The use of lanthanide ions to convert photons to different, more useful, wavelengths is well-known from a wide range of applications (e.g. fluorescent tubes, lasers, white light LEDs). Recently, a new potential application has emerged: the use of lanthanide ions for spectral conversion in solar cells. The main energy loss in the conversion of solar energy to electricity is related to the so-called spectral mismatch: low energy photons are not absorbed by a solar cell while high energy photons are not used efficiently. To reduce the spectral mismatch losses both upconversion and downconversion are viable options. In the case of upconversion two low energy infrared photons that cannot be absorbed by the solar cell, are added up to give one high energy photon that can be absorbed. In the case of downconversion one high energy photon is split into two lower energy photons that can both be absorbed by the solar cell. The rich and unique energy level structure arising from the 4fn inner shell configuration of the trivalent lanthanide ions gives a variety of options for efficient up- and downconversion. In this perspective an overview will be given of recent work on photon management for solar cells. Three topics can be distinguished: (1) modelling of the potential impact of spectral conversion on the efficiency of solar cells; (2) research on up- and downconversion materials based on lanthanides; and (3) proof-of-principle experiments. Finally, an outlook will be given, including issues that need to be resolved before wide scale application of up- and downconversion materials can be anticipated.

Lanthanide ions as spectral converters for solar cells

We review photonic applications of dielectric whispering-gallery mode (WGM) resonators-tracing the growth of the technology from experiments with levitating droplets of aerosols to ultrahigh-Q solid state crystalline and integrated on-chip microresonators.

Optical resonators with whispering-gallery modes-part II: applications

Upconversion spectroscopy and properties of NaYF4 doped with Er3+, Tm3+ and/or Yb3+

Optical delay lines have some important applications, notably in optical communication systems and in phased arrays. These devices are based on the concept of optical group delay, which, in turn, can be understood as the property of an optical filter. Optical filters are well-understood devices and, in particular, their dispersive properties determine the group delay response. We review these dispersive properties and point out some of the inherent tradeoffs involved in generating large group delay. Fiber Bragg gratings and recent results on optical all-pass filters are used as examples.

Optical delay lines based on optical filters

Embodiments of apparatus for providing distributed airborne wireless communications are provided herein. In some embodiments, a distributed communication payload includes: a payload control electronics module; an air-to-user link module to provide wireless communication links between airborne platforms and end-user devices; and an air-to-air link module to provide wireless communication links between airborne platforms; wherein the air-to-user and air-to-air link modules are subdivided into multiple sections that can be disposed on different airborne platforms.

Apparatus for distributed airborne wireless communications

A spectroscopic investigation of an extensive series of ${\mathrm{Er}}^{3+}$-doped and ${\mathrm{Er}}^{3+}{,\mathrm{Y}\mathrm{b}}^{3+}$-codoped soda-lime-silicate (SL) and aluminosilicate (AS) glasses is presented. Compared to SL glasses, $4f$ transitions in AS glasses show higher oscillator strengths, larger inhomogeneous broadening, and smaller crystal-field splittings of the respective excited-state multiplets. The ${\mathrm{Er}}^{3+}$ excited-state relaxation dynamics is adequately described by a combination of the Judd-Ofelt model and the energy-gap law. With the exception of ${}^{4}{I}_{13/2},$ multiphonon relaxation is dominant for all excited states, making it possible to efficiently pump the 1.55 \ensuremath{\mu}m ${}^{4}{I}_{13/2}{\ensuremath{\rightarrow}}^{4}{I}_{15/2}$ emission by excitation of ${}^{4}{I}_{11/2}$ at around 980 nm. The absolute ${}^{4}{I}_{13/2}$ luminescence quantum yield, for low 980-nm excitation density $(\ensuremath{\sim}5{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}),$ \ensuremath{\eta}, is $\ensuremath{\sim}0.9$ at 0.4 mol % ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$ and drops to about 0.65 upon increasing ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$ to 1.2 mol %, indicating the onset of energy-transfer processes. Samples with high ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ impurity concentration suffer from significantly higher quenching of ${}^{4}{I}_{13/2}$ luminescence at higher ${\mathrm{Er}}^{3+}$ concentrations. Energy migration to the minority of ${\mathrm{Er}}^{3+}$ ions coordinated to ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$, followed by efficient multiphonon relaxation accounts for this effect. At low excitation densities, the strong near-infrared absorption of ${\mathrm{Yb}}^{3+}$ in combination with efficient $\mathrm{Yb}\ensuremath{\rightarrow}\mathrm{Er}$ energy transfer increases the ${}^{4}{I}_{13/2}$ population density in ${\mathrm{Yb}}^{3+},{\mathrm{Er}}^{3+}$-codoped samples by up to 2 orders of magnitude compared to equivalent samples without ${\mathrm{Yb}}^{3+}$. The dependence of \ensuremath{\eta} on ${\mathrm{Yb}}^{3+}$ codotation of 0.4 mol % ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$-doped samples predicts that a minimum of $\ensuremath{\sim}0.8\mathrm{mol}%$ ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}$ is required to achieve efficient sensitization of ${\mathrm{Er}}^{3+}$ by ${\mathrm{Yb}}^{3+}$. The relative intensities of upconversion luminescence from ${}^{4}{S}_{3/2}$ and ${}^{2}{H}_{11/2}$ are used to analyze internal sample heating in detail. Due to the high absorption cross section of ${\mathrm{Yb}}^{3+}$, increasing the ${\mathrm{Yb}}^{3+}$ concentration in ${\mathrm{Yb}}^{3+},{\mathrm{Er}}^{3+}$-codoped samples of given length increases the absorbed power and subsequently the total density of multiphonon emission, leading to internal temperatures of up to 572 K in 0.4 mol % ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$ samples codoped with 4 mol % ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}$ and excited with $51{\mathrm{k}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$. Multiphonon relaxation from ${}^{4}{I}_{13/2}$ is shown to be inefficient even at these high internal sample temperatures. From upconversion luminescence spectra of a series of glasses, the thermal conductivity is estimated to be between $3.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ and $7.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{\mathrm{W}\mathrm{}\mathrm{m}}^{\mathrm{\ensuremath{-}}1}{\mathrm{}\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$, in good agreement with the known value of $4.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{\mathrm{W}\mathrm{}\mathrm{m}}^{\mathrm{\ensuremath{-}}1}{\mathrm{}\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ for soda-lime-silicate glass.

Spectroscopic properties of Er 3 + - and Yb 3 + -doped soda-lime silicate and aluminosilicate glasses

New integrated optical all-pass filters are presented that can be used for tunable dispersion compensation, dispersion slope compensation, and as building blocks in tunable bandpass filters. The dispersion slope compensation capability is demonstrated using ring resonators in /spl Delta/=2% Ge-doped silica planar waveguides. In addition, a tunable four-stage filter with free-spectral range (FSR)=25 GHz, a passband width of 14 GHz (0.56/spl times/FSR), and D=1800 ps/nm is reported.

Integrated all-pass filters for tunable dispersion and dispersion slope compensation

Embodiments of methods and apparatus for providing distributed airborne wireless communications are provided herein. In some embodiments, an airborne wireless communication node includes: an airborne fleet comprising a plurality of airborne platforms having flight control electronics configured to control flight of individual airborne platforms and coordinate a flight plan of the airborne fleet as a whole; and a distributed communication payload, wherein the communication payload is subdivided into constituent parts, wherein the parts are distributed and positioned on respective ones of the plurality of airborne platforms. In some embodiments, the distributed communication payload includes: air-to-user link equipment to provide communication links with end-users, the air-to-user link equipment further comprising an RF antenna; air-to-air link equipment to provide communications between individual airborne platforms; and payload control electronics to control the air-to-user and air-to-air link equipment and managing communication services.

Distributed airborne wireless networks

Uniform upconversion in erbium-doped silicate glasses is investigated as a function of glass composition, concentration, and fabrication method. Comparisons of upconversion coefficients are made among soda lime silicate and aluminosilicate bulk glasses and soda lime silicate waveguides. Comparisons are also made with studies performed by other researchers. The results indicate that both the composition and the preparation method of the glass affect the value of the upconversion coefficient, with as much as a factor-of-4 variation observed at fixed Er3+ concentration. Values of the upconversion coefficient are found to be consistent with the Forster–Dexter microscopic model.

Allan James Bruce

Papers

Apparatus for distributed airborne wireless communications

Spectroscopic properties of Er 3 + - and Yb 3 + -doped soda-lime silicate and aluminosilicate glasses

Integrated all-pass filters for tunable dispersion and dispersion slope compensation

Distributed airborne wireless networks

Uniform upconversion in high-concentration Er(3+)-doped soda lime silicate and aluminosilicate glasses.