Terahertz (THz) waves occupy a unique region of the electromagnetic spectrum between microwaves and infrared, with frequencies typically ranging from 0.1 to 10 THz. This band has garnered significant interest for applications in imaging, sensing, and high-speed communication due to its non-ionizing nature and ability to penetrate many non-conductive materials. However, efficient generation and detection of THz waves remain challenging, particularly in compact and scalable systems. Two-dimensional (2D) materials, particularly graphene, have emerged as promising candidates for THz applications due to their exceptional electronic and optical properties, including high carrier mobility, tunable conductivity, and strong light-matter interactions.

Graphene’s linear band structure near the Dirac points enables unique electronic transitions that are highly responsive to THz frequencies. One of the key mechanisms for THz wave generation in graphene is nonlinear conductivity, which arises under intense optical or electrical excitation. When graphene is driven by a high-frequency electric field, its charge carriers exhibit a nonlinear velocity-field relationship, leading to the emission of THz radiation. This effect is particularly pronounced in the presence of a DC bias, where the asymmetric distribution of carriers in momentum space results in a net THz photocurrent. Experimental studies have demonstrated THz emission from graphene under femtosecond laser excitation, with conversion efficiencies that can be optimized through electrostatic gating or chemical doping.

Plasmonic resonances in graphene further enhance its utility for THz applications. Surface plasmons—collective oscillations of electrons—in graphene can be tuned across the THz range by adjusting the Fermi level via electrostatic gating. Unlike conventional metals, graphene plasmons exhibit much tighter confinement and lower losses, enabling subwavelength THz wave manipulation. These plasmons can be excited by incident THz radiation, leading to enhanced near-field interactions that are useful for imaging and sensing. For instance, graphene-based THz detectors leverage plasmonic resonances to achieve high sensitivity, with reported noise-equivalent powers as low as a few pW/Hz^0.5. The ability to pattern graphene into nanostructures, such as ribbons or disks, allows for further control over plasmonic resonances, enabling tailored responses for specific THz frequencies.

In THz imaging, graphene’s plasmonic properties enable subdiffraction-limited resolution, overcoming the traditional limitations imposed by the long wavelengths of THz radiation. By coupling THz waves to graphene plasmons, it is possible to concentrate energy into nanoscale volumes, enhancing the interaction with target molecules or materials. This has been exploited in near-field THz microscopy, where graphene-based probes achieve spatial resolutions below 100 nm, far surpassing the diffraction limit. Such capabilities are particularly valuable for biomedical imaging, where THz waves can identify tissue abnormalities or chemical compositions without harmful ionizing radiation.

For communication systems, graphene’s high carrier mobility and ultrafast response make it suitable for THz modulators and detectors. THz communication promises data rates exceeding 100 Gbps, but conventional electronic devices struggle to operate efficiently at these frequencies. Graphene-based modulators exploit the material’s tunable conductivity to encode information onto THz carriers, with modulation speeds demonstrated up to several tens of GHz. Similarly, graphene detectors, which rely on photothermoelectric or plasmonic rectification mechanisms, can demodulate THz signals with high fidelity. Recent advancements have shown that heterostructures combining graphene with other 2D materials, such as hexagonal boron nitride (hBN), can further improve device performance by reducing scattering and enhancing carrier lifetimes.

Nonlinear effects in graphene also play a critical role in THz wave mixing and frequency conversion. Four-wave mixing and difference-frequency generation have been observed in graphene under optical excitation, enabling the synthesis of THz waves from near-infrared or visible lasers. These processes benefit from graphene’s large third-order nonlinear susceptibility, which is several orders of magnitude higher than that of traditional semiconductors. By integrating graphene with photonic structures, such as waveguides or metasurfaces, efficient THz generation can be achieved in on-chip configurations, paving the way for compact THz systems.

Despite these advantages, challenges remain in optimizing graphene-based THz devices. The coupling efficiency between free-space THz waves and graphene plasmons is often limited by impedance mismatch, requiring careful design of antenna structures or grating couplers. Additionally, the scalability of graphene synthesis and device fabrication must be addressed to enable mass production. Advances in chemical vapor deposition (CVD) growth and transfer techniques have improved the uniformity and quality of large-area graphene films, but further progress is needed to meet industrial standards.

The integration of graphene with other 2D materials offers additional opportunities for THz applications. For example, transition metal dichalcogenides (TMDCs) exhibit strong excitonic resonances that can hybridize with graphene plasmons, creating new pathways for THz light-matter interaction. Van der Waals heterostructures, which stack multiple 2D materials with atomic precision, enable the design of tailored electronic and optical properties that are not possible in single-material systems. These heterostructures have been used to demonstrate novel THz devices, such as tunable filters and polarizers, with performance metrics that surpass those of conventional materials.

In summary, 2D materials like graphene provide a versatile platform for THz wave generation and detection, leveraging nonlinear conductivity and plasmonic resonances to achieve high performance in imaging and communication systems. The ability to tune graphene’s electronic properties through electrostatic gating or nanostructuring allows for precise control over THz interactions, enabling applications that were previously unattainable with bulk materials. While challenges in device integration and scalability persist, ongoing advancements in material synthesis and nanofabrication are expected to unlock the full potential of graphene-based THz technologies. As research progresses, these innovations could lead to compact, high-efficiency THz systems that revolutionize fields ranging from medical diagnostics to wireless communications.