Spectrometers shrink to nanoscale for in-situ cell analysis

September 16, 2019 //By Julien Happich
A team of researchers from the University of Cambridge has engineered an ultracompact microspectrometer design based on a single semiconductor nanowire about 100μm-long with a graded composition along its length.

Artist's impression of single-nanowire spectrometer.
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.

Operational schematic of the nanowire spectrometer.
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.

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