Rydberg Excitons Research Articles

Signatures of Quantum Coherences in Rydberg Excitons.

Coherent optical control of individual particles has been demonstrated both for atoms and semiconductor quantum dots. Here we demonstrate the emergence of quantum coherent effects in semiconductor Rydberg excitons in bulk Cu_{2}O. Because of the spectral proximity between two adjacent Rydberg exciton states, a single-frequency laser may pump both resonances with little dissipation from the detuning. As a consequence, additional resonances appear in the absorption spectrum that correspond to dressed states consisting of two Rydberg exciton levels coupled to the excitonic vacuum, forming a V-type three-level system, but driven only by one laser light source. We show that the level of pure dephasing in this system is extremely low. These observations are a crucial step towards coherently controlled quantum technologies in a bulk semiconductor.

Electro-optical properties of Rydberg excitons

We show how to compute the electrooptical functions (absorption, reflection, and transmission) when Rydberg Excitons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. With the use of Real Density Matrix Approach numerical calculations applied for Cu$_2$0 crystal are performed. We also examine in detail and explain the dependence of the resonance displacement on the state number and applied electric field strength. We report a good agreement with recently published experimental data.

Giant Rydberg excitons: Probing quantum chaos.

Giant Rydberg excitons reveal signatures of quantum chaotic behaviour in the presence of time-reversal symmetry breaking enforced by the background solid-state lattice, and they provide a new mesoscopic platform for fundamental studies of quantum chaos.

Attractive Coulomb interaction of two-dimensional Rydberg excitons

We analyze theoretically the Coulomb scattering processes of highly excited excitons in the direct bandgap semiconductor quantum wells. We find that contrary to the interaction of ground state excitons the electron and hole exchange interaction between excited excitons has an attractive character both for $s$- and $p$-type 2D excitons. Moreover, we show that similarly to the three-dimensional (3D) highly excited excitons, the direct interaction of 2D Rydberg excitons exhibits van der Waals type long-range interaction. The results predict the linear growth of the absolute value of exchange interaction strength with an exciton principal quantum number, and point the way towards enhancement of optical nonlinearity in 2D excitonic systems.

Quantum chaos and breaking of all anti-unitary symmetries in Rydberg excitons.

Symmetries are the underlying principles of fundamental interactions in nature. Chaos in a quantum system may emerge from breaking these symmetries. Compared to vacuum, crystals are attractive for studying quantum chaos, as they not only break spatial isotropy, but also lead to novel quasiparticles with modified interactions. Here we study yellow Rydberg excitons in cuprous oxide which couple strongly to the vacuum light field and interact significantly with crystal phonons, leading to inversion symmetry breaking. In a magnetic field, time-reversal symmetry is also broken and the exciton states show a complex splitting pattern, resulting in quadratic level repulsion for small splittings. In contrast to atomic chaotic systems in a magnetic field, which show only a linear level repulsion, this is a signature of a system where all anti-unitary symmetries are broken simultaneously. This behaviour can otherwise be found only for the electro-weak interaction or engineered billiards.

Optical properties of Rydberg excitons and polaritons

We show how to compute the optical functions when Rydberg Excitons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. We use the Real Density Matrix Approach (RDMA), which, combined with Green's function method, enables one to derive analytical expressions for the optical functions. Choosing the susceptibility, we performed numerical calculations appropriate to a Cu$_2$0 crystal. The effect of the coherence is displayed in the line shape. We also examine in details and explain the dependence of the oscillator strength and the resonance placement on the state number. We report a good agreement with recently published experimental data.

Giant Rydberg excitons in the copper oxide Cu2O.

A highly excited atom having an electron that has moved into a level with large principal quantum number is a hydrogen-like object, termed a Rydberg atom. The giant size of Rydberg atoms leads to huge interaction effects. Monitoring these interactions has provided insights into atomic and molecular physics on the single-quantum level. Excitons--the fundamental optical excitations in semiconductors, consisting of an electron and a positively charged hole--are the condensed-matter analogues of hydrogen. Highly excited excitons with extensions similar to those of Rydberg atoms are of interest because they can be placed and moved in a crystal with high precision using microscopic energy potential landscapes. The interaction of such Rydberg excitons may allow the formation of ordered exciton phases or the sensing of elementary excitations in their surroundings on a quantum level. Here we demonstrate the existence of Rydberg excitons in the copper oxide Cu2O, with principal quantum numbers as large as n = 25. These states have giant wavefunction extensions (that is, the average distance between the electron and the hole) of more than two micrometres, compared to about a nanometre for the ground state. The strong dipole-dipole interaction between such excitons is indicated by a blockade effect in which the presence of one exciton prevents the excitation of another in its vicinity.

Motional stark effect on Rydberg states in crossed electric and magnetic fields

Acknowledging the finite size of the nucleus in a Rydberg atom subjected to a magnetic field introduces an additional term into the Hamiltonian. Though often neglected or negligible in atoms, this term, which produces the motional Stark effect, may be important, e.g., in the solid state to an exciton interacting with a strong magnetic field. The consequences of this term in the problem of a two-dimensional Rydberg atom or exciton in crossed external electric and magnetic fields are investigated. A rich dynamics is uncovered: depending on the mass ratio and external field strengths, new types of internal motion may arise as compared to the dynamics possible in the infinite nuclear mass approximation.

Observation of cluster-specific excitations in XeN clusters.

The absorption spectra of ${\mathrm{Xe}}_{\mathit{N}}$ clusters (N=50--500) exhibit extra absorption bands not seen in condensed Xe gas. They show up if the cluster radius is comparable to the radius of the electronic excitation. From the sharpness of the bands and from the energy-level positions it is concluded that these excitations have a character which combines features seen in molecular Rydberg states and excitons of the solid. This interpretation is supported by simple model calculations. For small clusters (N\ensuremath{\le}150) the number of extra absorption bands reflects the shell structure of the clusters.

On exciton absorption, band structure, and phase transformation of GaSe under pressure

AbstractThe spectra of the absorption coefficient (α) of GaSe at the photon energy range 1.94 to 2.64eV and at pressures between 1 bar and 12 kbar are investigated at 295°K. Data at 1 to 4360 bar confirm that the edge exciton of GaSe is the direct allowed one. The pressure coefficient of the band gap (dEg/dP = −6.2 × 10−6eV bar−1), one of the Rydberg exciton constant (dR/dP = −0.6 × 10−6eV bar−1), and one of the exciton effective mass [(1/μ) (dμ/dP) = −2.4 × 10−5 bar−1] are approximately determined. It is supposed that two interband exciton transitions at 2.001 and 3.25eV give the contribution into the α spectrum above 2.1eV. The assignment of these transitions is discussed in connection with the band structure calculation of Schlüter. At 5280 bar a discontinuous jump of the absorption edge shape is observed for GaSe, with a change of the sign and a value for dEg/dP of +2.0 × 10−6eV bar−1 within the range of 5280 to 11780 bar. This effect is interpreted as the transformation of one GaSe polytype into another, i.e. as a possible phase transformation close to that of second kind.

Regularities between Exciton States andF-Center Excited States of Alkali Halides

The extent to which the intrinsic and the $F$-center absorption for alkali halides have common features has been explored by plotting the respective band photon energies against each other. Of the many possible binary combinations of excitonic and $F$-center energy-level separations for the nine potassium, rubidium, and cesium chlorides, bromides, and iodides, two sets of corresponding bands can be recognized for each halide by the very nearly equal energy separations between the exciton fundamental and the $F$ band, i.e., $\ensuremath{\Delta}h\ensuremath{\nu}(\mathrm{intrinsic})=\ensuremath{\Delta}h\ensuremath{\nu}(F \mathrm{center})$. The remaining structure in the excitonic spectra can then be attributed to upper components of halogen doublets. Splittings are very nearly constant for the iodides. The first two bands of the modified Rydberg exciton series, which converges at the conduction-band edge, correlate with the $F$ and $K$ bands. The $L$ bands comprise another series which, with their corresponding excitonic bands, exhibit a linear dependence on the consecutive energy levels of the free alkali atom.