17 results
Search Results
Now showing 1 - 10 of 17
- ItemSub-cycle valleytronics: control of valley polarization using few-cycle linearly polarized pulses(Washington, DC : OSA, 2021) Jiménez-Galán, Álvaro; Silva, Rui E. F.; Smirnova, Olga; Ivanov, MishaSo far, it has been assumed that selective excitation of a desired valley in the Brillouin zone of a hexagonal two-dimensional material has to rely on using circularly polarized fields. We theoretically demonstrate a way to control the valley excitation in hexagonal 2D materials on a few-femtosecond timescale using a few-cycle, linearly polarized pulse with controlled carrier–envelope phase. The valley polarization is mapped onto the strength of the perpendicular harmonic signal of a weak, linearly polarized pulse, which allows to read this information all-optically without destroying the valley state and without relying on the Berry curvature, making our approach potentially applicable to inversion-symmetric materials. We show applicability of this method to hexagonal boron nitride and MoS2.
- ItemElectron dynamics in laser-driven atoms near the continuum threshold(Washington, DC : OSA, 2021) Liu, Mingqing; Xu, Songpo; Hu, Shilin; Becker, Wilhelm; Quan, Wei; Liu, Xiaojun; Chen, JingStrong-field ionization and Rydberg-state excitation (RSE) near the continuum threshold exhibit two phenomena that have attracted a lot of recent attention: the low-energy structure (LES) just above and frustrated tunneling ionization just below the threshold. The former becomes apparent for longer laser wavelengths, while the latter has been especially investigated in the near infrared; both have been treated as separate phenomena so far. Here we present a unified perspective based on electron trajectories, which emphasizes the very important role of the electron-ion Coulomb interaction as expected in this energy region. Namely, those trajectories that generate the LES can also be recaptured into a Rydberg state. The coherent superposition of the contributions of such trajectories with different travel times (each generating one of the various LES peaks) causes an oscillation in the intensity dependence of the RSE yield, which is especially noticeable for longer wavelengths. The theory is illustrated by RSE experiments at 1800 nm, which agree very well with the theory with respect to position and period of the oscillation. The wavelength scaling of the RSE oscillation is also discussed. Our work establishes a solid relationship between processes below and above the threshold and sheds new light on atomic dynamics driven by intense laser fields in this critical energy region.
- ItemFew-cycle 65-µJ pulses at 11.4 µm for ultrafast nonlinear longwave-infrared spectroscopy(Washington, DC : Optical Society of America, OSA, 2022) Fuertjes, Pia; Bock, Martin; Grafenstein, Lorenz von; Ueberschaer, Dennis; Griebner, Uwe; Elsaesser, ThomasLow-energy excitations can provide insight into the basic ultrafast nonequilibrium dynamics of condensed matter. High-energy femtosecond pulses in the long-wavelength infrared are required to induce such processes, and can be generated in an optical parametric chirped pulse amplification (OPCPA) system comprising three GaSe stages. A femtosecond Cr:ZnS laser serves as the front-end, providing the seed for the 2.0-µm pump and the 2.4-µm signal pulses without nonlinear conversion processes. The OPCPA system is pumped at 2.05 µm by a picosecond Ho:YLF regenerative amplifier at a 1-kHz repetition rate. The recompressed idler pulses at 11.4 µm have a duration of 185 fs and an unprecedented energy of 65 µJ, corresponding to a pump-to-idler conversion efficiency of 1.2%. Nonlinear transmission experiments in the range of the L2 infrared band of liquid water demonstrate the potential of the pulses for nonlinear vibrational spectroscopy of liquids and solids.
- ItemValley control by linearly polarized laser pulses: example of WSe2(Washington, DC : OSA, 2022) Sharma, S.; Elliott, P.; Shallcross, S.Electrons at the band edges of materials are endowed with a valley index, a quantum number locating the band edge within the Brillouin zone. An important question is then how this index may be controlled by laser pulses, with current understanding that it couples exclusively via circularly polarized light. Employing both tight-binding and state-of-the-art time dependent density function theory, we show that on femtosecond time scales valley coupling is a much more general effect. We find that two time separated linearly polarized pulses allow almost complete control over valley excitation, with the pulse time difference and polarization vectors emerging as key parameters for valley control. Our findings highlight the possibility of controlling coherent electronic excitation by successive femtosecond laser pulses, and offer a route towards valleytronics in two-dimensional materials.
- ItemIn situ temporal measurement of ultrashort laser pulses at full power during high-intensity laser–matter interactions(Washington, DC : OSA, 2020) Crespo, Helder M.; Witting, Tobias; Canhota, Miguel; Miranda, Miguel; Tisch, John W.GIn laser-matter interaction experiments, it is of paramount importance to characterize the laser pulse on target (in situ) and at full power. This allows pulse optimization and meaningful comparison with theory, and it can shed fundamental new light on pulse distortions occurring in or on the target.Here we introduce and demonstrate a new technique based on dispersion-scan using the concurrent third harmonic emission from the target that permits the full (amplitude and phase), in situ, in-parallel characterization of ultrashort laser pulses in a gas or solid target over a very wide intensity range encompassing the 1013-1015Wcm-2regime of high harmonic generation and other important strong-field phenomena, with possible extension to relativistic intensities presently inaccessible to other diagnostics. © 2020 OSA - The Optical Society. All rights reserved.
- Item35 W continuous-wave Ho:YAG single-crystal fiber laser(Cambridge : Cambridge Univ. Press, 2020) Zhao, Yongguang; Wang, Li; Chen, Weidong; Wang, Jianlei; Song, Qingsong; Xu, Xiaodong; Liu, Ying; Shen, Deyuan; Xu, Jun; Mateos, Xavier; Loiko, Pavel; Wang, Zhengping; Xu, Xinguang; Griebner, Uwe; Petrov, ValentinWe report on a high-power Ho:YAG single-crystal fiber (SCF) laser inband pumped by a high-brightness Tm-fiber laser at 1908 nm. The Ho:YAG SCF grown by the micro-pulling-down technique exhibits a propagation loss of at. A continuous-wave output power of 35.2 W is achieved with a slope efficiency of 42.7%, which is to the best of our knowledge the highest power ever reported from an SCF-based laser in the 2 spectral range. © 2020 The Author(s). Published by Cambridge University Press in association with Chinese Laser Press.
- ItemCompact intense extreme-ultraviolet source(Washington, DC : OSA, 2021) Major, Balázs; Ghafur, Omair; Kovács, Katalin; Varjú, Katalin; Tosa, Valer; Vrakking, Marc J. J.; Schütte, B.High-intensity laser pulses covering the ultraviolet to terahertz spectral regions are nowadays routinely generated in a large number of laboratories. In contrast, intense extreme-ultraviolet (XUV) pulses have only been demonstrated using a small number of sources including free-electron laser facilities [1-3] and long high-harmonic generation (HHG) beamlines [4-9]. Here we demonstrate a concept for a compact intense XUV source based on HHG that is focused to an intensity of $2 \times 10^{14}$ W/cm$^2$, with a potential increase up to $10^{17}$ W/cm$^2$ in the future. Our approach uses tight focusing of the near-infrared (NIR) driving laser and minimizes the XUV virtual source size by generating harmonics several Rayleigh lengths away from the NIR focus. Accordingly, the XUV pulses can be refocused to a small beam waist radius of 600 nm, enabling the absorption of up to four XUV photons by a single Ar atom in a setup that fits on a modest (2 m) laser table. Our concept represents a straightforward approach for the generation of intense XUV pulses in many laboratories, providing novel opportunities for XUV strong-field and nonlinear optics experiments, for XUV-pump XUV-probe spectroscopy and for the coherent diffractive imaging of nanoscale structures.
- ItemGain and lasing from CdSe/CdS nanoplatelet stripe waveguides(Amsterdam : Elsevier, 2022) Belitsch, Martin; Dirin, Dmitry N.; Kovalenko, Maksym V.; Pichler, Kevin; Rotter, Stefan; Ghalgaoui, Ahmed; Ditlbacher, Harald; Hohenau, Andreas; Krenn, Joachim R.Colloidal semiconducting nanocrystals are efficient, stable and spectrally tunable emitters, but achievable optical gain is often limited by fast nonradiative processes. These processes are strongly suppressed in slab-shaped nanocrystals (nanoplatelets), due to relaxed exciton Coulomb interaction. Here, we show that CdSe/CdS nanoplatelets can be engineered into (sub)microscopic stripe waveguides that achieve lasing without further components for feedback, i.e., just relying on the stripe end reflection. We find a remarkably high gain factor for the CdSe/CdS nanoplatelets of 1630 cm−1. In addition, by comparison with numerical simulations we assign a distinct emission peak broadening above laser threshold to emission pulse shortening. Our results illustrate the feasibility of geometrically simple monolithic microscale nanoplatelet lasers as an attractive option for a variety of photonic applications.
- ItemTm3+-doped calcium lithium tantalum gallium garnet (Tm:CLTGG): novel laser crystal(Washington, DC : OSA, 2021) Alles, Adrian; Pan, Zhongben; Loiko, Pavel; Serres, Josep Maria; Slimi, Sami; Yingming, Shawuti; Tang, Kaiyang; Wang, Yicheng; Zhao, Yongguang; Dunina, Elena; Kornienko, Alexey; Camy, Patrice; Chen, Weidong; Wang, Li; Griebner, Uwe; Petrov, Valentin; Solé, Rosa Maria; Aguiló, Magdalena; Díaz, Francesc; Mateos, XavierWe report on the development of a novel laser crystal with broadband emission properties at ∼2 µm – a Tm3+,Li+-codoped calcium tantalum gallium garnet (Tm:CLTGG). The crystal is grown by the Czochralski method. Its structure (cubic, sp. gr. 𝐼𝑎3¯𝑑, a = 12.5158(0) Å) is refined by the Rietveld method. Tm:CLTGG exhibits a relatively high thermal conductivity of 4.33 Wm-1K-1. Raman spectroscopy confirms a weak concentration of vacancies due to the charge compensation provided by Li+ codoping. The transition probabilities of Tm3+ ions are determined using the modified Judd-Ofelt theory yielding the intensity parameters Ω2 = 5.185, Ω4 = 0.650, Ω6 = 1.068 [10−20 cm2] and α = 0.171 [10−4 cm]. The crystal-field splitting of the Tm3+ multiplets is revealed at 10 K. The first diode-pumped Tm:CLTGG laser generates 1.08 W at ∼2 µm with a slope efficiency of 23.8%. The Tm3+ ions in CLTGG exhibit significant inhomogeneous spectral broadening due to the structure disorder (a random distribution of Ta5+ and Ga3+ cations over octahedral and tetrahedral lattice sites) leading to smooth and broad gain profiles (bandwidth: 130 nm) extending well above 2 µm and rendering Tm:CLTGG suitable for femtosecond pulse generation.
- ItemA general approach for all-visible-light switching of diarylethenes through triplet sensitization using semiconducting nanocrystals(London [u.a.] : RSC, 2022) Hou, Lili; Larsson, Wera; Hecht, Stefan; Andréasson, Joakim; Albinsson, BoCoupling semiconducting nanocrystals (NCs) with organic molecules provides an efficient route to generate and transfer triplet excitons. These excitons can be used to power photochemical transformations such as photoisomerization reactions using low energy radiation. Thus, it is desirable to develop a general approach that can efficiently be used to control photoswitches using all-visible-light aiming at future applications in life- and materials sciences. Here, we demonstrate a simple ‘cocktail’ strategy that can achieve all-visible-light switchable diarylethenes (DAEs) through triplet energy transfer from the hybrid of CdS NCs and phenanthrene-3-carboxylic acid, with high photoisomerization efficiency and improved fatigue resistance. The size-tunable excitation energies of CdS NCs make it possible to precisely match the clear spectral window of the relevant DAE photoswitch. We demonstrate reversible all-visible-light photoisomerization of a series of DAE derivatives both in the liquid and solid state, even in the presence of oxygen. Our general strategy is promising for fabrication of all-visible-light activated optoelectronic devices as well as memories, and should in principle be adaptable to photopharmacology.