We demonstrate an improvement in the wall plug efficiency in a bound-to-continuum quantum cascade laser operating below 2 THz. This was achieved by targeting one perceived weakness in a previous design, extraction from the lower laser state. This targeted improvement was made possible by employing an automated search algorithm to select a design giving rise to specified electronic states. The resulting laser design represents the highest wall plug efficiency below 2 THz and the lowest frequency devices to operate in the high-power single plasmon waveguide. The device displays a wall plug efficiency of 1.2% per facet, double the reference design.