開發具寬頻高效率之氮化鈮超導單光子偵測器: 共振可調之隙電漿子

Abstract

高靈敏度光子偵測器在光學通訊、光譜分析、生物醫學影像學、量子計算等領域具有廣泛的應用。其中，超導奈米線單光子偵測器(SNSPD)因其良好的偵測效率、極低的暗計數和極短的時間抖動而受到科學家們青睞。然而，奈米線由於偵測面積較小，需要較長且曲折的複雜結構來增加偵測面積，導致動態電感增加，恢復時間增長，限制了其快速量測下一次事件的能力。此外，大部分SNSPD僅限於通訊波段(1550 nm)，對於可見光範圍的研究相對較少。因此，為了克服動態電感的限制，我們致力於開發超導微米線單光子偵測器(SMSPD)。微米線具有較大的偵測面積和較小的動態電感，但其偵測效率不及奈米線。因此，本研究在微米線上引入奈米結構，使其產生隙電漿子共振，以提高其偵測效率，使超導微米線單光子偵測器成為一個更具競爭力的選擇。 在本研究中，我們選擇氮化鈮(NbN)作為超導單光子偵測器的材料。NbN是一種已知的超導材料，也是難熔電漿子材料，在低溫時仍保持超導狀態並具有激發電漿子模態的能力。通過引入隙電漿子共振結構，我們有效增強了微弱光子信號與NbN超導微米線之間的交互作用，從而提高其偵測效率。本實驗室已經透過隙電漿子共振增強氮化鈮超導單光子偵測器使波長532 nm的偵測效率從68%增加至98%，我們仍致力於開發寬頻和偏振敏感的超導單光子偵測器，然而銀奈米立方無法共振在藍光波段(450 nm)，所以我利用鋁奈米立方作為隙電漿子的共振腔使其在可見光波段有寬頻共振，利用有限時域差分法(FDTD)模擬不同形狀大小的鋁奈米立方結構，使其在特定波長、偏振與NbN發生隙電漿子共振，進而增強光子電場，使其提高偵測效率。寬頻SMSPD在量子通信和量子計算中，能夠在不同波長下檢測單光子，非常適合量子密鑰分發（QKD）和量子糾纏光子的檢測。此外，在生物醫學成像中，寬頻SMSPD能夠提高螢光壽命成像和單分子檢測的分辨率和靈敏度。在天文觀測中，這些探測器可以檢測來自遙遠天體的微弱光子信號，適用於高精度的天文觀測。同時，寬頻SMSPD在光譜分析中可提供高分辨率和高靈敏度的數據，應用於材料科學和化學分析。在高能物理實驗中，寬頻SMSPD也可用於檢測高能粒子與物質相互作用產生的光子，適合於粒子軌跡重建和能量測量。 在製程方面，我們利用超高真空射頻磁控濺鍍機在氧化鎂(MgO)基板上成長出15 nm的NbN薄膜，並通過橢圓偏振儀、原子力顯微鏡和超導量子干涉元件磁量儀來確認樣品的品質。在確定了NbN薄膜品質後，我們利用雷射直寫光刻機和反應式離子蝕刻機製備微米線結構，並通過原子層沉積法成長出5 nm的氧化鋁(Al2O3)絕緣層。最後，通過電子束微影並用熱蒸鍍機蒸鍍出鋁(Al)膜後進行掀離處理在微米線上製造奈米結構，完成樣品製備。在量測方面，我們將樣品放入低溫腔體中進行量測，通過引入可見光脈衝雷射作為光子源，並通過連接電路到示波器上觀察不同波長和光強的光信號，我們量測了波長450 nm、515 nm和640 nm以及波長532 nm的偏振敏感性的光子偵測效率，並比較了沒有奈米結構的SMSPD，最後通過MATLAB程式進行偵測效率分析。分析結果為有奈米結構的SMSPD在波長450 nm的偵測效率為96.5%、波長515 nm的偵測效率為94.4%、波長640 nm的偵測效率為98.3%，與沒有奈米結構的SMSPD相比光子敏感度增加約6.7倍、4倍以及8倍，而波長532 nm的偏振敏感性從1.1提升至10.1，以上可以證明利用奈米結構所產生的隙電漿子共振會提升SMSPD的光子敏感度。 最後，我們討論了SMSPD的未來發展方向。我們將整合光子源、波導和超導偵測器，在單一晶片上實現積體光路，以最大化超導單光子偵測器的效率。未來將進一步擴展到通訊波段的超導單光子偵測器，並應用於生物醫學、量子計算和高能粒子物理等領域。

High sensitivity photon detectors have wide applications in optical communications, spectroscopy, biomedical imaging, quantum computing, and other fields. Among them, superconducting nanowire single-photon detectors (SNSPDs) are favored by scientists due to their excellent detection efficiency, extremely low dark counts, and very short timing jitter. However, due to the small detection area of nanowires, complex, long, and convoluted structures are required to increase the detection area, leading to increased dynamic inductance, longer recovery times, and limitations in rapidly measuring subsequent events. Additionally, most SNSPDs are limited to the communication wavelength band (1550 nm), with relatively little research on the visible light range. Therefore, to overcome the limitations of kinetic inductance, we are dedicated to developing superconducting microwire single-photon detectors (SMSPDs). A microwire has a larger detection area and smaller dynamic inductance, but their detection efficiency is not as good as nanowires. Hence, in this study, we fabricated nanostructures on microwires to induce gap plasmon resonance, thereby improving detection efficiency and making SMSPDs a more competitive option. In this study, we selected niobium nitride (NbN) as the material for SMSPDs. NbN is a known superconducting material and a refractory plasmonic material that maintains superconductivity at low temperatures and has the ability to excite plasmon modes. By introducing a gap plasmon resonance structure, we effectively enhanced the interaction between weak photon signals and NbN superconducting microwires, thereby improving their detection efficiency. Our laboratory has successfully improved the single-photon detection efficiency of SMSPDs at a wavelength of 532 nm from 68% to 98%. To developing broadband and polarization-sensitive SMSPDs, We use the finite-difference time-domain (FDTD) to optimize the gap plasmon resonance of aluminum nanocubes and NbN at specific wavelengths and polarizations, promise the excellent performance detection efficiency. However, since silver nanocubes cannot resonate in the blue light (450 nm), I utilized aluminum nanocubes as the gap plasmon resonance cavity to achieve broadband resonance in the visible light range. Broadband SMSPDs perform excellently in quantum communication and quantum computing. They can detect single photons at different wavelengths, making them ideal for quantum key distribution (QKD) and quantum entangled photon pair detection. Additionally, in biomedical imaging, broadband SMSPDs can improve the resolution and sensitivity of fluorescence lifetime imaging and single-molecule detection. For high-precision observational astronomy, these detectors can detect weak photon signals from distant celestial bodies. Moreover, broadband SMSPDs can provide high-resolution and high-sensitivity data in spectroscopy, applicable in materials science and chemical analysis. In high-energy physics experiments, broadband SMSPDs can also be used to detect photons generated by the interaction of high-energy particles with matter, suitable for particle trajectory reconstruction and energy measurement. In sample preparation, we used an ultra-high vacuum RF magnetron sputtering machine to grow a 15 nm-NbN film on a magnesium oxide (MgO) substrate and confirmed the quality of the sample through ellipsometry, atomic force microscopy, and superconducting quantum interference device. After determining the quality of the NbN film, we used direct writing lithography and a reactive ion etching machine to fabricate the microwire structure and grew a 5 nm aluminum oxide (Al2O3) as a insulating layer through atomic layer deposition. Finally, we fabricated nanostructures on the microwires by electron beam lithography and thermal evaporation of an aluminum (Al) film followed by a lift-off process to complete sample preparation. To determine the detectivity, we cooled the SMSPD in a optical cryostat, illuminate it with pulsed visible laser, and measured the photon-induced electric signal at different wavelengths and intensities by connecting the circuit to an oscilloscope. We also discussed the polarization sensitivity at wavelengths of 450 nm, 515 nm, 532 nm , and 640 nm, and compared it with SMSPDs without nanostructures. Finally, we determined the detection efficiency by a MATLAB program. The analysis results show that the detection efficiency of the SMSPD with nanostructures is 96.45% at a wavelength of 450 nm, 94.39% at 515 nm, and 98.31% at 640 nm. Compared to the SMSPD without nanostructures, the photon sensitivity increases approximately 6.66 times, 4 times, and 8 times, respectively. Additionally, the polarization sensitivity at a wavelength of 532 nm increases from 1.05 to 10.06. This demonstrates that the plasmonic resonance generated by the nanostructures enhances the photon sensitivity of the SMSPD. In the end, we discussed the potential of SMSPDs. We plan to integrate the photon emitters, waveguides, and superconducting detectors on a single chip to maximize the efficiency of superconducting single-photon detectors. Future work will further extend to superconducting single-photon detectors in the communication wavelength band and applications in biomedical, quantum computing, and high-energy particle physics.