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Ultrafast Laser Physics and Precision Metrology Group

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Research Themes and News


The research of the Ultrafast Laser Physics and Precision Metrology Group is based on frequency comb lasers and ultrafast laser technology, and its many applications. The main themes of the group are listed below with a short explanation.


News


Latest News


Together with the collegues of the AML group, Prof. dr. Kjeld Eikema was awarded a NWO/FOM Program of 1.8 MEuro in December 2016 to investigate the Proton Radius Puzzle using precision metrology in several systems.


     nwo logo
Moreover, he also received an European Research Council (ERC) Advanced Grant of 2.5 MEuro in March 2016 to  investigate QED and the proton radius puzzle by precision spectroscopy on the 1S-2S transition in trapped helium+ ions. You can find a short description at the news section of the department website, and at the section about open PhD and Postdoc positions. ERC logo

Latest papers:

R.K. Altmann, S. Galtier, L.S. Dreissen, and K.S.E. Eikema,
High-Precision Ramsey-Comb Spectroscopy at Deep-Ultraviolet Wavelengths
Phys. Rev. Lett 117, 173201 (2016),
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.173201

G.S.M. Jansen, D. Rudolf, L. Freissem, K.S.E. Eikema, and S. Witte,
Spatially resolved Fourier-transform spectroscopy in the extreme ultraviolet
Optica 3, 1122 (2016)
https://www.osapublishing.org/optica/abstract.cfm?uri=optica-3-10-1122&origin=search


Main Research Themes


Frequency combs
Frequency combs lasers from IR to XUV wavelengths

Frequency comb lasers are responsible for a revolution in many fields of science because they enable to measure optical frequencies with extreme precision, and also provide control at an attosecond level over optical waves and ultrafast laser pulses. Frequency combs are based on modelocked lasers (usually operating in the near-infrared) in which easily a million modes can laser together, in phase. It looks like a 'comb' of frequencies, hence the name 'Frequency comb'.
We work on several projects to extend the wavelength range of these special lasers, and to apply them for precision spectroscopic applications ranging from the infrared range of the spectrum (wavelengths of 3-10 micron), to the extreme ultraviolet (XUV, wavelength<100 nm).
In a collaboration with the Technical University Eindhoven we also worked on miniaturization of frequency comb lasers using quantum-dot laser material.

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Precision spectroscopy for fundamental tests

Using spectroscopy with frequency comb lasers, we determine the energy structure of atoms and molecules with very high accuracy (typically > 9 decimal places). In this manner elements of the 'Standard Model' in physics, such as Quantum Electrodynamics (QED) can be tested by a comparison with the theoretical level energy structure. The comparison works best for simple atoms (such as helium) and small molecules (such as H2). Our measurement of the ground state energy of helium using an XUV frequency comb at 51 nm, is the most accurate measurement at the shortest wavelength ever performed. We have also demonstrated direct frequency comb measurements on ions (such as calcium) to test for a possible variation of the fine-structure constant alpha, and provided the calibration for measurements on meta-stable helium. Currently an experiment is carried out with the new concept of "Ramsey Comb Spectroscopy", which we demonstrated on a two-photon transition in Rb, and recently also on deep-UV two-photon transitions in krypton and molecular hydrogen. One of the next targets for precision spectroscopy is a determination of the 1S-2S transition in helium+ ions, to shed light on the 'proton-size puzzle'.

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precision spectroscopy


coh control
Coherent control

With coherent control it is possible to steer the outcome of processes such as laser excitation, ionization, and molecular dissociation, by 'spectral shaping' of ultrashort pulses. This spectral shaping is based on  modulators that imprint a phase and/or intensity function on the spectral components of the ultrashort pulses involved in the excitation process.
We are investigating new forms of coherent control, e.g. to create complex spatial excitation patterns. One application is the complete suppression of background signal in direct frequency comb spectroscopy of two-photon transitions.

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X-ray generation and lens-less imaging

The resolution of imaging, such as microscopy, ultimately depends on the wavelength that is used. With shorter wavelengths one can resolve smaller objects. With this project we are constructing an X-ray microscope operating in the so-called 'water window' between 2 and 4 nm. In this wavelength range  water is almost transparent, but the building blocks of life (such as carbon and nitrogen) produce significant absorption and therefore contrast. The idea is to reach a resolution near 10 nm.
The X-rays are produced via high-harmonic generation using ultrashort laser pulses (10 fs) with TW peak power at high repetition rate (300 Hz). The harmonic generating process is rather inefficient, but it leads to highly coherent X-rays with excellent timing properties. In parallel to this effort we are also developing lens-less imaging methods for X-rays to enable table- top high-resolution X-ray imaging.

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X-ray imaging


laser development
Ultra-stable lasers, fiber links, and ultra-intense laser development

Apart from our frequency comb laser development efforts, we also build and develop ultra-stable lasers and high peak power lasers (TW level).
The ultra-stable lasers all operate near 1550 nm; we have 2 with a few-kHz bandwidth, and one commercial laser from Menlo Systems with a bandwidth of 0.28 Hz at 1542 nm. These lasers are used for precision measurements and optical stabilization of our frequency combs (including Ti:sapphire based systems using frequency doubling of 1550 nm to 775 nm).
We also are involved in the development of fiber-link between the VU and the KVI-Groningen (operational since spring 2012) to transfer frequency and/or timing references with high precision over hundreds of km on a commercial channel.
We develop ultra-intense lasers based on parametric amplification, including the high-power pump laser required for this process. Two systems are under construction using QCW diode pumped Nd:YAG amplifier modules. Applications are the XUV comb for precision metrology, and X-ray generation for high-resolution microscopy with 2-4 nm light.

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Previous projects

In one of the previous projects we have been involved in a study on long absolute distance measurements in space. One application is formation flight of several satellites each with its own telescope to obtain a larger synthetic aperture and resolution.

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Older projects


Below you see a picture of the lab a few years ago. Since then the lab has doubled in size. In October 2016 we re-arranged the lab for building up the new vacuum system and ultra-stable Ramsey-comb laser setup for the He+ 1S-2S experiment.
When a bit more is finished, a new overview picture of the lab will be posted.

lab


And the picture below from May 2017 is the new ultra-stable ULN Frequency Comb from Menlo Systems that we are going to use for the He+ 1S-2S experiment:

ULN frequency comb




We gratefully acknowledge financial support from (including the Memphis Project):
NWO FOM Laser Europe STW VU ERC

Questions? Contact: k.s.e.eikema@vu.nl