半絕緣砷化镓與砷離子佈植砷化镓天線兆赫茲輻射實驗與模擬分析

Abstract

在此所呈現的工作之主要焦點在對於不同孔徑大小的偏壓光導天線的實驗分析和由光激發產生兆赫輻射脈衝之數值與解釋。孔徑大小1.5 公分的天線被定義為大孔徑天線。孔徑大小從 20微米到 0.5 公釐範圍的天線則是被區分為中型孔徑天線。製造天線所使用的光導材料是砷離子佈植的砷化鎵 （GaAs:As+) 和使用半絕緣的砷化鎵（SI-GaAs）。 理論上，我們採用描述電磁波傳輸的一組非線性偏微分方程式，包含了電磁波、 載子的漂移－擴散，和波頌方程式來解釋測量到的資料。這一個模型考慮到發生在兆赫天線內的空間－電荷電場，以及近場屛避電場的效應。在中型孔徑天線中，前者扮演了一個重要的角色，而後者在大孔徑天線中主要決定其特性。 在實驗上，我們從不同觀點分析這些測量到的兆赫輻射脈衝波形，包括尖峰峰值，尖峰寬度，負與正峰值的比，以及頻寬。我們發現來自中型孔徑天線中，測量到的兆赫脈衝的特性顯著地仰賴孔徑大小和光導材料。我們可以從我們的理論模擬推論，測量到的孔徑相依的行為源自於較大孔徑的天線在其陽極的附近有一個因陷阱而提高的偏壓電場是較強的，因此引起較大的空間－電荷電場屛避的效應，以及較大的頻寬。對於大孔徑天線的實驗結果來說，我們的砷離子佈植砷化鎵天線比半絕緣砷化鎵天線展現了更大的頻寬和更好的發射效率。由已知的實驗觀察可得知，砷離子佈植砷化鎵天線的光學吸收係數在某些條件下，可以是大於半絕緣砷化鎵天線的光學吸收係數，我們將此假設代入理論模型中，由模擬結果我們成功地驗證了砷離子佈植砷化鎵天線的優越特性確實能歸因於在離子佈植層中比起半絕緣砷化鎵有較大的吸收係數。對於兩者類型的材料，我們觀察到輻射出來的尖峰兆赫振幅顯示對於泵光辉度有一個異常的相關性，這相關性偏離了縮放比例規則所給的預測。分析那理論上的和模擬結果，我們推論出這種行為起因於帶填充和雙光子的吸收效果的發生。此外，在特定的泵光辉度之下，尖峰兆赫振幅對於偏壓電場的相關性，偏離了從被縮放比例規則預測的線性關係。

The work presented here is focused on the experimental analysis and numerical explanation of the terahertz radiation pulses from biased photoconductive antennas with different gap sizes. The antenna with gap sizes of 1.5 cm is divided into large-aperture antennas. The antennas with gap sizes ranging from to 0.5 mm are divided into mid-size-gap antennas. The photoconductive materials fabricated on the large-aperture antennas are arsenic-ion-implanted GaAs (GaAs:As+) and semi-insulating GaAs (SI-GaAs). The photoconductive material fabricated on the mid-size-gap antennas is multi-energy arsenic-ion-implanted GaAs (GaAs:As+). Theoretically, we adopt a set of nonlinear partial differential equations including an electromagnetic wave, drift-diffusion, and Poisson equations to interpret the measured data. This model considers the space-charge field and near-field screening effects within the THz antenna. The former plays an important role in the mid-size-gap antennas, whereas the latter is dominant in the large-aperture antennas. Experimentally, we analyze these measured terahertz radiation waveforms from different point of views, including the peak terahertz amplitude, the peak width, the ratio of negative peak to positive peak, and bandwidth. We find that the characteristics of measured terahertz pulses from the mid-size-gap antennas depend markedly on both the gap size and photoconductive material. We deduce from our simulation that the gap-dependent behavior stems from the fact that an antenna with larger gap has a stronger trap-enhanced bias field near the anode edge and thereby induces larger space-charge field screening effect and bandwidth. For the large-aperture antennas, our single-energy arsenic-ion-implanted GaAs antenna exhibits larger bandwidth and better emission efficiency in comparison with semi-insulating GaAs antenna. Our simulation verifies that the superior characteristics for the latter can be partly attributed to larger optical absorption in the ion-implanted layer. For both types of materials, we observe that the radiated peak terahertz amplitude displays an anomalous dependence on pump fluence, which deviates from the prediction given by the scaling rule. Analyzing the theoretical and simulation results, we infer that this behavior arises from band filling and two-photon absorption effects. Besides, at specific pump fluence, the dependence of peak terahertz amplitude on bias field is also distinct from the usual linear relationship predicted by the scaling rule.