Terahertz Plasmonics of Semiconductor Core-Shell Nanowires

Core-shell nanowires (NWs) made of III-V semiconductors are promising nanostructures for optoelectronic and photovoltaic applications. One of the advantages is the possibility to epitaxially grow the NWs on Si substrates despite the large lattice mismatch, since the Si-GaAs interface area is very small. Moreover, the large surface-to-volume ratio for the NWs’ core allows to impose a very large strain when the core is overgrown with a lattice-mismatched shell, without creating any misfit dislocations. In this way, the bandgap of the core semiconductor can be tuned in a broad range via strain engineering [1].
Pump-probe terahertz spectroscopy has been proven as a perfect tool for contactless probing of electrical properties of semiconductor NWs [2]. The analysis of the optical conductivity spectra using the localized surface plasmon model allows to estimate the carrier lifetime and the carrier mobility. Recently, using THz spectroscopy we have demonstrated that the mobility in highly strained GaAs NW cores can exceed the mobility in bulk GaAs by 30-50% [3]. We found out the particular importance of the geometrical factor that defines the rescaling of the localized surface plasmon frequency in NWs. Its dependence on the aspect ratio can be derived analytically in the approximation of the cylindrical NW shape [4]. However, for a dense ensemble of NWs, where some can form bundles or touch each other, the geometric factor may vary, leading to an inhomogeneous broadening of the plasmon resonances. We discuss the role of this effect and its impact on the estimation of carrier mobility.
Finally, we demonstrate a THz nonlinearity of photodoped GaAs/In0.2Ga0.8As core-shell NWs using single-cycle intense THz pulses with peak electric fields up to 0.6 MV/cm. We found that with increasing the peak THz field, the plasmon frequency demonstrates a redshift accompanied by a suppression of the spectral weight. Remarkably, the spectral weight does not remain proportional to the square of the plasmon frequency, indicating an onset of a spatially inhomogeneous carrier distribution across the NW. The observed behavior can be ascribed to nonlinear effects caused by the intervalley scattering occurring in the high electric field regime. However, in contrast to bulk semiconductors, this effect initially sets in at hot spots of the NW where the local electric field is enhanced by the plasmonic resonance [5].

[1] L. Balaghi et al., Nat. Commun. 10, 2793 (2019).
[2] H. J. Joyce et al., Semicond. Sci. Technol. 31, 103003 (2016)
[3] L. Balaghi et al., Nat. Commun. 12, 6642 (2021).
[4] I. Fotev et al., Nanotechnology 30, 244004 (2019).
[5] R. Rana et al., Nano Lett. 20, 3225 (2020).