Cavities for Intersubband Transitions

The different possible geometries for intersubband transitions in microcavities are discussed. A consequence of the selection rule governing intersubband transition is the vanishing interaction with the electromagnetic field inside a usual vertical cavity. The geometry of vertical planar cavities which has been used extensively with interband transitions is therefore useless in the case of intersubband transitions. Different solutions are reviewed to overcome this problem. The breakdown of the selection rule in a vertical cavity is first discussed. This can be done with the use of vertical quantum wells, or thanks to intracavity diffraction gratings. Second, the use of in plane cavities is discussed. Two solutions are here envisaged: Whispering gallery modes in microdisk cavities, and efficient etched air/GaAs Bragg mirrors. Concerning the latter attractive solution, the losses by diffraction into the substrate are evaluated theoretically and experimentally. The solution of the Maxwell equations by a finite element method in this three dimensional system shows the great importance of diffraction. These results are confirmed by waveguided Fourier transform spectroscopy. To overcome this difficulty, we propose the use of lower refractive index substrates, such as oxidized AlAs.