ABSTRACT
Exploitation of the terahertz (THz) electromagnetic resources is the fundamental need to envision real-world future smart society based on innovative information and communication technology (ICT) [1]. However, the THz region is yet to be well explored due to substantial physical limitations for both electron devices such as transistors and photonic devices such as lasers [1] (Figure 50.1). Then therefore, the development of compact, integrated, room-temperature operating novel solid-state THz laser devices is the critical demand for the brighter future society [2]. Among broad aspects and varieties of laser physics and device technology, THz frequency range constraints their applicability due to its low photon energy (on the order of meV to 100 meV) comparable to or rather weaker than the thermal energy at room temperature (~26 meV). Photon emission via interband radiative direct transitions of electrons in general semiconductor materials having bandgap energies above 100 meV does not meet the requirement for the THz lasers. Therefore, the THz lasers utilize the transitions of electrons in various fine structures of the energy bands such as absorption lines of gas molecules in gas lasers like He-Ne [3], CO2 [4], and HCN [5], subbands where the degenerated states are split by spatial confinements in quantum wells in quantum cascade lasers (QCLs) [6,7], valence bands for heavy/light holes in p-Ge lasers [8–10], and Landau levels split by the application of magnetic fields in Landau lasers [11]. Another limiting factor is a so-called “phonon decoherency.” The coherency of THz photons is easily broken in solid-state materials by the phonon scattering caused by thermally excited lattice vibrations (acoustic phonons) and/or electromagnetically excited lattice vibrations (optical phonons). To cope with this critical limit in the solid-state THz lasers, decrease of the operating temperature is a straightforward way. In fact, most of the currently existing solid-state THz lasers can only operate at cryogenic temperatures whose thermal energies are below the THz photon energies [1]. The THz p-Ge lasers are so-called inter-valence-band (IVB) transition-type THz lasers and exploit the direct transition of holes between the light hole (LH) valence band and the heavy hole (HH) valence band under high electric and magnetic fields to emit rather intense THz photons under cryogenic temperatures below ~20 K [8–10] (Figure 50.2). The THz QCLs are the most successful, industrialized current-injection-type laser devices that could be integrated into a single microchip and serve rather high wall-plug efficiency although still suffering 672from the phonon decoherency [7,12,13]. The Landau lasers need external magnetic fields to split the degenerated Landau levels [11]. It is performed in THz p-Ge lasers as one lasing mode [9]. Due to their cryogenic temperature operation and the needs of external magnetic field, the physical size and volume of the THz Landau lasers as well as p-Ge lasers are large so that they could not have been well industrialized so far. New trends with graphene-based THz lasers have been recently emerging [11,14–17], and some experimental demonstrations have been reported in the last 3 years [18,19].
