FEATURE SPECTROSCOPY
‘The 3D-printed miniature spectrometer marks an innovation for 3D-printed micro-optics in complexity’
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at 2 to 9µm, based on a single tuneable photodetector. The material is around 10nm thick, and allows users to tune the light-matter interaction to capture the different spectral components. Meanwhile, an algorithm is used to help shift the complexity involved in spectroscopy from hardware to software. At 9 by 16µm2 the Xia lab’s
spectrometer’s dimensions are smaller than the cross-section of a human hair, and comparable to the wavelength of the light that it measures. With conventional spectrometers, light is split up by the colours that compose the spectrum, whereas in this device, the light need not be split into different parts spatially. Where it also differs from more
conventional spectrometers is a lack of reliance on advanced optical components, such as interferometers or tuneable infrared lasers, which could allow extreme miniaturisation of spectrometers and on- chip, affordable mid-infrared spectroscopy and spectral imaging. The researchers noted that automobiles,
drones and satellites are often fitted with infrared cameras that take greyscale thermal images to detect pedestrians, vehicles and other hazards, while this spectrometer has a higher detection ability for such potential threats, since the spectral information can be continuously measured, albeit with moderate resolution. Researcher Doron Naveh, of Bar- Ilan University said: ‘It is very exciting to realise such a high-performance spectrometer with the ultimate compactness. We expect that the principle of leveraging advances in hardware and software simultaneously, as shown in this work, will lead to commercial applications in medicine, agriculture and food quality control.’
3D printing At the University of Stuttgart in Germany, 3D printing was used to achieve spectrometer miniaturisation2. The angle-insensitive 3D-printed miniature spectrometer has a direct separated spatial-spectral response and was fabricated via two-photon direct laser
20 Electro Optics July 2021
writing, combined with a superfine inkjet process. The device has a volume of less than
100 by 100 by 300µm3. Its tailored and chirped high-frequency grating enables strongly dispersive behaviour, and it has a wavelength range of 200nm in the visible range from 490 to 690nm. It has a spectral resolution of 9.2 ± 1.1nm at 532nm and 17.8 ± 1.7nm at a wavelength of 633nm. Researcher Andrea Toulouse explained: ‘Printing this spectrometer directly onto camera sensors is feasible, and can be replicated for use as a macro-pixel of a snapshot hyperspectral camera.’ The 3D-printed spectrometer collects
light with a numerical aperture (NA) that is larger than 0.4, a light collection area of 50 by 50µm2 at the first lens and redistributes the wavelengths with minimal losses. ‘Overall,’ said Toulouse, ‘the 3D-printed miniature spectrometer marks an innovation for 3D-printed micro-optics in terms of complexity, while exploiting, for the first time, the micrometre-size range for direct spectrometers.’ The size of the device is comparable to a standard smartphone lens, which the researchers believe highlights the potential of such technology. It means that the spectrometer can effectively be integrated non-invasively into a smartphone camera, i.e. printed directly onto the camera sensor, such as for single spectral measurements
or as individual pixels of a snapshot hyperspectral camera. In this study, the feasibility of the 3D printed device was theoretically assessed and experimentally proven. Explained Toulouse: ‘To the best of the authors’ knowledge, the presented spectrometer is the first with a direct spatial-spectral response that has been demonstrated in this size range. Compared to computational and filter-based approaches, its angular insensitivity and relatively large aperture of 50 by 50µm2 are emphasised. ‘A hybrid direct computational approach
could significantly improve the average noise level by 5 per cent to 20 per cent. Wavelength range extensions of 200nm per 100 by 100µm2 footprint were proposed in a multi-aperture approach by implementing the same design principle for adjacent parts of the spectrum,’ Toulouse said. Possible applications range from endoscopy devices and consumer electronics to chemical microreactors. EO
References 1
2
Form S-4 Rockley Photonics Holdings; https://sec. report/Document/0001193125-21-105235/
A. Toulouse et al., Light: Advanced Manufacturing (2020)1:5 Official journal of the JHL 2689-9620;
https://doi.org/10.37188/lam.2020.005
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