the multiexcitonic gain mechanism which leads to very high cw lasing thresholds that are well above the nc photostability limit for single exciton gain the intrinsic gain dynamics are determined by the radiative single exciton lifetime which are typically orders of magnitude longer than auger decay time constants the difference in relaxation behaviors for single and biexciton gain mechanisms is illustrated in the inset of fig5 a which shows relaxation dynamics of u03b4 u03b1 measured at the onset of optical gain for type i circles 25 and type ii circles 30 samples in type i ncs the measured decay time is 30 ps while it is more than 50 times longer 1700 ps for the type ii ncs thus the present invention provides the first practical demonstration of nc structures that produce optical amplification due to stimulated emission of single exciton states which eliminates complications associated with ultrafast multiexciton auger recombination this new approach makes use of dynamic splitting of the degeneracy of the lowest energy emitting transition by giant x x interactions that develop in type ii hetero structures upon spatial separation of electrons and holes implementation of the single exciton gain regime could allow reduction of the lasing threshold under cw excitation by orders of magnitude which could significantly enhance the technological potential of colloidal ncs as u201c soft u201d chemically processible optical gain media the preparation of materials used in the present invention is more particularly described in the following.