Source: Projects Magazine.

A team of scientists from the University of P¨¦cs who developed a method for producing ultra-short high-energy terahertz pulses, are now confident that they will be able to increase the electric field value of these pulses by a magnitude of 100. This development could lead to a variety of new and exciting applications, ranging from cancer therapy to semiconductor research. We spoke to J¨¢nos Hebling and J¨®zsef F¨¹löp to find out more.

Terahertz radiation is a specific type of electromagnetic radiation that lies between the frequency ranges of microwave and infrared radiation. Around 25 years ago, scientists began investigating the use of femtosecond laser pulses for the generation of THz radiation, resulting in the generation of THz pulses with a frequency one to three orders of magnitude larger than any electronic device can produce. Using this technique, THz pulses comprised of only a single oscillation of the electric field can be obtained (see Figure on next page).

THz radiation has a number of useful applications, such as for security devices in airports. It is able to penetrate through most materials that are used for packaging, so by using multispectral THz imaging, dangerous substances such as biological weapons or drugs can be detected without having to open the packaging. It is also able to do this incredibly quickly, making it extremely practical.

Professor J¨¢nos Hebling, director of the Institute of Physics at the University of P¨¦cs explains how he first became involved in THz research: ¡°Back in 2000, I was working at the Max Planck institute in Stuttgart where I was investigating the temporal behaviour of phonon polaritons in semiconductors. I developed a novel setup for the generation of such excitations using ultra-short laser pulses.

¡°When I later began work at the University of P¨¦cs, we realised that we could use this setup for the generation of high-energy THz pulses using nonlinear materials such as lithium-niobate.¡±

Nonlinear optical materials are materials in which the electric field of light output is not related to the electric field of light input by a simple proportionality constant. Because of this nonlinear behaviour, an intense light beam propagating through a nonlinear optical material can produce new frequency components not contained in the input light, which cannot be seen with weak light beams.

For example, an intense light beam propagating through a nonlinear material can generate, in addition, harmonics or overtones of the original light frequency such as its second harmonic, used also in green laser pointers. The difference of two input frequency components can also be generated if the velocity of the input laser radiation and that of the generated low-frequency (THz) radiation are equal. In lithium-niobate, a highly nonlinear material, this can be accomplished by tilting the intensity front of the input short laser pulses. Ultrashort laser pulses can be said to be like flying pancakes, since their spatial extension in the direction of propagation is very small. Hence, tilting the intensity front means a slanted flying pancake. This technique of tilted-pulse-front pumping developed by the team in P¨¦cs is now used by several groups around the world for THz pulse generation.

¡°By using high pump pulse energies, we were able to generate incredibly intense THz pulses, which we then used in the first THz pump¡ªTHz probe experiments at MIT, Cambridge,¡± continues Hebling. ¡°This method of THz generation is now used widely by research groups from around the world.¡±

Only a few years ago, an electric field value of 1 MV/cm was considered very high, but it has been predicted that field strengths as high as 100 MV/cm are possible in the future. This would be achieved with a modified setup by using longer pump pulses and cooling the lithium-niobate crystals to very low temperatures. This could allow for a number of exciting new applications, including the manipulation of ions and relativistic electrons.

Jozsef F¨¹löp, another member of the team, further explains what the implications of their THz research are: ¡°I was very keen to get involved in the development of this project, as it was clear that in the future there were a lot of things that could be done with this technology.

¡°We are now at a stage where some of these applications, such as non-linear THz spectroscopy, have started to be demonstrated. In the last two years we have seen that these pulses can introduce changes in the state of charged carriers in semiconductors, and they do this instantaneously because they are very short. This will help for a better understanding of ultrafast carrier dynamics of electron distribution in these materials, which will have applications in electronics.¡±

¡°One of the most interesting uses that we envisage for the future is the manipulation of charged particles, for instance the acceleration of electrons and protons,¡± he continues. ¡°This could eventually be used for medical applications such as cancer therapy.¡±

This sort of radiation therapy for treating cancer using ion beams is known as particle therapy, or more accurately, hadron therapy. This works by aiming beams of energetic hadrons, i.e. protons or ions, at a tumor. The main reason why ion beams are superior to x-ray or gamma radiation in cancer therapy is their different absorption characteristics. Ion beams are mainly absorbed at a well defined depth inside the tissue, while hardly effecting the tissue outside of this depth. Gamma rays follow the usual exponential attenuation law, therefore they are mainly absorbed close to the surface, thereby heavily damaging the healthy tissue outside the tumor.

The team from P¨¦cs has submitted a number of patents for the use of high field strength THz pulses for this application, and so hopefully in the future their research into this field could lead to reducing the suffering of cancer patients around the world.