Editorial Feature

Miniaturizing Raman Imaging Devices with Hybrid Metalenses

Raman imaging is a powerful technique that can provide distribution maps of both chemical species as well as particular material properties. By having access to spectral and spatial information, it is possible to reconstruct even complex objects in terms of their local chemical compositions.

raman imaging, metalenses

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Raman imaging is commonly used for pharmaceutical analysis, where the spatial resolution can be used to check that the active pharmaceutical ingredient is homogeneously distributed in a tablet.1 This helps ensure controlled dose release takes place on the desired timescale. Raman imaging is preferred over other techniques, such as infrared, as it offers better chemical identification of pharmaceutical compounds as well as good sensitivity and detection limits.

Threat detection for explosives, food contamination, and tissue imaging are all other common applications of Raman imaging. The chemical sensitivity means there is a good selectivity and a low false-positive rate for the detection of compounds such as trinitrotoluene (TNT) and its degradation products for explosive detection.

As a clinical diagnosis tool, Raman imaging helps locate regions where particular chemical species have built up and map out tissue construction with rapid analysis times.1

Raman Signatures

What makes Raman imaging a powerful technique is a relationship between the energy and intensities of the Raman signal and the underlying molecular or material structure.

Raman signals are sensitive to the frequencies at which the chemical bonds in a molecule or material vibrate. These frequencies are dependent on two factors: the masses of the atoms in the bond and the strength of the bonds between the oscillating species. The exact bond strengths are also influenced by the local chemical environment around the bond being probed.

These factors work together to give a unique fingerprint of the molecule that is used for identification. A non-linear molecule will have a total of 3N-6 vibrational modes, where N is the number of atoms. While not all these vibrational modes will give rise to spectroscopically bright Raman transitions, for complex molecular systems, this means there are enough unique lines to allow for confident identification of a given molecular species.

Imaging Constraints

There are many different types of Raman schemes that use one to multiple laser pulses to enhance the spectral discrimination between different types of Raman signals.

The most straightforward scheme is to excite the species of interest with a sufficiently intense laser pulse and record the Raman scattering signals. This is known as Spontaneous Raman. A common variation of the technique used in microscopy for better contrast is Coherent Raman Scattering, which uses two pulses, a pump pulse and the ‘Stokes’ pulse, typically of lower energy than the initial pump.

While coherent Raman scattering is very power-demanding as it is a non-linear optical process, it overcomes many of the spectral congestions issues of incoherent Raman techniques, and now work is underway to make more broadband versions of the technique.2 This would allow for visualization of multiple vibrational modes without the need for tuning the energies of the lasers and rapidly reduce acquisition times – making it a powerful tool for microscopy applications.

For imaging techniques, the achievable spatial resolution is determined by the spot size of the laser pulses on the sample. For most methods, the beam is focused to a point and then rastered over the object of interest for the full two- or three-dimensional object reconstruction. Full spectral information is recorded at each point of this scan.

Many Raman approaches make use of near-infrared pulses and so a suitable lens for focusing on coherent Raman imaging applications needs to be capable of focusing both the pump and any additional beams to a tight spot on the sample.

However, for microscopy applications, there are often size limits on how physically large these lenses can be and for handheld applications, where miniaturization is essential, there are additional challenges associated with the fabrication and thermal load on the very small optics to achieve similar focal spot sizes.

New Metalenses

A team at Waterloo has developed new hybrid metalenses for two coherent Raman microscopy techniques, coherent anti-Stokes Raman scattering (CARS) and stimulated Raman spectroscopy (SRS).3 The team has achieved focal spot sizes for near-infrared light that are nearly at the diffraction limit for this wavelength.

These hybrid metalenses are just 2 mm in diameter and consist of a plano-convex refractive lens which is then attached to a 1.5 mm diameter metasurface made from 1 μm-high silicon nanopillars.

The unusual construction offers a lens with excellent focusing capabilities but is small enough that it could be incorporated into endoscopes and handheld devices for performing Raman imaging on patients for diagnosis.

Such hybrid lens designs allow greater flexibility in optical properties, which is essential where multiple wavelengths must be focused by a single optic. They can also achieve nearly aberration-free focusing, which is essential for retrieving high-quality imaging data.

The current challenge is to continue to improve such lenses but also to find ways to incorporate them into fibers.

References and Further Reading

  1. Stewart, S., Priore, R. J., Nelson, M. P., & Treado, P. J. (2012). Raman imaging. Annual Review of Analytical Chemistry, 5, 337–360. https://doi.org/10.1146/annurev-anchem-062011-143152
  2. Polli, D., Kumar, V., Valensise, C. M., Marangoni, M., & Cerullo, G. (2018). Broadband Coherent Raman Scattering Microscopy. Laser and Photonics Reviews, 12(9). https://doi.org/10.1002/lpor.201800020
  3. Lin, P., Chen, W. T., Yousef, K. M. A., Marchioni, J., Zhu, A., Capasso, F., & Cheng, J. X. (2021). Coherent Raman scattering imaging with a near-infrared achromatic metalens. APL Photonics, 6(9). https://doi.org/10.1063/5.0059874

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Rebecca Ingle, Ph.D

Written by

Rebecca Ingle, Ph.D

Dr. Rebecca Ingle is a researcher in the field of ultrafast spectroscopy, where she specializes in using X-ray and optical spectroscopies to track precisely what happens during light-triggered chemical reactions.

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