Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

develops at f1.25 mJ/cm 2 (the lasing threshold is clearly seen in the pumpdependentmode intensity in the inset in Fig. 26b). The position of this mode(612.0 nm) corresponds to the optical-gain maximum (solid circles in Fig.26b). As the pump fluence is increased, additional WG modes develop on thelow-energy side of the 612.0-nm mode, which is consistent with an asymmetricshape of the gain band. From the intermode spacing (f0.75 nm), one canderive an effective index of the gain medium of 1.8, indicating that WG modesare confined primarily within the NQD layer.In Ref. 63, NQD lasing structures were fabricated by combining solgeltitania waveguides [11] containing large NQD volume fractions withDFB gratings (Fig. 27a, inset). The gratings were fabricated by patterningthermal oxide layers on silicon substrates. The structures were completed byspin-coating thin NQD/titania films on the top of the gratings.In the DFB/NQD structure, the refractive index is spatially modulateddue to the thickness modulation of the nanocrystal/titania film caused by theunderlying grating. This modulation leads to the formation of the stop bandwith a width of f10 nm. Above the lasing threshold, a sharp mode at theedge of the stop band grows superlinearly as the pump power is increased (seespectrum in Fig. 27a). The emission from the front of the structure collapsesinto a visible laser beam, and the luminescence spot on the film tightens into abright narrow line indicating a transition to the lasing regime. The DFBstructures were used to demonstrate lasing with a tunable emission colorusing NQDs of different sizes, as illustrated in Fig. 27b. This figure showssharp lasing lines (solid lines) that are compared to spontaneous emissionspectra (dashed lines) recorded in the ‘‘subthreshold’’ regime. The lasingspectrum was tunable from 621 to 560 nm by changing the NQD radii from2.7 to 1.7 nm.VI.CONCLUSIONS AND OUTLOOKOptical gain and lasing in strongly confined, colloidal quantum dots is a new,exciting area in NQD research. Due to very small sizes and the strongFigure 27 (a) The room-temperature laser spectrum of a DFB device fabricatedusing core-shell CdSe/ZnS NQDs with a 2.5-nm CdSe core radius. Inset: the scanningelectron microphotograph of a typical DFB device used in Ref. 63. (b) Spontaneousemission spectra (dashed lines) of CdSe NQDs with radii 2.7, 2.4, 2.1, and 1.7 nm (sizedecreases from left to right) in comparison with lasing spectra (solid lines) of thesame samples incorporated into a DFB device. The lasing emission occurs at 621 nm(R = 2.7 nm), 607 nm (R = 2.4 nm), 583 nm (R = 2.1 nm), and 560 nm (R = 1.7 nm).Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.