@article { ISI:000430337700008,
	title = {The harmonic state of quantum cascade lasers: origin, control, and prospective applications [Invited]},
	journal = {OPTICS EXPRESS},
	volume = {26},
	number = {8},
	year = {2018},
	month = {APR 16},
	pages = {9464-9483},
	publisher = {OPTICAL SOC AMER},
	type = {Article},
	address = {2010 MASSACHUSETTS AVE NW, WASHINGTON, DC 20036 USA},
	abstract = {The recently discovered ability of the quantum cascade laser to produce a harmonic frequency comb has attracted new interest in these devices for both applications and fundamental laser physics. In this review we present an extensive experimental phenomenology of the harmonic state, including its appearance in mid-infrared and terahertz quantum cascade lasers, studies of its destabilization induced by delayed optical feedback, and the assessment of its frequency comb nature. A theoretical model explaining its origin as due to the mutual interaction of population gratings and population pulsations inside the laser cavity will he described. We explore different approaches to control the spacing of the harmonic state, such as optical injection seeding and variation of the device temperature. Prospective applications of the harmonic state include microwave and terahertz generation, picosecond pulse generation in the mid-infrared, and broadband spectroscopy. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement},
	issn = {1094-4087},
	doi = {10.1364/OE.26.009464},
	author = {Piccardo, Marco and Chevalier, Paul and Mansuripur, Tobias S. and Kazakov, Dmitry and Wang, Yongrui and Noah A. Rubin and Meadowcroft, Lauren and Belyanin, Alexey and Federico Capasso}
}
