Uni Rostock Logo Physik Uni Rostock Logo

4th International Workshop on Rydberg Excitons in Semiconductors

Rydberg Exciton

26.-27. November 2020

Excitons – bound pairs of electrons and holes in a semiconducting material – can be produced in excited internal states by means of laser excitation. More than 60 years after their discovery in cuprous oxide semiconductors, the investigation of such Rydberg excitons is currently attracting increasing world-wide interest due to their vastly exaggerated properties. Just like for their atomic counterparts, the enhanced external-field sensitivity and strong mutual interactions of such Rydberg states makes them attractive systems for fundamental studies of basic quantum phenomena and suggests exciting opportunities for future applications, such as nonlinear optical interfaces.

The workshop will bring together international researchers to share latest results, to discuss current challenges, and to explore future perspectives and new opportunities in this growing and vibrant field of research. The unusual circumstances this year sadly prevented us from organizing an in-person meeting. As the field of Rydberg exciton physics is growing nonetheless, we will meet this year in virtual space.

CET
09:45
10:00
10:30
11:00
11:30
12:00
12:30
13:00
13:30
14:30
15:00
15:30
16:00
16:30
Thu 26. Nov 20
Opening remarks
Julian Heckötter (Dortmund)
Asymmetric Rydberg blockade of excitons in Cuprous Oxide
Asymmetric Rydberg blockade of excitons in Cuprous Oxide
(Download Talk)

Our work demonstrates the existence of strong spatially-correlated interactions between Rydberg excitons in Cuprous Oxide. We use an asymmetric pump-probe excitation scheme to optically generate Rydberg excitons in different quantum states and investigate the interaction between them.
The presence of pump excitons results in an excitation blockade of probe excitons that exhibits a characteristic dependence on the principal quantum number and unique spectral signatures of the underlying interaction potential.
We show that these signatures correspond to dipole-dipole interactions between pump- and probe excitons and identify the interaction to be of long-range van der Waals-type.
This analysis further allows us to separate the observed effects from a plasma interaction.
Stephen Lynch (Cardiff)
Comparing Rydberg Excitation in Synthetic and Natural Cuprous Oxide
Comparing Rydberg Excitation in Synthetic and Natural Cuprous Oxide
(Download Talk)

In this presentation, we report on our recent investigations of Rydberg excitation in bulk synthetic cuprous oxide grown by the optical float zone technique, and compare the result with natural samples. Samples have been assessed using several material characterisation techniques. X-ray diffraction patterns confirm both materials are mostly single crystal, and mid-infrared transmission spectroscopy revealed little difference between synthetic and natural material. The synthetic samples show principal quantum numbers up to n = 10, exhibit additional absorption lines, plus enhanced spatial broadening and spatial inhomogeneity. Room temperature and cryogenic photoluminescence measurements reveal a significant excess of copper vacancies in the synthetic material. Our measurements provide a route towards achieving high-n excitons in synthetic crystals, opening an avenue to scalable quantum devices.
Sai Kiran Rajendran (St. Andrews)
Rydberg excitons in Cu2O nanoparticles
Rydberg excitons in Cu2O nanoparticles
(Download Talk)

The strong exciton-exciton interactions [1] as well as exciton-plasma [2] interactions have a pronounced effect on Rydberg excitons in Cu2O especially at high principal quantum numbers. Confinement of these Rydberg excitons is an important step towards exploitation of their giant non-linearities for applications in quantum technologies. Here, we characterise the linear optical properties of excitons in a thin slab of natural Cu2O crystal at 4K. We then investigate these excitons in natural as well as synthetic Cu2O nanoparticles to reveal their fragile nature when confined to nanoscale. Our results will be of interest towards achieving spatial confinement [3] as well as making photonic structures [4].

[1] T. Kazimierczuk et al, Nature 514, 343 (2014)
[2] J. Heckötter et al, Physical Review Letters 121, 097401 (2018)
[3] A. Konzelmann et al, Journal of Physics B: Atomic, Molecular and Optical Physics 53, 024001 (2020)
[4] V. Walther et al, Nature Communications, 9, 1309 (2018)
Virtual coffee break
Pavel Belov (St. Petersburg)
Excited exciton states and resonances in GaAs-based quantum wells
Excited exciton states and resonances in GaAs-based quantum wells
(Download Talk | Password restricted)

Recent measurements of the reflectance spectra of the high-quality GaAs-based heterostructures show that the nonradiative broadening of the exciton ensemble can be of order of the radiative one [1]. To describe the measured spectra, we accurately calculated the exciton states and electron-hole resonances in a single GaAs-based QW as well as corresponding radiative decay rates [2,3]. We considered the three-dimensional Schroedinger equation for the exciton in a quasi two-dimensional structure and analytically described the spectrum of the corresponding differential operator for arbitrary QW width as well as in case of external perturbation [4,5]. In particular, we determined the energy position of the lower boundary of the continuous spectrum that allowed us to distinguish the bound Rydberg states from the resonant electron-hole states [6]. We identified in the measured reflectance spectra the heavy- and light-hole exciton states for different QW widths of the GaAs-based heterostructures. We obtained the radiative broadenings of the identified states using the exciton-light coupling model [7] as well as the nonradiative ones of the resonant states by the complex scaling technique [8]. The calculated linewidth broadenings were compared with those obtained from the fits to measured spectra.
Financial support from the RFBR, grants No. 19-02-00576 and No. 20-32-70131, and SPbU/DAAD Dmitrij-Mendeleev-Programme, grant No. 57516244, is acknowledged.

References
[1] A.V. Trifonov, S.N. Korotan, A.S. Kurdyubov et al. Phys. Rev. B. 91, 115307 (2015).
[2] E.S. Khramtsov, P.A. Belov, P.S. Grigoryev et al. J. Appl. Phys. 119, 184301 (2016).
[3] P.A. Belov, Physica E 112, 96 (2019).
[4] P.A. Belov, Semiconductors 52, 1791 (2018).
[5] D.K. Loginov, P.A. Belov, V.G. Davydov, et al. Phys. Rev. Res. 2, 033510 (2020).
[6] P.A. Belov, Semiconductors 53, 2049 (2020).
[7] E.L. Ivchenko, Optical spectroscopy of semiconductor nanostructures (Harrow: Alpha Science Int., 2005).
[8] N. Moiseyev, Phys. Rep. 302, 212 (1998).
Jan Ertl (Stuttgart)
Semiclassical approaches to excitons in cuprous oxide
Semiclassical approaches to excitons in cuprous oxide
(Download Talk)

Semiclassical approaches to excitons in cuprous oxide", "Abstract": "A complete theoretical description of excitons in cuprous oxide needs to include a description of its band structure. This can be done by introducing a quasispin and hole spin adding new degrees of freedom compared to the hydrogen-like case. While quantum mechanical calculations are limited to low principle quantum numbers, experimental techniques are also able to access the high energy regime of excitonic spectra. For atomic physics semiclassical theory is able to successfully describe this area by connecting classical trajectories to quantum spectra. In my talk I will introduce an adiabatic approach, considering quasispin and hole spin, which allows for the calculation of classical exciton orbits and their parameters by introducing energy surfaces in momentum space. This enables us to study the classical dynamics of excitons in cuprous oxide ultimately laying down the foundations for a semiclassical description of the system and providing deep physical insight into exciton physics [1].

[1] J. Ertl et al. PRB 101, 241201(R) (2020)
Lunch break
Andreas Farenbruch (Dortmund)
Second harmonic generation spectroscopy of the Rydberg series of ortho- and paraexcitons in cuprous oxide
Second harmonic generation spectroscopy of the Rydberg series of ortho- and paraexcitons in cuprous oxide
(Download Talk)

We report on second harmonic generation (SHG) spectroscopy of ortho- and paraexcitons in cuprous oxide. SHG can occur for light propagating along the low-symmetry directions [111] or [112]. Using spectrally broad femtosecond pulses we measure S and D excitons up to a principal quantum number of n = 9 with a spectral resolution limited by the spectrometer to 80 µeV. By application of a magnetic field up to 10 T in Voigt configuration, SHG gets also allowed for excitation along the [110]-axis and even the high-symmetry cubic direction [001] due to the Zeeman- and magneto-Stark effect. Using group theory we derive the polarization selection rules for both mechanisms. By setting a specific configuration of the linear polarization angles of the incoming and outgoing light, we are able to separate both SHG mechanisms.

The symmetry and spin forbidden paraexcitons are activated by a magnetic field admixture from the M = 0 component of the orthoexciton. By this method we demonstrate the Rydberg series of paraexcitons up to a quantum number of n = 6. Due to the electron-hole exchange interaction, the paraexcitons are generally located below the orthoexcitons. This order is reversed for n = 2, due to mixing of the 2S orthoexciton of the yellow series and the 1S orthoexciton of the green series.
Su-Fei Shi (Rensselaer Polytechnic Institute)
Rydberg excitons in TMDCs
Rydberg excitons in TMDCs
(Download Talk | Password restricted)

A strong Coulomb interaction could lead to a strongly bound exciton with high-order excited states, similar to the Rydberg atom. The interaction of giant Rydberg excitons can be engineered for a correlated ordered exciton array with a Rydberg blockade, which is promising for realizing quantum simulation. Monolayer transition metal dichalcogenides, with their greatly enhanced Coulomb interaction, are an ideal platform to host the Rydberg excitons in two dimensions. Here, we employ helicity-resolved magneto-photocurrent spectroscopy to identify Rydberg exciton states up to 11s in monolayer WSe2. Notably, the radius of the Rydberg exciton at 11s can be as large as 214 nm, orders of magnitude larger than the 1s exciton. The giant valley-polarized Rydberg exciton not only provides an exciting platform to study the strong exciton–exciton interaction and nonlinear exciton response but also allows the investigation of the different interplay between the Coulomb interaction and Landau quantization, tunable from a low- to high-magnetic-field limit.
Valentin Walther (Harvard)
A Quantum Nonlinear Mirror with Rydberg Excitons
A Quantum Nonlinear Mirror with Rydberg Excitons
(Download Talk | Password restricted)

Excitons, bound electron-hole pairs, in a monolayered semiconductor form a collective resonance that can fully and coherently reflect resonant light. We investigate how the reflection and transmission properties of such a system are modified by strong excitonic interactions. The nonlinear reflection of a Gaussian beam is shown to be strongly affected by interactions when the beam radius is smaller than a characteristic interaction length scale. In the limit of very strong interactions, we analytically solve the intensity-intensity correlation function of the reflected light, demonstrating that the reflected light is both sub-poissonian and non-classical. Oscillations in the photon correlations occur under strong driving at an intrinsic time scale, set only by the excitonic light matter coupling constant. This time scale can be controlled optically using an scheme of electromagnetically induced transparency. The resulting oscillations are shown to be slow and persistent, even in the presence of additional decoherence effects.
Breakout discussions
Fri 27. Nov 20
Sjard Ole Krüger (Rostock)
Rydberg excitons in cuprous oxide in the presence of impurities
Rydberg excitons in cuprous oxide in the presence of impurities
(Download Talk | Password restricted)

Interaction of charged impurities and Rydberg excitons in cuprous oxide", "Abstract": "Although the understanding of the single-exciton absorption spectra has come a long way, the spectra do still contain a few unexplained features. Among these are the exact line shapes of the highest resolvable Rydberg resonances and their deviation from the theoretical predictions. In this talk, we investigate the influence of a population of charged point defects as a possible explanation for some of these deviations [1], e.g. the observed change in the line asymmetry and the loss of oscillator strength. To this end, we model the statistical distribution of the local electric fields induced by the impurities by the Holtsmark microfield-distribution [2], calculate the Holtsmark-averaged Stark-spectra and compare them to experimental spectra.

[1] S. O. Krüger and S. Scheel, Phys. Rev. B 101, 235204 (2020).
[2] J. Holtsmark, Ann. Phys. 363, 577 (1919).
Dirk Semkat (Greifswald)
Interaction of Rydberg excitons with a low-density electron-hole plasma in cuprous oxide
Interaction of Rydberg excitons with a low-density electron-hole plasma in cuprous oxide
(Download Talk)

The influence of an electron-hole plasma on the transmission spectra of Rydberg excitons in cuprous oxide (Cu2O) is analyzed. Comparing measured spectra with the predictions of quantum many-body theory, an excellent agreement is demonstrated for parameter regions where competing mechanisms like ionization by residual charged impurities play a minor role.
Patric Rommel (Stuttgart)
Numerical investigation of complex exciton resonances and of the exchange splitting in cuprous oxide
Numerical investigation of complex exciton resonances and of the exchange splitting in cuprous oxide
(Download Talk)

The talk will focus on two subjects.
Topic 1: The complex-coordinate rotation method allows for the numerical calculation of resonance states with complex eigenenergies.
We give an overview of various results obtained for excitons in cuprous oxide, beginning with the application of an electric field to the yellow exciton series [1] and then moving on to the green exciton resonances [2]. The latter results serve as a preliminary step towards the investigation of interseries transition matrix elements between the yellow and green series.
Topic 2: Farenbruch et al. experimentally investigated the yellow paraexciton series in cuprous oxide using second harmonic generation spectroscopy [3], confirming the reversal of the 2S para- and orthoexcitons [4]. A detailed numerical analysis shows that this reversal originates from the strong mixing with the green 1S state [5]. We additionally investigate the dependence of the exchange splitting on the principal quantum number and on the choice of series.

[1] P. Zielinski et al., J. Phys. B 53, 054004 (2020)
[2] P. Rommel et al., Phys. Rev. B 101, 075208 (2020)
[3] P. Rommel et al., Phys. Rev. B 101, 115202 (2020)
[4] A. Farenbruch et al., Phys. Rev. Lett. 125 (2020), in press
[5] P. Rommel et al., Phys. Rev. B (to be submitted)
Virtual coffee break
Thomas Boulier (LPENS)
Towards Rydberg exciton-polaritons in Paris: introducing a new experimental group
Towards Rydberg exciton-polaritons in Paris: introducing a new experimental group
(Download Talk)

In this introductory talk I will present the perspectives and current status of a new experimental activity that is beginning at the Laboratoire de Physique des l'Ecole Normale Supérieure (LPENS). We are interested in using both visible and THz/MIR radiations to explore two-photons excitations of Rydberg excitons, starting with Cu2O. This approach is promising to reach the polariton regime, from which a cavity a QED setup could be developed.
Joshua Rogers (Durham)
Resonant coupling of Rydberg excitonic states to a microwave field in cuprous oxide
Resonant coupling of Rydberg excitonic states to a microwave field in cuprous oxide
(Download Talk)

Many of the aims of those of us studying Rydberg excitons in cuprous oxide echo results previously demonstrated in the Rydberg atomic physics community. The exquisite sensitivity of Rydberg atoms to external fields has seen them put to use in gravimeters, magnetometers, RF and THz field detectors and other quantum sensors and transducers. Rydberg Helium atoms have recently been coupled to a superconducting microwave resonator, a proven architecture for highly controllable qubit circuits. This result demonstrated a pathway to coherent optical readout, a critical requirement for long-range distribution of the quantum information encoded in microwave qubit circuits. In order to translate results like this into a solid state system, we present new evidence that demonstrates a significant coupling when a microwave field is applied to Rydberg excitons in cuprous oxide. In second-harmonic-generation experiments where the laser is resonant with excitons n > 5, Stokes and anti-Stokes sidebands appear in the spectrally-resolved emission. In broadband absorbance experiments, the optical density of the material is increased at even parity S and D excitonic states when microwaves of the same frequency are applied. Although the exact mechanism remains unclear, the observed effect bears hallmarks of both Brillouin scattering and microwave-driven intra-band transitions.
Marina Semina (Ioffe Institute)
Excited trions in atomically thin semiconductors
Excited trions in atomically thin semiconductors
(Download Talk)

Atomically thin semiconductors based on transition metal dichalcogenides (TMDCs) host Rydberg-like series of two-dimensional (2D) excitons. The deviations of their binding energies and oscillator strengths from the hydrogen model are mainly caused by the specifics of the Coulomb interaction screening in 2D semiconductors [1].

The analogues of the hydrogenic ions H+ and H2- are also observed in TMDC monolayers with significant binding energies of about 30 meV. These states are known as trions or charged excitons and are formed from two electrons and a hole (X--trion) or two holes and an electron (X+-trion). The multivalley band structure of the 2D TMDCs gives rise to the fine structure of the trion states which has not been observed in conventional semiconductors [2].

Recently, the excited states of the trions have been observed in the monolayer semiconductors [3,4,5,6], which calls for the development of the theoretical description of a situation where the resident charge carrier interacts with the excited exciton. We develop such formalism based on the scattering matrix approach and uncover the autoionization effect which produces an intrinsic contribution to the excited trion lifetime related to its dissociation into the free electron (hole) and the ground state exciton. We compare our findings with experimental results of Ref. [6].

The prospects for studying Rydberg series of trions are discussed.

[1] Gang Wang, Alexey Chernikov, Mikhail M. Glazov, Tony F. Heinz, Xavier Marie, Thierry Amand, and Bernhard Urbaszek, Rev. Mod. Phys. 90, 021001 (2018)
[2] E. Courtade, M. Semina, M. Manca, M. M. Glazov, C. Robert, F. Cadiz, G. Wang, T. Taniguchi, K. Watanabe, M. Pierre, W. Escoffier, E. L. Ivchenko, P. Renucci, X. Marie, T. Amand, and B. Urbaszek, Phys. Rev. B 96, 085302 (2017)
[3] A. Arora, T. Deilmann, T. Reichenauer, J. Kern,S. Michaelis de Vasconcellos, M. Rohlfing, and R. Bratschitsch, Phys. Rev. Lett. 123, 167401 (2019)
[4] Thomas Goldstein, Yueh-Chun Wu, Shao-Yu Chen, Takashi Taniguchi, Kenji Watanabe, Kalman Varga, and Jun Yan, J. Chem. Phys. 153, 071101 (2020)
[5] Erfu Liu, Jeremiah van Baren, Zhengguang Lu, Takashi Taniguchi, Kenji Watanabe, Dmitry Smirnov, Yia-Chung Chang, Chun Hung Lui, arXiv:2006.04895 (2020).
[6] Koloman Wagner, Edith Wietek, Jonas D. Ziegler, Marina Semina, Takashi Taniguchi, Kenji Watanabe, Jonas Zipfel, Mikhail M. Glazov, Alexey Chernikov, arXiv:2007.05396 (2020)
Lunch break
Poster session
  1. Poster flash
  2. Presentation in breakout rooms
Martin Bergen
Time Resolved Pump-Probe Measurements in Cuprous Oxide
Patrick Egenlauf
Search for exceptional points in cuprous oxide
Peter Grünwald
Decoupling Phonons from Excitons in Cu2O
Michael Marquardt
Stability properties of classical exciton orbits in cuprous oxide
Jon Pritchett
Characterisation of natural and synthetic Cu2O through room temperature photoluminescence spectroscopy
Valentin Walther
Phonons, plasmas and Rydberg excitons
Breakout discussions
Closing remarks
Jump to Program