In an article published in Optics and Laser Technology, researchers examined terahertz (THz) optoelectronic devices, including terahertz lasers, modulators, detectors, and phase shifters based on graphene materials.
The process of graphene's growth, transfer, and patterning was covered in the study, along with the details of related functions. It was found that graphene-based terahertz optoelectronic devices have excellent output bandwidth, detection bandwidth, responsivity, noise equivalent power (NEP), and sensitivity.
Terahertz Waves and its Significance in Optoelectronics
Between infrared and microwave radiation is the terahertz wave with a frequency range of 1 THz to 10 THz. Terahertz technology is currently demonstrating significant value in basic research, defense, communication, astronomy, medicine, environmental science, and industrial applications due to the rapid advancement of semiconductor integration and ultra-fast optoelectronics technologies.
The fundamental building block of fullerene, carbon nanotubes, and graphite is the novel substance known as graphene, which has a single-layer 2D honeycomb lattice construction. Graphene is an operational material with a zero-band gap structure, which leads to broadband absorption characteristics and has achieved relatively broad applications in photonics and nanoelectronics.
Graphene, a novel optoelectronic material, is closely connected primarily because of its distinct energy band shape and features of electron transport. Double-layer graphene or epitaxially produced graphene can be manufactured by manipulating graphene's restricted bandwidth artificially to get it into the terahertz wave band.
The interaction between the graphene epitaxial layer and the graphene substrate in graphene-based optoelectronics alters the material's electronic state, energy, structure, and optical phonons. Considering the newly discovered characteristics of graphene compounds, in addition to offering ideas for future research directions, graphene has the potential to serve as the foundation for the investigation of novel terahertz devices.
Graphene has a thickness of just one atomic layer made of tightly packed carbon atoms in a 2D hexagonal lattice. The covalent bonds between every two carbon atoms in each hexagonal lattice in the plane are spaced at 120 degrees, generating a sp2 hybrid orbital. The energy band of graphene structure possesses unique optical characteristics not present in other semiconductor materials.
Graphene-Based Terahertz Optoelectronic Devices
The analysis of the performance comparisons revealed that traditional terahertz lasers and graphene-based terahertz lasers could be contrasted in frequency range and output power. While the output frequency band was around 4-8 THz, the output power reached the order of mW, and the spectral resolution was between KHz and GHz. The primary cause of graphene THz's outstanding performance was graphene's distinctive visual and electrical characteristics.
The properties of materials made of graphene could make the overall improvement of THz control efficiency. Therefore, the most efficient ways to control graphene's conductivity include light field regulation and gate voltage. It was frequently necessary for a THz modulator to have strong modulation performance, including modulation frequency and bandwidth.
The study's findings indicated that graphene is a material with great potential for advancing flexible THz technology. Graphene-based materials assisted in the downsizing and utility of THz detectors. The THz detector with the graphene structure had a quicker response time, a lower NEP, and higher sensitivity. The THz detector's maximum responsiveness was about 800 V/W, and its lowest NEP was at 3×10-11 W/Hz1/2.
Different findings were drawn by examining the issues encountered during the research process for graphene THz optoelectronic devices. By employing electron beam excitation to transform graphene into coherent THz Cherenkov radiation, THz radiation power might be significantly increased. A graphene micro or nano resonant cavity array was proposed to realize a THz plasma laser. The excited plasma excitations were closely coupled with THz radiation, resulting in more significant THz radiation to increase THz radiation gain.
Instead of a metal gate or even a 2D semiconductor layer, graphene provided better THz modulation capability. Due to Drude scattering, the optical conductivity of graphene diminished as the input THz wave frequency increased. Graphene's plasma effect could be used to modulate THz frequencies and employing patterned meta-surface graphene structures could further enhance the coupling effect with THz waves.
Significance of the Study
Graphene is a new class of 2D functional material with unique optical and electrical characteristics that can enhance the functionality of THz devices. It is anticipated to have significant uses in the THz industry.
With the development of science and technology, it has become possible to create large-area, high-quality graphene materials using the CVD process, providing the basis for THz optoelectronics research.
Advances in graphene material preparation technology have laid a solid foundation for graphene layer control, functionalization, physical properties research, and the exploration of new-generation optoelectronic devices.
In summary, graphene-based THz optoelectronic device research is still in its early stages. The research history demonstrated that graphene THz devices had size, low loss, and simple integration advantages. As a result, graphene was simple to mix with other THz components to enhance the device's overall performance. It could even be integrated into a single-chip THz system, which offered new research opportunities and directions for creating THz optoelectronic devices.
Zhou, Q., Qiu, Q., Huang, Z. (2022) Graphene-based terahertz optoelectronics. Optics & Laser Technology. https://www.sciencedirect.com/science/article/pii/S0030399222007083