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Left: computed Bragg spectra of dual-slit gratings with a period and different slit spacing. An optical mode created by distributed feedback from the peak at is coupled out vertically by the peak at . Their relative strength is determined by the slit spacing. Right: top views of the corresponding devices.
(a) Computed eigenfrequencies of a dual-slit grating as a function of slit spacing. Solid lines and points show the two band-edge modes of an infinite and finite length grating, respectively. The period is varied with the slit spacing (dashed line), in order to keep the frequency of the lower band-edge constant. (b) Computed far-field in the plane formed by device-axis and surface normal for the symmetric and antisymmetric modes of a single-slit (black) DFB and for the symmetric mode of a dual-slit (gray, red online) device with (normalized to the resonator energy).
(a) Surface loss rates of the symmetric mode at constant eigenfrequency, computed with different models. Blue squares are obtained by integration of the far-field (b) resulting from the near-field extracted from a 2D finite-length, finite-width model (c). The shown eigenmode is computed for a device with . The far-field is computed on a spherical wedge extending ±30° from the surface normal. While the various models show similar values up to a ratio of 0.6, the blue squares have a very different trend for larger slit separation, which is in better agreement with the experiment.
Light-current characteristics of devices with different slit spacing, measured for 400 ns long pulses at a 25 kHz repetition rate (1% duty cycle). Narrow and broad devices are plotted in the left and right panel, respectively, on different scales. In both cases, the slope efficiency seems to peak around . The maximum power of the first order DFB is about four orders of magnitude below the other devices . In the case of the largest slit spacing (0.7), the asymmetric mode reaches threshold first (power collected below 2.3 A), so the slope efficiency is not to be compared. The inset shows the spectrum of the device with slit spacing of 0.60 at 2.45 A on a logarithmic scale.
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