An international team of researchers has invented a simple way of creating ultrashort pulses of extreme ultraviolet (EUV) light. The system uses a new 3D metallic waveguide, or "nanofunnel", that coverts pulses of infrared light to EUV.
EUV light has a wavelength of around 5–50 nm, which is about 100–10 times shorter than that of visible light. As a result, ultrashort pulses of EUV light are ideal for studying fundamental physics phenomena – such as how electrons move in atoms, molecules and solids.
However, it is difficult to produce EUV radiation using conventional methods that rely on using amplified light pulses from an oscillator (a source of laser light) to ionize noble gas atoms. The electrons liberated during this process are accelerated in the light field and their surplus energy is freed as attosecond (10–18 s) pulses of light of different wavelengths. The shortest wavelengths of light can then be "filtered out" to produce a single EUV pulse – a complicated process.
Simpler way of making pulses
Now, researchers at the Korea Advanced Institute of Science and Technology (KAIST), the Max Planck Institute of Quantum Optics (MPQ) in Germany and Georgia State University (GSU) in the US have come up with a different – and much simpler – way of doing things.
The new technique works by converting femtosecond (10–15 s) infrared pulses into femtosecond EUV pulses. The process exploits surface-plasmon polaritons (SPPs), which are particle-like collective oscillations that occur when light interacts with a metal's conduction electrons.
The nanofunnel made by the KAIST-MPQ-GSU team was devised so that it concentrated incident infrared light pulses into a spot that is smaller than the wavelength of the incident light. The funnel is a metallic nanostructure made of silver that contains a hollow hole shaped like a tapered cone. The cone is just a few micrometres long and filled with xenon gas. The tip of the funnel is around 100 nm across.
The researchers sent infrared light pulses (at a rate of 75 MHz) into the funnel, which is designed so that it contains patches of metal that are positively charged, followed by patches that are negatively charged. This arrangement produces electromagnetic fluctuations on the inside walls of the funnel, which result in the creation of SPPs. These particles then travel towards the tip, where the conical shape of the funnel concentrates their fields.
"The field on the inside of the funnel can become a few hundred times stronger than the field of the incident infrared light," explains Mark Stockman of GSU. "This enhanced field results in the generation of EUV light in the Xe gas."
An important feature of the nanofunnel is that it can be produced at frequencies of up to about 75 MHz. Seung-Woo Kim, team leader at KAIST, where the experiments were carried out, adds: "Due to their short wavelength and potentially short pulse duration, EUV light pulses can be an important tool for exploring electron dynamics in atoms, molecules and solids. Electrons move very fast – on the attosecond timescale – and light flashes that are shorter than attoseconds long are therefore needed to image these particles. Although scientists routinely use attosecond light flashes for such studies, they have much lower frequencies. Our new nanofunnel could change all this."
The results are detailed in Nature Photonics.