Showing papers by "Andrius Baltuška published in 2017"

Parametric amplification of 100 fs mid-infrared pulses in ZnGeP 2 driven by a Ho:YAG chirped-pulse amplifier.

Abstract: We report on the parametric generation of 100 fs sub-6-cycle 40 μJ pulses with the center wavelength at 5.2 μm using a 1 ps 2.1 μm pump laser and a dispersion management scheme based on bulk material. Our optically synchronized amplifier chain consists of a Ho:YAG chirped-pulse amplifier and white-light-seeded optical parametric amplifiers providing simultaneous passive carrier-envelope phase locking of three ultrashort longwave pulses at the pump, signal, and idler wavelengths corresponding, respectively, to 2.1, 3.5, and 5.2 μm. We also demonstrate bandwidth enhancement and efficient control over nonlinear spectral phase in the regime of cascaded χ2 nonlinearity in ZnGeP2.

Disentangling Intracycle Interferences in Photoelectron Momentum Distributions Using Orthogonal Two-Color Laser Fields.

Abstract: We use orthogonally polarized two-color (OTC) laser pulses to separate quantum paths in the multiphoton ionization of Ar atoms. Our OTC pulses consist of 400 and 800 nm light at a relative intensity ratio of 10∶1. We find a hitherto unobserved interference in the photoelectron momentum distribution, which exhibits a strong dependence on the relative phase of the OTC pulse. Analysis of model calculations reveals that the interference is caused by quantum pathways from nonadjacent quarter cycles.

Self-compression of high-peak-power mid-infrared pulses in anomalously dispersive air

Abstract: 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.

Localizing high-lying Rydberg wave packets with two-color laser fields

Abstract: We demonstrate control over the localization of high-lying Rydberg wave packets in argon atoms with phase-locked orthogonally polarized two-color laser fields. With a reaction microscope, we measure ionization signals of high-lying Rydberg states induced by a weak dc field and blackbody radiation as a function of the relative phase between the two-color fields. We find that the dc-field-ionization yield of high-lying Rydberg argon atoms oscillates with the relative two-color phase with a period of $2\ensuremath{\pi}$ while the photoionization signal by blackbody radiation shows a period of $\ensuremath{\pi}$. Accompanying simulations show that these observations are a clear signature of the asymmetric localization of electrons recaptured into very elongated (low angular momentum) high-lying Rydberg states after conclusion of the laser pulse. Our findings thus open an effective pathway to control the localization of high-lying Rydberg wave packets.

Enhanced ionisation of polyatomic molecules in intense laser pulses is due to energy upshift and field coupling of multiple orbitals

Abstract: We present the results of a combined experimental and numerical study on strong-field ionisation of acetylene performed with the aim of identifying the mechanism behind the previously reported surprisingly large multi-electron ionisation probabilities of polyatomic molecules. Using coincidence momentum imaging techniques and time-dependent density functional simulations, we show that the reported efficient ionisation is due to the combined action of a significant geometrically induced energy upshift of the most relevant valence orbitals as the C–H distance stretches beyond about two times the equilibrium distance, and a strong increase in the coupling between multiple molecular orbitals concomitant with this stretch motion. The identified enhanced ionisation mechanism, which we refer to as EIC-MOUSE, is only effective for molecules aligned close to parallel to the laser polarisation direction, and is inhibited for perpendicularly aligned molecules because of a suppression of the C–H stretch motion during the onset of ionisation.

Molecular oxygen observed by direct photoproduction from carbon dioxide

Abstract: We report experiments on the direct observation of molecular oxygen formation from ${\mathrm{CO}}_{2}$ in strong laser fields with a reaction microscope. Our accompanying simulations and pump-probe measurements suggest that ${\mathrm{CO}}_{2}$ molecules undergo bending motion during strong-field ionization which supports the molecular oxygen formation process. The observation of molecular oxygen formation from ${\mathrm{CO}}_{2}$ may trigger further experimental and theoretical studies on such processes with laser pulses, and provides hints in studies of the ${\mathrm{O}}_{2}$ and ${\mathrm{O}}_{2}{}^{+}$ abundance in ${\mathrm{CO}}_{2}$-dominated planetary atmospheres.

High-energy pulse stacking via regenerative pulse-burst amplification.

Abstract: Here we present a coherent pulse stacking approach for upscaling the energy of a solid-state femtosecond chirped pulse amplifier We demonstrate pulse splitting into four replicas, amplification in a burst-mode regenerative Yb:CaF2 amplifier, designed to overcome intracavity optical damage by colliding pulse replicas, and coherent combining into a single millijoule level pulse The thresholds of pulse-burst-induced damage of optical elements are experimentally investigated The scheme allows achieving an enhancement factor of 262 using a single-stage stacker cavity and, potentially, much higher enhancement factors using cascaded stacking

Numerical investigation of the sequential-double-ionization dynamics of helium in different few-cycle-laser-field shapes

Abstract: We investigate sequential double ionization of helium by intense near-circularly polarized few-cycle laser pulses using a semiclassical ionization model with two independent electrons. Simulated ${\mathrm{He}}^{2+}$ ion momentum distributions are compared to those obtained in recent benchmark experiments [M. S. Sch\"offler, X. Xie, P. Wustelt, M. M\"oller, S. Roither, D. Kartashov, A. M. Sayler, A. Baltuska, G. G. Paulus, and M. Kitzler, Phys. Rev. A 93, 063421 (2016)]. We study the influence of a number of pulse parameters such as peak intensity, carrier-envelope phase, pulse duration, and second- and third-order spectral phase on the shape of the ion momentum distributions. Good agreement is found in the main features of these distributions and of their dependence on the laser pulse duration, peak intensity, and carrier-envelope phase. Furthermore, we find that for explaining certain fine-scale features observed in the experiments, it becomes important to consider subtle timing variations in the two-electron emissions introduced by small values of chirp. This result highlights the possibility of measuring and controlling multielectron dynamics on the attosecond time scale by fine tuning the field evolution of intense close-to-single-cycle laser pulses.

Bismuth Ferrite Dielectric Nanoparticles Excited at Telecom Wavelengths as Multicolor Sources by Second, Third, and Fourth Harmonic Generation

Abstract: We demonstrate the simultaneous generation of second, third, and fourth harmonic from a single dielectric Bismuth Ferrite nanoparticle excited by a telecom fiber laser at 1560 nm. We first characterize the signals associated with different nonlinear orders in terms of spectrum, excitation intensity dependence, and relative signal strengths. Successively, on the basis of the polarization-resolved emission curves of the three harmonics, we discuss the interplay of susceptibility tensor components at the different orders and we show how polarization can be used as an optical handle to control the relative frequency conversion properties.

Laser wakefield acceleration with mid-IR laser pulses

Abstract: We present the first results from laser plasma wakefield acceleration driven by ultrashort mid-infrared laser pulses (100fs, λ=3.9µm). The onset of relativistic self-focusing and electron acceleration scale with critical laser power and target width.

Mapping anomalous dispersion of air with ultrashort mid-infrared pulses

Abstract: 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.

Optimizing pulse compressibility in completely all-fibered Ytterbium chirped pulse amplifiers for in vivo two photon laser scanning microscopy.

Abstract: A simple and completely all-fiber Yb chirped pulse amplifier that uses a dispersion matched fiber stretcher and a spliced-on hollow core photonic bandgap fiber compressor is applied in nonlinear optical microscopy. This stretching-compression approach improves compressibility and helps to maximize the fluorescence signal in two-photon laser scanning microscopy as compared with approaches that use standard single mode fibers as stretcher. We also show that in femtosecond all-fiber systems, compensation of higher order dispersion terms is relevant even for pulses with relatively narrow bandwidths for applications relying on nonlinear optical effects. The completely all-fiber system was applied to image green fluorescent beads, a stained lily-of-the-valley root and rat-tail tendon. We also demonstrated in vivo imaging in zebrafish larvae, where we simultaneously measure second harmonic and fluorescence from two-photon excited red-fluorescent protein. Since the pulses are compressed in a fiber, this source is especially suited for upgrading existing laser scanning (confocal) microscopes with multiphoton imaging capabilities in space restricted settings or for incorporation in endoscope-based microscopy.

Nonlinear optics in the mid-infrared: new morning

Abstract: 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.

Generation of a single UV pulse from a near-IR pulse burst

Abstract: High harmonic generation (HHG) requires high peak-power laser sources. Most of the well-known high peak power lasers are operating in the near-IR wavelength region. Recently it was demonstrated that HHG can be effectively phase matched in the soft X-ray region by using very high intensity UV lasers and multiply charged ions [1]. High average and high peak power UV sources operating around and below 280 nm are required for many other applications, such as ablation in ophthalmology, materials processing and photoelectron spectroscopy. Due to lack of ultrafast high peak power lasers operating in UV, generation of ultrashort UV pulses is possible by up-converting frequency of near-IR laser. This can be done by cascaded harmonic generation in nonlinear crystals with efficiency higher than 40% [2]. However, to obtain high pulse energies in UV region, high energy IR pump is necessary. This becomes increasingly difficult for femtosecond laser pulses because of the optical damage problem in CPA systems. Very high pulse stretching rates in the CPA become unfeasible due to the limited size of dispersive optics. Alternatively, the intensity in the laser cavity can be decreased by using a pulse burst which effectively increases the pulse duration. Therefore, this approach is also suitable for increasing energy throughput in fiber delivery and fiber post compression schemes [3].

Efficient few-cycle mid-IR pulse generation in the 5-11 µm window driven by an Yb amplifier

Abstract: We demonstrate efficient difference frequency generation in the 5–11 µm range using AGS crystal pumped at wavelengths beyond two-photon absorption limit. A simple and efficient 15 mJ Yb-based driver and a cascaded KTA/AGS parametric down-conversion allow generation of 150 µJ pulses with bandwidth spanning 7–10 µm.

Efficient 170 eV source directly driven by an Yb laser amplifier

Abstract: High brightness XUV sources in the 140–170 eV spectral range are of great demand as this spectral range covers important magnetic and molecular materials, such as the N-edge of Gadolinium and Terbium (145 and 155 eV) and L 2 , 3 edge of sulphur [1]. High-order harmonic generation (HHG) allows performing ultrafast dynamics measurements with high temporal resolution in this spectral range because of extremely short, potentially sub-fs, temporal envelopes that are intrinsically synchronized to the near-IR driving laser pulses. We present HHG in the 170 eV range using 8.5 mJ post-compressed 20 fs 1.03 μm pulses from a 14 mJ Yb:CaF 2 laser chirped pulse amplifier operating at 0.5 kHz repetition rate.

Creating and Dissipating Clouds in the Atmosphere with Ultrashort Lasers

Abstract: 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.

X-ray emission from nanostructured targets irradiated by a relativistically intense mid-infrared driver

Abstract: We report the first experimental results on multi-keV X-ray generation from relativistic laser-solid interaction using a mid-IR (3.9 μm) high power femtosecond laser source and nanostructured solid targets. Regimes of relativistic laser-plasma interaction with a long wavelength driver qualitatively differ from experiments with conventional near-IR or visible laser sources. The dynamics of the laser-particle interaction by relativistic dimensionless parameter a 0 ∝ √Iλ where I — the laser intensity, λ — the wavelength. The same number can be achieved for a long wavelength laser source with much less energy fluence than for a short-wavelength one. This scaling results in high efficiency of X-ray generation from solid plasmas at rather moderate intensities [1]. Also, the relatively small photon energy of the mid-IR radiation reduces the multi-photon absorption probability, and sample with the bandgap of several eV will be ionized closer to the peak intensities, whereas for near-IR or visible laser sources a few photon ionization will occur on the leading front of the laser pulse. We investigate these new regimes of relativistic laser-plasma interaction for different nanostructured solid targets, where the morphology enables an efficient volumetric heating mechanism of solid density plasma [2].

Burst-mode pumping scheme for longwave parametric amplification

Abstract: High-peak-power longwave OPAs operating above the wavelength of 5μm [1] require high-energy picosecond pump pulses >2 μm. These can be generated directly in laser amplifiers, e.g. Ho-doped [1, 2] or via cascaded OPAs [3]. Burst-mode pump lasers offer great advantages over single-pulse amplifiers because they help overcoming the challenges of chirped-pulse as well as direct picosecond pulse amplifiers operating in the singlepulse amplification mode. The techniques of coherent pulse combining [4] and coherent pulse stacking of pulse bursts [5] offer an opportunity, at the expense of interferometric phase stabilization, for a multifold energy increase in a single pulse without modifying the laser amplifier itself. Simple non-interferometric combining techniques, such as energy combination via frequency doubling in a non-collinearly phase-matched nonlinear crystal also exist, but they are not suitable for pumping LWIR OPAs.

180 fs High power megahertz Ytterbium fiber chirped pulse amplifier for in-vivo high-speed functional imaging

Abstract: Imaging tools that allow for high resolution high-speed network-level brain activity mapping are of high interest in neuroscience and in other biomedical applications studying biological population dynamics in scattering tissues in vivo. Currently nonlinear optical microscopy plays a key role in imaging neural activity in behaving animals. In scattering tissue, two-photon microscopy in combination with raster scanning approaches have been widely applied, but one of the most difficult challenges is to achieve sufficient image contrast deep in scattering samples[1]. Due to limited number of emitted photons available for image reconstruction, accessing neuronal dynamics with sufficient temporal resolution for large-scale volumetric measurements of neural activity in scattering tissue is challenging. For high-speed raster scanning multiphoton imaging, fundamentally the speed limit is set by the pulse repetition rate of the femtosecond laser used, i.e. at least one laser pulse must be used per image pixel. In practice, this limit is also ruled by a compromise of the speed and inertia of the imaging and beam steering hardware, as well as the maximum allowed incident laser power on the sample to avoid damage or excessive heating. On the other hand, the available pulse peak intensity and operation wavelength of the driving laser source play also a significant role for achieving deep tissue imaging. To counteract the power decrease with depth, in order to maintain sufficient intensity at the focus at significant depths in scattering media, it is key to increase the energy of the excitation pulse. Yb-fiber chirped pulse amplifiers providing femtosecond, multi μJ-level pulses at very high repetition rates are very attractive for high-speed functional imaging. In this work, we present a μJ-level, MHz repetition rate all-fiber integrated chirped pulse amplifier delivering clean ∼180 fs pulses (Fig. 1 (a-c)), that enabled us to demonstrate a new approach for fast volumetric raster scanning microscopy, using temporal focusing [2] to produce an enlarged point spread function of 5×5×10 μm (matching the typical size of neurons in the mouse cortex) to allow for single laser pulse per vowel excitation [3]. This is important because it allows to sample the imaging volumes with the minimally required number of voxels, which leads to faster volume sampling and higher signal-to-noise ratios for the same average laser power. The large stretching ratio of the pulses in the Yb-fiber amplifier allows us to change the repetition rate and output energy of the system over a large range, without compromising the pulse fidelity. At MHz repetition rate, we have obtained up to 10 W of average power from the system using less than 60% of the available pump power, and at 100 kHz, compression of 1 W of output power (10 μ J pulse energy) from a similar system, using the same stretcher, but less efficient gratings yielded 160 fs pulses [4]. With the system set to a repetition rate of 4 MHz and 600 nJ output energy, we have performed high-speed volumetric functional imaging in the brain of living, awake mice, using a microscope with a sculpted focal volume of 5×5×10 μm3 (xyz) through temporal focusing. We demonstrate high-speed single neuron-resolution three-dimensional imaging in mouse brain in-vivo, on GCamp6 and jRGECKO1 labelled neurons, over large volumetric FOVs (up to 500×500×500 μm) at multi-Hertz (3–6 Hz) update rate, see Fig. 1 (d) and (e).

Enhanced ionization of acetylene in intense laser pulses is due to energy upshift and field coupling of multiple orbitals

Abstract: Laser-ionization of molecules is one of the most important processes in the strong-field and attosecond sciences. One of the key mechanisms governing molecular ionization is enhanced ionization, where for diatomic molecules the ionization probability becomes strongly enhanced at a critical internuclear distance [1, 2]. Experiments on polytatomic molecules have revealed remarkably high charge states and indicated the existence of a different ionization-enhancement mechanism [3-5]. However, until now this mechanism has not been fully understood and has been a subject of intense debate.

Localizing high-lying Rydberg wave packets by orthogonally-polarized two-color laser pulses

Abstract: Highly excited Rydberg atoms and molecules, in comparison with normal atoms and molecules, have unique properties and can be exploited in the studies of the quantum phenomena on human-sized level and the transition from the quantum to the classic world [1]. In a strong laser pulse, valence electrons of an atom or a molecule can be detached through tunnelling or barrier suppression ionization. After conclusion of the pulse, some of the released electrons may be recaptured by the ionic Coulomb potential and populate highly excited Rydberg states[2]. Recently, we have demonstrated the detection of such states with the electron-ion coincidence spectroscopy [3]In this submission, we report on the control of the formation of spatially localized high-lying Rydberg wave packets by waveform controlled orthogonally-polarized two-color (OTC) fields in argon atoms.

Role of CO 2 in filamentation of 3.9-μm Mid IR pulses in ambient air

Abstract: 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.

Localizing High-Lying Rydberg Wave Packets with Orthogonally-Polarized Two-Color Laser Fields

Abstract: ∗Photonics Institute, Technische Universität Wien, A-1040 Vienna, Austria †State Key Laboratory for Mesoscopic Physics and Collaborative Innovation Center of Quantum Matter, School of Physics, Peking University, Beijing 100871, China ‡Institute for Theoretical Physics, Technische Universität Wien, A-1040 Vienna, Austria §Institute of Theoretical Chemistry, University of Vienna, A-1010 Vienna, Austria

Filamentation of chirped mid-IR pulses in ambient air in the vicinity of zero dispersion

Abstract: Variation of the sign of dispersion in air in the vicinity of 3.6 pm [1] makes propagation and filamentation of mid-IR pulses extremely sensitive to the chirp, which allows to take control over the temporal pulse splitting, spectral broadening, energy losses and spatial profile. Here we report on filamentation of 30-mJ, 3.9-pm pulses produced by a hybrid OPA/OPCPA system [2]. Filamentation in ambient air is assisted by loose 7-m focusing and controlled by adjusting the chirp of the output of the OPCPA system.

Losses and intensity clamping during filamentation of mid-IR pulses in ambient air

Abstract: 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.

Direct observation of laser-induced O 2 + production from CO 2

Abstract: On the Earth, oxygen molecules are generated mostly via the photosynthesis by green plants and algae from carbon dioxide and water: nCO 2 + nH 2 O light→ (CH 2 O)n + nO 2 [1]. Although, theoretical studies showed the possibility of O 2 production via CO 2 fragmentation [2] and experimental investigations achieved the detection of C+ (referring the creation of O 2 on the other side) [3], the direct observation of O2 formation did not yet happen to our knowledge. In this submission, we report the direct experimental observation of molecular oxygen formation from carbon dioxide molecule after being doubly ionized by a strong laser pulse.

From three-photon to tunnel ionization pumped ZnO nanolasers

Abstract: We report on experimental investigation of stimulated near ultraviolet (NUV) emission from a disordered array of nanowires pumped in a very broad range of pumping wavelengths ranging from near to mid-IR. The nanowires (NWs) are made of ZnO, which is a well-known wide-bandgap (3.3 eV) semiconductor material and one of a few, efficient NUV laser materials directly driven by UV pumping sources [1, 2]. In addition, two-photon pumped lasing has been demonstrated for thin films of nanocrystalline ZnO [3] as well as three-photon pumped lasing for nanowires [4]. Here, we report on stimulated emission in the 390 nm range from disordered ZnO nanowire arrays pumped via three-photon absorption of intense femtosecond laser pulses centred at 0.8 μm wavelength (near-IR) and in the tunnel ionization regime when the nanowires are excited by a mid-IR source operating at 3.9 μm central wavelength.