Intrinsic mode coupling in mirror-symmetric whispering gallery resonators.

Rotationally symmetric micro-cavities with disk, ring or toroidal shape displaying whispering gallery modes (WGMs) play an essential role in modern-day photonics. Due to the reduced symmetry of such resonators compared to spheres, an exact analytical model yielding WGMs as solutions does not exist. The established WGM classification scheme based on approximated analytical solutions is generally useful but neglects a possible interaction between the different modes. In this paper, we assess the limitation of the validity of this established classification based on extensive finite element method (FEM) simulations. We investigate respective mode couplings as well as underlying selection rules based on avoided crossings of the modes' resonance wavelengths. We propose conserved mode properties solely based on true symmetries of the underlying refractive-index distribution and deduce a novel WGM classification scheme.

Comparison of various excitation and detection schemes for dye-doped polymeric whispering gallery mode micro-lasers.

We compare different excitation and collection configurations based on free-space optics and evanescently coupled tapered fibers for both lasing and fluorescence emission from dye-doped doped polymeric whispering gallery mode (WGM) micro-disk lasers. The focus of the comparison is on the lasing threshold and efficiency of light collection. With the aid of optical fibers, we localize the pump energy to the cavity-mode volume and reduce the necessary pump energy to achieve lasing by two orders of magnitude. When using fibers for detection, the collection efficiency is enhanced by four orders of magnitude compared to a free-space read-out perpendicular to the resonator plane. By enhancing the collection efficiency we are able to record a pronounced modulation of the dye fluorescence under continuous wave (cw) pumping conditions evoked by coupling to the WGMs. Alternatively to fibers as a collection tool, we present a read-out technique based on the detection of in-plane radiated light. We show that this method is especially beneficial in an aqueous environment as well as for size-reduced micro-lasers where radiation is strongly pronounced. Furthermore, we show that this technique allows for the assignment of transverse electric (TE) and transverse magnetic (TM) polarization to the observed fundamental modes in a water environment by performing polarization-dependent photoluminescence (PL) spectroscopy. We emphasize the importance of the polarization determination for sensing applications and verify expected differences in the bulk refractive index sensitivity for TE and TM WGMs experimentally.

Size-optimized polymeric whispering gallery mode lasers with enhanced sensing performance.

Integration of optically active materials into whispering gallery mode (WGM) cavities enables low-threshold laser emission. In contrast to their passive counterparts, the WGMs of these microlasers can be pumped and read out easily via free-space optics. The WGMs interact with the cavity environment via their evanescent field, and thus lend themselves to label-free bio-sensing. The detection limit of such sensors, given as the ratio of the resolution of the whole measurement system to the sensitivity of the WGMs, is an important figure of merit. In this work we show that the detection limit of polymeric microdisk lasers can be improved by more than a factor of seven by optimizing their radius and thickness. We use the bulk refractive index sensitivity, the magnitude of the sensor reaction towards refractive index changes of the bulk environment, to quantify the sensing performance and show that it can be enhanced while the spectral resolution is maintained. Furthermore, we investigate the effect of the size of the cavity on the quality factor and the lasing threshold in an aqueous environment, hence allowing optimization of the cavity size for enhanced sensor performance. For all considered quantities, numerically computed expectations are verified by experimental results.

3D whispering-gallery-mode microlasers by direct laser writing and subsequent soft nanoimprint lithography.

We demonstrate the realization of 3D whispering-gallery-mode (WGM) microlasers by direct laser writing (DLW) and their replication by nanoimprint lithography using a soft mold technique ("soft NIL"). The combination of DLW as a method for rapid prototyping and soft NIL offers a fast track towards large scale fabrication of 3D passive and active optical components applicable to a wide variety of materials. A performance analysis shows that surface-scattering-limited Q-factors of replicated resonators as high as 1×105 at 635 nm can be achieved with this process combination. Lasing in the replicated WGM resonators is demonstrated by the incorporation of laser dyes in the target material. Low lasing thresholds in the order of 15 kW/cm2 are obtained under ns-pulsed excitation.

Efficient free-space read-out of WGM lasers using circular micromirrors.

Lasing from whispering-gallery mode (WGM) resonators occurs omnidirectional in azimuthal plane. Most applications of WGM resonators require spectral analysis with off-chip detectors, where in-plane emission and beam divergence hinder efficient detection. We demonstrate redirecting WGM laser emission from all azimuthal angles using a circular micromirror placed around the cavity. By collecting reflections off the micromirror via free-space optics, read-out intensity improved by one order of magnitude. Blocking vertically emitted spontaneous emission and recording reflections off the micromirror only, signal-to-noise ratio improved from 4.6 dB to 15 dB. Our read-out concept may be applied to arbitrary WGM cavity geometries without deteriorating the cavity's quality factor.

Large-scale parallel surface functionalization of goblet-type whispering gallery mode microcavity arrays for biosensing applications.

A novel surface functionalization technique is presented for large-scale selective molecule deposition onto whispering gallery mode microgoblet cavities. The parallel technique allows damage-free individual functionalization of the cavities, arranged on-chip in densely packaged arrays. As the stamp pad a glass slide is utilized, bearing phospholipids with different functional head groups. Coated microcavities are characterized and demonstrated as biosensors.

Assessment of crystal quality and unit cell orientation in epitaxial Cu2ZnSnSe4 layers using polarized Raman scattering.

We use polarization-resolved Raman spectroscopy to assess the crystal quality of epitaxial kesterite layers. It is demonstrated for the example of epitaxial Cu2ZnSnSe4 layers on GaAs(001) that "standing" and "lying" kesterite unit cell orientations (c'-axis parallel / perpendicular to the growth direction) can be distinguished by the application of Raman tensor analysis. From the appearance of characteristic intensity oscillations when the sample is rotated one can distinguish polycrystalline and epitaxial layers. The method can be transferred to kesterite layers oriented in any crystal direction and can shed light on the growth of such layers in general.

Revealing non-genetic adhesive variations in clonal populations by comparative single-cell force spectroscopy.

Cell populations often display heterogeneous behavior, including cell-to-cell variations in morphology, adhesion and spreading. However, better understanding the significance of such cell variations for the function of the population as a whole requires quantitative single-cell assays. To investigate adhesion variability in a CHO cell population in detail, we measured integrin-mediated adhesion to laminin and collagen, two ubiquitous ECM components, by AFM-based single-cell force spectroscopy (SCFS). CHO cells generally adhered more strongly to laminin than collagen but population adhesion force distributions to both ECM components were broad and partially overlapped. To determine the levels of laminin and collagen binding in individual cells directly, we alternatingly measured single cells on adjacent microstripes of collagen and laminin arrayed on the same adhesion substrate. In repeated measurements (≥60) individual cells showed a stable and ECM type-specific adhesion response. All tested cells bound laminin more strongly, but the scale of laminin over collagen binding varied between cells. Together, this demonstrates that adhesion levels to different ECM components are tightly yet differently set in each cell of the population. Adhesion variability to laminin was non-genetic and cell cycle-independent but scaled with the range of α6 integrin expression on the cell surface. Adhesive cell-to-cell variations due to varying receptor expression levels thus appear to be an inherent feature of cell populations and should to be considered when fully characterizing population adhesion. In this approach, SCFS performed on multifunctional adhesion substrates can provide quantitative single-cell information not obtainable from population-averaging measurements on homogeneous adhesion substrates.

Flexible coupling of high-Q goblet resonators for formation of tunable photonic molecules.

We report on a method for a highly flexible arrangement of polymeric high-Q whispering gallery mode resonators. Parallel on-chip fabricated goblet resonators are detached from the substrate by bonding a gold wire to the field-free center of their polymeric cavity. This enables the precise control of the resonator's spatial position. The modal spectrum of the detached resonator reveals preservation of its high optical quality. Manipulation of the resonators' position allows for designing coupled resonators geometries and tuning the coupling properties dynamically after batch fabrication. The properties of the modal spectrum evidence the successful optical coupling.

Direct laser writing for nanoporous liquid core laser sensors.

We report the fabrication of nanoporous liquid core lasers via direct laser writing based on two-photon absorption in combination with thiolene-chemistry. As gain medium Rhodamine 6G was embedded in the nanoporous polybutadiene matrix. The lasing devices with thresholds of 19 µJ/mm(2) were measured to have bulk refractive index sensitivities of 169 nm/RIU at a laser wavelength of 600 nm, demonstrating strongly increased overlap of the modes with the analyte in comparison to solid state evanescent wave sensors.

Strongly confined, low-threshold laser modes in organic semiconductor microgoblets.

We investigate lasing from high-Q, polymeric goblet-type microcavities covered by an organic semiconductor gain layer. We analyze the optical modes in the high-Q cavities using finite element simulations and present a numerical method to determine the cutoff thickness of the gain layer above which the whispering gallery modes are strongly confined in this layer. Fabricated devices show reduced lasing thresholds for increasing gain layer thicknesses, which can be explained by a higher filling factor of the optical modes in the gain layer. Furthermore, reduced lasing threshold is accompanied by a red-shift of the laser emission.

Direct laser writing for active and passive high-Q polymer microdisks on silicon.

We report the fabrication of high-Q polymeric microdisks on silicon via direct laser writing utilizing two-photon absorption induced polymerization. The quality factors of the passive cavities are above 10(6) in the 1300 nm wavelength region. The flexible three-dimensional (3D) lithography method allows for the fabrication of different cavity thicknesses on the same substrate, useful for rapid prototyping of active and passive optical microcavities. Microdisk lasers are realized by doping the resist with dye, resulting in laser emission at visible wavelengths.

Highly linear self-assembled porphyrin wires.

An efficient noncovalent assembly process involving high geometrical control was applied to a linear bis(imidazolyl zinc porphyrin) 7Zn, bearing C(18) substitutents, to generate linear multiporphyrin wires. The association process is based on imidazole recognition within the cavity of the phenanthroline-strapped zinc porphyrin. In chlorinated solvents, discrete soluble oligomers were obtained after (7Zn)(n) was end-capped with a terminal single imidazolyl zinc porphyrin derivative 4Zn. These soluble species, as well as their destabilization in the presence of protic solvents, were studied by UV-visible and time-resolved luminescence. In the solid state, assemblies as long as 480 nm, which corresponds to 190 iterative units or a total of 380 porphyrins, were observed by atomic force microscopy measurements on mica. The length and linearity of the porphyrin wires obtained illustrate the potential of phenanthroline-strapped porphyrins for the directional control of self-assembly processes.

The supramolecular organization of self-assembling chlorosomal bacteriochlorophyll c, d, or e mimics.

Bacteriochlorophylls (BChls) c, d, and e are the main light-harvesting pigments of green photosynthetic bacteria that self-assemble into nanostructures within the chlorosomes forming the most efficient antennas of photosynthetic organisms. All previous models of the chlorosomal antennae, which are quite controversially discussed because no single crystals could be grown so far from these organelles, involve a strong hydrogen-bonding interaction between the 3(1) hydroxyl group and the 13(1) carbonyl group. We have synthesized different self-assemblies of BChl c mimics having the same functional groups as the natural counterparts, that is, a hydroxyethyl substituent, a carbonyl group and a divalent metal atom ligated by a tetrapyrrole. These artificial BChl mimics have been shown by single crystal x-ray diffraction to form extended stacks that are packed by hydrophobic interactions and in the absence of hydrogen bonding. Time-resolved photoluminescence proves the ordered nature of the self-assembled stacks. FT-IR spectra show that on self-assembly the carbonyl frequency is shifted by approximately 30 cm(-1) to lower wavenumbers. From the FT-IR data we can infer the proximal interactions between the BChls in the chlorosomes consistent with a single crystal x-ray structure that shows a weak electrostatic interaction between carbonyl groups and the central zinc atom.

Lasing dynamics in single ZnO nanorods.

We study the lasing dynamics of individual ZnO nanorods by time-resolved mu-photoluminescence. The distinct laser modes show gain competition and pronounced shifts as a function of excitation density. This behavior can be understood in terms of many-particle effects within an inverted electron-hole plasma and of the calculated mode spectra of the particular nanorod, whose geometry is known from electron microscope investigations.

Programmed metalloporphyrins for self-assembly within light-harvesting stacks: (5,15-dicyano-10,20-bis(3,5-di-tert-butylphenyl)porphyrinato)zinc(II) and its push-pull 15-N,N-dialkylamino-5-cyano congeners obtained by a facile direct amination.

The title dicyano compound was synthesized via cyanation and it self-assembles in nonpolar solvents giving red-shifted and broad absorption maxima just as the bacteriochlorophylls which are encountered in the light-harvesting organelles of early photosynthetic bacteria. In the crystal, stacks are formed through a hierarchic combination of pi-stacking and a CN-Zn electrostatic interaction. Push-pull 15-N,N-dialkylamino-5-cyano congeners could be obtained in high yields using a solvent- and catalyst-free direct amination of meso-bromoporphyrins. Importantly, the fluorescence of the self-assembled species due to the very orderly manner in which the chromophores are arranged is not entirely quenched and has a surprisingly long lifetime of over 1 ns. This lends hope of using the trapped energy in biomimetic hybrid solar cells.

High Quality 3D Photonics using Nano Imprint Lithography of Fast Sol-gel Materials.

A method for the realization of low-loss integrated optical components is proposed and demonstrated. This approach is simple, fast, inexpensive, scalable for mass production, and compatible with both 2D and 3D geometries. The process is based on a novel dual-step soft nano imprint lithography process for producing devices with smooth surfaces, combined with fast sol-gel technology providing highly transparent materials. As a concrete example, this approach is demonstrated on a micro ring resonator made by direct laser writing (DLW) to achieve a quality factor improvement from one hundred thousand to more than 3 million. To the best of our knowledge this also sets a Q-factor record for UV-curable integrated micro-ring resonators. The process supports the integration of many types of materials such as light-emitting, electro-optic, piezo-electric, and can be readily applied to a wide variety of devices such as waveguides, lenses, diffractive elements and more.

Photonic molecules with a tunable inter-cavity gap.

Optical micro-resonators have broad applications. They are used, for example, to enhance light-matter interactions in optical sensors or as model systems for investigating fundamental physical mechanisms in cavity quantum electrodynamics. Coupling two or more micro-cavities is particularly interesting as it enlarges the design freedom and the field of application. In this context, achieving tunability of the coupling strength and hence the inter-cavity gap is of utmost importance for adjusting the properties of the coupled micro-resonator system. In this paper, we report on a novel coupling approach that allows highly precise tuning of the coupling gap of polymeric micro-resonators that are fabricated side by side on a common substrate. We structure goblet-shaped whispering-gallery-mode resonators on an elastic silicone-based polymer substrate by direct laser writing. The silicone substrate is mechanically stretched in order to exploit the lateral shrinkage to reduce the coupling gap. Incorporating a laser dye into the micro-resonators transforms the cavities into micro-lasers that can be pumped optically. We have investigated the lasing emission by micro-photoluminescence spectroscopy, focusing on the spatial localization of the modes. Our results demonstrate the formation of photonic molecules consisting of two or even three resonators, for which the coupling strengths and hence the lasing performance can be precisely tuned. Flexibility and tunability are key elements in future photonics, making our approach interesting for various photonic applications. For instance, as our coupling approach can also be extended to larger cavity arrays, it might serve as a platform for tunable coupled-resonator optical waveguide devices.

Template-assisted large-scale ordered arrays of ZnO pillars for optical and piezoelectric applications.

Spatially separated ZnO pillars, typically 300 nm in diameter and 2 microm in height, are fabricated via a template-directed approach that leads to long-range hexagonal order. The templates of Au nanodisk arrays are obtained by using metal membranes as a lithography mask. The growth of ZnO pillars is performed in a double-tube system through vapor diffusion-deposition. The growth mechanism of the pillars is studied in detail and is proposed to be a combination of vapor-liquid-solid and vapor-solid models. The piezoelectric and optical properties of single pillars are characterized using piezoresponse force microscopy and micro-photoluminescence spectroscopy, respectively. The pillars show strong excitonic emissions up to room temperature, which indicate a relatively low defect density and good crystalline quality. The obtained piezoelectric coefficient d(33) is (7.5+/-0.6) pm V(-1), which is to our knowledge the first reported value for a single nanopillar.

A switchable self-assembling and disassembling chiral system based on a porphyrin-substituted phenylalanine-phenylalanine motif.

Artificial light-harvesting systems have until now not been able to self-assemble into structures with a large photon capture cross-section that upon a stimulus reversibly can switch into an inactive state. Here we describe a simple and robust FLFL-dipeptide construct to which a meso-tetraphenylporphyrin has been appended and which self-assembles to fibrils, platelets or nanospheres depending on the solvent composition. The fibrils, functioning as quenched antennas, give intense excitonic couplets in the electronic circular dichroism spectra which are mirror imaged if the unnatural FDFD-analogue is used. By slightly increasing the solvent polarity, these light-harvesting fibres disassemble to spherical structures with silent electronic circular dichroism spectra but which fluoresce. Upon further dilution with the nonpolar solvent, the intense Cotton effects are recovered, thus proving a reversible switching. A single crystal X-ray structure shows a head-to-head arrangement of porphyrins that explains both their excitonic coupling and quenched fluorescence.