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For 500 years optical resolution has been limited by physical constraints. An EMPIR project has now pushed this towards the ultimate quantum limit

High technology sectors are increasingly working at the nanoscale. As a measurement tool, optical systems have a large role to play as they offer the advantages of speed, non-invasiveness and reliability.
Traditional optical-based measurement systems do not often have the spatial resolution or sensitivity required, limiting innovation in this area.

Despite how complex or sophisticated classical optical measurements are the current maximum resolution is related to the wavelength (lambda, λ) of the light used. For air-based measurements this equates to around half the wavelength of the light (~ ^1/2 λ). One method to increase resolution and see smaller details is to use very short wavelength light – such as UV. However, short wavelength light only has a small depth of penetration (~ 100 nm), and as UV interacts with oxygen, requires vacuum conditions. Moreover, another fundamental limitation of conventional measurements is the random noise directly coming from the light sources, that can affect the final recognition of samples features such as the shape, the transmission to light, or the thickness.

The now complete project Light-matter interplay for optical metrology beyond the classical spatial resolution limits (17FUN01, BeCOMe) has used quantum-based techniques to improve the spatial resolution beyond the ½ λ limit and the sensitivity to sample features.

Project impacts

Resolution towards the quantum limit

A major barrier to imaging is what is known as the ‘shot noise’, which is a type of random noise fundamentally related to the quantum fluctuations of the light itself. If the number of photons hitting a detector is too low, then signals cannot be discriminated from random or shot noise - so data such as images can’t be resolved.
The project has pushed these measurements towards the ultimate quantum limit using twin photon beams. One beam is used to probe the object while the other is used to measure the random, yet identical noise. Subtracting the noise path from the signal path allows sub-shot noise resolution – the project has done this and pushed these measurements towards the limit using twin photon beams - with the best sensitivity per photon ever achieved so far.  Dr Bechera, one of the researchers involved commented: “This could have applications in the enhanced imagining of photo-sensitive or photo-reactive biological or material samples”. This work has been described in the paper Unbiased estimation of an optical loss at the ultimate quantum limit with twin-beams. It is  one of an impressive list of over 47 papers  in peer reviewed papers by the project, including one in Nature Communications regarding photon-entanglement and time travel and one on a new class of single-photon emitters in optically active diamond defects for use in such things as quantum enhanced imaging.

Scattering beyond the weak regime

How an object scatters electromagnetic radiation – such as light – can be used to retrieve the shape or physical properties of an unknown object. For small particles that scatter weakly the mathematical Born series can be used to retrieve shape or physical properties. As the particles get larger or scatter strongly then this approach begins to breakdown. This form of ‘inverse problem’ - working out the effect (image) from the cause (particles scattering from the object) – is also often too complicated or computationally expensive to perform.
The BeCOMe project solved this by employing Padé approximants,  as described in the publication in Physical Review Research.
This approach can represent an important building block to the application of the Born series.

Resolution beyond the half-lambda limit (super-resolution)

The technique of Tip-enhanced photoluminescence (TEPL) was improved to allow use with high resolution imaging, demonstrating that the spatial resolution achieved by TEPL can be more than 20 times smaller than the excitation wavelength ((~ ^1/25 λ).

During the project INRiM, the National Metrology Institute of Italy, developed the possibility of merging classical and quantum methods that can provide a further improvement in microscopy with potential important application in bio-imaging. INRiM aims to offer a proof of principle in the near future.

Finally, to support user uptake the open-source software linked to the Finite Difference Time Domain (FDTD) calculations developed in the project has been made publicly available.

The software code is a rigorous and powerful tool for modelling nano-scale optical devices and was adapted for the modelling of scattering on sub-wavelength gratings and its performance was compared to results obtained by Finite Element Methods (FEM) modelling. Notes on the comparison are available here.

The work performed in the BeCOMe project will aid European photonics industries to characterise materials, such as nano-products, in an improved way and help exploit new emerging market opportunities, particularly by using the novel opportunities provided by functional nano-optical materials.

The coordinator of the highly successful project Lauryna Siaudinyte (VSL) said about the project:

”Project "Become" targeted optical measurement instrumentation and methods which are relevant for multiple R&D sectors ranging from software development to biotechnology and computer electronics. Besides exploring the light-matter interaction, the project also addressed quantum properties of light and provided quantum-based metrology schemes. Enhancing the resolution and providing optimized supper-resolution options, had a significant contribution to the latest developments in microscopy.  The consortium is proud to perform and continue in the current project "POLight" such an innovative research as well as provide novel solutions to metrology problems emerging for the Semiconductor, Optics and other hi-tech industries.”

Current EMPIR project 20FUN02 POLight builds upon this work.

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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