臺灣博碩士論文加值系統

我們利用磁光陷阱產生被冷卻及捕捉的低溫銣原子，再以光譜量測的方法研究低溫銣原子的同調性導致現象，所研究的現象包括電磁波導致透明(Electromagnetically induced transparency, 簡稱EIT)、受激拉曼躍遷(stimulated Raman transitions, 簡稱SRT)、反衝導致共振(recoil induced resonances, 簡稱RIR)、電磁波導致吸收(Electromagnetically induced absorption, 簡稱EIA)、及交互作用的暗共振(Interacting dark resonance, 簡稱IDR)。
電磁波導致透明是在一強耦合雷射光的作用下，原本對一弱探測光會吸收的原子會變得不吸收的現象，其原因是強耦合雷射光的存在使探測光的吸收多了很多可能的躍遷路徑，並且這些路徑間產生破壞性干涉而使吸收減小。通常電磁波導致透明只在三能階系統探討，但真實的原子包含了許多簡併的黎曼能階，我們研究簡併的黎曼能階對電磁波導致透明的影響，我們的研究表明當考慮簡併的黎曼能階後，雷射光的偏極及原子的能階組合須恰當選擇否則電磁波導致透明的效應會顯著變差。在一個特別挑選的能階系統下我們得到很好的電磁波導致透明光譜，其線寬可低於100kHz。在此系統中，我們研究磁場對光譜的影響，並且也利用電磁波導致透明現象將光的群速度減慢至每秒600公尺。
我們系統性的研究了不同偏極組態下低溫原子的激發-探測光譜，不同的偏極組態會導致非常不同的光譜特徵，這些光譜結構均小於激發態的自然線寬，但其光譜線寬、形狀和對激發雷射的頻率之相依關係差益很大。我們的研究釐清了產生這些不同特徵的光譜背後的物理機制，包含了受激拉曼躍遷、反衝導致共振及二波混合(Two-wave mixing)。
電磁波導致吸收是原子在一強耦合雷射光的作用下，會使一弱探測光的吸收加強的現象，其原因是因耦合光造成在激發態黎曼能階間的同調性經由自發輻射而轉移至基態的黎曼能階而造成。我們所得到低溫原子的電磁波導致吸收光譜線寬可低於100kHz，並且理論跟實驗光譜很一致。我們也指出電磁波導致吸收光譜可用於精密的磁場量測，並且設計了一實驗用以驗證電磁波導致吸收現象的成因。
交互作用的暗共振是電磁波導致透明效應的推廣，在磁波導致透明的三能階系統中加入一微波驅動第四個能階及強耦合光所驅動的基態，理論預測在此系統中兩個暗共振間的干涉將會在電磁波導致透明的光譜中間產生一極窄的吸收峰，我們首次在實驗上實現此預測，這個實現將開啟操控原子反應的更多可能性。
我們也提出並驗證了一個可以直接及準確地量測磁光陷阱中的原子數的簡單方法，此方法所決定的原子數不決定於雷射的偏極、強度、頻率及線寬以及其它系統參數如環境磁場等。
另外，我們也演示了此方法可應用於同調性居數侷限(Coherent population trapping, 簡稱CPT)的研究，它將可擴展CPT的研究範疇。

This thesis reports our studies on the quantum coherence and interference induced phenomena in cold 87Rb atoms The cold atoms are produced with a magneto-optical trap (MOT). The phenomena include electromagnetically induced transparency (EIT), stimulated Raman transitions (SRT), recoil-induced resonances (RIR), electromagnetically induced absorption (EIA), and interacting dark resonances (IDR).
EIT is a significant reduction of absorption experienced by a weak probe field, due to the presence of a strong coupling field on a linked transition. Destructive interference of probe absorption among different transition pathways results in the transparency. EIT is usually modeled in a three-level system. However, multi-Zeeman levels are usually involved in real atomic systems. We study the role of degenerate Zeeman levels in EIT. We demonstrate that improper choused energy-levels scheme may degrade the performance of EIT. With a better EIT scheme, we obtain very narrow EIT spectra with linewidths below than 100 kHz. We also demonstrate the slow light experiment using EIT and reduce the group velocity of light to 600 m/s.
We systematically study pump-probe spectroscopy of cold 87Rb atoms. The pump-probe spectra are measured without the presence of the trapping beams or any optical molasses. Various polarization configurations of the probe and pump fields result in very different spectra of probe absorption. The observed spectra exhibit a dispersive profile, a dispersionlike profile, a Lorentzian profile, or a dispersive and a Lorentzian profiles. The widths of all the spectral profiles are narrower than the natural linewidth of the excited state. Our work clarifies the mechanisms behind these different spectral profiles and provides essential information for the pump-probe spectroscopy of cold atoms. The mechanisms involved stimulated Raman transitions, recoil-induced resonances, and two-wave mixing.
EIA is a phenomenon that absorption of a weak probe field is enhanced by the presence of a coupling field. The physical origin of EIA is the spontaneous transfer of the light-induced coherence among degenerate Zeeman levels of excited states to those of ground states. We systematically study the EIA spectra in cold 87Rb atoms. The measured EIA linewidth can be as narrow as 100 kHz. Our work demonstrates the EIA spectrum can be applied to precision detection of magnetic fields. We also perform an experiment to support that spontaneous coherence transfer is an essential process in generating EIA phenomenon.
IDR is a phenomenon in a four-level system. The system is based on three-level -type EIT system with an additional microwave driving the magnetic dipole transition of the fourth level to the ground state that is also drove by the coupling field. Constructive interference between two dark resonances in the system produce the spectrum of a sharp and high-contrast absorption peak emerging inside the narrow EIT transparency window. We report the first experimental observation of the IDR spectrum. The success of this experiment opens more possibilities in manipulating the atomic response.
We propose and demonstrate a simple technique that accurately determines number of atoms in a magneto-optical trap. Absorption energy of a laser field that interacts with cold atoms is a direct measurement of atom number. The measured energy neither depends on the detuning, intensity, and polarization of the laser field nor on other system parameters. Our work also demonstrates that such technique can be applied to and diversify the study of coherent population trapping (CPT).

CHAPTER 1. INTRODUCTION 3
1.1 Laser cooling and trapping of atoms 3
1.2 Coherence-induced phenomena 4
1.3 Theoretical basics 5
1.4 Atomic properties of 87Rb atom 7
1.5 Brief history of our experiments 10
1.6 Overview of this thesis 11
CHAPTER 2. EXPERIMENTAL SETUP 12
2.1 Lasers and their frequencies control 12
2.2 Magneto-optical trap 13
2.3 Timing control and spectroscopic measurement 13
2.4 Magnetic field control 15
2.5 Microwave spectroscopy 16
CHAPTER 3. ELECTROMAGNETICALLY INDUCED TRANSPARENCY 19
3.1 Introduction of EIT 19
3.2 Effect of Zeeman degeneracy on EIT 24
3.3 Theoretical calculations of EIT spectra in multi-Zeeman levels systems 32
3.4 The Zeeman states EIT 35
3.5 EIT with Zeeman shift 37
3.6 Slow light experiment 42
CHAPTER 4. PUMP-PROBE SPECTROSCOPY FOR VARIOUS POLARIZATION CONFIGURATIONS 46
4.1 Introduction 46
4.2 Experimental setup 48
4.3 Theoretical calculations 50
4.3.1 The Zeeman-state calculation 50
4.3.2 The momentum-state calculation 52
4.4 Experimental spectra and discussions 52
4.4.1 lin∥lin 54
4.4.2 lin ⊥ lin 57
4.4.3 σ∥lin 60
4.4.4 σ⊥ lin 63
4.4.5 Others 65
4.5 Conclusion 67
CHAPTER 5. ELECTROMAGNETICALLY INDUCED ABSORPTION 69
5.1 Introduction 69
5.2 Experimental results and discussions 72
5.3 Theoretical calculation 76
5.4 Spontaneous coherence transfer and EIA 78
CHAPTER 6. INTERACTING DARK RESONANCE 80
6.1 Introduction 80
6.3 Theoretical treatment of IDR 82
6.4 Experimental results and discussions 85
CHAPTER 7. SIMPLE TECHNIQUE FOR DIRECTLY AND ACCURATELY MEASURING NUMBER OF ATOMS IN A MAGNETO-OPTICAL TRAP 91
7.1 Introduction of methods to measure atom number 91
7.2 Basic principle of the optical pumping method 94
7.3 Experimental results and discussions 95
7.4 Applications to the study of coherent population trapping 99
CHAPTER 8. CONCLUSIONS AND PROSPECTS 102

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