Quantum cascade lasers (QCLs) are unipolar optoelectronic devices that exploit optical transitions between engineered electronic subbands created in multiple coupled semiconductor quantum wells. The large number of nanometre-thick semiconductor layers and the complexity of the overall structure, makes the QCL the most impressive demonstration of the capabilities offered by bandstructure engineering and state-of-the-art materials growth technologies.

Now 20 years from their first experimental realization, QCLs have proven to provide outstanding performance across the mid-infrared and far-infrared (terahertz) spectral ranges. The QCL is the most efficient and compact source in the mid-infrared spectral range with multi-Watt output powers, continuous wave and room temperature operation, and with wall-plug efficiencies exceeding 20% at room temperature. Although still requiring cryogenic cooling, terahertz frequency QCLs have a significant opportunity to impact technological applications in the far-infrared, owing to their high output powers (1 W), broad operating frequency range, compact footprint, the use of photonic and plasmonic approaches to engineer the output beam profile, and the ability to stabilize their frequency, phase and amplitude. The recent development of room temperature THz sources based on intra-cavity frequency mixing in mid-infrared QCLs, with the latest devices achieving nearly 2 mW of peak output power, and tunable across the 1–6 THz spectral range, further demonstrates the versatility and potential of QCL technology.

The QCL has already been commercialized by a number of companies internationally, and is the core photonic component in technology underpinning a variety of applications in, for example, fields such as: medical, environmental and security sensing; process and quality control; telecommunications; and, metrology. The QCL is also an exciting and versatile vehicle for the pursuit of fundamental blue skies research, including developing coherent control in condensed matter systems, developing quantum technologies, and understanding the origins of the lasing process itself.

This was the seventh conference in the successful IQCLSW series. Previous meetings comprised:

IQCLSW 2014 took place in Policoro, Italy, and was co-chaired by Miriam Vitiello (CNR, Italy), Giacomo Scalari (ETH Zürich, Switzerland), and Benjamin Williams (UCLA, USA).

IQCLSW 2012 took place in Baden, Austria, and was co-chaired by Claire Gmachl (Princeton University, USA) and Karl Unterrainer (TU Wein, Austria).

IQCLSW 2010 took place in Villa Finaly, Florence, Italy, and was co-chaired by Carlo Sirtori (Université Paris Diderot, France) and Gottfried Strasser (TU Wein, Austria).

IQCLSW 2008 took place in Monte Verita, Switzerland, and was co-chaired by Jérôme Faist ETH Zürich, Switzerland), Alessandro Tredicucci (NEST CNR-INFM, Italy), and Claire Gmachl (Princeton University, USA).

The 2006 QCL workshop took place in Brindisi (Italy) and was co-chaired by Qing Hu (MIT, USA) and Gaetano Scamarcio (University of Bari, Italy).

The 2004 QCL workshop took place in Seville (Spain) and was co-chaired by Federico Capasso (Harvard University, USA) and Serge Luryi (Stony Brook, USA).