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Black phosphorous (BP) is a unique, puckered structure, 2-dimensional (2D) material with excellent opto-electronic properties that is currently being researched across many scientific fields for electronic and optical applications. Utilising BP, the team of researchers from South Korea has managed to demonstrate efficient optical switching by FWM-based wavelength conversion within a BP-deposited fibre optic device.

2-dimensional (2D) crystals have recently emerged as promising materials for electrical and optical applications. Many 2D materials can be stacked on top of each other to create novel devices which can be tailored to many given applications. The nonlinear optical effects and ultrafast optical response has allowed for many advanced optical systems to be designed. Other 2D materials such as graphene and transition metal dichalcogenides (TMDCs) exhibit a zero-band gap and very large band gap, respectively, so there has been a need to find a material whose band gap lies in between these two extremes- This is where black phosphorous (BP) comes in.

BP is one of the most recent materials to be studied in the 2D-research world. BP has a direct and controllable bandgap that can facilitatewide-ranging, tuneable, optical responses. It also fills the gap of extreme band gaps traditionally associated with 2D materials and can be tuned from 2 eV to 0.3 eV by the addition of extra BP layers.This, coupled with the high anisotropic in-plane properties, has made BP a great candidate for nonlinear optical materials in optical broadband applications.

BP is also one of the most thermodynamically stable allotropes of phosphorous and it possess a monoelemental and anisotropic atomic structure. In addition to band gap tunability, BP also exhibits a high carrier mobility, large on/off ratios in BP-based electronic devices and anisotropic properties across many applications. The stacking of multiple layers allows BP to interact with light across a wide-range of the electromagnetic spectrum.

Recent advancements in BP technologies has shown it to exhibit four-wave-mixing (FWM). FWM is phenomenon of the nonlinear optical Kerr effect, which occurs when high power optical signals from twoor more different wavelengths are transmitted through a nonlinear optical media.

The Device

The device is a nonlinear optical device, composed of either a side polished fibre, or a D-shaped fibre, deposited with BP. The device was employed with an evanescent field interaction of BP, with propagating light to showcase the FWM phenomenon. The device experimentally demonstrated a FWM-based wavelength conversion for modulated signals up to a frequency of 20 GHz.

The fibre optic device was fabricated by a continuous wavelength (CW) laser, multiple external cavity laser (ECL), a pump laser, which utilised a mixture of tuneable and fixed lasers. Band-pass filters and polarisation controllers were also used in the setup to suppress theamplified spontaneous emission and optimize the state of polarization (SOP), respectively. A 3-dB directional coupler, and combined laser, generated and propagated the FWM through the BP-deposited device. The output light spectrum is measured by an optical spectral analyser, and Raman Spectroscopy (Renishaw inVia confocal Raman microscope, 532 nm, 2 mW), XRD (Rigaku, D-MAX 2500-PC) and UV-vis-IR-Spectroscopy (VARIAN Cary 5000 UV-Vis-NIR spectrophotometer, 175-3000 nm) were used to analyse the device and its properties.

The device was tested with both BP and bare (i.e. non-BP-deposited) fibres, with BP fibres showing greater results all-round. There is a marked improvement in the signal generation by 1.6 dB when the device is employed with BP. The FMW-based wavelength conversion performance parameter was also increases by up to 33% with the BP-based optical device.

The research has clearly deciphered that the optical devices deposited with BP show an efficient optical switching in the evanescent field interaction regime at ultrafast speeds. The ability of BP to produce superior nonlinearity in this device, alongside its already established opto-electronic properties, is anticipated to open-up BP to new commercial opto-electronic/photonic applications and further research in the future.

Source:

Uddin S., Debnath P. C., Park K., Song Y-W., Nonlinear Black Phosphorus for Ultrafast Optical Switching, Scientific Reports, 2017, 7, 43371