Low-threshold anisotropic polychromatic emission from monodisperse quantum dots.

Colloidal quantum dots (QDs) are solution-processable semiconductor nanocrystals with favorable optoelectronic characteristics, one of which is their multi-excitonic behavior that enables broadband polychromatic light generation and amplification from monodisperse QDs. However, the practicality of this has been limited by the difficulty in achieving spatial separation and patterning of different colors as well as the high pumping intensity required to excite the multi-excitonic states. Here, we have addressed these issues by integrating monodisperse QDs in multi-excitonic states into a specially designed cavity, in which the QDs exhibit an anisotropic polychromatic emission (APE) characteristic that allows for tuning the emission from green to red by shifting the observation direction from perpendicular to lateral. Subsequently, the APE threshold under 300-ps pulsed excitation has been reduced from 32 to 21 μJ cm by optimizing the cavity structure. Based on the manipulation of multi-excitonic emission and angle-dependent wavelength selectivity of the developed cavity, we have fabricated a full-color micro-pixel array with a pixel size as small as 23 μm by combining cavity-integrated monodisperse QDs and blue backlight. Furthermore, the threshold of APE under quasi-continuous-wave pumping was as low as 5 W cm, indicating its compatibility with commercial LEDs and/or laser diodes. Since APE arises from the multi-excitonic behavior of QDs that supports optical gain, its unprecedentedly low threshold suggests the feasibility of the diode-pumped colloidal QD laser. This work demonstrates a novel method of manipulating the QDs' optical properties beyond controlling their size, composition or structure, and reveals great potential for achieving full-color emission using monodisperse QDs.