Here we show some simulations and experimental data. All examples have been calculated for a poled 12-micrometer PVDF film (24 microns for the bimorph).
At low frequencies, the temperature distribution is uniform throughout the sample. The real part is nearly zero, while the imaginary part gives a temperature amplitude of about 0.36 K. Thus, the temperature lags behind the modulated laser light by p/2. With this temperature distribution, the average pyroelectric coefficient is measured.
At high modulation frequencies, the temperature distribution is highly non-uniform. The highest amplitude is reached near the heated surface. As a result of this localized temperature distribution, we probe the pyroelectric coefficient in the surface layers only. Most lock-in amplifiers allow modulation frequencies of up to 100 kHz, equivalent to a spatial resolution of 0.5mm. Note that the amplitudes are much smaller (sub-mK) than in the 2 Hz example.