Abstract

The recently demonstrated quantum cascade laser (QCL) relies on only one type of carrier (unipolar laser) and on quantum jumps of electrons between discrete conduction band energy levels of quantum wells.^1,2 As such the wavelength can be tailored over a very wide range by varying layer thicknesses. In the original structure the laser transition is between states centered in neighboring wells to facilitate population inversion, i.e. the transition is diagonal.^1,2 In this design, however, the luminescence linewidth is relatively broad due to the interface roughness since electrons traverse several heterointerfaces in the photon emission process, thus reducing the peak gain. To circumvent this problem we designed the structure of Fig. 1 where electrons make a vertical radiative transition essentially in the same well. This should reduce considerably the threshold current density. To prevent electron escape in the continuum, the digitally graded superlattice injector is designed to act as a Bragg reflector for electrons in the higher excited state and to siphon out electrons from the lower states via a miniband facing the latter (Fig. 1). The lower state of the laser transition is separated by an optical phonon (~30 meV) from the n = 1 state to ensure population inversion. Each of the 25 active regions (see Fig. 1) consists of a 4.5 nm GaInAs well coupled to a 3.6 nm well by a 2.8 nm AlInAs barrier. Injection into the n = 3 state is through a 6.5 nm AlInAs barrier and electrons escape out of the n = 1 state through a 3.0 nm AlInAs barrier. The waveguiding cladding regions are similar to those of the diagonal transition structure.^1,2 Current pulses of 30 ns with a 20 kHz repetition rate were injected into the ridge mesa devices. Figure 2 displays the peak optical power versus drive current. The peak optical power is ~80 mW at 80 K. The measured slope efficiency is 300 mW/A per facet and essentially temperature independent and corresponds to a differential quantum efficiency per period of × 10⁻² when corrected for the collection efficiency of the apparatus. The threshold density has a value J_th = 1.7 kA/cm² at 10 K and 3 kA/cm² at 100 K.

© 1995 Optical Society of America