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2023 Vol. 3, No. 5

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High-Sensitivity Visualization of Ultrafast Carrier Diffusion by Wide-Field Holographic Microscopy
Martin Hörmann, Federico Visentin, Andrea Zanetta, Johann Osmond, Giulia Grancini, Niek F. van Hulst, Matz Liebel, Giulio Cerullo, Franco V. A. Camargo
2023, 2023(5) doi: 10.34133/ultrafastscience.0032
Ultrafast transient microscopy is a key tool to study the photophysical properties of materials in space and time, but current implementations are limited to ≈1-μm fields of view, offering no statistical information for heterogeneous samples. Recently, we demonstrated wide-field transient imaging based on multiplexed off-axis holography. Here, we perform ultrafast microscopy in parallel around a hundred diffraction-limited excitation spots over a ≈60-μm field of view, which not only automatically samples the photophysical heterogeneity of the sample over a large area but can also be used to obtain a 10-fold increase in signal-tonoise ratio by computing an average spot. We apply our microscope to study the carrier diffusion processes in methylammonium lead bromide perovskites. We observe strong diffusion due to the presence of hot carriers during the first picosecond and slower diffusion afterward. We also describe how many-body kinetics can be misleadingly interpreted as strong diffusion at high excitation densities, while at weak excitation, real diffusion is observed. Therefore, the vast increase in sensitivity offered by this technique benefits the study of carrier transport not only by reducing data acquisition times but also by enabling the measurement of the much smaller signals generated at low carrier densities.
Fourier-Limited Attosecond Pulse from High Harmonic Generation Assisted by Ultrafast Magnetic Fields
Rodrigo Martín-Hernández, Hongtao Hu, Andrius Baltuska, Luis Plaja, Carlos Hernández-García
2023, 2023(5) doi: 10.34133/ultrafastscience.0036
One of the main constraints for reducing the temporal duration of attosecond pulses is the attochirp inherent to the process of high-order harmonic generation (HHG). Though the attochirp can be compensated in the extreme-ultraviolet using dispersive materials, this is unfeasible toward x-rays, where the shortest attosecond or even sub-attosecond pulses could be obtained. We theoretically demonstrate that HHG driven by a circularly polarized infrared pulse while assisted by an strong oscillating ultrafast intense magnetic field enables the generation of few-cycle Fourier-limited few attosecond pulses. In such a novel scenario, the magnetic field transversally confines the ionized electron during the HHG process, analogously to a nanowire trapping. Once the electron is ionized, the transverse electron dynamics is excited by the magnetic field, acting as a high-energy reservoir to be released in the form of phase-locked spectrally wide high-frequency harmonic radiation during the electron recollision with the parent ion. In addition, the transverse breathing dynamics of the electron wavepacket, introduced by the magnetic trapping, strongly modulates the recollision efficiency of the electronic trajectories, thus the attosecond pulse emissions. The aftermath is the possibility of producing high-frequency (hundreds of eV) attosecond isolated few-cycle pulses, almost Fourier limited. The isolated intense magnetic fields considered in our simulations, of tens of kT, can be produced in finite spatial volumes considering structured beams or stationary configurations of counter-propagating state-of-the-art multi-terawatt/petawatt lasers.
Conical Intersection versus Avoided Crossing: Geometric Phase Effect in Molecular High-Order Harmonics
Guanglu Yuan, Ruifeng Lu, Shicheng Jiang, Konstantin Dorfman
2023, 2023(5) doi: 10.34133/ultrafastscience.0040
Nonadiabatic dynamics around an avoided crossing or a conical intersection play a crucial role in the photoinduced processes of most polyatomic molecules. The present work shows that the topological phase in conical intersection makes the behavior of pump-probe high-order harmonic signals different from the case of avoided crossing. The coherence built up when the system crosses the avoided crossing will lead to the oscillatory behavior of the spectrum, while the geometric phase erodes these oscillations in the case of conical intersection. Additionally, the dynamical blueshift and the splitting of the time-resolved spectrum allow capturing the snapshot dynamics with the sub-femtosecond resolution.
Recent Advances and Perspective of Photonic Bound States in the Continuum
Guizhen Xu, Hongyang Xing, Zhanqiang Xue, Dan Lu, Jinying Fan, Junxing Fan, Perry Ping Shum, Longqing Cong
2023, 2023(5) doi: 10.34133/ultrafastscience.0033
Recent advancements in photonic bound states in the continuum (BICs) have opened up exciting new possibilities for the design of optoelectronic devices with improved performance. In this perspective article, we provide an overview of recent progress in photonic BICs based on metamaterials and photonic crystals, focusing on both the underlying physics and their practical applications. The first part of this article introduces 2 different interpretations of BICs, based on far-field interference of multipoles and near-field analysis of topological charges. We then discuss recent research on manipulating the far-field radiation properties of BICs through engineering topological charges. The second part of the article summarizes recent developments in the applications of BICs, including chiral light and vortex beam generation, nonlinear optical frequency conversion, sensors, and nanolasers. Finally, we conclude with a discussion of the potential of photonic BICs to advance terahertz applications in areas such as generation and detection, modulation, sensing, and isolation. We believe that continued research in this area will lead to exciting new advancements in optoelectronics, particularly in the field of terahertz devices.
Ultrafast Laser Filamentation in Transparent Solids
Tianyang Yan, Lingfei Ji
2023, 2023(5) doi: 10.34133/ultrafastscience.0023
Ultrafast laser filamentation results from the interaction of ultrafast laser with Kerr media. During filamentary propagation, the transparent medium is altered by numerous linear and nonlinear effects of ultrashort laser pulses. Filamentation can cause material modification in solids through laser energy deposition and ionization processes, which creates a new opportunity for ultrafast laser processing of materials when combined with filamentary propagation characteristics, such as intensity champing and long propagation distance. This paper reviews the research on ultrafast laser filamentation in solids for micro- and nano-processing, including the fundamental physics, filamentation characteristics, and applications in solids for ultrafast laser filamentation-induced processing. Additionally highlighted are the difficulties and potential applications for solid-based filamentation-induced processing.
Hearing the Heartbeat of Atoms: Unveiling Attosecond Horizons
Jing Li, Yunquan Liu
2023, 2023(5) doi: 10.34133/ultrafastscience.0049
The 2023 Nobel Prize in Physics spotlights the techniques to generate attosecond light pulses. The generation of attosecond pulses heralds a new era in understanding electron dynamics. This perspective traces the evolution of ultrafast science, from early microwave electronics to the recent breakthroughs in attosecond pulse generation and measurement. Key milestones, such as high harmonic generation, the RABBITT method, attosecond streaking camera, etc, illuminate our journey toward capturing the transient electron motions in atoms. Recent discoveries, including zeptosecond delays in H2 single-photon double ionization and the potential of attosecond “electron” pulses despite challenges, etc., hint at an exciting future for ultrafast studies.