Ultrashort pulse | EPFL Graph Search

In optics, an ultrashort pulse, also known as an ultrafast event, is an electromagnetic pulse whose time duration is of the order of a picosecond (10−12 second) or less. Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators. Amplification of ultrashort pulses almost always requires the technique of chirped pulse amplification, in order to avoid damage to the gain medium of the amplifier. They are characterized by a high peak intensity (or more correctly, irradiance) that usually leads to nonlinear interactions in various materials, including air. These processes are studied in the field of nonlinear optics. In the specialized literature, "ultrashort" refers to the femtosecond (fs) and picosecond (ps) range, although such pulses no longer hold the record for the shortest pulses artificially generated. Indeed, x-ray pulses with durations on the attosecond time scale have been reported. The 1999 Nobel Prize in Chemistry was awarded to Ahme

PHz Electronic Device Design and Simulation for Waveguide-Integrated Carrier-Envelope Phase Detection

Yujia Yang

Carrier-envelope phase (CEP) detection of ultrashort optical pulses and low-energy waveform field sampling have recently been demonstrated using direct time-domain methods that exploit optical-field photoemission from plasmonic nanoantennas. These devices are compact and integratable solid-state detectors operating at optical frequencies under ambient conditions using low pulse energies (picojoule-level). Potential applications include frequency-comb stabilization, optical time-domain spectroscopy, compact tools for attosecond science and metrology, and petahertz-scale information processing. However, to date these devices have been driven by free-space optical waveforms and their implementation within integrated photonic platforms has yet to be demonstrated. In this work, we design and simulate fully-integrated plasmonic nanoantennas coupled to a Si3N4-core waveguide for CEP detection. We find that when coupled to realistic on-chip, few-cycle supercontinuum sources, these devices are suitable for direct time-domain CEP detection within integrated photonic platforms. We estimate a signal-to-noise ratio of 30 dB at 50 kHz resolution bandwidth, and address technical details, such as the tuning of the nanoantennas plasmonic resonance, CEP slippage in the waveguide, optical losses, and sensitivity to driving pulse characteristics such as energy and duration. Our results provide the basis for future design and fabrication of time-domain CEP detectors and allow for the development of fully-integrated attosecond science applications, frequency-comb stabilization and light-wave-based PHz electronics.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2022

Ultrafast Photoelectron and Optical Spectroscopies of Photoinduced Processes in Molecular Systems in Solution

Luca Longetti

Investigating molecular excitations with femtosecond time resolution is of pivotal importance to understand the out-of-equilibrium processes taking place in molecular systems upon light absorption. The photochemistry of solvated species is heavily determined by the early steps of the relaxation dynamics. In this thesis, ultrafast electronic spectroscopies are exploited to study relaxation dynamics of photoexcited molecules in solution by probing the transient electronic structure. Vacuum-ultraviolet (VUV) ultrashort monochromatized light pulses (HARMONIUM, 15-100 eV) are exploited for steady-state photoelectron spectroscopy (PES) and time-resolved PES in a pump-probe scheme. This is a powerful tool for investigating electronic structure and ultrafast processes of solvated molecules with high time (50fs) and energy (0.12eV) resolution. PES retrieves binding energies of valence and core electronic states with elemental specificity and without the constraints of selection rules, and is here applied to liquids and solutions by means of a liquid micro-jet. With the aim of extending this technique to a larger class of solutes, we demonstrate the capability to perform studies on volatile liquids such as organic solvents. PES spectra of gas phase and liquid phase aromatic compounds are presented and their valence orbitals are compared and identified. The solvation is discussed and the vertical ionization energies are reported. This retrieves important parameters for electrochemistry and opens new perspectives towards PES studies of molecules in organic solvents. Upon photoexcitation, molecules can undergo several relaxation processes such as nonradiative relaxation processes which include dissociation, internal conversion and intersystem crossing. Energy can also be released to the solvent environment by intermolecular processes such as vibrational energy transfer. All of these phenomena are reflected in a transient electronic configuration. PES can hence probe electronic and structural dynamics along the entire reaction coordinates. The ferric trisoxalate complex represents an ideal coordination compound showing an intriguing cascade of processes upon photoexcitation. We have directly observed the metal photoreduction due to an instantaneous intramolecular charge transfer. We then suggest a branching between photodissociation and a back-electron transfer channels, which completes the picture of the compoundâs photochemistry. This experiment shows PES sensitivity to oxidation state changes, providing an interesting perspective for photoredox reaction studies in solution and for intra- and inter-molecular charge-transfer phenomena. Vibrational wave packets (WP) are interesting observables to map non-adiabatic dynamics, relaxation pathways and coupling to solvent of the excited molecule. We have investigated molecular iodine in ethanol, which we have studied first by optical transient absorption spectroscopy using a white light continuum probe. The electronic absorption spectrum of I2 in ethanol is strongly modified compared to the case of other organic solvents, due to a strong coupling of covalent and ionic states. We have mapped the vibrational WP dynamics, in particular its trajectory and damping along the I-I bond axis. This study is intended to prepare for an ultrafast PES experiment aimed at linking the response of inner shells to the WP launched in the valence states. Preliminary steady-state PES spectra are reported and discussed.

EPFL2020

Roadmap on optical rogue waves and extreme events

Krassimir Panajotov, Yiqi Zhang

The pioneering paper 'Optical rogue waves' by Solli et al (2007 Nature 450 1054) started the new subfield in optics. This work launched a great deal of activity on this novel subject. As a result, the initial concept has expanded and has been enriched by new ideas. Various approaches have been suggested since then. A fresh look at the older results and new discoveries has been undertaken, stimulated by the concept of 'optical rogue waves'. Presently, there may not by a unique view on how this new scientific term should be used and developed. There is nothing surprising when the opinion of the experts diverge in any new field of research. After all, rogue waves may appear for a multiplicity of reasons and not necessarily only in optical fibers and not only in the process of supercontinuum generation. We know by now that rogue waves may be generated by lasers, appear in wide aperture cavities, in plasmas and in a variety of other optical systems. Theorists, in turn, have suggested many other situations when rogue waves may be observed. The strict definition of a rogue wave is still an open question. For example, it has been suggested that it is defined as 'an optical pulse whose amplitude or intensity is much higher than that of the surrounding pulses'. This definition (as suggested by a peer reviewer) is clear at the intuitive level and can be easily extended to the case of spatial beams although additional clarifications are still needed. An extended definition has been presented earlier by N Akhmediev and E Pelinovsky (2010 Eur. Phys. J. Spec. Top. 185 1-4). Discussions along these lines are always useful and all new approaches stimulate research and encourage discoveries of new phenomena. Despite the potentially existing disagreements, the scientific terms 'optical rogue waves' and 'extreme events' do exist. Therefore coordination of our efforts in either unifying the concept or in introducing alternative definitions must be continued. From this point of view, a number of the scientists who work in this area of research have come together to present their research in a single review article that will greatly benefit all interested parties of this research direction. Whether the authors of this 'roadmap' have similar views or different from the original concept, the potential reader of the review will enrich their knowledge by encountering most of the existing views on the subject. Previously, a special issue on optical rogue waves (2013 J. Opt. 15 060201) was successful in achieving this goal but over two years have passed and more material has been published in this quickly emerging subject. Thus, it is time for a roadmap that may stimulate and encourage further research.

Iop Publishing Ltd2016