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Classical concept of quantum-cascade lasers

A quantum-cascade (QC) laser is based on intersubband-transitions of electrons inside a quantum-well structure. Therefore, unlike other semiconductor light sources, the emitted wavelength is not determined by the band gap of the used material but on the thickness of the constituent layers.

Fig. 1: Typical structure of a quantum-cascade laser

The classical concept of QC lasers is a periodic repetition of active sections and so-called injector regions, in which a miniband is formed. A typical structure is shown in Fig. 1. From the injector miniband the electrons are injected into the upper laser energy level (4) of the active section. Here the laser transition takes place. After that, the lower laser energy level (3) is emptied by LO-phonon emissions and the electrons enter the next stage by tunnelling.

 

Concept of quantum-cascade lasers without injector regions

Injector regions have been considered as an essential requirement for laser action in QC lasers. In the active sections the photons are generated, while the miniband in the injector regions enables the transfer of the electrons from the lower levels of one active section into the upper level of the next section. Due to the doping of these regions, they act as an electron reservoir and provide stable current flow. Furthermore, the injector miniband helps to avoid a thermal backfilling of the lower laser level. However, apart from these benefits, the main disadvantage of these structures is the lengthening of the active stage with optically passive and slightly absorbing material and therefore a reduced overlap of the waveguide mode with the active sections. QC lasers without injector regions are expected to yield improved performance, provided that problems like thermal backfilling can be avoided and the electron transfer can be managed otherwise.

Fig. 2: Concept of an injectorless QC laser, based on a five-level staircase.

The concept of our QC laser without injector miniband is based on a five-level staircase (Fig.2). At a certain bias field, levels 1 and 2 of one active section are resonant with levels 4 and 5 of the following section and a double LO-phonon resonant condition occurs. Therefore, after the radiative transition (between levels 4 and 3), the electrons are transferred directly from one active section into the next, without the need for a bridging miniband. The bias field can be increased until level 1 is brought into resonance with level 5 and a triple LO-Phonon condition is realized. Thus, a thermal backfilling of the lower laser level can be avoided.

Quantum-cascade lasers without injector miniband:
Active region design

Most quantum cascade laser (QCL) designs use only two different material compositions. One material composition (i. e. GaInAs or GaAs) is used as the well and the other (i. e. AlInAs or AlGaAs ) is used as the barrier. In our group, we have implemented high bandgap AlAs into the barrier to suppress carrier escape into the continuum. Additionally, we included thin layers of InAs into the transition well for increased dipole matrix element and carrier lifetime in the upper state. Fig. 3a illustrates the schematic band diagram of the device with four material compositions, which demonstrated record low threshold current densities at room temperature in pulsed operation mode. The device was emtting around 7 µm.

Fig. 3b illustrates the conduction band profile for the mid infrared device with two InAs layers in the transition well. The device had an emission wavelength of 5.6 µm and optical output power of 1.3 W with a slope efficiency of 1.3 W/A at room temperature in pulsed operation.

Fig.5: Conduction band profile and wavefunctions at a bias field of 103 kV/cm for the four material based mid infrared QCL.
Fig.3b: Conduction band profile and wavefunctions at a bias field of 127 kV/cm
 

Structure and processing

Fig.4: Optical microscope image of a THz-DFG device

Our QCL structures are grown by solid source molecular beam epitaxy (SS-MBE). The epitaxial structures are then processed in our class 100 cleanroom environment. The process involves optical lithography, dry-etching, wet-chemical etching, dielectric deposition via PECVD or sputtering. For contacts, e-beam evaporation in ultra-high vacuum environment is used. Additional Au electroplating is applied for improved heatsinking purposes. When needed, re-growth is performed in MOVPE.

 

Intracavity nonlinear frequency mixing in quantum cascade lasers

Quantum cascade lasers are extremely interesting research subjects within the field of nonlinear optics. It has been demonstrated that intersubband transitions in quantum wells can be tuned to posses a giant nonlinear optical response, which posseses resonant properties. Such synthetic nonlinearities can be monolithically integrated with active Mid-IR QCLs to make efficient nonlinear devices with a spectral coverage in the Near-IR (2 - 4 µm) and Far-IR (15 - 200 µm).

In our group, we work in a close collaboration with Prof. M. A. Belkin's group at the University of Texas at Austin. Our research areas include development of room-temperature operating devices in the Near-IR by intracavity quasi-phase-matched second-harmonic-generation and terahertz devices by difference-frequency-generation. Our research breakthroughs include room-temperature lasing in AlInAs/GaInAs/InP based devices down to 2.7 µm by second-harmonic generation (Fig. 5), and 210 K lasing at 70 µm wavelength by difference-frequency-generation (DFG) (Fig. 6).

Fig.5: Room-temperature light-output vs. current density data for a Near-IR device, emitting around 2.7 µm. Emission spectrum of the pump and the nonlinear signals can be seen in the inset.
Fig.6: Difference-frequency signal (~70 µm) at 78 K and 210 K. The corresponding pump emission is shown in the inset.
 
TUM Technische Universität München TUM Technische Universität München Physik Department Elektrotechnik und Informationstechnik TUM Technische Universität München
 

Research Staff

Prof. Dr.-Ing.
Markus-Christian Amann

Dipl.-Ing. Gerhard Böhm

Frederic Demmerle

Contact

Frederic Demmerle 
Walter Schottky Institut E26
Am Coulombwall 4
D-85748 Garching
Germany

Phone: +49-(0)89-289-12785
Fax: +49-(0)89-289-12704
eMail: frederic.demmerle@wsi.tum.de

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