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Scanning electron microscopy close-up of a coupled ring-CSEL pair with a coupling gap of . The outer sidewalls of the coupling region are kept free from any silicon nitride (SiN) and gold (Au) depositions over a distance of around the point of closest distance of both cavities.
The LI-characteristics of the phase locked ring-CSEL pair with a coupling gap of and for the noncoupled structure ( coupling gap). For the mapping see the legend. The curves show the collected optical power for a combined operation of the ring-CSELs (driven in parallel from the same power supply, thick lines) and the sum of single device LI curves when operated individually (dashed-dotted lines). The considerable deviation in threshold and optical power of these two values for the coupled structure (black lines) is due to the occurring intercavity losses.
(a) Emission spectrum for combined operation (solid line) and individual operation of ring-CSELs (dashed and dotted lines). Inset depicts the simulated radial mode distribution in the wide ring ridge. The field leaking into the air extends over a distance of . (b) A close up of the two dominating emission peaks between 1255 and . Due to equivalent geometries of both devices, the peaks are situated at roughly the same wavenumbers. (c) In order to identify phase-locking, 40 mA offset current was added to ring-CSEL 2, which lead to a redshift of (dashed line). In spite of this spectral separation of the single peaks, they still couple to common emission lines at combined operation.
The ring-CSEL pair with a coupling gap of exhibits shoulders (double peaks) in its emission spectrum for combined operation (see inset). Due to fabrication irregularities the single devices already emit at different wavenumbers, making the application of a spectral redshift for statements about coherent coupling redundant. The appearing double peaks are an indication for independent (nonphase-locked) operation.
(a) Far field of the coherent coupled ring-CSEL pair with a coupling gap of . Coherence is observed by the star shape interference pattern around the central bright ring. For comparison (b) shows the far field of a noncoherently emitting device pair, showing the same geometry but a coupling gap of .
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