量子點雷射之NICE-OHMS光譜及共振腔增強原子宇稱不守恆之可行性研究

本論文主要是研究以光學共振腔架構增強線偏振旋轉方法量測原子中宇稱不守恆效應的可能性。在共振腔偏振儀架構下,我們首次提出利用雷射光與原子束非正交時造成的都卜勒頻移(Doppler shift),將線性共振腔中將原本會來回抵消的線偏振旋轉特性巧妙地轉換成可來回連續增強。這樣的共振腔架構與 NICE-OHMS(噪音自動消除共振腔與頻率解調之分子光譜儀)技術結合後便是一個擁有絕佳靈敏度且有機會到達散粒噪音基準(shot-noise limit)的偏振儀。本論文中,我們以1.28微米量子點雷射做為光源,並用精細度高達18500的光學共振腔建立了NICE-OHMS系統,這是在量測宇稱不守恆前重要的初步結果。 1.28微米是以線偏振旋轉方式量測原子宇稱不守恆中重要的波段,因它的能量剛好滿足幾種不同原子宇稱不守恆相關的磁偶極矩躍遷(magnetic dipole moment transition),例如鉛、鉈和鐿。為了產生1.1到1.3微米波段的穩定光源, 我們研究了新型量子點二極體雷射(Quantum-dot laser)在外腔式共振腔架構下的光譜特性,其中我們觀察到其因發光區域不均勻增益造成過多的殘餘殘餘自發輻射光,在實驗架設中我們加入額外光柵將其濾除因而增加進入高精細度光學共振腔的耦合效率。在NICE-OHMS中我們使用光纖耦合的相位調制器(fiber-coupled electro- optical modulator)達成頻率調制以及鎖頻,並完成800 kHz寬頻帶的穩頻電路將量子點外腔式二極體雷射成功鎖在高精細度光學共振腔上,雷射鎖頻後量得的穩定度為1 kHz(1微秒的積分時間),隨後加入頻率調制成功建立了NICE- OHMS用以量測微弱吸收訊號。作為系統靈敏度的檢驗,我們以二氮化氧在1.283 微米附近的微弱躍遷作為校正訊號,在訊噪比30以及氣壓54微托耳下,回推得最小雜訊等效吸收係數(noise-equivalent absorption coefficient)為5.3×10-10 cm-1Hz-1,其對應在一秒積分時間下的最小吸收能力為6×10-9 程度,這結果與目前其他研究群所建立的NICE-OHMS系統相比是相當的。

關鍵字

宇稱不守恆；光譜；量子點二極體；外腔式二極體雷射；光學共振腔；穩頻

並列摘要

This work is to demonstrate the feasibility of a cavity-enhanced parity non-conservation (PNC) induced optical rotation experiment. It proposes that, with a non-orthogonal atomic beam system, the reciprocal circular birefringence of PNC induced optical rotation can be cleverly converted to non-reciprocal circular birefringence and hence provide a continued increase of optical rotation in a linear cavity. Such an enhancement cavity configuration can be an ultrasensitive polarimeter based on noise-immune cavity enhanced optical heterodyne molecular spectrometer (NICE-OHMS) technique. In this thesis, a 1.28 um NICE-OHMS apparatus has been constructed for the future PNC experiment. The wavelength region of 1.28 um is particularly important for several magnetic dipole transitions of various atomic systems for PNC optical rotation measurements, such as lead, thallium, ytterbium and iodine. To access the wavelength of the 1.1-1.3 um region, the spectroscopic characteristics of a quantum-dot (QD) laser with the external cavity configuration were investigated. The residual electroluminescence, due to the inhomogeneous broadening of QD gain medium, was observed and filtered out using a grating. While a fiber-coupled electro-optical modulator (EOM) was employed, a wide-bandwidth (~800 kHz) locking of a QD-ECDL to a high-finesse (F=18500) cavity had been demonstrated using the Pound-Drever-Hall technique. The laser linewidth of the QD-ECDL has been narrowed to 1 kHz level (1 ms), then the laser was applied to NICE-OHMS for observing weak transitions. The spectrum of weak nitrous oxide transitions at 1.283 um are obtained with a signal-to-noise ratio of 30 for gas pressure of 54~mTorr. For system diagnosis, the minimum noise-equivalent absorption coefficient, alpha_{min}, 5.3*10^{-10} cm^{-1}Hz^{-1}, had been reached. That infers the minimum fractional absorption to be 6*10^{-9} at 1 s integration time, comparable to the other Doppler-broadened NICE-OHMS experiments.

並列關鍵字

NICE-OHMS ； parity non-conservation ； cavity-enhanced ； quantum-dot ； spectroscopy ； laser stabilization ； atomic physics

參考文獻

[1] T. D. Lee and C. N. Yang, “Question of parity conservation in weak interactions,” Phys. Rev. 104, 254–258 (1956).

[2] V. A. Alekseev, B. Y. Zel’dovich, and I. I. Sobel’man, “Parity non-conservation effects in atoms,” Soviet Physics Uspekhi 19, 207 (1976).

[3] M.-A. Bouchiat and C. Bouchiat, “Weak neutral currents in atomic physics,” Phys. Lett. B 48, 111 – 114 (1974).

[4] M. J. D. Macpherson, K. P. Zetie, R. B. Warrington, D. N. Stacey, and J. P. Hoare, “Precise measurement of parity nonconserving optical rotation at 876 nm in atomic bismuth,” Phys. Rev. Lett. 67, 2784–2787 (1991).

[5] D. M. Meekhof, P. Vetter, P. K. Majumder, S. K. Lamoreaux, and E. N. Fortson, “High- precision measurement of parity nonconserving optical rotation in atomic lead,” Phys. Rev. Lett. 71, 3442–3445 (1993).

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量子點雷射之NICE-OHMS光譜及共振腔增強原子宇稱不守恆之可行性研究

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