ABSTRACT

A similar scheme involving an optical parametric oscillator was recently presented by Kiessling et al. [15]. Let’s now turn to optoelectronic schemes. Quantum cascade lasers (QCLs) represent a very promising technology toward the realization of room-temperature, monolithic, and compact currentinjected sources in the 1-5 THz range. The first QCL was demonstrated in 1994 by Faist et al. [16]. These lasers rely on the emission from transitions between subbands in quantum wells. When a voltage is applied to a complicated sequence of repeating quantum well structures, electrically injected electrons stream down a potential staircase and emit a THz photon at each step. Early QCLs needed cryogenic cooling, worked only in pulsed mode, and emitted in the mid-infrared [16]. Since then tremendous progress has been made. In 2002 Tredicucci et al. presented the first real THz QCL working at 4.4 THz [17]. This device still required cryogenic cooling. However, in 2013 Lu et al. reported the first high-power, room-temperature THz QCL based on difference frequency generation [18].