Near- and mid-infrared intersubband absorption in top-down GaN/AlN nano- and micro-pillars.

We present a systematic study of top-down processed GaN/AlN heterostructures for intersubband optoelectronic applications. Samples containing quantum well superlattices that display either near- or mid-infrared intersubband absorption were etched into nano- and micro-pillar arrays in an inductively coupled plasma. We investigate the influence of this process on the structure and strain-state, on the interband emission and on the intersubband absorption. Notably, for pillar spacings significantly smaller (≤1/3) than the intersubband wavelength, the magnitude of the intersubband absorption is not reduced even when 90% of the material is etched away and a similar linewidth is obtained. The same holds for the interband emission. In contrast, for pillar spacings on the order of the intersubband absorption wavelength, the intersubband absorption is masked by refraction effects and photonic crystal modes. The presented results are a first step towards micro- and nano-structured group-III nitride devices relying on intersubband transitions.

Well-ordered ZnO nanowires with controllable inclination on semipolar ZnO surfaces by chemical bath deposition.

Controlling the formation of ZnO nanowire (NW) arrays on a wide variety of substrates is crucial for their efficient integration into nanoscale devices. While their nucleation and growth by chemical bath deposition (CBD) have intensively been investigated on non-polar and polar c-plane ZnO surfaces, their formation on alternatively oriented ZnO surfaces has not been addressed yet. In this work, the standard CBD technique of ZnO is investigated on [Formula: see text] and [Formula: see text] semipolar ZnO single crystal surfaces. A uniform nanostructured layer consisting of tilted ZnO NWs is formed on the [Formula: see text] surface while elongated nanostructures are coalesced into a two-dimensional compact layer on the [Formula: see text] surface. By further combining the CBD with selective area growth (SAG) using electron beam-assisted lithography, highly tilted well-ordered ZnO NWs with high structural uniformity are grown on the [Formula: see text] patterned surface. The structural analysis reveals that ZnO NWs are homoepitaxially grown along the polar c-axis. The occurrence of quasi-transverse and -longitudinal optical phonon modes in Raman spectra is detected and their origin and position are explained in the framework of the Loudon's model. These results highlight the possibility to form ZnO NWs on original semipolar ZnO surfaces. It also opens the way for comprehensively understanding the nucleation and growth of ZnO NW arrays on poorly and highly textured polycrystalline ZnO seed layers composed of nanoparticles with a wide range of non-polar, semipolar, and polar plane orientations. Eventually, the possibility to tune both the inclination and dimensions of well-ordered ZnO NW arrays by using SAG on semipolar surfaces is noteworthy for photonic and optoelectronic nanoscale devices.

Polarization-insensitive fiber-coupled superconducting-nanowire single photon detector using a high-index dielectric capping layer.

Superconducting-nanowire single photon detectors (SNSPDs) are able to reach near-unity detection efficiency in the infrared spectral range. However, due to the intrinsic asymmetry of nanowires, SNSPDs are usually very sensitive to the polarization of the incident radiation, their responsivity being maximum for light polarized parallel to the nanowire length (transverse-electric (TE) polarization). Here, we report on the reduction of the polarization sensitivity obtained by capping NbN-based SNSPDs with a high-index SiNx dielectric layer, which reduces the permittivity mismatch between the NbN wire and the surrounding area. Experimentally, a polarization sensitivity below 0.1 is obtained both at 1.31 and 1.55 µm, in excellent agreement with simulations.

Polarity-Dependent Growth Rates of Selective Area Grown ZnO Nanorods by Chemical Bath Deposition.

Polarity is known to affect the growth and properties of ZnO single crystals and epitaxial films, but its effects are mostly unknown in ZnO nanorods. To leave polarity as the only varying parameter, ZnO nanorods are grown by chemical bath deposition under identical conditions and during the same run on O- and Zn-polar ZnO single crystals patterned by electron beam lithography with the same pattern consisting of 15 different domains. The resulting well-ordered O- and Zn-polar ZnO nanorod arrays with high structural uniformity are formed on all the domains. The comparison of their typical dimensions unambiguously reveals that Zn-polar ZnO nanorods have much higher growth rates than O-polar ZnO nanorods for all the hole diameter and period combinations. The distinct growth rates are explained in the framework of the surface reaction-/diffusive transport-limited elongation regime analysis, which yields a much larger surface reaction rate constant for Zn-polar ZnO nanorods. The origin of the difference is attributed to polarity-dependent dangling bond configurations at the top polar c-faces of ZnO nanorods, which may further be affected by polarity-dependent interactions with the ionic species in aqueous solution. These findings show the relevance of considering polarity as an important quantity in ZnO nanorods.