Semiconductor-based terahertz (THz) sources are critical components in security imaging systems, offering non-invasive detection capabilities for concealed objects, explosives, and other threats. Among the most prominent semiconductor THz sources are quantum cascade lasers (QCLs) and photomixers, which leverage advanced material systems like InGaAs and GaAs to generate tunable, high-frequency radiation. These devices operate within the 0.1–10 THz range, bridging the gap between microwave and infrared technologies. Their performance is governed by factors such as frequency tuning, output power, and material limitations, which directly impact their suitability for security applications.

Quantum cascade lasers are unipolar semiconductor devices that exploit intersubband transitions in multiple quantum wells to produce THz radiation. The active region typically consists of alternating layers of InGaAs and GaAs or InGaAs and AlInAs, grown using molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). The bandgap engineering of these materials allows for precise control over the emitted THz frequency, which is determined by the thickness and composition of the quantum wells. Frequency tuning in QCLs is achieved through external cavity configurations, bias voltage adjustments, or temperature modulation. For instance, varying the applied electric field can shift the emission frequency by several gigahertz, enabling fine-tuning for specific imaging applications. However, the output power of THz QCLs is highly temperature-dependent, with most devices requiring cryogenic cooling to maintain milliwatt-level output. Room-temperature operation remains a challenge due to increased non-radiative scattering and thermal backfilling of lower energy states. Current state-of-the-art QCLs deliver output powers in the range of 1–100 mW at frequencies between 2–5 THz, though this drops significantly at higher frequencies due to optical phonon scattering in the active region.

Photomixers, another class of semiconductor THz sources, rely on the heterodyne mixing of two near-infrared lasers in a photoconductive material, typically low-temperature-grown GaAs (LT-GaAs) or InGaAs. The beat frequency of the two lasers determines the THz output, allowing for continuous-wave (CW) generation with broad tunability. Photomixers excel in applications requiring frequency agility, as they can be tuned across a wide range (0.1–3 THz) simply by adjusting the wavelength difference between the two pump lasers. However, their output power is comparatively lower than QCLs, typically in the microwatt range, due to limitations in photoconductive gain and thermal dissipation. The use of nanostructured electrodes and plasmonic antenna designs has improved power extraction, but material properties remain a limiting factor. LT-GaAs, for example, offers sub-picosecond carrier lifetimes essential for high-speed operation but suffers from low carrier mobility, which restricts the maximum achievable power. InGaAs-based photomixers provide higher mobility but require careful engineering to minimize dark current and thermal effects.

Material selection plays a pivotal role in the performance of semiconductor THz sources. InGaAs-based systems are favored for their high electron mobility and tunable bandgap, making them suitable for both QCLs and photomixers. However, the narrow bandgap of InGaAs limits the maximum operating temperature and output power due to increased thermal carrier generation. GaAs, with its wider bandgap, offers better thermal stability but at the cost of reduced mobility and higher effective mass, which can impede high-frequency operation. The trade-offs between these materials dictate the design and operational limits of THz devices. For instance, InGaAs/AlInAs QCLs can achieve higher frequencies but require sophisticated cooling systems, while GaAs-based photomixers are more practical for portable systems but lack the power needed for long-range imaging.

Frequency tuning is a critical requirement for security imaging, as different materials exhibit distinct absorption fingerprints in the THz range. QCLs offer discrete tuning through mode hopping or continuous tuning via external cavities, but their range is constrained by the gain bandwidth of the active medium. Photomixers provide broader tunability but with lower spectral purity and power. Advanced techniques such as distributed feedback (DFB) gratings in QCLs and dual-laser frequency combs in photomixers have been employed to enhance tuning precision and stability. These developments are essential for applications like standoff detection, where the ability to sweep across multiple frequencies improves identification accuracy.

Output power is another key consideration, as it directly affects the signal-to-noise ratio (SNR) in imaging systems. While QCLs deliver higher power, their reliance on cryogenic cooling limits their deployment in field applications. Photomixers, though less powerful, operate at room temperature and are more compact, making them suitable for handheld or portable devices. Recent advancements in plasmonic waveguides and photonic crystal cavities have boosted the power output of both technologies, but material-induced losses remain a bottleneck. For example, free-carrier absorption in heavily doped semiconductor layers can significantly attenuate THz waves, reducing overall efficiency.

Security imaging systems demand robust, reliable, and tunable THz sources capable of operating in diverse environments. Semiconductor-based devices like QCLs and photomixers meet these requirements to varying degrees, with each offering distinct advantages and challenges. The ongoing development of novel material systems, such as strained InGaAs heterostructures and GaN-based QCLs, promises to push the boundaries of THz generation, enabling higher power, broader tunability, and room-temperature operation. As these technologies mature, their integration into real-world security platforms will become increasingly feasible, enhancing the ability to detect and identify concealed threats with unprecedented precision.