利用可見光波段超短脈衝雷射研究新穎量子物質之超快動力學

Abstract

在凝態物理的領域中,準粒子之間的交互作用扮演了極為重要的角色。例如,在強 關聯系統中,電子之間的強交互作用甚至使得電子組態看似是導體的材料實際上是絕緣 體。因此,研究諸如電子、聲子、電子自旋等自由度之間的交互作用有著其不可或缺的 重要性。瞬態時間解析光譜可解析準粒子之間的能量流動,進而得到準粒子間的交互作 用強度,由於其具有飛秒、皮秒等級的時間解析能力,以超短脈衝雷射為光源的瞬態時 間解析光譜能給予我們傳統電子儀器因受限於電子傳輸速度所無法提供的超快動力學 資訊。在本論文中,我們獨立發展並建構了超短脈衝雷射光源與技術: 包含了單一波長 的光參數放大器(OPA)和超寬頻的非共線光參數放大器(NOPA)。其所產生的超短脈衝雷 射使我們的時間解析能力得以提升至飛秒等級,寬廣的頻譜和可調性更提高了我們對於 各類材料研究的自由度。 在多鐵性六方晶系錳氧化物中,獨特的磁、電有序現象使其具有廣泛的應用價值。 在線性吸收光譜、二倍頻光譜、中子繞射以及拉曼光譜中均呈現了此材料在各種物理量 之間的交互作用。其中,錳離子因環境晶體場造成能階分裂的 d 軌域在磁有序發生的溫 度下出現了劇烈的變化,在前人所發表的時間解析光譜中,我們已清楚了解 d 軌域中的e2g 能階在受到磁有序影響時的動力學行為。本論文利用超寬頻時間解析光譜可同時觀 測六方晶系鈥錳氧薄膜中 e2g 和 e1g 能態的載子動力學行為,清楚地解析出 e2g 能態除了 受到電有序的影響,更在磁有序的轉變溫度產生了劇烈變化;而 e1g 能態主要受到長程 磁有序的影響。載子在 e2g 和 e1g 能態的弛緩時間分別為 340 ± 80 和 380 ± 80 飛秒,在 e1g 能態的電子聲子耦和作用下的弛緩時間則為 2.7 ± 1.5 皮秒。由此時間解析光譜的結 果,我們推論短程磁有序和電有序與 e2g 能態緊密相關;而長程磁有序則與 e2g 和 e1g 能 態高度地連結。 在太陽能電池材料中,了解光激載子的產生、再結合、以及收集對於提升太陽能電 池的效率十分重要。其中,有機高分子聚合物/富勒烯複合物(P3HT:PCBM)在製程過程 中,於覆蓋上鋁電極之後退火的元件相較於先將複合物退火再覆蓋鋁電極的元件具有較 高的光電轉換效率,然而其微觀原因一直未被深入的討論。為了對太陽能電池的未來發 展提供具體的方向,本論文利用可見光波段超短脈衝雷射對此有機複合物太陽能電池進 行了載子動力學的研究。研究結果顯示,利用後退火製程製備的元件表現出較長的受激 載子弛緩時間。另外,藉由獨立分析各項弛緩時間的強度我們發現後退火製程製備的元 件具有較多的載子分離數目和較低的再結合程度,這給予了後退火元件具有較高光電轉 換效率一個清楚的解釋。 最後,我們利用波長可調的光參數放大器研究新興二維材料-二氧化鉬中電子自旋 和能谷之間的耦和關係。我們成功地建立完整的二氧化鉬載子動力學模型,包含各向異 性的電子自旋弛緩、光激子解離、谷間散射、受激載子的谷內散射等過程的具體時間尺 度。另外,藉由仔細比對室溫和低溫下各個弛緩過程的比重,我們也發現在室溫下的各 向異性電子自旋分布亦和低溫下的極化強度近乎相同,這些研究成果為實現室溫自旋電 子元件帶來了令人振奮的前景。

In condensed matter physics, correlations between quasiparticles play an extremely important role to understand the behavior of a material. For example, the strong interaction between electrons with the metal-like electron configuration makes the strongly correlated insulators, rather than conductors which were predicted by the band theory. Therefore, scientists are eager to figure out the interaction strength between different degrees of freedom, e.g. electron, phonon, and spin etc. The transient spectroscopy is one of the desired techniques which is capable to observe the energy transfer between quasiparticles and specify the strength of mediated interactions. In this dissertation, we have successfully constructed the ultrashort pulse OPA and NOPA systems in the visible range. The light sources with the extremely high time resolution and broadband spectrum enhance our capability to explore several kinds of materials. Taking the advantage of the broadband ultrashort pulses, the Mn3+ on-site d-d transitions in the multiferroic hexagonal HoMnO3 thin films can be unambiguously revealed. The ultrafast responses of the e1g and e2g states differed significantly in the hexagonal HoMnO3. The e2g state exhibited ferroelectric-like behavior and underwent a marked change at the magnetic transition temperature. By contrast, the e1g state strongly correlated with the appearance of a long-range antiferromagnetic phase. Moreover, the primary thermalization times of the e2g and e1g states were determined to be 340 ± 80 and 380 ± 80 fs, respectively. The electron-phonon relaxation in the a1g state was 2.7 ± 1.5 ps. The organic solar cells of heterojunction system, ITO/PEDOT:PSS/P3HT:PCBM/Al, with a thermal annealing after deposition of Al exhibit better performance than those with a annealing process before deposition of Al. In this dissertation, ultrafast time-resolved spectroscopy is employed to reveal the underlying mechanism of annealing effects on the performance of P3HT:PCBM solar cell devices. The decomposed relaxation processes show that the post-annealed devices exhibit an increase in charge transfer, in the number of separated polarons and a reduction in the amount of recombination between excited carriers. Moreover, the longer lifetime for the excited carriers in post-annealed devices indicates it is more likely to be dissociated into photo-carriers and result in a larger value for photo-current, which demonstrates the underlying mechanism for the improvement of device performance. In the spin-valley coupled monolayer MoS2, carriers in the K and K’ valleys can be selectively excited by circularly polarized optical fields. The comprehensive dynamics of spin- valley coupled polarization and polarized exciton are completely resolved in this dissertation. We present a systematic study of the ultrafast dynamics of monolayer MoS2 including spin randomization, exciton dissociation, free carrier relaxation, and electron-hole recombination by helicity- and photon energy-resolved transient spectroscopy. The time constants for these processes are 60 fs, 1 ps, 25 ps, and ~300 ps, respectively. The ultrafast dynamics of spin polarization, valley population, and exciton dissociation provides the desired information about the mechanism of radiationless transitions in various applications of 2D transition metal dichalcogenides.