Spectrometers shrink to nanoscale for in-situ cell analysis
Credit: Ella Maru Studio
Because the platform is independent of complex optical components or cavities, it can be used pretty-much as-is at the micrometer-scale, as an in-situ materials characterization technique.
As an example, the paper “Single-nanowire spectrometers” published in the Nanophotonics journal reports about the high-resolution lensless spectral imaging of biological cells, using a shift register scanning strategy whereby a single-nanowire spectrometer is moved in micrometer-steps across a x-Y plane above a sample, providing in-situ mapping.
Source: Science’s Nanophotonics journal.
The active element consists of a compositionally graded semiconducting CdSxSe1-x nanowire in which one end is composed mainly of elements Cd and S and the other mainly of Cd and Se, offering a continuous gradient of bandgaps spanning from 1.74 to 2.42 eV along their length. Transferred to a Si/SiO2 substrate, it is then contacted through an array of parallel In/Au electrodes so as to measure the wavelength-dependent photocurrent at different spots along the nanowire. Cross-referencing the measurements with a pre-calibrated response function for each of these points allows for the computational reconstruction of the incident-light wavelengths.
The entire active element is only a few hundreds of nanometres wide and tens of micrometers in length and operates without the need for any complex or dispersive optics which typically increase a system’s footprint. The nanowire is grown in a single bottom-up process and is by essence unidimensional. It doesn’t suffer from lattice mismatch even with large compositional gradients across its length, explains the paper, that is because the nanowire growth interface is independent of the substrate once nucleated.
Hence, although the devices reported in the paper were designed to operate over the full visible spectrum, they could be engineered with an almost arbitrary number of material systems, simply through adjusting source vapours during nanowire growth, yielding systems that could operate across any wavelength range from the infrared to ultraviolet, the authors highlight.
The two different nanowire spectrometers presented in the paper contained 30 and 38 photodetector units and the authors were able to resolve monochromatic light at 8.5nm and 7nm minimum reconstructed full width at half maximum (FWHM), respectively.
Despite a reduction in footprint of about two to three orders of magnitude compared to centimetre-scale spectrometers, the proof-of-concept device offered a resolution comparable to that of other visible-range spectral reconstruction micro-spectrometers.
incorporated into a packaged chip. Scale bar: 10 mm.
Source: Science’s Nanophotonics journal.
It was demonstrated for spectral imaging, first through spatial point-scanning (scanning an image focused by a lens onto the device, across the whole focal plane). The scanning resolution was defined by the 0.3 mm mapping steps used (which could have been as small as the nanowire itself). Then the nanowire spectrometer was demonstrated in lensless spectral imaging, standing only a few micrometers over the sample and scanning at steps smaller than the nanowire itself (but always matching a fixed integer multiple of the electrode array’s pitch).
Such devices could find their way in any miniaturized spectroscopic application, including lab-on-a-chip systems, drones, implants, and wearable devices, the authors conclude.
They have applied for a patent in the UK and hope to commercialise the platform in the near future.
University of Cambridge – www.cam.ac.uk
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