(a) Illustration of a packaged 6 mm long THz QCL. (b) Layer structure within the QCL. Simulated cross-sectional fundamental mode profiles within (c) a standard SI-SP waveguide at 2.85 THz and (d) the same structure after the removal of the metal overlayers. Scale bars 50 μm.
(a) Composite SEM image of an entire 6 mm long THz QCL ridge, showing electrical contacts, bond wires and ADFB region (dashed line). Scale bar 500 μm. (b) ADFB grating design. Vertical lines represent milled slits with a minimum separation of Λ, dashes represent additional Λ/2 lengths. (c) Composite SEM image of the ADFB grating milled into a THz QCL ridge. Scale bar 200 μm. (d) Short section of an ADFB grating. Scale bar 50 μm. (e) High-magnification image of a milled slit. Scale bar 500 nm. (f) Cross-section through a milled slit of similar depth (though wider) in a second device. Scale bar 1 μm. (g) Dimensions of markers (cross and squares) milled alongside the QCL ridge with two overlapping ADFB sections (solid and dashed lines) (h) Typical marker alignment after ADFB milling, with sub-μm stitching errors in x- and y-directions. Scale bar 5 μm.
THz QCL emission spectra before (dashed) and after (solid) introduction of ADFB gratings, along with the frequency placement of the grating reflectivity responses (lower panels). Vertical dotted lines indicate the centre frequency (f N) of the grating responses.
(a) Electrical and output power characteristics of three QCLs before and after ADFB introduction. (b) Top: thickness of the upper SI-SP waveguide layers. Below: representative SEM images of various ADFB device slits (scale bars 500 nm), along with illustrations of their estimated cross-sectional milling profiles. (c) Number of lasing modes (above a 10 dB cut-off on a normalised power scale) versus estimated milling depth. (d) Average SMSR of the dominant post-FIB lasing modes. (e) Additional losses introduced to various QCLs by FIB-milled ADFB gratings, calculated from the elevated threshold driving current densities. Trend lines (dashed) in (c)-(e) have the same exponent (d = z − 235, δ = 110), revealing similar non-linearities in depth dependence of the presented device parameters.
(a) Emission spectra from device G, before and after ADFB introduction. (b) Emission spectra from device H, before and after ADFB introduction. Vertical dotted lines indicate the centre frequency (f N) of the grating responses. Lower panels: ADFB reflectivity responses.
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