We have demonstrated a new approach to realize a continuous wave room temperature THz laser source: a low-pressure molecular gas is optically pumped by a solid-state mid-infrared source. A tunable quantum cascade laser allows to have access to transitions that are not accessible to CO 2 lasers. Here we present our last results concerning NH3: pure inversion transitions allows emission around 1 THz. At 1.073 THz a new cavity design is used and a power of 0.67 mW is obtained with a high differential efficiency of 6.4 mW/W. This improved output power will permit imaging and local oscillator applications.