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Evidence is given that Rényi entropies of macroscopic thermodynamic systems defined on the bases of probabilities of microstates cannot be related to observables. The notion of observable is clarified.

Electrothermal poling of soft glasses (soda lime and borosilicate glass) at relatively high currents led to the creation of a second-order optical nonlinearity. The second-harmonic generation efficiency decays in time, and the decay rate can be accelerated by exposure to intense infrared radiation.

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The etching rate of silica glass in 40% hydrofluoric acid under the influence of electric fields is studied with the help of an interferometric technique. A linear dependence of etching rate with field strength was found, with a ϳ2.5% etching rate change for an applied field of 20 MV͞m. The experimental results are compared with those of a simple model that attributes this dependence to partial orientation of the HF molecules in the electric field. The measured dependence is sufficiently significant to account for the selective etching observed in frequency doubling glasses and fibers.[S0031-9007 (97)02547-7] PACS numbers: 81.65.CfEtching of silica glass ͑SiO 2 ͒ in hydrofluoric acid is an important step in the processing of microelectronic components, and this motivated studies on etching mechanisms [1] and on the role played by dopants [2] in the glass matrix. Etching in the presence of an electric field has also been investigated, and contradictory results exist in the literature. While the field created by a pn junction was observed to be capable of stopping the etching of a film of silica [3], recent results were reported where fringing fields of intensities as high as 10 MV͞m were applied to silica samples and the etching rate in a solution of HF was found to be unaffected [4]. The effect of an electric field on the etching rate of silica is of interest from a fundamental point of view and also with a view to technological applications. It has been shown that it is possible to pole by optical [5] or by electrothermal means [6] both bulk silica samples and optical fibers, creating a permanent electric field in the glass that gives rise to a strong optical nonlinearity capable of frequency doubling light. Etching is a powerful tool to study the physics behind poling, and has been used to reveal the distribution of the electric field in glass fibers [7] and in bulk glasses [8][9][10]. In order to extract further information from etching experiments it is necessary to learn what causes the unequal etching rates observed in poled glasses and fibers. In pure SiO 2 systems (such as found in poled bulk samples or in a frequency doubling fiber cladding) unequal etching can have two possible origins: (1) The etching rate may be influenced by embedded charges causing structural or chemical changes in the glass material and (2) a strong electric field alone could alter the etching velocity. In this paper an etching experiment of SiO 2 in HF is described where only an electric field can be responsible for an alteration of the etching rate. The dependence of etching rate with field strength was measured. The experimental results are compared with those of a simple model that attributes the alteration to partial orientation of the HF molecules in the electric field.An interferometric optical method was used to determine in real time the thickness of glass etched that avoided low precision mechanical measurements and the necessity to interrupt the etching process [10]. The SiO 2 samples were fixed approximately horizontall...