Characterization of Terahertz Radiation Generated in an Organic Crystal
Författare
Summary, in English
The terahertz region of the electromagnetic spectrum is an important research area allowing insights into fundamental science such as inter-molecular interactions, and with applications in the environmental, medical and security field. Using femtosecond lasers, it is possible to design compact and efficient terahertz generation schemes, resulting in few-cycle broadband pulses. This master thesis is concerned with generating terahertz radiation through optical rectification in the organic crystal DSTMS. This crystal has a high second order nonlinear coefficient and is phase matched for a 1500 nm pump laser. Terahertz radiation was successfully generated, with a maximum energy of 3.75 $\mu$J, corresponding to a generation efficiency of 0.71$\%$ at a pump laser energy of 530 $\mu$J. The theoretical background of this process, as well as the practical work is presented in this thesis.
Summary, in English
Terahertz light can interact with matter and cause molecules to vibrate and undergo structural changes. This can be used to investigate phenomena that are tiny in space, and occur on an extremely small time scale. Terahertz light must however first be generated, and this can be done by using intense laser light propagating through a crystal.
Light can be used as a tool to measure and investigate our surroundings. Different kinds of light can be used to explore different phenomena, and a vast quantity of different techniques have been developed. Light can be used to measure properties that are impossible to access in other ways. For example, extremely short distances can be measured using light, because the wavelengths of light itself can be very short, and work as "mini yardsticks". Light can also interact with different materials in a way that enables us to analyze them. Measuring with light involves finding the right wavelength for what you want to measure, and figuring out the right technique to do it.
The electromagnetic spectrum includes all different wavelengths of light, and stretches from the very long radiowaves, with wavelengths that are hundreds of kilometers long, to the very short gamma rays, with wavelengths at merely picometers (10^(-12) m). In addition to wavelengths, one can also describe light by its frequency. There is a specific part of the electromagnetic spectrum called the teraherts band, which is comprised of light with frequencies around 10^(12) Hz, or 1 THz. This light typically has wavelengths of a couple of tenths of mm. Terahertz frequencies are useful in a number of different applications. A key feature of terahertz light is that it is highly absorbed by water. This means that terahertz frequencies are well suited to use in both medical and environmental research, where water content is an important aspect. Terahertz light can interact with materials in a number of different ways. It can cause molecules to start vibrating, and induce electronic and structural changes in the material. These changes can then be observed by using a different kind of light, for example X-rays. These kinds of experiments, where light is used to "poke" and observe materials, are powerful investigative tools in science, and are part of what will be done at the new MAX IV facility in Lund.
In order to use terahertz light, one must first generate it. This has been the focus of a degree project at the Atomic Physics Division at Lunds University. There are many ways of generating terahertz light, and methods differ in efficiency as well as properties of the generated light. One method is using a laser to illuminate a crystal. When very intense light, like the light from a laser, interacts with matter a number of strange, so called nonlinear, effects occur. One of these is that light of a certain frequency can convert into light of a different frequency. The underlying mechanism is that laser light will cause the polarization in the crystal to oscillate over time, and these oscillations act as a source of electromagnetic radiation.
Typical efficiencies (how much of the input laser energy is converted into terahertz light) of this kind of generation method are between a few tenths of per mille and a few percent. Using the short pulse laser at the FemtoMAX beamline at MAX IV, and an organic crystal called DSTMS, terahertz light was successfully generated at the relatively high efficiency 0.71 %.
Light can be used as a tool to measure and investigate our surroundings. Different kinds of light can be used to explore different phenomena, and a vast quantity of different techniques have been developed. Light can be used to measure properties that are impossible to access in other ways. For example, extremely short distances can be measured using light, because the wavelengths of light itself can be very short, and work as "mini yardsticks". Light can also interact with different materials in a way that enables us to analyze them. Measuring with light involves finding the right wavelength for what you want to measure, and figuring out the right technique to do it.
The electromagnetic spectrum includes all different wavelengths of light, and stretches from the very long radiowaves, with wavelengths that are hundreds of kilometers long, to the very short gamma rays, with wavelengths at merely picometers (10^(-12) m). In addition to wavelengths, one can also describe light by its frequency. There is a specific part of the electromagnetic spectrum called the teraherts band, which is comprised of light with frequencies around 10^(12) Hz, or 1 THz. This light typically has wavelengths of a couple of tenths of mm. Terahertz frequencies are useful in a number of different applications. A key feature of terahertz light is that it is highly absorbed by water. This means that terahertz frequencies are well suited to use in both medical and environmental research, where water content is an important aspect. Terahertz light can interact with materials in a number of different ways. It can cause molecules to start vibrating, and induce electronic and structural changes in the material. These changes can then be observed by using a different kind of light, for example X-rays. These kinds of experiments, where light is used to "poke" and observe materials, are powerful investigative tools in science, and are part of what will be done at the new MAX IV facility in Lund.
In order to use terahertz light, one must first generate it. This has been the focus of a degree project at the Atomic Physics Division at Lunds University. There are many ways of generating terahertz light, and methods differ in efficiency as well as properties of the generated light. One method is using a laser to illuminate a crystal. When very intense light, like the light from a laser, interacts with matter a number of strange, so called nonlinear, effects occur. One of these is that light of a certain frequency can convert into light of a different frequency. The underlying mechanism is that laser light will cause the polarization in the crystal to oscillate over time, and these oscillations act as a source of electromagnetic radiation.
Typical efficiencies (how much of the input laser energy is converted into terahertz light) of this kind of generation method are between a few tenths of per mille and a few percent. Using the short pulse laser at the FemtoMAX beamline at MAX IV, and an organic crystal called DSTMS, terahertz light was successfully generated at the relatively high efficiency 0.71 %.
Avdelning/ar
Publiceringsår
2016
Språk
Engelska
Fulltext
Dokumenttyp
Examensarbete för masterexamen (Två år)
Ämne
- Physics and Astronomy
Nyckelord
- Nonlinear optics
- Terahertz radiation generation
- Optical rectification
- Difference frequency generation
- Organic crystal
- DSTMS
Handledare
- Jörgen Larsson