Coherence and Population Decay Measurements in Molecular Iodine Using Amplitude-Crafted Phase-Specific Laser Pulses.

Richard G. Brewer, Satoru S. Kano

W. S. Warren, Luciano Mueller, J. L. Bates +2 more

In molecular iodine, and most other gas-phase molecules as well, generation of a π/2 or π pulse is theoretically impossible. This result can be understood by analogy with microwave spectroscopy, since the rotational contribution to the transition dipole moment is an explicit function of the projection of the total angular momentum F = J + I along the electric-field axis. The only solution is the generation of phase-modulated pulses that inherently compensate for Rabi-frequency inhomogeneity. We present theoretical and experimental evidence that crafted phase- and amplitude-modulated pulse shapes can excite a uniform, localized, and nearly complete inversion in nuclear magnetic resonance and laser spectroscopy.

Dipankar Bhattacharyya, Jyotirmoy Guha

Mark A. Banash, J. L. Bates, C. P. Lin +1 more

We discuss experimental applications of specific phase and amplitude modulated laser pulse shapes. Electronic transitions in real molecules generally do not have a well-defined Rabi frequency because of the distribution of the orientation of the transition dipole, and because the density of states for all but the very simplest molecules causes many Doppler broadened transitions (including hyperfine components) to overlap. Crafted pulse shapes generate complete population inversions regardless of Rabi frequency, as well as provide a uniform inversion over the pulse bandwidth.1 These experiments directly observe population transfer between frequency ranges by giving two different frequency π pulses at different times—the use of crafted pulses increases signal to noise by now inverting the entire Rabi distribution. Experimental modifications which select a single velocity range in an arbitrarily complex molecule while retaining the other advantages of pulse shaping are presented, and the velocity dependence of fluorescence and coherence lifetimes in molecular gases is analyzed.2 Applications to monitoring velocity-changing collisional dynamics and generating large vibrational population inversions in l2 are presented, as will experimental methods to extend these sequences to more complex molecules, where velocity selection and frequency resolution are not equivalent.

Saadia Kemal

This thesis looks into the aspects of molecular rot-vibrational energy level relaxation of iodine when it is excited by a resonant laser. In addition, the effects of Doppler detuning of molecular levels are explored. The proposed Active Hyperspectral Imaging Project [1] will detect in the atmosphere, low concentrations of a specific element's molecules by using remote sensing optical techniques. In order to improve the signal to noise ratio, a 'high field' coherent excitation technique is proposed. However such coherent excitation should take less time than the typical relaxation time of the system to avoid de-coherence effects. The purpose of the present study is to assess the values of the de-population and de-phasing relaxation times in iodine. The result of this experiment will be used for the design of an intra-cavity coherent excitation experiment in which the power needed for excitation would be obtained by a resonant enhancement inside a low-loss optical storage cavity. We can estimate the minimum power required for coherent excitation based on our findings of the relaxation times. This high field experiment would gain information regarding the coupling between the molecule and the laser field. In the present 'low field' experiment, output from a low-powered tunable diode laser was sent through a Pockels cell to create a top hat pulse which was used to excite gaseous iodine, located in an evacuated cell. The time dependence of a spectrally resolved fluorescence signal was used to observe the relaxation behavior of some targeted excited state and the behavior was fitted to a model resulting in estimates of de-phasing and depopulation relaxation parameters. The experiment was also set-up to observe the effects of Doppler broadening due to molecular velocities. Fluorescence was collected from a narrow strip inside the iodine cell (see figure 3.3) whose short axis was parallel to the laser beam and its length was altered to assess the Doppler contribution. The information of de-phasing and de-population collected from the low field experiment would be useful for the high field experiment in which enough intensity will be created to induce Rabi oscillations [2] by the placement of a custom iodine cell inside a short near-confocal cavity. These oscillations would aid in the estimation of the dipole matrix element (which quantifies the coupling between the molecule and the laser field). The time duration of de-phasing and de-population will determine the minimum field intensity required to produce a Rabi oscillation whose period is less than the de-phasing and de-population times. The situation for the low field experiment was modeled by a two-level energy system interacting with a single mode field. In chapter 2, equation of motion is developed which includes the detuning effect. An output intensity model designed to fit the data is presented that would be used to calculate the rise and decay times helping us to ascertain the relaxation effects. The practical setup is presented in chapter 3. Details regarding the beam line, stabilization of the diode laser, selection of the iodine absorption line by tuning the laser and selection of a fluorescence line of iodine are all described in that chapter. The procedure of placement of the Pockels cell, production of a 1 µs laser pulse and data collection are also given. The chapter concludes with a representative data-set. For the Analysis, the model from the second chapter is used to fit to the experimental data. Values of rise and fall times are presented and the relationship between these values is explored. The average value of de-phasing due to molecular collision is calculated and the power requirements for the Rabi oscillations are discussed in this chapter. The thesis concludes with a summary and suggestions for further work.

Hiroki Nakatsuka, 修治 浅香, Makoto Tomita +1 more