The challenge for lasers in the THz spectral region is the realization of compact sources, which operate in single mode, with sufficient optical output power (typically between several mW and several tens of mW), and at temperatures, which do not require cooling with liquid helium. For the frequency range 1–5 THz, quantum-cascade lasers have been developed, which can fulfill these requirements. They are so-called intersubband emitters, since the lasing transition takes place within the conduction band and not across the energy gap as in conventional interband semiconductor lasers.
Therefore, quantum-cascade lasers are unipolar lasers, i. e., only one type of carrier, typically electrons, are injected into the laser structure. In order to obtain population inversion between subbands of the conduction band, a rather complex semiconductor heterostructure with typically 6 to 20 layers with thicknesses in the range of a few to about 20 nanometers has to be realized, which is repeated about 100 times forming a semiconductor superlattice with a complex unit cell. The total thickness of the complete structure typically amounts to about 10 micrometer. The realization of such a structure requires a highly accurate growth technique such as molecular beam epitaxy with a very good stability of the growth parameters over 10 to 20 hours, which corresponds to one of the core competences of the institute. The materials of choice for the THz region are GaAs for the quantum wells and Al(x)Ga(1-x)As for the barriers with x ranging from 0.10 to 0.25. After growth, the wafers are processed to form edge-emitting ridge waveguide lasers. Typical dimensions of the laser ridges are a width of 50 to 200 micrometer and a length of 1 to 4 micrometer.
Our activities in the field of THz sources based on quantum-cascade lasers cover the design, the growth, the fabrication, and characterization of the lasers. For single-mode operation, distributed-feedback lasers are fabricated by dry etching techniques. Currently, there are several challenges for the development of THz quantum-cascade lasers. One challenge is the extension of the frequency range beyond 5 THz, i. e. emission frequencies close to the frequency of the longitudinal optical phonon in GaAs at 8.8 THz. Another one is single-mode operation with sufficient output powers, for which we use first-order lateral distributed-feedback gratings. In contrast, the realization of multi-frequency or broad-band emission is essential for absorption spectroscopy. A general challenge is the increase of the operating temperature beyond 100 K for continuous-wave operation with sufficient output powers.
For THz quantum-cascade lasers, we are currently investigating the frequency region around 4.75 THz, where quantum-cascade lasers are needed as local oscillators in a heterodyne receiver to detect the oxygen fine structure (OI) line at 4.745 THz in interstellar oxygen. Because the detection system is used in an airborne mission, the requirements with regard to operating temperature, output power, single-mode operation, and beam profile are quite demanding. First, we achieved the realization of a quantum-cascade laser with sufficient output power and an emission frequency less than 50 GHz away from the target frequency. Second, we accomplished single-mode operation using a first-order lateral distributed feedback grating. Third, the operating temperature was sufficiently high to operate the laser in a mechanical cryocooler so that this laser is ready to be used as a local oscillator in a heterodyne receiver.