A new approach to optical microscopy
A nanostructured LED array is the basis for a novel approach to sub diffraction limited microscopy on a chip-size level. A first prototype microscope has been constructed.
In the Chipscope project funded by the EU, a completely new strategy towards optical microscopy is explored by a team of researchers from different European institutions. In classical optical microscopy, the analysed sample area is illuminated simultaneously, collecting the light which is scattered from each point with an area-selective detector, for example the human eye or the sensor of a camera.
In the Chipscope idea instead, a structured light source with individually addressable elements is utilized. The specimen is located on top of this light source, in close vicinity. Whenever single emitters are activated, the light propagation depends on the spatial structure of the sample, very similar to what is known as shadow imaging in the macroscopic world. To obtain an image, the overall amount of light which is transmitted through the sample region is sensed by a detector, activating one light element at a time and thereby scanning across the sample space. If the light elements have sizes in the nanometer regime and the sample is in close contact to them, the optical near field is of relevance and super resolution imaging may become possible with a chip-based setup.
Several partners in the project bring in expertise in the according research fields. The structured light source is realized by light-emitting diodes, which are developed at the University of Technology in Braunschweig, Germany. To the present point, no structured LED arrays with individually addressable pixels down to the sub-µm regime are commercially available. First LED arrays with pixel sizes down to 1 µm have already been demonstrated by the researchers. They are based on gallium nitride (GaN), a semiconductor material which is commonly used for blue and white LEDs. Controlled structuring of such LEDs down to the sub-µm regime is extremely challenging. It is conducted by photo- and electron beam lithography, where structures in the semiconductor are defined with high precision by optical shadow masks or focussed electron beams.
As a further component, highly sensitive light detectors are required for the microscope prototype. Here, a group at the University of Barcelona, Spain, develops single-photon avalanche detectors (SPADs), which can detect very low light intensities down to single photons. First tests with those detectors integrated into a prototype of the ChipScope microscope have already been conducted and have shown promising results.
Moreover, a way to bring specimens into close vicinity of the structured light source is vital for proper microscope operation. An established technology to realize this utilizes microfluidic channels, where a fine system of channels is structured into a polymer matrix. Using high-precision pumps, a micro-volume liquid is driven through this system and carries the specimen along to the target position. This part of the microscope assembly is contributed by the Austrian Institute of Technology AIT. Further partners in the Chipscope project comprise a team of the Medical University of Vienna, the Italian University of Rome Tor Vergata, the Ludwig-Maximilians University in Munich and the FSRM, Switzerland.
The Chipscope project, funded in the framework of the EU’s Horizon 2020 programme, was launched in 2017 and will run until the end of 2020. Up to now, a lot of progress has already been achieved in the different subtopics involved in the project, including a prototype of the proposed microscope. The involved research groups are confident that the technology can be pushed forwards during the final period of the project and that the fundamentals of the Chipscope microscope technology will be explored as well as a more powerful prototype with higher resolution can be presented by the end of the project.