High-intensity focused ultrasound (HIFU) is increasingly getting in focus of interest for polymer layer joining [1]. It has been found that HIFU is able to heat polymer layers with a temperature rise of over 680 K · s^-1 [1]. However the physical mechanisms causing the heating of thin polymer layers using ultrasound with a solid wave guide ultrasonic transducer are still unknown. In this paper, the correlation between sample thickness, particle velocity at the top of the solid wave guide and heating behaviors are investigated by means of scanning Doppler laser vibrometry and infrared thermography. The study showed that the heat generation induced by high-intensity focused ultrasound with a solid wave guide depends on the distribution of the particle velocities at the top of the wave guide for samples thinner than a half wavelength in PMMA. However for the samples thicker than a half wavelength, the heating of the sample depends on the particle velocities distribution at the top of the wave guide, as well as on the sound waves focused by the transducer. The findings are relevant for processes in which solid polymers need to be heated quickly and locally such as in welding or forming.