## 4 Conclusions and Open Questions  
In this article, we reviewed the recent results in the emergent field of fluorescence and plasmonic nanoparticle-enhanced sonoluminescence with a focus on potential applications of light emitted by collapsing bubbles in the fields of photonics and biophotonics and adjacent areas. Given the ability of ultrasound waves that cause the bubble collapse to readily pass through the skin and bodily fluids and tissues and due to the presence of natural and artificial gas bubbles inside a living body, it has been argued that sonoluminescence light could be used to activate certain drugs inside the body, also providing a means for the inactivation of pathogens with UV radiation and for imaging of individual cells without the application of chemicals that can pose ethical issues. While rigorous in vivo tests of these potential applications of sonoluminescence have not been conducted yet, the results obtained in a number of recent experimental works, where commercial medical ultrasound imaging microbubble contrast agents and medical-grade ultrasound wave intensities were employed, strongly speak in favour of the plausibility of the concept of sonoluminescence-based photonics and biophotonics. However, extra research studies are required for sonoluminescence-based processes to attract the interest of a large number of scientists working on photonics and biophotonics. Here, we outlined the current challenges and open questions, the resolution of which, in our opinion, will shape the future research efforts in the area of sonoluminescence and gas bubble photonics. Firstly, the experiments involving the interaction of sonoluminescence light with nanoparticles have thus far been conducted in purified liquids and liquids that contain chemicals that facilitate observations of bright sonoluminescence. However, in biologically relevant situations, the composition of fluids may be unfavourable for the production of sonoluminescence from the point of view of both fluid dynamics and optics. Therefore, additional studies of sonoluminescence have to be conducted in different types of biofluids or fluids that have similar properties, and the research questions such as the impact of the biofluid viscosity and of light absorption and scattering have to be rigorously addressed. Indeed, while in single-bubble sonoluminescence experiments, microbubbles can be trapped in water using standing acoustic pressure waves, achieving a stable bubble position in liquids with a higher viscosity has proven to be challenging, since, in such liquids, bubbles undergo a quasiperiodic circular translational motion on the time scale of seconds [106]. Secondly, it is known that the intensity of light emitted by a single sonoluminescing bubble can be increased using several driving ultrasound waves of different frequencies. The optimal values of the pressure amplitude and relative phase of the ultrasound waves were reported in [107], where a two-harmonic-frequency bubble-forcing regime --26.5 kHz and 53 kHz--was shown to result in increased temperatures inside a sonoluminescing bubble, which maintained its spherical stability despite the action of the two acoustic pressure waves. Subsequently, a combination of this sonoluminescence regime with nanoparticles should allow achieving higher sonoluminescence light intensities. It is also plausible that the use of more than two excitation frequencies or even a quasi-continuum of frequencies would result in strong sonoluminescence, provided that the bubble maintains its spherical shape during the oscillation. One potential test of this idea could employ acoustic frequency combs--signals with spectra consisting of equidistant frequency peaks [29]. Despite the multifrequency content of an acoustic frequency comb signal, a bubble driven by it can still maintain its spherical shape [108], which is favourable for producing sonoluminescence [35].