The Coherence-Induced Phenomena in Cold Atoms

• Main Authors: Ying-Cheng Chen, 陳應誠
• Other Authors: Ite Albert Yu
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/71733352222848939491

博士 === 國立清華大學 === 物理學系 === 90 === 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).