Experimental Study Of Photoassociation Spectroscopy And Superradiance Based On ⁸⁷Rb Bose-Einstein Condensate

The 20 th century saw a multitude of progress in every field of science and technology,especially in theoretical and experimental physics.One of the progresses in the field of atomic,molecular and optical physics was the prediction and the realization of the BoseEinstein condensation of atomic gas at very low temperatures.The 87 Rb BEC was the first to be realized in 1995 and many other atomic species were condensed shortly.The field of BEC later branched into several directions/disciplines while the BEC was acting as a starting point.These disciplines use the BEC to simulate complex physical systems in the laboratory with exceptional control over the tuning of the system parameters that would be difficult or sometimes impossible in the real world systems.This thesis also presents a series of such experiments which do not fall under a single specialty of physics but all are performed in the 87 Rb BEC as a starting point and thus qualify under the umbrella of ultra-cold atomic physics.In the starting two chapters,several theoretical and experimental topics(relevant to the field of BEC in general and to our laboratory setup in special)are discussed for the sake of completeness and then the three experiments performed as part of my Ph D work are discussed in the remaining chapters.The necessary electronic circuit sketches and optical beam setups are listed in the appendix A in the end of the thesis.The first of these experiments is the realization of the N-type system in the BEC by the application of three lasers to the BEC.Two of the lasers are phase locked by the optical phase lock loop system while the third one is locked independently and can still introduce and maintain the coherent effects in the 87 Rb BEC.The N-type system is studied under the absorption imaging detection technique and all of the band structure/absorption spectral features are successfully retrieved using this method.This study serves as an important step in the use of BEC in applications like quantum information,atomic coherent experiments and the quantum memory.The behavior of the N-type system in the BEC under different driving and coupling laser detuning and intensities is both theoretically(using the density matrix method)and experimentally studied and its features are discussed.A very good qualitative agreement of the theoretical and experimental data shows the power of absorption imaging as a probing tool and also of the BEC as an ideal platform for such atomic coherence experiments.The second experiment is related to the formation of the ⁸⁷Rb2 excited-state nearthreshold molecules using the 52P1/2 level as the threshold point,a process called photoassociation.This is done by shining a single laser tuned below and above the two excited hyperfine states of the said level.In our study,more than 40 quantized excited states are created and detected using two detection methods.The 87 Rb BECs prepared in both the lower as well as the upper hyperfine ground state are excited to these excited molecular(dimer)states and both of the detection methods can detect the created molecular states but with variable accuracy.These states are never detected before and several of the previous attempts using magneto-optic traps(MOTs)or hotter atoms showed a continuous loss(in contrast to the quantized loss that we have detected)in this region of the frequency spectrum.The very low temperature of the BEC and the low power requirements by the BEC enabled us to create and detect these states with great precision.The effect of power broadening and energy shift of these states with the increasing laser power is also studied.An interesting feature is observed near the atomic resonance of the 87 Rb which becomes anomalously broadened due to the mixing of the nearby excited molecular states.This broadening is so strong that the typical Lorentzian line-shape of the atomic resonance is destroyed and which is retrieved back only when a very weak photoassociation laser power is used.Similar broadening was observed in the pioneering experiments of atomic spectroscopy but the nearby quantized states were never detected at that time.We have observed from the experimental data that the use of Bragg scattering from an optical lattice in the BEC for the detection of these excited molecular states produces better results as compared to the trap loss/absorption imaging method.The possible reasons for this are also discussed in this thesis.I do not give any theoretical treatment of the detected molecular states and leave it for future study for the theoretical physics community.The last experiment is performed with a superradiance lattice superposed on a phase modulated BEC.First the BEC is exposed to a blue detuned lattice for a variable time interval and then the superradiance lattice is turned ON to collect the phase information using the superradiance scattering light.The BEC wave-function evolves under the first blue detuned lattice and at variable time delays the phase acquired by the wave-function is different.The superradiance lattice scattering carries this phase modulation information and acts as an in-situ method for the measurement of the phase imprinted on the BEC without waiting for the BEC to expand for a certain time of flight(TOF).The phase modulation affects the superradiance scattering in the form of oscillations in the intensity as a function of time and agrees well with the previously existing theory qualitatively.This experiment also includes the study and comparison of the effects of a moving lattice in two opposite directions on the net phase imprinted on the BEC.The phase imprinted by two simultaneous optical lattices(a super-lattice)at different powers of each lattice is also studied and the phase information revealed by the oscillation scattering light intensity agrees very well with what the theory should predict.The in-situ probing of the BEC phase removes the possible noise sources that are typically inherent to the TOF imaging probing experiments.This experiment is suitable for the study of the depths of complex optical lattice potentials which are hard to measure using the conventional methods.