Optical Spectroscopy of Single Nanowires

This thesis describes optical spectroscopy on III-V semiconductor nanowires. The nanowires were grown by metal-organic vapor phase epitaxy (MOVPE) and chemical beam epitaxy (CBE). Photoluminescence and photocurrent spectroscopy are used as tools to investigate issues such as the size of the band gap, the effects of surface states, and the charge carrier transport in core-shell nanowires. The band gap of InAs1-xPx nanowires with wurtzite crystal structure is measured as a function of the composition for 0.15<x<0.48. The band gap is measured using photocurrent spectroscopy on single InAs nanowires with a centrally placed InAs1-xPx segment. The wurtzite band gap is found to be about 120 meV larger than the corresponding zinc blende band gap over the entire composition range. The photocurrent spectrum is measured for excitation polarized parallel and perpendicular to the nanowire axis. The nanowires are found to have a large polarization dependence of the photocurrent, which is explained by the difference in dielectric constant of the nanowire and the surrounding air. The large polarization dependence in combination with the tunable band gap and the low dark current due to the band edge offset in the heterostructure, makes such nanowires possible candidates for polarization-sensitive photodetectors in the infrared. The effect on the optical properties of the crystal structure is further investigated by comparing the spectral excitation power dependence of InP nanowires with zinc blende crystal structure and InP nanowires with a high density of rotational twins. The difference in excitation power dependence is explained by interpreting the rotational twins as monolayer thick wurtzite segments. The rotationally twinned structure responds to the light as a type II heterostructure due to the type II offset between the zinc blende and wurtzite energy bands. p- and n-doped InP nanowires are studied with photoluminescence spectroscopy. The radial band bending caused by the Fermi level pinning at the surface, causes the electrons and holes to be separated radially and this is observed as a lowering of the photoluminescence energy. This is further investigated by applying a gate voltage on the nanowire sample to change the band bending, and observe the changes in the photoluminescence signal. This could potentially be used for investigating the doping concentration in such nanowires. Core-shell nanowires with GaAs core and a larger band gap GaxIn1-xP shell are studied by photoluminescence and time-resolved photoluminescence spectroscopy. It is observed that the photoluminescence decay is fast, indicating that the decay is dominated by non-radiative recombination also with a passivating shell on the nanowire. The charge carrier transport from the shell to the core is partially hindered at the low temperatures used (10~K). The photoluminescence decay is modelled by simple rate equations, with qualitative agreement with the experiments. It is also studied how the strain from the lattice mismatched shell, and the choice of substrate (Si or GaP) affects the photoluminescence intensity and decay time. It is found that the maximum PL intensity is obtained for unstrained nanowires. A smaller part of the thesis describes photoluminescence measurements on the conjugated polymer MEH-PPV (poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene]). The measurements are performed on single polymer chains dispersed in a PMMA matrix. The polymer spectra acquired at room temperature and 20~K are compared to obtain information about the conformational dynamics of the polymer chain. It is observed that at 20 K, the photoluminescence spectrum has a narrow line width and there is a large spread in the distribution of the spectral maxima. This was explained by assuming that at this low temperature, the thermal energy was not enough to allow conformational changes, and each single chain is frozen in a specific conformation. At room temperature conformational changes are possible, resulting in the single chain spectra being broad with only small inhomogeneous broadening of the ensemble spectrum

Ultrafast second-stokes diamond raman laser

We report a synchronously-pumped femtosecond diamond Raman laser operating with a tunable second-Stokes output. Pumped using a mode-locked Ti:sapphire laser at 840-910 nm with a duration of 165 fs, the second-Stokes wavelength was tuneable from 1082 - 1200 nm with subpicosecond duration. Our results demonstrate potential for cascaded Raman conversion to extend the wavelength coverage of standard laser sources to new regions

Label-free CARS microscopy through a multimode fibre endoscope

Multimode fibres have recently been employed as high-resolution ultra-thin endoscopes, capable of imaging biological structures deep inside tissue in vivo. Here, we extend this technique to label-free non-linear microscopy with chemical contrast using coherent anti-Stokes Raman scattering (CARS) through a multimode fibre endoscope, which opens up new avenues for instant and in-situ diagnosis of potentially malignant tissue. We use a commercial 125 µm diameter, 0.29 NA GRIN fibre, and wavefront shaping on an SLM is used to create foci that are scanned behind the fibre facet across the sample. The chemical selectivity is demonstrated by imaging 2 µm polystyrene and 2.5 µm PMMA beads with per pixel integration time as low as 1 ms for epi-detection.Multimode fibres have recently been employed as high-resolution ultra-thin endoscopes, capable of imaging biological structures deep inside tissue in vivo. Here, we extend this technique to label-free non-linear microscopy with chemical contrast using coherent anti-Stokes Raman scattering (CARS) through a multimode fibre endoscope, which opens up new avenues for instant and in-situ diagnosis of potentially malignant tissue. We use a commercial 125 µm diameter, 0.29 NA GRIN fibre, and wavefront shaping on an SLM is used to create foci that are scanned behind the fibre facet across the sample. The chemical selectivity is demonstrated by imaging 2 µm polystyrene and 2.5 µm PMMA beads with per pixel integration time as low as 1 ms for epi-detection

A novel optical microscope for imaging large embryos and tissue volumes with sub-cellular resolution throughout

Current optical microscope objectives of low magnification have low numerical aperture and therefore have too little depth resolution and discrimination to perform well in confocal and nonlinear microscopy. This is a serious limitation in important areas, including the phenotypic screening of human genes in transgenic mice by study of embryos undergoing advanced organogenesis. We have built an optical lens system for 3D imaging of objects up to 6 mm wide and 3 mm thick with depth resolution of only a few microns instead of the tens of microns currently attained, allowing sub-cellular detail to be resolved throughout the volume. We present this lens, called the Mesolens, with performance data and images from biological specimens including confocal images of whole fixed and intact fluorescently-stained 12.5-day old mouse embryos

Label-free imaging of thick tissue at 1550nm using a femtosecond optical parametric generator

We have developed a simple wavelength tunable optical parametric generator (OPG), emitting broad band ultrashort pulses with peak wavelengths at 1530-1790 nm, for nonlinear label-free microscopy. The OPG consists of a periodically poled lithium niobate crystal, pumped at 1064 nm by a ultrafast Yb:fiber laser with high pulse energy. We demonstrate that this OPG can be used for label-free imaging, by third harmonic generation, of nuclei of brain cells and blood vessels in a >150 µm thick brain tissue section, with very little decay of intensity with imaging depth and no visible damage to the tissue at an incident average power of 15 mW

Polarization-resolved second-harmonic generation imaging through a multimode fiber

Multimode fiber-based endoscopes have recently emerged as a tool for minimally invasive endoscopy in tissue, at depths well beyond the reach of multiphoton imaging. Here, we demonstrate label-free second-harmonic generation (SHG) microscopy through such a fiber endoscope. We simultaneously fully control the excitation polarization state and the spatial distribution of the light at the fiber tip, and we use this to implement polarization-resolved SHG imaging, which allows imaging and identification of structural proteins such as collagen and myosin. We image mouse tail tendon and heart tissue, employing the endoscope at depths up to 1 mm, demonstrating that we can differentiate these structural proteins. This method has the potential for enabling instant and in situ diagnosis of tumors and fibrotic conditions in sensitive tissue with minimal damage

In situ etching for total control over axial and radial nanowire growth

We report a method using in situ etching to decouple the axial from the radial nanowire growth pathway, independent of other growth parameters. Thereby a wide range of growth parameters can be explored to improve the nanowire properties without concern of tapering or excess structural defects formed during radial growth. We demonstrate the method using etching by HCl during InP nanowire growth. The improved crystal quality of etched nanowires is indicated by strongly enhanced photoluminescence as compared to reference nanowires obtained without etching

Widefield two-photon excitation without scanning : live cell microscopy with high time resolution and low photo-bleaching

We demonstrate fluorescence imaging by two-photon excitation without scanning in biological specimens as previously described by Hwang and co-workers, but with an increased field size and with framing rates of up to 100 Hz. During recordings of synaptically-driven Ca2+ events in primary rat hippocampal neurone cultures loaded with the fluorescent Ca2+ indicator Fluo-4 AM, we have observed greatly reduced photo-bleaching in comparison with single-photon excitation. This method, which requires no costly additions to the microscope, promises to be useful for work where high time-resolution is required

Temperature effect on single chain MEH-PPV spectra

We report single molecule photoluminescence spectra of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) in PMMA-matrix at 293 and 20 K with 90 s acquisition time. All spectra at 293 K are very similar, contrary to literature results with acquisition times < 5 s. We propose that fluctuations of conformations with distinct emission properties cause broadening of the spectra and reduce the differences in fluorescence spectra between separate chains. At 20 K the spectrum from a single chain consists of a few narrow peaks with absence of major spectral evolution within tens-of-minutes. The emission from distinct chains is spread over 3000 cm(-1) characterizing different frozen conformations. (C) 2004 Elsevier B.V. All rights reserved