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straight topological waveguide located just below the triangular laser
cavity (Fig. 4a). Each CW (CCW) cavity mode evanescently couples to
the straight waveguide, propagates to the right (left) and then outcou-
ples via a second-order grating. This sample is found to support three
topological lasing modes with frequencies near 3.2 THz. By selectively
covering the left or the right side of the device, we observe that each
lasing mode emits with approximately equal intensities from the two
facets (Fig. 4c and Extended Data Fig. 10a), indicating that the CW and
CCW cavity modes have equal weights. For comparison, we observe
that the same sample also supports non-topological lasing modes in
a neighbouring frequency range, just above the photonic bandgap
(around 3.4 THz), at high pumping currents, for example at 2.96 A.
The non-topological lasing modes are observed to emit with very dif-
ferent intensities from the two output facets (Fig. 4d and Extended
Data Fig. 10b). This demonstrates a qualitative difference in behaviour
between topological and non-topological lasing modes in a single
device.
In summary, we have implemented electrically pumped lasers based
on the topological edge states of a valley photonic crystal, operating
in the THz frequency regime. By investigating several different device
configurations, we have established a chain of evidence demonstrating
the running-wave features of the topological lasing modes. The most
noteworthy observation is the regular mode spacing, which arises
because the modes have running-wave characteristics despite the
sharp corners of the cavity and various other disturbances. Looking
ahead, there are further opportunities in using the valley degree of
freedom in other active photonic devices, and the realization of an
electrically pumped topological laser points the way towards incor-
porating topological protection into practical device applications.
Apart from promising applications as a robust THz light source, this
QCL platform may find immediate use in exploring the dynamical and
nonlinear features of topological laser modes^33.
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