Colloidal quantum dot and hybrid lasers

Student thesis: Doctoral Thesis

Abstract

This thesis presents research on solution-processed semiconductor lasers, with an emphasis on colloidal quantum dot (CQD) lasers, aimed at progressing the state of the art of the technology. A particular focus is placed on demonstrating such lasers under long (>nanosecond) pulse pumping in order to bring this device technology closer to applications. Alloyed-core CdSSe CQDs with a ZnS outer shell that emit in the visible are studied. These were prepared in a solid-state thin-film overcoated with a polymeric cladding (polyvinyl alcohol or PVA) to demonstrate a distributed feedback(DFB) laser having a bilayer waveguide structure. It is shown that the PVA symmetrises the laser structure and reduces the modal losses, in turn enabling the lowest threshold operation in the nanosecond regime for CQD lasers at the time;13.5 μJ/cm2 at 5 ns. The format is also beneficial for the photostability of the laser. The CQDs are also demonstrated as the gain material of a robust, orange-emitting Vertical-Cavity Surface-Emitting Laser (VCSEL). Oscillation threshold comprised between 1 and 20 mJ/cm2 for 5ns pump pulse duration are obtained. For a broad area excitation (250 μm at 1/e2 radius), the operation is highly multimode and characterised by spatio-temporal instabilities.When pumped with a spot size at or below 100μm in radius, the VCSEL is observed to be single mode for low pump energies while additional longitudinal modes, as well as transverse mode,appear as the pumping level is increased. A study of the polarisation of the CQD laser emission for different polarisation of the pump was carried out and the Stokes parameters calculated. Results show that the pump polarisation has no significant effect on the VCSEL polarisation. While the bulk of the research concerns CQDs, two types of organic semiconductors were also applied to the aforementioned DFB laser structure. Poly(paraphenylenevinylene) (PPV) copolymer Super Yellow leading to what is to the best of our knowledge, the only case of completely ambient lasing using Super Yellow. Refractive index sensing with the laser was attempted and results are summarised. The second type in fact consisted of two organic semiconductor materials from the same novel family of autofluorescence molecules respectively named Blue 4 and Blue 6. These are investigated for the first time as laser materials. Both materialsshow signs of random lasing when prepared in a solid-state film. Single modelasing in distributed feedback format is also demonstrated with Blue 4.
Date of Award1 Apr 2017
LanguageEnglish
Awarding Institution
  • University Of Strathclyde
SupervisorNicolas Laurand (Supervisor) & Martin Dawson (Supervisor)

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