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- ItemStable coherent mode-locking based on π pulse formation in single-section lasers([London] : Macmillan Publishers Limited, part of Springer Nature, 2021) Arkhipov, Rostislav; Pakhomov, Anton; Arkhipov, Mikhail; Babushkin, Ihar; Rosanov, NikolayHere we consider coherent mode-locking (CML) regimes in single-section cavity lasers, taking place for pulse durations less than atomic population and phase relaxation times, which arise due to coherent Rabi oscillations of the atomic inversion. Typically, CML is introduced for lasers with two sections, the gain and absorber ones. Here we show that, for certain combination of the cavity length and relaxation parameters, a very stable CML in a laser, containing only gain section, may arise. The mode-locking is unconditionally self-starting and appears due to balance of intra-pulse de-excitation and slow interpulse-scale pump-induced relaxation processes. We also discuss the scaling of the system to shorter pulse durations, showing a possibility of mode-locking for few-cycle pulses.
- ItemStrong-field ionization of clusters using two-cycle pulses at 1.8 μm([London] : Macmillan Publishers Limited, part of Springer Nature, 2016) Schütte, Bernd; Ye, Peng; Patchkovskii, Serguei; Austin, Dane R.; Brahms, Christian; Strüber, Christian; Witting, Tobias; Ivanov, Misha Yu; Tisch, John W. G.; Marangos, Jon P.The interaction of intense laser pulses with nanoscale particles leads to the production of high-energy electrons, ions, neutral atoms, neutrons and photons. Up to now, investigations have focused on near-infrared to X-ray laser pulses consisting of many optical cycles. Here we study strong-field ionization of rare-gas clusters (103 to 105 atoms) using two-cycle 1.8 μm laser pulses to access a new interaction regime in the limit where the electron dynamics are dominated by the laser field and the cluster atoms do not have time to move significantly. The emission of fast electrons with kinetic energies exceeding 3 keV is observed using laser pulses with a wavelength of 1.8 μm and an intensity of 1 × 1015 W/cm2, whereas only electrons below 500 eV are observed at 800 nm using a similar intensity and pulse duration. Fast electrons are preferentially emitted along the laser polarization direction, showing that they are driven out from the cluster by the laser field. In addition to direct electron emission, an electron rescattering plateau is observed. Scaling to even longer wavelengths is expected to result in a highly directional current of energetic electrons on a few-femtosecond timescale.
- ItemTailoring optical properties and stimulated emission in nanostructured polythiophene([London] : Macmillan Publishers Limited, part of Springer Nature, 2019) Portone, Alberto; Ganzer, Lucia; Branchi, Federico; Ramos, Rodrigo; Caldas, Marília J.; Pisignano, Dario; Molinari, Elisa; Cerullo, Giulio; Persano, Luana; Prezzi, Deborah; Virgili, TersillaPolythiophenes are the most widely utilized semiconducting polymers in organic electronics, but they are scarcely exploited in photonics due to their high photo-induced absorption caused by interchain polaron pairs, which prevents the establishment of a window of net optical gain. Here we study the photophysics of poly(3-hexylthiophene) configured with different degrees of supramolecular ordering, spin-coated thin films and templated nanowires, and find marked differences in their optical properties. Transient absorption measurements evidence a partially-polarized stimulated emission band in the nanowire samples, in contrast with the photo-induced absorption band observed in spin-coated thin films. In combination with theoretical modeling, our experimental results reveal the origin of the primary photoexcitations dominating the dynamics for different supramolecular ordering, with singlet excitons in the nanostructured samples superseding the presence of polaron pairs, which are present in the disordered films. Our approach demonstrates a viable strategy to direct optical properties through structural control, and the observation of optical gain opens the possibility to the use of polythiophene nanostructures as building blocks of organic optical amplifiers and active photonic devices. © 2019, The Author(s).
- ItemRole of hole confinement in the recombination properties of InGaN quantum structures([London] : Macmillan Publishers Limited, part of Springer Nature, 2019) Anikeeva, M.; Albrecht, M.; Mahler, F.; Tomm, J. W.; Lymperakis, L.; Chèze, C.; Calarco, R.; Neugebauer, J.; Schulz, T.We study the isolated contribution of hole localization for well-known charge carrier recombination properties observed in conventional, polar InGaN quantum wells (QWs). This involves the interplay of charge carrier localization and non-radiative transitions, a non-exponential decay of the emission and a specific temperature dependence of the emission, denoted as “s-shape”. We investigate two dimensional In0.25Ga0.75N QWs of single monolayer (ML) thickness, stacked in a superlattice with GaN barriers of 6, 12, 25 and 50 MLs. Our results are based on scanning and high-resolution transmission electron microscopy (STEM and HR-TEM), continuous-wave (CW) and time-resolved photoluminescence (TRPL) measurements as well as density functional theory (DFT) calculations. We show that the recombination processes in our structures are not affected by polarization fields and electron localization. Nevertheless, we observe all the aforementioned recombination properties typically found in standard polar InGaN quantum wells. Via decreasing the GaN barrier width to 6 MLs and below, the localization of holes in our QWs is strongly reduced. This enhances the influence of non-radiative recombination, resulting in a decreased lifetime of the emission, a weaker spectral dependence of the decay time and a reduced s-shape of the emission peak. These findings suggest that single exponential decay observed in non-polar QWs might be related to an increasing influence of non-radiative transitions.
- ItemGigaGauss solenoidal magnetic field inside bubbles excited in under-dense plasma([London] : Macmillan Publishers Limited, part of Springer Nature, 2016) Lécz, Z.; Konoplev, I.V.; Seryi, A.; Andreev, A.This paper proposes a novel and effective method for generating GigaGauss level, solenoidal quasi-static magnetic fields in under-dense plasma using screw-shaped high intensity laser pulses. This method produces large solenoidal fields that move with the driving laser pulse and are collinear with the accelerated electrons. This is in contrast with already known techniques which rely on interactions with over-dense or solid targets and generates radial or toroidal magnetic field localized at the stationary target. The solenoidal field is quasi-stationary in the reference frame of the laser pulse and can be used for guiding electron beams. It can also provide synchrotron radiation beam emittance cooling for laser-plasma accelerated electron and positron beams, opening up novel opportunities for designs of the light sources, free electron lasers, and high energy colliders based on laser plasma acceleration.
- ItemOcean rogue waves and their phase space dynamics in the limit of a linear interference model([London] : Macmillan Publishers Limited, part of Springer Nature, 2016) Birkholz, Simon; Brée, Carsten; Veselić, Ivan; Demircan, Ayhan; Steinmeyer, GünterWe reanalyse the probability for formation of extreme waves using the simple model of linear interference of a finite number of elementary waves with fixed amplitude and random phase fluctuations. Under these model assumptions no rogue waves appear when less than 10 elementary waves interfere with each other. Above this threshold rogue wave formation becomes increasingly likely, with appearance frequencies that may even exceed long-term observations by an order of magnitude. For estimation of the effective number of interfering waves, we suggest the Grassberger-Procaccia dimensional analysis of individual time series. For the ocean system, it is further shown that the resulting phase space dimension may vary, such that the threshold for rogue wave formation is not always reached. Time series analysis as well as the appearance of particular focusing wind conditions may enable an effective forecast of such rogue-wave prone situations. In particular, extracting the dimension from ocean time series allows much more specific estimation of the rogue wave probability.
- ItemInfluence of tunnel ionization to third-harmonic generation of infrared femtosecond laser pulses in air([London] : Macmillan Publishers Limited, part of Springer Nature, 2020) Tamulienė, Viktorija; Juškevičiūtė, Greta; Buožius, Danas; Vaičaitis, Virgilijus; Babushkin, Ihar; Morgner, UweHere we present an experimental as well as theoretical study of third-harmonic generation in tightly focused femtosecond filaments in air at the wavelength of 1.5μm. At low intensities, longitudinal phase matching is dominating in the formation of 3rd harmonics, whereas at higher intensities locked X-waves are formed. We provide the arguments that the X-wave formation is governed mainly by the tunnel-like ionization dynamics rather than by the multiphoton one. Despite of this fact, the impact of the ionization-induced nonlinearity is lower than the one from bound–bound transitions at all intensities. © 2020, The Author(s).
- ItemFemtosecond X-ray absorption study of electron localization in photoexcited anatase TiO 2([London] : Macmillan Publishers Limited, part of Springer Nature, 2015) Santomauro, F.G.; Lübcke, A.; Rittmann, J.; Baldini, E.; Ferrer, A.; Silatani, M.; Zimmermann, P.; Grübel, S.; Johnson, J.A.; Mariager, S.O.; Beaud, P.; Grolimund, D.; Borca, C.; Ingold, G.; Johnson, S.L.; Chergui, M.Transition metal oxides are among the most promising solar materials, whose properties rely on the generation, transport and trapping of charge carriers (electrons and holes). Identifying the latter’s dynamics at room temperature requires tools that combine elemental and structural sensitivity, with the atomic scale resolution of time (femtoseconds, fs). Here, we use fs Ti K-edge X-ray absorption spectroscopy (XAS) upon 3.49 eV (355 nm) excitation of aqueous colloidal anatase titanium dioxide nanoparticles to probe the trapping dynamics of photogenerated electrons. We find that their localization at Titanium atoms occurs in <300 fs, forming Ti3+ centres, in or near the unit cell where the electron is created. We conclude that electron localization is due to its trapping at pentacoordinated sites, mostly present in the surface shell region. The present demonstration of fs hard X-ray absorption capabilities opens the way to a detailed description of the charge carrier dynamics in transition metal oxides.
- ItemEven harmonic generation in isotropic media of dissociating homonuclear molecules([London] : Macmillan Publishers Limited, part of Springer Nature, 2016) Silva, R.E.F.; Rivière, P.; Morales, F.; Smirnova, O.; Ivanov, M.; Martín, F.Isotropic gases irradiated by long pulses of intense IR light can generate very high harmonics of the incident field. It is generally accepted that, due to the symmetry of the generating medium, be it an atomic or an isotropic molecular gas, only odd harmonics of the driving field can be produced. Here we show how the interplay of electronic and nuclear dynamics can lead to a marked breakdown of this standard picture: a substantial part of the harmonic spectrum can consist of even rather than odd harmonics. We demonstrate the effect using ab-initio solutions of the time-dependent Schrödinger equation for and its isotopes in full dimensionality. By means of a simple analytical model, we identify its physical origin, which is the appearance of a permanent dipole moment in dissociating homonuclear molecules, caused by light-induced localization of the electric charge during dissociation. The effect arises for sufficiently long laser pulses and the region of the spectrum where even harmonics are produced is controlled by pulse duration. Our results (i) show how the interplay of femtosecond nuclear and attosecond electronic dynamics, which affects the charge flow inside the dissociating molecule, is reflected in the nonlinear response, and (ii) force one to augment standard selection rules found in nonlinear optics textbooks by considering light-induced modifications of the medium during the generation process.
- ItemPopulation difference gratings created on vibrational transitions by nonoverlapping subcycle THz pulses([London] : Macmillan Publishers Limited, part of Springer Nature, 2021) Arkhipov, Rostislav; Pakhomov, Anton; Arkhipov, Mikhail; Babushkin, Ihar; Demircan, Ayhan; Morgner, Uwe; Rosanov, NikolayWe study theoretically a possibility of creation and ultrafast control (erasing, spatial frequency multiplication) of population density gratings in a multi-level resonant medium having a resonance transition frequency in the THz range. These gratings are produced by subcycle THz pulses coherently interacting with a nonlinear medium, without any need for pulses to overlap, thereby utilizing an indirect pulse interaction via an induced coherent polarization grating. High values of dipole moments of the transitions in the THz range facilitate low field strength of the needed THz excitation. Our results clearly show this possibility in multi-level resonant media. Our theoretical approach is based on an approximate analytical solution of time-dependent Schrödinger equation (TDSE) using perturbation theory. Remarkably, as we show here, quasi-unipolar subcycle pulses allow more efficient excitation of higher quantum levels, leading to gratings with a stronger modulation depth. Numerical simulations, performed for THz resonances of the H20 molecule using Bloch equations for density matrix elements, are in agreement with analytical results in the perturbative regime. In the strong-field non-perturbative regime, the spatial shape of the gratings becomes non-harmonic. A possibility of THz radiation control using such gratings is discussed. The predicted phenomena open novel avenues in THz spectroscopy of molecules with unipolar and quasi-unipolar THz light bursts and allow for better control of ultra-short THz pulses.