# Temperature-Insensitive Imaging Properties of a Broadband Terahertz Nonlinear Quantum Cascade Laser
## 1 Introduction  
Terahertz (THz) frequency range radiation (0.3-10 THz) has been used to demonstrate the imaging of objects that are opaque at optical frequencies [1; 2]. There are many THz applications, including the screening of chemical substances [3; 4; 5], and nondestructive imaging for industrial materials [6; 7; 8; 9; 10] and historical arts objects [11; 12]. THz quantum cascade lasers (THz-QCLs) are the most promising light source in the THz region [13; 14], and many studies on THz imaging technology have been performed using THz-QCLs [15].  
Alongside the development of THz-QCL technology, THz-QCL sources based on intra-cavity difference-frequency generation (DFG) have been developed as a new type of room temperature semiconductor THz light source [16]. These devices, known as THz DFG-QCLs or THz NL-QCLs (THz nonlinear quantum cascade lasers), use two-color mid-IR laser active regions engineered to exhibit a large intersubband nonlinear susceptibility \(\chi^{2}\) for an efficient THz DFG process. Currently, THz NL-QCLs based on the InGaAs/InAlAs/InP material system are the only electrically pumped monolithic THz semiconductor sources operable at room temperature in the 0.6 THz to 6 THz range [17; 18; 19; 20]. Since 2012, the performance of THz NL-QCLs has been improved by adopting a Cherenkov emission scheme [21]. In addition, recent efforts in wavefunction engineering using a dual-upper-state (DAU) active region [22; 23] possessing a broad gain bandwidth led to a significant improvement in terms of device performance, as well as the higher optical nonlinearity of the active region for efficient terahertz generation. In fact, without the integration of two QCL active regions for the dual-wavelength emission, THz generation is achieved in mid-infrared QCLs, based on a DAU active region with a device structure almost identical to commercialized, typical mid-infrared QCLs [24; 25]. As a result of the enhanced optical nonlinearity, this approach has expanded the lower limit of the frequency range down to 615 GHz [20]. This is the first demonstration of a sub-terahertz QCL source without an external magnetic field [19].  
In addition to the features that THz NL-QCLs are compact, room temperature semiconductor light sources, these devices show a single-lobed Gaussian-like beam pattern due to the Cherenkov emission scheme [21; 26]. Such good beam quality has major advantages in terms of practical applications. Furthermore, their THz emission spectra, associated with the mid-IR pumps used in the devices, can be exploited to achieve ultra-broadband bandwidth, as well as single-mode operation, with various device configurations. Therefore, THz NL-QCLs are suitable for imaging applications, and in fact, we previously demonstrated high-quality non-destructive imaging using a broadband THz NL-QCL [26]. Although THz NL-QCLs are operable at room temperature, the output power can be increased by decreasing the operating temperature; this is attributed to the enhancement of the mid-IR pump power from QCLs, which strongly depends on the operating temperature [24] in general. Accordingly, cooling is also useful when requiring higher THz output power for specific imaging applications. However, both the emission frequency and output direction of the THz radiation from THz NL-QCLs may be influenced by changing the operating temperature. In this work, we investigated the temperature dependence of the imaging properties (far-field pattern and THz imaging results) of a broadband THz NL-QCL.