Podcasts about Femtosecond

This is a recap of the top 10 posts on Hacker News on May 14th, 2024.This podcast was generated by wondercraft.ai(00:37): VeoOriginal post: https://news.ycombinator.com/item?id=40358041&utm_source=wondercraft_ai(02:18): Glider – open-source eInk monitor with an emphasis on low latencyOriginal post: https://news.ycombinator.com/item?id=40358309&utm_source=wondercraft_ai(03:46): Gemini FlashOriginal post: https://news.ycombinator.com/item?id=40358071&utm_source=wondercraft_ai(05:26): Sir, there's a cat in your mirror dimensionOriginal post: https://news.ycombinator.com/item?id=40357141&utm_source=wondercraft_ai(07:10): VMware Fusion Pro: Now available free for personal useOriginal post: https://news.ycombinator.com/item?id=40357271&utm_source=wondercraft_ai(08:56): Model Explorer: intuitive and hierarchical visualization of model graphsOriginal post: https://news.ycombinator.com/item?id=40357681&utm_source=wondercraft_ai(10:47): Femtosecond lasers create 3D midair plasma displays you can touch (2015)Original post: https://news.ycombinator.com/item?id=40356751&utm_source=wondercraft_ai(12:19): Firefox search updateOriginal post: https://news.ycombinator.com/item?id=40355982&utm_source=wondercraft_ai(13:55): The notifier pattern for applications that use PostgresOriginal post: https://news.ycombinator.com/item?id=40352686&utm_source=wondercraft_ai(15:30): Fast linked listsOriginal post: https://news.ycombinator.com/item?id=40355227&utm_source=wondercraft_aiThis is a third-party project, independent from HN and YC. Text and audio generated using AI, by wondercraft.ai. Create your own studio quality podcast with text as the only input in seconds at app.wondercraft.ai. Issues or feedback? We'd love to hear from you: team@wondercraft.ai

In this enlightening episode of 'The History of Eyecare,' host Dr. Morgan Micheletti discovers the inspiring journey of Dr. Denise Visco, an esteemed ophthalmologist known for her dedication to cataract surgery and innovation in eye care. Dr. Visco recounts her unique path into the world of ophthalmology, sharing the pivotal moments and decisions that shaped her career. Her passion for eye care shines through as she discusses the early challenges she faced and the exhilarating experience of starting her own practice. Dr. Micheletti and Dr. Visco engage in a fascinating conversation about the latest advancements in eye care technology and surgical techniques. Dr. Visco shares her insights into the evolution of procedures like LASIK and SMILE, highlighting the transformative impact these techniques have had on patient care. The discussion illuminates the constant progression of eye care, underscoring the importance of embracing new technologies to enhance patient outcomes. Concluding the episode, Dr. Visco reflects on the broader implications of these advancements for the field of ophthalmology. The dialogue emphasizes the significance of continuous learning, mentorship, and the drive to push the boundaries of what's possible in eye care. Listeners will leave with a deeper appreciation of the dynamic and ever-evolving nature of ophthalmology, inspired by Dr. Visco's commitment to excellence and innovation in the field.

Femtosecond Laser Assisted Cataract Surgery (FLACS) has been around for over a decade, yet there isn't necessarily consensus amongst the eye care community as to how this technology should be implemented within cataract surgery. Some advocate strongly for FLACS, citing greater reliability and precision than traditional cataract surgery. Others have argued, however, that even if the femtosecond technology has improved accuracy, this doesn't necessarily mean that it translates to better cataract surgery results. Dr. Eric Donnenfeld joins the podcast.

Chemistry is complicated but it had to start somewhere. The origins of complex chemistry had to be built up from scratch. How did complex compounds form on early earth. How can we replicate the conditions of early earth and watch complex chemistry develop? Peering into chemical reactions is tricky because they can happen so fast.  Zhong Yin, Yi-Ping Chang, Tadas Balčiūnas, Yashoj Shakya, Aleksa Djorović, Geoffrey Gaulier, Giuseppe Fazio, Robin Santra, Ludger Inhester, Jean-Pierre Wolf, Hans Jakob Wörner. Femtosecond proton transfer in urea solutions probed by X-ray spectroscopy. Nature, 2023; DOI: 10.1038/s41586-023-06182-6  

Welcome to The ohmTown Daily News Show (ODNS). The show is held live on https://www.twitch.tv/ohmTown/ at 11AM Eastern. I cover a selection of aggregated news articles and discuss them briefly with a perspective merging business, technology, and society. Episode: ohmTown Daily News Show for August 22nd, 2022. (Episode 234) Links Discussed: https://www.ohmtown.com/groups/hatchideas/f/d/nasa-released-an-audio-clip-of-a-black-hole-and-its-pretty-spooky/ (nasa-released-an-audio-clip-of-a-black-hole) https://www.ohmtown.com/groups/the-word-in-tech/f/d/ebay-is-acquiring-tcgplayer-one-of-the-largest-trading-card-marketplaces/ (ebay-is-acquiring-tcgplayer) https://www.ohmtown.com/groups/warcrafters/f/d/bitcoin-mining-company-recoups-67m-in-debt-selling-26200-old-rigs/ (bitcoin-mining-company-recoups-67m) https://www.ohmtown.com/groups/realityhacker/f/d/townscaper-vr-releasing-on-quest-pico-headsets-in-october/ (townscaper-vr) https://www.ohmtown.com/groups/warcrafters/f/d/slime-rancher-2-wrangles-its-way-onto-pc-next-month/ (slime-rancher-2) https://www.ohmtown.com/groups/ofthegrape/f/d/how-is-brandy-created-infographic/ (how-is-brandy-created) https://www.ohmtown.com/groups/the-word-in-tech/f/d/electron-slow-motion-ion-physics-on-the-femtosecond-scale/ (electron-slow-motion-ion-physics) https://www.ohmtown.com/groups/mobble/f/d/scientists-are-unraveling-the-mystery-of-the-arrow-of-time/ (the-mystery-of-the-arrow-of-time) https://www.ohmtown.com/groups/the-continuity-report/f/d/knives-out-sequel-glass-onion-gets-release-date-reveals-first-look-images/ (knives-out-sequel-glass-onion) https://www.ohmtown.com/groups/the-word-in-tech/f/d/electrical-currents-to-the-brain-improve-memory-for-older-adults-study-finds/ (electrical-currents-to-the-brain-improve) https://www.ohmtown.com/groups/thedailynewsshow/f/d/ford-cutting-3000-white-collar-jobs-in-bid-to-lower-costs/ (ford-cutting-3000-white-collar-jobs) https://www.ohmtown.com/groups/the-word-in-tech/f/d/thor-love-and-thunder-is-coming-to-disney-plus-in-just-a-few-weeks/ (thor-love-and-thunder-is-coming-to-disney-plus)

In this podcast episode, MRS Bulletin's Laura Leay interviews Nate Hohman from The University of Connecticut about the structure of two chalcogenolates his group uncovered. By combining serial femtosecond crystallography —usually used to characterize large molecules—and a clique algorithm, Hohman's group was able to analyze the structure of small molecules. With serial femtosecond crystallography, large molecules like proteins produce thousands of spots on the detector; in contrast, small molecule crystals only a produce a few spots. The algorithm uses the pattern that the spots make on the detector to determine the orientation of as many crystals in the liquid jet as possible. The data from each crystal can then be merged together to find the structure. 

Chunlei Guo, from the High-Intensity Femtosecond Laser Laboratory in the Institute of Optics at the University of Rochester, discusses his work in laser processing, leading to the discovery of highly functionalized materials. Guo's research, including with black and colored metals, is addressing societal issues including global access to clean water. Chrys Panayiotou, executive director of LASER-TEC, joins us to talk about developing a highly skilled optics and photonics workforce.Sponsored by:Photonics Spectra Conference - www.PhotonicsSpectraConference.comAll Things Photonics is produced by Photonics Media and airs biweekly, on Tuesdays. Find links to the stories mentioned in the episode on our website, www.photonics.com/podcast.

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.11.13.382218v1?rss=1 Authors: Hosseinizadeh, A., Breckwoldt, N., Fung, R., Sepehr, R., Schmidt, M., Schwander, P., Santra, R., Ourmazd, A. Abstract: The structural dynamics of a molecule are determined by the underlying potential energy landscape. Conical intersections are funnels connecting otherwise separate energy surfaces. Posited almost a century ago [1], conical intersections remain the subject of intense scientific investigation [2-4]. In biology, they play a pivotal role in vision, photosynthesis, and DNA stability [5,6]. In ultrafast radiationless de-excitation [1,7], they are vital to ameliorating photon-induced damage. In chemistry, they tightly couple the normally separable nuclear and electronic degrees of freedom, precluding the Born-Oppenheimer approximation [8]. In physics, they manifest a Berry phase, giving rise to destructive interference between clockwise and anti-clockwise trajectories around the conical intersection [9]. Accurate theoretical methods for examining conical intersections are at present limited to small molecules. Experimental investigations are challenged by the required time resolution and sensitivity. Current structure-dynamical understanding of conical intersections is thus limited to simple molecules with around 10 atoms, on timescales of about 100 fs or longer [10]. Spectroscopy can achieve better time resolution, but provides only indirect structural information. Here, we present single-femtosecond, atomic-resolution movies of a 2,000-atom protein passing through a conical intersection. These movies, extracted from experimental data by geometric machine learning, reveal the dynamical trajectories of de-excitation via a conical intersection, yield the key parameters of the conical intersection controlling the de-excitation process, and elucidate the topography of the electronic potential energy surfaces involved. Copy rights belong to original authors. Visit the link for more info

Dr. Keith Walter and Dr. Gary Wortz discuss why Femto is beneficial in 2020, how they have been very successful in integrating Femto into their busy practices, and their most common use for the Femtosecond laser.

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.09.09.287987v1?rss=1 Authors: Durdagi, S., Dag, C., Dogan, B., Yigin, M., Avsar, T., Buyukdag, C., Erol, I., Ertem, F. B., Calis, S., Yildirim, G., Orhan, M., Guven, O., Aksoydan, B., Destan, E., Sahin, K., Besler, S. O., Oktay, L., Shafiei, A., Tolu, I., Ayan, E., Yuksel, B., Peksen, A. B., Gocenler, O., Yucel, A. D., Can, O., Ozabrahamyan, S., Olkan, A., Erdemoglu, E., Aksit, F., Tanisali, G. H., Yefanov, O. M., Barty, A., Tolstikova, A., Ketawala, G. K., Botha, S., Dao, E. H., Hayes, B., Liang, M., Seaberg, M. H., Hunter, M. S., Batyuk, A., Mariani, V., Su, Z., Poitevin, F., Yoon, C. H., Kupitz, C. J., Sierra, R. G., Sn Abstract: The COVID19 pandemic has resulted in 25+ million reported infections and nearly 850.000 deaths. Research to identify effective therapies for COVID19 includes: i) designing a vaccine as future protection; ii) structure-based drug design; and iii) identifying existing drugs to repurpose them as effective and immediate treatments. To assist in drug repurposing and design, we determined two apo structures of Severe Acute Respiratory Syndrome CoronaVirus-2 main protease at ambient-temperature by Serial Femtosecond X-ray crystallography. We employed detailed molecular simulations of selected known main protease inhibitors with the structures and compared binding modes and energies. The combined structural biology and molecular modeling studies not only reveal the dynamics of small molecules targeting main protease but will also provide invaluable opportunities for drug repurposing and structure-based drug design studies against SARS-CoV-2. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.08.21.257170v1?rss=1 Authors: Mehrabi, P., Buecker, R., Bourenkov, G., Ginn, H., von Stetten, D., Mueller-Werkmeister, H. M., Kuo, A., Morizumi, T., Eger, B., Ou, W.-L., Oghbaey, S., Sarracini, A., Besaw, J., Pare-Labrosse, O., Meier, S., Schikora, H., Tellkamp, F., Marx, A., Sherrell, D. A., Axford, D., Owen, R. L., Ernst, O. P., Pai, E. F., Schulz, E. C., Miller, R. J. D. Abstract: For the two proteins myoglobin (MB) and fluoroacetate dehalogenase (FAcD), we present a systematic comparison of crystallographic diffraction data collected by serial femtosecond (SFX) and serial synchrotron crystallography (SSX). To maximize comparability, we used the same batch of crystals, the same sample delivery device, as well as the same data analysis software. Overall figures of merit indicate that the data of both radiation sources are of equivalent quality. For both proteins reasonable data statistics can be obtained with approximately 5000 room temperature diffraction images irrespective of the radiation source. The direct comparability of SSX and SFX data indicates that diffraction quality is rather linked to the properties of the crystals than to the radiation source. Time-resolved experiments can therefore be conducted at the source that best matches the desired time-resolution. Copy rights belong to original authors. Visit the link for more info

Pulin Shah, MD, joins Marguerite McDonald, MD, FACS, for a deep dive into the topic of femtosecond laser cataract surgery. They discuss the growing market for the surgery, and the ins and outs of its successful implementation.

Pulin Shah, MD, joins Marguerite McDonald, MD, FACS, for a deep dive into the topic of femtosecond laser cataract surgery. They discuss the growing market for the surgery, and the ins and outs of its successful implementation.

Guests: Nick Mamalis, MD Professor University of Utah Department of Ophthalmology and Visual Sciences Salt Lake City, UT Michael Greenwood, MD  Clinical Instructor of Surgery University of North Dakota School of Medicine and Health Sciences Fargo, N.D.

This episode sees 3 interviews with 3 different experts in the field of Femtosecond lasers:   Richard Packard talks to Zoltan Nagy, who performed the first femtosecond laser-cut anterior capsulotomy in a human patient in August 2008.   Thomas Kohnen talks to Julian Stevens about the femto-laser assisted LASIK approach.What is the best treatment for low to moderate myopia? Surgeons have an increasing number of options to choose among.  Sean Henehan asks Boris Malyugin is femtosecond laser-assisted cataract surgery (FLACS) a fad or is it here to stay? Is there any evidence from larger studies to support any particular advantages for FLACS over conventional surgery?   

Guests: George O. Waring IV, MD Assistant Professor of Ophthalmology Director of Refractive Surgery Storm Eye Institute Medical University of South Carolina (MUSC) Charleston, South Carolina Peter A. Karth, MD Oregon Eye Consultants Eugene, Oregon

Guests: Soon Phaik Chee, MD Associate Professor Singapore National Eye Centre Singapore Mingguang He, MD, PhD Professor of Ophthalmic Epidemiology University of Melbourne and Centre for Eye Research Australia Director of WHO Collaborating Centre for Prevention of Blindness (Australia) Melbourne, Australia

Mon, 19 Jan 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/18006/ https://edoc.ub.uni-muenchen.de/18006/1/Bucher_Dominik.pdf Bucher, Dominik ddc:530, ddc:500, Fakultät für Physik

Chirped pulse amplification in solid-state lasers is currently the method of choice for producing high-energy ultrashort pulses, having surpassed the performance of dye lasers over 20 years ago. The third generation of femtosecond technology based on short-pulse-pumped optical parametric chirped pulse amplification (OPCPA) holds promise for providing few-cycle pulses with terawatt-scale peak powers and kilowatt-scale-average powers simultaneously, heralding the next wave of attosecond and femtosecond science. OPCPA laser systems pumped by near-1-ps pulses support broadband and efficient amplification of few-cycle pulses due to their unrivaled gain per unit length. This is rooted in the high threshold for dielectric breakdown of the nonlinear crystals for even shorter pump pulse durations. Concomitantly, short pump pulses simplify dispersion management and improve the temporal contrast of the amplified signal. This thesis covers the main experimental and theoretical steps required to design and operate a high-power, high-energy, few-cycle OPCPA. This includes the generation of a broadband, high-contrast, carrier envelope phase (CEP)-stable seed, the practical use of a high-power thin-disk regenerative amplifier, its efficient use for pumping a multi-stage OPCPA chain and compression of the resulting pulses. A theoretical exploration of the concept and its extension to different modes of operation, including widely-tunable, high-power multi-cycle pulse trains, and ultrabroadband waveform synthesis is presented. Finally, a conceptual design of a field synthesizer with multi-terawatt, multi-octave light transients is discussed, which holds promise for extending the photon energy attainable via high harmonic generation to several kiloelectronvolts, nourishing the hope for attosecond spectroscopy at hard-x-ray wavelengths.

Die grundlegenden Funktionsprinzipien der Natur zu verstehen, ist seit jeher Antrieb der Naturwissenschaften. Verhalten und Eigenschaften von Festkörpern werden dabei häufig von dynamischen Prozessen auf atomarer Skala (< 10^-10 m) bestimmt, welche typischerweise auf Zeitskalen im Bereich von zehn Femtosekunden (10^-15 s) bis hin zu vielen Picosekunden (10^-12 s) ablaufen. Zeitaufgelöste Elektronenbeugung an kristallinen Festkörpern ermöglicht die direkte Beobachtung solcher Prozesse in Raum und Zeit. Die bislang mit diesem Verfahren erreichte Zeitauflösung von etwa 100 fs eignet sich jedoch nicht zur Beobachtung der schnellsten Prozesse in Festkörpern. Auch die, zur zuverlässigen Auflösung von großen Elementarzellen molekularer Kristalle erforderliche, transversale Kohärenz ist unzureichend. Eine wesentliche Ursache für diese beiden Probleme liegt in der gegenseitigen Coulomb-Abstoßung der Elektronen innerhalb eines Pulses und den daraus resultierenden Veränderungen der Geschwindigkeitsverteilungen in radialer und longitudinaler Richtung. Während erstere zu verringerter transversaler Kohärenz führt, hat letztere längere Elektronenpulsdauern und damit eine begrenzte Zeitauflösung zur Folge. In dieser Arbeit wird ein Messaufbau zur zeitaufgelösten Elektronenbeugung vorgestellt, welcher auf der Erzeugung von nur einem Elektron pro Puls basiert. Aufgrund der Vermeidung von Coulomb-Abstoßung innerhalb der Pulse ist dieser Ansatz eine vielversprechende Basis zur konzeptionell nahezu unbegrenzten Verbesserung der Zeitauflösung. Eine hier eigens entwickelte, thermisch stabilisierte Elektronenquelle garantiert einen hohen Grad an Kohärenz bei gleichzeitig hervorragender Langzeitstabilität der Photoelektronenausbeute. Insbesondere letzteres ist für zeitaufgelöste Beugungsexperimente mit Einzeleelektronen aufgrund der längeren Integrationszeit unerlässlich, konnte jedoch durch vorhergehende Quellen nicht erreicht werden. Darüber hinaus werden in dieser Arbeit die besonderen Ansprüche der Einzelelektronenbeugung an die zu untersuchenden Materialien diskutiert und Strategien zur Vermeidung von Schäden an der Probe durch akkumulierte Anregungsenergie entwickelt. Diese erfordern neue Schwerpunkte bei der Probenpräparation, welche entwickelt und diskutiert werden. Die Beobachtung der komplexen Relaxationsdynamik in Graphit-Dünnfilmen mit zeitaufgelöster Einzelelektronenbeugung demonstriert abschließend die generelle Eignung dieses Verfahrens als zuverlässige Methodik zur Untersuchung von reversibler, struktureller Dynamik in Festkörpern mit atomarer Auflösung. Nicht-relativistische Einzelelektronenpulse können mit Hilfe von zeitabhängigen Feldern bei Mikrowellenfrequenzen bis in den 10 fs-Bereich komprimiert werden, eventuell sogar bis in den Attosekundenbereich. Die hier demonstrierte langzeitstabile und hochkohärente Elektronenquelle, sowie die Methodiken zur Probenpräparation und zeitaufgelösten Beugung mit Einzelelektronenpulsen liefern die Basis für zukünftige Experimente dieser Art.

Wed, 16 Oct 2013 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/16299/ https://edoc.ub.uni-muenchen.de/16299/1/Krebs_Nils.pdf Krebs, Nils ddc:530, ddc:500, Fakultät für Physik

Guests: Zoltan Z. Nagy, MD Professor at the Department of Ophthalmology Semmelweis University Budapest, Hungary John Kanellopoulos, MD  Professor of Ophthalmology  NYU School of Medicine New York, NY Director, Laservision.gr Institute Athens, Greece

The goal of this thesis is to develop a compact high-power solid-state oscillator capable of superseding existing ultrafast technology based on low-power Ti:sapphire oscillators. Different applications such as extra- or intra-cavity XUV generation, seeding of high-energy low-repetition-rate amplifier systems and femtosecond enhancement cavities can be dramatically influenced by the availability of such a reliable, compact femtosecond source. We applied, for the first time, Kerr-lens mode-locking to a thin-disk Yb:YAG oscillator, resulting in an unprecedented combination of an average power 45 W and pulse duration of 250 fs directly available from the oscillator with repetition rate of 40 MHz and a footprint of only 1*0.4 m^2. Even shorter emission-bandwidth-limited 200-fs pulses have been generated with the reduced output coupler transmission of 5.5% at an average power of 17 W. Moreover, the oscillator was operating not only in the negative dispersion regime common to solid-state oscillators but also in the positive dispersion regime, resulting in a spectrum spanning a range of 20 nm, which is the broadest hitherto reported for Yb:YAG material in high-power operation. First attempts towards CE phase-stabilized high-power pulses from such an oscillator are also described. State-of-the-art XUV generation driven by high-power NIR femtosecond systems requires methods of separating generated XUV from NIR radiation. Such a method has been proposed and realized. It constitutes a glass substrate having a low-loss anti-reflection coating for NIR wavelengths at grazing incidence of >70° and serving simultaneously as a high reflector for radiation in the range of 1-100 nm with reflectivity >60%. The device can be used for both extra- and intra-cavity XUV generation.

Femtosecond stimulated Raman microscopy (FSRM) is an upcoming technique in vibrational microscopy that combines optical imaging with the chemical sensitivity of Raman scattering. It allows for the quantitative, space resolved representation of microscopic samples. In FSRM, chemical contrast relies on femtosecond stimulated Raman scattering (FSRS), a nonlinear Raman process providing signal levels, that are strongly amplified with respect to spontaneous Raman scattering. Therefore, FSRM benefits from shortened exposure times and improved signal/noise-ratios. Even though FSRS is an optically nonlinear effect, its spectral signature resembles the (polarised) spontaneous Raman spectra. Autofluorescence of the sample does not contribute to the FSRS signature. FSRM turns out to be suitable for chemical imaging of biological systems, as cells and tissues. Since their signal strengths are quite low, data with high signal/noise-ratios is essential for the specific analysis of the sample. These requirements can be provided by FSRM. Moreover, the linear dependency of the FSRS signal on the sample concentration facilitates the quantitative analysis of its molecular composition. Given that tissue shows a wide heterogeneity on the molecular level, spectrally broad information, which is indeed provided by FSRM, is indispensable for chemically sensitive imaging. The intention of the present work was to establish FSRM as a tool for biological and medical imaging. FSRM is a scanning technique that relies on the nonlinear interaction of two ultrashort laser pulses with a Raman active medium. One of them is a spectrally narrow and intense picosecond pulse (Raman pump pulse), the other a spectrally broad and weak femtosecond pulse (Raman probe pulse). Both pulses are coupled collinearly into a scanning microscope where stimulated Raman scattering occurs in the sample positioned at the focus. This interaction leads to a spectral modification of the Raman probe pulse, which is recorded by the aid of a multi-channel detector. Its FSRS spectrum can be obtained by referencing the femtosecond pulse in presence of the Raman pump pulse to the one in absence. By raster-scanning the sample space-resolved FSRS spectra are retrieved. In the first FSRM setup a laser/amplifier system served as laser source. Its peak intensities are not compatible with the requirements of biological systems. Therefore a new light source has been developed for FSRM. It is based on a laser oscillator running at high repetition rate, which provides ultrashort femtosecond pulses with a spectral width over more than 3000 cm-1. These serve directly as Raman probe pulses. The Raman pump pulse is generated out of the laser spectrum by means of a ytterbium based fiber amplifier. Stimulated Raman microscopy is considered as a disturbance-free, nonlinear microscopy technique. In the context of this work, a further nonlinear contribution to the FSRS signal is reported for the first time. It shows up as a strong oscillation of the baseline in the spectrum and deteriorates the signal/noise-ratio as well as the spectral selectivity. This background can be identified as a spectral interference between the Raman probe pulse and an additional electric field, which is generated by a four-wave-mixing process between the Raman pump and Raman probe pulse. Properties and methods to suppress the spectral interferences are presented.

NOTE: we are sorry that there is distracting background noise in the audio in different parts of Dr. Jones' lecture. Abstract: The fs frequency comb has proven to be a powerful tool in both precision spectroscopy and ultrafast science. By providing a direct phase-coherent link between optical and microwave frequencies it has dramatically simplified the precision measurement of optical transitions while simultaneously enabling access to sub-cycle control and synchronization f optical fields. It’s use has thus far been limited to wavelengths in the deep-UV or longer (>200 nm). In this talk, I will discuss 2 experiments utilizing the fs frequency comb for precision spectroscopy at UV and vacuum-ultraviolet (VUV) wavelengths. The first experimental effort is in the development of an optical atomic clock based on a narrow transition in neutral mercury atoms. A laser-cooled ensemble of mercury offers excellent prospects as a next generation optical frequency standard. The second effort is focused on extending precision spectroscopy into the VUV using intra-cavity high harmonic generation (HHG) with the fs frequency comb. I will discuss current results, simulations that highlight the limitations of this approach, and planned experiments utilizing this source. Dr. R. Jason Jones is Assistant Professor of Optical Sciences at the University of Arizona. His current research interests are in ultrafast laser science, femtosecond frequency combs, extreme nonlinear light/matter interactions and generation, optical frequency metrology, and high-resolution spectroscopy. His lecture was given on January 21, 2011.

A conversation among Richard Lindstrom, MD, David F. Chang, MD, William W. Culbertson, MD, and Michael C. Knorz, MD. Dr. Richard Lindstrom moderates a panel of 3 experts who share their impressions on LASIK-assisted cataract surgery. The participants discuss the current and potential benefits of applying Femtosecond laser to the cataract procedure, including precise control of the size and location of the capsulorrhexis, an integrated imaging system, reproducible incisions for astigmatism correction, and the ability to soften or prechop a dense nucleus. (May 2011)

Tue, 27 Jan 2009 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/9620/ https://edoc.ub.uni-muenchen.de/9620/1/Gollub_Caroline.pdf Gollub, Caroline ddc:540, ddc:500, Fakultät für Chemie und

Femtosecond pump-probe spectroscopy in the UV to near infrared region has been implemented to monitor the energetic changes of valence electrons in molecules during chemical reaction, and to identify the intermediates and their stabilization by the surrounding. New laser flash-techniques with semiconductor probe sources have been developed to access reaction steps on the nano- to (milli-)second scale. As the complementary methods use the same spectrally tunable femtosecond pump, the reaction course can be followed for the first time over 14 magnitudes under identical starting conditions. With these techniques the influence of substituents and solvent on the reaction of medium sized molecules is studied to relate the structure to the reactivity and to investigate the physical basis of this relation. Based on a systematic variation of substituents the ground state reactivity of polar reactants can be classified in a general scale that allows the quantitative prediction of reaction speed: The reactivity of nucleophiles is parameterized via the kinetics of their combinations with a reference set of diarylcarbenium cations. Here the underlying empiric linear free energy relation is confirmed for fast bimolecular reactions from milliseconds to the diffusion limit. Three highly reactive nucleophiles are classified. For anions with two reaction centres, strong limitations of the HSAB principle and related theoretical concepts are revealed as the observed reactivity contradicts their predictions. In contrast to these bimolecular reactions, ultrafast structural reorganisation within one molecule initiated by a femtosecond light pulse are not superposed by diffusive and orientational motions or thermal activation. Such unimolecular reactions are studied in three molecular families that exhibit electron transfer (ET) between donor and acceptor units arranged via a central sp3-hybridized carbon. In spite of the common reaction centre the photo-processes span pure physical deactivation to complete chemical conversion. ET without bond cleavage is studied in low to high-polar solvents for a lactonic derivative of triarylmethane. The optically populated charge transfer state is converted into a highly polar charge transfer state. The observed kinetics of this conversion on the picosecond-scale is strongly dependent on the solvent but not directly correlated to the known solvation times. To explain these findings a quantitative model is established that takes into account the relaxation processes of the solute-solvent system after optical excitation. According to their unequal dipole moments, the donor and acceptor level of the ET are stabilized during the solvation to a different extent. In sufficient polar solvents the product state becomes that way energetically accessible and even favoured. This dynamic behaviour of the energetics renders the barrier of the ET and in turn its rates in forward and backward direction time-dependent. So the efficiency and kinetics of the ET are fully determined by the solvation. The dissociative ET of diarylmethane derivatives towards its ionic fragments is resolved for the first time also spectrally – here for the precursor compounds of the nitrite ion and diarylcarbenium cations of low and medium reactivity in the afore mentioned scale. The main component of the absorption of the sp2-hybridized cation emerges within a few 100 fs after excitation. The final band shape develops on the scale of 10 ps predominantly by vibrational cooling as revealed by the spectral signatures. While the sub-ps dynamics of the heterolytic dissociation is comparable with the reported one of diphenylmethyl chloride, the channel via the radical pair and successive intermolecular ET is reduced or even suppressed. The photochromic transformation from dihydroazulene (DHA) derivatives to their vinylhaptafulvene (VHF) isomers is found to proceed via the ring-opening on the fs- to ps-scale followed by internal conversion on the VHF geometry. Due to its dissociative ET character, the ring-opening is accelerated by the dynamic solvation of the VHF excited state in analogy to the solvent control in the lactonic triarylmethane derivative. Coherent wave packet oscillations reveal motions of the molecular frame around its central tetrahedral carbon. These vibrations bring the electron acceptor unit in plane with the excited conjugated system and promote the ET to the sigma-bond to be broken. In spite of the common general reaction path, DHA derivatives can differ strongly in their switching properties. A comparative study for compounds from three DHA classes correlates the substitution pattern with the reactivity. Electronically unsaturated substituents cause a delocalisation of the DHA wave function reducing the driving force and speed of the initial dissociative ET. For such compounds a non-reactive deactivation channel of the DHA excitation is observed in the UV bleaching region. The structural constraint of the rotation of the ring-opened isomer is shown to introduce a conical intersection on the deactivation pathway of the excited VHF. These insights explain the occurrence of solvent dependent quantum yields or the lack of the photoinduced reaction from VHF on and open the route towards an optical DHA switch. Host-guest-systems of 2-(2’-hydroxyphenyl)benzothiazole (HBT) in zeolites are studied in the form of colloidal suspension by optical transmission spectroscopy. Depending on the acid-base properties of the surrounding, two different species are found in the zeolite voids: enol-HBT is converted into the keto-form in a base catalyzed reaction with the anion of HBT as intermediate. Photoexcitation converts the ketotautomer back to the enol-tautomer. This transformation was observed with a for zeolite samples unprecedented resolution of sub-200 fs to proceed via ultrafast deprotonation mediated by the protic surrounding on the ps-scale. The photo-cycle is completed by nanosecond relaxation of the excited anion to the ground state. As this first application shows, the colloidal zeolite suspensions are well suited for developing host-guest-systems with medium sized molecules. Particularly, femtosecond transient transmission spectroscopy has proven to resolve the photodynamics in real-time, which opens new perspectives for the design of nano-structured supramolecular functional materials.

The scope of the work presented is the investigation of photochemical reactions by means of ultrafast spectroscopy. Naturally these reactions start off in an optically bright excited state. Femtosecond time-resolved fluorescence spectroscopy is thus the method of choice to track the spectral and temporal dynamics of these emissive states. Here, an ultrafast fluorescence spectrometer based on the optical Kerr-effect serves as the appropriate tool to pursue this task. Additional information on dark states and ground states is provided by Uv-Vis transient absorption experiments. The first part of the thesis deals with a fundamental concept of mechanistic chemistry – the pericyclic reactions. The spectroscopic consequences implied within this theoretical framework are investigated by means of emission and absorption spectroscopy. The molecular probe is an indolyl-substituted fulgimide which undergoes a light-induced cyclization or cycloreversion, respectively. Both reactions feature a bi-phasic emission decay (cyclization: 0.06 ps, 0.4 ps, cycloreversion: 0.09 ps, 2.4 ps) whereas the slower component goes along with the product formation. The large difference in the slower time constants as well as the spectral properties of the corresponding emission point to the existence of different excited state pathways for both reactions. These results challenge the basic one-dimensional reaction scheme commonly used to describe pericyclic reactions. Referring to theoretical investigations, a two-dimensional reactive space is proposed to hold responsible for the different behaviour of the two isomers. The second part of the studies focuses on the dynamics of a certain type of photolabile protecting groups. These molecules are intramolecularly sensitised by a triplet energy donor, namely thioxanthone, and feature an ortho-substituted nitroaromatic as the reactive core. Investigations on the closely related energy donor xanthone reveal that photo-excitation is followed by a rapid (~ 1 ps) equilibration between the emissive singlet and a triplet state. This equilibrium holds responsible for a delayed fluorescence with a lifetime of ~ 0.1 − 1 ns and is ”switched off” by an internal conversion within the triplet manifold. These results can be directly transferred to thioxanthone and the sensitised protecting groups. The energy transfer in the latter molecules features a fast component from the initially populated triplet state (~ 100 ps) and a further slower contribution from the relaxed triplet state. Finally, the photo-reactive ortho-nitrobenzaldehyde (o-NBA) is compared with its non-reactive isomers m- and p-NBA as model systems to obtain information on the reactive core of the protecting groups. These first fluorescence experiments on monocyclic nitrated aromatics feature bi-phasic emission decays in all three cases – each with time constants of 1npi* relaxation.

The maximum energy achievable directly from conventional Ti:sapphire oscillators has been limited by the onset of instabilities such as cw-generation and pulse splitting because of the high intensity in the laser medium. Generation of microjoule pulses at megahertz repetition rates are of special interest in many areas of science and technology. The main subject of this thesis is the development of high energy Ti:sapphire oscillators at megahertz repetition rate. The main concept that was applied to overcome the difficulties pointed out above was to operate the laser in the positive dispersion regime. By operating the laser in this regime, intracavity picosecond pulses are generated that can be externally compressed down to femtosecond pulse durations. The long pulse duration inside the laser offers an elegant way to reduce pulse instabilities by decreasing the intracavity intensity via pulse stretching. Drawing on this concept, Ti:sapphire chirped-pulse oscillators delivering sub-50-fs pulses of 0.5 uJ and 60 nJ energy are demonstrated at average power levels of 1 and 4 W (repetition rate: 2 MHz and 70 MHz), respectively. The 0.5 uJ pulses have a peak power in excess of 10 MW. By locking a 76-MHz chirped-pulse oscillator to a femtosecond-enhancement cavity 7.8 uJ pulses with 55 fs pulse duration were achieved. High harmonic generation was demonstrated in a Xenon target placed close to the enhancement cavity focus.

Aromatic nitro-compounds are known to photochemically abstract hydrogen atoms from adjacent hydrocarbon moieties. For ortho-substituted nitro-aromatics these abstractions proceed intra-molecularly and trigger secondary processes. Due to these processes aromatic nitro-compounds are used as, e.g., photo-labile protecting groups and as the leaving group in caged compounds. Here, a prototypical nitro-compound, ortho-nitrobenzaldehyde (o-NBA), which is photochemically transformed into ortho-nitrosobenzoic acid is studied. This reaction is known for more than 100 years but is still not totally specified. Tracing these photochemical processes requires high temporal resolution down to about 100 fs and a detection technique that is sensitive to structural changes after starting the photoreaction. Femtosecond vibrational spectroscopy fulfils these conditions in observation of ultrafast chemical processes. So transient IR spectroscopy and the recently developed femtosecond stimulated Raman spectroscopy (FSRS) are used to record the ultafast structural changes in this photoreaction. In addition the kinetics of the o-NBA photoreaction are monitored by means of visible femtosecond absorption spectroscopy. The novel implementation of FSRS uses a white light continuum as the Stokes probe pulse in the stimulated Raman process. The reaction is started by a actinic laser pulse thereby the transient Raman spectra can be recorded in pump-probe fashion and are compared with calculated spectra of possible intermediates of the reaction. With the use of these techniques the photoreaction of NBA into intermediates can be followed and a reaction mechanism is described.

Coherent control of molecular dynamics deals with the steering of quantum mechanical systems with suitably shaped ultrashort laser fields. The coherence properties of the laser field are exploited to achieve constructive interference for a predefined target wave function via a phasecorrect superposition of wave functions. The goal of coherent control, the selective preparation of a target state, is an important prerequisite for mode-selective chemistry. The laser pulse tailored to drive the system from the initial to the target state as a perturbation can in general not be determined by a quantum-mechanical calculation, since usually even the Hamiltonian of the system is unknown. A practical alternative is to determine the required shape of the laser field in a feedback-controlled regulation loop which uses a signal derived from the experiment as feedback. The loop is repeated until a pulse that suits the requirements is obtained. Experiments in this area have until recently mostly been limited to the wavelength regime of Ti:Sa lasers and their fundamentals. This work deals with the fundamentals of feedback-controlled shaping of ultrashort laser pulses with respect to both establishment of its technical prerequisites and its application to suitable model systems. The feedback loop has been tested using a simple optimization experiment with known outcome; then it was applied to experiments of progressively increasing complexity. From the optimized pulses, physical insight into the optimization process has been gained. In the first part of this work, the required technology has been implemented and standardized such that control experiments might employ it as a standard tool. One of the technical prerequisites was the frequency conversion of the 800 nm Ti:Sa laser pulses to a wavelength range suited to the particular systems. To this end, non-collinear optical parametric amplifiers have been built in different designs that routinely produce tunable sub-20 fs pulses in the visible. The characterization techniques for ultrashort pulses have been implemented as well. Pulse shapers with cylindrical instead of spherical mirrors have been implemented for the modulation of broadband pulses, and their functionality has been explained both theoretically and experimentally. A new liquid crystal device, the core of our pulse shapers, has been developed in cooperation with the group of Thomas Feurer at the Universit¨at Jena and the Jenoptik GmbH which allows for the generation of more complex pulse shapes than with other commercially available devices to date. Using a pulse shaper to modulate the white light continuum that serves as the seed for the non-collinear optical parametric amplifier, generation of phase-locked two-color double pulses has been achieved, with tunable wavelengths, delays, and relative carrier phases between the single pulses. The basic principle, phase conservation during optical parametric amplification, has been demonstrated. With this setup, control experiments which require pulses with the above described attributes in electronically controllable form are possible for the first time. An evolutionary strategy used as the optimization algorithm in the feedback loop has been programmed and characterized both in simulation and experiment using a simple optimization experiment, namely pulse recompression by phase compensation. In the second part of this work, pulse recompression of ultra-broadband spectra in the sub-20fs regime serves as an example of utility of feedback-controlled optimization. This experiment simultaneously served as a further test of the feedback loop in the limit of a physically unreachable optimization goal. It has been demonstrated that a suitable parameterization of the electric field, implemented by a mapping of the optimization parameters adjusted by the algorithm to the physical parameterscontrolling the liquid crystal mask affords a means of acquiring physical knowledge from the retrieved optimal electric fields. A parameterization helps to dissect the physical processes mediating the control process, thereby assuring fast, secure convergence and robustness against signal noise. So-called ”bright” and ”dark” pulses, i.e. pulses that are absorbed by a medium or transmitted, respectively, have been demonstrated for the case of the two-photon transition Na(3s→→5s). The physical constraints responsible for pulses being either ”bright” or ”dark”, namely a symmetric or anti-symmetric spectral phase, have been incorporated in the parameterization with the purpose of testing the concept of parameterization for such studies. An example of mode-selective preparation of vibrational states in a polyatomic molecule is the control of the ground state dynamics in polydiacetylene. In a Raman step with a shaped Stokes pulse, the population of the backbone vibrations of polydiacetylene in its ground state could be controlled. A consecutive probe pulse in a CARS (coherent anti-Stokes Raman scattering) arrangement generates an anti-Stokes signal which, once frequency-resolved, served as feedback. Of the three or four modes, respectively, accessible within the pulse bandwidth, single modes as well as combinations of modes could be excited with high selectivity. Again, suitable parameterizations helped to identify one of the processes responsible for the control as a Tannor-Rice scheme. Since both the amplitude and the phase of each mode could be influenced, the focusing of a wave packet at a predefined time, or, equivalently, the generation of local modes represents the control of a unimolecular reaction. Starting from the control of a unimolecular reaction, the possibilities of controlling a bimolecular reaction were addressed. The NaH2 collision complex was chosen as a suitable system for the control of bimolecular reactions generally and a conical intersection in particular. First timeresolved experiments have been presented.

Polarization pump-probe femtosecond spectroscopy was used to investigate photoinduced optical density changes in allophycocyanin (APC) trimers at 635–690 nm after excitation with 230-fs pulses at 618 nm. The initial bleaching observed at λ < 645 nm is followed by subpicosecond absorption recovery corresponding to 430 ± 40 fs recovery kinetics measured at 615 nm with 70-fs pulses. Only the red part of the APC absorption band remains strongly bleached at 3 ps after excitation. The spectral and kinetic results can be described in terms of two different models of interaction between neighbouring α-80 and β-81 chromophores of APC trimers. According to the first one, the observed subpicosecond kinetics corresponds to relaxation between the levels of excitonically coupled, spectrally identical α-80 and β-81 chromophores. Excited state absorption to doubly excited excitonic state should in this case contribute to the measured difference spectra. According to the second one, the femtosecond excitation energy transfer in APC trimers takes place between a donor chromophore absorbing predominantly at 620 nm and an acceptor chromophore absorbing at 650 nm. The high anisotropy value observed at 615 nm during the first 1.2 ps is in good agreement with the donor-acceptor model. Anisotropy values calculated in the 635–675 nm spectral region at 3 ps after excitation are in the 0.1–0.25 range corresponding to an angle of 30°–45° between donor and acceptor transition dipole orientations. The high anisotropy obtained at 658 nm during the excitation is probably due to stimulated emission of the donor chromophore.

Fri, 1 Jan 1993 12:00:00 +0100 http://epub.ub.uni-muenchen.de/2406/ http://epub.ub.uni-muenchen.de/2406/1/2406.pdf Lauterwasser, Christoph; Finkele, Ulrich; Struck, A.; Scheer, Hugo; Zinth, Wolfgang Lauterwasser, Christoph; Finkele, Ulrich; Struck, A.; Scheer, Hugo und Zinth, Wolfgang (1993): Femtosecond spectroscopy of the primary electron transfer in photosynthetic reaction centers. In: Shima, A.; Ichihashi, M.; Fujiwara, Y. und Takebe, H. (Hrsg.), Frontiers of Photobiology. Excerpta Medica: Amsterdam, pp. 209-214.

Ultrafast energy-transfer processes in allophycocyanin (APC) trimers from Mastigocladus laminosus have been examined by a femtosecond absorption technique. Isotropic absorption recovery kinetics with τ=440±30 fs were observed in APC trimers at 615 nm. In APC monomers such a fast process was not observed. The anisotropy in both samples was constant and close to 0.4 during the first few picoseconds. The results are consistent with a model of the APC trimer in which the two APC chromophores have different absorption spectra with maxima about 600 and 650 nm. The transfer of energy from the 600 nm chromophore to the 650 nm chromophore occurs in 440 fs and is dominated by the Förster dipole—dipole energy-transfer mechanism.

Wed, 1 Jan 1992 12:00:00 +0100 https://epub.ub.uni-muenchen.de/2357/1/2357.pdf Scheer, Hugo; Fischer, R.; Gillbro, T.; Sharkov, A. V.; Kryukov, P. G.; Kryukov, I. V.; Khoroshilov, E. V. ddc:

Wed, 1 Jan 1992 12:00:00 +0100 https://epub.ub.uni-muenchen.de/3764/1/3764.pdf Lauterwasser, Christoph; Finkele, Ulrich; Dressler, K.; Hamm, P.; Zinth, Wolfgang

Femtosecond spectroscopy was used in combination with site-directed mutagenesis to study the influence of tyrosine M210 (YM210) on the primary electron transfer in the reaction center of Rhodobacter sphaeroides. The exchange of YM210 to phenylalanine caused the time constant of primary electron transfer to increase from 3.5 f 0.4 ps to 16 f 6 ps while the exchange to leucine increased the time constant even more to 22 f 8 ps. The results suggest that tyrosine M210 is important for the fast rate of the primary electron transfer.

The influence of phase-modulation on femtosecond time-resolved coherent Raman scattering is investigated theoretically and experimentally. The coherent Raman signal taken as a function of the spectral position shows unexpected temporal oscillations close to time zero. A theoretical analysis of the coherent Raman scattering process indicates that the femtosecond light pulses are amplitude and phase modulated. The pulses are asymmetric in time with more slowly decaying trailing wings. The phase of the pulse amplitude contains quadratic and higher-order contributions.

The photodynamics of bacteriorhodopsin were studied by transient absorption and gain measurements after excitation with femtosecond pulses at 620 nm. With probing pulses at longer wavelengths (λ > 770 nm) the previously reported formation of the J intermediate (with a time constant of 500±100 fs) was confirmed. With probing pulses around 700 nm, a faster process with a relaxation time of 200±70 fs was observed. The data analysis strongly suggests that this kinetic constant describes the reactive motion of the polyatomic molecule on its excited-state potential energy surface, i.e. one observes directly the incipient isomerization of the retinal molecule. The minimum of the S1 potential energy surface reached in 200 fs lies approximately 13300 cm−1 above the ground state of bacteriorhodopsin and from this minimum the intermediate J is formed with a time constant of 500 fs.

Fri, 1 Jan 1988 12:00:00 +0100 https://epub.ub.uni-muenchen.de/3564/1/3564.pdf Kaiser, Wolfgang; Dressler, K.; Dobler, J.; Zinth, Wolfgang ddc:530,

Fri, 1 Jan 1988 12:00:00 +0100 https://epub.ub.uni-muenchen.de/3557/1/3557.pdf Leonhardt, R.; Holzapfel, Wolfgang; Zinth, Wolfgang ddc:530, Physik

Thu, 1 Jan 1987 12:00:00 +0100 https://epub.ub.uni-muenchen.de/3549/1/3549.pdf Kaiser, Wolfgang; Zinth, Wolfgang; Holzapfel, Wolfgang; Leonhardt, R.

Mon, 22 Dec 1986 12:00:00 +0100 https://epub.ub.uni-muenchen.de/3535/1/45.pdf Kaiser, Wolfgang; Zinth, Wolfgang; Nuss, M. C. ddc:530, Physik

Femtosecond light pulses tunable between 840 nm and 880 nm are generated in a synchronously pumped ring dye laser. The laser emits nearly bandwidth-limited pulses (Δv tp = 0.45) with pulse durations down to 65 fs. At a pumping power of 450 mW of a mode-locked Ar-ion laser (λ = 514 nm) the infrared femtosecond dye laser has an output of up to 15 mW.

Wed, 1 Jan 1986 12:00:00 +0100 https://epub.ub.uni-muenchen.de/3539/1/3539.pdf Kaiser, Wolfgang; Dobler, J.; Zinth, Wolfgang ddc:530, Phy

Wed, 1 Jan 1986 12:00:00 +0100 https://epub.ub.uni-muenchen.de/3538/1/3538.pdf Kaiser, Wolfgang; Zinth, Wolfgang; Nuss, M. C. ddc:530, Physik

The first steps in the photochemistry of bacteriorhodopsin (BR) are investigated with light pulses of 160 fs duration. Four samples are studied: (i) the purple membrane, (ii) deuterated purple membrane, (iii) BR trimers and (iv) BR monomers. In all samples the first intermediate J is formed within 430±50 fs. No isotope effect is observed in the formation of J upon deuteration, in contrast to previous reports with much higher excitation energies. Thus proton movement to or from the retinal Schiff's base is not relevant during the first step. Comparing the data for trimeric and monomeric BR suggests an upper limit of 50 fs for the transfer of excitation energy from the excitonically coupled trimer to a single retinal chromophore.

Tue, 1 Jan 1985 12:00:00 +0100 http://www.aps.org/meetings/baps https://epub.ub.uni-muenchen.de/3298/1/3298.pdf Kaiser, Wolfgang; Polland, Hans-Joachim; Nuss, M. C.; Zinth, Wolfgang ddc:530, Physik

Tue, 1 Jan 1985 12:00:00 +0100 https://epub.ub.uni-muenchen.de/3250/1/3250.pdf Kaiser, Wolfgang; Franz, M. A.; Nuss, M. C.; Zinth, Wolfgang