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Nature photonics 17, XX (2022). doi:10.1038/s41566-022-01121-9, The serrodyne principle enables an electromagnetic signal to be frequency shifted by applying a linear phase ramp in the time domain. This phenomenon has been exploited to frequency shift signals in the radiofrequency, microwave and optical regions of the electromagnetic spectrum over ranges of up to a few gigahertz, for example, to analyse the Doppler shift of radiofrequency signals for noise suppression and frequency stabilization. Here we employ this principle to shift the centre frequency of high-power femtosecond laser pulses over a range of several terahertz with the help of a nonlinear multi-pass cell. We demonstrate our method experimentally by shifting the central wavelength of a state-of-the-art 75 W frequency comb laser from 1,030 nm to 1,060 nm and to 1,000 nm. Furthermore, we experimentally show that this wavelength-shifting technique supports coherence characteristics at the few hertz-level while improving the temporal pulse quality. The technique is generally applicable to wide parameter ranges and different laser systems, enabling efficient wavelength conversion of high-power lasers to spectral regions beyond the gain bandwidth of available laser platforms., Published by Nature Publ. Group, London [u.a.]

High energy tunable ultrafast UV pulses are of great interest for a variety of applications. These pulses are also required for high repetition rate, fully coherent UV seeded free electron lasers [1] . In the absence of proper laser gain materials, optical parametric chirped pulse amplification (OPCPA) is considered as a foremost technique to generate high power tunable ultra-short laser pulses in various spectral ranges [2] . A typical example is the broadband green pumped OPCPA system driven by frequency doubled Ytterbium based chirped pulse laser amplifiers (CPA). The excellent power scalability of the OPCPAs allows generation of multi-millijoule, few femtosecond pulses with hundreds of watts of average power harnessing high energy and repetition rate commercially available CPA pump systems [3] . The absence of proper nonlinear crystals hinders operating OPCPAs directly in the UV spectral range. Therefore, frequency doubling, tippling or quadrupling of conventional OPCPAs are the usual techniques of generating tunable UV pulses. However, these schemes suffer from the strong two-photon absorption, narrow phase matching bandwidth, and non-linear phase transfer to UV range. As a consequence, for ultra-short pulses the maximum conversion efficiency of such schemes is typically limited to below 10%, and usually accompanied by pulse compression complications.

We show that a combination of OPCPA and cascaded SFM can efficiently generate 100 µJ, MHz repetition rate, 50 fs, tunable pulses from 279 nm to 317 nm for seeding superconducting X-ray free electron lasers.

Journal of synchrotron radiation 28(1), - (2021). doi:10.1107/S1600577520015052, This paper reports on nonlinear spectral broadening of 1.1 ps pulses in a gas-filled multi-pass cell to generate sub-100 fs optical pulses at 1030 nm and 515 nm at pulse energies of 0.8 mJ and 225 µJ, respectively, for pump–probe experiments at the free-electron laser FLASH. Combining a 100 kHz Yb:YAG laser with 180 W in-burst average power and a post-compression platform enables reaching simultaneously high average powers and short pulse durations for high-repetition-rate FEL pump–probe experiments., Published by Wiley-Blackwell, [S.l.]

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.

We present a HHG-based vacuum ultraviolet (VUV) source at the free electron laser FLASH2. The source provides ultrashort pulses from 10 to 40eV, coupled to the REMI end-station (beamline FL26) for VUV-FEL pump-probe experiments.

We present a VUV beamline installed as pump-probe source at the free-electron laser FLASH. The source is based on high-order harmonic generation driven by femtosec-ond near-infrared laser pulses synchronized with the FEL burst mode.

We present experimental studies of long-distance transmission of ultrashort mid-infrared laser pulses through atmospheric air, probing air dispersion in the 3.6–4.2-μm wavelength range. Atmospheric air is still highly transparent to electromagnetic radiation in this spectral region, making it interesting for long-distance signal transmission. However, unlike most of the high-transmission regions in gas media, the group-velocity dispersion, as we show in this work, is anomalous at these wavelengths due to the nearby asymmetric-stretch rovibrational band of atmospheric carbon dioxide. The spectrograms of ultrashort mid-infrared laser pulses transmitted over a distance of 60 m in our experiments provide a map of air dispersion in this wavelength range, revealing clear signatures of anomalous dispersion, with anomalous group delays as long as 1.8 ps detected across the bandwidth covered by 80-fs laser pulses.

We examine mid-IR light bullets generated in ambient air. 2-optical cycle pulses confined in space are generated in filamentation regime. Few-fold solitonic self-compression is achieved for strongly chirped mid-IR pulses.

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.

We present time-resolved ultraviolet-pump x-ray probe Auger spectra of 2-thiouracil. An ultraviolet induced shift towards higher kinetic energies is observed in the sulfur 2p Auger decay. The difference Auger spectra of pumped and unpumped molecules exhibit ultrafast dynamics in the shift amplitude, in which three phases can be recognized. In the first 100 fs, a shift towards higher kinetic energies is observed, followed by a 400 fs shift back to lower kinetic energies and a 1 ps shift again to higher kinetic energies. We use a simple Coulomb-model, aided by quantum chemical calculations of potential energy states, to deduce a C–S bond expansion within the first 100 fs. The bond elongation triggers internal conversion from the photoexcited S2 to the S1 state. Based on timescales, the subsequent dynamics can be interpreted in terms of S1 nuclear relaxation and S1-triplet internal conversion.

Physical review / X Expanding access X 9(2), 021029 (2019). doi:10.1103/PhysRevX.9.021029, We report on experimental results in a new regime of relativistic light-matter interaction employing midinfrared (3.9-mu m wavelength) high-intensity femtosecond laser pulses. In the laser-generated plasma, electrons reach relativistic energies already for rather low intensities due to the fortunate lambda(2) scaling of the kinetic energy with the laser wavelength. The lower intensity efficiently suppresses optical field ionization and creation of the preplasma at the rising edge of the laser pulse, enabling an enhanced efficient vacuum heating of the plasma. The lower critical plasma density for long-wavelength radiation can be surmounted by using nanowires instead of flat targets. Numerical simulations, which are in a good agreement with experimental results, suggest that approximate to 80% of the incident laser energy has been absorbed resulting in a long-living, key-temperature, high-charge-state plasma with a density more than 3 orders of magnitude above the critical value. Our results pave the way to laser-driven experiments on laboratory astrophysics and nuclear physics at a high repetition rate., Published by APS, College Park, Md.

Physical review letters 120(9), 093002 (2018). doi:10.1103/PhysRevLett.120.093002, Recent advances in high-order harmonic generation have made it possible to use a tabletop-scale setup to produce spatially and temporally coherent beams of light with bandwidth spanning 12 octaves, from the ultraviolet up to x-ray photon energies >1.6 keV. Here we demonstrate the use of this light for x-ray-absorption spectroscopy at the K- and L-absorption edges of solids at photon energies near 1 keV. We also report x-ray-absorption spectroscopy in the water window spectral region (284–543 eV) using a high flux high-order harmonic generation x-ray supercontinuum with 10$^9$ photon/s=s in 1% bandwidth, 3 orders of magnitude larger than has previously been possible using tabletop sources. Since this x-ray radiation emerges as a single attosecond-to-femtosecond pulse with peak brightness exceeding 10$^{26}$ photon$/s/mrad^{2}/mm^{2}/1$% bandwidth, these novel coherent x-ray sources are ideal for probingthe fastest molecular and materials processes on femtosecond-to-attosecond time scales and picometer length scales., Published by APS, College Park, Md.

We present laser-induced condensation as a method for detecting of ultra-low concentrations of volatile organic compounds featuring a detection threshold well below 1 ppb. The method was tested in lab scale experiments and compared with quantitative photo-ionization detection measurements. The method was shown to be sensitive to the laser excitation wavelength between 1.6’2.0 um range.

Subterawatt ultrashort pulses in the mid-infrared (mid-IR) range delivered by a multistage optical parametric chirped pulse amplifier are used to study laser-induced filamentation and optical-harmonic generation in the field of a mid-IR driver. Conditions and physical scenarios enabling filamentation-assisted compression of subterawatt mid-IR pulses to few-cycle pulse widths are identified.

Filamentation of Mid-IR pulses in ambient air is significantly more complicated and challenging as compared to near-IR filaments, due the changing sign of dispersion in the vicinity of 3.6 μm, substantially lower ionization rates and hence plasma density [1], as well as because of the presence of molecular resonances around the atmospheric transparency windows. Experimental observations reveal that despite its low concentration in ambient air, which is only 400–600 ppm (0.04–0.06%), CO2 strongly affects filamentation of 3.9-μm pulses by influencing spectral and temporal dynamics, as well as spatial beam transformations and propagation losses. As a main loss mechanism during filamentation of loosely focused 3.9-μm pulses in ambient air, dynamic absorption by CO2 is identified. Since in the case of loose focusing plasma density in the filament is negligible, pulses during filamentation in both main constituents, O2 and N2, experience pronounced red-sided broadening (Fig.la), which is governed by stimulated Raman scattering. In ambient air, newly generated spectral components roll over CO2 resonant absorption band situated in the vicinity of 4.2-μm and are effectively absorbed, which results in up to 50% filamentation losses. To our surprise, an increase of CO2 concentration to ∼5% level (by exhaling air [3] into 2-m long open-ended tube) leads to complete disappearance of losses.

Few-cycle high peak power mid-IR sources are required for strong field applications, such as driving HHG in the keV range [1] and high efficiency THz generation [2]. In order to expose target to a peak field without causing excessive ionization, ultrashort pulses consisting of a few-cycles are needed. Parametric sources based on KTA crystals have been successfully used for the generation of multi-mJ pulses in the 3–4 μm range [3], however, the duration is typically limited to sub-100 fs duration due to phase matching bandwidth limitations. A robust and popular scheme for the generation of few-cycle pulses is via post-compression in a hollow core fiber (HCF) filled with noble gas [4] that yields uniform pulse compression across the beam and the perfect beam profile at the output due to spatial filtering effects.

Different regimes of filamentation of multi-millijoule mid-infrared pulses in ambient air can be achieved by varying focusing conditions and chirp of the driving pulses. Dynamic absorption losses are identified as a possible mechanism of observed plasma-less filamentation.

Ultrashort intense lasers are able to efficiently condensate water vapor in air into droplets, even with mid-IR laser pulses. In addition, existing droplets can be expelled from the beam to transmit information through fog.

The electric conductivity, HV discharge triggering and guiding capabilities of filaments at 3.9 micrometer in air are investigated in the perspective of lightning control applications, and compared to near-IR filaments in identical conditions

We report on the generation of high-energy few-optical-cycle pulses in mid-infrared spectral region via self-compression in transparent dielectrics and filaments as well as via four wave parametric amplification in gas-filled hollow waveguides.

We present post-compression of 22-mJ 90-fs pulses at 3.9 µm via spectralbroadening in a noble-gas-filled 1 mm core diameter 3 meter long capillary with over 60% throughput and recompressed in a bulk BaF2 plate down to 33 fs.

We study properties of filaments ignited by multi-millijoule, 90-fs mid-IR pulses centered at 3.9 μm. The studies are performed by monitoring plasma density distribution and losses as well as beam profiles at different focusing conditions.

We report self-action of 0.5-TW peak-power few-cycle 4-um pulses in air and bulk dielectrics that is strikingly different compared to the case of near-IR drivers. An example of scalable nonlinear self-compression of 20-mJ is highlighted.

We demonstrate the generation of 10-mJ 1.5-μm few-cycle pulses from a 4-stage OPCPA system. The system is based on a fusion of femtosecond DPSS Yb technology and a picosecond Nd:YAG pump laser. In a first preliminary filamentation experiment in argon at 5 atm, we observe significant spectral broadening of the recompressed multi-mJ 1.5-μm pulses. The filamentation output spectrum supports the generation of 8 fs pulses equivalent to sub-to-cycle pulses at 1.5 μm. We foresee that our terawatt-peak-power sub-to-cycle pulse source will open the door to exciting new experiments in attosecond high-field science in the near future.

We identify a physical scenario whereby soliton transients generated in freely propagating laser beams within the regions of anomalous dispersion in air can be compressed as a part of their free-beam spatiotemporal evolution to yield few-cycle mid- and long-wavelength-infrared field waveforms, whose peak power is substantially higher than the peak power of the input pulses. We show that this free-beam soliton self-compression scenario does not require ionization or laser-induced filamentation, enabling high-throughput self-compression of mid- and long-wavelength-infrared laser pulses within a broad range of peak powers from tens of gigawatts up to the terawatt level. We also demonstrate that this method of pulse compression can be extended to long-range propagation, providing self-compression of high-peak-power laser pulses in atmospheric air within propagation ranges as long as hundreds of meters, suggesting new ways towards longer-range standoff detection and remote sensing.

We discuss a four-stage optical parametric chirped-pulse amplifier that delivers carrier-envelope phase-stable ~1.5 μm pulses with energies up to 12.5 mJ before recompression. The system (previously reported in Opt. Lett. 34, 2498 (2009)) is based on a fusion of femtosecond diode-pumped solid-state Yb technology and a picosecond 100-mJ Nd:YAG pump amplifier. Pulses with 62 nm bandwidth are recompressed to a 74.4 fs duration, which is close to the transform limit. Here, to show the way towards a TW-peak-power single-cycle IR source, we perform detailed investigations of single-filament IR supercontinuum generation via femtosecond filamentation in noble gases. Depending on the experimental conditions, two filamentation regimes can be achieved: (i) in the filamentation regime without plasma-induced pulse self-compression, we generate 4-mJ 600-nm-wide IR supercontinua of high spatial quality supporting 8-fs pulse durations, which corresponds to less than two optical cycles at 1.5 μm; (ii) in the self-compression regime, we demonstrate self-compression of 2.2 mJ pulses down to 19.8 fs duration in a single filament in argon with a 1.5 mJ output energy and 66% energy throughput. By adapting the experimental conditions, further energy upscaling of the self-compressed pulses seems feasible. Keywords: optical parametric amplification, filamentation, pulse self-compression, intense few-cycle infrared pulses

We identify and experimentally demonstrate a physical scenario whereby high-peak-power mid-infrared (mid-IR) pulses can be compressed as a part of their free-beam spatiotemporal evolution within the regions of anomalous dispersion in air to yield few-cycle subterawatt field waveforms. Unlike filamentation-assisted pulse compression, the pulse-compression scenario identified in this work does not involve any noticeable ionization of air, enabling a whole-beam self-compression of mid-IR laser pulses without ionization-induced loss. Ultrashort high-peak-power 3.9 μm laser pulses are shown to exhibit such self-compression dynamics when exposed to the dispersion anomaly of air induced by the asymmetric-stretch rovibrational band of carbon dioxide. Even though the group-velocity dispersion cannot be even defined as a single constant for the entire bandwidth of mid-IR laser pulses used in experiments, with all soliton transients shattered by high-order dispersion, 100–200 GW, 100 fs, 3.9 μm laser pulses can be compressed in this regime to 35 fs subterawatt field waveforms.

We report on theoretical and experimental investigations of two crystal traveling-wave optical parametric generator (OPG). It is shown that the efficiency and angular structure of traveling-wave tunable OPG output depends on the distance between the two crystals. Air dispersion driven efficiency modulation at different OPG output wavelengths is observed experimentally and confirmed by theoretical calculations.

Midinfrared powerful 90 fs pulses at a wavelength of λ=3.9μm drive a femtosecond hard x-ray source (Cu~Kα: \hbar ω =8.05 keV). Up to 10^8 X-ray photons/pulse are generated which is twice as many as with 800 nm drivers of a 100 times higher peak intensity.

We discuss prospects of the generation of high power mid-IR pulses by analyzing soliton self-compression of multi-mJ 4-μm pulses and the development of few-optical-cycle 6-μm optical parametric amplifier. Scalability of pump lasers is addressed.

we present the results of experimental and theoretical investigations for remote initiation of lasing in nitrogen and air by femtosecond filaments generated by laser pulses with different wavelengths, polarization and applying adaptive control methods

Virtually lossless self-compression of 10-mJ 3.9-um sub-100 fs pulses in bulk YAG resulting in 9-mJ 33-fs pulses is reported. Generated peak power exceeds 250 GW which is suitable for filamentation in ambient air.

We perform a proof-of-principle demonstration of chemically specific standoff gas sensing, in which a coherent stimulated Raman signal is detected in the direction anticollinear to a two-color laser excitation beam traversing the target volume. The proposed geometry is intrinsically free space as it does not involve back-scattering (reflection) of the signal or excitation beams at or behind the target. A beam carrying an intense mid-IR femtosecond (fs) pulse and a parametrically generated picosecond (ps) UV Stokes pulse is fired in the forward direction. A fs filament, produced by the intense mid-IR pulse, emits a backward-propagating narrowband ps laser pulse at the 337 and 357 nm transitions of excited molecular nitrogen, thus supplying a counter-propagating Raman pump pulse. The scheme is linearly sensitive to species concentration and provides both transverse and longitudinal spatial resolution.

The first table-top hard X-ray plasma source driven by a mid-infrared source provides 10^9 photons per pulse.

The results of theoretical and experimental investigations of two crystal travelling-wave optical parametric generator(TOPG) are presented. It is shown that the efficiency and angular structure of TOPG output depends on the distance be-tween crystals. The experimentally observed efficiency modulation is attributed to air dispersion and this is confirmed bytheoretical calculations.

Recent breakthroughs in ultrafast photonics in the mid-IR help understand complex interactions of high-intensity mid-IR field waveforms with matter, offer new approaches for x-ray generation, enable mid-IR laser filamentation in the atmosphere, facilitate lasing in filaments, give rise to unique regimes of laser-matter interactions, and reveal unexpected properties of materials in the mid-IR range.

Filamentation of femtosecond pulses in transparent solids with anomalous group-velocity dispersion is investigated for a broad range of the peak powers. Solitonic self-compression of multi-millijoule mid-IR pulses is achieved using a 1.5-mm-thick CaF2 plate.

Strong narrow-band coherent emission lines of the \(\mathrm{B}^{2} \varSigma_{\mathrm{u}} ^{+} \mbox{--} \mathrm{X}^{2} \varSigma_{\mathrm{g}} ^{+}\) transition of \(\mathrm{N}_{2} ^{+}\) in air and the A2 Π u–X2 Π g transition of \(\mathrm{CO}_{2} ^{+}\) in CO2 gas are observed from filaments generated by intense infrared femtosecond laser pulses (1150–2000 nm

Powerful 90 fs pulses at a midinfrared wavelength of l=3.9mm drive a femtosecond hard x-ray source (Cu Ka: hbaro=8.05 keV). Up to 10^8 X-ray photons/pulse are generated which is twice as many as with 800 nm drivers of a 100 times higher peak intensity.

We report the observation of forward-propagating UV stimulated emission in atmospheric nitrogen pumped, at a standoff distance of 2.6 m, by an energetic laser pulse at a wavelength of 1053 nm and with the duration in the picosecond range. The generated optical gain at 337 nm is seeded by third harmonic of the pump beam. This demonstration is an important step towards the development of techniques for standoff optical sensing in the atmosphere.

Summary form only given. The gain bandwidth of Ho-doped crystals is sufficiently broad to support femtosecond operation, which would allow pumping of ultrashort-pulse mid-IR optical parametric amplifiers (OPAs) [1,2] and, upon reaching adequate peak powers, eye-safe filamentation in the >2-μm transparency window of the atmosphere. In this contribution, we report on, to our knowledge, the first demonstration of a femtosecond cw-pumped Ho:YAG chirped pulse amplifier (CPA) system operating at room temperature and producing broadband pulses supporting ~440 fs pulse durations at the energy level of up to 3 mJ at a 5-kHz repetition rate. The CPA system consists of a seed source, stretcher, Ho:YAG regenerative amplifier (RA) and compressor. The RA is based on an AR coated Ho:YAG rod end-pumped by 70-W Tm fiber laser (IPG Photonics). Due to the unavailability of Faraday rotators capable to withstand high peak intensities, we have designed a ring-shaped RA cavity, in which the injection and output beam ports are separated and the amplification of the injected beam is unidirectional. Because of the ring-cavity design, the Pockels cell (RTP) is operated in the λ/2 regime.The RA is seeded with 2.1-μm pulses generated in a white-light-seeded two-stage OPA based on type I BBO and type II KTA crystals. In early experiments the OPA was pumped by a commercial 0.4-mJ Yb:KGW DPSSL system. More recently, we have demonstrated that the 10-μJ, 190-fs pulses from our monolithic Yb fiber amplifier [3] are sufficient to generate the 0.7-μJ, 260-fs pulses at 2.1 μm in the OPA. The output of the OPA is directed to the positive dispersion Offner stretcher. When seeded with an unshaped spectrum at low pump powers, the RA generates narrowband, 2.4-nm FWHM, pulses centered at around 2090 nm. For higher pump powers, a weaker second band appears which is centered at the 2097 nm gain peak of Ho:YAG (Fig.1a). In order to compensate for gain narrowing in the RA cavity, we have installed a simple mechanical amplitude shaper in the Fourier plane of the stretcher. The shaping, although at a cost of 85% of the seed pulse energy, results in a substantially broadened, > 12 nm FWHM, amplified spectrum. The amplified pulses are recompressed with a grating compressor based on a 600 l/mm grating pair. The pulse duration inferred from frequency-resolved optical gating (FROG) is 530 fs (Fig. 1b), whereas the calculated spectrum-limited pulse duration is ~440 fs. The output of the RA exhibits a rather good spatial profile (inset in Fig.1d) with the beam quality factor M2=1.3. In the ring cavity, picosecond pulses propagate only in one direction, while the Q-switched radiation develops in both directions, which allows monitoring of the Q-switched background in the direction opposite to the seed propagation and, consequently, evaluating the contrast of the amplified picosecond pulses (Fig. 1c). Measurements reveal that for achieving high contrast a strong seed is essential. Measurements using the full available pump power, presented in Fig. 1d, have proven the scalability of the RA toward a multi-mJ operation. The saturation of the output energy seen in Fig. 1d is due to worsened pump beam quality at high pump powers.

We observed the filamentation of mid-infrared ultrashort laser pulses (3.9 μm, 80 fs) in molecular gases. It efficiently generates a broadband supercontinuum over two octaves in the 2.5–6 μm spectral range, with a red-shift up to 500 nm due to the Raman effect, which dominates over the blue shift induced by self-steepening and the gas ionization. As a result, the conversion efficiency into the Stokes region (4.3–6 μm) 65% is demonstrated.

We review the status in the development of intense mid-IR ultrashort pulse sources and highlight several applications in extreme non-linear optics. Scalability of average power, pulse energy, duration and wavelength will be addressed. Article not available.

We present experimental and numerical investigation of a new filamentation regime of mid-infrared femtosecond pulses in solids. Efficient emission of dispersive waves without fundamental spectrum broadening and continuum generation is observed in this regime.