‘Currently, there is no real three-dimensional fabrication method on the nanoscale,’ said lead investigator Joachim Fischer of the Karlsruhe Institute of Technology (KIT). ‘The only thing you can do is you can stack two-dimensional stuff on top of each other, which is very time consuming and error prone.’
The team’s method is a variation of the relatively new 3D additive manufacturing technique of two-photon polymerisation (2PP).
Here, a liquid resin — or ‘photoresist’ — is polymerised at certain points by a pulsed laser, leaving behind 3D features as a solid polymer.
However, the minimum resolution in which features can be created is around 100nm in the lateral plane and 250nm in the axial plane, which does not meet the strict definition of nanoscale.
The team developed a technique that uses two lasers: an excitation laser to start polymerisation and a depletion laser to almost immediately stop it, thus limiting the size of the resultant features.
‘The photoresist is made out of monomer and there are photo-initiators — these are little molecules that do the job of light absorption,’ Fischer said.
‘So light comes in, excites these photo-initiator molecules and usually they start the chemical reaction. What we do is try to immediately de-excite them so they do not even have a chance to start the chemical reaction — we effectively reduce the extent of the excitation.’
The team has now achieved a spatial resolution of 65nm and is hoping to get down to 30–40nm soon, with an ultimate goal of 10nm. This threshold could exploit a whole range of phenomenon, such as the cloaking principles of metamaterials, all within reach through relatively simple fabrication rather than using exotic crystal.
‘If you want to fool light to move around an object using metamaterials, you always have to create a substructure that is so small the wavelength of light doesn’t feel it anymore,’ Fischer said.
The method could also be used for tissue scaffolds — for example, it has been demonstrated that stem cells differentiate into different mature tissue types depending on their nanoscale environment.
‘You can also think about surface structures that are found in nature, such as the lotus effect or the feet of a gecko; these are all nanostructured things you could exploit and explore in greater detail. Right now, you simply can’t play around and find new stuff on the nanoscale,’ Fischer said.