Fig. 1 | Nature Communications

Fig. 1

From: High power surface emitting terahertz laser with hybrid second- and fourth-order Bragg gratings

Fig. 1

The hybrid second-order and fourth-order grating scheme. a Schematic depicting Bragg diffraction due to periodic gratings in an optical waveguide. ki is the incident wavevector of the guided wave with a wavelength λwg and kd is the wavevector of a diffracted wave, θd is the angle of diffracted wave. b DFB due to second-order gratings, in which first-order Bragg diffraction \(( {k_{\rm {d}}^{(1)}} )\) is in the surface-normal direction and second-order Bragg diffraction \(( {k_{\rm {d}}^{(2)}} )\) is in the opposite direction to the guided incident wave that establishes DFB. Symmetric (large radiative out-coupling) and antisymmetric (small radiative out-coupling) resonant modes are established at the edges of the photonic bandgap due to DFB. c A hybrid grating concept that combines second-order and fourth-order Bragg gratings. The second-order grating provides stronger DFB, and leads to establishing resonant optical modes with similar phase relationships to the grating as in b. An added fourth-order grating serves to enhance the out-coupling of the antisymmetric mode and reduce that of the symmetric mode. By adjusting design parameters, either of those modes could be excited in a DFB laser cavity that has a greater radiative efficiency compared to the excited mode for a second-order DFB cavity

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