Phase seeding of a terahertz quantum cascade laser

Abstract

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.

Terahertz pulse generation from quantum cascade lasers

“…Here we demonstrate that this is not the case: the dominant factor necessary for active pulse generation is in fact the microwave cavity and the synchronization between the propagating electronic microwave modulation and the THz pulse in the QCL. By using phase resolved detection [3] of the electric field in QCLs embedded in MM waveguides, we demonstrate that active mode locking requires the phase velocity of the microwave round trip modulation to equal the group velocity of the generated THz pulse. This allows the THz pulse to propagate in phase with the microwave modulation along the gain medium, permitting pulse generation [4].…”

Carrier envelope phase retrieval of a multi-cycle pulse by heterodyne mixing of a pulse containing a few cycles

“…The generation of laser pulses which last only for tens of attoseconds has opened the door to attosecond science [1][2][3][4][5][6]. Such ultrashort pulses allow researchers to resolve and control the fast motions of the electrons on the time scale of attoseconds.…”

Quantum Confinement in High Electron Mobility Transistors

“…Stacking of quantum wells can further enhance the response of intersubband resonance (ISR), and such designs are the key for various applications like photodetectors or intersubband lasers [6]. One of the very common and sophisticated examples in this regard is the quantum cascade laser [7][8][9], which is based on the cascade phenomena and intersubband transitions across many layers of quantum wells. Such compact and powerful lasers are used for practical applications in THz spectroscopy [10][11][12][13], sensing technology [14,15], biomedical applications [16,17] and also in security applications [11,18].…”