Scientists have developed frequency comb light sources with an extended spectrum in the mid infrared (MIR) wavelength band, which could allow for new applications such as real-time, high resolution spectroscopy. According to the researchers, the new light sources are an important step in developing sensitive, cheap gas sensors for environmental and medical testing.
The research, which has been described in Nature Communications, was carried out by Belgium scientists from Ghent University and imec, in collaboration with the Max Planck Institute for Quantum Optics in Germany and Auckland University in New Zealand.
Frequency combs are laser-generated tools made up of a large number of very precisely defined frequencies that are evenly spaced. They can be compared to teeth on a pocket comb, with each ‘tooth’ representing an individual colour or frequency.
Their development has enabled the construction of a link between the optical and radio parts of the electromagnetic spectrum, allowing researchers to determine optical frequencies with unprecedented precision.
In recent years, frequency combs have been used in various environmental applications to measure gases. For example, last year, the United States’ National Institute of Standards and Technology (NIST) demonstrated laser frequency combs that could analyse the concentrations of greenhouse over a distance of two kilometres.
But although it has been shown that frequency combs can be used to carry out fast, high-resolution spectroscopy over a broad spectrum, traditional comb sources are not at the right wavelength spectrum for use in spectroscopy.
However, the international team of scientists managed to develop mid-infrared frequency combs by combining the strong light-matter interaction in silicon with its broad transparency window.
The researchers first fabricated nanowire silicon photonics waveguides to confine the light in a very small area waveguide, which further enhanced the strong light-matter interaction so that the spectrum of the frequency combs could be widened mid-infrared.
The achievements were possible through the use of a unique pump laser source, previously developed by the Institute of Photonic Sciences in Spain.
The development of frequency combs in the mid-infrared wavelength region is an important step towards a small-footprint chip scale mid-infrared frequency comb source. Such sources could act as sensitive cheap gas sensors for use in environmental monitoring for measuring air-pollution, for example, or in medical diagnostics as a cheap device to carry out breath analysis.