Quantum Cascade lasers
Quantum cascade lasers (QCLs) are made up of nanometer-sized quantum wells and barriers. Due to their small size (~1mm) and high output powers (>10 mW), terahertz (THz) QCLs have the potential to revolutionize THz technology. However, room temperature operation has yet to be achieved and as of 2012 the maximum reported operating temperature was 200K. Although the energy of a THz photon is less than kT at room temperature, this does not prevent population inversion in a multi-level system. In fact bulky molecular gas lasers have been operating at THz frequencies since the 1960's. Much less is known about the optimal designs for THz QCLs than their mid-infrared QCL counter-parts, which are entering into commercial production. Gain measurements can reveal valuable information on laser design and operation. Recently we (and several other teams) have begun to study the spectral gain of THz QCLs using THz time-domain spectroscopy, which enables the gain to be measured as a function of both time and frequency. By measuring the spectral gain of QCLs, we have studied transitions between states before band structure alignment, and observed coherent coupling between the injector and upper laser states. We have also begun to study the temporal response of QCLs on picosecond time scales. These studies demonstrate that TDS can give valuable insights on the operation of THz QCLs that are not possible with other techniques.
Left: A SEM picture of a terahertz quantum cascade laser.
Right: The square modulus of the wave functions in a terahertz quantum cascade laser.