September 1, 2020.
Infrared radiation and applications
Nowadays, infrared radiation is widely used in many fields such as chemistry, biology, materials science, agriculture, food industry and medical applications. The infrared radiation can be categorized into 3 frequency regimes: near infrared (NIR), mid infrared (MIR) and far infrared (FIR). The most popular regimes for applications are the MIR radiation with wavelengths of 2 – 30 micron and the FIR radiation including terahertz (THz) radiation with wavelengths of 0.03 - 3 mm.
The MIR radiation has frequencies corresponding to finger prints of covalent bonds in many molecules. It is popularly used to study chemical compounds and structure of biomolecule samples utilizing infrared spectroscopy technique. For example, it can be used to detect the change of proteins, fatty acids, or nucleic acids, which are important components in DNA. It can also be applied in characterizing cells and tissues, which is very useful for disease diagnostics such as fatty liver disease and disorder of brain cells or stem cell. For the FIR/THz radiation, this frequency regime is presently utilized in researches and applications related to spectroscopic technique for both Fourier transform infrared spectroscopy (FTIR) and time-domain spectroscopy (TDS). The THz radiation is also widely used in noninvasive inspection THz imaging to distinguish compounds of sample that have different densities. Therefore, it is very useful in e.g. security, agriculture, electronics, medicine and pharmacy applications.
Essential properties of MIR and FIR/THz radiation, which can be efficiency utilized in advanced researches and applications, are coherent, high brightness and ultra-short pulse in order of femtosecond or picosecond. With these properties, the radiation can be used in applications for complicate systems such as molecular structure of biomolecules, phonon vibration in solids and semiconductors, electron excitation in quantum materials and hydrogen-bonding in gas and liquids molecules as shown in Figure 1. To generate the radiation with these specific properties, special design and development of tools, technology and procedures are required.
Fig. 1 Diagram of interaction responses in THz spectrum  and MIR spectrum .
The infrared radiation with aforementioned properties can be produced from electron linear accelerator-based free-electron lasers (FEL). The produced radiation is coherent, high-brightness and short pulse. In addition, its wavelength can be tuned by varying the electron beam properties and the radiation production procedure. This is an advantage of the FEL over the conventional lasers, which the radiation wavelength depends on the binding energy of bound electrons in atoms or molecules. The number of such FEL laboratories is limited worldwide, especially for the MIR and THz regimes. One of outstanding laboratory is the Free Electron Lasers for Infrared eXperiments (FELIX) Facility at Radboud University, the Netherlands. At this laboratory, the users around the world can come to use the IR FEL in various applications, especially molecular dynamic study of intermolecular interaction in samples using FTIR, TDS and pump-probe experiment. Moreover, combination of IR FEL with external FTIR and mass spectrometer can be used in infrared multiple photon dissociation to deeply characterize structure, mass and absorption property of molecules in the same time. This leads to high efficiency and precise characterization technique, especially for samples with low density contents.
There are several techniques to generate FEL from linear accelerator depending on radiation frequency and its properties. For the infrared regime, the FEL can be produced by transporting relativistic electrons into magnetic field of undulator, which has series of dipole magnets with different field direction between the adjacent poles. When electrons moving in such field, they travel with periodic trajectory and emit electromagnetic radiation as illustrated in Figure 2.
Fig. 2 Diagram of radiation emission from relativistic electrons travelling in undulator magnetic field .
Electrons in the beam that has pulse length equal or shorter than the radiation wavelength will emit radiation at the same phase leading to constructive interference. This results in high intense coherent radiation with power proportional to number of electrons squared. This light-source is called “super-radiant free-electron laser”. In case that the electron beam has the pulse length longer than the radiation wavelength, we can generate the coherent radiation by transporting the electron beam through the undulator magnet equipped with optical cavity. Emitted radiation from electrons is reflected back and forth in the optical cavity leading to modulation of electrons’ energies and microbunching phenomena. This results in electron micro-bunches with a length shorter than radiation wavelength. The coherent radiation with high brightness is therefore generated. A fraction of FEL is transmitted through an open hole of the optical mirror installed at one side of undulator magnet and transported to the experimental station through the optimized radiation beamline. This type of light-source is called “oscillator free-electron laser” as illustrated in Figure 3. There are also other types of FELs, which are not focused in this article.
Fig. 3 Diagrams of system  and principle  of oscillator free-electron laser.
Development of infrared free-electron laser in Thailand
Under this project with the support from Thailand Center of Excellence in Physics, the electron linear accelerator system at the PBP-CMU Electron Linac Laboratory (PCELL) of the Plasma and Beam Physics Research Facility at Chiang Mai University is upgraded to generate free-electron lasers in MIR and THz regimes. Research activities are carried out also through the collaboration with the PITZ group of the Deutsches Elektronen-Synchrotron (DESY) in Germany, the Institute of Advanced of Kyoto University in japan, the Research Center for ELectron Photon Science (ELPH) of Tohoku University in japan, and the National Astronomical Research Institute of Thailand (Public Organization). The project was started by moving, modifying and installing the accelerator system in the radiation-shield hall as shown in Figure 4. Addition subsystems including magnetic bunch compressors, undulator magnets, optical cavity, electron and radiation beamlines are developed and purchased. Moreover, experimental stations for utilizing both MIR and THz radiation are designed. At our laboratory, three types of coherent radiation are planned: coherent THz transition radiation (THz-TR), mid infrared free-electron laser (MIR-FEL) and super-radiant THz free-electron laser (THz-FEL).
Fig. 4 The system to produce and accelerate electron beam installed in the radiation-shield hall.
The THz-TR is generated by transporting electron beam with energy in a range of 10 – 25 MeV and a pulse length of femtosecond scale from vacuum to thin aluminum foil at the experimental station. Since the pulse length of electron bunches is very short, the emitted coherent radiation is in the THz regime with the frequency range of 0.3 – 3 THz. This radiation can be used in THz-FTIR spectroscopy as shown in Figure 5. Currently, the TR experimental station is already installed in the electron beamline. The THz-FTIR spectroscopy is under the purchase process.
Fig. 5 Diagram of the IR FEL system and experimental stations for THz-FTIR spectrometer and pump-probe experiment. The part in blue dotted line frame is already installed in the radiation-shielding hall as shown in Figure 4.
Currently, several accelerator components as well beamline and vacuum parts have been designed, constructed and purchased. There are two sets of components for MIR-FEL and THz-FEL, respectively. The important components consist of 6 dipole magnets, 18 quadrupole magnets, 20 steering magnets, 9 screen stations and 8 current transformers. All these components are designed by researchers and students in the project. The construction is mainly carried out in Thailand to reduce the cost and to develop this kind of technology in the country. Examples of these components are shown in Figures 6 and 7.
Fig. 6 Computer model and picture of dipole magnets, which are developed under this project. The construction of the magnetic yoke and poles are under the supported from the National Astronomical Research Institute of Thailand (Public Organization).
Fig.7 Computer model and picture of quadrupole magnets, which are developed under this project.
The MIR-FEL will be generated by using the oscillator free-electron laser technique. The permanent undulator magnet with properties listed in Table 1 was transferred from the Institute of Advanced Energy of Kyoto University. The design of the optical cavity system was done. The engineering drawings are prepared. Then, the construction of this system will be proceeded. From computer simulation results, properties of the expected MIR-FEL are also shown in Table 1.
Table 1 Expected properties of electron beam, undulator magnet and MIR-FEL.
The THz-FEL will be generated by using the super-radiant free-electron laser technique. The electromagnetic undulator with properties listed in Table 2 is under the design to optimize three-dimensional magnetic field distribution. Then, the construction will be proceeded. The design of special beamline to produce high brightness electron beam with femtosecond pulse length is ongoing. There are many parameters to be optimized in order to get the electron beam with required properties. From computer simulation results, properties of the expected THz-FEL are listed in Table 2.
Table 2 Expected properties of electron beam, undulator magnet and THz-FEL.
Presently, we can generate and accelerate electron beam through the linear accelerator. Beam diagnostics and characterizations are carried on. The installation of all components in the MIR-FEL beamline will be started soon. The first radiation from the MIR undulator is expected before the end of this year. In parallel to the construction and installation of the accelerator components, the design and development of experimental stations are also performed. The first experimental stations will be the THz FITR and THz time-domain spectroscopy (TDS). Then, the pump-probe experiment using MIR-FEL and THz-FEL will be proceeded.
Pilot experiments using the THz-TR, THz-FEL and MIR-FEL will be established at PCELL. These experiments are study of ionic liquids for electrochemical energy storage device applications, study on THz radiation effects in DNA, and study of structure and molecular interaction of organic molecules in space. These experiments will be conducted under the collaboration with the Terahertz Technology Research Team at the National Electronics and Computer Technology Center (NECTEC) and the research teams from Suranaree University of Technology and the National Astronomical Research Institute of Thailand (NARIT). In the future, more experimental stations will be further developed for advance research using IR FEL. The experiments will also be opened to all users from Thailand and South East Asia.
Asst. Prof. Dr. Sakhorn Rimjaem
Electron Linear Accelerator Laboratory, Plasma and Beam Physics Research Facility, Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai - 50200, Thailand