A curious way with light


Rebecca Pool

Monday, January 7, 2019 - 13:30
From bending it backwards to sorting drugs, Professor Jennifer Dionne uses light to take microscopies and materials to extraordinary levels.
A nanoparticle chewing nematode wouldn't spring to most researchers' minds when building biosensors to detect early disease, but for Stanford University's Professor Jennifer Dionne, it all makes perfect sense.
Heart disease, cancer and other diseases alter cellular-level forces within our bodies. So developing a nanoparticle that emits different coloured light in response to micronewton and nanonewton level forces is an obvious, first step.
But then feeding the nanoparticle to the millimetre-long worm and recording the light that is emitted from the sensors through its transparent body during digestion is not so obvious; or is it?
"As the worm chews the nanoparticle and it moves through the digestive tract, forces are exerted by organs and cells," explains Dionne. "We can make the nanoparticles sensitive to these forces, capture the dynamic emission [of the particles] and so use the particles as a readout of mechanical forces in biology."
Dionne's unlikely biosensor development stems from close collaboration with Stanford molecular and cellular physiologist Professor Miriam Goodman, who uses nematodes to understand human sensation.
But while unconventional, her project is symptomatic of an open-minded approach to research that has seen Dionne delivering breakthrough after breakthrough.
Stress-sensitive upconverting nanoparticles are used to image mechanical processes in biology. These force sensors to shed light on the neuromuscular pump action in C. elegans. [Alice Lay et al, Nano Lett., 2017, 17 (7), pp 4172–4177]
In 2007, she hit the headlines and contradicted hundreds of years of scientific understanding by bending visible light backwards with a negative-index material she had developed with her Caltech supervisor, Professor Harry Atwater, and colleagues. 
Only months later, still in Atwater's group, she unveiled a nanoscale plasmonic modulator for switching light, that set the field of on-chip, optical communications alight.
Building on her negative refractive index material research, in 2013 she delivered a broadband negative index metamaterial that held promise for invisibility cloaks as well as a 'perfect lens' to directly image individual proteins via light microscopy.
And from upconverting nanoparticles to optical filters of chiral nanoparticles, much more has followed. 
"My research has been all about developing new optical materials and new methods to directly visualise nanoscale chemical and biological processes," she says. "This theme runs through my whole lab, and we really want to visualise processes at the highest possible resolution and in real time."
Light fascination
From an early age, Dionne was intrigued by light. She recalls a high school astronomy experiment where she used emitted spectra from stars to understand the composition of the celestial bodies.
"I realised that you could get so much information about a material, just from the light it emits," she says. "And even then, I thought it would be really unique if you could create materials that would actually allow you to control how light interacts with them."
From her all-girls Catholic high school, Dionne went to Washington University in St Louis, to study Physics, and Systems Science and Engineering.
Jennifer Dionne as a young graduate.
Her interests were diverse, with undergraduate projects including developing nuclear magnetic resonance imaging systems for lung imaging of small animals and studying fluid flow in wave tanks to understand how ocean fronts form.
Torn between oceanography and physics, she came across the emerging field of nanoscience during a tour of Caltech and was hooked.
Taking on her Masters and then PhD at the Atwater Research Lab, she explored how to overcome diffraction using plasmonics; her future in light matter interactions was sealed.
It was 2003, and nanophotonics and controlling light at the nanoscale were emerging research fields.
As part of her research, Dionne wanted to see if she could alter the structure and composition of metals to better control light.
Structured metals were considered to be a great platform for this, and by directing light at the surface, wavelike motions of electrons - plasmons - could be created at that surface.
Crucially, these plasmons could be 'squeezed' to reduce their wavelength to less than that of the light in air or a dielectric, and create a host of unusual phenomena.
Dionne's negative index materials and chip-based plasmonic systems followed, fuelling her research ever further.
Manipulating light in unnatural ways: this metamaterial exhibits negative index of refraction, which skews the light to the left, different from what would have been expected from any natural material, which have a positive angles of refraction. [Image: Ashwin Atre, Stanford Engineering]
With PhD in tow, in 2008, Dionne moved to the University of California, Berkeley and Lawrence Berkeley National Lab, to work with 'the very inspirational' Professor Paul Alivisatos.
Here, she was investigating the electro-optic and photochemical properties of semiconductor and metallic nanocrystals for solar fuel generation.
"Alivisatos pioneered research in quantum dots and his approach to science was incredibly thoughtful and insightful," she says. "I focused on photocatalysis in his lab and learnt so much about chemistry; this is where I really started getting interested in the intersection between physics and chemistry."
Dionne quickly moved on to Stanford, in pursuit of the University's open laboratory policy, cutting-edge environmental TEM and Palo Alto sunshine.
But as she points out: "I've taken what I learnt from the Alivisatos lab and now look at photocatalysts using light-coupled electron microscopes."
"Paul and I have since worked together, trying to create these very bright upconverting nanoparticles that can convert infrared light to visible light, for solar energy applications," she adds.
So come 2010, Dionne took the position of Assistant Professor of Materials Science and Engineering at Stanford University, to lead a growing team of materials scientists, electrical engineers, physicists and chemists, intent on developing materials to control light in improbable  ways.
And the results have been as diverse as they have been spectacular.
Dionne and researchers design optical materials and microscopies to study processes as they unfold at nanometre resolution.
From word go, metamaterial and metasurface development has continued at speed, with researchers delivering their broadband metamaterial with negative indices across hundreds of nanometres in the visible and near-infrared spectrums, in 2013.
Myriad metamaterials have followed with researchers looking to develop optical diodes, in which light can travel in one direction.
Dionne's lab has also become renowned for designing and synthesizing bright, stable, highly efficient luminescent nanoparticles that can both fluoresce and upconvert near-infrared photons to higher energies.
Crucially, these upconverting nanoparticles can be used to harvest more of the solar spectrum to create efficient solar panels, and also boost resolution in biological imaging. 
"You will always find unexpected surprises if you keep looking, so remain open and curious, and always ask 'what if', and 'why'?" Jennifer Dionne.
The young associate professor also regards her lab's development of chiral nanophotonics as a key achievement, including the development of a nanostructured filter that sorts chiral molecules with light, according to their left- or right-handedness.
Early demonstration of the filter used a nano-fabricated spiral on an atomic force microscope tip to quantify the optical forces through the filter.
Shining circularly polarised light through the filter exerts a force onto the tip, according to the handedness of the light and tip.
Given this, Dionne and colleagues have used the AFM probe to map chiral forces, showing that the optical forces produced by the filter are strong enough to separate certain chiral molecules.
"Many agrichemical and pharmaceutical products are chiral and its is very difficult to separate these," highlights Dionne. "But our vision is to use the chiral light to preferentially interact with one handedness of the drug or agrichemical, and efficiently separate the chiral structures."
And importantly, in-situ TEM features heavily in Dionne's lab, with researchers striving to image chemical transformations as well as light- and plasmon-mediated catalysis in nanoparticles, in real-time and at nanometre scale resolution.
Industry links
Back in 2012, Dionne and colleagues used aberration-corrected TEM with STEM electron energy-loss spectroscopy to investigate the plasmon resonances of individual, silver nanoparticles with diameters as small as 2 nm.
As Dionne puts it: "I thought we could use the shift in plasmon resonance to determine when a photocatalytic reaction was occurring, or to be able to monitor reactions with single electron precision."
The researchers have since used electron diffraction, electron energy loss spectroscopy and dark-field contrast in the environmental TEM to study hydrogenation reactions in metallic nanoparticles, critical to energy storage devices.
And now Dionne is very excited about recent work with Gatan, to develop a custom cathodoluminescence holder, to couple light into and out of the environmental TEM and study individual catalysts with nanometre resolution for better photocatalyst design.
"This has been quite an engineering challenge as we have to surround a sample with parabolic mirrors and other components to focus light onto it," she says. "But with this prototype set-up, we've been able to show how illumination at different wavelengths can modify the rate and mechanism of a reaction... eventually we might be able to use light to control which products of a photochemical reaction are generated."
Right now, Dionne and colleagues are also working on coupling the light out of the TEM, which holds vast potential for cell biologists. "You could fluorescently tag a protein in the environmental TEM, look at the light emission, and then simultaneously get information about the location of those proteins," she says, "So, it's kind of a tool that brings together the best of both optical and electron microscopy."
So where next for Dionne? Earlier this year, she became Director of a new Department of Energy centre called 'Photonics at Thermodynamic Limits' that aims to develop new nanomaterials that reflect, radiate and otherwise interact with light with applications in the energy sector and beyond.
 "There are so many new materials that are redefining textbook operations of light matter interactions," she highlights. "We want to explore these materials using cutting-edge experimental and theoretical techniques and achieve radiative efficiencies that approach thermodynamic limits for energy generation and storage, and information applications."
Professor Jennifer Diones and her students use advanced electron microscope techniques to image atoms, nanoparticles and more.
Yet, on top of this and her endless optical materials development, Dionne's unconventional and interdisciplinary research on nematodes with fellow Stanford researchers continues.
As she highlights, the C. elegans and humans share many similar genes, so the hope is to use the nematode platform to better understand human biological systems.
And if her research team can hone their nanonewton-detecting nanoparticles to detect even tinier piconewton forces, then studying subcellular forces, including the mechanical forces between neurons, becomes feasible.
"We engage in curiousity-driven research in my lab, and I believe you should never stop asking questions," she says. "You will always find unexpected surprises if you keep looking, so remain open and curious, and always ask 'what if', and 'why'?"
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