以飛秒雷射建構奈米音波系統:原理及應用

Abstract

藉由壓電效應與超快光譜技術，奈米結構的壓電半導體可當成光學壓電換能器 (optical piezoelectric transducer) 產生與偵測奈米音波。利用光壓電換能器，我們研究奈米超音波學 (nano-ultrasonics) 中的物理問題及科技應用。本論文中，我們討論光學壓電換能器中奈米音波的產生與偵測原理，並用實驗方法驗證理論模型。在理論與光學技術的基礎下，我們用聲波工程的觀點去探討壓電換能器的設計原理及特性。我們討論了幾個議題，如聲波偵測器、窄頻與寬頻聲波產生器、可調頻聲波換能器。

利用聲米音波系統，我們在論文中示範了幾個創新的應用。首先，我們發現在奈米音波以兆赫 (THz) 頻段調制能帶時，電子與電洞的能量分佈也以兆赫頻率跟著被調制。這個研究顯示未來利用奈米音波控制電子元件的可行性。再者，我們將奈米音波應用在奈米超音波影像科技。在我們的一維奈米音波掃瞄量測技術中，精確度可小於一奈米。而在量測樣品之二維結構的實驗中，我們證明此技術的縱向解析度可以小於二十奈米。若對二維影像做進一步處理，橫向解析度可小於一百奈米。實驗上，我們證明這個創新的奈米超音波技術擁有非破壞性、三維解析能力及奈米解析度的優點，這些優點是目前其他影像技術所無法同時擁有的。為了不需後處理技術而直接取得三維奈米音波影像，我們示範如何產生奈米大小的聲波源。利用設計過的二維光場分佈在光學壓電換能器中預先激發電子電洞，我們可以產生不受光學繞射極限所限制的奈米聲波源。

論文中，我們也示範如何用奈米音波系統來研究奈米音波元件特性。利用單量子井結構的光學壓電換能器，我們直接在時間上量測兆赫頻段的奈米音波波形。聲波元件中，聲子奈米晶體在可當作奈米音波反射鏡。利用這個方法，我們在實驗上直接量得聲子奈米晶體的反射轉換函數 (transfer function)。此外，我們也研究能隙 (bandgap) 音波在聲子奈米晶體中能量傳播的行為。

除了異質結構的壓電半導體，我們示範在P型與N型接面的空乏區(depletion region) 中，也可以透過壓電效應產生奈米音波脈衝。由於外在電場可控制空乏區大小，而音波脈衝的波形是由空乏區所決定，這意味著未來結合電子系統控制音波脈衝波形的可能性。此研究中，我們另一方面也直接用實驗證明在壓電半導體中，壓電效應是產生音波最有效率的方法。

Piezoelectric semiconductor nanostructures can be treated as optical piezoelectric transducers to generate and detect acoustic nanowaves through the piezoelectric effects by femtoseocond pump-probe techniques. With the optical piezoelectric transducer, several physical issues and applications in nano-ultrasonics can be investigated. In this thesis, the theories of generation and detection of acoustic nanowaves in the optical piezoelectric transducers have been presented, and verified by experiments. With the theories and optical techniques, the design principles and characterization methods of the optical piezoelectric transducers have been illustrated from the viewpoint of acoustic engineering. We have discussed several topics such as acoustic sensors, narrowband and broadband acoustic wave generator, and frequency-tunable acoustic transducers.

Based on the nano-acoustic system, several novel applications have been demonstrated in this thesis. Firstly, we have found that the energy distributions of electrons and holes can follow the band modulation due to the acoustic nanowaves in THz regime. These investigations imply the feasibility to utilize acoustic nanowaves for controlling electronic devices. Secondly, we have demonstrated that acoustic nanowaves can be utilized for ultrasonic imaging. The accuracy of one-dimensional ultrasonic scan measurement is less than one nanometer. For resolving a two-dimensional subsurface structure, the axial resolution is less than 20 nm while the transverse resolution can be less than 100 nm if the images are post-processed. We have demonstrated that the novel nano-ultrasonic technology has the advantages of non-destructive measurement, three-dimensional imaging capability, and nanometer resolutions, of which another state-of-the-art imaging technology does not possess all. Thirdly, to directly obtain a three-dimensional ultrasonic image with nanometer resolutions, the technique of generating acoustic nanospots has been demonstrated. With beam shaped optical pulses for exciting saturation carriers, the acoustic spot size can be smaller than the optical spot size, which is on the micron scale due to the diffraction limit.

On the other hand, we have also used the nano-acoustic system to characterize nanowave devices. By using a single-quantum-well structure optical piezoelectric transducer, the waveforms of sub-THz or THz acoustic nanowaves can be directly measured in the time domain. We have used this method to experimentally obtain the reflection transfer function of a phononic bandgap nano-crystal, which can be served as a mirror for acoustic nanowaves. Moreover, we have investigated the energy propagation of bandgap waves in the phononic bandgap nano-crystal.

Besides the piezoelectric semiconductor heterostructures, we have also demonstrated that acoustic nano-pulses can be generated in the depletion region of a p-n junction through the piezoelectric effects. The shape and width of the acoustic nano-pulses are determined by the depletion region, which can be controlled by an external bias. This interesting demonstration has shown the possibility to integrate electronic technologies for controlling acoustic nanopulses. In this study, the high optoacoustic conversion efficiency of the piezoelectric effects has also been experimentally confirmed in piezoelectric semiconductors.