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Near-field spectroscopy of colloidal quantum dots and vertical cavity surface emitting lasers. / Mark David Holton

Swansea University Author: Mark David Holton

Abstract

A scanning near-field optical microscope (SNOM) has been employed for developments and measurements that allow spectroscopic characterisations on the nanoscale. To obtain spectral properties on spatial resolutions beyond the diffraction limit a spectrometer has been integrated into the SNOM system,...

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Published: 2007
Institution: Swansea University
Degree level: Doctoral
Degree name: Ph.D
URI: https://cronfa.swan.ac.uk/Record/cronfa42728
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Abstract: A scanning near-field optical microscope (SNOM) has been employed for developments and measurements that allow spectroscopic characterisations on the nanoscale. To obtain spectral properties on spatial resolutions beyond the diffraction limit a spectrometer has been integrated into the SNOM system, together with various avalanche photodiode detectors, a cooled charge-coupled device and various filters, suitable to the experiments conducted. The system was optimised such that it allowed subsequent probe-positioning to perform point spectroscopy, using localised stimulation and collection techniques in the near-field. This spectral detection scheme has been applied to two areas of study, laser devices and quantum dot systems. The simultaneous topographical and optical study of semiconductor lasers, specifically vertical cavity surface emitting lasers (VCSEL), was carried out to spatially and spectrally map individual transverse mode emissions at the aperture surfaces in the nearfield. These measurements showed the clear presence of modulation of the intensity of the transverse modes in the form of concentric rings. The effect was attributed to a subsurface defect within the aperture of the device, clipping the Gaussian emission profile of the fundamental transverse mode. Higher order transverse modes were also found to be affected, revealing a more complex modulation structure due to their non-Gaussian emission. Structural defects, in or at the surface of such devices, have been shown to have significant effects on far-field characteristics. It is therefore important to spatially map the spectral source of such effects to gain insight into their origin. Spectral SNOM studies on quantum dots were conducted on mixed colour cadmium selenide/zinc sulphide (CdSe/ZnS), and cadmium selenide/hexadecylamine CdSe(HDA) quantum dots. Low concentrations were immobilised within a 2-3 nm thick layer of a PMMA polymer matrix, spin coated onto cleaved mica. Optical stimulation in the nearfield revealed simultaneous topographic and optical detection of single and small clusters of quantum dots. Spectroscopic measurements of single and small clusters of quantum dots in the near-field, showed a minimum spectral full width half maximum (FWHM) of ~14-16 nm. Repeated imaging of single quantum dots also showed fluorescence intermittency events on a range of time scales from below the time resolution of the set-up, to over an hour, in addition to fluorescence brightening. Conclusions as to the potential for quantum dots in biological imaging are discussed.
Keywords: Optics.;Electrical engineering.
College: College of Science