## 3 Results and Discussion
The peaks, indicated by filled circles with labels 3 and 4 in Figure 1b, correspond to light absorption by the three upper interface phonon-plasmon modes. An additional increase of the electron concentration to \(N_{e}\) = 2 \(\times\) 10\({}^{18}\) and to 3 \(\times\) 10\({}^{18}\) cm\({}^{-3}\) brought the system into the regime of enhanced phonon-plasmon interaction (marked in Figure 1b by a pink-shaded area). The resonant amplification of the absorption observed in this range of electron concentrations was due to the enhanced hybridization of interface phonon-plasmon modes. This led to an increase by a factor of \(\sim\)3 of the low-energy absorption peak intensity for \(N_{e}\) = 2 \(\times\) 10\({}^{18}\) cm\({}^{-3}\) (indicated with label 5 in Figure 2c) and by a factor of \(\sim\)2 for \(N_{e}\) = 3 \(\times\) 10\({}^{18}\) cm\({}^{-3}\) (indicated with label 8 in Figure 2d) as compared to the intensity of the peak labeled with 1 at \(N_{e}\) = 0 (see Figure 2a). The intensity of the absorption peaks labeled 6 and 9 remained nearly constant within the analyzed electron concentration range. A further increase of the electron concentration led to an attenuation of the interface phonon-plasmon interaction. The spectral weight of the absorption peaks decreased, and the peak positions shifted to the energies of the interface phonons (see Figure 2e). Figure 3: Light absorption cross section versus the photon energy for the u-GaAs/d-InAs/Hexane nanostructure. The absorption spectrum of nanostructures with an undoped InAs shell shows three peaks (see Figure 3a), whose positions correspond to three interface phonon energies in the u-GaAs/u-InAs/Hexane nanoshell. Also, for this nanostructure, resonant amplification of the absorption took place for electron concentrations corresponding to enhanced interface phonon-plasmon interaction (shown in Figure 1c by pink-shaded areas). The first resonance near \(N_{e}=10^{18}\) cm\({}^{-3}\) (see Figure 3b) is characterized by an increase of the intensity of the absorption peak near 33 meV with a factor of \(\sim\)8. This increase resulted from the interaction of interface phonons with interface plasmons in the upper plasmon branch. As before, we find that an increase of the electron concentration from \(N_{e}=10^{18}\) cm\({}^{-3}\) to \(N_{e}=3\times 10^{18}\) cm\({}^{-3}\) led to a weakening of the absorption. The spectral weight became approximately the same as that for \(N_{e}=0\) (see Figure 3c). In the electron concentration range from \(N_{e}=5\times 10^{18}\) to \(15\times 10^{18}\) cm\({}^{-3}\), there was a second resonant amplification of the absorption, now associated with the hybridization of the interface phonons with the plasmons from the lower interface plasmon branch. In this case, the nanostructure efficiently absorbed light in a broad electron concentration range from \(5\times 10^{18}\) to \(15\times 10^{18}\) cm\({}^{-3}\) (see Figure 3d,e). At higher electron concentrations, light absorption in the nanostructure was significantly reduced. A strong phonon-plasmon coupling and a resonant amplification of the absorption can also be realized in doped semiconductor quantum dots of different materials and shapes, including InAs/GaAs core/shell quantum dots grown on Si substrate [57], colloidal CdSe/CdS and CdSe/ZnS quantum dots [58], or semiconductor quantum dots without a shell [31; 59; 60]. However, core/shell nanocrystals allow for better control over the absorption spectrum through the selection of the core/shell thickness ratio and the material of the shell.