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Optics letters 48(4), 984 (2023). doi:10.1364/OL.478790, Published by OSA, Washington, DC

Light-matter strong coupling in the vacuum limit has been shown to enhance material properties over the past decade. Oxide nanoparticles are known to exhibit weak ferromagnetism due to vacancies in the lattice. Here we report the 700-fold enhancement of the ferromagnetism of YBa$_2$Cu$_3$O$_{7-x}$ nanoparticles under cooperative strong coupling at room temperature. The magnetic moment reaches 0.90 $\mu_{\rm B}$/mol, and with such a high value, it competes with YBa$_2$Cu$_3$O$_{7-x}$ superconductivity at low temperature. This strong ferromagnetism at room temperature suggest that strong coupling is a new tool for the development of next generations of magnetic and spintronic nanodevices., Comment: 24 pages, 4 figures - difference with v1 version: revised Supplementary Information file

Frequency combs present a unique tool for high-precision and rapid molecular spectroscopy. Difference frequency generation (DFG) of near-infrared sources is a common approach to generate passively stabilized mid-infrared combs. However, only little attention has been paid so far to precisely measure the coherence properties of such sources. Here, we investigate these using a Raman-soliton based DFG source driven by an Yb:fiber frequency comb. A heterodyne beat between the second harmonic of the phase-locked DFG comb near 4 µm and a 2 µm Tm:fiber frequency comb locked to the same optical reference is performed. Using this method, we measure the relative phase noise power spectral density of both combs. This results in a sub-Hz relative linewidth between the DFG comb and the Tm:fiber comb. We also introduce a new pump/seed delay locking mechanism based on interferometry for long-term stable intensity noise suppression.

We discuss basic pulse energy scaling principles for multi-pass cells (MPCs) and introduce a novel multi-pass cell type which we employ to numerically predictpost-compression of 125 mJ pulses using a 2 m setup.

Vibrational strong coupling (VSC) has recently emerged as a completely new tool for influencing chemical reactivity. It harnesses electromagnetic vacuum fluctuations through the creation of hybrid states of light and matter, called polaritonic states, in an optical cavity resonant to a molecular absorption band. Here, we investigate the effect of vibrational strong coupling of water on the enzymatic activity of pepsin, where a water molecule is directly involved in the enzyme's chemical mechanism. We observe an approximately 4.5‐fold decrease of the apparent second‐order rate constant kcat/Km when coupling the water stretching vibration, whereas no effect was detected for the strong coupling of the bending vibration. The possibility of modifying enzymatic activity by coupling water demonstrates the potential of VSC as a new tool to study biochemical reactivity., Angewandte Chemie. International Edition, 58 (43), ISSN:1433-7851, ISSN:1521-3773, ISSN:0570-0833

Modern X-ray Free Electron Lasers (XFEL) enable a variety of scientific applications [1] . For driving their electron gun photocathode, they require a robust deep ultraviolet (DUV) laser source. To enable the capability for generating both 1 ps electron bunches for few femtoseconds, single spike self-amplified stimulated emission (SASE) X-ray pulses and high charge 20 ps electron bunches for standard operation, we demonstrate a 257.5 nm photocathode laser with widely tunable output pulse durations. The schematic of the set-up is shown in Fig. 1 . The laser front-end consists of a pm-Yb-fiber oscillator and 3-stage amplifier, which generates a 20 mW, 1 MHz pulse train at 1030 nm center wavelength to seed a 4-stage Yb:YAG amplifier chain. An additional nonlinear Yb-fiber amplifier generates 100 mW, 150 fs for cross-correlation reference [2] . The Yb:YAG amplifier chain boosts the laser pulse energy to more than 180 µJ. The front-end contains an additional fiber stretcher and a programmable high-resolution spectral shaper for tuning the amplifier output spectrum. This allows after passing a fixed dispersion transmissive grating compressor to tune the NIR output pulse duration between 1 ps and 20 ps.

We demonstrate a compact, bulk multi-pass cell with high-transmission, for compressing the output of a 30W, 250fs Ytterbium source to 31fs with a resulting peak power of 2.5GW.

We introduce the combination of multi-pass cell and multi-plate spectral broadening. We demonstrate the compression of 110-μJ pulses from 900-fs to 60-fs in a single stage and report broadening to 38-fs transform-limit by nonlinear mode-matching.

Broadband optical spectra are employed in ultrafast time-resolved studies, seeding of tunable parametric amplifiers, and selectively addressing transitions in absorption spectroscopy. A strong extension of spectral bandwidth is commonly achieved in nonlinear fiber by means of self-phase modulation. In terms of energy scalability and achievable bandwidth, the method is, however, limited by the damage threshold of the core material. Here, we study spectral broadening in four different single-mode normal dispersive photonic crystal fibers length of 8–10 cm. They are pumped by a thin-disk oscillator emitting 250 fs pulses at peak powers close to the critical power of silica. We demonstrate mode-field diameter-dependent broadening factors from 19 to 51, which are obtained at power spectral densities from 200 to 5 mW/nm. We explain the results by a relation between peak power and broadening factor. This will serve as a fiber selection guideline transferable to lasers emitting at wavelengths different from 1030 nm. In addition, we examine to which extend prechirp leads to energy scalability of spectral broadening in solid-core fiber. By contrast to previous reports, we point out that the applicability of the technique is strongly limited through the requirement of bell-shaped input pulses and the pulse duration dependent material damage threshold.

We demonstrate post-compression of 1.2 ps pulses to the few-cycle regime via multi-pass spectral broadening. We achieve compression factors of 40 via single and >90 via dual stage compression employing mJ pulses.

9th EPS-QEOD Europhoton Conference on Solid-State, Fibre, and Waveguide Coherent Light Sources, EUROPHOTON 2020, Online, Online, 30 Aug 2020 - 4 Sep 2020; The European physical journal / Web of Conferences 243, 21002 (2020). doi:10.1051/epjconf/202024321002, Published by EDP Sciences, Les Ulis

We report post-compression of 1.2 ps pulses into the few-cycle regime via multi-pass spectral broadening. We achieve compression factors of 40 in single and 93 in a dual stage scheme using a compact setup.

Nonlinear optical responses provide a powerful way to understand the microscopic interactions between laser fields and matter. They are critical for plenty of applications, such as in lasers, integrated photonic circuits, biosensing and medical tools. However, most materials exhibit weak optical nonlinearities or long response times when they interact with intense optical fields. Here, we strongly couple the exciton of cyanine dye J-aggregates to an optical mode of a Fabry-Perot (FP) cavity, and achieve an enhancement of the complex nonlinear refractive index by two orders of magnitude compared with that of the uncoupled condition. Moreover, the coupled system shows an ultrafast response of ~120 fs that we extract from optical cross-correlation measurements. The ultrafast and large enhancement of the optical nonlinar coefficients in this work paves the way for exploring strong coupling effects on various third-order nonlinear optical phenomena and for technological applications., Nature Communications, 12 (1), ISSN:2041-1723

The first chapter has pointed out important breakthroughs of solid-state laser architectures. In particular Ti:sapph lasers have revolutionized frequency metrology and ultrafast optics. The introduction has also emphasized the potential of the thin-disk technology to become the foundation of the upcoming power-scalable laser oscillator generation.

Three application ideas are derived from the knowledge and the achievements which have been accomplished through the experiments and simulations presented in the Chaps. 2– 4 of this thesis. The author expects that each of them could be realized on the time scale of about one year.

The previous chapters of this dissertation have demonstrated the progress in the development of KLM TD oscillators towards waveform control and few-cycle operation. So-far all research was concentrated on the optical octave from about 700–1400 nm.

This chapter of the dissertation will discuss the average and peak power scalability of the achievements described in Chap. 2, i.e. few-cycle pulse generation and CEP stabilization. In the thesis’ introduction, this has been proclaimed as the general goal of current femtosecond laser development. However, there are often application dependent preferences in scaling either pulse energy or repetition rate, rather than both. A few examples shall be sketched to enable a more specific discussion of the results presented in the ensuing sections.

In this work, we demonstrate postcompression of 1.2 ps laser pulses to 13 fs via gas-based multipass spectral broadening. Our results yield a single-stage compression factor of about 40 at 200 W in-burst average power and a total compression factor > 90 at reduced power. The employed scheme represents a route toward compact few-cycle sources driven by industrial-grade Yb:YAG lasers at high average power.

One-micrometer wavelength ultrafast laser emission is transformed to a powerful tool for ultrabroadband mid-infrared spectroscopy., Spectroscopy in the wavelength range from 2 to 11 μm (900 to 5000 cm−1) implies a multitude of applications in fundamental physics, chemistry, as well as environmental and life sciences. The related vibrational transitions, which all infrared-active small molecules, the most common functional groups, as well as biomolecules like proteins, lipids, nucleic acids, and carbohydrates exhibit, reveal information about molecular structure and composition. However, light sources and detectors in the mid-infrared have been inferior to those in the visible or near-infrared, in terms of power, bandwidth, and sensitivity, severely limiting the performance of infrared experimental techniques. This article demonstrates the generation of femtosecond radiation with up to 5 W at 4.1 μm and 1.3 W at 8.5 μm, corresponding to an order-of-magnitude average power increase for ultrafast light sources operating at wavelengths longer than 5 μm. The presented concept is based on power-scalable near-infrared lasers emitting at a wavelength near 1 μm, which pump optical parametric amplifiers. In addition, both wavelength tunability and supercontinuum generation are reported, resulting in spectral coverage from 1.6 to 10.2 μm with power densities exceeding state-of-the-art synchrotron sources over the entire range. The flexible frequency conversion scheme is highly attractive for both up-conversion and frequency comb spectroscopy, as well as for a variety of time-domain applications.

A compact source that generates sub-two-cycle-duration pulses with an average power of 0.1 W spanning 6.8–16.4 μm combines the properties of power scalability, high repetition rate and phase coherence for the first time in this spectral region. Powerful coherent light with a spectrum spanning the mid-infrared (MIR) spectral range is crucial for a number of applications in natural as well as life sciences, but so far has only been available from large-scale synchrotron sources1. Here we present a compact apparatus that generates pulses with a sub-two-cycle duration and with an average power of 0.1 W and a spectral coverage of 6.8–16.4 μm (at −30 dB). The demonstrated source combines, for the first time in this spectral region, a high power, a high repetition rate and phase coherence. The MIR pulses emerge via difference-frequency generation (DFG) driven by the nonlinearly compressed pulses of a Kerr-lens mode-locked ytterbium-doped yttrium–aluminium–garnet (Yb:YAG) thin-disc oscillator. The resultant 100 MHz MIR pulse train is hundreds to thousands of times more powerful than state-of-the-art frequency combs that emit in this range2,3,4, and offers a high dynamic range for spectroscopy in the molecular fingerprint region4,5,6,7 and an ideal prerequisite for hyperspectral imaging8 as well as for the time-domain coherent control of vibrational dynamics9,10,11.

We report a 2-octave mid-infrared continuum simultaneously spanning from 500 cm-1 to 2250 cm-1 at 24 mW of average power. It is based on difference frequency generation driven by a newly developed femtosecond Ho:YAG thin-disk oscillator.

The bandwidth of gain materials for high-power femtosecond lasers, mainly Yb:YAG, is fairly limited, allowing only > 100 fs long pulses to be generated efficiently in oscillators [1] or near ps pulses from amplifiers [2]. Therefore, additional pulse-compression, commonly employing solid or gas-filled fibres, is necessary to permit e.g. compact, MHz XUV sources [3] or brilliant mid-infrared (MIR) sources for spectroscopy applications [4]. The use of bulk materials offers some distinct advantages over fiber-based spectral broadening such as insensitivity to coupling or alignment as well as low cost.

High peak power, few cycle pulses at 2 um are highly sought after for their ability to generate ultrabroad, passively CEP stabilized continua in the mid-infrared (IR) [1] using highly nonlinear non-oxide crystals, which promises numerous applications in physics, environmental and life sciences. Recently, a Kerr-lens mode-locked Ho:YAG thin disk (based on TRUMPF technology) oscillator has directly generated high power femtosecond pulses at 2.1 micron [2], and the output can be further compressed for applications requiring few-cycle pulses. Soliton self-compression is an elegant scheme that can deliver such pulses directly from the fiber, and has previously been applied to pulse compression of a Yb:YAG thin-disk oscillator [3] and to ∼2 micron fiber sources [4,5]. Here we demonstrate the compression of 260 fs pulses down to 15 fs using a silica glass fiber, delivering, to the best of our knowledge, the shortest pulses from any holmium or thulium-based laser system.

Peak and average power scalability is the key feature of advancing femtosecond laser technology. Today, near-infrared light sources are capable of providing hundreds of Watts of average power. These sources, however, scarcely deliver pulses shorter than 100 fs which are, for instance, highly beneficial for frequency conversion to the extreme ultraviolet or to the mid- infrared. Therefore, the development of power scalable pulse compression schemes is still an ongoing quest. This article presents the compression of 90 W average power, 190 fs pulses to 70 W, 30 fs. An increase in peak power from 18 MW to 60 MW is achieved. The compression scheme is based on cascaded phase-mismatched quadratic nonlinearities in BBO crystals. In addition to the experimental results, simulations are presented which compare spatially resolved spectra of pulses spectrally broadened in self-focusing and self-defocusing media, respectively. It is demonstrated that balancing self- defocusing and Gaussian beam convergence results in an efficient, power-scalable spectral broadening mechanism in bulk material.

9th EPS-QEOD Europhoton Conference on Solid-State, Fibre, and Waveguide Coherent Light Sources, EUROPHOTON 2020, Online, Online, 30 Aug 2020 - 4 Sep 2020; The European physical journal / Web of Conferences 243, 21001 (2020). doi:10.1051/epjconf/202024321001, Published by EDP Sciences, Les Ulis

1.3 W of average power at 8.5 μm central wavelength are generated through collinear optical parametric amplification in LiGaSt The crystal is pumped by a 45 W femtosecond oscillator operating at 1.03 μm wavelength. The seed is generated by a normally dispersive photonic crystal fibre. This ensures passive carrier-envelope-offset stability of the idler. The concept is expected to be peak and average power scalable, opening an exciting perspective towards applications in spectroscopy of molecular fingerprints as well as in high field physics.

Spectral broadening in bulk material is a simple, robust and low-cost method to extend the bandwidth of a laser source. Consequently, it enables ultrashort pulse compression. Experiments with a 38 MHz repetition rate, 50 W average power Kerr-lens mode-locked thin-disk oscillator were performed. The initially 1.2 μJ, 250 fs pulses are compressed to 43 fs by means of self-phase modulation in a single 15 mm thick quartz crystal and subsequent chirped-mirror compression. The losses due to spatial nonlinear effects are only about 40 %. A second broadening stage reduced the Fourier transform limit to 15 fs. It is shown that the intensity noise of the oscillator is preserved independent of the broadening factor. Simulations manifest the peak power scalability of the concept and show that it is applicable to a wide range of input pulse durations and energies.

450 mW of average power at 8.5 μm central wavelength are generated through optical parametric amplification in LiGaS 2 . The crystal is pumped by a mode-locked thin-disk oscillator and seeded by the output of a photonic crystal fiber.

Waveform-stabilized laser pulses have revolutionized the exploration of the electronic structure and dynamics of matter by serving as the technological basis for frequency-comb and attosecond spectroscopy. Their primary sources, mode-locked titanium-doped sapphire lasers and erbium/ytterbium-doped fibre lasers, deliver pulses with several nanojoules energy, which is insufficient for many important applications. Here we present the waveform-stabilized light source that is scalable to microjoule energy levels at the full (megahertz) repetition rate of the laser oscillator. A diode-pumped Kerr-lens-mode-locked Yb:YAG thin-disk laser combined with extracavity pulse compression yields waveform-stabilized few-cycle pulses (7.7 fs, 2.2 cycles) with a pulse energy of 0.15 μJ and an average power of 6 W. The demonstrated concept is scalable to pulse energies of several microjoules and near-gigawatt peak powers. The generation of attosecond pulses at the full repetition rate of the oscillator comes into reach. The presented system could serve as a primary source for frequency combs in the mid infrared and vacuum UV with unprecedented high power levels., Frequency combs have revolutionized the study of electronic structures and dynamics of matter but currently used lasers systems are limited in terms of achievable pulse energies. Here, Pronin et al. demonstrate few cycle pulse emission from a thin-disk laser with 150 nJ pulse energy and 7.7 fs pulse duration.

Compression of 250-fs, 1-μJ pulses from a KLM Yb:YAG thin-disk oscillator down to 9.1 fs is demonstrated. A kagomé-PCF with a 36-μm core-diameter is used with a pressure gradient from 0 to 40 bar of krypton. Compression to 22 fs is achieved by 1200 fssup2/supgroup-delay-dispersion provided by chirped mirrors. By coupling the output into a second kagomé-PCF with a pressure gradient from 0 to 25 bar of argon, octave spanning spectral broadening via the soliton-effect is observed at 18-W average output power. Self-compression to 9.1 fs is measured, with compressibility to 5 fs predicted. Also observed is strong emission in the visible via dispersive wave generation, amounting to 4% of the total output power.

The demand for and usage of broadband coherent mid-infrared sources, such as those provided by synchrotron facilities, are growing. Since most organic molecules exhibit characteristic vibrational modes in the wavelength range between 500 and 4000 cm−1, such broadband coherent sources enable micro- or even nano-spectroscopic applications at or below the diffraction limit with a high signal-to-noise ratio1, 2, 3. These techniques have been applied in diverse fields ranging from life sciences, material analysis, and time-resolved spectroscopy. Here we demonstrate a broadband, coherent and intrinsically carrier-envelope-phase-stable source with a spectrum spanning from 500 to 2250 cm−1 (−30 dB) at an average power of 24 mW and a repetition rate of 77 MHz. This performance is enabled by the first mode-locked thin-disk oscillator operating at 2 μm wavelength, providing a tenfold increase in average power over femtosecond oscillators previously demonstrated in this wavelength range4. Multi-octave spectral coverage from this compact and power-scalable system opens up a range of time- and frequency-domain spectroscopic applications.

3.4-W average power at 4.1-μm wavelength is generated through optical parametric amplification in lithium niobate. The crystal is directly pumped by a mode-locked thin-disk oscillator and seeded with a continuum from an all-normal dispersion fiber.

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We describe an Yb:YAG KLM thin-disk oscillator with enhanced nonlinearity directly providing 140 fs 160 W. This output is successfully compressed down to 30 fs in a scalable crystalline bulk-material setup, without need for fibers.

The concept of distributed Kerr-lens mode-locking and a thin-disk Yb:YAG oscillator based on this concept are presented. The described oscillator directly generates pulses with a duration of 49 fs and spectral width of 33 nm

Spectral broadening of 1.3-μJ, 250-fs pulses in crystalline quartz is presented. Pulse compression to 43-fs was achieved by mans of broadening in a single 15-mm crystal. A second stage reduced the transform limit to 15-fs.

0.9-W average power at 4.1-μm wavelength is generated through optical parametric amplification in lithium niobate. The crystal is directly pumped by a mode-locked thin-disk oscillator and seeded with a continuum from an all-normal dispersion fiber.

The goal of this contribution is to provide a guideline for Kerr-lens mode-locking (KLM) of thin-disk oscillators. This includes cavity design, hard and soft-aperture optimization, handling of thermal effects in intra-cavity optics as well as methods of average power and energy scaling. The main differences and similarities between mode-locking of Ti:sapphire bulk and Yb:YAG thin-disk oscillators are presented.