Articles you may be interested inMacroporous silicon: Homogeneity investigations and fabrication tolerances of a simple cubic three-dimensional photonic crystal

We report on the fabrication of periodic arrays of deep nanopores with high aspect ratios in crystalline silicon. The radii and pitches of the pores were defined in a chromium mask by means of deep UV scan and step technology. The pores were etched with a reactive ion etching process with SF(6), optimized for the formation of deep nanopores. We have realized structures with pitches between 440 and 750 nm, pore diameters between 310 and 515 nm, and depth to diameter aspect ratios up to 16. To the best of our knowledge, this is the highest aspect ratio ever reported for arrays of nanopores in silicon made with a reactive ion etching process. Our experimental results show that the etching rate of the nanopores is aspect-ratio-dependent, and is mostly influenced by the angular distribution of the etching ions. Furthermore we show both experimentally and theoretically that, for sub-micrometer structures, reducing the sidewall erosion is the best way to maximize the aspect ratio of the pores. Our structures have potential applications in chemical sensors, in the control of liquid wetting of surfaces, and as capacitors in high-frequency electronics. We demonstrate by means of optical reflectivity that our high-quality structures are very well suited as photonic crystals. Since the process studied is compatible with existing CMOS semiconductor fabrication, it allows for the incorporation of the etched arrays in silicon chips.

We have studied the reflectivity of CMOS-compatible three-dimensional silicon inverse woodpile photonic crystals at near-infrared frequencies. Polarization-resolved reflectivity spectra were obtained from two orthogonal crystal surfaces using an objective with a high numerical aperture. The spectra reveal broad peaks with maximum reflectivity of 67% that are independent of the spatial position on the crystals. The spectrally overlapping reflectivity peaks for all directions and polarizations form the signature of a broad photonic band gap with a relative bandwidth up to 16%. This signature is supported with stopgaps in plane wave bandstructure calculations and with the frequency region of the expected band gap. arXiv:1012.5263v2 [physics.optics]

Near-infrared emissive lanthanide complexes were synthesized with covalently attached sensitizers that absorb in the visible. This functionalization was designed such that the sensitizer is in close proximity to the lanthanide ion, which is a prerequisite for efficient energy transfer from the excited sensitizer to the lanthanide ion. The sensitizers used were fluorescein, eosin, and erythrosin, which were linked via a β-alanine spacer to the polydentate chelate. The sensitizers were chosen because they absorb visible light and are structurally very similar, but the intrinsic intersystem crossing quantum yields of the sensitizers vary significantly, because of the presence of the heavy atoms (bromine in eosin and iodine in erythrosin). It was expected that an intrinsic high intersystem crossing would be beneficial in the sensitization process, because energy transfer occurs through the triplet state of sensitizers. However, because of the enhanced intersystem crossing of the sensitizers by the nearby heavy and paramagnetic lanthanide ions, these intrinsic differences were largely diminished. It was even found that fluorescein acts as a more efficient sensitizer for the NIR emission than eosin and erythrosin. The donating triplet state of fluorescein is higher in energy than that of eosin and erythrosin, resulting in less energy back transfer and therefore in a higher efficiency of sensitized emission. This and considerations of selection rules for energy transfer to the lanthanide ions made it possible to distinguish the 4 F 9/2 level of Nd 3+ as the main acceptor channel for energy transfer. In the Er 3+ complexes, the enhancement in intersystem crossing was lower in the eosin and erythrosin complexes than in the fluorescein complex, which was concluded from the remaining complex fluorescence. Furthermore, it is tentatively concluded that additional pathways other than those allowed in Dexter energy transfer play a role in the sensitization of Er 3+ . In the Yb 3+ complexes, the higher efficiency of sensitization by fluorescein is due to the enhanced intersystem crossing that is larger in the fluorescein complex than in the eosin or erythrosin complex.

Three dimensional photonic band gap crystals with a cubic diamond-like symmetry are fabricated. These so-called inverse-woodpile nanostructures consist of two perpendicular sets of pores in single-crystal silicon wafers and are made by means of complementary metal oxide-semiconductor (CMOS)-compatible methods. Both sets of pores have high aspect ratios and are made by deep reactive-ion etching. The mask for the first set of pores is defined in chromium by means of deep UV scan-and-step technology. The mask for the second set of pores is patterned using an ion beam and carefully placed at an angle of 90° with an alignment precision of better than 30 nm. Crystals are made with pore radii between 135-186 nm with lattice parameters a = 686 and c = 488 nm such that a/c = √2; hence the structure is cubic. The crystals are characterized using scanning electron microscopy and X-ray diffraction. By milling away slices of crystal, the pores are analyzed in detail in both directions regarding depth, radius, tapering, shape, and alignment. Using optical reflectivity it is demonstrated that the crystals have broad reflectivity peaks in the near-infrared frequency range, which includes the telecommunication range. The strong reflectivity confirms the high quality of the photonic crystals. Furthermore the width of the reflectivity peaks agrees well with gaps in calculated photonic band structures.