Frequency comb lasers make the jump to extreme ultraviolet light
Researchers from the Institute for Lasers, Life and Biophotonics Amsterdam at the VU University of Amsterdam, have developed a method for making the high precision of frequency comb lasers available for experiments that use very short wavelengths (< 100 nm in the extreme ultraviolet). Previously, frequency comb lasers were only available for experiments with larger wavelengths (infrared to visible light). Based on a specially developed optical amplifier and non-linear optical conversion through high-harmonic generation, a frequency comb of 51 nm has now been realised. This comb, at an optical frequency of 6 PHz, has made it possible to carry out absolute frequency measurements in the extreme ultraviolet for the first time. The first application of this was determining the ionisation energy of the ground state of helium with an accuracy ten times greater than that previously achieved . The value found concurs well with the most recent theoretical predictions based on advanced Quantum Electrodynamics calculations (QED, the best-tested theory in physics). The researchers' findings were published on 2 August 2010 in the renowned journal Physical Review Letters (PRL). The research was subsequently highlighted in both PRL and Nature.
Frequency comb lasers explained
Frequency comb lasers have brought about a real revolution in many scientific areas over the past 10 years, such as precise frequency metrology and attosecond physics. It is therefore hardly surprising that one of the reasons why researchers Ted Haensch and John Hall received the Nobel Prize for Physics in 2005 was their work on the principle behind these lasers. Frequency comb lasers derive their name from the characteristic that they transmit hundreds of thousands of frequencies simultaneously, whereby the distance between the frequencies is precisely known and is extremely regular. Consequently the spectrum can be viewed as a 'comb' of frequencies. At the same time all of the frequencies interfere together to produce an endless series of extremely short laser pulses, the electromagnetic wave of which can also be controlled with considerable precision (the position in time and the optical phase of the pulses). This Fourier relationship is determined by just two parameters in the form of radio frequencies, which are equivalent to a so-called starting frequency and the repeating frequency of the pulses. Frequency combs therefore work as a sort of transmission between optical frequencies of hundreds of THz, and radio frequencies in the MHz range that can be measured extremely accurately with the help of atomic clocks. In this manner the accuracy of atomic clocks (at least 15 figures are possible) also becomes available via a frequency comb at high (optical) frequencies that cannot be directly measured electronically. This is, for example, extremely important for precision spectroscopy and the development of optical atomic clocks. For the latter the frequency comb functions the other way round: an optical transition is used as the frequency standard, which is reduced to measurable frequencies using a frequency comb. Due to the high frequency of optical transitions, a far higher accuracy can be achieved (in a shorter time) than is possible with traditional atomic clocks based on a microwave transition. Thanks to these developments, there is nothing that can be measured as accurately as frequencies. For example, nowadays this is used to test if the physical constants are indeed constant. At the same time, the ability to control the electromagnetic wave of light pulses with the aid of comb lasers also provides the following possibility: the control of processes with a time resolution better than a femtosecond. This has led to rapid developments in the field of attosecond physics.
Jump to extreme ultraviolet light
Extreme ultraviolet light has such a short wavelength that it is immediately absorbed in air and optical materials. Therefore non-linear optical methods must be used to construct a frequency comb at these wavelengths. The new research method is based on the amplification (more than 10 million times) of two laser pulses from a comb laser of 773 nm. The phase between the pulses must be kept constant to a very high level of accuracy for the effect to be maintained. The amplified pulses, with an energy of several mJ, are then strong enough to initiate a non-linear process in krypton gas that produces high-harmonic frequencies of the original light frequencies. One of the harmonics is the 15th at 51 nm. The two converted pulses at 51 nm together constitute a frequency comb in the form of a cosine-modulated spectrum. The maxima of the spectrum are equivalent to the expected transformed positions of the original comb spectrum in the infrared.
To demonstrate that precise measurements can indeed be made, the comb light at 51 nm was used to excite helium from the ground state to the 4p and 5p singlet levels. The analysis of the excitation signal was then used to determine the ground state energy with a frequency accuracy of 6 MHz.
Future perspectives
The high contrast of the helium signal (up to 55%) reveals that with this method frequency combs can probably be converted to even shorter wavelengths and that the resolution can also be significantly improved. This opens up perspectives for the testing of QED in highly charged ions where these effects are many times greater. Another interesting perspective is the realisation of optical clocks based on extreme ultraviolet or even X-ray transitions, with which it might be possible to determine time and frequencies even more accurately still.
This research was carried out under the auspices of the FOM Industrial Partnership Programme 'Metrology with Frequency Comb Lasers'.
Reference
"Extreme ultraviolet frequency comb metrology", D.Z. Kandula, Ch. Cohle, T.J. Pinkert, W. Ubachs, K.S.E. Eikema, Phys. Rev. Lett. 105, 063001 (2010).
Highlighted in PRL:
http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.105.063001
Highlighted in Nature: Nature 466, 798 (2010), doi:10.1038/466798c.
Information
For further information please contact:
Prof. Kjeld Eikema, telephone +31 (0)20 598 79 57.