Since the first demonstration of quantum-cascade lasers (QCLs) in 1994, remarkable progress has been made from the mid-infrared (mid-IR) to terahertz (THz) spectral range. The 1–6 THz spectral range is very attractive for many applications, such as imaging, chem-/bio-sensing, heterodyne detection, and spectroscopy. However, this spectral range still lacks high-performance compact continuous-wave (CW) light sources operable at room temperature. Recently, THz sources based on intracavity difference-frequency generation (DFG) in dual-wavelength mid-IR QCLs have been demonstrated. These devices, known as THz DFG-QCLs, have their active region engineered to exhibit giant intersubband nonlinear susceptibility χ<sup>(2)</sup> for THz DFG. Recently, we developed THz DFG-QCLs containing an homogeneous active region with dual-upper states (DAU), which exhibit a THz output power of 301 μW with a high mid-IR-to-THz conversion efficiency of 1.2 mW/W<sup>2</sup>. The DAU active region approach provides a broadband gain bandwidth, and as a result, two wavelength emissions can be obtained without a heterogeneous cascade that has been used previously; this leads to a low threshold current density compared with that obtained from the use of a heterogeneous active region. Here, we present a low threshold THz DFG-QCL based on a λ~6.8 μm DAU active region. The λ~6.8 μm DAU-QCLs have exhibited very low threshold current density as well as broad gain bandwidth. By applying the λ~6.8 μm DAU design approach, the device demonstrates room temperature CW operation without an epidown mounting scheme, where a threshold current density for THz emission has been shown to be low, at 1.3 kA/cm<sup>2</sup>. Besides, ultra-broadband emission covering 1.6–3.5 THz has been obtained in CW mode below 200 K.
We discuss novel approaches to improve the tuning bandwidth and power output of terahertz (THz) sources based on difference-frequency generation (DFG) in mid-infrared quantum cascade lasers (QCLs). Using a double Littrow external-cavity system, we experimentally demonstrate that both doubly-resonant terms and optical rectification terms in the expression for the intersubband optical nonlinearity contribute to THz generation in DFG-QCLs and report THz DFG-QCLs with the optimized optical rectification terms. We also demonstrate a hybrid DFG-QCL device on silicon that enables significant improvement on THz out-coupling efficiency and results in more than 5 times higher THz output power compared to that of a reference device on its native semi-insulating InP substrate. Finally, we report for the first time the THz emission linewidth of a free-running continuous-wave THz DFG-QCL.
A high photoresponse in a room-temperature quantum cascade detector (QCD) based on a coupled quantum-well design is demonstrated with a peak detection wavelength of 5.4 μm. In this design, forward electron transfer is engineered to be five times as large as relaxation back to ground level. In this situation, the coupled quantum-well QCD indicates a high responsivity of 22 mA/W as well as a specified detectivity (D*) of 8.0×10<sup>7</sup> cmW<sup>-1</sup>Hz<sup>1/2</sup>, both at room-temperature with commonly used 45° wedge configuration. Applying a waveguide configuration for the proposed QCD, an elevated responsivity of ~130 mA/W with a D* of 1.1×10<sup>8</sup> cmW<sup>-1</sup>Hz<sup>1/2</sup> was obtained at room-temperature. A laser absorption spectroscopy for N<sub>2</sub>O gas with proposed QCD and a distributed feedback quantum cascade laser has been also demonstrated.
Quantum cascade (QC) lasers are promising light sources for many chemical sensing applications in the mid-infrared
spectral range. For industrial applications, broadband wavelength tuning of external-cavity QC lasers with very broad
gain-width has been demonstrated. QC lasers based on anti-crossed dual-upper-state (DAU) designs are one of the
promising candidates because of its broad bandwidth as well as high device performances. In fact, wide wavelength
tuning of external cavity QC lasers with the anti-crossed DAU designs has been exhibited in several wavelengths: the
tuning range of ~25% in pulsed mode and <17% in cw mode at room temperature. Here we report conspicuous
temperature performances of continuous wave quantum cascade lasers with broad gain bandwidths. The lasers with the
anti-crossed DAU designs, characterized by strong super-linear current-light output curves, exhibit the extremely high
characteristic temperature for threshold current density, T0~750 K above room temperature. In addition, its slope
efficiency is growing with increasing temperature (negative T1-value). For the pulsed operation of a short 1 mm length
laser, the temperature coefficient reaches the surprisingly high value of 1085 K over 340-380 K temperature range. The
distinctive characteristics of the DAU lasers are attributable to the optical absorption quenching which has been clarified
to take place in indirect pumped QC lasers. Such high characteristic temperatures of the DAU-QC lasers provide great
advantages for practical applications, in addition to its potential of broadband tuning.
A wide wavelength tuning of an external cavity quantum-cascade (QC) laser based on the anticrossed dual-upper-state to
multiple-lower-state design is demonstrated in continuous wave (cw) operation at room temperature. The tuning ranges
of 321 cm<sup>-1</sup> (Δλ/λ~22%) in pulsed operation and 248 cm<sup>-1</sup> (Δλ/λ~17%) in cw operation are achieved, despite
employment of the active region with translational symmetry. The present tuning range in cw operation substantially
exceeds the values obtained with the QC lasers based on conventional broadband active region designs. In addition, the
continuous, single mode tuning is realized with its widely homogeneous gain spectrum.
We report a high performance operation of quantum cascade lasers based on Single Phonon-Continuum depopulation
(SPC) structures. The lasers exhibit low CW threshold current densities and high characteristic temperatures in the wide
wavelength range between 4.5 and 10.8 μm. An 8.2 μm laser, despite a bare ridge structure which is extremely simple
configuration without any intentional thermal dissipation equipments and any HR coatings, demonstrates the high device
performance: a low threshold current density of 1.66 kA/cm<sup>2</sup> and a high maximum output power of 76 mW (from one
facet) at room temperature in CW operation. Our shortest wavelength 4.5 μm laser with HR coating reveals a low
threshold current density of 1.7 kA/cm<sup>2</sup> and maximum output power of 161 mW at room temperature in CW operation.
For long wavelength, we present the first room temperature, CW operation of DFB QCL with top grating. The DFB laser
emits ~9.6 μm single mode spectra at temperatures between -5 °C and 50 °C. The wide tuning range is obtained to be
from 1031 to 1039 cm<sup>-1</sup>.