Robert Klas

Energetic sub-2-cycle laser with 216 W average power.

Few-cycle lasers are essential for many research areas such as attosecond physics that promise to address fundamental questions in science and technology. Therefore, further advancements are connected to significant progress in the underlying laser technology. Here, two-stage nonlinear compression of a 660 W femtosecond fiber laser system is utilized to achieve unprecedented average power levels of energetic ultrashort or even few-cycle laser pulses. In a first compression step, 408 W, 320 μJ, 30 fs pulses are achieved, which can be further compressed to 216 W, 170 μJ, 6.3 fs pulses in a second compression stage. To the best of our knowledge, this is the highest average power few-cycle laser system presented so far. It is expected to significantly advance the fields of high harmonic generation and attosecond science.

High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules

Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 1011 photons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CH3I are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses.

Table-top milliwatt-class extreme ultraviolet high harmonic light source

Extreme ultraviolet (XUV) lasers are essential for the investigation of fundamental physics. Especially high repetition rate, high photon flux sources are of major interest for reducing acquisition times and improving signal to noise ratios in a plethora of applications. Here, an XUV source based on cascaded frequency conversion is presented, which delivers due to the drastic better single atom response for short wavelength drivers, an average output power of (832 +- 204) {\mu}W at 21.7 eV. This is the highest average power produced by any HHG source in this spectral range surpassing precious demonstrations by more than a factor of four. Furthermore, a narrow-band harmonic at 26.6 eV with a relative energy bandwidth of only {\Delta}E/E= 1.8 x 10E-3 has been generated, which is of high interest for high precision spectroscopy experiments.

High-power nonlinear compression stage delivering sub-50 fs, 0.25 mJ pulses, 15 W at 2 µm wavelength for HHG

We present the nonlinear compression of ultrashort pulses at 2 µm wavelength to sub-50 fs, >3 GW at 15.4 W of average power. This source was used for HHG in an argon gas jet.

200 W Average Power Energetic Few-cycle Fiber Laser

A state-of-the-art 8 channel fiber-chirped-pulse-amplifier system delivers 680 W of average power. Two-stage nonlinear compression in gas-filled capillaries yields 400 W, 30fs, >300µJ pulses and 220W, sub-7fs, 170 µJ pulses, respectively.

High Photon Flux 70 eV HHG Source for Applications in Molecular and Solid State Physics

A 100 kHz high harmonic source with record high >1011 photons/s in single harmonics between 55-73 eV is presented. The unique capabilities are underlined by using it for coincidence experiments and measurements on magnetic samples.

High resolution XUV Fourier transform holography on a table top

Today, coherent imaging techniques provide the highest resolution in the extreme ultraviolet (XUV) and X-ray regions. Fourier transform holography (FTH) is particularly unique, providing robust and straightforward image reconstruction at the same time. Here, we combine two important advances: First, our experiment is based on a table-top light source which is compact, scalable and highly accessible. Second, we demonstrate the highest resolution ever achieved with FTH at any light source (34 nm) by utilizing a high photon flux source and cutting-edge nanofabrication technology. The performance, versatility and reliability of our approach allows imaging of complex wavelength-scale structures, including wave guiding effects within these structures, and resolving embedded nanoscale features, which are invisible for electron microscopes. Our work represents an important step towards real-world applications and a broad use of XUV imaging in many areas of science and technology. Even nanoscale studies of ultra-fast dynamics are within reach.

Temporal contrast enhancement of a femtosecond fiber CPA system by filtering of SPM broadened spectra

We present a novel approach for temporal contrast enhancement of energetic laser pulses by filtered SPM broadened spectra. A measured temporal contrast enhancement by at least 7 orders of magnitude in a simple setup has been achieved. This technique is applicable to a wide range of laser parameters and poses a highly efficient alternative to existing contrast-enhancement methods.

Scaling ultrashort laser pulse induced glass modifications for cleaving applications

Ultrashort laser pulses allow for in-volume processing of glass through non-linear absorption. This results in permanent material changes, largely independent of the processed glass, and it is of particular relevance for cleaving applications. In this paper, a laser with a wavelength of 1030 nm, pulse duration of 19 ps, repetition rate of 10 kHz, and burst regime consisting of either four or eight pulses, with an intra-burst pulse separation of 12.5 ns, is used. Subsequently, a Gaussian-Bessel focal line is generated in a fused silica substrate with the aid of an axicon configuration. We show how the structure of the modifications, including the length of material disruptions and affected zones, can be directly influenced by a reasonable choice of focus geometry, pulse energy, and burst regime. We achieve single-shot modifications with 2 μm in diameter and 7.6 mm in length, exceeding an aspect ratio of 1:3800. Furthermore, a maximum length of 10.8 mm could be achieved with a single shot.

Milliwatt-class high harmonic generation with an high average power short wavelength fiber laser

Table-top extreme ultraviolet (XUV) light sources with laser like properties are of interest in a vast variety of applications. To achieve such light pulses with femtosecond pulse durations, high harmonic generation (HHG) is an established technique. It delivers ultrashort pulses and a wide range of photon energies, suitable for the study of core level transitions, highly excited states, or transitions in highly-charged ions [1], as well as for experiments in solid state physics such as photoelectron spectroscopy [2]. To achieve short acquisition times as well as acceptable signal to noise ratios, a high repetition rate and high photon flux is beneficial. Therefore, average power scalable fiber-based lasers are an ideal driver [3]. Since the HHG single atom response scales with ∼λ−5.5 [4] and phase matching conditions are more beneficial for a shorter driving wavelength, a much higher overall conversion efficiency can be achieved for short wavelength drivers [5]. By using the second harmonic of an infrared laser for HHG we demonstrate the highest average power high harmonic source to date. This cascaded frequency conversion yields a record high average power of (832 + 204) μW, contained in a single harmonic line at 21.7 eV with a relative energy bandwidth of ΔΕ/E ≈ 10−2, shown in Fig. 1a) [6].