Spectroscopic study of transparency current in mid-infrared quantum cascade lasers

Area of Science:

• Semiconductor Physics
• Optoelectronics
• Quantum Electronics

Background:

• Quantum cascade lasers (QCLs) are vital semiconductor devices for mid-infrared applications.
• Understanding the factors influencing their threshold current is key to improving performance.
• The role of transparency current in QCL operation, particularly in InGaAs/AlGaAs systems, requires further investigation.

Purpose of the Study:

• To directly measure and analyze the origins of transparency current in 5 µm InGaAs/AlGaAs quantum cascade lasers.
• To compare the impact of different active region designs on transparency current.
• To quantify the contribution of transparency current to the threshold current over a range of temperatures.

Main Methods:

• Broadband transmission measurements were performed on laser waveguides under sub-threshold conditions.
• Measurements were conducted across a temperature range of 80-280 K.
• Two active region designs were evaluated: one with double-LO-phonon relaxation and another without resonant LO-phonon assisted depopulation.

Main Results:

• Transparency current values were successfully determined for both active region designs.
• The study found that transparency current constitutes over 70% of the threshold current at high temperatures.
• This significant contribution was observed irrespective of the specific active region design employed.

Conclusions:

• Transparency current is a dominant factor in determining the threshold current of 5 µm InGaAs/AlGaAs QCLs at elevated temperatures.
• Active region design has a limited effect on the substantial contribution of transparency current to the threshold current.
• These findings provide direct insight into QCL operational mechanisms and inform future device optimization.