Chirp-controlled filamentation and formation of light bullets in the mid-IR.

Formation of light bullets-tightly localized in space and time light packets, retaining their spatiotemporal shape during propagation-is, for the first time, experimentally observed and investigated in a new regime of mid-infrared filamentation in ambient air. It is suggested that the light bullets generated in ambient air by multi-mJ, positively chirped 3.9-µm pulses originate from a dynamic interplay between the anomalous dispersion in the vicinity of CO2 resonance and positive chirp, both intrinsic, carried by the driver pulse, and accumulated, originating from nonlinear propagation in air. By adjusting the initial chirp of the driving pulses, one can control the spatial beam profile, energy losses, and spectral-temporal dynamics of filamenting pulses and deliver sub-3-cycle mid-IR pulses in high-quality beam on a remote target.

Filamentation of mid-IR pulses in ambient air in the vicinity of molecular resonances.

Properties of filaments ignited by multi-millijoule, 90 fs mid-infrared pulses centered at 3.9 µm are examined experimentally by monitoring plasma density, losses, spectral dynamics and beam profile evolution at different focusing strengths. By changing from strong (f=0.25 m) to loose (f=7 m) focusing, we observe a shift from plasma-assisted filamentation to filaments with low plasma density. In the latter case, filamentation manifests itself by beam self-symmetrization and spatial self-channeling. Spectral dynamics in the case of loose focusing is dominated by the nonlinear Raman frequency downshift, which leads to the overlap with the CO2 resonance in the vicinity of 4.2 µm. The dynamic CO2 absorption in the case of 3.9 µm filaments with their low plasma content is the main mechanism of energy losses and, either alone or together with other nonlinear processes, contributes to the arrest of intensity.

Stimulated Raman gas sensing by backward UV lasing from a femtosecond filament.

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.

Mid-infrared laser filamentation in molecular gases.

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.

High energy and average power femtosecond laser for driving mid-infrared optical parametric amplifiers.

We have developed the first (to our knowledge) femtosecond Tm-fiber-laser-pumped Ho:YAG room-temperature chirped pulse amplifier system delivering scalable multimillijoule, multikilohertz pulses with a bandwidth exceeding 12 nm and average power of 15 W. The recompressed 530 fs pulses are suitable for broadband white light generation in transparent solids, which makes the developed source ideal for both pumping and seeding optical parametric amplifiers operating in the mid-IR spectral range.

FLASH free electron laser pump-probe laser concept based on spectral broadening of high-power ytterbium picosecond systems in multi-pass cells.

Within the FLASH2020+ upgrade, the pump-probe laser capabilities of the extreme ultraviolet and soft x-ray free-electron laser (XFEL) FLASH in Hamburg will be extended. In particular, providing wavelength tunability, shorter pulse durations, and reduced arrival time jitter will increase the scientific opportunities and the time resolution for the XFEL-optical laser pump-probe experiments. We present here a novel concept for the pump-probe laser at FLASH that is based on the post-compression of picosecond pulses emitted from high-power Ytterbium:YAG slab amplifiers. Flexible reduction of the pulse duration is facilitated by spectral broadening in pressure-tunable multi-pass cells. As an application, we show the pumping of a commercial optical parametric amplifier with 150 fs post-compressed pulses. By means of an additional difference frequency generation stage, tunable spectral coverage from 1.3 to 16 µm is reached with multi-µJ, sub-150 fs pulses. Finally, a modular reconfiguration approach to the optical setups close to the free-electron laser instruments is implemented. This enables fast installation of the nonlinear frequency converters at the end stations for user operation and flexibility between different instruments in the two experimental halls.

Ultrafast-laser-induced backward stimulated Raman scattering for tracing atmospheric gases.

By combining tunable broadband pulse generation with the technique of nonlinear spectral compression we demonstrate a prototype scheme for highly selective detection of air molecules by backward stimulated Raman scattering. The experimental results allow to extrapolate the laser parameters required for standoff sensing based on the recently demonstrated backward atmospheric lasing.

Third- and fifth-harmonic generation by mid-infrared ultrashort pulses: beyond the fifth-order nonlinearity.

Third- and fifth-harmonic generation by ultrashort laser pulses in the mid-infrared (mid-IR) reveals nonlinear-optical effects beyond the fifth-order nonlinearity and enables, because of an extraordinarily long coherence length, efficient multiplex frequency upconversion of ultrashort mid-IR pulses. We show that harmonic generation by mid-IR pulses provides an access to the key optical constants of gas media, allowing metrology of linear and nonlinear-optical susceptibilities in the mid-IR and offering a tool for the remote sensing of the atmosphere.

Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy.

The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an electronically excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The method allows us to discover that a major part of the population relaxes to the molecular ground state within 220-250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states.

Synchronized beamline at FLASH2 based on high-order harmonic generation for two-color dynamics studies.

We present the design, integration, and operation of the novel vacuum ultraviolet (VUV) beamline installed at the free-electron laser (FEL) FLASH. The VUV source is based on high-order harmonic generation (HHG) in gas and is driven by an optical laser system synchronized with the timing structure of the FEL. Ultrashort pulses in the spectral range from 10 to 40 eV are coupled with the FEL in the beamline FL26, which features a reaction microscope (REMI) permanent endstation for time-resolved studies of ultrafast dynamics in atomic and molecular targets. The connection of the high-pressure gas HHG source to the ultra-high vacuum FEL beamline requires a compact and reliable system, able to encounter the challenging vacuum requirements and coupling conditions. First commissioning results show the successful operation of the beamline, reaching a VUV focused beam size of about 20 µm at the REMI endstation. Proof-of-principle photo-electron momentum measurements in argon indicate the source capabilities for future two-color pump-probe experiments.

Mapping anomalous dispersion of air with ultrashort mid-infrared pulses.

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.

Multi-millijoule few-cycle mid-infrared pulses through nonlinear self-compression in bulk.

The physics of strong-field applications requires driver laser pulses that are both energetic and extremely short. Whereas optical amplifiers, laser and parametric, boost the energy, their gain bandwidth restricts the attainable pulse duration, requiring additional nonlinear spectral broadening to enable few or even single cycle compression and a corresponding peak power increase. Here we demonstrate, in the mid-infrared wavelength range that is important for scaling the ponderomotive energy in strong-field interactions, a simple energy-efficient and scalable soliton-like pulse compression in a mm-long yttrium aluminium garnet crystal with no additional dispersion management. Sub-three-cycle pulses with >0.44 TW peak power are compressed and extracted before the onset of modulation instability and multiple filamentation as a result of a favourable interplay between strong anomalous dispersion and optical nonlinearity around the wavelength of 3.9 µm. As a manifestation of the increased peak power, we show the evidence of mid-infrared pulse filamentation in atmospheric air.

Mid-infrared laser filaments in the atmosphere.

Filamentation of ultrashort laser pulses in the atmosphere offers unique opportunities for long-range transmission of high-power laser radiation and standoff detection. With the critical power of self-focusing scaling as the laser wavelength squared, the quest for longer-wavelength drivers, which would radically increase the peak power and, hence, the laser energy in a single filament, has been ongoing over two decades, during which time the available laser sources limited filamentation experiments in the atmosphere to the near-infrared and visible ranges. Here, we demonstrate filamentation of ultrashort mid-infrared pulses in the atmosphere for the first time. We show that, with the spectrum of a femtosecond laser driver centered at 3.9 µm, right at the edge of the atmospheric transmission window, radiation energies above 20 mJ and peak powers in excess of 200 GW can be transmitted through the atmosphere in a single filament. Our studies reveal unique properties of mid-infrared filaments, where the generation of powerful mid-infrared supercontinuum is accompanied by unusual scenarios of optical harmonic generation, giving rise to remarkably broad radiation spectra, stretching from the visible to the mid-infrared.

Scalable Yb-MOPA-driven carrier-envelope phase-stable few-cycle parametric amplifier at 1.5 microm.

Carrier-envelope phase-stable 4 microJ pulses at approximately 1.5 microm are obtained from a femtosecond Yb:KGW-MOPA-pumped two-stage optical parametric amplifier. This novel technology represents a highly attractive alternative to traditional Ti:sapphire front-ends for seeding multimillijoule-level optical parametric chirped-pulse amplifiers. For this task, we demonstrate stretching of the OPA output to approximately 40 ps and recompression to 33 fs pulse duration. As a stand-alone system, our tunable two-stage OPA might find numerous applications in time-resolved spectroscopy and micromachining.

Multi-mJ, 200-fs, cw-pumped, cryogenically cooled, Yb,Na:CaF2 amplifier.

Using a novel (to our knowledge) broadband Yb-doped Yb3+,Na+:CaF2 crystal cooled in a closed loop to 130 K we demonstrate a chirped pulse regenerative laser amplifier delivering the energy of up to 3 mJ at a repetition rate of 1 kHz and an average output power of 6 W at 20 kHz. The gain narrowing in the laser crystal is compensated by shaping the amplitude of the seed pulse spectrum. As the result, at the highest amplified pulse energy we obtain a 12 nm FWHM bandwidth supporting a 130 fs pulse duration, assuming ideal compression. Amplified pulses were recompressed from 250 ps to 195 fs with a 1700 lines/mm transmission grating compressor.