TECHNOLOGY FOCUS - ELECTRONICS - Preferred Circuit: In the microwave circuit, the heterostructure is an alternation of two different semiconductor materials, selectively n-doped, the modulation of the conduction band offset resulting in a quantum potential consisting of a sequence of quantum wells for the n-type carriers. The heterostructure is made with materials selected from GaAs-AlxGa1-xAs where x is typically 0.1-0.3, grown on a gallium arsenide (GaAs) substrate, or In0.53Ga0.47As-Al0.48In0.52As grown on indium phosphide (InP) substrates. The active region is a gain medium capable of serving as the active region of a quantum cascade laser, where the active region is specifically adapted to be alimented with a bias current to bring the active region into lasing mode. The microwave circuit has a resonance frequency of 0.5-100 THz, preferably between 1-10 THz. The inductive portion has an inductance L, the capacitive portion has a capacitance C, and the resonance frequency of the circuit is 1/(2 pi square root(LC)). The inductive and capacitive portions have a volume that is smaller than the cube of the characteristic free space wavelength at the resonance frequency, preferably smaller by a factor of 100000-10000000. The active region is alimented with a bias current applied parallel or perpendicular to the heterostructure layers. The active region is emitting at the resonance frequency of the microwave circuit, where the active region is specifically adapted to be alimented with a bias current to bring the active region into emission mode. The active region is absorbing at the resonance frequency of the microwave circuit. The active region is specifically adapted to be alimented with a bias current to bring the active region into the absorption mode. The active region consists of parabolic wells. Preferred Laser: The laser has an emission frequency, the emission frequency being adjustable by adjusting the geometry of an inductive and/or a capacitive portion of the microwave circuit.