Colloidal CdSe/ZnS Quantum Dots- Application to Microdisk Lasers and Study on Resonance Energy Transfer

• Main Authors: Yu-Ming Hsiang, 向育民
• Other Authors: Ming-Hua Mao
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/nve7he

碩士 === 國立臺灣大學 === 光電工程學研究所 === 107 === In this thesis, we use CdSe/ZnS colloidal quantum dots as active medium to fabricate microdisk resonators with sandwiched structure, and use atomic layer deposition (ALD) technique to passivate Al2O3 on quantum dots, which can prevent quantum dots from being destroyed by plasma gas and enhance light stability in the air. Finally, we produce red-emitting microdisk lasers with low threshold power density and high photo-stability operating in continuous wave excitation at room temperature. A 10μm-diameter microdisk laser with quantum-dot active medium encapsulated by Al2O3 in all directions exhibits a threshold power density of 153 kW/cm2 and quality factor of 1280. Lasing characteristics can be observed by changing the excitation power. At below threshold, we can observe small whispering gallery modes. The spectral width narrows as the excitation power density increases. It confirms the transition from spontaneous emission to stimulated emission of the laser device. In addition, whispering gallery modes can also be observed from the microdisks with yellow-emitting quantum dots as active medium. However, luminescence intensity of yellow-emitting quantum dots has been significantly reduced after processed by ALD and PECVD, resulting in gain in the resonant cavity insufficient to overcome loss. No lasing action has been observed from these yellow-emitting quantum dots. The microdisk with yellow-emitting and red-emitting quantum dots mixed as the active layer doesn’t show laser action because the red-emitting quantum dot density becomes lower after mixture. If higher concentration of yellow-emitting quantum dots is used to compensate their luminescence loss during the process, luminescence intensity of yellow-emitting quantum dots can be comparable to that of red-emitting quantum dots at low excitation power. As the excitation power increases, luminescence intensity of yellow-emitting quantum dots can become higher than that of red-emitting quantum dots due to band-filling effect. In order to investigate Förster resonance energy transfer effect, we perform time resolved and spectrally resolved photoluminescence measurement. It is found that quantum dots with longer emission wavelength exhibit a delayed photoluminescence decay and their carrier lifetime is longer. These phenomena indicate the existence of FRET. Furthermore, with increasing the average distance between quantum dots by diluting quantum dot solution, FRET effect will be reduced. Finally, using yellow-emitting and red-emitting quantum dots mixed as active medium, we observe much shortened carrier lifetime in a microdisk cavity in comparison with that in the thin film. This phenomenon is attributed to the Purcell effect of the cavity and the increased nonradiative recombination due to the small cavity size. FRET effect is less likely to be observed in this case.