arXiv subjects

Julian Heckötter

Publications and source records attributed to Julian Heckötter.

12 recordsLinked to original sources

arXiv2025

Rydberg excitons in Cu$_2$O at millikelvin temperatures

Rydberg excitons in the semiconductor Cu$_2$O have been observed in absorption experiments up to a principal quantum number of n = 28 at millikelvin temperatures [1]. Here, we extend the experimental parameter space by variing both temperature and excitation power. In particular, we show that the P excitons close to the band gap react more sensitively to an increase of the excitation power than states of the associated D exciton multiplet, even though the latter are located at comparatively higher energy. This finding is similar to the one observed when applying an external electric field, suggesting that the observed behavior arises from internal electric fields created by charged impurities that are optically ionized. At laser intensities below 1 $μ$W/cm$^2$, absorption lines of excitons with n=29 are observed.

cond-mat.mes-hall↗

arXiv2025

Distinct terahertz third-harmonic generation of many-body excitonic states

The dynamics of an electron-hole plasma governed by strong Coulomb interaction is a challenging many-body problem.We report on experimental realization of electron-hole many-body states in the picosecond time scale, with tunable densities in a representative semiconductor Cu$_2$O. By using time-resolved optical-pump terahertz third-harmonic-generation spectroscopy, we study the nonlinear terahertz dynamical characteristics of the many-body electron-hole states. We find not only efficient and nonperturbative terahertz third-harmonic yield associated with the excitonic formation, but also a nonmonotonic dependence of the excitonic nonlinear response on the electron-hole density, reflecting the exciton dissociation at high charge density. Our results provide an efficient excitonic sensing of the far-from-equilibrium electron-hole many-body states.

cond-mat.str-el↗

arXiv2024

Effective interaction dynamics of Rydberg excitons

Rydberg states of excitons in Cu$_2$O are known to interact strongly with other carriers in their surrounding such as other excitons, free charge carriers and static charged impurities. In cw-experiments, discriminating the individual processes is notoriously challenging. Here, we use a time-resolved two-color pump-probe setup and investigate the temporal evolution of Rydberg excitons with a resolution of 15 ns. The dynamic response of the Rydberg system to pump pulses with different energies is analyzed and the elementary processes that constitute the complex decay dynamics are compared among the different excitation scenarios. Our analysis reveals mainly four different timescales on which physical processes occur.

cond-mat.mes-hall↗

arXiv2024

Nonlinear Rydberg exciton-polaritons in Cu$_2$O microcavities

Rydberg excitons (analogues of Rydberg atoms in condensed matter systems) are highly excited bound electron-hole states with large Bohr radii. The interaction between them as well as exciton coupling to light may lead to strong optical nonlinearity, with applications in sensing and quantum information processing. Here, we achieve strong effective photon-photon interactions (Kerr-like optical nonlinearity) via the Rydberg blockade phenomenon and the hybridisation of excitons and photons forming polaritons in a Cu$_2$O-filled microresonators. Under pulsed resonant excitation polariton resonance frequencies are renormalised due to the reduction of the photon-exciton coupling with increasing exciton density. Theoretical analysis shows that the Rydberg blockade plays a major role in the experimentally observed scaling of the polariton nonlinearity coefficient as $\propto n^{4.4 \pm 1.8}$ for principal quantum numbers up to n = 7. Such high principal quantum numbers studied in a polariton system for the first time are essential for realisation of high Rydberg optical nonlinearities, which paves the way towards quantum optical applications and fundamental studies of strongly-correlated photonic (polaritonic) states in a solid state system.

cond-mat.mes-hall↗

arXiv2023

Neutralization of impurities by cw-excitation of high-$n$ Rydberg-excitons

We investigate the neutralization of charged defects in a Cu$_2$O crystal experimentally with a cw two-color pump-probe setup. We demonstrate that the excitation of high-lying Rydberg excitons at low densities generates an enhancement of exciton absorption up to a factor of 3. The result is an overall improvement of the absorption spectrum's quality in terms of increased oscillator strengths and narrower linewidths, referred to as purification. The reduction of the amount of charged impurities in the crystal is further verified by a thorough analysis of impurity photoluminescence signals.

cond-mat.mes-hall↗

arXiv2023

Taming Charged Defects: Large Scale Purification in Semiconductors using Rydberg Excitons

We investigate the interaction between highly excited Rydberg excitons and charged impurities in the semiconductor Cuprous Oxide. We find that it is long-ranged and based on charge-induced dipole interactions. We demonstrate that we can utilize this interaction to neutralize impurities throughout the whole crystal - an effect we call purification. Purification reduces detrimental electrical stray fields drastically. We find that the absorption of the purified crystal increases by up to 25 % and that the purification effect is long-lived and may persist for hundreds of microseconds or even longer. Using a time-resolved pump-probe technique, we can discriminate purification from Rydberg blockade, which has been a long-standing goal in excitonic Rydberg systems.

cond-mat.mes-hall↗

arXiv2021

Scrutinizing the Debye plasma model: Rydberg excitons unravel the properties of low-density plasmas in semiconductors

For low-density plasmas, the classical limit described by the Debye-Hückel theory is still considered as an appropriate description even though a clear experimental proof of this paradigm is lacking due to the problems in determining the plasma-induced shift of single-particle energies in atomic systems. We show that Rydberg excitons in states with a high principal quantum number are highly sensitive probes for their surrounding making it possible to unravel accurately the basic properties of low-density non-degenerate electron-hole plasmas. To this end, we accurately measure the parameters of Rydberg excitons such as energies and linewidths in absorption spectra of bulk cuprous oxide crystals in which a tailored electron-hole plasma has been generated optically. Since from the absorption spectra exciton energies, as well as the shift of the single-particle energies given by the band edge, can be directly derived, the measurements allow us to determine the plasma density and temperature independently, which has been a notoriously hard problem in semiconductor physics. Our analysis shows unambiguously that the impact of the plasma cannot be described by the classical Debye model, but requires a quantum many-body theory, not only for the semiconductor plasma investigated here, but in general. Furthermore, it reveals a new exciton scattering mechanism with coupled plasmon-phonon modes becoming important even at very low plasma densities.

cond-mat.mes-hall↗

arXiv2020

Asymmetric Rydberg blockade of giant excitons in Cuprous Oxide

The ability to generate and control strong long-range interactions via highly excited electronic states has been the foundation for recent breakthroughs in a host of areas, from atomic and molecular physics [1, 2] to quantum optics [3, 4] and technology [5-7]. Rydberg excitons provide a promising solid-state realization of such highly excited states, for which record-breaking orbital sizes of up to a micrometer have indeed been observed in cuprous oxide semiconductors [8]. Here, we demonstrate the generation and control of strong exciton interactions in this material by optically producing two distinct quantum states of Rydberg excitons. This makes two-color pump-probe experiments possible that allow for a detailed probing of the interactions. Our experiments reveal the emergence of strong spatial correlations and an inter-state Rydberg blockade that extends over remarkably large distances of several micrometers. The generated many-body states of semiconductor excitons exhibit universal properties that only depend on the shape of the interaction potential and yield clear evidence for its vastly extended-range and power-law character.

cond-mat.mes-hall↗

arXiv2020

Coherent transfer matrix analysis of the transmission spectra of Rydberg excitons in Cuprous Oxide

In this study we analyze the transmission spectrum of a thin plate of Cuprous Oxide in the range of the absorption of the yellow exciton states with the coherent transfer matrix method. We demonstrate that, in contrast to the usual analysis using Beer's law, which turns out to be a rather good approximation only in the spectral region of high principal quantum numbers, a consistent quantitative description over the whole spectral range under consideration is possible. This leads to new and more accurate parameters not only for the Rydberg exciton states, but also for the strengths of indirect transitions. Furthermore, the results have consequences on the determination of the density of electron-hole pairs after optical excitation.

cond-mat.mes-hall↗

arXiv2018

Magneto-Stark-effect of yellow excitons in cuprous oxide

We investigate and compare experimental and numerical excitonic spectra of the yellow series in cuprous oxide Cu$_2$O in the Voigt configuration and thus partially extend the results from Schweiner et al. [Phys. Rev. B 95, 035202 (2017)], who only considered the Faraday configuration. The main difference between the configurations is given by an additional effective electric field in the Voigt configuration, caused by the motion of the exciton through the magnetic field. This Magneto-Stark effect was already postulated by Gross et al. and Thomas et al. in 1961 [Sov. Phys. Solid State 3, 221 (1961); Phys. Rev. 124, 657 (1961)]. Group theoretical considerations show that the field most of all significantly increases the number of allowed lines by decreasing the symmetry of the system. This conclusion is supported by both the experimental and numerical data.

cond-mat.mtrl-sci↗

arXiv2016

Magnetoexcitons in cuprous oxide

Two of the most striking experimental findings when investigating exciton spectra in cuprous oxide using high-resolution spectroscopy are the observability and the fine structure splitting of $F$ excitons reported by J. Thewes et al. [Phys. Rev. Lett. 115, 027402 (2015)]. These findings show that it is indispensable to account for the complex valence band structure and the cubic symmetry of the solid in the theory of excitons. This is all the more important for magnetoexcitons, where the external magnetic field reduces the symmetry of the system even further. We present the theory of excitons in $\mathrm{Cu_{2}O}$ in an external magnetic field and especially discuss the dependence of the spectra on the direction of the external magnetic field, which cannot be understood from a simple hydrogen-like model. Using high-resolution spectroscopy, we also present the corresponding experimental spectra for cuprous oxide in Faraday configuration. The theoretical results and experimental spectra are in excellent agreement as regards not only the energies but also the relative oscillator strengths. Furthermore, this comparison allows for the determination of the fourth Luttinger parameter $κ$ of this semiconductor.

cond-mat.mtrl-sci↗

arXiv2016

Deviations of the exciton level spectrum in cuprous oxide from the hydrogen series

Recent high-resolution absorption spectroscopy on excited excitons in cuprous oxide [Nature (London) 514, 343 (2014)] has revealed significant deviations of their spectrum from that of the ideal hydrogen-like series. Here we show that the complex band dispersion of the crystal, which determines the kinetic energy of electrons and holes, strongly affects the exciton binding energy. Specifically, we show that the nonparabolicity of the band dispersion is the main cause of the deviation from the hydrogen series. Experimental data collected from high-resolution absorption spectroscopy in electric fields validate the assignment of the deviation to the nonparabolicity of the band dispersion.

cond-mat.mes-hall↗