Control of Spontaneous Emission from Colloidal Quantum Dots in a Polymer Microcavity
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Control of Spontaneous Emission from Colloidal Quantum Dots in a Polymer Microcavity Vinod Menon1, Nikesh Valappil1, Iosef Zeylikovich2, Taposh Gayen2, Bidyut Das2, and Robert R. Alfano2 1 Physics, Queens College - CUNY, 65-30 Kissena Blvd, Flushing, NY, 11367 2 Physics, City College- CUNY, 138 St & Convent Avenue, New York, NY, 10031
ABSTRACT We report the fabrication of a one dimensional microcavity structure embedded with colloidal CdSe/ZnS core/shell quantum dots using solution processing. The microcavity structures were fabricated by spin coating alternating layers of polymers of different refractive indices (poly-vinylcarbazole, and poly-acrylic acid) to form the distributed Bragg reflectors (DBRs). Greater than 90% reflectivity was obtained using ten periods of the structure. The one dimensional microcavity was formed by sandwiching a λ/n thick defect layer between two such DBRs. The microcavity demonstrated directionality in emission and well behaved dispersion characteristics. Room temperature time-resolved photoluminescence measurements carried out on this structure showed six fold enhancement of spontaneous emission rate. The photoluminescence decay time of the quantum dots was found to be ~ 1 ns while for the quantum dots embedded in the microcavity it was ~150 ps. INTRODUCTION Quantum dots (QD) are attractive as optically active materials for semiconductor lasers, light emitting diodes, amplifiers and chip-scale photonic signal processors due to their delta function like density of states, strong carrier confinement, inherently large optical nonlinearities, low optical power thresholds for absorption saturation and fast recovery times. These properties of QDs are expected to significantly improve device performance across different application platforms. Specifically, from an emitter stand point, the QD based devices are expected to have ultra pure spectral characteristics, high operational temperature and stability, low timing jitter and broad gain spectrum. Traditionally, QDs have been grown using molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD) or other related epitaxial processes. While these techniques have provided good quality QDs both in terms of optical and electronic properties, it has proved difficult and expensive to change the material properties and also to maintain such elaborate and costly fabrication systems. An alternative scheme that has been developed more recently is the colloidal chemistry route for synthesis of QDs [1-5]. The colloidal chemistry technique offers more versatility and lower production cost. In addition, the technique enables spin-coating based processing, possibility of self-assembly, compatibility with silicon platform and tunability for a wide array of materials with specific absorption and emission spectra. By providing a wide range of functional wavelengths without having to change material systems, the colloidal chemistry route gives the photonic device designer a new and very valuable degree of freedom.
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