Abstract
The recently demonstrated quantum cascade laser is a fundamentally new semiconductor laser.1-3 It relies on only one type of carrier (unipolar laser) and on quantum jumps of electrons between discrete conduction band energy levels of quantum wells. As such the wavelength can be tailored over a very wide range from the mid-ir (a few microns) to the far-ir (~100 μm) by simply varying layer thicknesses. Two types of quantum cascade lasers will be discussed. In the original structure the relevant intersubband radiative transition is between states centered in different neighboring wells to facilitate population inversion, i.e. the transition is diagonal (Fig. 1). In this design, however, the width of the luminescence transition is relatively broad (FWHM~22 meV) due to the interface roughness since electrons traverse several heterointerfaces in the photon emission process. As a consequence the peak gain is reduced. To circumvent this problem we designed the structure of Fig.2 where electrons make a vertical radiative transition essentially in the same well. This reduces considerably the width of the gain spectrum (FWHM ≈ 10 meV) and therefore the laser threshold current density. To prevent electron escape in the continuum, which is greatly reduced in the case of the diagonal transition, the superlattice of the digitally graded injector is designed to act as a Bragg reflector for electrons in the higher excited state and to simultaneously ensure swift electron escape from the lower states via a miniband facing the latter (Fig. 2).
© 1995 Optical Society of America
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