The quantum cascade laser is a powerful, narrow linewidth, and continuous wave source of terahertz radiation. The authors have implemented a distributed feedback device in a spectrometer for high-resolution gas phase spectroscopy. Amplitude as well as frequency modulation schemes have been realized. The absolute frequency was determined by mixing the radiation from the quantum cascade laser with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519THz were measured in order to demonstrate the performance of the spectrometer.

Although lasers have found numerous applications, their design is often still based on the concept of a gain medium within a mirror cavity. Exceptions to this are distributed feedback lasers(1), in which feedback develops along a periodic structure, or random lasers, which do not require any form of cavity(2). Random lasers have very rich emission spectra, but are difficult to control. Distributed feedback devices, conversely, have the same limited design possibilities of regular lasers. We show, by making use of a quasi-crystalline structure in an electrically pumped device, that several advantages of a random laser can be combined with the predictability of a distributed feedback resonator. We have constructed a terahertz quantum cascade laser based on a Fibonacci distributed feedback sequence, and show that engineering of the self-similar spectrum of the grating allows features beyond those possible with traditional periodic resonators, such as directional output independent of the emission frequency and multicolour operation

Terahertz quantum cascade lasers have been investigated with respect to their performance as a local oscillator in a heterodyne receiver. The beam profile has been measured and transformed in to a close to Gaussian profile resulting in a good matching between the field patterns of the quantum cascade laser and the antenna of a superconducting hot electron bolometric mixer. Noise temperature measurements with the hot electron bolometer and a 2.5 THz quantum cascade laser yielded the same result as with a gas laser as local oscillator.

In microdisk lasers(1-3) a ring resonator is formed by successive total internal reflections inside a circularly shaped waveguide. The photon lifetime of the resulting whispering gallery optical modes is limited mainly by the waveguide absorption. Light is usually coupled out by tunnelling owing to the disk curvature or through imperfections at the border, but the output power is hard to exploit in a potential application because the emission is mainly in the disk plane and isotropic. Here we realize vertically emitting whispering gallery lasers by implementing appropriate diffraction gratings along the disk circumference. We use terahertz quantum cascade structures(4,5) and demonstrate a 50-fold increase in the optical power compared to devices without gratings, while at the same time engineering the lasing spectrum according to the grating rotational symmetry. This concept will allow the fabrication of compact arrays of single-mode terahertz sources with regular beam profiles and high output power

A periodic array of thin slits opened on a metallic surface can act as a one-dimensional photonic crystal for the propagation of surface-plasmon waves. We have used such structure for the implementation of distributed feedback resonators in quantum cascade lasers emitting near 2.5THz. Single-mode emission, stable at all injection currents and operating temperatures, was achieved both in pulsed and continuous wave. The devices exhibited output powers of several milliwatts with low threshold current densities of ∼100A∕cm2.