## 3 Experimental Section
### Analysis on Ti-Si Thin-Film Specimens with the Acousto-Optical Method
#### 3.2.3 Detailed Analysis on Ti-Pt Resonance  
The observation of resonance-like behavior in the audible frequency range was somewhat surprising. In particular, the prominent valleys in the treated Ti-Pt specimen in the lower driving frequency range in Figure 7 was not expected. It was not observed in the previous study [17], where the lowest driving frequency was 10 kHz. We tested the Ti-Pt specimens a number of times using different interferometric setups with a similar interferometric setup in a different laboratory in a different country and still observed resonance-like behavior at the same frequency range. To confirm these observations more precisely, we made two modifications on the experimental arrangement. First, we added a second interferometric beam pair to the interferometer. This allowed us to take the measurement of two specimens simultaneously, thereby removing all temporal fluctuations common to the two interferometric pairs, such as intensity and polarization fluctuation of the laser beam and floor vibration noise, in comparing experimental data from the two specimens. Figure 8 illustrates the dual beam arrangement. The incident beam from the laser was split into two paths before the beam splitter of the interferometer. The two pairs of laser beams were delivered in parallel to each other to the end of the two interferometric arms. At the end of the \(X\)-arm (the horizontal arm in Figure 8), the first beam reflected off the first specimen, and the second beam reflected off the \(X\)-end mirror. At the end of the \(Y\)-arm (the vertical arm), the first beam reflected off the \(Y\)-end mirror and the second beam reflected off the second specimen.  
Second, we calibrated the acoustic transducer by measuring the transducer's oscillation amplitude as a function of the operating frequency and applied voltage. This ensured that the specimens were oscillated by the transducer with the same oscillation amplitude independent of the driving frequency. The constant transducer oscillation amplitude facilitated the measurement in the lower  
Figure 8: Dual beam Michelson interferometer. Beam Splitter 1 (BS1) splits the incoming laser beam into two passes for two pairs of interferences. Beam 2 is reflected on the rear surface of BS1. Light reflected on the Si/Pt specimen surface interferes with light reflected on M2/M1. Two interferometric fringe patterns are formed on the screen simultaneously. Respective specimens and corresponding mirrors are aligned for fringe formation, as explained in Figure 2. The screen is placed at a distance for better alignment of interfering light beams.  
driving frequency end where the optical alignment was much more difficult than the higher frequency range because the oscillation amplitude was larger for the same applied voltage.  
Figure 9a shows the raw data of the driving frequency sweep of the Fourier spectrum peak measured for the untreated and treated Ti-Pt specimens. Also shown in this figure are the Fourier spectrum peaks of a pair of bare Si specimens. One of the bare Si specimens was placed at the \(X\)-end of the interferometer, and the other was placed at the \(Y\)-end (Figure 8). The signal levels of the raw Fourier spectrum peak values for the two bare Si specimens are different because the first beam splitter did not split the laser beam exactly by 50%. Notice that the treated data converges to the bare Si data of the same arm at the low frequency end (7 kHz) and that the untreated data converges to the bare Si data of the same arm at the low and high frequency ends. This indicates that at these frequencies, the film surface oscillated for the same amount as the bare Si. In Figure 9b, the untreated and treated signals are normalized to the respective bare Si data. The valleys at 8 kHz in the treated specimen and those at 12.5 kHz and 13.5 kHz in the untreated specimen are more clearly seen than in Figure 9a.