ABSTRACT
As an improvement of the active layer structure, an indirect pump (IDP) structure has been proposed and demonstrated [8]. As a result of an introduction of the IDP scheme at the carrier injection barrier of the active layer, the characteristic temperature T0 of the device reaches up to 303 K. The upper energy level for the laser oscillation (Lup) exists at the same energy of the injector in the case of the conventional resonant-longitudinal-optical-phonon (RP)-type active structure (see Fig. 4.3) [13]; however, with the IDP structure, a new laser upper oscillation level (Lup-new) is established below this level at a point equivalent to the energy of the longitudinal optical (LO) phonon, and immediately after the carriers (electrons) tunnel the injection barrier, ultrahigh-speed relaxation to the Lup-newoccurs at the top energy level in the emission well due to LO phonon resonance scattering. The carriers tunneling the injection barrier in the reverse direction are reduced by reducing the carrier density at the energy level immediately after tunneling the injection barrier in the normal direction. As a result, current density is approximately doubled with the same amount of doping of the injection layer. At the same time, since the carrier density of the injection layer is reduced, Drude absorption of light is controlled by this amount, the threshold current is reduced, and a high T0 is obtained. As a result of above-mentioned effects, the threshold current becomes lower and T0 becomes higher. In the actual IDP QCL of l = 13.5 mm using the lattice-matched InGaAs/InAlAs/InP material system, T0 reaches 400 K [9] at room temperature. This value is almost equivalent to the T0(around 500 K [10]) of the latest 1.3 µm band quantum dot laser operating at room temperature. Distributed feedback lasers [11] for single-mode operation, and plazmonic antenna loading [12] on the laser end facet of the waveguide to improve the quality of the beam emitted from the end facet, are used to improve the waveguide resonator structure and have the practical effect of improving ease of use. Gas analysis applications (CO, CO2, NO, NO2, SO2, CH4, NH3, C2H4, etc.) are proceeding in the mid-IR region. Examples of the development of gas analysis applications include environmental monitoring, waste gas analysis, and diagnoses based on exhaled air. Furthermore, light detection and ranging (LIDAR) in the atmospheric window (3.4-5.5 µm and 8-14 µm in the mid-IR band) is under investigation. These lasers hold promise as high-power (watt-
class) light sources for use in plastic fabrication in light of the low transmittance of plastics in the 8-14 µm band. A number of other applications have been proposed, including medical applications and noncontact visualization of matter distribution combining multicolor laser arrays and microbolometer cameras. 4.1.3 Terahertz-Band Quantum Cascade LasersIn the THz frequency range, the GaAs/AlGaAs material system is primarily used for the THz QCL. GaAs/AlGaAs lasers. The types of active layer structure of the THz QCLs can generally be classified as the RP type [13] described above, the bound-to-continuum (BTC) type [14], and the interminiband type: interminiband transitioning chirped superlattice [15] [(IM-CSL]) (see Fig. 4.4). The IDP structure, which is a kind of improved RP-type active structure, has also been applied to the THz QCL. In both the BTC and IM-CSL, a population inversion for laser action can be realized by using the ultrafast carrier relaxation process from the top of the miniband to the bottom of the miniband. The carrier distribution at the top of the miniband decreased against the upper energy levels of the laser oscillation. In the RP, the resonant LO phonon scattering process is utilized instead of intraminiband scattering. The carriers in the level 2 (L2) are scattered by the LO phonon to L1. This process is extremely fast (t32 >> t2) and depopulates the L2. Then, a population inversion arises among levels L3 and L2. Finally, the carriers in L1 are injected into the next active unit.
