562 research outputs found

    Efficient excitation of self-collimated beams and single Bloch modes in planar photonic crystals

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    Using finite-difference time-domain calculations, we investigate out-of-plane coupling between a square-lattice planar photonic crystal and a conventional waveguide located above the photonic crystal. We couple a waveguide oriented in the GX direction to a photonic crystal mode in the second band and show that anticrossing takes place. In this way, a self-collimated beam is launched in the planar photonic crystal, with full power transfer. Furthermore, we investigate the coupling between a waveguide oriented in the GM direction and a photonic crystal and show that single photonic crystal modes can be selectively excited

    Self-Collimation in Planar Photonic Crystals

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    We analyze, in three dimensions, the dispersion properties of dielectric slabs perforated with two-dimensional photonic crystals (PCs) of square symmetry. The band diagrams are calculated for all -vectors in the first Brillouin zone, and not only along the characteristic high-symmetry directions. We have analyzed the equal-frequency contours of the first two bands, and we found that the square lattice planar photonic crystal is a good candidate for the self-collimation of light beams. We map out the group velocities for the second band of a square lattice planar PC and show that the group velocity is the highest in the region of maximum self-collimation. Such a self-collimated beam is predicted to show beating patterns due to the specific shape of the equal-frequency contours. A geometrical transformation maps the region of the first and second photonic bands where self-collimation takes place one onto the other and gives additional insights on the structural similarities of self-collimation in those two bands

    Thermal management in microfluidics using micro-Peltier junctions

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    We report refrigeration and heating of nanoliter fluid volumes with micro-Peltier junctions. The temperature of small liquid reservoirs can be rapidly changed and controlled within a range between -3 degrees C to over 120 degrees C with good long-term stability. These thermal management systems enable the fabrication of complex chip-based chemical and biochemical reaction systems in which the temperature of many processes can be controlled independently

    Near-field scanning optical microscopic transient lens for carrier dynamics study in InGaN/GaN

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    Time-resolved microscopic transient lens (TR-M-TL) and near-field scanning optical microscopic transient lens (NSOM-TL) were performed to reveal temporal and spatial behavior of carrier dynamics in InGaN/GaN quantum wells. The carrier and thermal dynamics were observed through the time profile of the TR-M-TL signal. Also, NSOM-photoluminescence and NSOM-TL images were observed at the same time. By comparing these two images, both radiative and nonradiative recombination centers in InGaN active layer were unambiguously discriminated with submicrometer scale. Such nonradiative carrier dynamics has been difficult to observe by conventional techniques in spite of its importance

    Methods for controlling positions of guided modes of photonic-crystal waveguides

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    We analyze different methods for controlling positions of guided modes of planar photonic-crystal waveguides. Methods based both on rearrangements of holes in the photonic-crystal lattice and on changes of hole sizes are presented. The ability to tune frequencies of guided modes within a frequency bandgap is necessary to achieve efficient guiding of light within a waveguide, as well as to match frequencies of eigenmodes of different photonic-crystal-based devices for the purpose of good coupling between them. We observe and explain the appearance of acceptor-type modes in donor-type waveguides

    Surface-plasmon enhanced bright emission from CdSe quantum-dot nanocrystals

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    We obtained very bright light emission from CdSe quantum dots (QDs) by using the surface-plasmon (SP) coupling technique. 23-fold enhanced photoluminescence (PL) intensities and two-fold increased PL decay rates are observed when the QDs are located on evaporated gold films. This enhancement is not effective for CdSe cores with ZnS shells (ZnS/CdSe). The reason for this difference can be explained by using the SP dispersion diagram and by considering the SP coupling mechanism. We discuss the inherent merits and demerits of this technique to increase the emission efficiency. This technique will enable high-speed and efficient light emission for optically as well as electrically pumped light emitters

    Modal Gain Analysis of Transverse Bragg Resonance Waveguide Lasers With and Without Transverse Defects

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    We use a transfer matrix method to analyze the modal gain of transverse Bragg resonance (TBR) structures. We show that these TBR structures can support two types of modes characterized by different modal angles: small mode angle (SMA) modes and TBR modes. We discuss the origin, modal properties and field distributions of both the TBR modes and SMA modes. Three different feedback mechanisms are proposed to select the desired TBR modes

    Surface plasmon enhanced light-emitting diode

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    A method for enhancing the emission properties of light-emitting diodes, by coupling to surface plasmons, is analyzed both theoretically and experimentally. The analyzed structure consists of a semiconductor emitter layer thinner than λ/2 sandwiched between two metal films. If a periodic pattern is defined in the top semitransparent metal layer by lithography, it is possible to efficiently couple out the light emitted from the semiconductor and to simultaneously enhance the spontaneous emission rate. For the analyzed designs, we theoretically estimate extraction efficiencies as high as 37% and Purcell factors of up to 4.5. We have experimentally measured photoluminescence intensities of up to 46 times higher in fabricated structures compared to unprocessed wafers. The increased light emission is due to an increase in the efficiency and an increase in the pumping intensity resulting from trapping of pump photons within the microcavity

    Surface micromachined membranes for tunnel transducers

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    We have developed low-temperature surface micromachining procedures for the fabrication of suspended SiO2/Si3N4 membranes. This fabrication method was integrated with electron beam lithography, anisotropic ion etching, and electroplating to construct electrostatically deflectable tunnel transducers. We show the structures and some preliminary measurements on the performance of these monolithic devices
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