In this article there is the following text:

In the linear regime, a laser beam incident at the boundary is exponentially attenuated by the medium. 10,11 The water medium absorbs the optical energy, which heats the water. As the laser is pulsed, local temperature variations are generated in the water column. The water density differs between the areas of warmer and cooler water, giving rise to regions of compression and rarefaction. These volume density fluctuations represent a propagating pressure wave. The effect of the medium’s attenuation on the laser light is to produce an arraylike structure of thermoacoustic sources that generate a modulated pressure wave at the laser amplitude modulation frequency. Typical modulation frequencies are from several kilohertz to several tens of kilohertz with optical to acoustic energy conversion increasing with frequency. However, acoustic absorption increases nonlinearly with increasing frequency, therefore mitigating this gain to some degree.

What does the bolded part mean? What are "arraylike structure of thermoacoustic sources"? I understand the part that says the laser beam's light when focused to a point on the water's surface will be attenuated, but then it goes on to say that this attenuation of light produced something called "arraylike structure of thermoacoustic sources". What does this mean? I'm lost.

The context of this article is a laser beam is focused onto the surface of water, which forms a plasma filament below the water surface. This plasma undergoes thermal expansion and generates acoustic sound / pressure waves.


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