Miniaturized spectrometer proves suitable for compact electronics. Research & Technology | November 2022

scheduled tribe. Galen, Switzerland, November 9, 2022 – Scientists in Europe have collaborated to develop an ultracompact spectrometer Design that provides large bandwidth, moderate spectral resolution and spectral sensitivity in the infrared (IR) region. According to the team, its design for a Fourier-transform waveguide spectrometer will allow optical measurement instruments to be integrated into compact instruments such as consumer ones. Electronics and Ultra Small Satellite.

The extreme miniaturization of IR spectrometers is critical for their integration into next-generation smartphones, wearables and space devices. Although teams have performed miniaturization efforts on various elements of spectrometers, such as dispersive elements, narrow band-pass filters, and Fourier-transform and reconstruction spectrometers, scaling of spectrometers has traditionally been described as a tradeoff between spectral bandwidth, resolution, and – Shutdown is required. Limited to the visible spectral range.

Fourier-transform IR spectrometers combine large spectral bandwidth and resolution in the IR range and have not yet been fully miniaturized. Waveguide-based Fourier-transform spectrometers offer a reduced device footprint, but rely on bulky, expensive external imaging sensors such as InGaAs cameras. At present, the size of the composite waveguide spectrometer cannot be smaller than that of commercially available detectors.

Experimental setup for a compact Fourier-transform waveguide spectrometer that, according to its designers and developers, will allow optical measurement instruments to be integrated into consumer electronics and ultrasmall satellites. The team used a red alignment laser to visualize the beam path from the fiber to the optical waveguide and its reflection in the gold mirror. Two microprobes were used to contact the photoconductor, whose size is in the subwavelength range. Courtesy of Empa

The research team – scientists from the Swiss Federal Laboratories for Materials Science and Technology (Empa), ETH Zurich, cole Polytechnique Fédérale de Lausanne (EPFL), the University of Salamanca, the European Space Agency (ESA), and the University of Basel – produced a proof-of-concept , small Fourier transform waveguide spectrometer incorporating a subwavelength photodetector as a light sensor. The photodetector was based on colloidal mercury telluride quantum dots (HgTe CQDs) and was CMOS-compatible. The photodetector operated at room temperature exhibits a spectral response up to a wavelength of 2 µm.

In addition, the wire-shaped, subwavelength-shaped photodetector was monolithically integrated with the optical waveguide. The monolithic integration of the photodetector reduced the thickness of the imaging sensor by a factor of 1000. The result was a large-bandwidth, ultracompact (below 100 × 100 × 100 µm) IR micro-spectrometer with a spectral resolution of 50 cm.^-1,

IR photodetectors based on solution-processable QDs offer several advantages: they can be fabricated on various substrates, and their spectral response can be tuned by the size and composition of the QDs. For example, the absorption spectrum of HgTe QDs can be extended to cover the visible and IR regions and reach the THz region by changing the QD shape. Subwavelength IR photodetectors traditionally rely on nonscalable device fabrication or require cryogenic cooling. Reducing commercial IR detectors such as InGaAs and mercury cadmium tellurides to subwavelength dimensions and their integration with optical waveguides is challenging.

The device’s photodetector fabricated on top of a surface optical waveguide consists of a gold electrode on the bottom serving as a scattering center, a photoactive layer containing colloidal mercury telluride (HGTe) quantum dots, and a top gold electrode. When the mirror is moved, the measured photocurrent maps the standing wave’s light intensity, that is, IR light. The Fourier transform of the measured signal gives the optical spectra. Courtesy of Lars Luder.

HgTe QD-based photodetectors are typically fabricated as either photoconductors or photodiodes. To the best of the team’s knowledge, HgTe QD-based photodetectors have not been monolithically integrated into waveguide spectrometers so far.

The miniaturization of IR spectrometers could lead to their widespread use in consumer electronics – for example, they could be implemented in smartphones to monitor food quality. They can be used to quickly detect certain chemicals without the need for laboratory equipment. Miniaturized spectrometers can also help users detect counterfeit medical drugs or greenhouse gases such as methane and carbon dioxide.

The demonstrated scaling could also be of interest for the development of miniature Raman spectrometers, biosensors, lab-on-a-chip devices and high-resolution hyperspectral cameras, according to Empa researcher Evan Shorbulko. In addition, femtosatellites – space instruments whose maximum weight is less than 100 grams – would require ultracompact spectrometers.

research was published in Nature Photonics ,www.doi.org/10.1038/s41566-022-01088-7,