Abstract:

Recent advances and new directions in quantum cascade (QC) lasers are discussed in this paper. Invented in 1994 following many years of research on band-structure engineered semiconductors and devices grown by molecular beam epitaxy, this fundamentally new laser has rapidly advanced to a leading position among midinfrared semiconductor lasers in terms of wavelength agility as well as power and temperature performance. Because of the cascaded structure, QC lasers have a slope efficiency proportional to the number of stages. Devices with 100 stages having a record peak power of 0.6 W at room temperature are reported here. QC lasers in the AlInAs-GaInAs lattice matched to InP material system can now be designed to emit in the whole midinfrared range from 4 to 20 mum by appropriately choosing the thickness of the quantum wells in the active region. Using strained AlInAs-GaInAs, wavelengths as short as 3.4 mum have been produced, New results on QC lasers emitting at 19 mum, the longest ever realized in a III-V semiconductor laser, are reported. These devices use innovative plasmon waveguides to greatly enhance the mode confinement factor, thereby reducing the thickness of the epitaxial material. By use of a distributed feedback (DFB) geometry, QC lasers show single-mode emission with a 30-dB side-mode suppression ratio. Broad continuous single-mode tuning by either temperature or current has been demonstrated in these DFB QC lasers at wavelengths in two atmospheric windows (3-5 and 8-13 mum), with continuous-wave linewidths <1 MHz when freerunning and 10 KHz with suitable locking to the side of a molecular transition. These devices have been used in a number of chemical sensing and spectroscopic applications, demonstrating the capability of detecting parts per billion in volume of several trace gases. Sophisticated band-structure engineering has allowed the design and demonstration of bidirectional lasers, These devices emit different wavelengths for opposite bias polarities. The last section of the paper deals,vith the high-speed operation of QC lasers, Gain switching with pulse widths similar to 50 ps and active modelocking with a few picosecond-long pulses have been demonstrated. Finally a new type of passive modelocking has been demonstrated in QC lasers, which relies on the giant and ultrafast optical Kerr effect of intersubband transitions.