Single-photon nonlinearity at room temperature.

The recent progress in nanotechnology1,2 and single-molecule spectroscopy3-5 paves the way for emergent cost-effective organic quantum optical technologies with potential applications in useful devices operating at ambient conditions. We harness a π-conjugated ladder-type polymer strongly coupled to a microcavity forming hybrid light-matter states, so-called exciton-polaritons, to create exciton-polariton condensates with quantum fluid properties. Obeying Bose statistics, exciton-polaritons exhibit an extreme nonlinearity when undergoing bosonic stimulation6, which we have managed to trigger at the single-photon level, thereby providing an efficient way for all-optical ultrafast control over the macroscopic condensate wavefunction. Here, we utilize stable excitons dressed with high-energy molecular vibrations, allowing for single-photon nonlinear operation at ambient conditions. This opens new horizons for practical implementations like sub-picosecond switching, amplification and all-optical logic at the fundamental quantum limit.

Low-loss optical waveguides made with a high-loss material.

For guiding light on a chip, it has been pivotal to use materials and process flows that allow low absorption and scattering. Based on subwavelength gratings, here, we show that it is possible to create broadband, multimode waveguides with very low propagation losses despite using a strongly absorbing material. We perform rigorous coupled-wave analysis and finite-difference time-domain simulations of integrated waveguides that consist of pairs of integrated high-index-contrast gratings. To showcase this concept, we demonstrate guiding of visible light in the wavelength range of 550-650 nm with losses down to 6 dB/cm using silicon gratings that have a material absorption of 13,000 dB/cm at this wavelength and are fabricated with standard silicon photonics technology. This approach allows us to overcome traditional limits of the various established photonics technology platforms with respect to their suitable spectral range and, furthermore, to mitigate situations where absorbing materials, such as highly doped semiconductors, cannot be avoided because of the need for electrical driving, for example, for amplifiers, lasers and modulators.

Molecular structure elucidation with charge-state control.

The charge state of a molecule governs its physicochemical properties, such as conformation, reactivity, and aromaticity, with implications for on-surface synthesis, catalysis, photoconversion, and applications in molecular electronics. On insulating, multilayer sodium chloride (NaCl) films, we controlled the charge state of organic molecules and resolved their structures in neutral, cationic, anionic, and dianionic states by atomic force microscopy, obtaining atomic resolution and bond-order discrimination using carbon monoxide (CO)-functionalized tips. We detected changes in conformation, adsorption geometry, and bond-order relations for azobenzene, tetracyanoquinodimethane, and pentacene in multiple charge states. Moreover, for porphine, we investigate the charge state-dependent change of aromaticity and conjugation pathway in the macrocycle. This work opens the way to studying chemical-structural changes of individual molecules for a wide range of charge states.

Large Pockels effect in micro- and nanostructured barium titanate integrated on silicon.

The electro-optical Pockels effect is an essential nonlinear effect used in many applications. The ultrafast modulation of the refractive index is, for example, crucial to optical modulators in photonic circuits. Silicon has emerged as a platform for integrating such compact circuits, but a strong Pockels effect is not available on silicon platforms. Here, we demonstrate a large electro-optical response in silicon photonic devices using barium titanate. We verify the Pockels effect to be the physical origin of the response, with r42 = 923 pm V-1, by confirming key signatures of the Pockels effect in ferroelectrics: the electro-optic response exhibits a crystalline anisotropy, remains strong at high frequencies, and shows hysteresis on changing the electric field. We prove that the Pockels effect remains strong even in nanoscale devices, and show as a practical example data modulation up to 50 Gbit s-1. We foresee that our work will enable novel device concepts with an application area largely extending beyond communication technologies.

Lasing Supraparticles Self-Assembled from Nanocrystals.

One of the most attractive commercial applications of semiconductor nanocrystals (NCs) is their use in lasers. Thanks to their high quantum yield, tunable optical properties, photostability, and wet-chemical processability, NCs have arisen as promising gain materials. Most of these applications, however, rely on incorporation of NCs in lasing cavities separately produced using sophisticated fabrication methods and often difficult to manipulate. Here, we present whispering gallery mode lasing in supraparticles (SPs) of self-assembled NCs. The SPs composed of NCs act as both lasing medium and cavity. Moreover, the synthesis of the SPs, based on an in-flow microfluidic device, allows precise control of the dimensions of the SPs, i.e. the size of the cavity, in the micrometer range with polydispersity as low as several percent. The SPs presented here show whispering gallery mode resonances with quality factors up to 320. Whispering gallery mode lasing is evidenced by a clear threshold behavior, coherent emission, and emission lifetime shortening due to the stimulation process.

Control of the interaction strength of photonic molecules by nanometer precise 3D fabrication.

Applications for high resolution 3D profiles, so-called grayscale lithography, exist in diverse fields such as optics, nanofluidics and tribology. All of them require the fabrication of patterns with reliable absolute patterning depth independent of the substrate location and target materials. Here we present a complete patterning and pattern-transfer solution based on thermal scanning probe lithography (t-SPL) and dry etching. We demonstrate the fabrication of 3D profiles in silicon and silicon oxide with nanometer scale accuracy of absolute depth levels. An accuracy of less than 1nm standard deviation in t-SPL is achieved by providing an accurate physical model of the writing process to a model-based implementation of a closed-loop lithography process. For transfering the pattern to a target substrate we optimized the etch process and demonstrate linear amplification of grayscale patterns into silicon and silicon oxide with amplification ratios of ∼6 and ∼1, respectively. The performance of the entire process is demonstrated by manufacturing photonic molecules of desired interaction strength. Excellent agreement of fabricated and simulated structures has been achieved.

Air and dielectric bands photonic crystal microringresonator for refractive index sensing.

We present the experimental and numerical analysis of a microring resonator with an integrated one-dimensional photonic crystal fabricated on a silicon-on-insulator platform and show its applicability in bulk refractive index sensing. The photonic crystal is formed by periodically patterned, partially etched cylindrical perforations, whose induced photonic bandgap is narrower than the range of measurable wavelengths (1520-1620 nm). Of particular interest is that the microring operates in both air and dielectric bands, and the sensitivities of the resonances on both edges of the bandgap were investigated. We showed that a higher field localization inside the volume of the perforations for the air band mode leads to an increase in sensitivity.

Ultra-wide free spectral range, enhanced sensitivity, and removed mode splitting SOI optical ring resonator with dispersive metal nanodisks.

A refractive index sensor with a free spectral range that is unlimited by neighboring mode spacing (10 fold increase with respect to 20 nm of an unmodified ring), based on an optical silicon-on-insulator microring resonator patterned with periodically arranged set of gold nanodisks, is presented and numerically verified. It is shown that the particular periodic arrangement of nanodisks selects a single resonance from a wide set of ring resonator modes and removes mode splitting. Extraction of the waveguided electromagnetic energy into evanescent plasmonic modes enhances light-analyte interaction and increases device sensitivity to variation of refractive index up to 176 nm/RIU (about 2-fold increase compared to the unmodified ring), which is useful for sensor applications. Proof of the concept is presented by finite-difference time-domain simulations of a design readily practicable by means of modern nanotechnology.

Numerical and experimental analysis of optical response of sub-wavelength period structure in carbonaceous film for refractive index sensing.

The resonance structure coupling the light into the leaky guided modes, which are visible in the reflection spectra as sharp peaks (Wood's anomalies), is analyzed experimentally and numerically. The guided mode resonance structure of 428 nm period patterned in a carbonaceous film demonstrated sensitivity of 70 nm/RIU. The calculated mode diagram explained the nature and positions of the peaks registered experimentally. The reflection spectra, near/far field distributions and field penetration depth for the analyzed structure were simulated employing three numerical solvers. The set of weak Rayleigh's anomalies was indentified from the simulations and the experimental data.