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(Color online) Superpositioned HL interferograms with exposure dose ratio of 1:1 (a) and 1.5:1 (c) and the truncated square gratings after dry etching. (b) and (d) Frequency analyses of the interferograms and truncated gratings in (a) and (c). (e) Coupling coefficients of the truncated gratings in (a) and (c) with different grating depths. The shaded region is the targeted grating depth. The insets in (e) are two eigenmodes for the dual-period grating. (f) and (g) SEM images of dual-period grating in SiO2 hard mask after RIE (top view) and the grating shape after ICP etching and top Ti/Au contact deposition (cross-sectional view).
(Color online) Room temperature mid-IR (a) and THz (b) spectra at different currents of a 2-mm-long DFB device and its FP counterpart with a taper angle of 2.3°. Ith is the threshold current of the FP device.
(Color online) Room temperature mid-IR P-I-V characteristics for a 3-mm-long DFB device with a taper angle of 1°. The inset is the total-power far fields for the two wavelengths at low current (I = 8 A) and high current (I = 11 A), respectively.
(Color online) Room temperature THz peak power and mid-IR-power product characteristics for a 3-mm-long DFB device with a taper angle of 1°. The inset is the THz peak powers as a function of the mid-IR-power product. The THz power conversion efficiency is deduced from a linear fit.
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