Abstract
Waves of any kinds, including sound waves and light waves, can interfere constructively or destructively when they are overlapped, allowing for myriad applications. However, unlike continuous waves of a single frequency, interference of photoacoustic pulses is often overlooked because of their broadband characteristics and short pulse durations. Here, we study cancellation of two symmetric photoacoustic pulses radiated in the opposite direction from the same photoacoustic sources near a free surface. The cancellation occurs when one of the two pulses is reflected with polarity reversal from the free surface and catches up with the other. The cancellation effect, responsible for reduced signal amplitudes, is systematically examined by implementing a thin transparent matching medium of the same acoustic impedance. By changing the thickness of the transparent layer, the overlap of the two symmetric pulses is controlled. For optimized matching layers, the cancellation effect can be significantly reduced, while the resulting output waveform remains unchanged. Similar to the planar absorber, different dimensional absorbers including cylinders and spheres also exhibit the cancellation between the outward and inward waves. This work could provide further understanding of photoacoustic generation and a simple strategy for increasing photoacoustic signal amplitudes.
Highlights
We have systematically investigated the cancellation effect that is responsible for significantly low longitudinal photoacoustic (PA) amplitude for the soft boundary
The cancellation effect is influenced by the optical penetration depth
For small optical penetration depths, the cancellation effect is significant such that the matching layer is very effective in recovering the PA amplitude
Summary
Mediums with strong light absorption (or smaller optical penetration depths dop cτ) have low out-coupling amplitudes and significant cancellation effect. Unlike the matching layer that can only change the time delay, the optical penetration depth can broaden the acoustic pulse duration. The effect of the broadened acoustic pulse duration can be seen, where the out-coupling amplitude is saturated at large optical penetration depths dop > cτ. The cancellation is detrimental to the output acoustic amplitude when light absorption is highly confined near the interface, e.g., metal absorbers irradiated by a nanosecond pulsed laser In these cases, adding the matching layer will effectively mitigate the cancellation and increase signal amplitude. Impedance-mismatched transparent coverings, a thin impedance-matched layer is used to effectively control the overlap of two symmetric photoacoustic pulses, and to minimize the cancellation effect. This finding could offer further understanding of photoacoustic generation and an idea for designing photoacoustic contrast agents
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