## 2 Discussion
### Photonically Enhanced Sonoluminescence  
Since the amplitude of ultrasound waves used for biomedical applications cannot be increased beyond certain limits dictated by particular applications [80], it has been suggested that low-intensity sonoluminescence light generated using ultrasound waves of a relatively low amplitude can be effectively amplified using some of the methods of nanophotonics, where nanoparticles with certain optical properties have been employed both to increase the intensity of light and to manipulate light, for example, by changing its wavelength (colour) [8]. This approach was validated in the work [58], where functionalised semiconductor ZnO nanoparticles were employed to increase the intensity of sonoluminescence light, reduce the cavitation threshold, and promote sonochemical reactions. Combined together, these effects of ZnO nanoparticles enabled decreasing the amplitude of the ultrasound waves needed to observe bright sonoluminescence. It is also noteworthy that ZnO nanocrystals have been used in various biomedical fluorescence assays [81], which additionally justifies their application in the experiments in [58] and their suitability for applications in the fields of biology and medicine. ZnO nanoparticles were synthesised using a microwave-assisted synthesis technique, and then, they were amine-functionalised to obtain ZnO-NH\({}_{2}\) nanoparticles that exhibited stable colloidal suspensions in both ethanol and distilled water. In the first series of experiments, the sonoluminescence light emission was detected using a PMT under dark conditions. The cavitation threshold was recorded by increasing the amplitude of the ultrasound wave until the appearance of spikes in the PMT-recorded optical spectrum caused by sonoluminescence. The so-recorded cavitation threshold in pure water was about 20% of the maximum power of the ultrasonic transducer used in the measurements, and in agreement with the previous work [82], it was observed that the presence of ZnO-NH\({}_{2}\) nanoparticles in the liquid resulted in a noticeable decrease in the cavitation threshold. As a next step, sonoluminescence emission in the presence of ZnO-NH\({}_{2}\) nanoparticles was measured using a standard spectrometer capable of detecting light in the spectral range from UV to infrared wavelengths. Since the sensitivity of the spectrometer is lower than that of the PMT device used at the first stage of the experimentation, the acoustic pressure was increased fourfold (however, it constituted about 80% of the maximum power deliverable by the ultrasound transducer used in the measurements). Figure 5a shows the sonoluminescence spectra obtained in pure water as a function of the intensity of the ultrasound wave. One can see a broad frequency peak that is centred on the wavelength of 450 nm and covers the UV and visible spectral ranges. The lowest level of ultrasound intensity that leads to the observation of a clear spectrum--1.2 W/cm\({}^{2}\)--corresponds to 40% of the maximum ultrasound intensity attainable in the experiment in [58]. However, in the presence of ZnO-NH\({}_{2}\) nanoparticles, a clear sonoluminescence spectrum can be observed at a lower ultrasound intensity of 0.9 W/cm\({}^{2}\) (Figure 5b). Moreover, one can see that the UV component of the sonoluminescence light (the wavelengths from 250 to 350 nm) is absorbed by ZnO-NH\({}_{2}\) nanoparticles. Subsequently, it was verified whether the presence of ZnO-NH\({}_{2}\) could result in the complete absorption of light in the UV range with a concomitant re-emission of light in the visible light region. To verify this hypothesis, the measurements were repeated in an argon-saturated atmosphere (it is well known that the presence of argon results in brighter sonoluminescence [34; 35]). The obtained spectra are shown in Figure 5c,d, convincingly confirming a strong absorption of the UV light by ZnO-NH\({}_{2}\) nanoparticles.