Laser-intensity-modulation method: A technique for determination of spatial distributions of polarization and space charge in polymer electrets

Abstract

A new method for measuring the spatial distribution of combined polarization and space charges in poled polymer electrets has been developed. The technique, called the laser-intensity-modulation method (LIMM), is based upon measurement of the pyroelectric current generated by the electret when it is heated with a laser beam which is intensity-modulated at frequencies varying from 200 to 2500 Hz. A special analytical technique is used to transform the current-frequency measurements to spatial distributions. The method gives high resolution and is very stable with respect to measurement errors. The technique can be easily implemented in a laboratory with conventional equipment. The advantages and limitations of LIMM are illustrated by means of calculations based upon simulated data. Experimental results on a number of laboratory and commercially poled samples are presented.

Measurement of the polarization profile across a surface-stabilized ferroelectric liquid crystal cell using the pyroelectric laser-intensity-modulation method

“…Temperature increment ␦T, produced by the laser heating can be expressed as the sum of a term T c due to the constant component of the heat flux and a term T m caused by the modulated component of the heat flux. It is known 8 that T c distribution decays to a steady spatial distribution with exponential time dependence for a time of approximately 1 ms and therefore cannot contribute to the sinusoidal response measured in our experiment.…”

“…We have examined an applicability of several thermal models to our case, some of these are described in the literature, 8,9,11 and these were applied to the geometry of the liquid crystal cell. It was found, however, that the model given by Leister et al 9 gives the most reliable results for the deconvolution of the experimental data using the regularization procedure…”

“…A spatial dependence of the pyroelectric coefficient may be caused by a spatial dependence of the spontaneous polarization or by free charges, intrinsic or injected from the electrodes. 8 Only in the case when the polarization charge divP is locally compensated by a free charge density (x), i.e., divPϭ, Eq. ͑3͒ reduces 16 to the well-known expression p(x)ϭ␣ P P(x)ϭ‫ץ‬ P/‫ץ‬T.…”

“…In this article, the LIMM pyroelectric technique, developed by Lang and Das-Gupta, 8 was used for studying the spatial distribution of the local polarization and the molecular alignment in the ferroelectric SmC* phase. Recently this pyroelectric method was applied for studying the ferroelectric liquid-crystalline elastomers.…”

“…9 The principle of the LIMM is as follows. [8][9][10][11] An intensity-modulated laser beam irradiates an opaque electrode of a sample. Absorption of the laser light in the electrode produces thermal waves that extend into the sample which cause the pyroelectric response; this is a convolution of the pyroelectric coefficient distribution and the temperature profile across the sample.…”