Ultra-broadband high-resolution microdroplet spectrometers for the near infrared


Near-infrared spectroscopy (NIRS) is a powerful, fast, accurate and non-destructive analytical tools that can be considered as a replacement of traditional chemical analysis. It has been increasingly used in various applications, including medical, biophotonics imaging and food product analysis. With the continuous advancement in this field, developing feasible miniaturized setups to facilitate in situ analysis and the creation of application diffusion at consumer levels have recently attracted significant research attention. Unlike visible light which uses a low-cost silicon detector, the near infrared (NIR) requires Fourier transform (FT) analyzers. Unfortunately, mechanically scanning interferometer that forms a crucial component of traditional FT spectrometers is often very difficult to miniaturize. Thus, more efficient and easier to miniaturize FT spectrometers are highly desirable.

Due to the recent progress in photonic integration technology, the emergence of small-scale FT spectrometers with no moving parts has been deemed promising for solving the miniaturization challenges. These devices rely on either thermo/electro-optic tuning of the integrated interferometers, integrating several micro-interferometers with fixed path differences or optically sampling the interference of stationary waves. Previous findings revealed that a droplet evaporating at the tip of an optical fiber could be used as a micro-optofluidic FT spectrometer with less costly nanofabrication. During evaporation, the displacement of the liquid surface serves as a scanning interferometer. This revelation has formed the basis of current research in high-performance microdroplet spectrometers. Nevertheless, the relatively long scanning duration and small spectral span of operation required to prevent instabilities during evaporation limit the practicability of this technique.

To overcome these limitations, scientists from the optical sensors lab of Italian National research council designed a microfluidic FT spectrometer for the NIR. The authors designed an evaporation setup to realize an ultra-broadband and high-resolution spectrometer by overcoming the drawback of the previous evaporation setups. This device was demonstrated on the tip of an optical fiber operated at a resolution of 3.5 cm­-1 in the 1000-2500 nm window. This was achieved by stabilizing the evaporation dynamics of the fluorocarbon droplet. The resulting device was compared with other FT-NIR small-scale spectrometers. Their research work is currently published in the journal, Optics Letters.

The research team demonstrated the superior performance of the fluorocarbon droplet spectrometers compared with the FT near-infrared small-scale spectrometers available in the literature. It exhibited the largest span/resolution ratio and wavelength span ever to be reported. Moreover, the largest operation window ever to be reported for a small-scale and stationary FT spectrometer was comparable to that of an integrated device comprising of a microelectromechanical spectrometer.

By keeping the perfluorohexane droplet in the pinned evaporation mode, a 5s scan producing a high contrast and repeatable interferograms was realized. These remarkable properties made it easier and more convenient to perform the spectral analysis of the narrowband or broadband radiation. Interestingly, these devices could operate in harsh environments and even autonomously without electrical sources of electrical power.

In summary, the authors successfully demonstrated the low-cost design and operation of an optofluidic microspectrometer with superior properties. The findings provide the possibility of covering the whole near-infrared region with a rapid, less expensive and miniaturized spectrometer, enabling various applications at the consumer level. In a statement to Advances in Engineering, the authors stated that their findings provide a new direction for the design and application of droplet spectrometers as small size and low-cost optofluidic NIR analyzers that could potentially revolutionize different fields of application such as optical fiber sensors.

Ultra-broadband high-resolution microdroplet spectrometers for the near infrared - Advances in Engineering


Capezzuto, M., D’Ambrosio, D., Avino, S., Giorgini, A., Gagliardi, G., & Malara, P. (2021). Ultra-broadband high-resolution microdroplet spectrometers for the near infraredOptics Letters, 47(1), 102-105.

Go To Optics Letters

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