Showing papers by "Andrius Baltuška published in 2020"
TL;DR: Efficient sub-cycle THz pulse generation by using two-color midinfrared femtosecond laser filaments in ambient air using affordable table-top laser systems is experimentally demonstrated.
Abstract: Extreme nonlinear interactions of THz electromagnetic fields with matter are the next frontier in nonlinear optics. However, reaching this frontier in free space is limited by the existing lack of appropriate powerful THz sources. Here, we experimentally demonstrate that two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm allows one to generate ultrashort sub-cycle THz pulses with sub-milijoule energy and THz conversion efficiency of 2.36%, resulting in THz field amplitudes above 100 MV cm−1. Our numerical simulations predict that the observed THz yield can be significantly upscaled by further optimizing the experimental setup. Finally, in order to demonstrate the strength of our THz source, we show that the generated THz pulses are powerful enough to induce nonlinear cross-phase modulation in electro-optic crystals. Our work paves the way toward free space extreme nonlinear THz optics using affordable table-top laser systems. Powerful terahertz pulses are generated during the nonlinear propagation of ultrashort laser pulses in gases. Here, the authors demonstrate efficient sub-cycle THz pulse generation by using two-color midinfrared femtosecond laser filaments in ambient air.
148 citations
20 Oct 2020
TL;DR: In this paper, a continuous red shift provided by stimulated Raman scattering (SRS) over a long propagation distance in nitrogen-filled hollow-core fibers (HCF) was used to achieve a continuous tunability of the laser wavelength from 1030 nm up to 1730 nm with a conversion efficiency higher than 70%.
Abstract: Ultrashort high-energy pulses at wavelengths longer than 1 µm are now desirable for a vast variety of applications in ultrafast and strong-field physics. To date, the main answer to the wavelength tunability for energetic, broadband pulses still relies on optical parametric amplification (OPA), which often requires multiple and complex stages, may feature imperfect beam quality, and has limited conversion efficiency into one of the amplified waves. In this work, we present a completely different strategy to realize an energy-efficient and scalable laser frequency shifter. This relies on the continuous red shift provided by stimulated Raman scattering (SRS) over a long propagation distance in nitrogen-filled hollow-core fibers (HCF). We show a continuous tunability of the laser wavelength from 1030 nm up to 1730 nm with a conversion efficiency higher than 70% and high beam quality. The highly asymmetric spectral broadening, arising from the spatiotemporal nonlinear interplay between higher-order modes of the HCF, can be readily employed to generate pulses (∼20fs) significantly shorter than the pump ones (∼200fs) with high beam quality, and the pulse energy can further be scaled up to tens of millijoules. We envision that this technique, coupled with the emerging high-power Yb laser technology, has the potential to answer the increasing demand for energetic multi-TW few-cycle sources tunable in the near-IR.
34 citations
07 Jan 2020
TL;DR: In this paper, a new approach to study the electron recapture process during double and multiple ionization of atoms was proposed, which is applicable to non-dissociative processes in molecules, which are not accessible with existing methods based on the measurement of kinetic energy released from fragmentation processes.
Abstract: This works shows a new approach to study the electron recapture process during double and multiple ionization of atoms. This scheme is applicable to non-dissociative processes in molecules, which are not accessible with existing methods based on the measurement of kinetic energy released from fragmentation processes. The experiments show a transition of frustrated double ionization in argon from a non-sequential scenario to a sequential scenario.
22 citations
TL;DR: In this article, the authors studied the effect of the pump laser wavelength on the demagnetization dynamics of Co/Pt multilayers by tuning the pump wavelength to 0.4, 0.8, and 1.8 µm.
Abstract: Ultrafast demagnetization, a phenomenon of utmost interest in the context of optical control of magnetically recorded data, has been extensively studied in a variety of different materials. However, only a limited number of studies have investigated the impact of the pump laser wavelength on the process, and only within a narrow spectral range. Performing resonant scattering experiment at the cobalt ${M}_{2,3}$ edges, using extreme ultraviolet radiation photons from a high harmonic source, we studied the ultrafast demagnetization dynamics of Co/Pt multilayers by tuning the pump wavelength to 0.4, 0.8, and 1.8 \ensuremath{\mu}m. We show that the degree of demagnetization at short time scale (100s of fs) is stronger at longer wavelengths. This is explained by the wavelength dependence of both the laser induced heating of the electrons (${T}_{e}\ensuremath{\propto}{\ensuremath{\lambda}}^{2}$) and the spatial distribution of the electromagnetic energy deposited into the multilayer sample.
18 citations
TL;DR: The unambiguous observation of the subcycle ionization bursts in sequential strong-field double ionization of H_{2} and their disentanglement in molecular frame photoelectron angular distributions is reported.
Abstract: We report on the unambiguous observation of the subcycle ionization bursts in sequential strong-field double ionization of H_{2} and their disentanglement in molecular frame photoelectron angular distributions. This observation was made possible by the use of few-cycle laser pulses with a known carrier-envelope phase, in combination with multiparticle coincidence momentum imaging. The approach demonstrated here will allow sampling of the intramolecular electron dynamics and the investigation of charge-state-specific Coulomb distortions on emitted electrons in polyatomic molecules.
13 citations
TL;DR: In this article, a pulse energy-governed nonlinear self-switching of femtosecond pulses at wavelength of 1700nm in a highly nonlinear high index contrast dual-core fiber was demonstrated.
13 citations
Posted Content•
TL;DR: In this paper, a continuous red shift provided by stimulated Raman scattering (SRS) over a long propagation distance in nitrogen-filled hollow core fibers (HCF) was used to achieve a continuous tunability of the laser wavelength from 1030 nm up to 1730 nm with conversion efficiency higher than 70% and high beam quality.
Abstract: Ultrashort high-energy pulses at wavelengths longer than 1 $\mu$m are nowadays desired for a vast variety of applications in ultrafast and strong-field physics. To date, the main answer to the wavelength tunability for energetic, broadband pulses still relies on optical parametric amplification (OPA), which often requires multiple and complex stages, may feature imperfect beam quality and has limited conversion efficiency into one of the amplified waves. In this work, we present a completely different strategy to realize an energy-efficient and scalable laser frequency shifter. This relies on the continuous red shift provided by stimulated Raman scattering (SRS) over a long propagation distance in nitrogen-filled hollow core fibers (HCF). We show a continuous tunability of the laser wavelength from 1030 nm up to 1730 nm with conversion efficiency higher than 70% and high beam quality. The highly asymmetric spectral broadening, arising from the spatiotemporal nonlinear interplay between high-order modes of the HCF, can be readily employed to generate pulses (~20 fs) significantly shorter than the pump ones (~200 fs) with high beam quality, and the pulse energy can further be scaled up to tens of millijoules. We envision that this technique, coupled with the emerging high-power Yb laser technology, has the potential to answer the increasing demand for energetic multi-TW few-cycle sources tunable in the near-IR.
13 citations
TL;DR: A generalized picture of the phase sensitivity of laser-induced directional bond breaking is established using the H_{2} molecule as the example and a strong electron-proton correlation is revealed that may open up a new approach to experimentally accessing the laser-sub-cycle intramolecular electron dynamics also in larger molecules.
Abstract: We establish a generalized picture of the phase sensitivity of laser-induced directional bond breaking using the H_{2} molecule as the example. We show that the well-known proton ejection anisotropy measured with few-cycle pulses as a function of their carrier-envelope phases arises as an amplitude modulation of an intrinsic anisotropy that is sensitive to the laser phase at the ionization time and determined by the molecule's electronic structure. Our work furthermore reveals a strong electron-proton correlation that may open up a new approach to experimentally accessing the laser-sub-cycle intramolecular electron dynamics also in larger molecules.
12 citations
TL;DR: It is found that laser-induced alignment with durations comparable to the characteristic rotational time scale TRotAlign enhances the efficiency of redshifted spectral broadening compared to noble gases.
Abstract: We demonstrate an efficient approach for enhancing the spectral broadening of long laser pulses and for efficient frequency redshifting by exploiting the intrinsic temporal properties of molecular alignment inside a gas-filled hollow-core fiber (HCF). We find that laser-induced alignment with durations comparable to the characteristic rotational time scale TRotAlign enhances the efficiency of redshifted spectral broadening compared to noble gases. The applicability of this approach to Yb lasers with (few hundred femtoseconds) long pulse duration is illustrated, for which efficient broadening based on conventional Kerr nonlinearity is challenging to achieve. Furthermore, this approach proposes a practical solution for high energy broadband long-wavelength light sources, and it is attractive for many strong field applications.
12 citations
20 Dec 2020
TL;DR: In this article, the Vernier effect was used to combine amplitude and phase shaping of femtosecond near-infrared pulses to produce tunable terahertz (THz) frequency pulses.
Abstract: Amplified bursts of laser pulses are sought for various machining, deposition, spectroscopic, and strong-field applications. Standard frequency- and time-domain techniques for pulse division become inadequate when intraburst repetition rates reach the terahertz (THz) range as a consequence of inaccessible spectral resolution, requirement for interferometric stability, and collapse of the chirped-pulse amplification (CPA) concept due to the loss of usable bandwidth needed for safe temporal stretching. Avoiding the burst amplification challenge and resorting to lossy post-division of an isolated laser pulse after CPA leaves the limitations of frequency- and time-domain techniques unsolved. In this Letter, we demonstrate an approach that successfully combines amplitude and phase shaping of THz bursts, formed using the Vernier effect, with active stabilization of spectral modes and efficient energy extraction from a CPA regenerative amplifier. As proof of concept, the amplified bursts of femtosecond near-infrared pulses are down-converted into tunable THz-frequency pulses via optical rectification.
11 citations
TL;DR: This paper presents a theoretical and experimental analysis of active feedback methods to stabilize otherwise unstable operational regimes of regenerative amplifiers and develops suitable feedback strategies utilizing measurements of the output pulse energies or the transmitted pump light.
Abstract: The performance of regenerative amplifiers at high repetition rates is often limited by the occurrence of bifurcations induced by a destabilization of the pulse-to-pulse dynamics. While bifurcations can be suppressed by increasing the seed energy using dedicated pre-amplifiers, the availability of adjustable filters and control electronics in modern pulse amplifiers allows to exploit feedback strategies to cope with these instabilities. In this paper, we present a theoretical and experimental analysis of active feedback methods to stabilize otherwise unstable operational regimes of regenerative amplifiers. To this end, the dynamics of regenerative amplifiers are investigated starting from a general space-dependent description to obtain a generalization of existing models from the literature. Suitable feedback strategies are then developed utilizing measurements of the output pulse energies or the transmitted pump light, respectively. The effectiveness of the proposed approach is highlighted by experimental results for a Yb:CaF2-based regenerative amplifier.
TL;DR: In this article, the authors investigated the ellipticity dependence of high-order harmonic generation from femtosecond mid-IR laser pulses in ZnO single crystal and polycrystalline thin film.
Abstract: The generation of high order harmonics from femtosecond mid-IR laser pulses in ZnO has shown great potential to reveal new insight into the ultrafast electron dynamics on a few femtosecond timescale. In this work we report on the experimental investigation of photoluminescence and high-order harmonic generation (HHG) in a ZnO single crystal and polycrystalline thin film irradiated with intense femtosecond mid-IR laser pulses. The ellipticity dependence of the HHG process is experimentally studied up to the 17th harmonic order for various driving laser wavelengths in the spectral range 3-4 µm. Interband Zener tunneling is found to exhibit a significant excitation efficiency drop for circularly polarized strong-field pump pulses. For higher harmonics with energies larger than the bandgap, the measured ellipticity dependence can be quantitatively described by numerical simulations based on the density matrix equations. The ellipticity dependence of the below and above ZnO band gap harmonics as a function of the laser wavelength provides an efficient method for distinguishing the dominant HHG mechanism for different harmonic orders.
TL;DR: A strong-field route to controlling the dynamics in molecular compounds through the excitation of electronic dynamics on a distant molecule by driving intermolecular electron-transfer processes is disclosed.
Abstract: We report on an experimental and theoretical study of the ionization-fragmentation dynamics of argon dimers in intense few-cycle laser pulses with a tagged carrier-envelope phase. We find that a field-driven electron transfer process from one argon atom across the system boundary to the other argon atom triggers subcycle electron-electron interaction dynamics in the neighboring atom. This attosecond electron-transfer process between distant entities and its implications manifests itself as a distinct phase-shift between the measured asymmetry of electron emission curves of the Ar^{+}+Ar^{2+} and Ar^{2+}+Ar^{2+} fragmentation channels. This letter discloses a strong-field route to controlling the dynamics in molecular compounds through the excitation of electronic dynamics on a distant molecule by driving intermolecular electron-transfer processes.
TL;DR: In this article, photoelectron angular distributions generated by double ionization of van der Waals dimers with ultrashort, intense laser pulses are measured using four-body coincidence imaging with a reaction microscope.
Abstract: We describe the results of experiments and simulations performed with the aim of extending photoelectron spectroscopy with intense laser pulses to the case of molecular compounds. Dimer frame photoelectron angular distributions generated by double ionization of ${\mathrm{N}}_{2}\ensuremath{-}{\mathrm{N}}_{2}$ and ${\mathrm{N}}_{2}\ensuremath{-}{\mathrm{O}}_{2}$ van der Waals dimers with ultrashort, intense laser pulses are measured using four-body coincidence imaging with a reaction microscope. To study the influence of the first-generated molecular ion on the ionization behavior of the remaining neutral molecule we employ a two-pulse sequence comprising of a linearly polarized and a delayed elliptically polarized laser pulse that allows distinguishing the two ionization steps. By analysis of the obtained electron momentum distributions we show that scattering of the photoelectron on the neighboring molecular potential leads to a deformation and rotation of the photoelectron angular distribution as compared to that measured for an isolated molecule. Based on this result we demonstrate that the electron momentum space in the dimer case can be separated, allowing us to extract information about the ionization pathway from the photoelectron angular distributions. Our work, when implemented with a variable pulse delay, opens up the possibility of investigating light-induced electronic dynamics in molecular dimers using angularly resolved photoelectron spectroscopy with intense laser pulses.
TL;DR: Compression of low energy ∼40fs pulses to ∼8fs using saturated hydrocarbons is demonstrated, suggesting the feasibility of this media for high repetition rate laser pulse compression.
Abstract: A conventional hollow core fiber (HCF) scheme is implemented to investigate spectral broadening of Titanium:Sapphire (Ti-Sa) femtosecond laser pulses in saturated hydrocarbon molecules compared to unsaturated ones. While the saturated molecules exhibit a spectral broadening similar to noble gases, for the unsaturated ones with π bonds, broadening towards blue is restrained. Numerical simulations underpin that it is a combination of group velocity dispersion (GVD) and Raman scattering which limits the spectral broadening for the unsaturated molecules. Compression of low energy ∼40fs pulses to ∼8fs using saturated hydrocarbons is demonstrated, suggesting the feasibility of this media for high repetition rate laser pulse compression.
08 Nov 2020
TL;DR: In this article, the authors investigated the optical rectification (OR) in the organic crystal BNA with respect to the crystal thickness and central wavelength of the driving pulse in the near-IR spectral range.
Abstract: We investigate THz generation by optical rectification (OR) in the organic crystal BNA with respect to the crystal thickness and central wavelength of the driving pulse in the near-IR spectral range. We report on high optical- to THz conversion efficiencies around 1 J.μm and broad spectra exceeding 5 THz.
14 Sep 2020
TL;DR: In this article, the angular dependence of laser-induced electron rescattering in CO2 with a novel method from the measured rotational half revival signals of filed-free aligned molecules using a reaction microscope was reported.
Abstract: We report on the retrieval of the angular dependence of laser-induced electron rescattering in CO2 with a novel method from the measured rotational half revival signals of filed-free aligned molecules using a reaction microscope.
Posted Content•
TL;DR: In this paper, the Vernier effect was used to combine amplitude and phase shaping of femtosecond near-infrared pulses to produce tunable terahertz (THz) frequency pulses.
Abstract: Amplified bursts of laser pulses are sought for various machining, deposition, spectroscopic and strong-field applications. Standard frequency- and time-domain techniques for pulse division become inadequate when intraburst repetition rates reach the terahertz (THz) range as a consequence of inaccessible spectral resolution, requirement for interferometric stability, and collapse of the chirped pulse amplification (CPA) concept due to the loss of usable bandwidth needed for safe temporal stretching. Avoiding the burst amplification challenge and resorting to a lossy post-division of an isolated laser pulse after CPA leaves the limitations of frequency- and time-domain techniques unsolved. In this letter, we demonstrate an approach that successfully combines amplitude and phase shaping of THz bursts, formed using the Vernier effect, with active stabilization of spectral modes and efficient energy extraction from a CPA regenerative amplifier. As proof of concept, the amplified bursts of femtosecond near-infrared pulses are down-converted into tunable THz-frequency pulses via optical rectification.
16 Nov 2020
TL;DR: In this paper, a conventional hollow core fiber (HCF) scheme is implemented to investigate the effect of group velocity dispersion (GVD) and Raman scattering in spectral broadening in molecular gases for low energy pulse compression application.
Abstract: A conventional hollow-core fiber (HCF) scheme is implemented to investigate the effect of group velocity dispersion (GVD) and Raman scattering in spectral broadening in molecular gases for low energy pulse compression application.
16 Nov 2020
TL;DR: In this article, the authors demonstrate generation of continuously-tunable, narrowband femtosecond pulses from phase-locked multi-millijoule femto-cond pulse bursts by optical rectification in a tilted-pulsefront setup.
Abstract: We demonstrate generation of continuously-tunable, narrowband THz pulses from phase-locked multi-millijoule femtosecond pulse bursts by optical rectification in a tilted-pulse-front setup. Experimental results indicate advances in both, femtosecond pulse burst and high-energy THz source technology.
10 May 2020
TL;DR: In this article, the authors report on unexpected efficient THz generation in organic crystals (DAST and DSTMS) by optical rectification (OR) of intense mid-infrared pulses.
Abstract: We report on unexpected efficient THz generation in organic crystals (DAST and DSTMS) by optical rectification (OR) of intense mid-infrared pulses. The crystals experience superior damage threshold and an ultra-broadband THz spectrum reaching 10 THz.
10 Oct 2020
TL;DR: In this paper, the authors demonstrate that temporally confined spatial solitons can be realized by space-time coupled propagation of strong femtosecond pulses in a nonlinear optical resonator, consisting of periodic layered Kerr media (PLKM).
Abstract: We experimentally demonstrate that temporally confined spatial solitons can be realized by space-time coupled propagation of strong femtosecond pulses in a nonlinear optical resonator, consisting of periodic layered Kerr media (PLKM). A universal relationship between the characteristic beam size and the critical nonlinear phase of the solitary modes is revealed, defining different regions of soliton stability. Taking advantage of the unique characters of these solitary modes, we demonstrate supercontinuum generation and pulse compression of initially 260 μJ, 170 fs pulses down to 22 fs in a single-stage PLKM resonator with an efficiency >90%.
10 May 2020
TL;DR: In this paper, the authors demonstrate generation of continuously-tunable, narrowband femtosecond pulses from phase-locked multi-millijoule femto-cond pulse bursts by optical rectification in a tilted-pulsefront setup.
Abstract: We demonstrate generation of continuously-tunable, narrowband THz pulses from phase-locked multi-millijoule femtosecond pulse bursts by optical rectification in a tilted-pulse-front setup. Experimental results indicate advances in both, femtosecond pulse burst and high-energy THz source technology.
Posted Content•
29 Jun 2020
TL;DR: In this paper, the authors investigate the space-time coupling of optical solitons in a nonlinear resonator and reveal a universal relationship between the beam size and the critical nonlinear phase of the solitary modes, defining different regions of the resonator stability.
Abstract: The dynamics and stabilization of solitons are essential for a large variety of fundamental processes in nonlinear optics, condensed matter physics and biology Taking the solitary propagation of femtosecond pulses in periodic layered Kerr media as an example, we investigate the space-time coupling of optical solitons in a nonlinear resonator A universal relationship between the beam size and the critical nonlinear phase of the solitary modes is revealed, defining different regions of the resonator stability Space-time coupling is shown to strongly influence the spectral, spatial and temporal profiles of femtosecond pulses The sustainable propagation of femtosecond pulses with a GW peak power is demonstrated on resonance, which can be regarded as temporally confined spatial solitons Taking advantage of the unique characters of these solitary modes, we demonstrate single-stage supercontinuum generation and compression of femtosecond pulses from initially 170 fs down to 22 fs with an efficiency >90% We also provide evidence of efficient mode self-cleaning which suggests rich spatial-temporal self-organization processes of laser beams in a nonlinear resonator
16 Nov 2020
TL;DR: Stimulated Raman scattering over ~6m-long nitrogen-filled hollow core fibers enables the efficient generation of high energy (mJ), few-cycle (~20-fs), shortwave-IR (1-1.73 µm) pulses as discussed by the authors.
Abstract: Stimulated Raman Scattering over ~6-m-long nitrogen-filled hollow-core fibers enables the efficient generation of high-energy (mJ), few-cycle (~20-fs), shortwave-IR (1-1.73 µm) pulses. Full three-dimensional model is also required to predict the observed extremely asymmetric spectral broadening.
16 Nov 2020
TL;DR: In this paper, a high harmonics source is used to perform x-ray resonant magnetic scattering on magnetic samples and the dynamics of laser-induced ultrafast demagnetization are probed and longer pump wavelengths are found to increase the initial quenching levels
Abstract: A high harmonics source is used to perform x-ray resonant magnetic scattering on magnetic samples The dynamics of laser-induced ultrafast demagnetization are probed and longer pump wavelengths are found to increase the initial quenching levels
19 Jul 2020
TL;DR: All-optical switching of 77 fs pulses centered at 1550 nm, driven by 270 fs, 1030 nm pulses in dual-core optical fibre exhibiting high index contrast is presented.
Abstract: All-optical switching of 77 fs pulses centered at 1550 nm, driven by 270 fs, 1030 nm pulses in dual-core optical fibre exhibiting high index contrast is presented. The switching is achieved by nonlinear balancing of dual-core asymmetry. High switching contrast exceeding 20 dB is attained with driving pulses of only few nanojoule energy, which is due to the fibre core made of soft glass possessing high nonlinearity.
TL;DR: In this article, an energy-efficient and scalable technique to realize energetic sub-100-fs pulses, continuously tunable in the 1030÷1730 nm range, by exploiting stimulated Raman scattering in long N2-filled hollow-core fibers is presented.
Abstract: We present an energy-efficient and scalable technique to realize energetic sub-100-fs pulses, continuously tunable in the 1030÷1730 nm range, by exploiting stimulated Raman scattering in long N2-filled hollow-core fibers.
08 Nov 2020
TL;DR: In this article, the authors demonstrated the generation of continuously-tunable, narrowband THz pulses from phase-locked multi-millijoule femtosecond pulse bursts by optical rectification.
Abstract: Optical rectification in lithium niobate and semiconductor nonlinear optical materials, utilizing the tilted-pulse-front technique, has become a versatile source of intense single- and multicycle THz pulses. Novel approaches recently proposed, and some of them demonstrated, are promising to scale the technology to unprecedented field strengths by enabling uniform interaction length for THz generation across a large beam. The generation of continuously-tunable, narrowband THz pulses from phase-locked multi-millijoule femtosecond pulse bursts by optical rectification is demonstrated.
DOI•
24 Apr 2020
TL;DR: In this article, a two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm was proposed to achieve sub-cycle THz pulses with multi-millijoule energies and record conversion efficiency.
Abstract: Extreme nonlinear interactions of THz electromagnetic fields with matter are the next frontier in nonlinear optics. However, reaching this frontier in free space is limited by the existing lack of appropriate powerful THz sources. Here we demonstrate both theoretically and experimentally the realization of a novel THz source with high peak power performance based on two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm. Our theory predicts that under this scheme sub-cycle THz pulses with multi-millijoule energies and record conversion efficiencies can be produced. Besides, we elucidate the origin of this high efficiency, which is made up of several factors, including a novel mechanism where the harmonics produced by the mid-infrared pulses strongly contribute to the field symmetry breaking and enhance the THz generation. In our experiments we verify the theoretical predictions by demonstrating ultrashort sub-cycle THz pulses with sub-millijoule energy and THz conversion efficiency of 2.36%, resulting in THz field amplitudes above 100 MV cm-1. Moreover, we show that these intense THz fields can drive nonlinear effects in bulk semiconductors (ZnSe and ZnTe) in free space and at room temperature. Our numerical simulations indicate that the observed THz yield can be significantly upscaled by further optimizing the experimental setup leading to even higher field strengths. Such intense THz pulses enable extreme field science, including into other, relativistic phenomena.