arXiv subjects

Alfred Leitenstorfer

Publications and source records attributed to Alfred Leitenstorfer.

At least 19 recordsLinked to original sources

arXiv2025

Field-tuning of ultrafast magnetization fluctuations in Sm$_{0.7}$Er$_{0.3}$FeO$_{3}$

The properties of spin fluctuations in antiferromagnets are largely unexplored, in particular at ultrafast timescales. Here, we employ femtosecond noise correlation spectroscopy (FemNoC) to experimentally study magnetization fluctuations in the canted antiferromagnet Sm$_{0.7}$Er$_{0.3}$FeO$_{3}$ across its spin reorientation transition and under external magnetic fields. By comparing our measurements to atomistic spin noise and Monte Carlo simulations, we find that the amplitude of the spin noise is governed by the free energy, with stronger fluctuations in regions where the potential landscape softens. We furthermore demonstrate that external magnetic fields suppress spin fluctuations and enhance the quasi-ferromagnetic magnon frequency by effectively stiffening the potential. These results highlight an effective route for tuning ultrafast magnetization fluctuations via external parameters.

cond-mat.mtrl-sci↗

arXiv2025

Modulation of sub-optical cycle photocurrents in an ultrafast near-infrared scanning tunnelling microscope

Lightwave-driven scanning tunnelling microscopy (STM) at near-IR frequencies promises an unprecedented combination of atomic spatial resolution and temporal resolution approaching the attosecond range. To achieve this goal, high-sensitivity optical control and detection of sub-cycle tunnelling currents must be achieved at the STM junction. Here, we demonstrate the generation and detection of coherent ultrafast currents across the junction of an STM illuminated by near-infrared single-cycle pulses. We introduce a novel modulation scheme that avoids time-dependent thermal loading while selectively isolating carrier-envelope phase (CEP)-dependent photocurrents. All artifacts arising from periodic modulation of laser power and thermal coupling are efficiently suppressed, enabling a clean readout of the coherent portion of the ultrafast tunneling current.

physics.optics↗

arXiv2024

Dynamical renormalization of the magnetic excitation spectrum via high-momentum nonlinear magnonics

Controlling macroscopic properties of quantum materials requires the ability to induce and manipulate excited states. The set of collective excitations of a solid is encoded in its dispersion relations. We find that the spectra of the low-momentum eigenmodes are renormalized by resonantly driving the high-momentum excitations. Our experimental data rule out laser-induced thermal processes as an origin of the renormalization. The photoinduced changes of the amplitudes and frequencies can be explained theoretically and numerically with a resonant light-scattering mechanism that couples high- and low-momentum eigenmodes in the dispersion relation across momentum space. While we demonstrate the renormalization of the magnetic spectrum in a quantum material, our experimental approach can be further generalized to lattice modes in semiconductors.

cond-mat.str-el↗

arXiv2023

Electric-field fluctuations as the cause of spectral instabilities in colloidal quantum dots

Spectral diffusion (SD) represents a substantial obstacle towards implementation of solid-state quantum emitters as a source of indistinguishable photons. By performing high-resolution emission spectroscopy for individual colloidal quantum dots at cryogenic temperatures, we prove the causal link between the quantum-confined Stark effect and SD. Statistically analyzing the wavelength of emitted photons, we show that increasing the sensitivity of the transition energy to an applied electric field results in amplified spectral fluctuations. This relation is quantitatively fit to a straightforward model, indicating the presence of a stochastic electric field on a microscopic scale whose standard deviation is 9 kV/cm, on average. Compensating the commonly observed intrinsic electric bias with an external one, we find that SD can be suppressed by up to a factor of three in CdSe/CdS core/shell nanorods. The current method will enable the study of SD in multiple types of quantum emitters, such as solid-state defects or organic lead-halide perovskite quantum dots, for which spectral instability is a critical barrier for applications in quantum sensing.

cond-mat.mes-hall↗

arXiv2023

Nonlinear Generation, Compression and Spatio-Temporal Analysis of GV/cm-Class Femtosecond Mid-Infrared Transients

A thin-disk regenerative amplifier with 3 kHz repetition rate pumps a second-order nonlinear mixing scheme providing femtosecond transients of maximum electric field strength beyond 330 MV/cm at a center frequency of 45 THz. This value surpasses field conditions present at sub-molecular dimensions of matter. The inherent competition between efficiency and bandwidth in parametric downconversion is overcome with a third-order nonlinear step of self-phase modulation. The high repetition rate, versatility, beam quality and passive phase stability of our system support advanced spectroscopic approaches like electro-optic sampling. With this technique, we study solitonic self-compression and octave-spanning supercontinua directly on a subcycle scale.

physics.optics↗

arXiv2023

Controlling Dual-Comb Soliton Motion inside a single Fiber Laser Cavity

Ultrafast science builds on dynamic compositions of precisely timed light pulses1 and evolving sequences of pulses are observed inside almost every mode-locked laser. However, the underlying physics remains barely controlled and utilized until now. Here, we demonstrate the fast and deterministic control of soliton motion for the generation of programmable ultrashort pulse patterns from a dual-comb mode-locked Er:fiber laser. Specifically, we harness intra-cavity modulation of individual solitons and their laser-intrinsic dynamics to facilitate the rapid tuning of two interlaced soliton combs. Upon extra-cavity temporal recombination of both combs, we obtain reconfigurable pulse patterns at arbitrary delays. Using high-throughput real-time spectral interferometry, we resolve the short-range inter-soliton motion upon external stimuli and we demonstrate the high-speed sweeping of picosecond pump-probe-delays and programmable free-form trajectories. This work introduces a novel approach to soliton control and paves the way for ultrafast instruments at unprecedented high tuning, cycling and acquisition speeds.

physics.optics↗

arXiv2023

Subcycle tomography of quantum light

Quantum light is considered to be one of the key resources of the coming second quantum revolution expected to give rise to groundbreaking technologies and applications. If the spatio-temporal and polarization structure of modes is known, the properties of quantum light are well understood. This information provides the basis for contemporary quantum optics and its applications in quantum communication and metrology. However, thinking about quantum light at the most fundamental timescale, namely the oscillation cycle of a mode or the inverse frequency of an involved photon, we realize that the corresponding picture has been missing until now. For instance, how to comprehend and characterize a single photon at this timescale? To fill this gap, we demonstrate theoretically how local quantum measurements allow to reconstruct and visualize a quantum field under study at subcycle scales, even when its temporal mode structure is a priori unknown. In particular, generation and tomography of ultrabroadband squeezed states as well as photon-subtracted states derived from them are described, incorporating also single-photon states. Our results set a cornerstone in the emerging chapter of quantum physics termed time-domain quantum optics. We expect this development to elicit new spectroscopic concepts for approaching e.g. fundamental correlations and entanglement in the dynamics of quantum matter, overcoming the temporal limitation set by the oscillation cycles of both light and elementary excitations.

quant-ph↗

arXiv2023

Few-femtosecond phase-sensitive detection of infrared electric fields with a third-order nonlinearity

Measuring an electric field waveform beyond radio frequencies is often accomplished via a second-order nonlinear interaction with a laser pulse shorter than half of the field's oscillation period. However, synthesizing such a gate pulse is extremely challenging when sampling mid- (MIR) and near- (NIR) infrared transients. Here, we demonstrate an alternative approach: a third-order nonlinear interaction with a relatively long multi-cycle pulse directly retrieves an electric-field transient whose central frequency is 156 THz. A theoretical model, exploring the different nonlinear frequency mixing processes, accurately reproduces our results. Furthermore, we demonstrate a measurement of the real part of a sample's dielectric function, information that is challenging to retrieve in time-resolved spectroscopy and is therefore often overlooked. The new method paves the way towards experimentally simple MIR-to-NIR time-resolved spectroscopy that simultaneously extracts the spectral amplitude and phase information, an important extension of optical pump-probe spectroscopy of, e.g., molecular vibrations and fundamental excitations in condensed-matter physics.

physics.optics↗

arXiv2023

Tracing attosecond electron emission from a nanometric metal tip

Solids exposed to intense electric fields release electrons through tunnelling. This fundamental quantum process lies at the heart of various applications, ranging from high brightness electron sources in DC operation to petahertz vacuum electronics in laser-driven operation. In the latter process, the electron wavepacket undergoes semiclassical dynamics in the strong oscillating laser field, similar to strong-field and attosecond physics in the gas phase. There, the sub-cycle electron dynamics has been determined with a stunning precision of tens of attoseconds, but at solids the quantum dynamics including the emission time window has so far not been measured. Here we show that two-colour modulation spectroscopy of backscattering electrons uncovers the sub-optical-cycle strong-field emission dynamics from nanostructures, with attosecond precision. In our experiment, photoelectron spectra of electrons emitted from a sharp metallic tip are measured as function of the relative phase between the two colours. Projecting the solution of the time-dependent Schrödinger equation onto classical trajectories relates phase-dependent signatures in the spectra to the emission dynamics and yield an emission duration of $710\pm30$ attoseconds by matching the quantum model to the experiment. Our results open the door to the quantitative timing and precise active control of strong-field photoemission in solid state and other systems and have direct ramifications for diverse fields such as ultrafast electron sources, quantum degeneracy studies and sub-Poissonian electron beams, nanoplasmonics and petahertz electronics.

physics.optics↗

arXiv2023

Ultrafast spontaneous spin switching in an antiferromagnet

Owing to their high magnon frequencies, antiferromagnets are key materials for future high-speed spintronics. Picosecond switching of antiferromagnetic order has been viewed a milestone for decades and pursued only by using ultrafast external perturbations. Here, we show that picosecond spin switching occurs spontaneously due to thermal fluctuations in the antiferromagnetic orthoferrite Sm0.7Er0.3FeO3. By analysing the correlation between the pulse-to-pulse polarisation fluctuations of two femtosecond optical probes, we extract the autocorrelation of incoherent magnon fluctuations. We observe a strong enhancement of the magnon fluctuation amplitude and the coherence time around the critical temperature of the spin reorientation transition. The spectrum shows two distinct modes, one corresponding to the quasi-ferromagnetic mode and another one which has not been previously reported in pump-probe experiments. Comparison to a stochastic spin dynamics simulation reveals this new mode as smoking gun of ultrafast spontaneous spin switching within the double-well anisotropy potential.

cond-mat.mtrl-sci↗

arXiv2021

Realizing an Unruh-DeWitt detector through electro-optic sampling of the electromagnetic vacuum

A new theoretical framework to describe the experimental advances in electro-optic detection of broadband quantum states, specifically the quantum vacuum, is devised. By making use of fundamental concepts from quantum field theory on spacetime metrics, the nonlinear interaction behind the electro-optic effect can be reformulated in terms of an Unruh-DeWitt detector coupled to a conjugate field during a very short time interval. When the coupling lasts for a time interval comparable to the oscillation periods of the detected field mode (i.e. the subcycle regime), virtual particles inhabiting the field vacuum are transferred to the detector in the form of real excitations. We demonstrate that this behavior can be rigorously translated to the scenario of electro-optic sampling of the quantum vacuum, in which the (spectrally filtered) probe works as an Unruh-DeWitt detector, with its interaction-generated photons arising from virtual particles inhabiting the electromagnetic vacuum. We discuss the specific working regime of such processes, and the consequences through characterization of the quantum light involved in the detection.

quant-ph↗

arXiv2021

Quantum susceptibilities in time-domain sampling of electric field fluctuations

Electro-optic sampling has emerged as a new quantum technique enabling measurements of electric field fluctuations on subcycle time scales. Probing a second-order nonlinear material with an ultrashort coherent laser pulse imprints the fluctuations of a terahertz field onto the resulting near-infrared electrooptic signal. We describe how the statistics of this time-domain signal can be calculated theoretically, incorporating from the onset the quantum nature of the electric fields involved in the underlying interactions. To this end, a microscopic quantum theory of the electro-optic process is developed using an ensemble of non-interacting three-level systems as a model for the nonlinear material. We find that the response of the nonlinear medium can be separated into a classical part sampling the terahertz field and quantum contributions independent of the state of the probed terahertz field. The quantum response is caused by interactions between the three-level systems mediated by the terahertz vacuum fluctuations. It arises due to cascading processes and contributions described by quantum susceptibilities solely accessible via quantum light. We show that the quantum contributions can be substantial and might even dominate the total response. We also determine the conditions under which the classical response serves as a good approximation of the electro-optic process and demonstrate how the statistics of the sampled terahertz field can be reconstructed from the statistics of the electro-optic signal. In a complementary regime, electro-optic sampling can serve as a spectroscopic tool to study the pure quantum susceptibilities of materials.

quant-ph↗

arXiv2021

Femtosecond Transfer and Manipulation of Persistent Hot-Trion Coherence in a Single CdSe/ZnSe Quantum Dot

Ultrafast transmission changes around the fundamental trion resonance are studied after exciting a p-shell exciton in a negatively charged II-VI quantum dot. The biexcitonic induced absorption reveals quantum beats between hot trion states at 133 GHz. While interband dephasing is dominated by relaxation of the P-shell hole within 390 fs, trionic coherence remains stored in the spin system for 85 ps due to Pauli blocking of the triplet electron. The complex spectro-temporal evolution of transmission is explained analytically by solving the Maxwell-Liouville equations. Pump and probe polarizations provide full control over amplitude and phase of the quantum beats.

cond-mat.mes-hall↗

arXiv2020

Up to 70 THz bandwidth from implanted Ge photoconductive antenna excited by a fibre laser

Phase-stable electromagnetic pulses in the THz frequency range offer several unique capabilities in time-resolved spectroscopy. However, the diversity of their application is limited by the covered spectral bandwidth. In particular, the upper frequency limit of photoconductive emitters - the most widespread technique in THz spectroscopy - reaches only up to 7 THz in regular transmission mode due to the absorption by infrared-active optical phonons. Here, we present ultra-broadband (extending up to 70 THz) THz emission from Au implanted Ge emitter which is compatible with a fibre laser operating at 1.1 and 1.55 μm wavelengths at a repetition rates of 10 and 20 MHz, respectively. This opens a perspective for the development of compact THz photonic devices operating up to multi-THz frequencies and compatible with Si CMOS technology.

physics.optics↗

arXiv2019

Control of excitonic absorption by thickness variation in few-layer GaSe

We control the thickness of GaSe on the level of individual layers and study the corresponding optical absorption via highly sensitive differential transmission measurements. Suppression of excitonic transitions is observed when the number of layers is smaller than a critical value of 8. Through ab-initio modelling we are able to link this behavior to a fundamental change in the band structure that leads to the formation of a valence band shaped as an inverted Mexican hat in thin GaSe. The thickness-controlled modulation of the optical properties provides attractive resources for the development of functional optoelectronic devices based on a single material.

cond-mat.mes-hall↗

arXiv2018

Subcycle squeezing of light from a time flow perspective

Light as a carrier of information and energy plays a fundamental role in both general relativity and quantum physics, linking these areas that are still not fully compliant with each other. Its quantum nature and spatio-temporal structure are exploited in many intriguing applications ranging from novel spectroscopy methods of complex many-body phenomena to quantum information processing and subwavelength lithography. Recent access to subcycle quantum features of electromagnetic radiation promises a new class of time-dependent quantum states of light. Paralleled with the developments in attosecond science, these advances motivate an urgent need for a theoretical framework that treats arbitrary wave packets of quantum light intrinsically in the time domain. Here, we formulate a consistent time domain theory of the generation and sampling of few-cycle and subcycle pulsed squeezed states, allowing for a relativistic interpretation in terms of induced changes in the local flow of time. Our theory enables the use of such states as a resource for novel ultrafast applications in quantum optics and quantum information.

quant-ph↗

arXiv2018

Self-control of a passively phase-locked Er:fibre frequency comb

Femtosecond frequency combs have boosted progress in various fields of precision metrology. Nevertheless, demanding applications such as front-end frequency and time standards, ultrastable microwave generation or high-resolution spectroscopy still necessitate improved stability. The spectral bandwidth and absolute position of individual comb lines are crucial in this context. Typically, both parameters are controlled on short and long time scales by tight locking to external optical and microwave references which represent costly and cumbersome additions to the entire setup. Here, we demonstrate fully self-controlled stabilization of a fibre-based femtosecond frequency comb requiring neither optical nor radio frequency external references. In the first step, this technology allows us to optically eliminate the carrier-envelope phase slip via ultrabroadband difference frequency generation. The resulting amplification of intrinsically quantum-limited phase noise from the mode-locked oscillator is elegantly addressed in the second step. We efficiently suppress these excess fluctuations by a direct transfer of the superior short-time noise properties of the fundamental oscillator to the offset-free comb. Our combined scheme provides a high-precision frequency reference operating completely autonomously, thus marking a new era for fibre-based sources in advanced applications ranging from space exploration to tests of the invariability of fundamental constants.

physics.optics↗

arXiv2018

Mid Infrared Nonlinear Plasmonics using Germanium Nanoantennas on Silicon Substrates

We demonstrate third harmonic generation in plasmonic antennas made of highly doped germanium and designed to be resonant in the mid infrared. Owing to the near-field enhancement, the result is an ultrafast, sub-diffraction, coherent light source tunable between 3 and 5 micrometer wavelength on a silicon substrate. To observe nonlinearity in this challenging spectral region, a high-power femtosecond laser system equipped with parametric frequency conversion in combination with an all-reflective confocal microscope setup is employed. We show spatially resolved maps of the linear scattering cross section and the nonlinear emission of single isolated antenna structures. A clear third order power dependence as well as the mid-infrared emission spectra prove the nonlinear nature of the light emission. Simulations support the observed resonance length of the double rod antenna and demonstrate that the field enhancement inside the antenna material is responsible for the nonlinear frequency mixing.

physics.app-ph↗