In this thesis we studied coherent effects in a cold atomic gas of rubidium isotope 87Rb interacting with two lasers. A theoretical background on the interaction of two lasers with an atom in lambda configuration is presented. One of the lasers is a continuous-wave laser, while the other is a pulsed, femtosecond laser. Rubidium transitions 52S1/2(F = 1) → 52P3/2(F’ = 2) and 52S1/2(F = 2) → 52P3/2(F’ = 2) were chosen and used, as they correspond to a lambda configuration. Also presented is a short theory of laser cooling and trapping, which is used to make a cloud of cold rubidium atoms for the experiment of coherent effects. We explain the methods of frequency stabilization of continuous-wave and femtosecond lasers, needed for measurements of high precision. The results were acquired by measuring laser radiation force on a rubidium cloud, as well as by measuring laser induced fluorescence of the cloud interacting with the lasers. We compared the measurements to the theoretical predictions and they correspond very well to the numerical models. We conclude that the femtosecond laser is a very useful tool which can be used in the experiments focused on coherent effects, as well as continuous-wave lasers have been used.
In this thesis we studied coherent effects in a cold atomic gas of rubidium isotope 87Rb interacting with two lasers. A theoretical background on the interaction of two lasers with an atom in lambda configuration is presented. One of the lasers is a continuous-wave laser, while the other is a pulsed, femtosecond laser. Rubidium transitions 52S1/2(F = 1) → 52P3/2(F’ = 2) and 52S1/2(F = 2) → 52P3/2(F’ = 2) were chosen and used, as they correspond to a lambda configuration. Also presented is a short theory of laser cooling and trapping, which is used to make a cloud of cold rubidium atoms for the experiment of coherent effects. We explain the methods of frequency stabilization of continuous-wave and femtosecond lasers, needed for measurements of high precision. The results were acquired by measuring laser radiation force on a rubidium cloud, as well as by measuring laser induced fluorescence of the cloud interacting with the lasers. We compared the measurements to the theoretical predictions and they correspond very well to the numerical models. We conclude that the femtosecond laser is a very useful tool which can be used in the experiments focused on coherent effects, as well as continuous-wave lasers have been used.
In this thesis, we studied radiation force of a femtosecond pulse train on cold rubidium atoms. Cooling of rubidium atoms is established by magneto-optic trap. We also studied theory of radiation force on atoms made by one femtosecond pulse train and two counter-propagating pulse trains. In case of two counter-propagating pulse trains we studied two cases of phase differences between pulses, case when pulses are in phase and when they are out of phase. Radiation force is measured for one femtosecond pulse train and for two counter-propagating pulse trains. In case of one pulse train measured radiation force matches theoretical model. In case of two pulse trains we made measurement in two configurations: ”in phase” and ”out of phase”. For ”out of phase” configuration, it is expected that there is no radiation force on atoms. For ”in phase” configuration, displacement of the cold atoms cloud center of mass is effect of femtosecond beams imbalance. We also measured spectral width of lasers that are used in experiment.
In this thesis, cooling of 87Rb atoms using an optical frequency comb resulting from highly stabilized femtosecond laser radiation was studied. Cooling of atoms was obtained by driving 52S1/2 (F = 2) → 52P3/2 (F' = 3) first resonant electronic transition of 87Rb atom with the single frequency comb line. Temperature of the cold atoms was determined using the time of flight technique. Temperature measurements were performed for various polarization configurations of counter propagating femtosecond laser beams. Temperature dependence of magnetic field and time of interaction with femtosecond laser was also measured. Cooling down to sub-Doppler temperatures was observed, with lowest measured temperature of 66 μK. According to our knowledge, this represents first experimental realization of sub-Doppler temperatures using optical frequency comb. For comparation with standard laser cooling techniques based on continuous-wave lasers, measurements were repeated using continuous-wave laser instead frequency comb. Well agreed results confirmed possibility of frequency comb laser cooling of atoms. That opens up a number of new possibilities in laser cooling, such as laser cooling of atoms with first resonant transitions in ultraviolet/vacuum ultraviolet spectrum range, where continuous-wave radiation is unavailable.
In this dissertation a cloud of cold rubidium atoms confined in a magnetooptical trap (MOT) has been realized. The laser cooling and trapping technique was implemented in the Laboratory for femtosecond frequency comb spectroscopy at Institute of Physics for the first time, so this work opens a new area of research. The cloud of cold atoms, containing ~ 10^9 87^Rb, F_g = 2, in volume ~ 3 mm3, density of ~ 1011 atom/cm3 in typical MOT operating conditions was obtained. Detailed characterization of the cloud has been done and the Doppler model for the real atomic system of rubidium 87 atoms was applied to describe the radiative force. The main focus of this work is investigation of the coherent continuous (cw) or pulsed (femtosecond) radiation effects on rubidium atoms in MOT. Atomic motion in the trap can be described with a damped harmonic oscillator model. A new technique for monitoring cold cloud center of mass oscillations induced by cw laser, tuned to rubidium resonant transition has been introduced. With this method the basic trap parameters, such are spring constant, damping coefficient and atom temperature are determined. For various trap working conditions an effective saturation intensity of 87^Rb D1 (F_g = 2 → F_e = 2) transition is, for the first time, determined through experimentally and theoretically investigated rubidium excited F_e = 2 state population depending on the probe beam intensity. Also, the radiative force of the probe D1 laser on cold cloud is measured and theoretically modelled. Interaction of cold rubidium cloud with femtosecond (fs) pulses has been studied in three different experimental configurations; interaction of cold atoms with single fs train of pulses, with two in-phase counterpropagating fs pulses and with two out-of-phase counterpropagating fs pulses. In all three described geometries the LIF signal and radiative force are experimentally determined and compared with theoretical model, based on optical Bloch equations (OBE) for two energy level atom.
Presented in this thesis are results of research on two topics in the field of cold atoms. These topics are connected by the fact they both employ laser induced forces caused by momentum transfer from photons to atoms. In both cases the laser induced forces change the velocity distribution of the atomic ensemble. In the first part of the thesis a new way to implement a synthetic Lorentz force into a cold atomic gas is presented. The synthetic Lorentz force (SLF) is based on radiation pressure and the Doppler effect, making it straightforward to implement in a large volume, for a broad range of velocities, and can be extended to different geometries. The force is perpendicular to the velocity of an atom, and zero for an atom at rest. This SFL is experimentally demonstrated in a system of cold rubidium atoms in two scenarios: first, by observing the center-of-mass motion of a cold atomic cloud and second, by observing the angular deflection of a rotationally asymmetrical cloud when released from a magneto-optical trap. The introduction of synthetic magnetism into the system of cold thermal atoms makes it an excellent candidate to emulate numerous complex classical systems, for example a tokamak fusion reactor or a star. In the second part of the thesis, the possibility of laser cooling with a frequency comb (FC) is explored. For this purpose a scheme for full stabilization of a fiber based FC that does not require traditional self-referencing is developed and implemented; the repetition frequency is locked to a stable microwave reference while the offset frequency is indirectly stabilized by referencing the frequency comb to a continuous wave laser that is stabilized by polarization spectroscopy in rubidium vapor. The FC stabilized in this way is used to cool rubidium atoms on a dipole-allowed transition at 780 nm to sub-Doppler temperatures. Temperatures as low as 55 μK were measured in a one-dimensional FC cooling geometry using the time-of-flight method. Laser cooling with FCs could enable achieving sub-Doppler temperatures for atoms with dipoleallowed transitions in the vacuum ultraviolet. This can significantly improve the precision of optical frequency standards, enable measurements of fundamental constants with unprecedented accuracy, and open up the possibility to reach quantum degeneracy with atoms that have optical transitions unreachable by continuous wave lasers such as hydrogen, deuterium and antihydrogen.
The subject of this thesis is the study of coherent interaction of light with matter (atoms) to improve the precision measurements and techniques. Special attention is drawn to get the narrow subnatural electromagnetically induced transmission (EIT), electromagnetically induced absorption (EIA) and nonlinear magneto-optic rotation (NMOR) caused by alkali atoms contained in a vapor cell. Subnatural polarization rotation introduces by a strong circularly polarized light in the absence of any external magnetic field was also studied. A detailed theoretical treatment, given in this dissertation, allows to associate each of the features of the spectra with a special physical mechanism. Many quantum phenomena related to interferences, coherences, optical pumping etc. experiments are studied using home-built diode lasers. This thesis also describes laser cooling and trapping of rubidium atoms using two techniques. Deflection of cold atoms horizontally from MOT using pushing beams are discussed in close consideration for the improvements in the precision measurements.
This thesis is organized as follows. In Chapter 1, an introduction to the importance of sub-natural narrow resonance and simplified technique in the precision measurement will be discussed.
In Chapter 2, an introduction to EIT, EIA and NMOR resonance are discussed. This chapter will provide a basic theoretical background of atom-field interactions, especially for Λ-type and N -type systems and its steady state solution using density matrix analysis and experimental tools. The most important notion of laser cooling of atoms, ions or molecule i.e., exchange of momentum between light and atoms combining with the Doppler effect will be discussed.
In Chapter 3, the observation of subnatural EIT and subnatural EIA in closed and open degenerate two-level system using room temperature vapor cell filled with Rb will be presented. Physical mechanisms that contribute to EIT and EIA, and the contrast of our results from the coherent population trapping (CPT)–type resonances are discussed in detail in appendix A.2 according to our experimental results.
In Chapter 4, the narrowing of subnatural EIT and subnatural EIA linewidth in closed and open systems again in degenerate two level transition, using the “Laguerre-Gaussian” control beam instead of generally studied Gaussian beam, will be discussed in detail.
In Chapter 5, the conversion between subnatural EIT to subnatural EIA in a degenerate Λ system will be discussed. The physical mechanism created by the introduction of a coherent counter propagating control laser to the co-propagating probe and the control laser are studied. The effect of polarization and axial velocity Doppler averaging will also be studied.
In Chapter 6, we will discuss the sensitive technique for precise measurement of small magnetic fields using the NMOR, by chopping the resonant laser beam. We will study the sensitivity and the potential application of this technique in the measurement of an atomic electric-dipole moment.
In Chapter 7, we will…
The aim of this research is to pave the way for making a trap for cold neutral atoms based on the force generated by pulses of a mode-locked laser. As an onset towards such a trap we decided to use a far-off-resonance trap (FORT) loaded with cold rubidium atoms from a magneto-optical trap (MOT). Our FORT setup consists of a titanium-sapphire laser that can operate both in continuous wave mode and in mode-locked mode. Our MOT setup uses a magnetic field gradient, and two diode lasers whose frequency is locked very accurately to the atomic transition using polarisation spectroscopy. Since the measurements we do require very precise timing of the order of less than one millisecond between the various phases of the experiment, a time-sequencing program was developed. Using this sophisticated setup, several characteristics of FORTs made with continuous and pulsed lasers were analysed. For example, we have investigated the loading behaviour of the traps and found that under general circumstances there is no significant difference. However, under special conditions it is possible to load more atoms into a pulsed trap. The lifetime of the number of atoms in the trap shows no dependence on the power of the trap laser in the continuous case. For the pulsed trap, however, the increased scattering due to the high peak intensities of the pulses does limit the achievable lifetimes. Moreover, due to the presence of a large number of photoassociation lines, i.e. lines were loosely bound molecules are formed by the interaction of two atoms an a photon, the two-particle losses depend heavily on the wavelength of the trap laser. Furthermore we have done measurements of the temperature of the trapped atoms, which revealed that the temperature is a constant fraction of the trap depth. We also looked at the effects of parametric excitation of the atoms in the FORT. The anharmonicity of the trap formed by the titanium-sapphire laser results in interesting physics, including a change in the average temperature of the atoms, depending on the modulation frequency. We also investigated the trapping behaviour of the FORT at low pulse repetition rates by switching the laser intensity on and off on a kHz scale. We investigated the dependence of the number of atoms on the polarisation of the FORT laser light, both for continuous and pulsed traps. Atoms can be trapped at elliptical polarisations as well, although the number of trapped atoms is less. As the laser is detuned further to the red of the rubidium D line, this effect becomes less and less pronounced, as predicted by our model.