In this work we have developed a new beam pattern measurement setup using a room-temperature pyrodetector and two PC controlled stepper motors. We measured the far-field beam patterns of surface plasmon QCLs, which are 217 μm or 157 μm wide, 1500 μm long, emitted at 2.84 THz in single mode. We discover that the beams contain two types of interference fringes. The type one has a spacing between two fringes which decreases with increasing angle, and occurs in the positive space (opposite to the substrate of the QCL). Type two has nearly equally, closely spaced rings and is observed in the negative space.
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