Negative refraction at telecommunication wavelengths through plasmon-photon hybridization.

We demonstrate negative refraction at telecommunication wavelengths through plasmon-photon hybridization on a simple microcavity with metallic mirrors. Instead of using conventional metals, the plasmonic excitations are provided by a heavily doped semiconductor which enables us to tune them into resonance with the infrared photon modes of the cavity. In this way, the dispersion of the resultant hybrid cavity modes can be widely adjusted. In particular, negative dispersion and negative refraction at telecommunication wavelengths on an all-ZnO monolithical cavity are demonstrated.

Ultrafast Nonlinear Response of Bulk Plasmons in Highly Doped ZnO Layers.

Longitudinal bulk plasmons in an n-doped ZnO layer system are studied by two-color femtosecond pump-probe spectroscopy in the midinfrared. The optical bulk plasmon resonance identified in linear reflectivity spectra undergoes a strong redshift and a limited broadening upon intraband excitation of electrons. The nonlinear changes of plasmon absorption decay on a time scale of 2 ps and originate from the intraband redistribution of electrons. Theoretical calculations explain the plasmon redshift by the transient increase of the ensemble-averaged electron mass and the concomitantly reduced plasma frequency in the hot electron plasma. The observed bulk plasmon nonlinearity holds strong potential for applications in plasmonics.

Strong coupling and laser action of ladder-type oligo(p-phenylene)s in a microcavity.

We investigate the coupling of ladder-type quarterphenyl to the photon modes of a dielectric ZrOx /SiOx microcavity at ultraviolet wavelengths. For a relatively long cavity (≈10 µm) with high-reflectivity mirrors (0.998), optically pumped laser action is demonstrated in the weak-coupling regime. We observe single-mode operation with a threshold of 0.4 mJ cm(-2) . Strong coupling is achieved by using a short λ/2 cavity. We find pronounced anti-crossing features of the molecular (0,0) and (0,1) vibronic transitions and the cavity mode in angle-dependent reflectivity measurements providing Rabi splittings of (90±10) meV. All these features occur spectrally resonant to the exciton transition of ZnO demonstrating the potential of ladder-type oligo(p-phenylene)s for the construction of inorganic/organic hybrid microcavities.

Morphology-, synthesis- and doping-independent tuning of ZnO work function using phenylphosphonates.

The work function (WF) of ZnO is modified by two types of dipole-bearing phenylphosphonate layers, yielding a maximum WF span of 1.2 eV. H3CO-phenyl phosphonate, with a positive dipole (positive pole pointing outwards from the surface), lowers the WF by ∼350 meV. NC-phenyl phosphonate, with a negative dipole, increases the WF by ∼750 meV. The WF shift is found to be independent of the type of ZnO surface. XPS data show strong molecular dipoles between the phenyl and the functionalizing (CN and OMe) tail groups, while an opposite dipole evolves in each molecular layer between the surface and the phenyl rings. The molecular modification is found to be invariant to supra-bandgap illumination, which indicates that the substrate's space charge-induced built-in potential is unlikely to be the reason for the WF difference. ZnO, grown by several different methods, with different degrees of crystalline perfection and various morphologies and crystallite dimensions, could all be modified to the same extent. Furthermore, a mixture of opposite dipoles allows gradual and continuous tuning of the WF, varying linearly with the partial concentration of the CN-terminated phosphonate in the solution. Exposure to the phosphonic acids during the molecular layer deposition process erodes a few atomic layers of the ZnO. The general validity of the treatment and the fine-tuning of the WF of treated interfaces are of interest for solar cells and LED applications.

Space-charge transfer in hybrid inorganic-organic systems.

We discuss density functional theory calculations of hybrid inorganic-organic systems that explicitly include the global effects of doping (i.e., position of the Fermi level) and the formation of a space-charge layer. For the example of tetrafluoro-tetracyanoquinodimethane on the ZnO(0001[over ¯]) surface we show that the adsorption energy and electron transfer depend strongly on the ZnO doping. The associated work function changes are large, for which the formation of space-charge layers is the main driving force. The prominent doping effects are expected to be quite general for charge-transfer interfaces in hybrid inorganic-organic systems and important for device design.

Band-offset engineering in organic/inorganic semiconductor hybrid structures.

Control over the electronic structure of organic/inorganic semiconductor interfaces is required to realize hybrid structures with tailored opto-electronic properties. An approach towards this goal is demonstrated for a layered hybrid system composed of p-sexiphenyl (6P) and ZnO. The molecular orientation can be switched from "upright-standing" to "flat-lying" by tuning the molecule-substrate interactions through aggregation on different crystal faces. The morphology change has profound consequences on the offsets between the molecular frontier energy levels and the semiconductor band edges. The combination of ZnO surface dipole modification through molecule adsorption and the orientation-dependence of the ionization energy of molecular layers shift these offsets by 0.7 eV. The implications for optimizing hybrid structures with regard to exciton and charge transfer are discussed.

Calculating Optical Absorption Spectra of Thin Polycrystalline Organic Films: Structural Disorder and Site-Dependent van der Waals Interaction.

We propose a new approach for calculating the change of the absorption spectrum of a molecule when moved from the gas phase to a crystalline morphology. The so-called gas-to-crystal shift Δ[Formula: see text] m is mainly caused by dispersion effects and depends sensitively on the molecule's specific position in the nanoscopic setting. Using an extended dipole approximation, we are able to divide Δ[Formula: see text] m = -QWm in two factors, where Q depends only on the molecular species and accounts for all nonresonant electronic transitions contributing to the dispersion while Wm is a geometry factor expressing the site dependence of the shift in a given molecular structure. The ability of our approach to predict absorption spectra is demonstrated using the example of polycrystalline films of 3,4,9,10-perylenetetracarboxylic diimide (PTCDI).