Waveguide Generates Coherent EUV

Anne Fischer Lent

Nonlinear optics can convert laser light from one wavelength to another, but the process often requires phase matching to be efficient. Unfortunately, in high-harmonic generation of coherent extreme-ultraviolet radiation, it has not been possible to phase-match above approximately 80 eV, reducing efficiency and limiting its applications. Now researchers from JILA in Boulder, Colo., and Sofia University in Bulgaria have applied concepts from visible light conversion that allow them to extend the range of efficient EUV generation by nearly 100 eV.

A modulated fiber enables researchers to generate coherent extreme-ultraviolet radiation by quasi-phase-matched frequency conversion.

The new method uses a quasi-phase-matched frequency conversion of laser light into the EUV. Firing a femtosecond laser into a gas-filled waveguide creates ions and electrons by ripping apart the gas atoms. Then it accelerates the electrons to very high energies and releases photons at EUV wavelengths when it slams the electrons back into the ions. This conversion process is efficient only if the laser waves and EUV waves travel at the same speed through the gas, which usually happens only at low levels of ionization.

By creating ripples in the diameter of the hollow-core waveguide, the team solved the phase-matching problem. The ripples force the lightwaves from the laser and the EUV beams to travel at effectively the same speed in the tube.

One advantage of this setup is size. Intense EUV sources such as synchrotrons are the size of buildings. The waveguide used in this design, however, fits in one hand, and the laser fits on a desktop.

This method could be used for metrology or for mask inspection in support of advanced lithography. It also may enable high-resolution imaging applications of materials and biological samples.

The next step in the research is to extend the beam's range into the water window: the region of the spectrum below 4 nm. Producing an EUV source at these wavelengths would be useful in designing a compact microscope for imaging biological structures within a wet or frozen cell.