Two groups working with ultrafast lasers have shown they can generate a regularly spaced range of frequencies in the extreme UV (EUV), creating the potential for applying spectroscopy on an extremely tiny scale.
Working separately, groups at JILA (Boulder, CO) and the Max Planck Institute for Quantum Optics (Garching, Germany) produced frequency combs in the EUV. In a frequency comb, discrete wavelengths of light are spaced at regular intervals. Jun Ye led the group at JILA, a research institute run jointly by the National Institute of Standards and Technology and the University of Colorado, while Theodor Haensch directed the group at Max Planck.
A spectrum attained using an ultrafast laser and a resonator cavity shows clear peaks down to the 13th-order harmonic of the fundamental wavelength (Graph courtesy Max Planck Institute for Quantum Optics).
The setup was similar in both experiments. They started with a Ti:sapphire laser producing femtosecond pulses and fired those into a resonator. The resonator consists of mirrors that store the pulses while more laser energy is fired in. The pulses interfere constructively and increase their pulse energy. The higher energy photons then travel through a thin cloud of xenon gas, which generates harmonics of the fundamental laser frequency, producing output at shorter wavelengths.
"Usually from [continuous-wavelength] lasers with a crystal you can achieve second-order harmonic generation or third-order harmonic generation," says Christoph Gohle, a PhD student in Haensch's group. In this case, he says, researchers reached 13th-order harmonics. "The idea is pretty straightforward, but there [are] a few technical difficulties to get around," Gohle says.
For one, they needed special mirrors for the resonator cavity. The mirrors had to have very high reflectivity over a broad spectral range of about 20 or 30 nm. They also had to deal with dispersion that could cause pulses to change shape and interfere with one another. The third challenge was getting the radiation out of the resonator, which they achieved by using sapphire windows with a small refractive index.
Ye says the two experiments were nearly identical. "Really, the only difference comes down to a couple technical details in terms of the pulse width we used."
Here, the EUV laser setup: ICp, coupling mirror with 1% transmission; CM, chirped mirrors; PD, photodiode; W, Brewster-angled sapphire window of 1-mm thickness; PZT, piezoelectric transducer. Inset: The reflectance of sapphire for p-polarized EUV light at an incidence angle of 60.48 (Brewster angle for 800-nm radiation).
"This is really great work," says Erich Ippen, a physics professor at the Massachusetts Institute of Technology (Cambridge, MA). "Jun Ye and Ted Haensch are the two leading scientists in the world in the area of frequency combs and high-precision spectroscopy. Their success at pushing comb technology into the [EUV] will certainly open up a wide range of new opportunities for important spectroscopy in both atomic and molecular physics."
Both groups will have to increase the power of their output by about 1000 times to actually perform spectro-scopy. Ye says his group is considering using a fiber laser for input to increase the initial energy of the pulses. They are also looking at different designs for the resonator cavity.
If the groups are successful, they will have a photon source for spectroscopy with enough resolution to observe atomic transitions in helium, allowing them to actually see basic properties of physics that have been predicted by theory but have been experimentally out of reach. Ye says the frequency combs might be used for illumination in EUV microscopy. Gohle says it might also be possible to use the technology to create ultradense holographic storage because the size of the wavelengths is so small.
Ye says it should take less than a year to discover if they can reach the higher power levels. "We are confident we can get there, but it's not going to be easy," he says.