Events

Host: Physical Chemistry
The ability to precisely design Å-scale plasmonic cavities has boosted the sensitivity and spatial resolution of surface- and tip-enhanced Raman scattering (SERS and TERS). In this context, low-temperature scanning probe microscopy (LT-SPM) offers great advantages to perform nanoscale vibrational spectromiscroscopy (TER-SM). Along with nanofabrication techniques of plasmonic tips, LT-SPM now allows to examine light–matter interactions in plasmonic “picocavities” down to the sub-molecular level. However, the underlying mechanisms behind the large enhancement factors present in such cavities remain unclear. We reveal how TERS evolves at vanishing tip–sample distances including the transition from a tunneling to conductive coupled regime. Upon atomic-point contact (APC) formation, a dramatic TERS enhancement is observed. In order to shed light on the mechanisms behind, we examined different model systems: an Ag tip with ultrathin ZnO films and single C molecules on the Au(111), Ag(111), and Cu(111) surfaces at 10 K. A pronounced electromagnetic enhancement of Raman scattering is commonly observed for a few Å gaps. The sudden increase of the TERS intensity upon APC formation is attributedto the chemical interaction between the tip and the sample which provides additional charge transfer enhancement. Furthermore, intense anti-Stokes signals can be observed, allowing us to perform Raman thermometry in electrically-fused plasmonic junctions. The results reveal pronounced non-thermal contributions, which underlines the necessity to better understand atomic-scale light–matter interactions. [more]
A central prospect of antiferromagnetic spintronics is to exploit magnetic properties that are unavailable with ferromagnets. However, this poses the challenge of accessing such properties for readout and control. To this end, light-induced manipulation of the transient ground state, e.g. by changing the magnetic anisotropy potential, opens promising pathways towards ultrafast deterministic control of antiferromagnetism. In this talk I will show how we use this approach to trigger a coherent rotation of the entire long-range antiferromagnetic spin arrangement about a crystalline axis in GdRh2Si2 and demonstrate deterministic control of this rotation upon ultrafast optical excitation. I will also show that our observations can be explained by a displacive excitation of the Gd spins' local anisotropy potential by the optical excitation, allowing for a full description of this transient magnetic anisotropy potential. See also: https://arxiv.org/abs/2002.01398 [more]

Real-time Rotation of Encapsulated Ortho and Para Water in Fullerene-C60

Copious physical, chemical and thermodynamic properties make water a unique material. For instance, it is known that confined water in nano-cages behaves differently from bulk water. Recent studies even indicate on quantum behavior and incipient ferroelectricity of water in nano-cages. To further study the behavior of confined water molecules, we use H2O@C60 system: encapsulated single water molecule in fullerene-C60 and study the distinct rotational dynamics of water’s spin isomers at cryogenic temperatures. We employ single-cycle terahertz (THz) pulses to coherently excite the low-frequency rotational motion of ortho- and para-water. The excitation leads to the slight orientation of water’s permanent dipoles towards the field polarization and consequently to the emission of electromagnetic waves, which we resolve via the field-free electro-optic sampling technique. We discuss our results on the real-time conversion of ortho- to para-water at 4 K and further show the direct impact of temperature on rotational degrees of freedom of entrapped water inside its cage. [more]
Coupling phase-stable single-cycle terahertz (THz) pulses to scanning tunneling microscope (STM) junctions enables spatio-temporal imaging with femtosecond temporal and Ångstrom spatial resolution. The time resolution achieved in such THz-gated STM is ultimately limited by the sub-cycle temporal variation of the tip-enhanced THz field acting as an ultrafast voltage pulse, and hence by the ability to feed high-frequency, broadband THz pulses into the junction. In this talk, I will present our results on the coupling of ultrabroadband (1-30 THz) single-cycle THz pulses from a spintronic THz emitter (STE) into a metallic STM junction. We demonstrate broadband phase resolved detection of the tip-enhanced THz waveform via THz-field-induced modulation of ultrafast photocurrents across the junction. Comparison to the unperturbed far-field THz waveform reveals the antenna response of the STM tip. Despite tip-induced low-pass filtering, frequencies up to 15 THz can be detected in the enhanced near-field, resulting in THz transients with a half-cycle period of 115 fs. Moreover, versatile phase and polarity control of the THz waveform can be achieved via the STE excitation conditions and magnetization, and few Volts THz bias at 1 MHz repetition rate can be reached in the current setup. Finally, we find a nearly constant THz voltage and waveform over a wide range of tip-sample distances, which by comparison to numerical simulations confirms the quasi-static nature of the THz pulses. Our results demonstrate the suitability of spintronic THz emitters for ultrafast THz-STM and provide insight into the femtosecond response of defined nanoscale junctions. [more]

Ultrafast Light-Induced Lifshitz Transition

Fermi surface is at the heart of our understanding of the properties of metals and strongly correlated many-body systems. An abrupt change in the Fermi surface topology, also called Lifshitz transition, can leads to the emergence of fascinating phenomena like colossal magnetoresistance and superconductivity. While Lifshitz transitions have been demonstrated for a broad range of materials using equilibrium tuning of macroscopic parameters like strain, doping, pressure, and temperature, a nonequilibrium route toward ultrafast and transient switching of the Fermi surface topology has not been demonstratedyet. Using time-resolved multidimensional photoemission spectroscopy combined with TDDFT+U simulations, we demonstrate a scheme based on ultrafast laser-driven band renormalization that drives a Lifshitz transition in the topological type-II Weyl semimetal Td-MoTe2, due to transient modification of effective electron-electron interactions. [more]

Layered materials beyond graphene – new possibilities and applications

Beyond graphene, which is intensively studied over more than one decade, the other related materials remain almost unexplored. The research activities in the field of other layered materials like phosphorene, arsenene, silicene and germanene are rapidly growing in the last few years. Compare to graphene, all these materials are non-zero band-gap semiconductors. This property opens new application possibilities in electronic and optoelectronic devices. The properties of 2D materials can be further controlled by their functionalization. The chemistry of materials beyond graphene is none explored and shows high application potential in many fields. Compare to the graphene and pnictogen group, the chemical exfoliation method mast be applied for synthesis of silicene / germanene derivatives using Zintl phase compounds like CaGe2 and CaSi2. Various methods well know from organic chemistry can be applied for synthesis of tetrel derivatives reaching almost complete derivatization of 2D material skeleton. [more]
Angle-resolved photoemission spectroscopy (ARPES) is one of the most powerful tools to study the electronic properties of solids. Besides providing a wealth of information on the momentum-dependent band structure, the impressive progress in high-resolution and multi-dimensional ARPES allows insights into the nature of the quantum states in the solid itself. [more]
Angle-resolved photoemission spectroscopy (ARPES) is often considered the best way to experimentally determine the ground-state electronic structure of materials. However, although applying ARPES to short-lived excited states via the pump/probe method (tr-ARPES) demands orders of magnitude more data than ground-state ARPES studies, measurements have been forced to work with orders of magnitude lower data rates due to the limits imposed by the repetition rate of available short-pulse extreme-ultraviolet (XUV) light sources and the collection efficiency of photoelectron analyzers. [more]

Ultrafast Science and Technologies at ALLS

The Advanced Laser Light Source (ALLS) is located at INRS-ÉMT near Montreal. It is the national laser facility of Canada offering access to a variety of laser systems and secondary sources. [more]
Single-molecule chemistry [1] has progressed together with the development of scanning probe microscopy and its related methods. Scanning tunneling microscopy (STM) has been widelyused for the observation and control of configurational changes and reactions for individual molecules on surfaces. [more]
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