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 μm by appropriately choosing the thickness of the quantum wells in the active region. Using strained AlInAs-GaInAs, wavelengths as short as 3.4 μm have been produced. New results on QC lasers emitting at 19 urn, 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 pm), 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 with the high-speed operation of QC lasers. Gain switching with pulse widths ∼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.
Original language | English (US) |
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Pages (from-to) | 931-946 |
Number of pages | 16 |
Journal | IEEE Journal on Selected Topics in Quantum Electronics |
Volume | 6 |
Issue number | 6 |
DOIs | |
State | Published - Dec 1 2000 |
Externally published | Yes |
All Science Journal Classification (ASJC) codes
- Atomic and Molecular Physics, and Optics
- Electrical and Electronic Engineering
Keywords
- Chemical sensing
- High-speed optoelectronic devices
- Injection lasers
- Intersubband transitions
- Mid-infrared
- Semiconductor heterostructures
- Unipolar semiconductor lasers