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.
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.
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