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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Giant oscillator strength ： ウィキペディア英語版 Giant oscillator strength Giant oscillator strength is inherent in excitons that are weakly bound to impurities or defects in crystals. The spectrum of fundamental absorption of direct-gap semiconductors such as Gallium arsenide (GaAs) and Cadmium sulfide (CdS) is continuous and corresponds to band-to-band transitions. It begins with transitions at the center of the Brillouin zone, $\backslash boldsymbol=0$. In a perfect crystal, this spectrum is preceded by a hydrogen-like series of the transitions to ''s''-states of Wannier-Mott excitons.〔R. J. Elliott, Intensity of optical absorption by excitons, Phys. Rev. 108, 1384 (1957).〕 In addition to the exciton lines, there are surprisingly strong additional absorption lines in the same spectral region.〔V. L. Broude, V. V. Eremenko and É. I. Rashba, The Absorption of Light by CdS Crystals, Soviet Physics Doklady 2, 239 (1957).〕 They belong to excitons weakly bound to impurities and defects and are termed 'impurity excitons'. Anomalously high intensity of the impurity-exciton lines indicate their giant oscillator strength of about $f\_i\backslash sim10$ per impurity center while the oscillator strength of free excitons is only of about $f\_\backslash sim10^$ per unit cell. Shallow impurity-exciton states are working as antennas borrowing their giant oscillator strength from wast areas of the crystal around them. They were predicted by Emmanuel Rashba first for molecular excitons〔E. I. Rashba, Theory of the impurity absorption of light in molecular crystals, Opt. Spektrosk. 2, 568-577 (1957)〕 and afterwards for excitons in semiconductors.〔E. I. Rashba and G. E. Gurgenishvili, To the theory of the edge absorption in semiconductors, Sov. Phys. - Solid State 4, 759 - 760 (1962)〕 Giant oscillator strengths of impurity excitons endow them with ultra-short radiational life-times $\backslash tau\_i\backslash sim1$ ns. ==Bound excitons in semiconductors: Theory== Interband optical transitions happen at the scale of the lattice constant which is small compared to the exciton radius. Therefore for large excitons in direct-gap crystals the oscillator strength $f\_$ of exciton absorption is proportional to $|\backslash Phi\_(0)|^2$ which is the value of the square of the wave function of the internal motion inside the exciton $\backslash Phi\_(\backslash boldsymbol\_e-\backslash boldsymbol\_h)$ at coinciding values of the electron $\backslash boldsymbol\_e$ and hole $\backslash boldsymbol\_h$ coordinates. For large excitons $|\backslash Phi\_(0)|^2\backslash approx\; 1/a^3\_$ where $a\_$ is the exciton radius, hence, $f\_\backslash approx\; v/a^3\_\backslash ll1$, here $v$ is the unit cell volume. The oscillator strength $f\_i$ for producing a bound exciton can be expressed through its wave function $\backslash Psi\_i(\backslash boldsymbol\_e,\backslash boldsymbol\_h)$ and $f\_$ as $f\_i=\backslash frac\backslash frac\_e,\backslash boldsymbol\_e))^2\}f\_$ . Coinciding coordinates in the numerator, $\backslash boldsymbol\_e=\backslash boldsymbol\_h$, reflect the fact the exciton is created at a spatial scale small compared with its radius. The integral in the numerator can only be performed for specific models of impurity excitons. However, if the exciton is weakly bound to impurity, hence, the radius of the bound exciton $a\_i$ satisfies the condition $a\_i$ ≥ $a\_$ and its wave function of the internal motion $\backslash Phi\_(\backslash boldsymbol\_e-\backslash boldsymbol\_h)$ is only slightly distorted, then the integral in the numerator can be evaluated as $(a\_i/a\_)^$. This immediately results in an estimate for $f\_i$ $f\_i\backslash approx\backslash fracf\_$ . This simple result reflects physics of the phenomenon of giant oscillator strength: coherent oscillation of electron polarization in the volume of about $a\_i^3\; >>\; v$. If the exciton is bound to a defect by a weak short-range potential, a more accurate estimate holds $f\_i=8\backslash left(\backslash frac\backslash frac\backslash right)^\backslash fracf\_$. Here $m=m\_e+m\_h$ is the exciton effective mass, $\backslash mu=(m\_e^+m\_h^)^$ is its reduced mass, $E\_$ is the exciton ionization energy, $E\_i$ is the binding energy of the exciton to impurity, and $m\_e$ and $m\_h$ are the electron and hole effective masses. Giant oscillator strength for shallow trapped excitons results in their short radiative lifetimes $\backslash tau\_i\backslash approx\backslash frac\; .$ Here $m\_0$ is the electron mass in vacuum, $c$ is the speed of light, $n$ is the refraction index, and $\backslash omega\_i$ is the frequency of emitted light. Typical values of $\backslash tau\_i$ are about nanoseconds, and these short radiative lifetimes favor the radiative recombination of excitons over the non-radiative one.〔E. I. Rashba, Giant Oscillator Strengths Associated with Exciton Complexes, Sov. Phys. Semicond. 8, 807-816 (1975)〕 When quantum yield of radiative emission is high, the process can be considered as resonance fluorescence. Similar effects exist for optical transitions between exciton and biexciton states. An alternative description of the same phenomenon is in terms of polaritons: giant cross-sections of the resonance scattering of electronic polaritons on impurities and lattice defects. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Giant oscillator strength」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.