LACUS Seminars

LACUS seminars

Prof. Christos Flytzanis  (Laboratoire de Physique, ENS; Université PSL, France)

Title: Amplification of coherent sub-THz acoustic phonons in superlattices. Fashioning and Pumping up the sound and the SASER issue

Abstract: We present and demonstrate a novel concept and scheme of acoustic phonon amplification at sub-THz frequency  and a few nanometers wavelength range based on the photodriven acoustoelectric(AE) effect with 100fs light pulses in an electrically biased semiconductor superlattice(SL) GaAs/AlGaAs; the amplification is due to stimulated Cerenkov folded zone ac-phonon emission by electrons undergoing intra-miniband transport. The process is accounted for with a detailed theoretical model and extensions are discussed.

With appropriate phononic microcavity configuration it can allow the realization of the SASER (Sound Amplification by Stimulated Emission of Radiation) operation the analogue of the Laser with sound. In the present configuration it is the analogue of the quantum cascade laser with sound in the sub-THz frequency and a few nm wavelength range. Such a coherent  phonon source allows for investigations with very high spatial resolution, almost comparable to that of an electron microscope, e.g. in microelectronic devices or in microbiology.

Thursday May 16th at 16:30 – Room CH G1 495

Prof. Jürgen Hauer (Technical University of Munich)

Title: Coherent multidimensional femtosecond spectroscopy – from basics to applications

Abstract: At the core of our perception of chemical transformation are ‘molecular movies’: we understand a reaction when there is a satisfactory picture of bonds breaking and forming and when the sequence of such events is known. The appropriate timescale to observe and interpret chemical reactions is given by the period of molecular vibrations, i.e. in the femtosecond (fs) range. A carbon-carbon double bond for instance vibrates with a period of roughly 20 fs. Accordingly, femtosecond spectroscopy has established itself as a fundamental tool in physical-chemical sciences. Since its advent nearly three decades ago, femtosecond spectroscopy has diversified greatly, with transient absorption as the most wide spread technique. The recently emerged two-dimensional electronic spectroscopy (2D-ES) reports on the same non-linear signal as transient absorption, but with a maximally enriched information content; in 2D-ES, the signal is resolved both in excitation and emission frequencies as well as in its time-dependence. In my talk, I will introduce 2D-ES and discuss several of its applications on molecular dynamics in biologically relevant systems as well as in molecular J-aggregates.

March 28th at 17:15 – Room BCH 2201

Dr. Philip Kurian (Howard University, Washington DC, USA)

Title : Ultrafast tryptophan-to-tryptophan energy transfer and superradiance in tubulin polymers.

Abstract: Oxidative stress from aerobic processes is a pathological hallmark of degenerative disorders such as Alzheimer’s disease and cancer. The precise role of reactive oxygen species (ROS) in the disease process, however, is poorly understood. It is known that the production of ROS by mitochondria can result in ultraweak photon emission (UPE) within cells, and UPEs in the UV and visible ranges have been observed with modern equipment during different stages of the mitotic cycle. Surrounding biomolecules can absorb these photons via aromatic amino acids (e.g., tryptophan and tyrosine), nucleobases (e.g., adenine, cytosine, guanine, thymine), and other chromophoric constituents, forming excited singlet or triplet transition states. One likely absorber is the microtubule cytoskeleton, as it forms a vast network spanning neurons, is highly co-localized with mitochondria, and shows a high density of aromatics, but DNA and the photoactive receptors in the mitochondrial membrane are also potential candidates. These networks may traffic ROS-generated endogenous photon energy for cellular signaling, or they may serve as dissipaters of such energy to protect the cell from potentially harmful effects. Recent modelling efforts based on ambient temperature experiment are presented, showing that such biopolymers can feasibly absorb and channel these photoexcitations via resonance energy transfer, on mesoscopic length scales of physiological significance. Additional simulations using a non-Hermitian Mukamel Hamiltonian demonstrate the possible existence of superradiant states in microtubules corresponding to similar observed phenomena in cylindrical chlorophyll complexes.

Thursday 5 July 2018 / 16:00 / CH G1 495

Dr. Jianbo Hu (California Institute of Technology and China Academy of Engineering Physics)

Title : 4D visualization of ultrafast lattice deformation in solids under laser excitation.

Abstract: The high scattering cross-section for electron/matter interaction, the atomic-scale spatial resolution, and the ultrafast temporal resolution of ultrafast electron diffraction (UED) represent the key elements that make this technique a unique tool for the dynamic investigation of nanomaterials. Here I introduce the general concept of UED, and then present a few applications on investigating the laser-induced lattice deformation in solids, including transient structures of phase change materials, energy localization in quantum dots, and rippling dynamics of free-standing graphene.

Monday 25 June 2018 / 11:00 / CH G1 495

Mrs. Selene Mor (Fritz Haber Institute, Berlin, Germany)

Title : Ultrafast electronic band gap control and inhibition of the photoinduced structural phase transition in an excitonic insultaor

Friday 15 June 2018 / 11:00 / CH G1 495

Dr. Wojciech Gawelda (European XFEL, Germany)

Title : Scientific opportunities and challenges for femtochemistry using X-ray Free Electron Lasers

Friday 04 May 2018 / 11:00 / CH G1 495

Mr. Paolo Usai (Università degli studi di Cagliari)

Title : Characterization of 2D perovskites via time-resolved ultrafast spectroscopy techniques

Thursday 3 May 2018 / 15:00 / CH G1 495

Dr. Jeremy Rouxel (University of California)

Title : Time-resolved nonlinear signals for X-ray spectroscopy and diffraction

Thursday 8 March 2018 / 14:00 / Room CH G1 495

Dr. Marino Marsi (Université Paris-Saclay France)

Title : Ultrafast study of out-of-equilibrium quantum materials

Tuesday 20 February 2018 / 11:00 / Room CH G1 495

LACUS Day 2018 –  Wednesday 14 February / Room CE11

Prof. Richard Friend (Université of Cambridge)

Title : Molecular semiconductors for LEDs and solar cells: designing around the Coulomb interaction

Monday 20 November 2017 / 16:30 / Room BCH 2201

Dr. Fulvio Parmigiani (University of Trieste and University of Cologne)

Title : A Non-equilibrium approach to the optical spectroscopy of high-temperature superconductors

Wednesday 8 November 2017 / 10:00 / Room CH G1 495

Prof. Nikolaus Ernsting (Humboldt-Universität Berlin)

Title : Hydration Dynamics Seen from the Inside: Possibilities of a Molecular THz Spectrometer

Thursday 2 November 2017 / 10:30 / Room CH G1 495

Prof. Laura Herz (University of Oxford)

Title : Mechanisms limiting charge-carrier recombination and mobilities in metal halide perovskites

Thursday 29 June 2017 / 16:30 / BCH 2201

Prof. Omar Mohammed (KAUST)

Title : Mapping Carrier Dynamics on Semiconductor Material Surfaces and at Interfaces using Laser Spectroscopy and 4D Electron Microscopy