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.

The interplay of vibrational motion and electronic charge relocation in an ionic hydrogen-bonded crystal is mapped by X-ray powder diffraction with a 100 fs time resolution. Photoexcitation of the prototype material KH 2 PO 4 induces coherent low-frequency motions of the PO 4 tetrahedra in the electronically excited state of the crystal while the average atomic positions remain unchanged. Time-dependent maps of electron density derived from the diffraction data demonstrate an oscillatory relocation of electronic charge with a spatial amplitude two orders of magnitude larger than the underlying vibrational lattice motions. Coherent longitudinal optical and tranverse optical phonon motions that dephase on a time scale of several picoseconds, drive the charge relocation, similar to a soft (transverse optical) mode driven phase transition between the ferro-and paraelectric phase of KH 2 PO 4 .charge density maps | charge transfer | femtosecond dynamics | X-ray diffraction I onic crystals are characterized by a periodic arrangement of positive and negative ions with the electronic charges essentially localized at the ionic sites. For a unit cell geometry with a finite electric dipole moment, such materials are ferroelectric and display a macroscopic electric polarization (1). Ferroelectrics have received strong attention both from the viewpoint of their basic properties (1, 2) and for device applications (3). A prototype class of ionic ferroelectrics are hydrogen-bonded potassium dihydrogen phosphate KH 2 PO 4 (KDP) and its isomorphs in which the electric polarization originates from a diplacement between the H 2 PO − 4 units and the K þ cations along the c-axis of the orthorhombic crystal structure (4-9). At a critical temperature of T C ¼ 123 K, the macroscopic polarization disappears and KDP transforms into a paraelectric tetragonal phase (Fig. 1A). The microscopic mechanisms behind this phase transition have remained controversial (cf. Materials and Methods).A fundamental issue for understanding the relation between structure and (ferro)electric properties consists in the interplay of a change of ionic positions and/or hydrogen bond geometries on the one hand with relocations of electronic charge on the other (9, 10, 11). So far, most experimental and theoretical work has addressed this problem under quasi-static conditions, close to equilibrium, and/or on time scales much longer than ionic motions (12). Here, we introduce femtosecond X-ray powder diffraction (13, 14) as a real-time probe of coupled vibrational and charge motions occurring on a time scale between approximately 100 fs and a few picoseconds. As X-rays interact with the electronic charges in the KDP crystallites, a series of diffraction patterns taken with a time resolution of 100 fs allows for deriving the momentary ionic positions and the charge distributions simultaneously. Results and DiscussionThe experiments make use of a pump-probe scheme in which ionic motions and the concomitant charge relocations are induced by electronic excitation via tw...

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.

Transient polarizations connected with a spatial redistribution of electronic charge in a mixed quantum state are induced by optical fields of high amplitude. We determine for the first time the related transient electron density maps, applying femtosecond x-ray powder diffraction as a structure probe. The prototype ionic material LiBH4 driven nonresonantly by an intense sub-40 fs optical pulse displays a large-amplitude fully reversible electron transfer from the BH4(-) anion to the Li+ cation during excitation. Our results establish this mechanism as the source of the strong optical polarization which agrees quantitatively with theoretical estimates.

The transient electronic and molecular structure arising from photoinduced charge transfer in transition metal complexes is studied by X-ray powder diffraction with a 100 fs temporal and atomic spatial resolution. Crystals containing a dense array of Fe(II)-tris(bipyridine) ([Fe(bpy)3](2 +)) complexes and their [Formula: see text] counterions display pronounced changes of electron density that occur within the first 100 fs after two-photon excitation of a small fraction of the [Fe(bpy)3](2 +) complexes. Transient electron density maps derived from the diffraction data reveal a transfer of electronic charge from the Fe atoms and-so far unknown-from the [Formula: see text] counterions to the bipyridine units. Such charge transfer (CT) is connected with changes of the inter-ionic and the Fe-bipyridine distances. An analysis of the electron density maps demonstrates the many-body character of charge transfer which affects approximately 30 complexes around a directly photoexcited one. The many-body behavior is governed by the long-range Coulomb forces in the ionic crystals and described by the concept of electronic polarons.