Bernd Löchel

Enhancement of the zero phonon line emission from a single nitrogen vacancy center in a nanodiamond via coupling to a photonic crystal cavity

Using a nanomanipulation technique a nanodiamond with a single nitrogen vacancy center is placed directly on the surface of a gallium phosphide photonic crystal cavity. A Purcell-enhancement of the fluorescence emission at the zero phonon line (ZPL) by a factor of 12.1 is observed. The ZPL coupling is a first crucial step toward future diamond-based integrated quantum optical devices.

Nanoassembled Plasmonic-Photonic Hybrid Cavity for Tailored Light-Matter Coupling

We propose and demonstrate a hybrid cavity system in which metal nanoparticles are evanescently coupled to a dielectric photonic crystal cavity using a nanoassembly method. While the metal constituents lead to strongly localized fields, optical feedback is provided by the surrounding photonic crystal structure. The combined effect of plasmonic field enhancement and high quality factor (Q approximately 900) opens new routes for the control of light-matter interaction at the nanoscale.

Controlled coupling of a single-diamond nanocrystal to a photonic crystal cavity

We demonstrate the controlled coupling of a single diamond nanocrystal to a planar photonic crystal double-heterostructure cavity. A dip-pen deposition method and subsequent manipulation with an atomic force microscope was used to precisely position the nanocrystal on top of the cavity. The optical properties of this combined system are investigated with regard to changes in the quality factor and resonance wavelength of the cavity mode as a function of the size and relative position of the diamond nanocrystal. These studies represent an important step toward well-controlled cavity-QED experiments with single-defect centers in diamond.

Modification of visible spontaneous emission with silicon nitride photonic crystal nanocavities

Photonic crystal (PC) nanocavities based on silicon nitride membranes are studied as tools for the manipulation of spontaneous emission in the wavelength range between 550 nm and 800 nm. We observe a strong modification of the fluorescence spectrum of dye molecules spin-cast on top of the PC, indicating an efficient coupling of the dye emission to the cavity modes. The cavity design is optimized with respect to the quality factor and values of nearly 1500 are achieved experimentally. Taking into account the small mode volume, which leads to a strong Purcell enhancement, these nanocavities enable the realization of efficient single photon sources in the visible region of the spectrum. Furthermore, their fabrication is fully compatible with existing CMOS technology, making an integration into more complex optoelectronic devices feasible.

Emission properties of high-Q silicon nitride photonic crystal heterostructure cavities

We report on the fabrication and optical characterization of photonic crystal (PC) double-heterostructure cavities made from silicon nitride (SiN). The intrinsic luminescence of the SiN membranes was used as an internal light source in the visible wavelength range (600–700nm) to study the quality factor and polarization properties of the cavity modes. Quality factors of up to 3400 were found experimentally, which represents the highest value reported so far in low-index PCs. These results highlight the role of SiN as a promising material system for PC devices in the visible.

Replica molding for multilevel micro-/nanostructure replication

The development of micro- and nanofabrication processes with the capability to achieve three-dimensional structures is of great interest for a wide variety of applications. In this paper, a replica molding process for the replication of combined micro- and nanostructures in an epoxy-based photo resin is reported. First, multilevel masters were realized using standard micro- and nanofabrication processes. The structures from these masters were transferred to poly(dimethylsiloxane) (PDMS) stamps by soft lithography. Finally, the PDMS stamps were used for the replication of nano- and microstructures in the epoxy-based resin by UV casting. With this process, micro- and nanostructures of minimal dimensions of 50 nm were successfully replicated. Furthermore, the capabilities of the process were confirmed by the fabrication of a microfluidic device. In this system, the surface of micro channels was structured to modify its wetting properties and create hydrophobic and hydrophilic areas without any additional chemical treatment.

Quasicrystalline-structured light harvesting nanophotonic silicon films on nanoimprinted glass for ultra-thin photovoltaics

We present nanophotonic light harvesting crystalline silicon (c-Si) thin films on glass exhibiting ten-fold transversely quasicrystalline lattice geometry on 6 x 8 mm2 area. The c-Si architectures with a nearest neighbor distance of 650 nm are fabricated by nanoimprinting the desired quasicrystalline geometry into sol-gel coated glass substrates followed by Si deposition of 240 nm to 270 nm thickness, self-organized solid phase crystallization and selective chemical etching. Broadband absorption measurements on these quasicrystalline-structured c-Si architectures yield a very significant improvement in light trapping in the near infrared regime and an enhanced light coupling due to a graded index effect in comparison to the unstructured sample. The average value of maximum achievable short circuit current density jsc, max of solar cells with such quasicrystalline-structured c-Si absorber geometry (19.3 mA/cm2) is more than double in comparison to the jsc, max of unstructured planar films of the same thickness (9.3 mA/cm2) and remains stable for light incident angles up to 60°. In comparison to a 320 nm thick c-Si film on textured ZnO:Al substrate as widely used for light trapping in amorphous-microcrystalline Si thin-film photovoltaics, still a 65% increased jsc, max is observable for the presented quasicrystalline c-Si structures. The nanophotonic light trapping efficiency of these transversely quasicrystalline c-Si nanoarchitectures is among the highest values for experimentally realized structures, revealing their promising influence for broadband and isotropic light trapping for economically viable and efficient ultra-thin solar cells.

A realistic fabrication and design concept for quantum gates based on single emitters integrated in plasmonic-dielectric waveguide structures.

Tremendous enhancement of light-matter interaction in plasmonic-dielectric hybrid devices allows for non-linearities at the level of single emitters and few photons, such as single photon transistors. However, constructing integrated components for such devices is technologically extremely challenging. We tackle this task by lithographically fabricating an on-chip plasmonic waveguide-structure connected to far-field in- and out-coupling ports via low-loss dielectric waveguides. We precisely describe our lithographic approach and characterize the fabricated integrated chip. We find excellent agreement with rigorous numerical simulations. Based on these findings we perform a numerical optimization and calculate concrete numbers for a plasmonic single-photon transistor.

Integration of moth-eye structures into a poly(dimethylsiloxane) stamp for the replication of functionalized microlenses using UV-nanoimprint lithography

The increasing demand for low cost camera modules for mobile devices requires technological solutions for the manufacturing process. One of the most promising fabrication processes for microlenses for camera modules is UV-nanoimprint lithography. In a typical fabrication process, an elastomer stamp is used to replicate microlenses. In this work, a method is presented to integrate moth-eye structures as an antireflective layer into a poly(dimethylsiloxane) (PDMS) stamp containing a microlens array. The integration of these structures is done by a thermoforming process. Due to the integration of the moth-eye structures into the PDMS stamp, the optical performance of the replicated microlenses can be improved and no additional processing steps are necessary after the replication process.

Assembly of fundamental photonic elements from single nanodiamonds

We demonstrate the ability to modify the emission properties and enhance the interaction strength of single emitters coupled to nanophotonic structures based on metals and dielectrics. Assembly of individual diamond nanocrystals, metal nanoparticles and photonic crystal cavities to meta-structures is introduced. Experiments concerning controlled coupling of single defect centers in nanodiamonds to silver nanowires with the goal to investigate quantum plasmonic effects are reported. Furthermore, we demonstrate the formation of a hybrid cavity system in which metal nanostructures are evanescently coupled to a dielectric photonic crystal cavity. This structure allows combined exploitation of both resonant dielectric as well as plasmonic enhancement.