keywords (EI-DFB) laser (EIT) ABSORPTION DFB LASERS DISTRIBUTED-FEEDBACK Engineering Engineering, Electrical & Electronic GAIN INDEX LAYER Optics Physical Sciences Physics Physics, Applied QUANTUM CASCADE LASERS Quantum Science & Technology Science & Technology Technology WELLS electromagnetically induced distributed feedback electromagnetically induced transparency enhancement of refractive index gain-coupled DFB laser intersubband laser optical defect phase-shifted DFB laser quantum well (QW) tunable lasers