Moving the optical resolution in liquid crystal to only 90 nanometers!

2^nd February 2019

Researchers of Soft Matter Lab in collaboration with researchers of the Biophysics Lab at JSI and Faculty of Pharmacy at University of Ljubljana, have used the super-resolution Stimulated Emission Depletion (STED) microscope to resolve structural details in liquid crystal  to 90 nanometers.

The goal was to compare the obtainable resolution of classical optical microscopy to that of fluorescent confocal (FCM) and STED microscopy. We studied several cholesteric LC samples with different pitch values confined tightly between two glass plates producing uniform lying helix (ULH) structures that appear as equidistant lines with half-pitch spacing. Observing these structures with a classical Nikon optical microscope, equipped with a high NA oil-immersion objective and near-UV narrow band illumination, it was possible to resolve ULH lines that were only 140 nm apart – not bad at all! Resolution of the FCM was significantly worse, because the emitted fluorescent light is in the red part of the spectrum, allowing us to resolve only lines more than 200 nm apart. However, switching on a far-red detuned doughnut shaped STED beam of high intensity, significantly reduced the volume from which fluorescent signal was collected and finally we were able to resolve lines that were more than 90 nm apart!

STED microscopy has extensively been used in life sciences ever since Stefan Hell introduced it in the 90s, but this is one of the first reports of its use in LC systems. As STED microscopy is an improved variant of FCM a very important step is the development of the probes (or dyes) that label the sample and produce optical contrast. The dyes used in our experiments were custom synthesised for use with the STED system available at JSI, mix with our samples and align well with the nematic director.

The paper by Jaka Pišljar, Gregor Posnjak, Boštjan Kokot, Rok Podlipec, Igor Muševič from JSI and Alan Godec and Stane Pajk from the Faculty of Pharmacy, University of Ljubljana was published online in Liquid Crystals.