Sample schematics with (a) top and (b) side view. Gain and loss cavities are sandwiched between TiO₂/SiO₂ distributed Bragg reflectors with 21 layers. They do overlap in the central part of the sample, separated by a third distributed Bragg reflector with 9 layers, leading to a coupled system. All reflector layers are of λ/4 optical thickness, cavity layers of λ/2 optical thickness, at design wavelength λ_D = 620 nm.