title: Quantum cascade lasers: 20 years of challenges.
abstract: We review the most recent technological and application advances of quantum cascade lasers, underlining the present milestones and future directions from the Mid-infrared to the Terahertz spectral range. Challenges and developments, which are the subject of the contributions to this focus issue, are also introduced.
authors.forename: Miriam Serena
authors.initials: MS
authors.lastname: Vitiello
authors.forename: Giacomo
authors.initials: G
authors.lastname: Scalari
authors.forename: Benjamin
authors.initials: B
authors.lastname: Williams
authors.forename: Paolo
authors.initials: P
authors.lastname: De Natale