Spectroscopy

Spectroscopy is one of the fastest growing research areas at MIKES. Because of the broad competence and extensive international contacts of MIKES researchers, this research area will grow in the near future, also. The research ranges from measurement methods, devices and applications for industry and environmental monitoring to fundamental physics research on cold atoms that serve the revision of International System of Units. Examples of our research are trace gas measurements, process measurements, emissions trading, water analysis and optical frequency metrology.

Methods, devices and applications for optical spectroscopy are in a rapidly developing phase. The dynamic nature of the field together with top level competence in optical metrology provide an excellent background for interdisciplinary research projects serving industry and research institutes.

Research projects in this area

Optical atomic clock

Time and frequency are the two quantities that can be measured the most accurately. Because of this the definition of the SI-second will have a central role in the forthcoming revision of the International System of Units. Traditional atomic clocks based on Cesium-133 atoms gain or lose approximately one second in 1000 million years. These clocks operating at a microwave frequency (9 GHz) are already close to the best possible stability and accuracy reachable by this technology. However, it has long been known that by increasing the operating frequency of a clock, its stability can be improved. Until recent years the main problem was how to measure these high frequencies, i.e. how to count the time from a clock that ticks a thousand billion times per second. At the turn of the millennium a practical solution for this problem was discovered: the optical frequency comb. This enabled the construction of optical clocks and their comparisons and although the development has just started, the best optical clocks are already an order of magnitude more stable than Cesium clocks.

MIKES is developing a single-ion strontium optical atomic clock. A single strontium ion 88Sr+ laser-cooled to a few millikelvins above absolute zero is trapped between electrodes shown in the figure on the right. A red clock laser excites the ion if and only if the frequency of the clock laser is correct with approximately an accuracy of 15 digits. The excitation can be detected and thus the laser frequency can be tuned to the resonance. One second has elapsed, when the laser frequency has oscillated 444 779 044 095 484.6 times.

Current status of the project:

The ion trap (in figure) and prototypes of the five lasers needed to trap the ion have been finished. The first ions were successfully trapped in spring 2013. The prestabili-zation of the clock laser is nearly finished, the frequency comb used for generating a countable signal is operational and the ultra-stable optical resonator used for stabilizing the clock laser has been assembled.

Recent publications:

[1] Lindvall et al., "Unpolarized, incoherent repumping light for prevention of dark states in a trapped and laser-cooled single ion", Phys. Rev. A, vol. 87, 013439, 2013
[2] Fordell et al., "Incoherent Repumper and Clearout Light Sources for Sr+ Ion Traps", EFTF 2013, Prague
[3] Lindvall et al., "Dark-State Suppression in a Trapped and Laser-Cooled Alkaline-Earth-Metal Single Ion", EFTF 2013, Prague
[4] Fordell et al., "Compact and Robust Repumper Light Source for 88Sr+ Single-Ion Traps", CLEO-Europe /IQEC 2013, Munich

High-precision spectroscopy in mid-infrared

In collaboration with the University of Helsinki, we have developed an accurate spectrometer for the mid-infrared wavelengths 3-4 µm [1]. The light source of the spectrometer is an optical parametric oscillator (OPO). The frequency of the infrared OPO beam is measured absolutely by using an optical frequency comb. The spectrometer is used to accurately measure absorption line frequencies of molecules that are important for climate studies, e.g. methane. The work is part of the EUMETRISPEC-project, which aim is to improve the quality of spectral databases.

[1] M. Vainio, M. Merimaa, and L. Halonen, “Frequency-comb-referenced molecular spectroscopy in the mid-infrared region,” Opt. Lett. 36, 4122-4124 (2011).

high-power supercontinuum light source for hyperspectral remote sensingLong-range hyperspectral remote sensing

Together with the Technical Research Centre of the Finnish Defence Forces and an industrial partner, MIKES has developed a high-power supercontinuum light source for hyperspectral remote sensing. The setup has been tested in field measurements up to 1.5 kilometers. The approach has good potential for hyperspectral identification and illumination of targets.

Real-Time Monitoring of Stable Methane Isotopologues in Ambient Air

Methane is one of the three most important greenhouse gases and is considered responsible for 20% of the observed enhanced greenhouse effect. In order to allow sensible emission mitigation strategies, accurate knowledge of methane's emission sources and sinks is necessary. A valuable tool for distinguishing amongst emission sources is isotopic measurement. The acquisition of real-time, high-precision isotopic methane isotope data, when coupled with back-trajectory analysis, can help identify methane emissions by geographic location, time and source type. The objective of this project is to develop a highly sensitive cavity ring-down spectrometer for use in the real-time measurement of three stable methane isotopologues in ambient air; 12CH4, 13CH4 and CH3D.

Mid-infrared spectroscopy for radiocarbon detection

A carbon isotope spectrometer based on mid-infrared spectroscopy of carbon dioxide isotopes is being developed in this project. The main purpose of the instrument is to detect 14C, which is present in modern carbon at a concentration of 1.2 ppt. By selecting a mid-infrared absorption line of 14CO2 with minimal interference from other isotopes of CO2, it is possible to accurately determine the 14C/12C ratio of the sample. The spectrometer is based on a quantum cascade laser, which allows for a very compact setup that can also be used for on-site measurements in real-time. However, due to the very low abundance of 14C, requirements on the instrument stability and sampling are particularly challenging. In order to achieve sensitivity close or below the ppt-level, cavity ring-down spectroscopy is used.

The primary application of the spectrometer is monitoring of elevated levels of 14C emitted from nuclear waste. When a sensitivity better than the natural abundance of 14C will be achieved, other applications are expected such as the determination of the biological origin of fuel, as fossil fuels have no 14C content, carbon capture and storage leaks detection or drug development using radiocarbon labelled drugs.

Methods for measuring airborne molecular contamination in industrial environments

AMCAirborne chemical contamination in the form of vapours and aerosols has adverse effects on products, processes and instruments, e.g. corrosion of metal surfaces on the wafer and formation of contamination layers on surfaces like optics and wafers. The problems arising from this kind of contamination are emphasized in productions requiring extreme purity, e.g. in the semiconductor industry and nanotechnology where the aim is to operate at an increasingly smaller scale.

At present the formation and behaviour of these contaminants in production environments is still largely unknown due to the lack of sensitive on-line on-site measurement methods. In the MetAMC project coordinated by MIKES, traceable laser methods capable for real-time monitoring of airborne molecular contamination in industrial environments, i.e. clean rooms environments at industrial sites, will be developed. Moreover, the project develops dynamic generation methods for trace level airborne molecular contaminants and a portable material generator for calibration of the methods developed as well as suitable sampling techniques for practical AMC monitoring.

MIKES is developing a photoacoustic spectroscopy analyser capable of detecting airborne molecular contamination at trace levels, suitable for field measurements. The device will be characterised and tested in real operating conditions, i.e. in industrial clean rooms, and its potential will be compared to other commercial analysers and other methods and devices developed in this project.​