Resonant enhancement of Raman scattering by surface phonon polaritons in GaAs nanowires

Abstract

Surface optical phonons are normally considered as subtle and poorly reproducible features in Raman spectra of nanostructured semiconductors, from which little or no information about the sample can be extracted. The present study demonstrates the potential for changing this situation. For a common type of GaAs semiconductor nanowires, we have shown that due to a combination of size-resonant light concentration, tapered shape and favourable scattering geometry, the surface phonon polariton Raman signal can be enhanced by orders of magnitude. The high signal gain enables routine detailed characterisation of the surface phonon polariton peak on an individual nanowire level, revealing its polarisation properties and spectral shift under variation of dielectric environment. This detailed characterisation was conducted using very low excitation power density despite high absorption of the excitation light in the nanowire material. The findings provide an effective way to use surface phonon polariton Raman scattering in the characterisation of dielectric nanowires and the prospect of developing novel surface sensors.

Polarization properties of Raman scattering by surface phonon polaritons in GaAsP nanowires

“…That obtained k z values are close to that of the free light dispersion, indicating that electromagnetic field of the excited resonant HE11 modes is still largely distributed outside the nanowire. This agrees with more detailed analysis of HE11 modes in GaAs nanowires [39].…”

“…The density of these maxima varies across the sample, but it is always less than 10% of the average NW density. This Raman hot spots phenomenon is similar to that observed on GaAs nanowire samples [39]. However, the intensity of the hot spots tends to be higher in GaAsP samples, which can be explained by the lower laser light absorption coefficient in this material.…”

“…The peak width value is close to that of the 'bulk' LO Ga-As and TO Ga-As modes. This is different from the SPhP peak characteristics in GaAs nanowires, where the SPhP peak width was significantly larger than that of the bulk modes [39].…”

“…Recently we have demonstrated [39] the amplification of Raman scattering by SPhPs on HE11 photonic resonance in GaAs nanowires, establishing the required experimental conditions. In this paper, we have reproduced the resonance amplification effect on GaAsP nanowires and utilised it for detailed characterisation of polarisation properties of SPhPs in nanowires of different cross section shape.…”

“…On the other hand, these spikes appear as dark areas on the silicon map (figures 1(e) and S3(d)), indicating local shadowing of the substrate due to strong absorption (or scattering) of the laser light on these objects similar to that on the hot spots. This implies that the standing up orientation (ensuring a backscattering geometry along the nanowire axis) is a principal condition for obtaining a strong Raman signal in the thick tapered nanowires, as suggested in [39].…”