Magnetotransport in quantum cascade detectors: analyzing the current under illumination.

Photocurrent measurements have been performed on a quantum cascade detector structure under strong magnetic field applied parallel to the growth axis. The photocurrent shows oscillations as a function of B. In order to describe that behavior, we have developed a rate equation model. The interpretation of the experimental data supports the idea that an elastic scattering contribution plays a central role in the behavior of those structures. We present a calculation of electron lifetime versus magnetic field which suggests that impurities scattering in the active region is the limiting factor. These experiments lead to a better understanding of these complex structures and give key parameters to optimize them further.

Phase seeding of a terahertz quantum cascade laser.

The amplification of spontaneous emission is used to initiate laser action. As the phase of spontaneous emission is random, the phase of the coherent laser emission (the carrier phase) will also be random each time laser action begins. This prevents phase-resolved detection of the laser field. Here, we demonstrate how the carrier phase can be fixed in a semiconductor laser: a quantum cascade laser (QCL). This is performed by injection seeding a QCL with coherent terahertz pulses, which forces laser action to start on a fixed phase. This permits the emitted laser field to be synchronously sampled with a femtosecond laser beam, and measured in the time domain. We observe the phase-resolved buildup of the laser field, which can give insights into the laser dynamics. In addition, as the electric field oscillations are directly measured in the time domain, QCLs can now be used as sources for time-domain spectroscopy.

Coherent combining of two quantum-cascade lasers in a Michelson cavity.

We developed a Michelson-type cavity to achieve coherent combining of two quantum-cascade lasers, emitting at around 4.5microm. We report a cw combining efficiency of 85% (up to 91% near threshold) with good beam quality (M(2)<1.4). Despite the interferometric nature of the coupling mechanism, this type of cavity can withstand disturbance from the laboratory environment without significant power fluctuations. Finally, the spectral behavior and the output power dependence on current are explored.

Optical parametric amplification of a distributed-feedback quantum-cascade laser in orientation-patterned GaAs.

We demonstrate what is to our knowledge the first realization of an optical parametric amplifier in orientation-patterned GaAs amplifying the emission of a quantum-cascade laser (QCL) with a distributed-feedback (DFB) structure. We report a gain as high as 53 dB at 4.5 mum, in good agreement with theoretical calculations. The narrow spectral linewidth and the good beam quality of this source are imposed by the DFB-QCL, while high-peak-power emission is achieved through the parametric amplification. These characteristics are of valuable interest for long-range spectroscopy.

Phase-resolved measurements of stimulated emission in a laser.

Lasers are usually described by their output frequency and intensity. However, laser operation is an inherently nonlinear process. Knowledge about the dynamic behaviour of lasers is thus of great importance for detailed understanding of laser operation and for improvement in performance for applications. Of particular interest is the time domain within the coherence time of the optical transition. This time is determined by the oscillation period of the laser radiation and thus is very short. Rigorous quantum mechanical models predict interesting effects like quantum beats, lasing without inversion, and photon echo processes. As these models are based on quantum coherence and interference, knowledge of the phase within the optical cycle is of particular interest. Laser radiation has so far been measured using intensity detectors, which are sensitive to the square of the electric field. Therefore information about the sign and phase of the laser radiation is lost. Here we use an electro-optic detection scheme to measure the amplitude and phase of stimulated radiation, and correlate this radiation directly with an input probing pulse. We have applied this technique to semiconductor quantum cascade lasers, which are coherent sources operating at frequencies between the optical (>100 THz) and electronic (<0.5 THz) ranges. In addition to the phase information, we can also determine the spectral gain, the bias dependence of this gain, and obtain an insight into the evolution of the laser field.