氧化鋅激子極子光電物理與動態特性之研究

Abstract

共振腔極子(Cavity polaritons)源自於激子與共振腔局限的光子之間的強耦合。它們的色 散相對於裸態激子和光子已被顯著地修改，導致極子色散曲線形成最低點為零動量的基態。 透過半光、半物質的準粒子的激子分量與晶格振動（聲和光聲子）之間的交互作用產生的散 射過程，激化激子的能量弛豫，達熱平衡化和極子之非線性交互作用現象。微腔中的受激的 極子散射的最後基態已經被用來實現極子雷射(polariton laser)和極子的玻色 - 愛因斯坦凝聚 (Polariton BEC)，導致許多有趣的的極子超流體(superfluidity)和極子旋渦(vortex)等宏觀量子 現象以及許多應用元件，例如極子雷射，它不需要達到雷射閾值，因此具有非常低的閾值。 然而，演變成這樣一個有用的極子雷射需要達到室溫的極子BEC。寬能隙半導體，如 GaN 和ZnO，已被提出可作為室溫操作極子雷射最適當候選材料；由於它們具有大的激子結 合能和振盪強度(oscillator strength)。室溫極子雷射在塊材和多量子阱GaN 的微腔中被實現； 然而，因ZnO 具優於GaN 的較大激子結合能和振盪強度，其拉比分裂(Rabi splitting) 在塊材 ZnO 平面腔中約120 meV，在氧化鋅微絲(microwires)中利用whispery gallery mode 共振可達 到200 meV。近日，透過本研究團隊與本校的盧廷昌教授和美國密歇根大學鄧輝教授合作， 我們利用ZnO 薄膜為基礎的混合微腔，在負detuning 下達成室溫極子雷射和光子雷射現象。 從本質上講，假使未凝聚的激子和低極子(lower polariton branch)底部的能量差與LO 聲子能 量共振的話，藉著LO 聲子輔助可有效降低ZnO 塊材微腔極子雷射閾值。 因此，本三年計劃的目標是結合不同研究專長的主持人從事大家共同感興趣的前瞻研 究課題來研究微腔極子非線性動力學。其研究主題包括：基於ZnO 材料和量子結構或其自組 成(self-formed)微腔中的極子-極子、極子-聲子、和聲光的散射過程，以及極子BEC 和超流 體的動力學研究等。因此，本子計畫(子計畫一)將集中精力從事下列主題之物理內涵和機制 研究，包括p 型摻雜ZnO epifilms 和ZnO / ZnMgO 多量子阱和嵌入DBR 腔和自形成的微腔 中的超快載子弛豫、同調聲子散射、和同調載子傳輸現象研究與理論模型。另外，我們也將 基於複數Gross-Pitaevskii 方程的Bogoliubov-de Genned 線性穩定性分析來從事極子凝聚態非 線性動力學研究，例如BKT 相變和渦旋晶格(vortex lattice)之形成等。

Cavity polaritons are quasiparticles resulting from the strong coupling of excitons with photons confined in a cavity. Their dispersion is remarkably modified with respect to the one of the bare excitons and photons leading to formation of a minimum at the polariton ground state at zero momentum. Scattering processes occurring through the interaction among the excitonic components of the half-light, half-matter quasiparticles and lattice vibrations (acoustic and optical phonons) are responsible for the energy relaxation, thermalization, and nonlinearities of polaritons. The stimulated polariton scattering to the final ground state has been used to realize the polariton laser and polariton Bose-Einstein condensate (BEC) in microcavities and lead to many interesting macro-quantum phenomena, such as the polariton superfluidity and vortices as well as many device proposals, e.g., the polariton laser, which does not require the achievement of the gain condition therefore has a very low threshold. However, the evolution of such a laser to a useful device requires the polariton BEC at room temperature (RT). Wide-band-gap semiconductors such as GaN and ZnO have been proposed as appropriate candidates for RT operation because of their large exciton binding energy and oscillator strength. RT polariton lasing has been effectively reported in bulk and multiquantum well GaN microcavities; whereas, the advantage of ZnO with respect to nitrides lies in the larger exciton binding energy and oscillator strength, leading to Rabi splitting of ~120 meV in bulk planar cavity and up to 200 meV in ZnO microwires. Recently, polariton lasing and photon lasing have been demonstrated through polariton scattering in our group with cooperation of Prof. TC Lu of NCTU and Prof. Hui Deng of U. Michigan using a ZnO-film-based hybrid microcavity with negative detuning. Essentially, the LO phonon can assist polariton lasing in a bulk ZnO-based microcavity, in which the lowest threshold of polariton lasing was achieved when the energy difference between the exciton reservoir and the bottom of the LPB is resonant with the LO phonon energy. Therefore, the goal of this three-years proposal is to coordinate PIs with different specialties to pursue a common research interests on the nonlinear dynamics of microcavity polaritons, including polariton-polariton, polariton-phonons and acousto-optic scattering processes, and dynamics of polariton BEC and superfluid, based on ZnO-based materials and quantum structures or self-formed microcavities. And in this Subproject 1, we will concentrate on the underlying physics and mechanisms of p-type doping ZnO epifilms and ZnO/ZnMgO multiple quantum wells embedded in ZnO based DBR cavities, ultrafast carrier relaxation, coherent phonon scattering, and coherent carrier transport of ZnO-based materials and quantum structures in Fabry-Perot and self-formed microcavities. Furthermore, we will also investigate the nonlinear dynamics of polariton condensates such as the Berezinskii-Kosterlitz-Thouless (BKT) transition and formation of vortex lattice based on the complex Gross-Pitaevskii equation (GPE) and Bogoliubov-de Genned linear stability analysis.