兆赫輻射波的產生與應用於釔鋇銅氧超導薄膜特性之研究

Abstract

在本論文中，我們研究利用飛秒級超快雷射光激發半導體或高溫超導體製作光致兆赫茲輻射脈衝產生源，且在國內外首次利用自由空間電光取樣技術量取高溫超導薄膜在以飛秒脈衝雷射激發下，產生之兆赫茲電磁輻射波的時域暫態波形。另外，由已建立之兆赫茲時域頻譜量測分析技術，研究氧化物薄膜與基板材料在兆赫茲波段之時域頻譜，以瞭解材料在此波段下之高頻電磁特性。 將半絕緣性之砷化鎵半導體微影蝕刻，鍍金後成為光導開關，在外加偏壓下此光導開關受雷射光激發後，由於光電流隨時間的瞬變，將產生高頻之兆赫茲輻射波，藉由利用自由空間電光取樣技術可將此高頻輻射波形完整取出，經由快速傅力葉轉換可得一輻射頻譜分佈。從改變不同的操作條件，我們已完整的研究分析利用半導體產生兆赫茲輻射。利用不同的外加偏壓、超快雷射光的能量密度與光導開關的孔隙大小，皆可獲得相同的兆赫茲輻射波形輸出。其頻率介於dc至3 THz之間。由此可知，利用光激發半導體產生高頻之兆赫茲輻射波的輻射機制在上述之操作條件下是一致的。 在偵測方式上，我們以厚度1釐米的ZnTe(110)電光晶體為偵測器來感應兆赫茲輻射訊號。作用在ZnTe晶體上之兆赫茲輻射電場強度與差分後之光電二極體所讀取到的信號有關。在我們的系統中，兆赫茲輻射與探測光沿ZnTe(110)面的法線分向傳播。隨著改變兆赫茲輻射電場與探測光的偏振方向相對於晶體的z軸之夾角時，輻射場強度大小和時域暫態波形的極性方向將會改變。當兆赫茲輻射電場與探測光的偏振方向皆垂直於晶體的z軸時，可偵測到最大的強度值。 我們完成以釔鋇銅氧高溫超導薄膜作兆赫茲輻射源的特性量測，包括改變不同雷射光能量、不同外加偏電流及不同量測溫度下之兆赫茲時域瞬變波形。另外，薄膜在超導態時，超導載子會因外受輻射光下，引起薄膜本身感應係數的變化，使得原來入射之輻射時域波形會有再成形的現象。藉由適當的轉換函數，可模擬出輻射時域波形的變化，並與實驗數據結果相符合。由結果顯示，光致超導體泰拉赫茲輻射的機制是來自於超導電流密度隨時間的調變，從這些時間解析量測所獲得的兆赫茲瞬間電場輻射波形，可以觀察到載子在非平衡狀態下的動力行為。在超快雷射光激發下，超導電流密度變化量會先減少，持續時間約為1.0 ps；接續，超導電流密度變化量開始回升，持續時間約為2.5 ps，此回復時間大小和超導能隙是相關的。 另外，我們將使用半導體材料來產生兆赫茲輻射源，並且從已建立之兆赫茲時域頻譜量測分析技術，研究氧化物薄膜與基板材料在兆赫茲波段下之高頻電磁特性。利用量測結果與理論的分析比較，可以獲到溫度與頻率相關之薄膜與基板材料之複數折射率，並進一步地求得氧化物薄膜之介電係數、光導係數、穿透深度、準粒子的散射時間等隨溫度、頻率的變化情形。

In this dissertation, three major parts are included. First, the observation of terahertz generation and detection from current-biased superconducting YBCO thin films excited by femtosecond optical pulses by using a free-space electro-optic sampling (FSEOS) technique is reported. The transient terahertz pulse was found to originate from the nonequilibrium superconductivity. Secondary, we also carried out systematic comparisons between the emissions generated with different operating parameters when using the semiconducting photoconductive switches as radiation source. Lastly, in relation to terahertz applications, we studied the terahertz frequency response in superconducting YBCO thin films by carrying out the terahertz time-domain spectroscopy (THz-TDS) measurements.

The characteristics of optically induced bipolar terahertz radiation in biased photoconductive switches and the emitted frequency spectrum distribution were found to remain unchanged even when the optical excitation fluence, strength of the biased field and the emitter gap spacing were varied. This suggests that the terahertz radiation obtained in the current setup must have originated from the same mechanism which is believed to be associated with the ultrafast charge transport process during pulsed laser illumination.

On the other hand, the efficiency of terahertz detection strongly depends on the orientation of the terahertz polarization with respect to the ZnTe crystal (001) axis and on the angle between the polarization of the probe beam and that of the terahertz radiation. These results give the optimal operating parameters for terahertz pulse detection using the ZnTe sensor crystal.

The origin of photogenerated terahertz radiation pulse emitted from current-biased superconducting YBCO thin films excited by femtosecond optical laser pulses is delineated using a FSEOS technique. Picosecond electromagnetic pulses about 450 fs wide were obtained. The frequency spectrum derived by Fourier transforming the picosecond pulses spans over 0.1-4 THz. By investigating the performances of the transient terahertz radiation generated under different operating parameters, pulse reshaping in the measured terahertz electric field caused by the kinetic inductance of the superconducting charge carriers is identified. After recovering the original waveforms of the emitted terahertz pulses, the transient supercurrent density directly correlated to the optically excited quasiparticle dynamics is obtained. A fast decreasing component of about 1.0 ps and a slower recovery process with a value of 2.5 ps are unambiguously delineated in the optically induced supercurrent modulation. The radiation mechanism of the transient terahertz pulse related to nonequilibrium superconductivity is discussed.

Finally, we have used the established coherent terahertz time-domain spectroscopy technique to investigate the properties of the electrodynamics of superconductor at terahertz frequencies. The temperature and frequency-dependent complex index of refraction of the materials, such as NdGaO3 substrate and YBCO thin films, were determined by analyzing the complex transmittance. We used these analysis to explain the variation of the amplitude and phase of pulse shape of terahertz generation in current-biased YBCO thin film a FSEOS technique, and in observing the effect of pulse reshaping of terahertz transmission in YBCO thin films detected in the THz-TDS measurements. The temperature dependence of the real component of complex conductivity revealed a large broad peak over the whole measured terahertz range. It is attributed to a competition between an increase in the normal carrier relaxation time and a decrease in the number of normal carriers with decreasing temperature below Tc. By fitting the measured complex optical conductivity with the aid of a two-fluid model, we were able to extract the temperature dependent of the London penetration depth and the temperature dependence of quasiparticle scattering rate