- 2D Crystals
There is an ultimate limit for electronic miniaturization imposed by electron-electron interactions and by the Pauli exclusion principle that forbids two electrons to occupy the same quantum state. This will eventually prevent the development of denser circuitry, as well as of multiplexed or parallel schemes using electrons as information carrier units. Photons, however, can share a same logical gate, without interacting among one another, unless mediated by the supporting material and its nonlinear optical properties. This suggests that, in the near future, technology based on all-optical photonic devices will partially take over electronics and, possibly, extend classical computing in use today to include new quantum protocols and techniques. In this photon controlling photon type of devices, materials that provide efficient nonlinear optical interaction will indubitably play a central role.
Current technologies use, as nonlinear media, crystals such as ferroelectric oxides (e.g., LiNbO3, BaTiO3, KNbO3, tungsten bronze family, etc.), compound semiconductors (e.g., GaAs, GaP, InP, CdS, CdSe, CdTe, etc.), silenites (e.g., Bi12SiO20, Bi12GeO20), or ceramics. Due to their tridimensional nature, these materials are usually brittle and hard to interface with typical photon carrying conduits, such as optical fibers and integrated optical waveguides (including on-chip silicon photonics). This makes them unsuitable to fulfill the above-mentioned holy grail of miniaturization and scalability.
2D materials, on the other hand, due to their atomic thicknesses, are soft and compliant with the substrate, and can be easily interfaced with waveguides for photonics and optoelectronics. Exciting examples in this direction were recently reported, with graphene-enhanced four wave mixing (Gu et al., Nature Photonics, 2012) and ultralow threshold lasing with a tungsten diselenide monolayer (Wu et al., Nature, 2015) being obtained with the 2D materials deposited on on-chip photonic crystal cavities.
In a paper just published in Advanced Materials, researchers from CA2DM and MackGraphe, the Graphene and Nanomaterials Research Center of the Mackenzie Presbyterian University, Brazil, have demonstrated, for the first time, optical frequency conversion in phosphorene, one of the most promising 2D materials, via the third-order nonlinear optical effect of third harmonic generation, which was found to exhibit a tunable and exceptional exciton-driven efficiency.
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