Nano illumination microscopy: a technique based on scanning with an array of individually addressable nanoLEDs

Abstract

In lensless microscopy, spatial resolution is usually provided by the pixel density of current digital cameras, which are reaching a hard-to-surpass pixel size / resolution limit over 1 µm. As an alternative, the dependence of the resolving power can be moved from the detector to the light sources, offering a new kind of lensless microscopy setups. The use of continuously scaled-down Light-Emitting Diode (LED) arrays to scan the sample allows resolutions on order of the LED size, giving rise to compact and low-cost microscopes without mechanical scanners or optical accessories. In this paper, we present the operation principle of this new approach to lensless microscopy, with simulations that demonstrate the possibility to use it for super-resolution, as well as a first prototype. This proof-of-concept setup integrates an 8 × 8 array of LEDs, each 5 × 5 μm2 pixel size and 10 μm pitch, and an optical detector. We characterize the system using Electron-Beam Lithography (EBL) pattern. Our prototype validates the imaging principle and opens the way to improve resolution by further miniaturizing the light sources.

Pursuing the Diffraction Limit with Nano-LED Scanning Transmission Optical Microscopy

“…In order to explore how scanning is affected by shifting the light source in steps smaller than the diffraction limit, the first prototype with a lens to provide demagnification was constructed. To do this, the sample was scanned with the downscaled image of a 5 µm LED from the 8 × 8 GaN array described formerly in [ 39 ]. Optical downscaling was performed with a x60 objective with an NA of 0.85.…”

“…A key improvement of this setup, following the results of [ 39 ], was the ability to locate the sample precisely where the spot was of minimum size, by placing it at the focal plane of the lens (see Figure 2 b1). As the spot size, as well as the pitch, is critical for image formation, this allows for improved resolutions.…”

“…The other key component of the microscope is an optical photodetector used to measure the light traversing the sample, and a camera can typically be used. The first prototype using this technique was already presented in [ 39 ], where every LED in an 8 × 8 gallium nitride (GaN) array (5 µm in size and 10 µm in pitch) was sequentially turned on and off, mapping the sample to prove the principle of operation of nano-illumination microscopy (NIM). The main features of this technique are that the resolution is given by the LED pitch and that the FOV is given by the LED array size when the sample is placed in direct contact with the array.…”

“…In this work, the second and third generations of NIM microscopes are presented. Both are based on [ 39 ], where the proof-of-concept and the characterization of the imaging technique were described. The first microscope presented was assembled with a single 5 µm LED, which was optically downscaled with a lens to further investigate the possibilities of the STOM technique.…”

“…Nevertheless, as the distance between LEDs, or their position, is also downscaled, the light can be shifted onto the sample with steps that are smaller than the diffraction limit, allowing for an oversampling of the image. Contrary to [ 39 ], where the sample is scanned by the light sequentially emitted by the 8 × 8 nano-LED array, a single LED and a set of nanopositioners are used in order to carry it out. Therefore, the nanopositioners are integrated to move the sample, i.e., they emulate the light scan, with nanometric precision.…”