## 2 Discussion  
In this section, we review the results of key works relevant to the mainstream discussion in this article. We note that multibubble sonoluminescence was discovered in the 1930s [62, 63] and that this phenomenon has been studied in great detail since then [38, 64]. Single-bubble sonoluminescence was discovered in the late 1980s [35, 65], although, according to some sources [38], the very first observation of this effect was reported in 1962 [66]. One remarkable observation made in single-bubble sonoluminescence measurements was the fact that the brightness of the light emitted by a bubble was detectable by the naked eye [35]. Yet, the equipment required to observe this effect employed relatively simple electronics. Compared with the very low brightness of the light emitted in typical multibubble sonoluminescence experiments that require more complex high-power acoustic power setups, the single-bubble sonoluminescence effect immediately attracted the considerable attention of researchers due to its promising potential practical applications. Thus, it is plausible that the ideas of using sonoluminescence light for biomedical purposes could be expressed at the same time. Furthermore, in the late 1980s, several investigators of bubble acoustics and fluid dynamics initiated the study of cavitation processes using therapeutic  
Figure 2: (a) Sketch of a potential proof-of-principle experiment involving a flask filled with water that contains a fluorescent material. When the flask is illuminated with visible light, the liquid does not change its colour. However, when the flask is illuminated with UV light, the liquid changes its colour due to fluorescence processes. In the rightmost image, the liquid in the flask contains bubbles, and it is irradiated with ultrasound, which leads to the emission of sonoluminescence light. Since the so-generated light has a significant UV component, fluorescence processes result in a change in the colour of the liquid. (b,c) The intensity of sonoluminescence light can be enhanced using plasmonic properties of metal nanoparticles that originate from the coupling of the light to a resonant oscillation of conduction electrons at the metal surface. This approach was theoretically demonstrated in [39] in the context of the disinfection of water contaminated by pathogens. Reproduced from [39] under the terms of a Creative Commons license.  
ultrasound [67; 68] (also note a relevant earlier work, where cavitation and sonochemiluminescence were investigated using high-kHz acoustic pressure waves [69]). This fact also speaks in favour of the assumption that the researchers aimed to use sonoluminescence for medical purposes in the 1980s. On the other hand, according to [41], the first evidence of a biomedical application of sonoluminescence was given in the work [70], which was published in a medical journal, where the authors proposed using sonoluminescence in blood plasma to detect tuberculosis and cancer of the lungs.  
However, even though the pioneering experiments on sonoluminescence employed sophisticated experimental equipment, the progress in the field of scientific instrumentation and data processing made in the past decades has enabled new approaches to the manipulation of bubbles and the detection of light emitted by them. Yet, the last three decades have also seen dramatic progress in the fields of medicine, chemistry, and pharmacology, where, for example, bubbles have been used as ultrasound contrast agents and as a means of targeted drug delivery (see Section 1). Alongside the advances in the adjacent areas of sonochemistry, sonoprocessing, and sonoporation [25; 26; 71; 72], these factors explain the recent surge in the interest in sonoluminescence from the point of view of biophotonics, imaging, and drug discovery [39; 40; 41; 58].