arXiv subjects

Martin Hoffmann

Publications and source records attributed to Martin Hoffmann.

At least 19 recordsLinked to original sources

arXiv2026

Ho3+-doped CALGO crystals for high-power ultrafast 2.1-{\mu}m lasers

Ho3+-doped disordered CaAlGdO4 (CALGO) crystals have recently emerged as a promising gain material platform for next-generation high-power ultrafast 2.1-{\mu}m laser systems. This laser gain material offers a unique combination of high-gain, small quantum defect, inhomogeneously broadened spectra, and good thermal conductivity, enabling ultrashort pulse generation and amplification at high-average power and high pulse energy. Many systems, including mode-locked oscillators and amplifiers with state-of-the-art performance, have been demonstrated in the last few years that promise to meet growing application demands for efficient ultrafast laser technology in this wavelength region. In this review paper, we summarize recent achievements using this gain material both in oscillators and amplifiers and place these results in the state-of-the-art of 2-{\mu}m ultrafast laser technology, present detailed spectroscopic characterization of this material, and discuss future perspectives of further performance scaling of Ho:CALGO lasers.

physics.optics↗

arXiv2025

Laser-driven few-cycle Terahertz sources with high average power

Ultrafast laser-driven terahertz sources are gaining in popularity in an increasingly wide range of scientific and technological applications. However, many fields continue to be severely limited by the typically low average power of these sources, which restricts speed, signal-to-noise ratio, and dynamic range in numerous measurements. Conversely, the past two decades have seen spectacular progress in high average power ultrafast laser technology based on Ytterbium lasers, rendering hundreds of watts to kilowatts of average power available to this community to drive THz sources. This has opened the young field of high-average-power laserdriven THz time-domain spectroscopy, which holds the potential to revolutionize the applications of THz time-domain systems. In this perspective article, we discuss this young field and emphasize recent advancements in broadband terahertz sources utilizing high-power Yb-based ultrafast lasers as drivers, which are nearing watt-level average power. We discuss various approaches explored thus far, current challenges, prospects for scaling, and future research areas that will accelerate their implementation in applications.

physics.optics↗

arXiv2025

Energy scaling of Kerr-lens modelocked Ho:YAG thin-disk oscillators to the microjoule level

We demonstrate peak power and pulse energy scaling of Kerr-lens modelocking (KLM) Ho:YAG thin-disk lasers (TDLs) emitting at 2.1-{\mu}m wavelength. We compare different laser configurations to reach a maximum pulse energy of 1.7 {\mu}J at an output power of 29 W with a pulse duration of 434 fs, corresponding to a peak power of 3.7 MW. This represents a 5-fold increase in pulse energy and 4-fold increase in peak power compared to previous KLM Ho:YAG TDLs, thus reaching record high peak power for 2.1-{\mu}m mode-locked oscillators. We discuss current limitations at this wavelength and guidelines for future work towards higher power and pulse energies.

physics.optics↗

arXiv2024

High peak-power 2.1-{\mu}m femtosecond Holmium amplifier at 100 kHz

High-power ultrafast laser sources in the short-wave infrared region are of great interest for numerous applications, including secondary sources of radiation and processing of materials commonly opaque in the near-infrared region. In this wavelength region, direct laser amplification around 2.1-{\mu}m wavelength within the atmospheric transparency window is particularly attractive for realizing compact and efficient high-power lasers. However, this wavelength region was widely underrepresented in femtosecond laser technology so far. Here, we report on a 2.1-{\mu}m laser system delivering 97-fs pulses with an unprecedented combination of high peak power of 525 MW and high repetition rate of 100 kHz. The amplifier system consists of a mode-locked oscillator seeding a regenerative amplifier (RA) using the novel broadband material Holmium (Ho)-doped CaAlGdO4 (CALGO), operating in the chirped pulse amplification (CPA) scheme and a nonlinear compression stage based on a Herriott-type multi-pass cell (MPC) with bulk material. We demonstrate the potential of this unique laser system by generating a micro plasma in ambient air, demonstrating its high intensity for future plasma-driven secondary sources. This system bridges the gap between conventional 1-to-10 kHz amplifiers and high-power MHz laser oscillators in this attractive wavelength range.

physics.optics↗

arXiv2023

Hybrid air-bulk multi-pass cell compressor for high pulse energies with full spatio-temporal characterization

Multi-pass cell (MPC) compressors have proven to be the method of choice for compression of high average power long-pulse Yb lasers. Yet, generating sub-30 fs pulses at high pulse energy with compact and simple components remains a challenge. This work demonstrates an efficient and cost-effective approach for nonlinear pulse compression at high pulse energy using a hybrid air-bulk MPC. By carefully balancing the relative nonlinear contributions of ambient air and fused silica, we achieve strong spectral broadening without dispersion engineering or pressure-control inside the cell at 400 uJ pulse energy. In this way, we compress pulses from 220 fs to 27 fs at 40.3 W of average power (100 kHz repetition rate), enhancing the peak power from 1.6 GW to 10.2 GW while maintaining 78% of the energy within the main pulse. Our approach combines the strengths of gas-filled and bulk compression schemes and exhibits excellent overall optical transmission (91%) and spectral uniformity. Moreover, we utilize the INSIGHT technique to investigate spatio-temporal couplings and geometrical aberrations of the compressed pulse. Our results demonstrate remarkable temporal homogeneity, with an average Strehl ratio of 0.97 consistently observed throughout the entire spectral profile. Additionally, all spectrally-integrated Zernike coefficients for geometrical aberrations maintain values below 0.02{\lambda}.

physics.optics↗

arXiv2023

GHz repetition rate, sub-100-fs Ho:CALGO laser at 2.1 um with watt-level average power

We report on a GHz fundamental repetition rate Kerr-lens mode-locked Ho:CALGO laser emitting at 2.1 um. The laser employs a ring-cavity to increase the fundamental repetition rate to 1.179 GHz and can be made to oscillate in both directions stably with nearly identical performance: for counterclockwise oscillation, it generates 93-fs pulses at 1.68 W of average power, whereas 92 fs and 1.69 W were measured for clockwise operation. Our current results represent the highest average power from a 2-um GHz oscillator and the first sub-100-fs pulse duration from a Ho-based oscillator.

physics.optics↗

arXiv2023

Low-noise, 2-W average power, 112-fs Kerr-lens mode-locked Ho:CALGO laser at 2.1 um

We report on an in-band pumped soft-aperture Kerr-lens mode-locked Ho:CALGO bulk laser at 2.1 um, generating 2 W of average power with 112 fs pulses at 91-MHz repetition rate. To the best of our knowledge, this is the highest average power from a 100-fs class mode-locked laser based on a Tm3+ or Ho3+ doped bulk material. We show that the laser has excellent noise properties with an integrated relative intensity noise of 0.02% and a timing jitter of 950 fs (RMS phase noise 0.543 mrad) in the integration interval from 10 Hz to 10 MHz. The demonstrated combination of high average power, short pulses, and low-noise make this an outstanding laser source for spectroscopy and many other applications at 2.1 um.

physics.optics↗

arXiv2022

Spectral broadening of 2 mJ femtosecond pulses in a compact air-filled convex-concave multi-pass cell

Multi-pass cell (MPC) based temporal pulse compressors have emerged in the last years as a powerful and versatile solution to the intrinsic issue of long pulses from Yb-based high-power ultrafast lasers. However, the spectral broadening of high-energy (typically more than 100 uJ) pulses has only been realized in complex setups, i.e., in large and costly, pressure-controlled vacuum chambers to avoid strong focusing, ionization, and damage on the mirrors. Here, we present spectral broadening of 2 mJ pulses in a simple and compact (60 cm long) multi-pass cell operated in ambient air. Instead of the traditional Herriott cell with concave-concave (CC/CC) mirrors, we use a convex-concave (CX/CC) design, where the beam stays large at all times allowing both to minimize damage and operate in ambient air. We demonstrate spectral broadening of 2.1 mJ pulses at 100 kHz repetition rate (200 W of average power) from 2.1 nm (pulse duration of 670 fs) to a spectral bandwidth of 24.5 nm, supporting 133 fs pulses with 96% transmission efficiency. We show the compressibility of these pulses down to 134 fs, and verify that the spectral homogeneity of the beam is similar to previously reported CC/CC designs. To the best of our knowledge, this is the first report of a CX/CC MPC compressor, operated at high pulse energies in air. Because of its simplicity, small footprint and low cost, we believe this demonstration will have significant impact in the ultrafast laser community.

physics.optics↗

arXiv2022

8.7-W average power, in-band pumped femtosecond Ho:CALGO laser at 2.1 um

We report on an in-band pumped SESAM mode-locked Ho:CALGO bulk laser with a record-high average power of 8.7 W and an optical-to-optical efficiency of 38.2% at a central wavelength of 2.1 um. At this power level, the bulk laser generates pulses with a duration of 369 fs at 84.4-MHz repetition rate, corresponding to a pulse energy of 103 nJ and a peak power of 246 kW. To the best of our knowledge, this is the highest average power and pulse energy directly generated from a mode-locked bulk laser in the 2-3 um wavelength region. Our current results indicate that Ho:CALGO is a competitive candidate for average power scaling of 2 um femtosecond lasers.

physics.optics↗

arXiv2022

50-W average power Ho:YAG SESAM-modelocked thin-disk oscillator at 2.1 um

Ultrafast laser systems operating with high-average power in the wavelength range from 1.9 um to 3 um are of interest for a wide range of applications for example in spectroscopy, material processing and as drivers for secondary sources in the XUV spectral region. In this area, laser systems based on holmium-doped gain materials directly emitting at 2.1 um have made significant progress over the past years, however so far only very few results were demonstrated in power-scalable high-power laser geometries. In particular, the thin-disk geometry is promising for directly modelocked oscillators with high average power levels that are comparable to amplifier systems at MHz repetition rate. In this paper, we demonstrate Semiconductor Saturable Absorber Mirror (SESAM) modelocked Ho:YAG thin-disk lasers (TDLs) emitting at 2.1 um wavelength with record-holding performance levels. In our highest average power configuration, we reach 50 W of average power, with 1.13 ps pulses, 2.11 uJ of pulse energy and ~1.9 MW of peak power. To the best of our knowledge, this represents the highest average power, as well as the highest output pulse energy so far demonstrated from a modelocked laser in the 2 um wavelength region. This record performance level was enabled by the recent development of high-power GaSb-based SESAMs with low loss, adapted for high intracavity power and pulse energy. We also explore the limitations in terms of reaching shorter pulse durations at high power with this gain material in the disk geometry and using SESAM modelocking, and present first steps in this direction, with the demonstration of 30 W of output power, with 692 fs pulses in another laser configuration.

physics.optics↗

arXiv2021

Moving towards high-power thin-disk lasers in the 2-{\mu}m wavelength range

Thin-disk lasers (TDLs) have made spectacular progress in the last decades both in continuous-wave and ultrafast operation. Nowadays, single thin-disk oscillators with > 16 kW of continuous-wave (CW)-power have been demonstrated and ultrafast amplifiers have largely surpassed the kilowatt milestone with pulse energies in the multi-100 mJ range. This amazing development has been demonstrated in the 1-{\mu}m wavelength range, using Yb-doped materials and supported by industrially available components. Motivated by both strong scientific and industrial applications, interest in expanding this performance to longer wavelength regions continues to increase. In particular, TDLs emitting directly in the short-wave mid-infrared (SW-MIR) region (2-3 {\mu}m) are especially sought after, and although many early studies have been reported, most remained in the proof-of-principle stage and the potential for multi-100-W operation remained undemonstrated. Here, we report on our recent results of a single fundamental-mode CW Ho:YAG thin-disk oscillator with >100 W of power, surpassing previous single-mode TDLs by a factor of >4, and marking a first milestone in the development of high-power SW-MIR TDLs. In optimized conditions, our laser system emitting at {\guillemotright} 2.1 {\mu}m reaches an output power of 112 W with 54.6-% optical-to-optical efficiency and an M2 = 1.1. This system is ideally suited for future direct modelocking at the 100 W level, as well as for ultrafast amplification. We start the discussion with a review of the state-of-the-art of TDLs emitting directly in the vicinity of 2 {\mu}m, and then discuss difficulties and possible routes both towards ultrafast operation and next possible steps for power scaling.

physics.optics↗

arXiv2020

Domain wall induced spin-polarized flat bands in antiferromagnetic topological insulators

A flat band in fermionic system is a dispersionless single-particle state with a diverging effective mass and nearly zero group velocity. These flat bands are expected to support exotic properties in the ground state, which might be important for a wide range of promising physical phenomena. For many applications it is highly desirable to have such states in Dirac materials, but so far they have been reported only in non-magnetic Dirac systems. In this work we propose a realization of topologically protected spin-polarized flat bands generated by domain walls in planar magnetic topological insulators. Using first-principles material design we suggest a family of intrinsic antiferromagnetic topological insulators with an in-plane sublattice magnetization and a high N\'eel temperature. Such systems can host domain walls in a natural manner. For these materials, we demonstrate the existence of spin-polarized flat bands in the vicinity of the Fermi level and discuss their properties and potential applications.

cond-mat.mes-hall↗

arXiv2018

First-principles investigations of the magnetic phase diagram of Gd$_{1-x}$Ca$_{x}$MnO$_{3}$

We studied for the first time the magnetic phase diagram of the rare-earth manganites series Gd$_{1-x}$Ca$_{x}$MnO$_{3}$ (GCMO) over the full concentration range based on density functional theory. GCMO has been shown to form solid solutions. We take into account this disordered character by adapting special quasi random structures at different concentration steps. The magnetic phase diagram is mainly described by means of the magnetic exchange interactions between the Mn sites and Monte Carlo simulations were performed to estimate the corresponding transition temperatures. They agree very well with recent experiments. The hole doped region $x<0.5$ shows a strong ferromagnetic ground state, which competes with A-type antiferromagnetism at higher Ca concentrations $x>0.6$.

cond-mat.mtrl-sci↗

arXiv2018

Unique thickness-dependent properties of the van der Waals interlayer antiferromagnet $\mathrm{MnBi_2Te_4}$ films

Using density functional theory and Monte Carlo calculations, we study the thickness dependence of the magnetic and electronic properties of a van der Waals interlayer antiferromagnet in the two-dimensional limit. Considering $\mathrm{MnBi_2Te_4}$ as a model material, we find it to demonstrate a remarkable set of thickness-dependent magnetic and topological transitions. While a single septuple layer block of $\mathrm{MnBi_2Te_4}$ is a topologically trivial ferromagnet, the thicker films made of an odd (even) number of blocks are uncompensated (compensated) interlayer antiferromagnets, which show wide bandgap quantum anomalous Hall (zero plateau quantum anomalous Hall) states. Thus, $\mathrm{MnBi_2Te_4}$ is the first stoichiometric material predicted to realize the zero plateau quantum anomalous Hall state intrinsically. This state has been theoretically shown to host the exotic axion insulator phase.

cond-mat.mtrl-sci↗

arXiv2018

Prediction and observation of the first antiferromagnetic topological insulator

Magnetic topological insulators (MTIs) are narrow gap semiconductor materials that combine non-trivial band topology and magnetic order. Unlike their nonmagnetic counterparts, MTIs may have some of the surfaces gapped due to breaking the time-reversal symmetry, which enables a number of exotic phenomena having potential applications in spintronics. So far, MTIs have only been created by means of doping nonmagnetic TIs with 3d transition metal elements, however, such an approach leads to strongly inhomogeneous magnetic and electronic properties of these materials, restricting the observation of important effects to very low temperatures. Finding intrinsic MTI, i.e. a stoichiometric well-ordered magnetic compound, could be an ideal solution to these problems, but no such material was observed to date. Here, using density functional theory we predict and further confirm by means of structural, transport, magnetic, angle- and spin-resolved photoemission spectroscopy measurements the realization of the antiferromagnetic (AFM) TI phase, that is hosted by the van der Waals layered compound MnBi$_2$Te$_4$. An interlayer AFM ordering makes MnBi$_2$Te$_4$ invariant with respect to the combination of the time-reversal ($\Theta$) and primitive-lattice translation ($T_{1/2}$) symmetries, $S = \Theta T_{1/2}$, giving rise to the $Z_2$ topological classification of AFM insulators. We find $Z_2 = 1$ for MnBi$_2$Te$_4$, which confirms its topologically nontrivial nature. The $S$-breaking (0001) surface of MnBi$_2$Te$_4$ exhibits a giant bandgap in the topological surface state as evidenced by ab initio calculations and photoemission measurements. These results culminate almost a decade-long search of an AFMTI, predicted in 2010. Furthermore, MnBi$_2$Te$_4$ is the first intrinsic magnetic TI realized experimentally.

cond-mat.mtrl-sci↗

arXiv2018

Tuning the probability of defect formation via substrate strains in Sr$_2$FeMoO$_6$ films

Since oxide materials like Sr$_2$FeMoO$_6$ are usually applied as thin films, we studied the effect of biaxial strain, resulting from the substrate, on the electronic and magnetic properties and, in particular, on the formation energy of point defects. From our first-principles calculations, we determined that the probability of forming point defects - like vacancies or substitutions - in Sr$_2$FeMoO$_6$ could be adjusted by choosing a proper substrate. For example, the amount of anti-site disorder can be reduced with compressive strain in order to obtain purer Sr$_2$FeMoO$_6$ as needed for spintronic applications, while the formation of oxygen vacancies is more likely for tensile strain, which improves the functionality of Sr$_2$FeMoO$_6$ as a basis material of solid oxide fuel cells. In addition, we were also be able to include the oxygen partial pressure in our study by using its thermodynamic connection with the chemical potential. Strontium vacancies become for example more likely than oxygen vacancies at a pressure of 1$\,$bar. Hence, this degree of freedom might offer in general another potential method for defect engineering in oxides besides, e.g., experimental growth conditions like temperature or gas pressure.

cond-mat.mtrl-sci↗

arXiv2018

The Power of Color: A Study on the Effective Use of Colored Light in Human-Robot Interaction

In times of more and more complex interaction techniques, we point out the powerfulness of colored light as a simple and cheap feedback mechanism. Since it is visible over a distance and does not interfere with other modalities, it is especially interesting for mobile robots. In an online survey, we asked 56 participants to choose the most appropriate colors for scenarios that were presented in the form of videos. In these scenarios a mobile robot accomplished tasks, in some with success, in others it failed because the task is not feasible, in others it stopped because it waited for help. We analyze in what way the color preferences differ between these three categories. The results show a connection between colors and meanings and that it depends on the participants' technical affinity, experience with robots and gender how clear the color preference is for a certain category. Finally, we found out that the participants' favorite color is not related to color preferences.

cs.RO↗

arXiv2018

SwarmRob: A Toolkit for Reproducibility and Sharing of Experimental Artifacts in Robotics Research

Due to the complexity of robotics, the reproducibility of results and experiments is one of the fundamental problems in robotics research. While the problem has been identified by the community, the approaches that address the problem appropriately are limited. The toolkit proposed in this paper tries to deal with the problem of reproducibility and sharing of experimental artifacts in robotics research by a holistic approach based on operating-system-level virtualization. The experimental artifacts of an experiment are isolated in "containers" that can be distributed to other researchers. Based on this, this paper presents a novel experimental workflow to describe, execute and distribute experimental software-artifacts to heterogeneous robots dynamically. As a result, the proposed solution supports researchers in executing and reproducing experimental evaluations.

cs.RO↗