Sven M. Hein

Large-area 3D chiral plasmonic structures

We manufacture large-area plasmonic structures featuring 3-dimensional chirality by colloidal nanohole lithography. By varying the polar rotating speed of the samples during gold evaporation, we can fabricate spiral-type ramp nanostructures. The optical properties show chiroptical resonances in the 100 to 400 THz frequency region (750 to 3000 nm), with circular dichroism values of up to 13%. Our method offers a simple low-cost manufacturing method of cm(2)-sized chiral plasmonic templates for chiroptical applications such as stereochemical enantiomer sensors.

Feedback-Induced Steady-State Light Bunching Above the Lasing Threshold

We develop a full quantum-optical approach for optical self-feedback of a microcavity laser. These miniaturized devices work in a regime between the quantum and classical limit and are test beds for the differences between a quantized theory of optical self-feedback and the corresponding semiclassical theory. The light intensity and photon statistics are investigated with and without an external feedback: We show that in the low-gain limit, where relaxation oscillations do not appear, the recently observed photon bunching in a quantum-dot microcavity laser with optical feedback can be accounted for only by the fully quantized model. By providing a description of laser devices with feedback in the quantum limit, we reveal insights into the origin of bunching in quantized and semiclassical models.

Plasmon hybridization in stacked metallic nanocups

We investigate hybridized electric and magnetic plasmon modes in stacked nanocups. To elucidate the coupling mechanism we demonstrate the analogy between split-ring-resonators and nanocups in the case of dipolar excitation and compare the behavior of stacked nanocups to stacked split-ring-resonators. The interplay of electric coupling with the symmetric and antisymmetric coupling of magnetic moments in effective split-ring-resonator resonances in the nanocups leads to experimentally observed hybridized modes in the coupled nanocup system. Our stacked nanocups are easily manufacturable at low cost, they cover a large-area, and can serve as SERS or SEIRA substrates. They might also serve as novel plasmonic nanoantennas, as templates for nonlinear plasmonics, and as stacked meander surfaces for metamaterial-assisted imaging.

Time-delayed quantum coherent Pyragas feedback control of photon squeezing in a degenerate parametric oscillator

Quantum coherent feedback control is a measurement-free control method fully preserving quantum coherence. In this paper we show how time-delayed quantum coherent feedback can be used to control the degree of squeezing in the output field of a cavity containing a degenerate parametric oscillator. We focus on the specific situation of Pyragas-type feedback control where time-delayed signals are fed back directly into the quantum system. Our results show how time-delayed feedback can enhance or decrease the degree of squeezing as a function of time delay and feedback strength.

Detection of dark-state relaxation through two-dimensional nano-optical spectroscopy

The correct understanding of the electronic structure and relaxation behavior in nanosystems is essential for technical applications. We propose a spectroscopic method to measure the dipole-forbidden electronic transitions of quantum dots and trace their relaxation behavior. Therefore, we utilize two-dimensional coherent spectroscopy, which is an advantageous tool to get information about the dynamics of exciton densities and coherences in nanoscopic structures. In combination with nanoplasmonics, it enables excitation of dipole-forbidden states. A nanoplasmonic dolmen structure allows us to dynamically excite either dipole-allowed and dipole forbidden states selectively. In combination with two-dimensional spectroscopy, this gives us additional control over excitation and tracing relaxation involving dipole-forbidden states in nanoscopic systems.

Feedback control of optomechanical systems

We investigate an optomechanical system with an unstable steady state, which can be stabilized via Pyragas control. We will demonstrate this for low and high pump rates. The system contains a pumped cavity, where one mirror is movable. To obtain time delayed feedback, we feed back the cavity field with an external mirror. This way, we achieve a maximal cavity field in the low pumping regime, which also corresponds to a large displaced movable mirror.

Photon pairs from a biexciton cascade with feedback-controlled polarization entanglement

We propose to use a time-delayed quantum-coherent feedback mechanism to increase and control the entanglement of photon pairs emitted by a quantum dot biexciton cascade. The quantum dot biexciton cascade is a well-known source of entangled photons on demand, however excitonic fine-structure splitting decreases the achievable polarization entanglement. We demonstrate that feedback can change the spectrum of the emitted photons in a way that the entanglement is either strongly increased or decreased, depending on the feedback time and phase. We analyze the dependence on parameters such as the delay time and the robustness of the proposed mechanism.

Feedback-Enhanced Entanglement of Photons from a Biexciton Cascade

The entanglement of photons from a biexciton cascade is strongly diminished by exciton fine-structure splitting. We demonstrate an optical feedback mechanism to counteract this loss and to control the photon entanglement.

Modifying the emission of electric and magnetic dipoles with plasmonic split-ring resonators

The radiative properties of photon emitters strongly depend on their photonic environment, i.e., on the local density of states (LDOS). Due to the high electric and magnetic fields around a resonantly excited plasmonic nanostructure, the LDOS is strongly modified. This leads to a plethora of fascinating effects, ranging from enhanced decay rates to suppressed emission, dependent on the location of the emitter. Using a Discrete Dipole Approximation algorithm, we explore the emission behavior of electric as well as magnetic dipole emitters located next to resonant split ring resonators, which are well-known for their large optical magnetic moment. We then compare our results to the widely used “two-dipole” model of an SRR and explain the strong deviations.