Transmittance to visible light of transparent nanocellular polymethylmethacrylate was measured. • Samples with a cell size 14 nm and a relative density of 0.46 can present transmittances as high as 0.94. • A theoretical model predicting the transmittance to visible light of nanocellular polymers has been developed. • Rayleigh scattering is main mechanisms of light transmission in transparent nanocellular polymethylmethacrylate.

Semiconductor nanowires (NWs) are the building blocks of future nanoelectronic devices. Furthermore, their large refractive index and reduced dimension make them suitable for nanophotonics. The study of the interaction between nanowires and visible light reveals resonances that promise light absorption/scattering engineering for photonic applications. Micro-Raman spectroscopy has been used as a characterization tool for semiconductor nanowires. The light/nanowire interaction can be experimentally assessed through the micro-Raman spectra of individual nanowires. As compared to both metallic and dielectric nanowires, semiconductor nanowires add additional tools for photon engineering. In particular, one can grow heterostructured nanowires, both axial and radial, and also one could modulate the doping level and the surface condition among other factors than can affect the light/NW interaction. We present herein a study of the optical response of group IV semiconductor nanowires to visible photons. The study is experimentally carried out through micro-Raman spectroscopy of different group IV nanowires, both homogeneous and axially heterostructured (SiGe/Si). The results are analyzed in terms of the electromagnetic modelling of the light/nanowire interaction using finite element methods. The presence of axial heterostructures is shown to produce electromagnetic resonances promising new photon engineering capabilities of semiconductor nanowires.

We present a phenomenon concerningelectromagnetic enhancement at the heterojunction region of axially heterostructured Si/SiGe nanowires when the nanowire is illuminated by a focused laser beam. The local electric field is sensed by micro Raman spectroscopy, which allowsthe enhancement of the Raman signal arising from the heterojunction region to be revealed; the Raman signal per unit volume increases at least tentimes with respect to the homogeneous Siand SiGe nanowire segments. In order to explore the physical meaning of this phenomenon, a threedimensional solution of the Maxwell equations of the interaction between the focused laser beam and the nanowire was carried out by finite element methods. A local enhancement of the electric field at the heterojunction was deduced.However, the magnitude of the electromagnetic field enhancement only approaches the experimental one when the free carriers are considered, showing enhanced absorption at the carrier depleted heterojunction region. The existence of this effect promises a way ofimprovingphoton harvesting using axially heterostructured semiconductor nanowires.

Short title: About the physical meaning of the critical temperature for catastrophic optical damage AbstractIt is usually assumed that the catastrophic optical damage of high power laser diodes is launched when a critical local temperature (Tc) is reached; temperatures ranging from 120ºC to 200ºC were experimentally reported. However, the physical meaning of Tc in the degradation process is still unclear. In this work we show that, in the presence of a local heat source in the active region, the temperature of the laser structure, calculated using finite element methods, is very inhomogeneously distributed among the different layers forming the device. This is due to the impact that the low dimensionality and the thermal boundary resistances have on the thermal transport across the laser structure. When these key factors are explicitly considered, the quantum well (QW) temperature can be several hundred degrees higher than the temperature of the guides and cladding layers. Due to the size of the experimental probes, the measured critical temperature is a weighted average over the QW, guides and claddings. We show the existence of a great difference between the calculated average temperature, equivalent to the experimentally measured temperature, and the peak temperature localized in the QW. A parallel study on double heterostructure lasers is also included for comparison.

An experimental and theoretical analysis of the temperature profile in semiconductor laser diodes using the photodeflection technique

The vapour-liquid-solid (VLS) method is by far the most extended procedure for bottom-up nanowire growth. This method also allows for the manufacture of nanowire axial heterojunctions in a straightforward way. To do this, during the growth process, precursor gases are switched on/off to obtain the desired change in the nanowire composition. Using this technique, axially heterostructured nanowires can be grown, which are crucial for the fabrication of electronic and optoelectronic devices. SiGe/Si nanowires are compatible with complementary metal oxide semiconductor (CMOS) technology, which improves their versatility and the possibility of integration with current electronic technologies. Abrupt heterointerfaces are fundamental for the development and correct operation of electronic and optoelectronic devices. Unfortunately, the VLS growth of SiGe/Si heterojunctions does not provide abrupt transitions because of the high solubility of group IV semiconductors in Au, with the corresponding reservoir effect that precludes the growth of sharp interfaces. In this work, we studied the growth dynamics of SiGe/Si heterojunctions based on already developed models for VLS growth. A composition map of the Si-Ge-Au liquid alloy is proposed to better understand the impact of the growing conditions on the nanowire growth process and the heterojunction formation. The solution of our model provides heterojunction profiles that are in good agreement with the experimental measurements. Finally, an in-depth study of the composition map provides a practical approach to the drastic reduction of heterojunction abruptness by reducing the Si and Ge concentrations in the catalyst droplet. This converges with previous approaches, which use catalysts aiming to reduce the solubility of the atomic species. This analysis opens new paths to the reduction of heterojunction abruptness using Au catalysts, but the model can be naturally extended to other catalysts and semiconductors.

Low-density polyethylene (LDPE) film containing a polydimethylsiloxane (PDMS) additive has been investigated using x-ray photoelectron spectroscopy (XPS) and surface resistivity measurements. The samples measured were: as-received; plasma-treated; washed with ethyl acetate; aged in air; and combinations of these treatments. Analysis by XPS has been used to characterize surface species concurrently with resistivity in the surface region to assist in understanding the relationship between charge transport and surface electronic structure. Radiofrequency plasma treatment causes major oxidation effects and lowering of resistivity in the first 4 s. With the same treatment time, in the case of washed samples, surface segregation of PDMS is enhanced, causing an initial increase of surface region resistivity. Plasma treatment beyond 4 s causes saturation of the O : C ratio in the surface and a loss of charged bonds. These points also allow a description of resistivity change with ageing, which includes hydration effects on exposure to air.

Catastrophic optical degradation (COD) of high power laser diodes is a crucial factor limiting ultra high power lasers. The understanding of the COD process is essential to improve the endurance of the high power laser diodes. The COD is observed as a process in which the active part of the laser diode is destroyed, forming characteristic defects, the so called dark line defects (DLDs). The DLDs are formed by arrays of dislocations generated during the laser operation. Local heating associated with non-radiative recombination is assumed to be at the origin of the COD process. A summary of the methods used to assess the COD, both in real time and post-mortem is presented. The main approaches developed in the last years to model the heat transport in the laser structures under non homogeneous temperature distribution are overviewed. Special emphasis is paid to the impact of the low dimensionality of QWs in two physical properties playing a major role in the COD process, namely, thermal conductivity and mechanical strength. A discussion about the impact of the nanoscale in both physical properties is presented. Finally, we summarize the main issues of the thermomechanical modelling of COD. Within this model the COD is launched when the local thermal stresses generated around the heat source overcome the yield stress of the active zone of the laser. The thermal runaway is related to the sharp decrease of the thermal conductivity once the onset of plasticity has been reached in the active zone of the laser.