Band potential fluctuations in InGaN/GaN quantum wells (QWs) induce carrier localization that affects emission linewidth and carrier recombination rate. Alloy composition and well width variations are considered as main sources of the potential fluctuations and are often treated indiscriminately. However, their impact on the emission linewidth and the carrier lifetimes may be different. Besides, the impact of the QW width fluctuations on the linewidth could possibly be reduced via optimization of growth, while random alloy composition fluctuations can hardly be avoided. In this work, we have studied these effects in green-emitting semipolar (20-21) plane InGaN/GaN single QW structures of different well widths (2, 4 and 6 nm) and in structures with different number of QWs (1, 5 and 10). Experiments have been performed by scanning near-field photoluminescence (PL) spectroscopy. It has been found that the well width fluctuations, compared to the InGaN alloy composition variations, play a negligible role in defining the PL linewidth. In multiple QW structures, the alloy composition fluctuations are spatially uncorrelated between the wells. Despite that the 10 QW structure exceeds the critical thickness, no PL linewidth changes related to a structural relaxation have been detected. On the other hand, the well width fluctuations have a large impact on the recombination times. In-plane electric fields, caused by the nonplanarity of QW interfaces, separate electrons and holes into different potential minima increasing the lifetimes in wide QWs.
We report the growth of ultrasharp carbon whiskers onto apertured near-field optical glass fiber probes. The ultrasharp
carbon whiskers are produced by the electron-assisted dissociation of residual oil vapors present in the vacuum chamber
during the electron beam exposition of the tip. This cost effective manufacturing procedure is reproducible, fast and
allows controlling the shape of the carbon whisker. The radius of curvature of the whisker apex is approximately 10 nm
while its small total length is around 100 nm thus fulfilling the requirements of aperture Scanning Near-Field Optical
Microscope (SNOM) probes, i.e. to keep the distance between the sample and the optical aperture during the scanning at
subwavelength scale. Furthermore, due to the intrinsic properties of the amorphous carbon whisker, the probes are
The carbon whisker optical fiber probes are mounted on tuning-forks using the earlier discussed double-resonant
principle. This process ensures a high quality factor of the sensor in the range 2000-5500, which enables to cope with the
large stiffness of the tuning-fork actuator and obtain a characteristic noise-limited sensitivity smaller than 10pN
necessary to image soft biological samples without destroying them. To illustrate the sensor's performances,
transmission near-field optical images of SNOM calibration grating as well as high-resolution state-of-the-art
topographic images of single DNA molecules are presented. Prospects of further improvements of the fabrication method
enabling to achieve the lighting rod enhancement of the optical near-field (nano-antenna effect) are briefly discussed.