Scientists in Europe have collaborated to develop an ultracompact spectrometer design that offers large bandwidth, moderate spectral resolution, and a 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 devices such as consumer electronics and ultrasmall satellites. Extreme miniaturization of IR spectrometers is critical for their integration into next-generation smartphones, wearables, and space devices. Though teams have demonstrated miniaturization efforts on various elements of spectrometers, such as dispersive elements, narrow band-pass filters, and Fourier-transform and reconstructive spectrometers, the scaling of spectrometers has traditionally required a trade-off between spectral bandwidth, resolution, and being limited to the visible spectral range. Fourier-transform IR spectrometers combine large spectral bandwidth and resolution in the IR range and have yet to be fully miniaturized. Waveguide-based Fourier-transform spectrometers offer a low device footprint, but rely on bulky, expensive external imaging sensors such as InGaAs cameras. Currently, the size of the overall waveguide spectrometer cannot be smaller than commercially available detectors. The experimental setup for the compact Fourier-transform waveguide spectrometer, which, according to its designers and developers, will allow optical measurement instruments to be integrated in consumer electronics and ultrasmall satellites. The team used a red alignment laser to visualize the beam path from the fiber into the optical waveguide and its reflection at a gold mirror. Two microprobes were used to contact the photoconductor, the size of which is in the subwavelength range. Courtesy of Empa. The research team — scientists from Swiss Federal Laboratories for Materials Science and Technology (Empa), ETH Zurich, École Polytechnique Fédérale de Lausanne (EPFL), the University of Salamanca, the European Space Agency (ESA), and the University of Basel — built a proof-of-concept, miniaturized Fourier transform waveguide spectrometer that incorporated a subwavelength photodetector as a light sensor. The photodetector was based on colloidal mercury telluride quantum dots (HgTe CQDs) and was CMOS-compatible. The room-temperature-operated photodetector exhibits a spectral response up to a wavelength of 2 μm. In addition, the wire-shaped, subwavelength-size photodetector was monolithically integrated with an optical waveguide. The monolithic integration of the photodetector downscaled 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 approach the THz region by varying the QD size. Subwavelength IR photodetectors traditionally rely on nonscalable device fabrication or require cryogenic cooling. The scaling of commercial IR detectors such as InGaAs and mercury cadmium tellurides down to subwavelength dimensions and their integration with optical waveguides are challenging. The device’s photodetector, fabricated on top of a surface optical waveguide, consists of a gold electrode at the bottom functioning as a scattering center, a photoactive layer consisting of colloidal mercury telluride (HgTe) quantum dots, and a top gold electrode. By moving the mirror, the measured photocurrent maps the light intensity of the standing wave, that is, the IR light. A Fourier transformation of the measured signal gives the optical spectra. Courtesy of Lars Lüder. HgTe QD-based photodetectors are typically fabricated either as photoconductors or photodiodes. To the best of the team’s knowledge, HgTe QD-based photodetectors have not been monolithically integrated into waveguide spectrometers until now. The miniaturization of IR spectrometers could lead to their wider use in consumer electronics — for example, they could be implemented in a smartphone for monitoring food quality. They could be used to quickly detect certain chemicals without the need for laboratory equipment. Miniaturized spectrometers could also help users detect counterfeit medical drugs or greenhouse gases such as methane and carbon dioxide. According to Empa researcher Ivan Shorubalko, the demonstrated scaling could also be of interest to the development of miniaturized Raman spectrometers, biosensors, lab-on-a-chip devices, and high-resolution hyperspectral cameras. In addition, femtosatellites — space devices with a maximum weight below 100 g — will require ultracompact spectrometers. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-022-01088-7).
