The quantum cascade laser (QCL) is a powerful solid-state source of terahertz (THz) electromagnetic waves. We report a methodology by which the operating emission frequency of a THz QCL can be tuned by systematically altering the percentage thickness of the layers in a proven design. Two THz QCLs based on a GaAs/Al0.10Ga0.90. As heterostructure operating around 2 THz were investigated; the first a reference structure and the second a modified version of the reference's active region in which every layer of the design is reduced in thickness by 3%. High resolution X-ray diffraction confirmed that both structures' active regions were within 0.5% of the intended thickness, resulting in an actual thickness difference between the two structures of 2.1%. Both structures lased with similar operating performance figures; in pulsed operation, lasing threshold currents (similar to 110 A cm(-2)), peak output powers (similar to 30 mW) and maximum operating temperatures (similar to 70 K). In agreement with the predicted theoretical trend, the thinner structure lased at a higher frequency compared to the reference structure, with a shift of similar to 0.06 THz (0.25 meV) which is very close to the predicted 0.08 THz (similar to 0.3 meV) shift. (c) 2006 Elsevier B.V. All rights reserved.