利用飛秒脈衝形變技術研究光導天線輻射兆赫波之特性

Abstract

本論文中，我們探討了鎖模光脈衝中的啾頻對材料之影響進而了解在產生兆赫輻射中其對應角色。實驗上藉由整合脈衝整形術與泵探技術之啾頻控制泵探實驗，我們觀察到當啾頻量由負啾頻向正啾頻增加，載子鬆弛時間隨之變大，並透過頻率相關光載子剩餘動能行為解釋；此外，透過結合脈衝整形術與光導天線產生兆赫輻射實驗中，除了再次確認入射脈衝啾頻調變對兆赫輻射在啾頻符號對應輻射大小不對稱與正啾頻入射條件較佳輻射效果外，我們首度報導了光導天線在高激發密度下可觀察到時域訊號極大值與激發密度異常關係，此原因歸咎於雙光子吸收效應，該效應一開始雖為負貢獻，但當單光子吸收飽和時，價電子可透過雙光子吸收躍遷至導帶，天線輻射能量持續增加，且透過雙光子能帶可突破激發飽和限制產生高頻兆赫輻射，大幅突破目前光導天線頻譜極大值位在一兆赫之限制，此結果暗示使用倍頻光源激發光導天線在產生高頻兆赫輻射上之極大潛力。另外我們透過Drude-Lorentz模型與啾頻泵探量測所獲得之載子生命期，分析並解釋實驗現象，確立兆赫時析光譜系統理想天線標準與探討啾頻在取樣過程之影響，實驗上雖透過啾頻控制增輻時域訊號可提供訊號值增強，兆赫波頻寬卻將因低頻增強主導但高頻抑制相對變窄，故仍應利用材料成長條件調控載子鬆弛時間與動量時間；天線端為產生高頻寬兆赫光源需使用轉換極限脈衝激發天線；輻射偵測端為精準取樣兆赫波形與頻譜，建議使用短脈衝取樣並選用短載子鬆弛時間與短動量時間材料製作天線。

In this thesis, role of chirp properties within mode locked pulses to generate efficiency and broadband THz radiation is investigated. By integration of pulse shaping technique and pump-probe measurement, behavior of increasing carrier relaxation time with chirp and positive sign is observed and explained by frequency dependent excess energy. Meanwhile, chirp- and power- dependence of THz radiation are studied using chirp controlled PC antenna based THz radiation system. We confirm the nature of asymmetry and positive preference of THz radiation for mode locked chirp pulse. We further observe an anomalous and nonlinear increase of THz radiation with increasing power of transform-limited and tight focusing exciting pulse. This could be due to distribution of carries from two photon absorption and is the first experimental report of two photon absorption induced THz radiation enhancement which have been predicted theoretically. In addition, we analyze phenomenon observed by Drude-Lorentz model with obtained chirp- and power- dependent carrier lifetime and influence of chirp within sampling. Finally, we suggest possible approaches for generating high efficiency and broadband THz radiation：(1) positive chirp exciting can enhance radiation power but depress the high frequency part. (2) transform-limited exciting pulse can provide broader spectrum and further enhance power under highly tight focusing condition due to TPA contribution. (3) antenna with short carrier and momentum lifetime exciting by transform-limited pulse to decrease influence of PC sampling.