Figure 10.

Schematic presentation of important milestones in the evolution of room temperature THz imaging systems within the last two decades. The systems are plotted with respect to the systems size reduction, power consumption, and enhanced functionality. Pioneering works in continuous wave [1] and optoelectronic THz imaging [2], as well as THz microscopy [273], and THz metamaterials [199] are denoted via blue circles and light-green labels. Meanings of other colors are depicted in the top-left corner of the plot. The first THz image recorded using optically-pumped molecular THz laser which is more than 2 meters long and uses of about 7 kW of electrical power, while state-of-art modern electronic sources, e.g., CMOS emitters are compact, in cm scale, and require only up to 1 W of power. No cryogenic cooling is needed for their operation. Invention of THz QCLs (still cooled cryogenically, below 50 K) unveiled an elegant solid-state-based compact solution for THz emitters—a route to reduce dimensions and power consumption in THz imaging systems [40]. First experiments on non-resonant THz detection using nanometric field-effect transistors open a new trend in development of sensitive detectors [139]. THz real-time imaging system using uncooled microbolometers array [184] demonstrated their potential for real time THz image recording. Room-temperature THz detection using silicon nanoFETs [141] was a breakthrough paper in the development of silicon-based THz and their sensors. Room-temperature generation of THz radiation in nanometric InAlAs/InGaAs and AlGaN/GaN HEMTs stimulated further research on nanotransistors. Self-resistive mixing mechanism in THz detection and development of CMOS technology-based THz focal-plane arrays provided a deeper understanding in physics behind the detection and opened a route for future cost-effective THz imaging solutions [144]. Femtosecond fiber lasers-based optoelectronics THz systems [35] demonstrated compact realization for optoelectronic THz imaging and increased convenience in their use. Room-temperature CMOS- [58,59,60], SiGe HBT-based [63] THz sources, and InP heterojunction bipolar transistors [64] revealed an interesting purely electronic approach to develop imaging systems. Resistive-distributed-plasmonic mixing models allowed gaining of wider insight into THz detection mechanism in nanometric FETs and extend their detection range up to 9 THz [148]. CMOS-based sensors and metamaterials absorbers can successfully be monolithically integrated [238]. Silicon-based diffractive optics can be a rational way for the integration of passive components [211,212] with active devices. Room temperature intracavity frequency difference tunable THz QCLs [51] exhibited a monolithic solution for THz spectroscopy, sensing and imaging systems. All electronic realization of THz nanoscopy allowed for creation of laser- and cryogenic cooling-free electronics-based near-field optical microscope [315]. Conventional THz QCLs has reached an operating temperature of 250 K, with the size of just a few millimeters [49].