Figure 2.2: Timeline of significant QCL developments.
Progress advanced quickly after the first demonstration of an intersubband laser in 1994, designed in the Bell Laboratories to emit at 4.2 μm wavelength with peak powers in excess of 8 mW in pulsed operation and grown by molecular beam epitaxy [8]. This progress is facilitated basically by the advance in growth techniques and by the improving understanding of bandstructure engineering, resulting in a better control of the electron transport to enable an increase in population inversion and gain. A timeline of the most important achievements is shown in Figure 2.2.
In 1996, MIR QCLs were created that reach a high pulsed power at temperatures up to 320 K and a continuous wave (CW) operation up to 140 K [18]. An active region based on a three-well vertical transition design and a funnel injector were used to optimize the gain. At 300 K a peak pulsed optical power of 200 mW was obtained, where the emission wavelength was about 5.2 μm. In the same year, long wavelength single-mode QCLs based on an AlInAs/GaInAs heterostructure emitting at 11.2 μm were established [19]. Moreover, a continuous wave operation with powers of about 7 mW at a temperature of 10 K was sucessfully obtained.
One year later, a continuously tunable single-mode laser was provided by a distributed feedback (DFB) QCL operating above room temperature at 5.4 and 8 μm wavelengths [20]. Due to the alternating refractive index the periodically structured active region acts as a diffraction grating, where the wavelength is determined by the Bragg reflection condition. In the same year, a long wavelength QCL based on a superlattice active region was demonstrated [21]. An intrinsic inversion is achieved by minibands in the active region. Electrons injected by tunneling emit photons corresponding to the energy separation.
The first GaAs/AlGaAs QCL was reported in 1998 [9]. This work demonstrated the validity of QCL principles in a heterostructure material system different from the system InGaAs/AlInAs on InP used before. This QCL structure employed 33 % Al in the barriers and emitted at a wavelength of 9.4 μm under pulse operation up to 140 K. The threshold current density of this device was reduced to an average of 5 kA/cm2 at 77 K, and a maximum pulsed operation temperature of about 200 K was achieved with a low loss Al-free waveguide [22]. Since the first realization of GaAs/Al0.45 Ga0.55As QCLs [23] the device performance has improved significantly. For several active region designs a room temperature pulsed operation has been demonstrated, e.g. a superlattice active region design with an emission wavelength at 12.6 μm [24] and a bound-to-continuum design emitting at 82 μm [25]. Recently, a continuous wave operation with a maximum temperature of 150 K has been achieved due to optimized device processing [26].
The first QCLs with wavelengths larger than 20 μm, in particular at 21.5 and 24 μm, were reported in 2001 [27]. Originating from interminiband transitions in superlattice active regions, laser action is achieved up to 140 K with a peak power of a few mW. The structures were grown by molecular beam epitaxy using an In0.53Ga0.47As/Al0.48In0.52As lattice matched to an InP substrate. Up to this time, these semiconductor lasers had the longest emission wavelength.
One year later, a continuous wave operation of a MIR semiconductor laser above room temperature was demonstrated [28]. At an emission wavelength of 9.1 μm, the optical output power ranged from 17 mW at 292 K to 3 mW at 312 K. In the same year, a prototype of a QCL emitting in the THz region (30 - 300 μm) was reported [29]. Emitting a single mode at 4.4 THz, the device reached an output power of more than 2 mW.
Further notable milestones are the first broudly and continuously tunable external cavity QCL in 2004 [30], and the first high power CW external cavity QCL at room temperature [31].
In 2010, a 100 μm emission of a QCL to a 10∘ cone was reported [32]. Due to fabricating a metamaterial layer on the output facet, the heavily doped semiconductor acts like a metal at terahertz frequencies.
Nowadays, QCLs operate over a wide wavelength range of 2.9 - 250 μm [33]. Spanning the MIR and THz region, they also operate at room temperature and in continuous wave mode with up to 3 W of optical power.