Quantum cascade laser: a unipolar intersubband semiconductor laser operating at 125 K

A new semiconductor injection laser (Quantum Cascade Laser) which differs in a fundamental way fiom diode lasers has been demonstrated. It relies on only one type of carrier (it is a unipolar semiconductor laser), and on electronic transitions between conduction band energy levels of quantum wells. Such intersubband lasers were originally proposed 25 years ago, but despite considerable effort thip ;s the first structure to achieve laser action. The present device operates at a wavelength of 4.26 microns, but since the wavelength is entirely determined by quantum confinement, it can be tailored from the mid-infrared to the submillimeter region using the same heterostructure material. Electrons streaming down a potential staircase sequentially emit photons at the steps. The latter consist of coupled quantum wells in which population inversion between discrete conduction band excited states is achieved in a 4-level atomic like laser scheme using tunneling injection. The AlInAs/GaInAs structure comprises 25 stages, each consisting of a graded gap n-type injection layer and a three coupled-well active region, cladded by waveguiding layers. The undoped active region includes 0.8 nm and 3.5 nm thick GaInAs wells separated by 3.5 nm AlInAs barriers. The reduced spatial overlap between the states of the laser transition and the strong tunnel-coupling to a nearby 2.8 nm GaInAs well ensure population inversion. A dramatic narrowing of the emission spectrum and attendant order of magnitude increase of the optical power above a current threshold = 10 kA/cm2 provide direct evidence of laser action. Powers = 20 mW in pulsed operation have been obtained at 80 K. Operating temperatures up to 125 K have been achieved with 5 mW of power. An outstanding feature of this laser is that the gain is much less sensitive to temperature than conventional semiconductor lasers. A detailed study of the temperature dependence of the threshold indicates a To = 125 K. In addition, the intrinsic linewidth of these lasers is expected to be Schawlow-Townes limited, similar to atomic lasers, without the linewidth enhancement factor typical of diode lasers. For a preliminary account of the operation of this laser at 10 K see Ref. 2.