Smaller, more versatile optical filters on the horizon

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A rendering of the experimental setup used. Light is reflected down to the nanostructure of molybdenum disulfide (yellow and teal lattice) and PZT (blue and green). Wavelengths reflecting from the surface are captured by the top detector as transmitted wavelengths pass through the PZT to the bottom detector. (Image: Hong et al.)

Researchers at the University of Nebraska–Lincoln in the US could spur the development of smaller, more versatile optical filters that are especially adept at playing a trick of the light.

The term ‘trick’ might not be the most accurate noun to describe what is actually a complex physical phenomenon known as second-harmonic generation, where two photons strike a material and eject a third photon with double the energy and half the wavelength.

Due to the differing wavelengths of the electromagnetic spectrum, this phenomenon can be used to transform incoming infrared waves into waves of blue light, for instance, or transform incoming waves of visible light into ultraviolet light.

How ever second-harmonic generation cannot be observed using any material. This however, which is why Xia Hong, associate professor of physics and astronomy, and her fellow nanoscientists at the university, have spent years working with an atom-thin layer of molybdenum disulfide.

They have been exploring the phenomena that emerge from pairing molybdenum disulfide with so-called ferroelectric materials, whose alignment of positive and negative charges will flip when exposed to an electric field.

Unexpected observations

Last year the researchers were studying how the optical behaviour of single-layer molybdenum disulfide responded when it was placed atop a ferroelectric material called lead zirconate titanate, or PZT.

We weren’t expecting much, but we saw this very, very strange effect,’ remarked Hong.

Instead of observing second-harmonic generation uniformly across the surface, the team noticed that certain segments were boosting the phenomenon while others dampened it. They also realised that the unexpected pattern emerged at the PZT’s domain walls, where a section with positive polarisation – upward-facing positive charges separated from downward-facing negative charges – met a section with negative polarisation. Not only that: the intensity of the reflected second-harmonic generation alternated wall by wall, so that the first, third and fifth wall boosted it while the second, fourth and sixth were dampening it.

Given that the pattern was missing in either material alone, the researchers deduced that it must originate from some sort of interaction between the two. On closer inspection, they discovered that vortex-like swirls of the positive and negative charges at the top of the PZT walls – similar to the tornadic rotation that can occur when warm and cool air converge – were contributing to the effect.

When that rotation matched the polarisation of the overlying molybdenum disulfide, so that the former swirled clockwise as the latter was aligned left to right, or vice versa, the reflected second-harmonic signal nearly quadrupled in intensity. When those polarisations ran counter to one another, the reflected signal virtually disappeared.

This side view of PZT shows the meeting of segments with positive polarisation (left, red) and negative polarisation (right, blue). (Image: Hong et al.)

The polarisation of the incoming light mattered, too. An electric field surrounding a ray of unpolarised light, such as that coming from the sun, will haphazardly jut out in all directions. The electric field of polarised light, by contrast, will stick to one plane – vertical or horizontal – or rotate around the ray in a predictable, cyclical way. Though incoming light that was polarised at certain angles did produce a clear second-harmonic pattern upon reflecting, the signals disappeared when the team adjusted the light’s polarisation to other angles.

The team also found an intensification-mitigation pattern for the wavelengths that passed through the nanostructure, rather than reflecting from it. As opposed to depending on the match or mismatch of polarisation between the materials, though, the second-harmonic generation responded to the polarisation of the PZT patches alone. When light was polarised at certain angles, the PZT patches with positive polarisation boosted the signal, whereas the negatively polarised patches dampened it. And adjusting the light’s polarisation could reverse the relative strengths of those signals.

Optical filtering on the nanoscale

Hong remarked that the nanostructure’s sensitivity to polarised light, combined with the ability to flip the PZT’s polarisation either electrically or mechanically, makes for something unusual: an optical filter that could be programmed and reprogrammed in a matter of seconds.

‘It’s nanoscale, and it can be controlled, so you could say this is a smarter way of filtering, because you can reconfigure it,’ she said. ‘It’s not a done deal. I can write the polarisation like this, I can erase it, then I can write it in a different way...the key is really that it’s a very simple technique.’

The technique’s versatility could prove useful in quickly characterising materials or substances, Hong continued, especially the properties that influence second-harmonic generation or dictate responses to the polarisation of light.

She jested that while the technique isn’t really suited to the routine, macro-level applications of polarised filtering – such as making polarised glasses – if a 3D movie were ever made at the microscale, this would probably be the way to do it. On a more serious note, Hong explained to Electro Optics that the electrically programmable nanoscale optical filters enabled by the technique could have applications in spectroscopy, nonlinear nanophotonics and information technology.

‘The smaller, smarter optical filters can find applications in nanoscale optical switches, modulators, and even optically controlled logic operations for developing integrated photonic circuits and optical computing systems,’ she confirmed. ‘The MoS2/ferroelectric heterostructure has the distinct advantages, in terms of size scaling and being nanoscale reconfigurable, thus promising on-chip generation and smart filtering of nonlinear optical signals for nanophotonics.’

Hong added that she has also been inspired by a comment posted on the webpage of the Nature Communications paper in which the researchers reported their findings, where a fellow scientist asked: ‘Haven’t you just built an optically controlled programmable quantum computer? This may have much more reach than you realise.’

Looking forward

As for what’s next for the researchers, Hong explained that they will continue to combine nonlinear optical techniques with scanning probe approaches, to study the interfacial synergetic effects between emerging polar 2D materials and ferroelectric oxides. The gained knowledge will then be leveraged to design the optical and electronic responses of the 2D material at the nanoscale.

Light signals reflected from PZT (left), reflected from the molybdenum disulfide-PZT nanostructure (centre), and transmitted through the nanostructure (right). The centre image, taken without filtering for polarised light, illustrates the alternated boosting (red) and dampening (blue) of second-harmonic generation at the PZT’s domain walls. (Image: Hong et al.)

‘One 2D candidate of interest is the ferroelectric van der Waals material,’ Hong said. ‘In addition, high-harmonic generation [HHG] up to the 13th order has been observed from monolayer MoS2 [Nature Physics 13, 262-265, 2017]. It would be very interesting to explore the optical filtering effect for the HHG in monolayer MoS2/ferroelectric heterostructures, which can serve as a platform for studying strong fields and ultrafast phenomena.’

When asked if the researchers would eventually be looking for any industrial partners to work with, Hong replied: ‘We are currently focusing on gaining fundamental understanding of the emerging interfacial phenomena in the ferroelectric/2D hybrid systems. We’d be very glad to see the phenomena eventually being developed into industrial applications, but we don’t have any specific plan at this stage.’

Hong authored the Nature Communications paper with Yongfeng Lu, Evgeny Tsymbal, Dawei Li, Xi Huang, Zhiyong Xiao, Hanying Chen, Le Zhang, Yifei Hao, Jingfeng Song and Ding-Fu Shao. The team received support from the US Department of Energy’s Office of Science, the National Science Foundation and the Office of Naval Research.

Article in Nature Communications volume 11: Polar coupling enabled nonlinear optical filtering at MoS2/ferroelectric heterointerfaces

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