Abstract
We examine superradiant (SR) light or amplified spontaneous emission (ASE) from a whispering-gallery-mode (WGM) laser comprising various sized (CdSe) ZnS quantum dots (QDs) in colloidal liquid. Laser-induced fluorescence (LIF) with a full width at half maximum (FWHM) of 40 nm is observed when the colloidal QD system is pumped with 2 mJ of laser light at 355 nm (3rd harmonic of Nd:YAG laser). Under optimal conditions of pump energy and focusing, ASE at 520 nm with a bandwidth of Δλ=8 nm (FWHM) and divergence of 9 mrad is observed. When the QDs are embedded on a high-Q factor silica microsphere ((functionalized with an amine), SMA), they generate WGMs with random peak distributions. Finally, when all the QDs embedded in SMAs are mixed and placed in a cuvette, we obtain a “WGM laser” that is almost continuously tunable from 520 nm to 630 nm with a spectral width less than 2 nm (FWHM) in the WGM and less than 1.2 nm in the cavity mode. We believe that this is the first report on a frequency tunable laser obtained using (CdSe) ZnS QDs embedded in an SMA, exhibiting an efficiency of 0.06%.
Highlights
Quantum dots (QDs) are very small semiconductor particles of the order of few nanometers such that electron and holes are confined in all spatial dimensions [1], and this very small QD size affords optical and electronic properties different from those of larger particles
Here, we demonstrate a broadband frequency-tunable WGM laser based on QDs (CdSe/ZnS) embedded in a silica microsphere (QD@Si) with a tunable range from 510 to 640 nm
The (CdSe) ZnS QDs exhibit two distinct features. (i) There is a primary peak at longer wavelengths with a Stokes shift of ~20 nm with respect to the photoluminescence peak. (ii) The absorbance increases steadily in the shorter wavelength region and reaches a maximum in the UV region
Summary
Quantum dots (QDs) are very small semiconductor particles of the order of few nanometers such that electron and holes are confined in all spatial dimensions [1], and this very small QD size affords optical and electronic properties different from those of larger particles. If energy (for e.g., photons) that is greater than or equal to the band gap is supplied to such QDs, an electron-hole pair is created, and this electron-hole pair forms an exciton Such exciton acts like pseudohydrogen atoms, and for a brief period of time, they move together through a distance that is called the diffusion length [3,4,5]. In the begging years of QD discovery, researchers faced difficulties in gating laser from QDs, because of its eight-fold degeneracy [7,8,9,10,11]
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