arXiv subjects

Kerstin Volz

Publications and source records attributed to Kerstin Volz.

29 records · Page 2Linked to original sources

arXiv2022

Ultra-Broadband Visible and Infrared Light Generation Driven by Far Infrared Light in the Broad Region from 8μm to 240μm

The most commonly used nonlinear optical process is the conversion of infrared light at 1064nm to green light at 532nm, as performed in common laser pointers. However, more relevant for future applications are nonlinear optical processes that generate a broad spectrum, a so called supercontinuum. A desirable goal is generating a spectrum that covers the whole visible range (400 -900nm), i.e., white light. Nowadays, white-light generation is usually achieved in specially designed photonic fibres requiring high laser intensities. However, in previous studies we showed that amorphous powders of $(PhSn)_4S_6$ cluster-molecules generate white light when they are irradiated by low-intensity near-infrared light. In this study, we use the mid- and far-infrared radiation of a free-electron laser to investigate the same molecules. White-light generation is observed for excitation with wavelength between 8 and 240$μ$ m. While the emitted radiation shows only slight variations, its intensity strongly depends on the excitation wavelength. We then match the wavelength dependent efficiency with the infrared absorption spectra of the material. This comparison shows: whenever the excitation can introduce molecular vibrations, less white light is generated. For all other wavelengths the excitation interacts mostly with the electron system. This shows that the electron system and the molecular backbone are decoupled to a large extent. Our work contributes to the understanding of the nonlinear process that underlies white-light generation in $(PhSn)_4S_6$ cluster molecules. Additionally, it shows the high potential of this material in applications where a broad laser spectrum is desired.

physics.optics↗

arXiv2022

Octave-spanning 1 emission across the visible spectrum from single crystalline 1,3,5,7-tetrakis-($p$-methoxyphenyl)adamantane

Sustainable efficient light emitter based solely on elements-of-hope are needed to replace current compounds based on less-abundant materials. Functionalized diamondoids are a potential solution for this challenge as they offer efficient, octave-spanning emission across the visible spectrum in their single-crystalline form. Its large quantum efficiency increases towards higher-than-ambient temperatures to beyond 7 %. The stability beyond 200 °C renders such functionalized diamondoids as sustainable phosphors for LED applications. Detailed structural and theoretical investigations suggest a crucial role of exciton states accompanied by structural modifications (self-trapped excitons) in the emission process.

cond-mat.mtrl-sci↗

arXiv2022

Adamantanes as white-light emitters: Controlling arrangement and functionality by external Coulomb forces

Functionalized adamantane molecular cluster materials show highly transient nonlinear optical properties of currently unclear structural origin. Several interaction mechanisms in compounds comprising molecular clusters, their inter- and intramolecular interactions as well as the interplay of their electronic systems and vibrations of their backbone are viable concepts to explain these nonlinear optical properties. We show that transient Coulomb forces also have to be considered as they can lead to intramolecular structure transformations and intermolecular rearrangements in the crystal. Both strongly influence the nonlinear optical properties. Moreover, selective bromine functionalization can trigger a photochemical rearrangement of the molecules. The structure and chemical bonding within the compounds are investigated in dependence on the laser irradiation at different stages of their nonlinear emission by electron diffraction and electron energy loss spectroscopy. The transient structural and chemical states observed are benchmarked by similar observations during electron irradiation, which makes quantification of structural changes possible and allows the correlation with first principles calculations. The functionalization and its subsequent usage to exploit photochemical effects can either enhance two-photon absorption or facilitate white-light emission rather than second-harmonic generation.

cond-mat.mtrl-sci↗

arXiv2022

White-light generating molecular materials: correlation between the amorphous/crystalline structure and nonlinear optical properties

Amorphous materials are integral part of todays technology, they commonly are performant and versatile in integration. Consequently, future applications increasingly aim to harvest the potential of the amorphous state. Establishing its structure-property relationship, however, is inherently challenging using diffraction-based techniques yet is extremely desirable for developing advanced functionalities. In this article, we introduce a set of transmission electron microscopy-based techniques to locally quantify the structure of a material. This unique approach allows to clearly identify the spatial distribution of amorphous and crystalline regions and to quantify atomic arrangements of amorphous regions of a representative model system. We study an ensemble of well-defined, functionalized adamantane-type cluster molecules exhibiting exceptionally promising nonlinear optical properties of unclear origin. The nanoscopic structure for three model compounds ([(PhSn)4S6], [(NpSn)4S6], [(CpSn)4S6]) correlates with their characteristic optical responses. These results highlight the advantageous properties of amorphous molecular materials when understanding the microscopic origin.

cond-mat.mtrl-sci↗

arXiv2021

Separation of interface and substrate carrier dynamics at a heterointerface based on coherent optical phonons

Transient reflectivity spectroscopy is widely used to study ultrafast carrier- and phonon-dynamics in semiconductors. In their heterostructures, it is often not straightforward to distinguish contributions to the signal from the various layers. In this work, we perform transient reflectivity measurements on lattice-matched GaP/Si(001) using a near infrared pulse, to which GaP is transparent. The pump laser pulse can generate coherent longitudinal optical (LO) phonons both in the GaP overlayer as well as in the Si substrate which have distinct frequencies. This enables us to track the amplitude of the respective signal contributions as a function of GaP layer thickness $d$. The Si phonon amplitude in the signal exhibits an oscillatory behavior with increasing $d$. This can be quantitatively explained by the interference of the probe light reflected at the air/GaP/Si heterointerface. Based on this knowledge, we can then separate the interface- and the substrate-contributions in the carrier-induced non-oscillatory transient reflectivity signal. The obtained interface signal provides evidence for ultrafast carrier injection from the Si substrate into the GaP overlayer. This is also corroborated by examining the deviation of the polarization-dependence of the GaP coherent optical phonon signal from that of the bulk semiconductor.

cond-mat.mes-hall↗

arXiv2019

Ab-initio calculation of band alignments for opto-electronic simulations

A modified core-to-valence band maximum approach is applied to calculate band offsets of strained III/V semiconductor hetero junctions. The method is used for the analysis of (In,Ga)As/GaAs/Ga(As,Sb) multi-quantum well structures. The obtained offsets and the resulting bandstructure are used as input for the microscopic calculation of photoluminescence spectra yielding very good agreement with recent experimental results.

cond-mat.mtrl-sci↗

arXiv2018

Coherent optical and acoustic phonons generated at lattice-matched GaP/Si(001) heterointerfaces

Thin GaP films can be grown on an exact Si(001) substrate with nearly perfect lattice match. We perform all-optical pump-probe measurements to investigate the ultrafast electron-phonon coupling at the buried interface of GaP/Si. Above-bandgap excitation with a femtosecond laser pulse can induce coherent longitudinal optical (LO) phonons both in the GaP opverlayer and in the Si substrate. The coupling of the GaP LO phonons with photoexcited plasma is reduced significantly with decreasing the GaP layer thickness from 56 to 16 nm due to the quasi-two-dimensional confinement of the plasma. The same laser pulse can also generate coherent longitudinal acoustic (LA) phonons in the form of a strain pulse. The strain induces not only a periodic modulation in the optical reflectivity as they propagate in the semiconductors, but also a sharp spike when it arrives at the GaP layer boundaries. The acoustic pulse induced at the GaP/Si interface is remarkably stronger than that at the Si surface, suggesting a possible application of the GaP/Si heterostructure as an opto-acoustic transducer. The amplitude and the phase of the reflectivity modulation varies with the GaP layer thickness, which can be understood in terms of the interference caused by the multiple acoustic pulses generated at the top surface and at the buried interface.

cond-mat.mtrl-sci↗

arXiv2017

Valence band-anticrossing in GaP$_{1-x}$Bi$_{x}$ dilute bismide alloys: giant bowing of the band gap and spin-orbit splitting energy

Using spectroscopic ellipsometry measurements on GaP$_{1-x}$Bi$_{x}$/GaP epitaxial layers up to $x = 3.7$% we observe a giant bowing of the direct band gap ($E_{g}^Γ$) and valence band spin-orbit splitting energy ($Δ_{\textrm{SO}}$). $E_{g}^Γ$ ($Δ_{\textrm{SO}}$) is measured to decrease (increase) by approximately 200 meV (240 meV) with the incorporation of 1% Bi, corresponding to a greater than fourfold increase in $Δ_{\textrm{SO}}$ in going from GaP to GaP$_{0.99}$Bi$_{0.01}$. The evolution of $E_{g}^Γ$ and $Δ_{\textrm{SO}}$ with $x$ is characterised by strong, composition-dependent bowing. We demonstrate that a simple valence band-anticrossing model, parametrised directly from atomistic supercell calculations, quantitatively describes the measured evolution of $E_{g}^Γ$ and $Δ_{\textrm{SO}}$ with $x$. In contrast to the well-studied GaAs$_{1-x}$Bi$_{x}$ alloy, in GaP$_{1-x}$Bi$_{x}$ substitutional Bi creates localised impurity states lying energetically within the GaP host matrix band gap. This leads to the emergence of an optically active band of Bi-hybridised states, accounting for the overall large bowing of $E_{g}^Γ$ and $Δ_{\textrm{SO}}$ and in particular for the giant bowing observed for $x \lesssim 1$%. Our analysis provides insight into the action of Bi as an isovalent impurity, and constitutes the first detailed experimental and theoretical analysis of the GaP$_{1-x}$Bi$_{x}$ alloy band structure.

cond-mat.mtrl-sci↗

arXiv2017

Sub-picosecond acoustic pulses at buried GaP/Si interfaces

We report on the optical generation and detection of ultrashort acoustic pulses that propagate in three-dimensional semiconductor crystals. Photoexcitaiton of lattice-matched GaP layers grown on Si(001) gives rise to a sharp spike in transient reflectivity due to the acoustic pulse generated at the GaP/Si interface and detected at the GaP surface and vice versa. The extremely short width of the reflectivity spike, 0.5 ps, would translate to a spatial extent of 3 nm or 10 atomic bilayers, which is comparable with the width of the intermixing layer at the GaP/Si interface. The reflectivity signals are also modified by quasi-periodic Brillouin oscillations of GaP and Si arising from the acoustic pulses during the propagation in the crystals. The present results demonstrate the potential application of the simple optical pump-probe scheme in the nondestructive evaluation of the buried semiconductor interface quality.

cond-mat.mtrl-sci↗

arXiv2017

Ab initio calculations of the concentration dependent band gap reduction in dilute nitrides

While being of persistent interest for the integration of lattice-matched laser devices with silicon circuits, the electronic structure of dilute nitride III/V-semiconductors has presented a challenge to ab initio computational approaches. The root of this lies in the strong distortion N atoms exert on most host materials. Here, we resolve these issues by combining density functional theory calculations based on the meta-GGA functional presented by Tran and Blaha (TB09) with a supercell approach for the dilute nitride Ga(NAs). Exploring the requirements posed to supercells, we show that the distortion field of a single N atom must be allowed to decrease so far, that it does not overlap with its periodic images. This also prevents spurious electronic interactions between translational symmetric atoms, allowing to compute band gaps in very good agreement with experimentally derived reference values. These results open up the field of dilute nitride compound semiconductors to predictive ab initio calculations.

cond-mat.mtrl-sci↗

arXiv2016

Direct Characterization of Band Bending in GaP/Si(001) Heterostructures with Hard X-ray Photoelectron Spectroscopy

We apply hard X-ray photoelectron spectroscopy (HAXPES) to investigate the electronic structures in ~50-nm thick epitaxial GaP layers grown on Si(001) under different conditions. Depth profiles of the local binding energies for the core levels are obtained by measuring the photoemission spectra at different incident photon energies between 3 and 7 keV and analyzing them with simple numerical models. The obtained depth profiles are in quantitative agreement with the band bending determinations for the same samples in a previous coherent phonon spectroscopic study. Our results demonstrate the applicability of the HAXPES with varying incident photon energy to characterize the electric potential profiles at buried semiconductor heterointerfaces.

cond-mat.mes-hall↗