Nanostructures in motion
Optical systems capable of changing in response to external factors have a range of applications – from image processing to smart sensors. Of utmost interest at the moment are micro- and nano-scale systems. The possibility to change the structure of systems at the nano-scale allows researchers to change their optical properties.
“These days, a nanostructure is unlikely to change once manufactured. For instance, if we make an array of nanocylinders we won’t be able to change it in any way later. Our goal here was to develop a dynamic nanostructure that can change its response or state based on various external factors. This way, we’d be able to change its properties at any point in time,” explains Elena Gerasimova, the article’s lead author and a third-year PhD student at the Faculty of Physics.
In addition, the developed nanostructures are easily and quickly synthesized and don’t require any expensive equipment for additional procedures such as lithography. In order to create such materials, the researchers from ITMO use chemical methods. Another key feature of this new technology is that there is no limit to how many times the nanostructure can change its state.
The nanostructure consists of synthesized spherical hydrogel particles of poly-N-isopropylacrylamide (pNIPAM). It is one of the most thoroughly-studied and commonly used temperature-sensitive polymers. At room temperature, the particles absorb water (hydrophilia) and expand in size. But once the temperature reaches 33 degrees Celsius, the polymer goes through a phase change, causing it to expel the water (hydrophobia) and thus lose half of its volume.
The researchers also modified the surface of the polymer with various nanoparticles. In one case, the team used silicon nanoparticles; in another – a mixture of silicon and gold nanoparticles. Silicon is a dielectric with a high light refraction index, while gold is a plasmonic material. Together, they amplify the electromagnetic field and, in turn, the nanostructure’s optical properties. But how does one combine the nanostructure with the silicon nanoparticles if both have the same negative charge? This challenge was solved by another of the article’s authors, first-year PhD student Lidia Mikhailova.
“We pulled a little trick by modifying the surface of the particles with the cationic polyelectrolyte PAH (polyallylamine hydrochloride), which has allowed us to not only compensate for silicon’s negative charge, but also to enrich the resulting particles with a positive charge,” she says.
Both nanostructures were studied in their contracted and expanded forms. The phase change that affects their state can also regulate optical properties, such as second-harmonic generation. This is a process during which a nanostructure absorbs light in one wavelength and produces light on a wavelength that is twice less than the original. In other words, the nanostructure will convert infrared light into green light – all thanks to its non-linear properties.
“Combining nanoparticles with different properties has allowed us to increase the effectiveness of second-harmonic generation. When a nanostructure is in its condensed form, the distance between gold and silicon nanoparticles is reduced and the material shines 35 times brighter than when it’s expanded. The nanostructure with silicon nanoparticles only amplifies light by seven times, the reason being that there are no free electrons that would otherwise be produced through interaction with gold,” says Vitaly Yaroshenko, a junior researcher at the Faculty of Physics and one of the article’s authors.
New research environment
In the future, nanostructures with enhanced second-harmonic generation properties could be used in micro- and nano-scale optical systems. That includes automated temperature sensors, robotic devices, reconfigurable optical metasurfaces, and other systems in which mechanical changes in polymers can be activated through external influences.
“We wanted to try and make an automated nanostructure. At the moment, there are some materials capable of affecting second-harmonic generation, but they require you to measure the second harmonic in advance and manually extend an array from the nanostructure. The properties, therefore, change depending on the level of extension. But if the process is automated, we’ll be able to integrate it into more independent systems that don’t require human input,” explains Elena Gerasimova.
In the future, the scientists will need to determine how the nanostructure could be placed within a more suitable medium. The issue is that the polymer can only contract and expand in water, which is why the research was conducted in colloidal solutions. One of the possible ways around it is to change the synthesis method so as to make the polymer collect water from air rather than dry out.
This research received support from the Russian Science Foundation via grant No. 21-72-30018.
Reference: Elena Gerasimova, Vitaly Yaroshenko, Lidia Mikhailova, Dmitry Dolgintsev, Alexander Timin, Mikhail Zyuzin, Dmitry Zuev. Thermally Induced Mechanical Switching of the Second-Harmonic Generation in pNIPAM Hydrogels-Linked Resonant Au and Si Nanoparticles (Advanced Optical Materials, 2022).