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 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.
We report on laser cooling of neutral rubidium atoms by using a single mode of a frequency comb. Cooling is achieved on a dipole-allowed transition at 780 nm in a one-dimensional retro-reflected beam geometry. Temperatures are measured using standard time-of-flight imaging. We show the dependence of the temperature on the cooling time, intensity and detuning of the frequency comb. The lowest temperature achieved is approximately equal to the Doppler temperature and is limited by the intensity of the comb mode driving the cooling transition. Additionally, we verify the analogy between frequency comb and continuous-wave laser cooling. Our work is a step towards laser cooling of atoms with strong cycling transitions in the vacuum ultraviolet, such as hydrogen, deuterium and antihydrogen, where generation of continuous-wave laser light is limited by current laser technology. Achieving efficient cooling at these wavelengths would significantly improve the precision of optical frequency standards, enable measurements of fundamental constants with unprecedented accuracy, improve tests of charge, parity, and time reversal symmetry, and open the way to achieving quantum degeneracy width new atomic species.
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 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.
We demonstrate 1D simultaneous laser cooling of 87Rb and 85Rb atoms using an optical frequency comb. By adjusting the pulse repetition frequency and the offset frequency, the frequency comb spectrum is tuned to ensure that two distinct frequency comb modes are simultaneously red-detuned from the cooling transitions, one mode for each species. Starting from a precooled cloud of 85,87Rb atoms at above-Doppler temperatures, we show simultaneous cooling of both species down to the Doppler temperature using two counter-propagating ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}}$ polarized beams from the frequency comb. The results indicate that simultaneous dual-species frequency comb cooling does not affect the cooling characteristics of individual atomic species. The results of this work imply that several atomic species could be cooled simultaneously using a single frequency comb source. This comb-based multichannel laser cooling could bring significant advances in multispecies atom interferometers for space applications and in the study of multispecies interactions.
Despite the prominent role of laser cooling in modern atomic physics, with applications ranging from quantum sensors to high-precision spectroscopy and ultracold chemistry, laser cooling techniques are still limited to atoms with simple energy level structure and closed transitions that are accessible by currently available continuous wave (cw) laser sources. Two main obstacles prevent the extension of laser cooling techniques to a variety of atomic species and molecules. The first is associated with technical difficulties in creating cw laser light in the vacuum ultraviolet (VUV) spectral range - the absence of those laser sources prevents laser cooling of simple atoms such as hydrogen, deuterium, and antihydrogen. The second obstacle is associated with complex energy level structure of many atoms and almost all molecules - it permits decay of an excited state into a number of lower-lying states that can be widely separated in energy, which makes laser cooling of such species inefficient and experimentally very challenging.