| 1. |
- Carlström, Stefanos, et al.
(författare)
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Spatially and spectrally resolved quantum path interference with chirped driving pulses
- 2016
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 18:12
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Tidskriftsartikel (refereegranskat)abstract
- We measure spectrally and spatially resolved high-order harmonics generated in argon using chirped multi-cycle laser pulses. Using a stable, high-repetition rate laser we observe detailed interference structures in the far-field. The structures are of two kinds; off-axis interference from the long trajectory only and on-axis interference including the short and long trajectories. The former is readily visible in the far-field spectrum, modulating both the spectral and spatial profile. To access the latter, we vary the chirp of the fundamental, imparting different phases on the different trajectories, thereby changing their relative phase. Using this method together with an analytical model, we are able to explain the on-axis behaviour and access the dipole phase parameters for the short (${\alpha }_{{\rm{s}}}$) and long (${\alpha }_{{\rm{l}}}$) trajectories. The extracted results compare very well with phase parameters calculated by solving the time-dependent Schrödinger equation. Going beyond the analytical model, we are also able to successfully reproduce the off-axis interference structure.
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| 5. |
- Larsen, Esben Witting, et al.
(författare)
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Sub-cycle ionization dynamics revealed by trajectory resolved, elliptically-driven high-order harmonic generation
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- The sub-cycle dynamics of electrons driven by strong laser fields is central to the emerging field of attosecond science. We demonstrate how the dynamics can be probed through high-order harmonic generation, where different trajectories leading to the same harmonic order are initiated at different times, thereby probing different field strengths. We find large differences between the trajectories with respect to both their sensitivity to driving field ellipticity and resonant enhancement. To accurately describe the ellipticity dependence of the long trajectory harmonics we must include a sub-cycle change of the initial velocity distribution of the electron and its excursion time. The resonant enhancement is observed only for the long trajectory contribution of a particular harmonic when a window resonance in argon, which is off-resonant in the field-free case, is shifted into resonance due to a large dynamic Stark shift.
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| 6. |
- Lorek, Eleonora, et al.
(författare)
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High-Order Harmonic Generation and Plasmonics
- 2015
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Ingår i: Nano-Structures for Optics and Photonics : Optical Strategies for Enhancing Sensing, Imaging, Communication and Energy Conversion - Optical Strategies for Enhancing Sensing, Imaging, Communication and Energy Conversion. - Dordrecht : Springer Netherlands. - 9789401791328 - 9789401791335 ; , s. 531-531
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Bokkapitel (refereegranskat)abstract
- Attosecond pulses allow for imaging of very fast processes, like electron dynamics. Stockman et al. suggested to use these pulses in connection with a Photoemission electron microscope (PEEM) to study the ultrafast dynamics of plasmons (Stockman et al. Nat Photonics 1:539–544, 2007). For efficient plasmon studies, the repetition rate of the attosecond pulses used needs to be higher than a few kHz (Mikkelsen et al. Rev Sci Instrum 80:123703, 2009). Attosecond pulses are produced in a process called high-order harmonic generation (HHG) (Paul et al. Science 292(5522):1689–1692, 2001; Ferray et al. J Phys B At Mol Opt Phys 21:L31–L35, 1988). In HHG, a strong laser field allows an electron to tunnel out, get accelerated and recombine with a high kinetic energy resulting in extreme ultraviolet attosecond pulses. The large intensity needed to drive the process normally limits the repetition rate of the laser to a few kHz. Using a tight focusing scheme (Heyl et al. Phys Rev Lett 107:033903, 2011; Vernaleken et al. Opt Lett 36:3428–3430, 2011), we, however, generate harmonics at a repetition rate of 200 kHz, both with a commercial turn-key laser and with an advanced laser system. Suitable nanostructures for a strong field enhancement are produced in-house and the field enhancement is studied with PEEM in a non-time resolved manner. With high-order harmonics produced at a high repetition rate, we hope to be able to follow also the ultrafast dynamics of plasmons in these structures (Mårsell et al. Ann der Phys 525:162–170, 2013).
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| 7. |
- Lorek, Eleonora, et al.
(författare)
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High-order harmonic generation using a high-repetition-rate turnkey laser.
- 2014
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 1089-7623 .- 0034-6748. ; 85:12
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Tidskriftsartikel (refereegranskat)abstract
- We generate high-order harmonics at high pulse repetition rates using a turnkey laser. High-order harmonics at 400 kHz are observed when argon is used as target gas. In neon, we achieve generation of photons with energies exceeding 90 eV (∼13 nm) at 20 kHz. We measure a photon flux of up to 4.4 × 10(10) photons per second per harmonic in argon at 100 kHz. Many experiments employing high-order harmonics would benefit from higher repetition rates, and the user-friendly operation opens up for applications of coherent extreme ultra-violet pulses in new research areas.
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| 8. |
- Lorek, Eleonora, et al.
(författare)
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Size and shape dependent few-cycle near-field dynamics of bowtie nanoantennas
- 2015
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Ingår i: Optics Express. - 1094-4087. ; 23:24, s. 31460-31471
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Tidskriftsartikel (refereegranskat)abstract
- Metal nanostructures can transfer electromagnetic energy from femtosecond laser pulses to the near-field down to spatial scales well below the optical diffraction limit. By combining few-femtosecond laser pulses with photoemission electron microscopy, we study the dynamics of the induced few-cycle near-field in individual bowtie nanoantennas. We investigate how the dynamics depend on antenna size and exact bowtie shape resulting from fabrication. Different dynamics are, as expected, measured for antennas of different sizes. However, we also detect comparable dynamics differences between individual antennas of similar size. With Finite-difference time-domain simulations we show that these dynamics differences between similarly sized antennas can be due to small lateral shape variations generally induced during the fabrication.
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| 9. |
- Lorek, Eleonora
(författare)
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Time-resolved plasmonics in designed nanostructures
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A metal nanoparticle can be considered as consisting of a base of positive ion cores and a sea of free electrons. When the free electrons are displaced, for example, by an incident electric field, a restoring force acts on the electrons. The electrons may then oscillate back and forth until equilibrium is reached. This oscillation occurs at the natural frequency, or eigenfrequency, of the system. By matching the driving frequency with this frequency, the amplitude (the maximum electron displacement) can be made large - the system is in resonance. This resonance mode is a plasmon. The separation of charge on that small length scale will result in a large field in the vicinity of the nanoparticle. This large field, often oscillating at optical frequencies, on the spatial scale of nanometers, has many potential applications, such as high-resolution microscopy, photo-voltaics, light emission and coherent control. Because of the interest in manipulating light on the nanoscale, particles having their resonances in the optical domain are often used. The collective electron oscillation, when resonantly excited, therefore occurs on the femtosecond timescale. Due to this ultrashort timescale, the dynamics are difficult to follow in time. The spatial confinement of the oscillation to the nanometer scale makes it challenging to also image them. This thesis explores ways of studying the ultrafast dynamics of plasmons spatially and temporally, simultaneously. Two types of experiments are discussed. The first is autocorrelation experiments where the induced and enhanced field is autocorrelated with itself. For one of these experiments, bowtie nanoantennas were manufactured, using the focused ion beam technique. In the second kind of experiment an infrared laser pulse is used to excite the plasmon, and a short attosecond pulse probes it. The work described in this thesis deals with the fabrication of nanostructures and the implementation of attosecond pulse generation schemes suitable for this purpose.
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| 10. |
- Matyschok, Jan, et al.
(författare)
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Compact, high-repetition-rate OPCPA system for high harmonic generation
- 2014
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Ingår i: Frontiers in Ultrafast Optics : Biomedical, Scientific, and Industrial Applications XIV - Biomedical, Scientific, and Industrial Applications XIV. - : SPIE. - 9780819498854 ; 8972
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Konferensbidrag (refereegranskat)abstract
- A compact, high-repetition rate optical parametric chirped pulse amplifier system emitting CEP-stable, few-cycle pulses with 10 μJ of pulse energy is reported for the purpose of high-order harmonic generation. The system is seeded from a commercially available, CEP-stabilized Ti:sapphire oscillator, delivering an octave-spanning spectrum from 600-1200 nm. The oscillator output serves on the one hand as broadband signal for the parametric amplification process and on the other hand as narrowband seed for an Ytterbium-based fiber preamplifier with subsequent main amplifiers and frequency doubling. Broadband parametric amplification up to 17 μJ at 200 kHz repetition rate was achieved in two 5 mm BBO crystals using non-collinear phase matching in the Poynting-vector-walk-off geometry. Efficient pulse compression down to 6.3 fs is achieved with chirped mirrors leading to a peak power exceeding 800 MW. We observed after warm-up time a stability of < 0.5 % rms over 100 min. Drifts of the CE-phase in the parametric amplifier part could be compensated by a slow feedback to the set point of the oscillator phase lock. The CEP stability was measured to be better than 80 mrad over 15 min (3 ms integration time). The experimentally observed output spectra and energies could be well reproduced by simulations of the parametric amplification process based on a (2+1)-dimensional nonlinear propagation code, providing important insight for future repetition rate scaling of OPCPA systems. The system is well-suited for attosecond science experiments which benefit from the high repetition rate. First results for high-order harmonic generation in argon will be presented.
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