Cross-Kerr interaction in a four-level atomic system

We derive the form of the cross-Kerr interaction in a four-level atomic system in the N-configuration. We use time-independent perturbation theory to calculate the eigenenergies and eigenstates of the Schrodinger equation for the system. The system is considered as a perturbation of a Raman resonant three-level lambda scheme for which exact solutions are known. We show that within the strong control field limit the cross-Kerr interaction can arise between two weak probe fields. The strength of this nonlinear coupling is several orders of magnitude larger than that achievable using optical fibres.Comment: 5 pages, resubmitted to Physical Review A with clarified style and correction to Fig

Aspects of stimulated two-photon transitions in bulk semiconductors

This thesis aims to further the understanding of the two-photon interaction dynamic in bulk direct semiconductors in an electronically excited state. The central goal is to assess the practicability of controlling the two-photon transition strength using an ultrafast all-optical scheme and ultimately identify the key parameters and requirements necessary to observe degenerate two-photon gain in these media. Here, a transfer of stimulated two-photon emission to the semiconductor medium is very attractive because of its inherently large and fast optical nonlinearities and the capability to inject such materials with very high carrier densities, making it possible to macroscopically utilize this comparatively weak higher order nonlinear optical processes in the potential realization of a semiconductor-based two-photon laser and practical quantum information processing. Utilizing various (ultrafast) spectroscopic techniques, it is found that the application of a strong optical excitation on the order of Iexc>10 mJ/cm2 (yielding carrier densities approaching ne≈1019 cm-3) indeed produces a notable modification to the two-photon coupling for target sum photon energies within a typical bandwidth of ΔE≤80 meV above the optical band gap energy of the sample. Here, in obeying central experimental parameters regarding the excitation process, gain medium and two-photon configuration, a full amplitude inversion of the nonlinear coefficient β(ω), indicating a two-photon emission regime, is observed. Furthermore, the impact of accompanying effects related to the optical excitation on the semiconductor medium, e.g. free-carrier absorption, in competition to the nonlinear interaction have been investigated in this work

Selectively tunable optical Stark effect of anisotropic excitons in atomically thin ReS2

The optical Stark effect is a coherent light-matter interaction describing the modification of quantum states by non-resonant light illumination in atoms, solids and nanostructures. Researchers have strived to utilize this effect to control exciton states, aiming to realize ultra-high-speed optical switches and modulators. However, most studies have focused on the optical Stark effect of only the lowest exciton state due to lack of energy selectivity, resulting in low degree-of-freedom devices. Here, by applying a linearly polarized laser pulse to few-layer ReS2, where reduced symmetry leads to strong in-plane anisotropy of excitons, we control the optical Stark shift of two energetically separated exciton states. Especially, we selectively tune the Stark effect of an individual state with varying light polarization. This is possible because each state has a completely distinct dependence on light polarization due to different excitonic transition dipole moments. Our finding provides a methodology for energy-selective control of exciton states.111612Ysciescopu

Spektrale und statistische Eigenschaften der parametrischen Fluoreszenz bei hoher optisch-parametrischer Verstärkung

This thesis is devoted to high-gain parametric down-conversion (PDC). PDC is mostly known in the low-gain (spontaneous) regime, in which the correlated photon pairs are produced. Spontaneous PDC (SPDC) plays a very important role for quantum optics as a variety of quantum states is produced via SPDC, including, for instance, entangled Bell states or photon-number states. Moreover, SPDC finds its applications in metrology, cryptography, imaging, and lithography. In the high-gain case PDC leads to generation of bright states having up to hundreds mW mean power. With such states almost any nonlinear optical interaction or light-matter interaction becomes more efficient. Even being macroscopically bright, the produced states maintain nonclassical properties as, for example, the fluctuations of electric field quadratures are squeezed below the shot-noise level. The high-gain PDC could be used not only in the same applications as SPDC, it also can provide new ones. For example, PDC is in use in the LIGO and GEO600 gravitational-wave detectors, because squeezing increases the sensitivity of interferometry. High-gain PDC has many remarkable spectral and statistical properties, which are in the focus of this work. The thesis discusses them in detail, both theoretically and experimentally, and shows how high-gain PDC could be used. The description starts from the PDC generation in normal and anomalous group velocity dispersion ranges. The spectrum and mode content of high-gain PDC is considered as well as their change with the parametric gain are demonstrated. Then, there are the interference effects emerging from the PDC correlations presented, namely the macroscopic analogue of the Hong-Ou-Mandel interference. In addition, it is shown how spatial and temporal walk-off matching could be used for the generation of giant narrowband twin beams. Finally, the statistical properties of high-gain PDC are reviewed as well as their use for multiphoton effects is demonstrated. Photon-number fluctuations of PDC are studied via normalized correlation functions and probability distributions. These fluctuations enhance the generation efficiency for multiphoton effects by orders of magnitude and lead to tremendously fluctuating light described by heavy-tailed photon-number probability distributions.Diese Arbeit befasst sich mit der experimentellen und theoretischen Untersuchung der parametrischen Fluoreszenz bei hoher optisch-parametrischer Verstärkung. Im Gegensatz zur parametrische Fluoreszenz bei geringer Verstärkung, wobei die Photonenpaare erzeugt werden, ist das Regime hoher Verstärkung noch vergleichsweise wenig erforscht. Parametrische Fluoreszenz ist bedeutsam für die Quantenoptik und wird z.B. in der Metrologie, Kryptographie, Bildgebung und Lithografie angewendet. Bei hoher Verstärkung entstehen extrem helle Lichtzustände, die bis zu hundert Milliwatt mittlere Leistung haben können. Solchen Zustände sind viel effizienter für nichtlineare optische Effekte und Licht-Materie-Wechselwirkungen. Obwohl die bei hoher Verstärkung erzeugten Lichtzustände makroskopisch hell sind, zeigen sie nichtklassische Eigenschaften. Die bei hoher optisch-parametrischer Verstärkung erzeugte parametrische Fluoreszenz hat außergewöhnliche spektrale und statistische Eigenschaften, die im Mittelpunkt dieser Arbeit stehen. Die Arbeit beschreibt diese Eigenschaften nicht nur theoretisch und experimentell, sondern zeigt auch realisierbare Anwendungen