Three-dimensional thermal-pulse probing of polarization profiles with low thermal stress

Thermal-pulse tomography (TPT) has been shown to be a valuable tool for non-destructively measuring three-dimensional electrical polarization and space-charge distributions in electret polymers. However, one of its drawbacks is the high thermal stress imposed on the sample surface when short pulses of laser light are focused to a tight spot. Q-switched Nd:YAG lasers, which are frequently used as heat source in TPT experiments, have a pulse duration of approximately 5 ns. However, due to bandwidth limitations of the amplifier circuits, the stimulating heat pulse can be as long as a few μs without significant loss of depth resolution. Recently, high-power diode lasers have become available that can be electrically driven to provide light pulses of the desired length. The longer light pulses from the diode laser have a significantly lower peak power density, thus avoiding ablation damage to the electrode, even at higher pulse energies. We present spatially resolved polarization maps of poly(vinylidene-trifluoroethylene) samples showing the significantly enhanced signal-to noise ratio of the upgraded instrument.