Fast optical work-function tuning at an organic/metal interface

In a two-color experiment, we demonstrate how light can be used as an external control to continuously tune the work function of a gold substrate functionalized with a dilute azobenzene-based self-assembled monolayer (SAM). The work function is measured by two-photon photoelectron spectroscopy. While in the ground state the azobenzene moiety adopts the trans configuration, illumination with pulsed laser light at a wavelength of 368 nm results in a photostationary state (PSS) mainly comprising cis isomers. An additional 450 nm continuous-wave laser with tunable intensity serves to shift the PSS back towards the ground state. This way the work function is freely adjustable in real time over a range of ∼240 meV between the two PSS extrema. We furthermore relate the change in work function to the average change in dipole moment per azobenzene chromophore. Quantum-chemical calculations that take into account available structural data of the molecules in the SAM must consider at least two different trans and four different cis orientations. The computed respective perpendicular trans-cis dipole-moment changes indicate that in experiment the cis molecules adopt different orientations along with a very high cis azobenzene yield in the UV PSS

Terahertz displacive excitation of a coherent Raman-active phonon in V2O3

Nonlinear processes involving frequency-mixing of light fields set the basis for ultrafast coherent spectroscopy of collective modes in solids. In certain semimetals and semiconductors, generation of coherent phonon modes can occur by a displacive force on the lattice at the difference-frequency mixing of a laser pulse excitation on the electronic system. Here, as a low-frequency counterpart of this process, we demonstrate that coherent phonon excitations can be induced by the sum-frequency components of an intense terahertz light field, coupled to intraband electronic transitions. This nonlinear process leads to charge-coupled coherent dynamics of Raman-active phonon modes in the strongly correlated metal VO. Our results show an alternative up-conversion pathway for the optical control of Raman-active modes in solids mediated by terahertz-driven electronic excitation

Towards more efficient and specific driving and probing of coherent phonon dynamics in solids

Ultrafast spectroscopy is a powerful tool to study materials and their microscopic interactions in non-equilibrium states. With ultrafast methods, it is possible to reveal how electrons and atoms move and interact on very short timescales. Revealing the underlying mechanisms helps to understand how macroscopic phases form, which interaction channels are available and how to control materials with external stimuli to design specific properties. Depending on the type of interaction, the excitation energy for efficient coupling between the material and an external electric field comes from very different regions of the electro-magnetic spectrum. Nowadays, from x-rays to microwaves, powerful sources for most spectral ranges have been developed. They even cover the range from 0.3 THz to 30 THz (commonly referred to as the Terahertz gap), where the interaction of phonons – modes of lattice vibrations – with an external electric field can be studied directly. This has been difficult historically because this is the part of the spectrum where the radiation changes from electronic to photonic treatment. These sources are applied either as the pump or the probe pulse in an ultrafast pump–probe experiment. Inventions like the quantum cascade laser, the use of photoconducting antennas or the process of optical rectification in suitable nonlinear crystals now lead to sources of low frequencies. Pushing the development of sources in the meV energy range to higher stability, higher powers, and table-top setups to enhance possible excitation schemes for more advanced material control is still an ongoing challenge. Furthermore, the need for narrowband sources (the counterpart of the established broadband sources) has grown, enabling elaborate pump–probe schemes while minimizing energy losses. With narrowband sources, it is possible to study specific vibrations in solids and gases with high energy specificity. This work focuses on the excitation and detection of coherent phonons on ultrafast timescales. The need for high pulse intensities to excite phonons efficiently into a nonlinear regime calls for new pump and probe schemes. I refine two aspects of the recent developments: In the first part, I present the characterisation of a new high-power narrowband source in the far- to mid-infrared spectral regime. The narrowband characteristic is achieved by chirping two infrared (IR) pulses used in a difference frequency generation (DFG) process. Varying the applied amount of chirp in each IR beam allows for sensitive tuning of the frequency sweep over the duration of the mid-IR pulse. This opens up new excitation schemes, e.g. with the concept of capture into resonance. A flexible table-top setup with tunable center frequency and chirp will enhance the efficiency of phonon driving, making nonlinear excitation regimes accessible. With the extensive characterisation of the setup constructed during my thesis project we can tune the electric field properties precisely and identify the maximum tuning range and pulse parameters. This became possible by a comprehensive study on the influence of certain parameters on the amount of chirp of the obtained mid-IR pulse and by modelling the electric field with large qualitative agreement. In addition to the development of new pulsed sources, the capabilities of structure-sensitive probes have increased significantly, driven by the improvement of x-ray free electron lasers (XFELs). Reduced temporal jitter and enhanced brightness allow for detailed analysis even in complex materials, where vibrational signatures can be small. Sophisticated data treatment and advanced experimental techniques have enabled an increased understanding on electron–phonon coupling in complex materials. The second part investigates ultrafast x-ray diffraction in a prototypical quasi-1D charge-density-wave (CDW) material, the blue bronze K0.3MoO3. With x-rays as a structure-sensitive probe, we are able to distinguish atoms involved in the excitation of specific phonon modes. With the aid of a simulation for the structure factors of individually distorted atoms in the crystal structure, we can decompose the atomic motion associated with CDW phonon modes for the first time: An amplitude mode at 1.68 THz is directed mostly along the y axis, and a 2.5 THz phonon mode has its main contribution along the z axis. This analysis can help towards the understanding of electron–phonon coupling mechanisms, especially for materials with larger unit cells, where calculations are currently still infeasible.Ultrakurzzeitspektroskopie ist eine leistungsfähige Methode zur Untersuchung von Materialien und ihren mikroskopischen Wechselwirkungen in Nichtgleichgewichtszuständen. Mit ultraschnellen Methoden lässt sich untersuchen, wie sich Elektronen und Atome auf sehr kurzen Zeitskalen bewegen und miteinander wechselwirken. Die zugrundeliegenden Prozesse zu verstehen, hilft zu beschreiben, wie sich makroskopische Phasen bilden, welche Wechselwirkungspfade zur Verfügung stehen, und wie Materialien designt werden können, um bestimmte Eigenschaften zu erreichen. Je nach Art der Wechselwirkung stammt die Anregungsenergie für eine effiziente Kopplung zwischen dem Material und einem externen elektrischen Feld aus sehr unterschiedlichen Bereichen des elektromagnetischen Spektrums. Heutzutage sind für die meisten Spektralbereiche, von Röntgenstrahlung bis hin zu Mikrowellen, leistungsfähige Quellen entwickelt worden. Sogar der Bereich von 0.3 bis 30 THz (im Allgemeinen als “Terahertz-Lücke” bezeichnet), in dem die direkte Wechselwirkung von Phononen – Moden von Gitterschwingungen – mit externen elektrischen Feldern untersucht werden kann, ist mittlerweile zugänglich. Dies war in der Vergangenheit schwierig, weil dies der Teil des Spektrums ist, in dem die Strahlung von elektronischer zu photonischer Charakteristik übergeht. Diese Quellen werden entweder als “Pump”- oder als “Probe”-Puls in einem ultraschnellen Pump–Probe-Experiment eingesetzt. Erfindungen wie der Quantenkaskadenlaser, die Verwendung von photoleitenden Antennen oder der Prozess der optischen Gleichrichtung in geeigneten nichtlinearen Kristallen führen nun zu Quellen mit niedrigen Frequenzen. Die Entwicklung von Strahlungsquellen im meV-Energiebereich mit höherer Stabilität, höheren Leistungen und einfachen Laboraufbauten zur Verbesserung möglicher Anregungsschemata für eine erweiterte Materialkontrolle ist nach wie vor eine ständige Herausforderung. Darüber hinaus ist der Bedarf an Schmalbandquellen (als Gegenstück zu den etablierten Breitbandquellen), die ausgeklügelte Pump–Probe-Techniken bei gleichzeitiger Minimierung von Energieverlusten ermöglichen, gestiegen. Mit Schmalbandquellen ist es möglich, spezifische Schwingungen in Festkörpern und Gasen mit hoher Energiespezifität zu untersuchen. Diese Arbeit behandelt die Anregung und den Nachweis von kohärenten Phononen auf ultrakurzen Zeitskalen. Die Notwendigkeit hoher Pulsintensitäten, um Phononen effizient in einen nichtlinearen Bereich anzuregen, erfordert neue Pump- und Probe-Techniken. Ich entwickle zwei Aspekte der jüngsten Entwicklungen weiter: Im ersten Teil stelle ich die Charakterisierung einer leistungsstarken Schmalbandquelle im mittleren Infrarotbereich (mid-IR) vor. Die Schmalbandcharakteristik wird durch “Chirpen” (zeitliche Frequenzverschiebung innerhalb eines Pulses) zweier IR-Pulse in einem Differenzfrequenzerzeugungsprozess (DFG) erreicht. Die resultierende Strahlung liegt hierbei im mid-IR-Bereich. Durch Variation des Chirp-Anteils in jedem IR-Puls lässt sich die Frequenzverschiebung über die Dauer des mid-IR-Pulses feinmaschig einstellen, was neue Anregungsschemata eröffnet, z.B. mit dem Konzept des “Einfangens in Resonanz”. Dieser flexible Laboraufbau mit variabler Zentralfrequenz und Chirp kann die Anregungseffizienz von Phononen verbessern und nichtlinear Anregungsregime zugänglich machen. Mit dieser umfassenden Charakterisierung des im Rahmen dieser Arbeit konstruierten Aufbaus ist es uns gelungen, die Eigenschaften des elektrischen Feldes präzise abzustimmen und den maximalen Durchstimmbereich und die Pulseigenschaften zu ermitteln. Wir konnten nicht nur zeigen, wie bestimmte experimentelle Parameter den Chirp des erhaltenen mid-IR-Pulses beeinflussen, sondern waren auch in der Lage, das elektrische Feld mit grosser qualitativer Übereinstimmung zu simulieren, was eine genaue Vorhersage des erwarteten elektrischen Felds ermöglicht. Neben der Entwicklung neuer gepulster Quellen hat sich auch die Leistungfähigkeit von Detektionspulsen für die Strukturanalyse deutlich erhöht, was auf die Weiterentwicklung der Röntgen-Freie-Elektronen-Laser (XFEL) zurückzuführen ist. Eine höhere zeitliche Auflösung und eine verbesserte Lichtstärke ermöglichen eine detaillierte Analyse selbst komplexer Materialien, bei denen die Schwingungssignaturen gering sein können. Verbesserte Datenverarbeitung und fortschrittliche experimentelle Techniken haben das Wissen über die Elektron-Phonon-Kopplung in komplexen Materialien erheblich erweitert. Im zweiten Teil untersucht die ultraschnelle Röntgenbeugung an einem prototypischen Quasi-1D-Ladungsdichtewellen-Material (CDW), der blauen Bronze K0.3MoO3. Mit dieser Methode sind wir in der Lage, Atome zu unterscheiden, die an der Anregung bestimmter Phononenmoden beteiligt sind. Mit Hilfe einer Simulation für die Strukturfaktoren einzelner verschobener Atome in der Kristallstuktur können wir erstmals die mit CDW-Phononmoden verbundene Atombewegung aufschlüsseln: Eine Amplitudenmode ist hauptsächlich entlang der y-Achse gerichtet, und eine 2.5 THz-Mode hat ihren Hauptbeitrag entlang der z-Achse. Diese Analyse kann dazu beitragen, die Mechanismen von Elektron–Phonon-Kopplung besser zu verstehen, insbesondere bei Materialien mit grösseren Einheitszellen, bei denen Modellierungen derzeit noch nicht durchführbar sind

Comparison of coherent phonon generation by electronic and ionic Raman scattering in LaAlO3

In ionic Raman scattering, infrared-active phonons mediate a scattering process that results in the creation or destruction of a Raman-active phonon. This mechanism relies on nonlinear interactions between phonons and has in recent years been associated with a variety of emergent lattice-driven phenomena in complex transition-metal oxides, but the underlying mechanism is often obscured by the presence of multiple coupled order parameters in play. Here, we use time-resolved spectroscopy to compare coherent phonons generated by ionic Raman scattering with those created by more conventional electronic Raman scattering on the nonmagnetic and non-strongly-correlated wide-band-gap insulator LaAlO3. We find that the oscillatory amplitude of the low-frequency Raman-active Eg mode exhibits a sharp peak when we tune our pump frequency into resonance with the high-frequency infrared-active Eu mode, consistent with first-principles calculations. Our results suggest that ionic Raman scattering can strongly dominate electronic Raman scattering in wide-band-gap insulating materials. We also see evidence of competing scattering channels at fluences above 28mJ/cm2 that alter the measured amplitude of the coherent phonon response.ISSN:2643-156

Correlation between electronic and structural orders in 1T−TiSe2

The correlation between electronic and crystal structures of 1T−TiSe2 in the charge-density wave (CDW) state is studied by x-ray diffraction in order to clarify basic properties in the CDW state, transport properties, and chirality. Three families of reflections are used to probe atomic displacements and the orbital asymmetry in Se. Two distinct onset temperatures are found: T_CDW and a lower T* indicative for an onset of Se out-of-plane atomic displacements. T* coincides with a DC resistivity maximum and the onset of the proposed gyrotropic (chiral) electronic structure. However, no indication for chirality is found. The relation between the atomic displacements and the transport properties is discussed in terms of Ti 3d and Se 4p states that only weakly couple to the CDW order.ISSN:2643-156

Correlation between electronic and structural orders in 1T−TiSe21T−TiSe_{2}

The correlation between electronic and crystal structures of $1T−TiSe_{2}$ in the charge-density wave (CDW) state is studied by x-ray diffraction in order to clarify basic properties in the CDW state, transport properties, and chirality. Three families of reflections are used to probe atomic displacements and the orbital asymmetry in Se. Two distinct onset temperatures are found: $T_{CDW}$ and a lower $T^∗$ indicative for an onset of Se out-of-plane atomic displacements. $T^∗$ coincides with a DC resistivity maximum and the onset of the proposed gyrotropic (chiral) electronic structure. However, no indication for chirality is found. The relation between the atomic displacements and the transport properties is discussed in terms of Ti $3d$ and Se $4p$ states that only weakly couple to the CDW order

Strong modulation of carrier effective mass in WTe2 via coherent lattice manipulation

The layered transition-metal dichalcogenide WTe2 is characterized by distinctive transport and topological properties. These properties are largely determined by electronic states close to the Fermi level, specifically to electron and hole pockets in the Fermi sea. In principle, these states can be manipulated by changes to the crystal structure. The precise impact of particular structural changes on the electronic properties is a strong function of the specific nature of the atomic displacements. Here, we report on time-resolved X-ray diffraction and infrared reflectivity measurements of the coherent structural dynamics in WTe2 induced by femtosecond laser pulses excitation (central wavelength 800 nm), with emphasis on a quantitative description of both in-plane and out-of-plane vibrational modes. We estimate the magnitude of these motions, and calculate via density functional theory their effect on the electronic structure. Based on these results, we predict that phonons periodically modulate the effective mass of carriers in the electron and hole pockets up to 20%. This work opens up new opportunities for modulating the peculiar transport properties of WTe2 on short time scales.ISSN:2397-713

Kinetics of a Phonon-Mediated Laser-Driven Structural Phase Transition in Sn₂P₂Se₆

We investigate the structural dynamics of the incommensurately modulated phase of Sn 2 P 2 Se 6 by means of time-resolved X-ray diffraction following excitation by an optical pump. Tracking the incommensurable distortion in the time domain enables us to identify the transport effects leading to a complete disappearance of the incommensurate phase over the course of 100 ns. These observations suggest that a thin surface layer of the high-temperature phase forms quickly after photo-excitation and then propagates into the material with a constant velocity of 3.7 m/s. Complementary static structural measurements reveal previously unreported higher-order satellite reflection in the incommensurate phase. These higher-order reflections are attributed to cubic vibrational terms in the Hamiltonian