Absorption (logic)

About: Absorption (logic) is a research topic. Over the lifetime, 5733 publications have been published within this topic receiving 236302 citations.

Coherent instabilities in a semiconductor laser with fast gain recovery

Abstract: We report the observation of a coherent multimode instability in quantum cascade lasers (QCLs), which is driven by the same fundamental mechanism of Rabi oscillations as the elusive Risken-Nummedal-Graham-Haken (RNGH) instability predicted $40\phantom{\rule{0.3em}{0ex}}\text{years}$ ago for ring lasers. The threshold of the observed instability is significantly lower than in the original RNGH instability, which we attribute to saturable-absorption nonlinearity in the laser. Coherent effects, which cannot be reproduced by standard laser rate equations, can play therefore a key role in the multimode dynamics of QCLs, and in lasers with fast gain recovery in general.

Sensitive Detection of Nuclear Quadrupole Interactions in Solids

Abstract: The pure nuclear quadrupole resonance of a low-abundance spin species in zero field is observed for molar concentrations as low as one part in ${10}^{7}$ by the application of a nuclear-double-resonance method. The quadrupole resonance is measured in terms of a decrease in the magnetic order of abundant nuclei which are dipolar-coupled to the low-abundance nuclear species. The double-resonance process is analyzed from the point of view of two energy reservoirs coupled by a dipolar perturbation, and spin diffusion among abundant nuclei is included phenomenologically. Radio-frequency power, which causes the main quadrupole transitions of rare spins, is frequency-modulated to provide saturation in the rotating frame and to enable identification of unknown spin transitions. The pure nuclear quadrupole resonance of Na and Cl nuclei near impurities and imperfections in NaCl is measured. Particular attention is given to the resonances associated with ${\mathrm{K}}^{+}$ and ${\mathrm{Br}}^{\ensuremath{-}}$ ions injected as impurities into the NaCl lattice. An unsuccessful search in zero field has been made for the naturally abundant 0.0156% deuterium quadrupole resonance in CaS${\mathrm{O}}_{4}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O and ${\mathrm{C}}_{10}$${\mathrm{H}}_{8}$, where the proton resonance is monitored as the abundant nuclear species. The proton dipolar absorption in zero magnetic field is anomalously broad and overlaps too much with the deuterium quadrupole resonance to permit independent observation of the latter. A narrowing of proton dipolar absorption in zero field is observed for strong radio-frequency fields larger than internal dipole fields.

Recombination Luminescence from V K Centers in Potassium Iodide

Abstract: This paper reports experiments with KI to determine whether the near-uv emission characteristic of the "pure" crystal arises from the recombination of an electron with a ${V}_{K}$ center. KI crystals containing Tl or Eu, when irradiated at 77\ifmmode^\circ\else\textdegree\fi{}K with 1.7-MeV electrons, exhibit absorption bands due to ${V}_{K}$ and $F$ centers. Illumination of an irradiated crystal in the $F$ band or near infrared results in attenuation of ${V}_{K}$ absorption bands and in emission of the 302- and 371-m\ensuremath{\mu} bands characteristic of the pure crystal, as well as in ${\mathrm{Tl}}^{+}$ emission. Thermal or optical destruction of ${V}_{K}$ centers results in attenuation of the 302- and 371-m\ensuremath{\mu} bands, while the ${\mathrm{Tl}}^{+}$ emission remains. Preferential orientation of ${V}_{K}$ centers by bleaching with polarized light at liquid-helium temperature or at 77\ifmmode^\circ\else\textdegree\fi{}K results in a polarization of both these emission bands. Assuming that the orienting transition is $\ensuremath{\sigma}$ polarized, the 302-m\ensuremath{\mu} band is polarized parallel to the ${V}_{K}$ axis, and the 371-m\ensuremath{\mu} band perpendicular. It is concluded that these bands do arise from the radiative decay of a ${V}_{K}$-plus-electron center.

Infrared absorption of deep defects in molecular-beam-epitaxial GaAs layers grown at 200 °C: Observation of an EL 2-like defect

Abstract: Infrared optical absorption and Hall-effect techniques were employed to study deep defects in As-rich molecular-beam-epitaxial GaAs layers grown at very low temperature (200 \ifmmode^\circ\else\textdegree\fi{}C). A large ir absorption band was observed between 0.55 eV and the band edge. This band is composed of photoquenchable and photounquenchable components. Photoquenching, thermal recovery from the metastable state, and ir absorption properties of the quenchable defect, of estimated concentration of \ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, are identical to those of EL2. On the other hand, the unquenchable defect, of estimated concentration \ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, resembles the isolated ${\mathrm{As}}_{\mathrm{Ga}}$ antisite observed in neutron-irradiated GaAs. Both defects' concentrations, which show different isothermal annealing behavior are reduced by about an order of magnitude upon thermal annealing at 600 \ifmmode^\circ\else\textdegree\fi{}C for 10 min. This reduction is accompanied by an increase of sample resistivity by a few orders of magnitude.

Defects in unirradiated α-Al 2 O 3

Abstract: Single-crystal samples of $\ensuremath{\alpha}$-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ have been studied that were colored during growth. The samples initially contained a single absorption band at 6.1 eV with a half-width of 0.68 eV. The sample had to be heated to a over 1400\ifmmode^\circ\else\textdegree\fi{}C to remove this band. The 6.1-eV band could be optically bleached at 10 K, which introduced bands at 4.1, 4.8, and 5.4 eV, and which shifted the high-energy peak to 6.05 eV. Bleaching the 6.1-eV band (at 10 K also) produced photoconductivity and an emission band at 3.0 eV, with a half-life of about 10 msec. The spectral dependence of the photoresponse and the excitation spectrum of the 3.0-eV band were similar to the 6.1-eV absorption. After bleaching an emission band at 3.8 eV was produced which was similar to a band produced in particle-irradiated material. The lifetime of the 3.8-eV emission was less than 50 nsec, and the excitation spectrum contained peaks at 4.8 and 5.4 eV, with a small shoulder at 5.9 eV. The temperature dependence of the half-width of the 3.8-eV emission followed a hyperbolic cotangent relation with an effective frequency of 340\ifmmode\pm\else\textpm\fi{}20 ${\mathrm{cm}}^{\ensuremath{-}1}$.