# Directly Synthesized Graphene-Based Photonics and Optoelectronics Devices
## 5 Conclusions  
We present a brief review of recent advancements in the development of directly synthesized graphene-based optoelectronic and photonic devices. In addition to the exciting electronic and optical properties, we have first discussed the current state of the art for synthesis of graphene. Then, we presented a summary of directly synthesized graphene on optical fiber-based substrates that exploit its nonlinear optical responses. Combined with facile direct growth and no requirements for transferring the resultant graphene, these methods are promising for applications in electronics and nonlinear optical ultrafast photonic devices. In addition to the superior coverage of graphene by the direct method, conformal growth is a great benefit that nullifies interstices frequently observed in suspended graphene cases. Nevertheless, the distance (distance from catalyst to the target substrates) functional growth of graphene exhibits tunable semiconducting properties that can be exploited for nanoelectronic devices. Demonstrations of ultrafast optical pulses in both Q-switching and mode-locking domains with in-situ and ACS grown graphene-enabled devices were possible. Ultrafast optical switching up to 20 GHz modulation frequency has been demonstrated by graphene grown in situ, and ACS methods indicate that the optical nonlinearity is sufficiently high. In addition to their excellent electronic and optical properties, these direct growth methods strengthen the candidacy of graphene for optoelectronic and photonic devices. The verified optical nonlinearity with direct growth methods on fiber devices can pave the way for implementation in futuristic graphene-enabled hybrid optoelectronic and photonic devices.  
The in situ and ACS methods, evidently different from other conventional methods, can develop transfer-free, directly synthesized graphene on optical devices. Furthermore, the ACS method has exhibited potential as a practical solution for direct synthesis of graphene on arbitrary 3D substrates. This method is a breakthrough for future on-chip optical signal processing and optical interconnects along with the chip-scale integration of electro-optic hybrid devices. In recent studies, the possibility of graphene-based integrated devices has been analyzed. Thus, graphene can also be used in broad-ranging optical control schemes. Therefore, the fabrication of silicon-based integrated optical circuits, which are expected to be used in numerous operations, such as pulsed light generation, routing, modulation, computing, and detection, can be enabled. Moreover, the integration of graphene into hybrid silicon photonics could be possible by direct deposition of graphene onto silicon. Therefore, graphene research may spawn a technological revolution that could possibly turn into a paradigm shift for ultrafast optics.  
**Author Contributions:** Conceptualization, S.U. and Y.-W.S., Methodology, S.U. and Y.-W.S., Data collection S.U., Writing S.U. and Y.-W.S., Original draft preparations, S.U., review and editing S.U. and Y.-W.S. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Basic Science Research Program (Grant No. NRF-2019R1A2C2087693) of the National Research Foundation (NRF) funded by the Ministry of Science and ICT, Republic of Korea. Also, supported by the Institutional Program (2E31011) funded by the Korea Institute of Science and Technology (KIST), Republic of Korea.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data is contained within the article.  
The authors declare no conflict of interest.