Dear colleagues and friends of the HI Jena,

welcome to the June issue of the HI Jena newsletter. Below you find the latest research news and announcements from our institute.

In particular, we like to remember you that the HI Jena has been founded five years ago. We like to use this opportunity to thank all colleagues at our Institute for their successfull work.

Kind Regards,
Helmholtz Institute Jena

High-energy laser pulses from the last POLARIS amplifier

Diode-pumped, chirped pulse amplification using Yb³⁺-doped amplification materials is envisioned to be an efficient approach for generating high peak power laser pulses both with high repetition rate and high pulse energy [1, 2]. Using diode-pumping technology, these laser media have already been used for the amplification of ns-laser pulses to energies in excess of 10 J. However, significantly shorter pulse durations in the sub-ps regime are required for high-intensity laser matter experiments such as electron or ion acceleration.

Yb:CaF2 has proven to be a promising candidate for the amplification of sub-ps pulses, since it supports a broader bandwidth. In order to increase both the energy and the peak intensity of the laser pulses a new amplification stage A5 (cf. Fig. 1 a.) has been commissioned at the Helmholtz-Institute Jena [3]. With this stage we have reached an energy of 16.6 J at a center wavelength of 1034 nm using Yb:CaF₂ as the active medium. Pumped by a total optical power of 300 kW from high-power laser diodes, a gain factor of g = 6.1 was achieved in a nine-pass amplifier configuration. A measured spectral bandwidth of 10 nm full width at half maximum, FWHM promises a bandwidth-limited compression of the pulses down to a duration of 150 fs. These are to our knowledge the most energetic laser pulses achieved from a diode-pumped chirped-pulse amplifier so far.

For anticipated output energies of up to 50 J, the diameter of the laser pulses in the last amplifier had to be increased to 35mm (FWHM), requiring an amplification crystal with a diameter of more than 60 mm to avoid clipping. As the active material, we use a <111>-cut, 2.3 at.% doped high quality Yb:CaF₂ crystal made by Hellma Materials GmbH with a diameter of d = 65mm and a thickness of t = 34mm (90% pump absorption).

In order to realize a smooth pump profile, the 120 individual focal spots of the laser diode stacks (LDS) have to be aligned carefully into the Yb:CaF₂-crystal. As a target function for the lateral distribution, a circular, 4th-order super-gaussian profile with a FWHM diameter of 35 mm has been chosen. A set of 360 adjustable parameters (two for position and one for intensity) needs to be optimized for the homogenization of the pump profile. For this purpose we have developed a routine based on an evolutionary algorithm [4] using adaptive strategy from the software package LAB2 [5]. For the measurement of the pump profile, the Yb:CaF₂-crystal is imaged onto a CCD, equipped with an edge-filter. This filter attenuates the 940 nm pump light by 6 orders of magnitude, while the fluorescence light at 1030 nm, indicating the distribution of the population inversion, is transmitted. With this technique we were able to record the pump profile with its assigned position and intensity for each LDS as well as for the complete set of LDS.

The manual alignment of 120 pump spots would be a time-consuming task. Furthermore, operation of A5 means to control more than 1000 technical parameters. For this reason we developed a control system (CS) based on CS-Framework for experiments [6] designed by GSI. Our CS automatically aligns the pump engine and provides user interfaces and routines for daily operation.

Currently we focus our activities on increasing the maximal output energy from the last amplifier, which is currently limited by the maximum fluence of 2 J/cm² on the Yb:CaF₂-crystal in order to avoid laser induced damage of the AR-coating (Fdamage = 3.4 J/cm²). For this purpose, two new Yb:CaF₂ crystals with an improved damage fluence have been purchased, which will be sufficient to triple the output energy of this amplifier in near future.

After the compression of the amplified pulses in the POLARIS compressor, we measured a pulse duration of 160 fs. The next step will be to focus the amplified pulses in order to use them in high-intensity laser matter experiments.

References:
[1] M. Siebold et al., Appl. Phys. B 90, 431 (2008).
[2] M. Hornung et al., Optics Letters 38, 718 (2013).
[3] A. Kessler et al., Optics Letters 39, 1333 (2014).
[4] S. Keppler et al., Proc. SPIE 8780, 87800I-1 (2013).
[5] B. Schmidt et al., ”LAB2-A virtual femtosecond laser lab,” http://www.lab2.de .
[6] D. Beck et al., “CS-Framework for Experiments”, http://wiki.gsi.de/cgi-bin/view/CSframework

Semiannual Palaver of the HI Jena

On Thursday, 10th July 2014 we will have our next semiannual palavor. It will take place in the Main Lecture Hall of the physics faculty at Max-Wien-Platz 1 , start at 14:00 p.m. and end at 18:30 p.m. Below, you find a list of the speakers (in alphabetical order) and the titles of their talks.

• Rene Geithner: SQUID-based beam intensity measurements for accelerators.
• Daniel Hengstler: maX mirco-calorimeter: high-resolution x-ray spectroscopy.
• Felix Karbstein: Towards an all-optical probe of quantum vacuum nonlinearity.
• Robert Riedel: High power femtosecond lasers - from research to market.
• Sergey Rykovanov: Laser-plasma undulator based on wakefield excitation in a plasma channel.
• Alexander Sävert: Advanced diagnostics of plasma wake fields.
• Johannes Ullmann: Ground-state hyperfine splittings in highly charged bismuth ions.
• Michael Zürch: High resolution XUV imaging.

Afterwards we will have a barbecue and plenty of time for discussions in the garden area behind the main building of the HI Jena at Fröbelstieg 3.

It is a great pleasure to announce four guest scientists of the HI Jena:

• Professor Gerald V. Dunne (University of Connecticut, USA) is visiting the HI Jena for two months (June and July 2014) to collaborate and work on particle production in time-dependent electromagnetic fields.
• Professor Alexei N. Grum-Grzhimailo (Moscow State University, Russia) is visiting the HI Jena in July 2014 to collaborate on multi-photon processes in FEL radiation.
• Dr. Vladimir Yerokhin (St. Petersburg State University, Russia) is visting HI Jena from May 17th to September 30th 2014 to collaborate on the theoretical analysis of elastic Rayleigh scattering of x-rays by heavy neutral atoms.
• Alexander Zaitsev (St. Petersburg State University, Russia) is visiting the HI Jena from July 15th to August 15th 2014 to collaborate on decay processes of multiple-charged ions.