Fig. 1. Computed two-dimensional mode profiles of the considered waveguide configurations. The upper panel (a) shows the fundamental 2.5 THz eigenmode of the standard QCL waveguide described in [12] (see this reference for thicknesses and doping concentrations). The left panel (b) shows the same result for a ridge where the active region is covered by 200 nm of high-doped GaAs, but no metal was deposited. The two different SPs bound to the top and bottom contact layer are clearly distinguishable. The right panel (c) shows the case where the top contact layer was also etched away. The computed losses are α=9.5 cm^-1 in (a), α=105 cm^-1 in (b), α=395 cm^-1 in (c). Note the very high value of the latter configuration; this is due to the mode being tightly bound to the buried layer, close to a frequency singularity of the surface plasmon propagation.

Fig. 2. Finite elements calculation of the reflected power from various SP gratings as a function of the frequency of the waveguide mode. This allows a quantitative comparison of the efficiencies of different grating structures. The red line shows the reflectance from a grating where only the metal layer is discontinuous, and the surface plasmon can still propagate at the highly doped top-contact layer. If this contact layer is also removed by wet etching, the reflectance is considerably increased, as is shown by the black line. The other lines also correspond to etched gratings. For both the first and second order grating, a slit width of 2 µm is the optimum choice. The grating period for first and second order grating are 16.5 µm and 33 µm respectively, the total length of the domain is 400 µm.

Fig. 3. Light-current characteristics of a 2.5 mm long DFB laser with a grating period of 16.5 µm, collected at various heat sink temperatures. They were measured in pulsed operation at a repetition rate of 400 Hz, using burst of 200 ns pulses for an overall duty-cycle of 5 %. Power calibration was performed at 9 K driving the laser at 50 % duty-cycle. The inset shows an example spectrum recorded at 50 K under an injection current of 0.86 A. The observed laser linewidth is limited by the resolution of the experimental apparatus.

Fig. 4. Vertically emitted power of a 2 mm long and 300 µm wide laser ridge with a grating period of 36.2 µm, recorded at various heat sink temperatures without any collecting lens. The current-voltage characteristics measured at 10 K is also shown.

Fig. 5. Laser emission spectra recorded in the vertical direction from two different devices, with 36.2 µm and 36.4 µm gratings respectively. A scanning electron microscope (SEM) picture of the 36.2 µm grating is also reported. On the right the annealed AuGe stripe used for the ohmic contact on top of the laser ridge is clearly visible.

Fig. 6. Angular distribution of the vertical emission from the laser with a grating period of 36.2 µm. The grating extends over 1.5 mm, and the device is 2 mm long and 300 µm wide. As expected, the beam profile is significantly narrower in the direction of the grating. The deviation of the maximum from 90° may be due to the non-perfectly-parallel sample mount inside the cryostat, but may also originate from a slight asymmetric placement of the grating with respect to the laser facets.

Fig. 7. Output power as a function of drive current for a 2.5 THz QC laser with 16.4 µm period DBR at the back facet. They have been measured in pulsed mode at 5 % duty-cycle, collecting the output from the front facet with f/1 parabolic optics. Inset: L-I characteristics as measured at 8 K from both the front (normal) and back (DBR) facet of a 2.3 THz laser with 18.1 µm period DBR. Linear fits of the slopes are plotted in dashed lines.

Fig. 8. FDTD calculation of the grating effect on mode propagation within the 2.5 THz laser waveguide. The amplitudes of the magnetic field component Hy recorded during the FDTD simulation are displayed. In black we report the spectrum of the injected wave, in red the amplitude of the transmitted wave after 1030 µm of propagation in an unstructured waveguide. The other curves display the amplitude of the reflected wave for a varying number of grating slits.