激發-探测光譜顯微術應用於半導體奈米結構及生物樣本

Abstract

激發-探測光譜顯微術結合了遠場光學顯微鏡的成像模式和超快光譜的時間分辨特性，已成爲研究電子和振動動力學的有力技術，在材料科學和生醫影像的應用中日益廣泛。這促使發展激發-探測技術以解析相應的時域及光譜特性。目前激發-探測中的時間分辨特性，主要用以研究載子動力學特性，對超快非線性在時域中的行爲研究較少。至於激發-探測技術中來探究振動特性，由於電子躍遷效率低，其靈敏度往往不足。

本研究在激發-探測光譜學的範疇下，主要拓展兩個研究方向，即探究半導體奈米結構中的超快載子動力學，以探討瞬態非線性行爲，並透過電子預共振效應發展高靈敏度之觀測方法，用以觀測生醫樣品內的振動特性。

本研究的第一部分，利用共軛焦顯微鏡搭配激發-探測的時間分辨特性，對矽奈米結構中的超快載子動力學進行檢測，並揭示了一種基於俄歇機制的瞬態非線性行爲。以往的研究主要集中在多光子吸收、四波混頻等瞬時非線性行爲，即在脈衝持續時間內發生的事件。此研究則展示了瞬態非線性具有非常規的特性，即在非激發-探測光之時間零點，仍具有非線性行為，呈現了各種非線性行爲之間的時間演化，包括次線性、全飽和、超線性響應。論文的第一部分主要對基於非線性載子動力學引起的瞬態非線性進行了檢測和應用。

在第二部分，我們著重於發展先進的超連續光源來獲取分子的振動信息，此信息提供了化學鍵的成像對比。激發-探測光譜顯微術能夠在不對樣品進行螢光標定的情況下，仍可對化學和生物樣品進行振動特性定位。然而，儘管激發-探測技術具有化學分辨性，目前的靈敏度仍然遠遠落後於螢光技術，源自於雷射 的激發能量遠小於激發態與基態的能階差，不能有效地促進振動能階之間的電子躍遷。而發展激發-探測光譜顯微術中的先進光源，有助於優化電子預共振效應，即透過控制激發光之能量接近至樣本之激發能階來增強振動信號。

在本論文所述的研究中，我們開發之激發-探測光譜顯微術用以檢測半導體奈米結構的時域特性以及解析生醫樣品的光譜特性。藉由非線性載子複合過程引起的瞬態非線性之研究，為操縱非線性行爲增加了新的自由度，即藉由時域進行調變。另一方面，對電子預共振效應的研究爲高靈敏度振動譜測量和生醫成像提供了新的思路。

Pump-probe spectro-microscopy combining the imaging modality of far-field optical microscopy with the time-resolved property of ultrafast spectroscopy has become a valuable technique for studying electronic and vibrational dynamics toward the growing applications in material science and biomedical imaging. The development of pump-probe techniques to resolve temporal or spectral features is highly desirable. However, current time-resolved spectroscopy mainly focuses on carrier dynamics, and few reports address the ultrafast nonlinear optical behaviors in the temporal domain. As for vibrational properties in pump-probe techniques, they usually suffer from insufficient sensitivity due to low-efficiency electronic transition.

In this study, under the scope of pump-probe spectroscopy, two major aspects are expanded, which are (A) characterizing the ultrafast carrier dynamics in semiconductors nanostructures to explore transient nonlinear behaviors and (B) investigating the vibrational properties of biomedical samples with highly sensitive detection through electronic pre-resonance effects.

In the first part of the research, ultrafast carrier dynamics of silicon nanostructures are characterized based on a confocal time-resolved spectro-microscope, revealing an unusual transient nonlinear behavior based on Auger mechanisms. Most previous reports mainly focus on instantaneous nonlinear behaviors such as multi-photon absorption and four-wave mixing; that is, the nonlinearities occur within pulses duration. Here, we demonstrate transient nonlinearity featuring an unconventional off-time-zero property and presenting temporal evolution among various nonlinear behaviors, including sub-linear, full-saturation, and super-linear responses. The first part is devoted to the characterization and application of transient nonlinearity based on nonlinear carrier dynamics.

In the second part, we focus on the development of an advanced supercontinuum light source to acquire the vibrational information of molecules, offering the contrast on chemical bonds. Pump-probe spectro-microscopy is capable to perform vibrational mapping of chemicals and biological samples without fluorescence labeling. However, despite the chemical specificity, the sensitivity is still far behind those in fluorescence techniques. This is attributed to the far-off resonance of the laser excitation, not efficiently promoting electronic transition among vibration levels. The development of the advanced light source in pump-probe spectro-microscopy is crucial in optimizing the electronic pre-resonance effects, where vibrational signals are significantly enhanced via tuning laser excitation close to the excited energy levels.

In the research described herein, our developed pump-probe spectro-microscopy has been utilized to characterize the temporal and spectral features in semiconductor nanostructures and biomedical samples, respectively. The study of transient nonlinearity induced by the nonlinear recombination process adds new degrees of freedom in temporally manipulating nonlinear behaviors. On the other hand, the research on electronic pre-resonance effects sheds light on high-sensitive vibrational spectroscopic measurement and biomedical imaging.