Amplification-free GW-level, 150†W, 14†MHz, and 8†fs thin-disk laser based on compression in multipass cells.

We report an amplification-free thin-disk laser system delivering 0.9 GW peak power. The 120 fs pulses, at 14 MHz, centered around 1 µm, containing 12.8 µJ delivered by a thin-disk oscillator, were compressed by factor 15 down to 8.0 fs with 148 W average output power and overall 82% efficiency. Additionally, we showed that even a sub-two-cycle operation with 6.2 fs can be reached with this technology. The system will be a crucial part of the XUV frequency comb being developed and a unique high-repetition rate driver for attosecond pulse generation.

Free-space quasi-phase matching.

We report a new, to the best of our knowledge, approach to phase matching of nonlinear materials based on the free-space multipass cells. This technique is applicable to noncentrosymmetric nonlinear crystals, including crystals that cannot be birefringent phase-matched or quasi-phase matched by periodic poling. Notably, by using this approach, the crystalline quartz is quasi-phase matched with the demonstrated increase of the second harmonic generation efficiency by a factor of 40. The method can be extended toward UV and THz ranges. This promises to revolutionize experimental nonlinear optics and all applications by increasing the number of available crystals for quasi-phase matching by at least one order of magnitude and brings fresh motivation for developing novel nonlinear materials.

110†MW thin-disk oscillator.

A compact Kerr-lens mode-locked thin-disk oscillator reproducibly delivering 110 MW output peak power, the highest among all oscillators, is reported. This simple and stable femtosecond oscillator delivering a unique combination of high average power (202 W) and peak power, is an ideal driver and an important milestone for the development of extreme ultraviolet transportable frequency comb sources.

Spectral broadening in convex-concave multipass cells.

Since its first demonstration in 2016, the multi-pass spectral broadening technique has covered impressive ranges of pulse energy (3 µJ - 100 mJ) and peak power (4 MW - 100 GW). Energy scaling of this technique into the joule-level is currently limited by phenomena such as optical damage, gas ionization and spatio-spectral beam inhomogeneity. These limitations can be overcome by the novel multi-pass convex-concave arrangement, which exhibits crucial properties such as large mode size and compactness. In a proof-of-principle experiment, 260 fs, 15 µJ and 200 µJ pulses are broadened and subsequently compressed to approximately 50 fs with 90% efficiency and excellent spatio-spectral homogeneity across the beam profile. We simulate the proposed concept for spectral broadening of 40 mJ and 1.3 ps input pulses and discuss the possibility of further scaling.

Few-cycle pulse compression and white light generation in cascaded multipass cells.

We report supercontinuum generation and pulse compression in two stacked multipass cells based on dielectric mirrors. The 230 fs pulses at 1 MHz containing 12 µJ are compressed by a factor of 33 down to 7 fs, corresponding to 1.0 GW peak power and overall transmission of 84%. The source is particularly interesting for such applications as time-resolved angle-resolved photoemission spectroscopy (ARPES), photoemission electron microscopy, and nonlinear spectroscopy.

Dual-comb thin-disk oscillator.

Dual-comb spectroscopy (DCS) normally operates with two independent, relatively low power and actively synchronized laser sources. This hinders the wide adoption for practical implementations and frequency conversion into deep UV and VUV spectral ranges. Here, we report a fully passive, high power dual-comb laser based on thin-disk technology and its application to direct frequency comb spectroscopy. The peak power (1.2 MW) and the average power (15 W) of our Yb:YAG thin-disk dual-comb system are more than one-order-of-magnitude higher than in any previous systems. The scheme allows easy adjustment of the repetition frequency difference during operation. Both combs share all cavity components which leads to an excellent mutual stability. A time-domain signal recorded over 10 ms without any active stabilization was sufficient to resolve individual comb lines after Fourier transformation.

Scalable high-repetition-rate sub-half-cycle terahertz pulses from spatially indirect interband transitions.

Intense phase-locked terahertz (THz) pulses are the bedrock of THz lightwave electronics, where the carrier field creates a transient bias to control electrons on sub-cycle time scales. Key applications such as THz scanning tunnelling microscopy or electronic devices operating at optical clock rates call for ultimately short, almost unipolar waveforms, at megahertz (MHz) repetition rates. Here, we present a flexible and scalable scheme for the generation of strong phase-locked THz pulses based on shift currents in type-II-aligned epitaxial semiconductor heterostructures. The measured THz waveforms exhibit only 0.45 optical cycles at their centre frequency within the full width at half maximum of the intensity envelope, peak fields above 1.1 kV cm-1 and spectral components up to the mid-infrared, at a repetition rate of 4 MHz. The only positive half-cycle of this waveform exceeds all negative half-cycles by almost four times, which is unexpected from shift currents alone. Our detailed analysis reveals that local charging dynamics induces the pronounced positive THz-emission peak as electrons and holes approach charge neutrality after separation by the optical pump pulse, also enabling ultrabroadband operation. Our unipolar emitters mark a milestone for flexibly scalable, next-generation high-repetition-rate sources of intense and strongly asymmetric electric field transients.

Multipass spectral broadening and compression in the green spectral range.

Multipass spectral broadening and compression around 515 nm are experimentally demonstrated. A nonlinear multipass cell with a bulk medium is used to compress 250-fs pulses down to 38 fs. The same input pulses create a sufficient bandwidth for sub-20-fs pulse generation in a multipass cell with gaseous media. In both cases, the efficiency exceeds 85%. Dispersion management by reduction of the cell size and the thickness of the nonlinear medium allows an efficient generation of ultrashort pulses in the visible range and establishes a pathway for ultraviolet spectral broadening by means of multipass cells.

Field-resolved infrared spectroscopy of biological systems.

The proper functioning of living systems and physiological phenotypes depends on molecular composition. Yet simultaneous quantitative detection of a wide variety of molecules remains a challenge1-8. Here we show how broadband optical coherence opens up opportunities for fingerprinting complex molecular ensembles in their natural environment. Vibrationally excited molecules emit a coherent electric field following few-cycle infrared laser excitation9-12, and this field is specific to the sample's molecular composition. Employing electro-optic sampling10,12-15, we directly measure this global molecular fingerprint down to field strengths 107 times weaker than that of the excitation. This enables transillumination of intact living systems with thicknesses of the order of 0.1 millimetres, permitting broadband infrared spectroscopic probing of human cells and plant leaves. In a proof-of-concept analysis of human blood serum, temporal isolation of the infrared electric-field fingerprint from its excitation along with its sampling with attosecond timing precision results in detection sensitivity of submicrograms per millilitre of blood serum and a detectable dynamic range of molecular concentration exceeding 105. This technique promises improved molecular sensitivity and molecular coverage for probing complex, real-world biological and medical settings.

Intra-pulse difference-frequency generation of mid-infrared (2.7-20 µm) by random quasi-phase-matching.

We present a mid-infrared (MIR) source based on intra-pulse difference-frequency generation under the random quasi-phase-matching condition. The scheme enables the use of non-birefringent materials whose crystal orientations are not perfectly and periodically poled, widening the choice of media for nonlinear frequency conversion. With a 2 µm driving source based on a Ho:YAG thin-disk laser, together with a polycrystalline ZnSe element, an octave-spanning MIR continuum (2.7-20 µm) was generated. At over 20 mW, the average power is comparable to regular phase-matching in birefringent crystals. A 1 µm laser system based on a Yb:YAG thin-disk laser was also tested as a driving source in this scheme. The new approach provides a simplified way for generating coherent MIR radiation with an ultrabroad bandwidth at reasonable efficiency.

All-solid-state multipass spectral broadening to sub-20 fs.

In this work, we present a nonlinear spectral broadening and compression scheme based on self-phase modulation in bulk media inside a Herriott-type multipass cell. With this reliable approach, we achieved a spectral broadening factor of 22 while maintaining an efficiency of over 60% at an average input power of 100 W, and an excellent output beam quality with M2=1.2. The output pulses were compressed to 18 fs, with the broadest spectrum supporting a Fourier-transform limit of 10 fs. The high efficiency and approximately four-optical-cycle pulse duration mark an important milestone towards the realization of a compact, high power oscillator-based driver for XUV frequency combs and other nonlinear processes.

Efficient High-Power Ultrashort Pulse Compression in Self-Defocusing Bulk Media.

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.

Kerr effect in multilayer dielectric coatings.

We report the utilization of the optical Kerr effect in multilayer dielectric coatings, previously discussed only theoretically. We present the design and realization of multilayer dielectric optical structures with layer-specific Kerr nonlinearities, which permit tailoring of the intensity-dependent effects. The modulation depth in reflectance reaches up to 6% for the demonstrated examples of dielectric nonlinear multilayer coatings. We show that the nonlinearity is based on the optical Kerr effect, with the recovery time faster than the laser pulse envelope of 1 ps. Due to high flexibility in design, the reported dielectric nonlinear multilayer coatings have the potential to open hitherto unprecedented possibilities in nonlinear optics and ultrafast laser applications.

Highly-dispersive mirrors reach new levels of dispersion.

A highly-dispersive mirror with the unprecedented group delay dispersion of -10000 fs2 in the wavelength range of 1025-1035 nm is reported. Reproducible production of a coating with such a high dispersion was possible due to the recently developed robust synthesis technique. Successful employment of the new highly-dispersive mirror in an oscillator is demonstrated.