越快光學及兆赫頻波光譜奈米結構研究

Abstract

『超快光譜術』為一種強力的探測技術, 此技術不僅可以探測各種材料的光學與電 學性質，還可以取得材料內的動力學與傳輸特性。時間解析的吸收、反射與螢光實驗已 經被證實有效且實際運用於分析各種材料，不管是原子、分子或固態材料。飛秒雷射的 問世使得人們可以運用兆赫波段，此波段包含了大部分原子、分子的特徵頻率，因此兆 赫光譜術已經成為一個新的探測材料電性的工具，甚至於使用在非傳統材料的分析，例 如半導體奈米結構。近年來我們團隊使用光學激發探測技術(optical pump-probe technique)、時間解析發光光譜(time-resolved luminescence technique)以及兆赫波時域光譜 術(THz time-domain spectroscopy)，已經成功展示出三族氮化物奈米柱的超快載子動態特 性，兆赫波時域光譜術使我們了解半導體的電子性質與載子散射生命週期。此外，利用” 混合式氮化銦鎵/ 金奈米晶體” 金屬氧化物半導體結構(hybrid InGaN-nanorod/Au-nanocrystal metal-oxide-semiconductor structure），我們也成功的展示 出三維度次波長綠光電漿子奈米雷射，這項工作代表著跨出主動式電漿子元件的一步， 可以克服被動式電漿子元件所遇到的耗損難題。 本提案中，使用時間解析雷射光譜術，我們將研究與控制在金屬氧化物半導體結構 中的同調性表面電漿子，系統性的利用可見光/兆赫波超快光譜術可以讓我們分析載子的 激發與弛豫過程，然而在時間解析方面，目前實驗室所使用的時間解析度約200皮秒， 不足以研究半導體奈米結構的輻射動力學以及雷射機制，因此在本提案中，我們計畫建 構光學升頻技術(upconversion)，其時間解析會與雷射的時間解析在同一個數量級。實驗 室已有的可見光激發可見光探測光譜術(optical-pump optical-probe spectroscopy)，此技術 主要是可以靈敏的分析載子佔據於能帶中的位置，我們也計畫建構可見光激發─兆赫波 探測光譜術系統(optical-pump-THz-probe spectroscopy)，此技術不僅可以分析載子的密 度，還可以分析載子在帶內的分布情形，因此可用來研究被可見光激發的載子，同時得 到載子在帶內載子弛豫以及在帶間復合的動態過程，而時間解析度與所使用雷射的時間 解析度為同一數量級。我們也將利用可見光與兆赫波光譜系統全面性的研究兩種新的材 料: 具有獨特二維原子結構的石墨烯(grapheme)，以及一種有機半導體，鈦氧酞菁 (titanylphthalocyanine, TiOPc)。我們將研究多層石墨烯的電子以及聲子特性，並用來與 單層石墨烯及石墨比較。而利用計畫所架構的超快雷射光譜分析系統，研究新的鈦氧酞 菁有機半導體材料，可以探測其內部電荷的移轉機制，而了解鈦氧酞菁分子與其固態半 導體結構的電子特性關聯，將有助於未來有機半導體元件的優化。

Ultrafast optical spectroscopy is a powerful technique for investigating not only optical and electronic properties of various kinds of materials, but also their dynamics and transport properties. The time-resolved absorption, reflection, and luminescence measurement have been practiced to realize the insight of atoms, molecules, and solids. The advent of fs laser system makes it possible to access the THz spectral region of which most of atoms and molecules have their fingerprint modes and THz spectroscopy emerges as a new tool to investigate the electronic properties of even unconventional forms of materials such as semiconductor nanostructures. Recently, we have successfully demonstrated ultrafast carrier dynamics in group III-nitrides nanorods using optical pump-probe technique and the time-resolved luminescence technique. THz time-domain spectroscopy enabled us to realize the electronic properties and the carrier scattering lifetimes of semiconductors. We have also been able to demonstrate the first three-dimensional subwavelength green plasmonic nanolaser based on a hybrid InGaN-nanorod/Au-nanocrystal metal-oxides-semiconductor (MOS) structure. This work represents a significant step toward active plasmonic components, which are urgently needed to overcome the intrinsically lossy feature of passive plasmonic components. In this proposal, we will study and control the characteristics of coherent surface plasmons in MOS nanostructures using the techniques of time-resolved laser spectroscopy. Systematic measurement and analysis based on ultrafast optical/THz spectroscopy will enable us to understand the carrier excitation and relaxation processes. The poor temporal resolution (~200 ps) of our current time-resolve PL system prevents us from realization of the details of the radiative dynamics in semiconductor nanostructures and lasing mechanism. In this proposal, we will construct a time-resolved PL system based on upconversion scheme, the temporal resolution of which is of the order of laser pulses. While optical-pump optical-probe spectroscopy measurements are sensitive primarily to the carrier occupation of specific regions in the energy bands, optical-pump-THz-probe spectroscopy is sensitive to not only the total carrier density but also to the distribution of these carriers in energy within the bands. Optical-pump-THz-probe spectroscopy can therefore be used to simultaneously study both intraband relaxation and interband recombination dynamics of photoexcited electrons and holes on ultrafast time scales. As new target materials, graphene with a unique two-dimensional atomic structure and an organic semiconductor, titanylphthalocyanine (TiOPc) will be investigated with our comprehensive optical/THz spectroscopy system. The electronic as well phononic properties of graphene multilayer, which can be different from those of bulk graphite will be studied and compared with those of graphene monolayer. The optimization of electronic devices based on organic semiconductors demands a better understanding of the inherent charge-transfer mechanism and we will elucidate the relation between structure and electronic properties of TiOPc by using our ultrafast spectroscopic diagnosis system.