“In electronic circuits, devices such as electronic diodes allow the flow of electrons in just one direction (forward bias) but not in the other (reverse bias),” explains team leader Ramakrishna Podila of Clemson University in the US. “To achieve a similar diode action for photons is very challenging, however, because we know that light travels in both directions (if I see you then you see me). In our work, we have combined two materials (2D Ti3C2 MXene and fullerene, or C60) that have contrasting nonlinear optical properties to make a device that does allow for one-way transmission of light. In other words, we have made an optical diode.”

MXenes are a new class of 2D transition-metal carbides, nitrides and carbonitrides that interact with light in a unique way. Researchers recently found that 2D Ti3C2Tx (where Tx can be functional groups such as –OH and –F) has nonlinear saturable absorption that could be useful for mode locking in femtosecond lasers.

To better understand why these materials boast such a good property, Podila and colleagues decided to make Ti3C2Tx thin films of different thicknesses and systemically study their nonlinear optical properties.

Ti3C2Tx can withstand higher laser powers

“We made our Ti3C2Tx films using an ‘interfacial film’ formation technique in which we formed a thin layer of Ti3C2Tx at the interface of two immiscible liquids (toluene and water) and then transferred this film to a quartz substrate. We characterized its nonlinear optical properties using a pulsed nanosecond laser operating at 1064 nm using a method known as the Z-scan.”

The researchers say that Ti3C2Tx is more electrically conducting and more mechanically robust than other 2D materials, such as graphene. It is thus able to withstand higher laser powers. It also retains its saturable absorption even after being exposed to air or low-energy plasma irradiation, so the nonlinear properties of the material are stable under these conditions.

The team then made photodiodes by juxtaposing Ti3C2Tx MXene with C60 thin films. “We had already made a similar device in the past by directly coating C60 on graphene, so knew that this approach works,” Podila tells nanotechweb.org. “The most important feature of the new MXene-based photonic diode is that it is ‘passive’ and does not require active magnetic fields to operate (unlike existing Faraday rotors).”

Improving the non-reciprocity factor

One key application for the device would be in optical isolation for high-power lasers – given that Ti3C2Tx can withstand high laser powers, as mentioned, he says. It might also be used as a saturable absorber material for Q-switching laser components.

The team, reporting its work in Advanced Materials, says that it still has some way to go in its research. “For one, the non-reciprocity factor (that is, the degree of optical isolation, which is defined by the ratio of light transmission in forward versus reverse directions) for this device is only a modest 4dB,” explains Podila. “Our goal is to explore other MXene materials that may allow us to improve this factor by an order of magnitude. Devices made from these materials may then be able to compete with existing Faraday rotors.”

This work was performed in collaboration with Clemson Nanomaterials Institute (Apparao M Rao, Sriparna Bhattacharya, and Yongchang Dong), Drexel University (the Yury Gogotsi group) and Missouri University of Science and Technology (Vadym Mochalin's group).