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Researchers at SPIE have now produced a solution-processed, mechanically flexible vertical- cavity surface-emitting laser (VCSEL) and planar photonic devices using colloidal quantum dots as the active medium.


The entire VCSEL structure, including a distributed- Bragg-reflector (DBR) mirror, is fabricated by spin coating on a glass substrate.


Figure 1(a) shows a schematic drawing of the 1D microcavity. Alternating layers of polymers of two different refractive indices were stacked to form the DBR mirrors. The high- and low-refractive-index polymers chosen were poly(N-vinylcarbazole) (PVK) and cellulose acetate (CA), with refractive indices of 1.683 and 1.475 at 600nm, respectively.


The scientists chose solvents in a way that prevented the solvent for one polymer from dissolving the other ; PVK is soluble in nonpolar solvents (such as chlorobenzene), while CA is soluble in polar solvents, such as alcohol.


The half-wavelength-cavity layer was formed by Indium Gallium Phosphide (InGaP) quantum dots emitting at 650nm, embedded in a PVK host.


The fabricated microcavity structure was peeled off the glass substrate to form a freestanding structure.


Figure 1(b) shows photographs of the microcavity, both freestanding and on a cylindrical surface. The device demonstrated lasing under optical pumping. Figure 1(c) shows the average output power as a function of average pump power, indicating a lasing threshold of 27mW and a slope efficiency (top emission) of ~12%.


Figure 1. (a) Schematic drawing of a solution- processed, all-polymer microcavity. (b) Photograph of a freestanding microcavity containing embedded quantum dots. The scale bar corresponds to 2.54cm. (c) Output power of the flexible-microcavity surface-emitting laser as a function of input optical- pump power. The insets show the emission spectra above threshold and the full width at half maximum (FWHM) of the lasing spectra as a function of pump power. PVK: Poly(N-vinylcarbazole). CA: Cellulose acetate. QDs: Quantum dots.


In a different demonstration, the researchers realized vertically integrated, 3D photonic circuits using a bottom-up approach. This differs considerably from the top-down approach that is widely employed in traditional semiconductor-based PICs.


It was found that use of colloidal quantum dots in a polymer host matrix as the active medium for emission, amplification, and detection has the advantage of compatibility with silicon technology and coverage of a wide spectral range.


All circuits were fabricated on a silicon-on-insulator substrate. Employing silicon for passive structures, such as waveguides, resonators, and couplers, has the additional benefit of enabling smaller-footprint


54 www.compoundsemiconductor.net August/September 2010


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