Extended Data Fig. 8. Coherence measurement of edge-emitted EL conducted using Michelson interferometry.

a, Interferogram of the 1S ASE band of the BRW device driven at about 100 A cm⁻². The edge-emitted light is fibre coupled into a Michelson interferometer consisting of a cube beam splitter, a fixed arm and a variable-length arm controlled by a piezoelectric stage. The interference signal is detected with a Si avalanche photodiode. 1S ASE is isolated using a 620-nm (or 2-eV) long-pass filter and a linear polarizer set to the ‘vertical’ (TE) polarization (Fig. 3d, left). b, Fringe visibility extracted from the interferogram in a (black symbols) as a function of the time delay between the two arms of the interferometer. An exponential fit of the experimental data (red line) yields coherence time τ_c of 47 fs. c,d The interferometric measurements conducted for the ‘horizontal’ (TM) polarization in which ASE is suppressed (Fig. 3d, left) yield a much shorter coherence time of 16 fs. The observed polarization-dependent threefold difference in τ_c indicates the presence of two subsets of photons with distinct temporal coherences. As expected for the ASE effect, the photons produced by stimulated emission (they dominate the edge-emitted TE-polarized light) exhibit a longer τ_c compared with photons produced by spontaneous emission (they dominate the edge-emitted TM-polarized light). The measured coherence times are consistent with those inferred from τ_c ≈ (πΔν)⁻¹, in which Δν is the emission linewidth in the frequency domain. In particular, based on the linewidths of the 1S spontaneous and ASE bands (82 to 30 meV, respectively, for the sample used in the studies of coherence), the estimated coherence times are 16 fs and 44 fs. Both values are in excellent agreement with the results of the interferometric measurements (16 fs and 47 fs, respectively).