Variable Temperature Spectroscopy of As-Grown and Passivated CdS Nanowire Optical Waveguide Cavities

Abstract

Semiconductor nanowire waveguide cavities hold promise for nanophotonic applications such as lasers, waveguides, switches, and sensors due to the tight optical confinement in these structures. However, to realize their full potential, high quality nanowires, whose emission at low temperatures is dominated by free exciton emission, need to be synthesized. In addition, a proper understanding of their complex optical properties, including light-matter coupling in these subwavelength structures, is required. We have synthesized very high-quality wurztite CdS nanowires capped with a 5 nm SiO(2) conformal coating with diameters spanning 100-300 nm using physical vapor and atomic layer deposition techniques and characterized their spatially resolved photoluminescence over the 77-298 K temperature range. In addition to the Fabry-Pérot resonator modulated emission from the ends of the wires, the low temperature emission from the center of the wire shows clear free excitonic peaks and LO phonon replicas, persisting up to room-temperature in the passivated wires. From laser scanning measurements we determined the absorption in the vicinity of the excitonic resonances. In addition to demonstrating the high optical quality of the nanowire crystals, these results provide the fundamental parameters for strong light-matter coupling studies, potentially leading to low threshold polariton lasers, sensitive sensors and optical switches at the nanoscale.

Size-dependent chemical transformation, structural phase change, and optical properties of nanowires

“…Waveguide dispersion modelling, an understanding of propagation loss as a function of wavelength, and improved crystal quality due to surface passivation [202] combined led to the most complete investigation into the size-dependence of optical properties of dielectric nanowires to date [173]. Figure 14a, c, and e show photoluminescence spectra collected from the end facets of three different surface-passivated CdS nanowires, while b, d, and f show Fabry-Pérot maxima plotted along fitted numerical calculations for the fundamental waveguide mode.…”

“…(c) high-resolution transmission electron microscopy (HRTEM) image showing the single-crystalline structure of a GeTe nanowire with the electron diffraction pattern (inset) showing the [202] growth direction of rhombohedral GeTe. (d) Scanning TEM (STEM)-energy dispersive x-ray spectroscopy (EDS) elemental mapping of Ge (green) and Te (red).…”

Strain-Induced Large Exciton Energy Shifts in Buckled CdS Nanowires

“…CdS NWs (grown along the c-axis) were grown via the vapor-liquid-solid method as reported elsewhere [40] and then transferred on pre-strained flexible substrates. Figure 1(a) shows schematically the procedure for obtaining in-plane buckled NWs on polydimethylsiloxane (PDMS) substrates.…”

“…We also present the PL spectrum of CdS NW without any strain (Fig. 2(d) top) for reference; both A- and B-exciton peaks are clearly observed, which imply that the CdS nanowires have high optical quality [40]. Compared with the PL spectra of a buckled nanowire, we can see that the emission peak (red curve) from the node region of the buckled nanowire shows some broadening and is slightly blueshifted (Fig.…”

Tailoring light–matter coupling in semiconductor and hybrid-plasmonic nanowires

“…Furthermore, surface passivation has been shown to reduce surface exciton recombination in nanowires to the point where direct free exciton recombination is directly observed,[39] ultimately resulting in ideal crystals with extremely low defect emission.…”