Evidence of random laserFigure 4 di

Evidence of random laserFigure 4 displays laser emission spectra of the random laser deviceunder 40-mA injection current collected at different angles, spanningfrom −45° to +45° with respect to surface normal. The angle-dependentEL spectra show that the sharp lasing peaks appear in alldirections and exhibit the same feature with respect to the measuredangle. Broad-angle emission is a distinct evidence of random lasers,which distinguishes themselves from conventional lasers that arehighly directional. The angle-dependent measurement also excludesthe possibility that the observed sharp peaks arise from the cavityresonance in between the top and bottom electrodes. In addition, asin the cross-sectional SEM image shown in Fig. 1B, the thickness ofthe device in-between the top and bottom electrodes is much lessthan the emission wavelength; hence, the possibility of a single resonatorformed by multilayer stacking in the device can be eliminated.This result is also supported by the fact that as the QD-based devicecontains pure 630- or 540-nm QDs, no laser action and interferencepatterns are observed under high injection current, as respectivelyexhibited in figs. S1 and S2.