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

Femtosecond electron beam probe of ultrafast electronics

The need for ever-faster information processing requires exceptionally small devices that operate at frequencies approaching the terahertz and petahertz regimes. For the diagnostics of such devices, researchers need a spatiotemporal tool that surpasses the device under test in speed and spatial resolution. Consequently, such a tool cannot be provided by electronics itself. Here we show how ultrafast electron beam probe with terahertz-compressed electron pulses can directly sense local electro-magnetic fields in electronic devices with femtosecond, micrometre and millivolt resolution under normal operation conditions. We analyse the dynamical response of a coplanar waveguide circuit and reveal the impulse response, signal reflections, attenuation and waveguide dispersion directly in the time domain. The demonstrated measurement bandwidth reaches 10 THz and the sensitivity to electric potentials is tens of millivolts or -20 dBm. Femtosecond time resolution and the capability to directly integrate our technique into existing electron-beam inspection devices in semiconductor industry makes our femtosecond electron beam probe a promising tool for research and development of next-generation electronics at unprecedented speed and size.

Sub-10-fs pulse generation from 10 nJ Yb-fiber laser with cascaded nonlinear pulse compression

We demonstrate cascaded nonlinear pulse compression of a Yb-doped fiber laser. The system is based on two pulse compression stages with bare single-mode fiber (SMF) and ultra-high NA (UHNA) fibers combined with two pairs of chirped mirrors. The 10 nJ, 110 fs input pulses are compressed down to 9.1 fs at 90 MHz, revealing a broadband spectrum from 800 nm to 1350 nm. This technique provides a simple approach to sub-10-fs compact Yb-doped fiber lasers for a variety of applications.

Origin of the Efficient Nonlinear Optical Response of Two-Dimensional Layered CuFeTe2 Nanosheets

Two-dimensional ternary transition metal chalcogenides (TMCs) have aroused great research interest owing to outstanding semiconducting properties and diverse magnetic response. CuFeTe₂, as a typical TMC, exhibits ambiguous magnetic behavior and ground state of spin density waves nature. Herein, we first report efficient nonlinear absorption and superior nonlinear refraction of CuFeTe₂ nanosheets. The nonlinear absorption and refraction coefficients reach -0.22 cm/GW and -1.66 × 10^-12 cm²/W, respectively. Semiempirical theory for direct bandgap semiconductors was applied to estimate the nonlinearities of CuFeTe₂ nanosheets. The calculation results indicate that the efficient nonlinearities stem from the free carrier induced band filling effect.

Polarized phonons carry angular momentum in ultrafast demagnetization

Magnetic phenomena are ubiquitous in nature and indispensable for modern science and technology, but it is notoriously difficult to change the magnetic order of a material in a rapid way. However, if a thin nickel film is subjected to ultrashort laser pulses, it loses its magnetic order almost completely within femtosecond timescales¹. This phenomenon is widespread^2-7 and offers opportunities for rapid information processing^8-11 or ultrafast spintronics at frequencies approaching those of light^8,9,12. Consequently, the physics of ultrafast demagnetization is central to modern materials research^1-7,13-28, but a crucial question has remained elusive: if a material loses its magnetization within mere femtoseconds, where is the missing angular momentum in such a short time? Here we use ultrafast electron diffraction to reveal in nickel an almost instantaneous, long-lasting, non-equilibrium population of anisotropic high-frequency phonons that appear within 150-750 fs. The anisotropy plane is perpendicular to the direction of the initial magnetization and the atomic oscillation amplitude is 2 pm. We explain these observations by means of circularly polarized phonons that quickly absorb the angular momentum of the spin system before macroscopic sample rotation. The time that is needed for demagnetization is related to the time it takes to accelerate the atoms. These results provide an atomistic picture of the Einstein-de Haas effect and signify the general importance of polarized phonons for non-equilibrium dynamics and phase transitions.

Studying the Nonlinear Optical Properties of Fluoride Laser Host Materials in the Ultraviolet Wavelength Region

Fluoride host materials doped with trivalent cerium ions have previously been demonstrated as successful laser materials in the ultraviolet wavelength region. However, the nonlinear optical properties of the fluoride hosts in this wavelength region have not been investigated yet, although nonlinearity could result in undesirable effects such as self-focusing and pulse distortion when these fluoride materials are used as gain media in high-power, ultrashort pulse laser oscillator and amplifier systems. In this work, the nonlinear refractive index of lithium calcium aluminum fluoride (LiCaAlF6), lithium strontium aluminum fluoride (LiSrAlF6), lanthanum fluoride (LaF3), and yttrium lithium fluoride (YLiF4) fluoride host materials are determined using the Kramers–Krönig relation model in the ultraviolet wavelength region. Self-focusing conditions, particularly at the peak laser emission wavelength of these materials, are further analyzed. Results show that LiCaAlF6 has the smallest nonlinear refractive index and self-focusing, making it an ideal host material under the conditions of ultrashort pulse and ultrahigh-power laser generation.

Efficient nonlinear compression of a thin-disk oscillator to 8.5 fs at 55 W average power

We demonstrate an efficient hybrid-scheme for nonlinear pulse compression of high-power thin-disk oscillator pulses to the sub-10 fs regime. The output of a home-built, 16 MHz, 84 W, 220 fs Yb:YAG thin-disk oscillator at 1030 nm is first compressed to 17 fs in two nonlinear multipass cells. In a third stage, based on multiple thin sapphire plates, further compression to 8.5 fs with 55 W output power and an overall optical efficiency of 65% is achieved. Ultrabroadband mid-infrared pulses covering the spectral range 2.4-8µm were generated from these compressed pulses by intra-pulse difference frequency generation.

Photo-Switchable Nanoripples in Ti3C2Tx MXene

MXenes are two-dimensional materials with a rich set of chemical and electromagnetic properties, the latter including saturable absorption and intense surface plasmon resonances. To fully harness the functionality of MXenes for applications in optics, electronics, and sensing, it is important to understand the interaction of light with MXenes on atomic and femtosecond dimensions. Here, we use ultrafast electron diffraction and high-resolution electron microscopy to investigate the laser-induced structural dynamics of Ti₃C₂T_x nanosheets. We find an exceptionally fast lattice response with an electron-phonon coupling time of 230 fs. Repetitive femtosecond laser excitation transforms Ti₃C₂T_x through a structural transition into a metamaterial with deeply sub-wavelength nanoripples that are aligned with the laser polarization. By a further laser illumination, the material is reversibly photo-switchable between a flat and rippled morphology. The resulting nanostructured MXene metamaterial with directional nanoripples is expected to exhibit an anisotropic optical and electronic response as well as an enhanced chemical activity that can be switched on and off by light.

Cascaded nonlinearity induced spatial domain effects in a high power femtosecond optical parametric oscillator

We have investigated the effect of cascaded optical nonlinearity on the spatial beam properties of a femtosecond optical parametric oscillator (OPO). The OPO was operated with a tunable phase mismatch by varying the angle of the nonlinear crystal. The cascaded nonlinearity induced self-focusing and defocusing changed resonator's stability and impacted mode properties. With tuning of a phase mismatch, the calculated parabolic part of cascaded nonlinearity lens focal length changes from f ∼ 30 mm (D ∼ 33 m^-1 at Δθ ∼ -0.5^o) to infinity and back to f ∼ -110 mm (D ∼ -9 m^-1 at Δθ ∼ 0.9^o) in the LBO nonlinear crystal. Such high power nonlinear lenses in a cavity operated near its stability limit promoted the generation of axially asymmetric or pass-to-pass unstable resonator modes. It was shown that phase mismatched optical parametric oscillation changes the physical character of the resonator from linear to ring-like with two nonlinear crystals having two different focusing powers. Calculations showed that the QCN induced spatial nonlinear phase should lead to severe longitudinal chromatic aberrations for broad spectrum pulses. A numerical simulation in XYZ spatial domain and calculations using ABCD matrix approach confirmed the physical mechanisms underlying the experimental results and allowed for the interpretation of the observed effects.

Kerr-lens mode-locked thin-disk oscillator with 50% output coupling rate

Several approaches to power scaling of mode-locked thin-disk oscillators exist. One of these approaches is based on the increased gain provided by multiple passes through the thin-disk laser medium. For the first time, to the best of our knowledge, we applied this approach to a Kerr-lens mode-locked thin-disk oscillator. The so obtained additional gain allowed mode-locked operation with up to 50% output coupling rate. This first demonstration is of particular importance for gain media with inherently low-emission cross sections and paves the way to even more powerful Kerr-lens mode-locked thin-disk oscillators. Moreover, the experimental results indicate an increased self-amplitude modulation related to an overall increase in the soft-aperture Kerr-lens effect.

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.

Ultrafast extreme ultraviolet photoemission without space charge

Time- and Angle-resolved photoelectron spectroscopy from surfaces can be used to record the dynamics of electrons and holes in condensed matter on ultrafast time scales. However, ultrafast photoemission experiments using extreme-ultraviolet (XUV) light have previously been limited by either space-charge effects, low photon flux, or limited tuning range. In this article, we describe XUV photoelectron spectroscopy experiments with up to 5 nA of average sample current using a tunable cavity-enhanced high-harmonic source operating at 88 MHz repetition rate. The source delivers >1011 photons/s in isolated harmonics to the sample over a broad photon energy range from 18 to 37 eV with a spot size of 58 × 100 μm2. From photoelectron spectroscopy data, we place conservative upper limits on the XUV pulse duration and photon energy bandwidth of 93 fs and 65 meV, respectively. The high photocurrent, lack of strong space charge distortions of the photoelectron spectra, and excellent isolation of individual harmonic orders allow us to observe laser-induced modifications of the photoelectron spectra at the 10-4 level, enabling time-resolved XUV photoemission experiments in a qualitatively new regime.

Direct compression of 170-fs 50-cycle pulses down to 1.5 cycles with 70% transmission

We present a straightforward route for extreme pulse compression, which relies on moderately driving self-phase modulation (SPM) over an extended propagation distance. This avoids that other detrimental nonlinear mechanisms take over and deteriorate the SPM process. The long propagation is obtained by means of a hollow-core fiber (HCF), up to 6 m in length. This concept is potentially scalable to TW pulse peak powers at kW average power level. As a proof of concept, we demonstrate 33-fold pulse compression of a 1 mJ, 6 kHz, 170 fs Yb laser down to 5.1 fs (1.5 cycles at 1030 nm), by employing a single HCF and subsequent chirped mirrors with an overall transmission of 70%.

Nonlinear pulse compression based on a gas-filled multipass cell

We demonstrate nonlinear temporal compression of a high-energy Yb-doped fiber laser source in a multipass cell filled with argon. The 160 μJ 275 fs input pulses are compressed down to 135 μJ 33 fs at the output, corresponding to an overall transmission of 85%. We also analyze the output beam, revealing essentially no space-time couplings. We believe this technique can be scalable to higher pulse energies and shorter pulse durations, enabling access to a wider parameter range for a large variety of ultrafast laser sources.

Efficient middle-infrared generation in LiGaS2 by simultaneous spectral broadening and difference-frequency generation

We report a surprisingly broadband and efficient midinfrared pulse generation in LiGaS₂ (Langasite, LGS) by invoking a simultaneous interplay of intrapulse difference-frequency generation, self-phase modulation, and dispersion. This cascaded mechanism expands the output bandwidth and output power at the same time. With 30-fs driving pulses centered at 1030-nm wavelength we obtain a broadband middle-infrared spectrum of 8-11 μm with an LGS crystal as thick as 4 mm, which is eight times longer than the walk-off length.

Multi-watt, multi-octave, mid-infrared femtosecond source

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.

Filamentation-free self-compression of mid-infrared pulses in birefringent crystals with second-order cascading-enhanced self-focusing nonlinearity

We experimentally demonstrate virtually lossless, filamentation-free and energy-scalable more than three-fold self-compression of mid-infrared laser pulses at 2.1 μm in a birefringent medium (β-BBO crystal), which stems from favorable interplay between the second-order cascading-enhanced self-phase modulation and anomalous group velocity dispersion. By choosing an appropriate input beam diameter and intensity, the self-compression down to sub-30 fs pulse widths with gigawatt peak power is achieved without the onset of beam filamentation and associated nonlinear losses due to the multiphoton absorption, yielding the energy throughput greater than 86%.

Multi-pass-cell-based nonlinear pulse compression to 115 fs at 7.5 µJ pulse energy and 300 W average power

We demonstrate nonlinear pulse compression by multi-pass cell spectral broadening (MPCSB) from 860 fs to 115 fs with compressed pulse energy of 7.5 µJ, average power of 300 W and close to diffraction-limited beam quality. The transmission of the compression unit is >90%. The results show that this recently introduced compression scheme for peak powers above the threshold for catastrophic self-focusing can be scaled to smaller pulse energies and can achieve a larger compression factor than previously reported. Good homogeneity of the spectral broadening across the beam profile is verified, which distinguishes MPCSB among other bulk compression schemes.