provides a high degree of control over the induced photochemical effects. The finding implies that the photolysis efficiency of the incorporated additives is directly affected by the host polymeric material.
Chemical and mechanical modifications are expected to be the two major types of side effects in the UV laser ablationbased processing of strongly absorbing molecular substrates. For the systematic characterization of these effects, studies on model polymeric systems are presented. As far as photochemical effects are concerned, UV ablation is shown to promote chemical pathways over the ones observed in the sub-ablative regime. However, the extent of these effects can be limited by an optimal etching depth vs. effective optical penetration depth in substrates of high absorptivity. Concerning mechanical effects, UV ablation is similarly shown to result in structural defects that are not observed in the irradiation at low energy fluence values. In practice, these may be limited by the inhomogeneous and stratified structure of the substrates encountered in real-life applications. The applicability of the results to the implementations of UV laser ablation is exemplified using the procedures that have been defined in laser restoration of painted artworks as a study case. Besides their implications for laser material processing implementations, the present results indicate that UV ablation introduces new physical and chemical paradigms that are of scientific importance in their own right.
Sequential double exposure holographic interferometry is employed for examining the mechanical effects induced in the UV ablation of polymers. Deformations, evidenced by changes in the refractive index of the substrate, are observed to develop at unexpectedly long distances (approximately equals 2 - 5 cm) from the irradiation area. The morphology of the induced effects depends on the substrate properties, with the major types observed being tentatively ascribed to delaminations and local fractures. For accounting for the highly delocalized spatial spread of the effects, laser doppler vibrometry was used for monitoring the propagation of the stresses generated by the ablation process. The examination confirms the development of intense vibrations far from the ablation point. The present results suggest that in UV laser processing of molecular substrates, the photomechanical effects can be significant and special attention should be paid in their characterization and their minimization.
Several aspects of the applications of laser technology in cleaning and diagnostics of painted artworks are reviewed. Laser and material parameter studies leading to the optimization of the cleaning process are presented while the use of spectroscopic techniques for obtaining information on the mechanism of the ablation process and probe possible photochemical effects is discussed. Furthermore, holographic interferometric techniques for detecting possible mechanical stress induced by the laser during the cleaning process are employed and preliminary result are shown. Finally, examples of the applications of imaging techniques in artwork diagnostics are presented.
The biological response to PDT depends on the photosensitizer accumulation to the malignant cells, the tumor vasculature, the inherent cell sensitivity to the photodynamic effect, etc. The in-vivo measurement of physical parameters related with these factors can serve in the understanding of the mechanisms involved in the cell death, as well as in the optimization of the treatment procedure. In this paper we demonstrate a modular diffuse reflection and fluorescence emission imaging colorimeter. We have used it for the in-vivo on-line quantitative measurement and mapping the time course fluorescence intensity after ALA application and for the tissue color changes associated with the erythema developed during irradiation. Based on the obtained quantitative data, we investigate the correlation between the photosensitizer accumulation before light irradiation and the hemodynamic changes, revealed as erythema development, occurring during light irradiation. Their relative significance as predicting factors for the photodynamic treatment effectiveness is also evaluated.
In the present study it was found that human keratinocytes grown on collagen substrate, exhibited increased resistance to the hematoprophyrin-mediated photodynamic treatment, in comparison to keratinocytes grown on Petri dishes without collagen. Interestingly, no protection was afforded by the collagen gel to the cells in the corresponding control ('hematoprophyrin only' and 'light only') experiments. This observation was found to be independent of light dose and drug concentration, and the relative degree of resistance was the same for both normal and malignant cells. The degree of keratinocyte resistance was found to be closely related to the duration of cell attachment on the collagen substrate. These results are indicative of an active interference of collagen gel with the cellular evolution of the photodynamic phenomenon and they are also suggestive of variation in the photodynamic treatment efficacy according to the cellular environment.
Twenty nine skin malignancies, 14 solar keratoses (SK) and 15 basal cell carcinomas (BCC) were treated with ALA-PDT. The overall cure response rate for SK was 85.7% and 66.6% for BCC. Specific histologic findings in sequential histology were indicative of the destructive and restoration mechanisms. A multispectral imaging system (MUSIS) was developed to perform diffuse reflection and fluorescence studies. Most of the lesions developed peak fluorescence 1.5 - 4 hours after ALA application. Hue was the most sensitive parameter to record the kinetics of the photosensitizer concentration in the atypical cells. It was worth noticing that the peak fluorescence differed among patients and this was an indication to start the irradiation more objectively. Erythema inspection and quantification during irradiation and afterwards was achieved by means of MUSIS. Erythema development was shown to include a new peak in the Hue histogram corresponding to a color range between red and violet. Time course erythema elicitation was found to differ between SK and BCC in accordance with recorded histologic difference relative to PDT-destruction mechanisms. Erythema development was shown to correlate significantly with the phototoxic effect and might be considered a reliable predictor of PDT efficacy.
The photodissociation of titanium tetrachloride in the gas-phase and from cryogenic films is investigated by mass-spectrometric detection of the products. The observations relate to recent studies of the excimer laser-induced titanium deposition from the named precursor molecule.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.