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Measurements on release-recapture of cold 85Rb atoms using an optical nanofibre in a magneto-optical trap

Published Aug 17, 2016• 31 citations
OpenAlex

Abstract

We have performed release-recapture temperature measurements of laser-cooled 85Rb atoms using an optical nanofibre (ONF) in a magneto-optical trap (MOT). The effects of changing the cooling laser light-shift parameter on the temperature of the cold atoms and spring constant of the trap are studied. By varying the cold atom number density near the ONF, the onset of the multiple scattering regime is observed without the need for an estimation of the atom cloud size. Moreover, this sensitive ONF assisted release-recapture technique is easily able to detect any optical misalignment of the cooling laser beams in the MOT. Keywords: laser cooling, optical nanofiber, rubidium, release-recapture, temperature

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In this thesis, a magneto-optical trap setup is used to laser cool and confine a cloud of 85Rb. The cloud typically contains 108 atoms in a 1 mm3 volume at a temperature in the region of the Doppler Limit (146 _K for 85Rb). To study the cold cloud, a subwavelength optical fibre - a nanofibre, or ONF - is positioned inside the cloud. The ONF can be used in two ways. Firstly, it is an efficient fluorescence collection tool for the cold atoms. Loading times, lifetimes and temperatures can be measured by coupling the atomic fluorescence to the evanescent region of the ONF. Secondly, the ONF is used as a probe beam delivery tool using the evanescent field properties of the device, allowing one to perform spectroscopy on few numbers of near-surface atoms. With improvements in optical density of the cloud, this system is an ideal candidate in which to generate electromagnetically induced transparency and slow light. A theoretical study of the van der Waals and Casimir-Polder interactions between an atom and a dielectric surface is also presented in this work in order to understand their effects in the spectroscopy of near-surface atoms.

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We present a method for measuring the average temperature of a cloud of cold 85 Rb atoms in a magneto-optical trap using an optical nanofibre. A periodic spatial variation is applied to the magnetic fields generated by the trapping coils and this causes the trap centre to oscillate, which, in turn, causes the cloud of cold atoms to oscillate. The optical nanofibre is used to collect the fluorescence emitted by the cold atoms, and the frequency response between the motion of the centre of the oscillating trap and the cloud of atoms is determined. This allows us to make measurements of cloud temperature both above and below the Doppler limit, thereby paving the way for nanofibres to be integrated with ultracold atoms for hybrid quantum devices.

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Experimental studies on a dark magneto-optical trap for 85Rb

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This work is concerned with density enhancements in a laser-cooled sample of 85Rb using a dark magneto-optical trap (MOT). The density of atoms in a standard MOT is limited to approximately ~1011 atoms/cm3 due to radiation pressure and collisions. The dark MOT (DMOT) circumvents problems associated with radiation pressure in the atom cloud. To create a DMOT, a donut-shaped repump beam is projected on to the cloud of 85Rb. The dark region in the center of the donut is attenuated by ~10^(-4) compared to the bright outer ring. Atoms which encounter this dark region in the repump beam are scattered to the lower hyperfine ground level, F = 2, otherwise known as the dark hyperfine level. Additionally, a depump beam is used to force more atoms into F = 2. Results using free-space spectroscopy are presented to demonstrate the effect of the donut-shaped repump beam and the depump beam on the population distribution of the atoms in the hyperfine ground states. A tapered optical fiber is used to measure the quality of the DMOT via a parameter, p, and a strong population transfer from the bright, F = 3, to the dark, F = 2, hyperfine ground level is demonstrated. The tapered fiber, with a waist of ~ 1 micron, is made from commercially-available optical fiber and is used to detect fluorescence from the cold 85Rb coupled into the fiber guided modes. This work lays the foundation for optical nanofiber absorption spectroscopy with a small number of atoms in an optically-dense system.

•Jun 25, 2013•0 citations
We have performed release-recapture temperature measurements of laser-cooled 85Rb atoms using an optical nanofibre (ONF) in a magneto-optical trap (MOT). The effects of changing the cooling laser light-shift parameter on the temperature of the cold atoms and spring constant of the trap are studied. By varying the cold atom number density near the ONF, the onset of the multiple scattering regime is observed without the need for an estimation of the atom cloud size. Moreover, this sensitive ONF assisted release-recapture technique is easily able to detect any optical misalignment of the cooling laser beams in the MOT. Keywords: laser cooling, optical nanofiber, rubidium, release-recapture, temperature
In this thesis, a magneto-optical trap setup is used to laser cool and confine a cloud of 85Rb. The cloud typically contains 108 atoms in a 1 mm3 volume at a temperature in the region of the Doppler Limit (146 _K for 85Rb). To study the cold cloud, a subwavelength optical fibre - a nanofibre, or ONF - is positioned inside the cloud. The ONF can be used in two ways. Firstly, it is an efficient fluorescence collection tool for the cold atoms. Loading times, lifetimes and temperatures can be measured by coupling the atomic fluorescence to the evanescent region of the ONF. Secondly, the ONF is used as a probe beam delivery tool using the evanescent field properties of the device, allowing one to perform spectroscopy on few numbers of near-surface atoms. With improvements in optical density of the cloud, this system is an ideal candidate in which to generate electromagnetically induced transparency and slow light. A theoretical study of the van der Waals and Casimir-Polder interactions between an atom and a dielectric surface is also presented in this work in order to understand their effects in the spectroscopy of near-surface atoms.
Optical nanofibres are ultrathin optical fibres with a waist diameter typically less than the wavelength of light being guided through them. Cold atoms can couple to the evanescent field of the nanofibre-guided modes and such systems are emerging as promising technologies for the development of atom-photon hybrid quantum devices. Atoms within the evanescent field region of an optical nanofibre can be probed by sending near or on-resonant light through the fibre; however, the probe light can detrimentally affect the properties of the atoms. In this paper, we report on the modification of the local temperature of laser-cooled 87 Rb atoms in a magneto-optical trap centred around an optical nanofibre when near-resonant probe light propagates through it. A transient absorption technique has been used to measure the temperature of the affected atoms and temperature variations from 160 μ k to 850 μ k, for a probe power ranging from 0 to 50 nW, have been observed. This effect could have implications in relation to using optical nanofibres for probing and manipulating cold or ultracold atoms.
We present a method for measuring the average temperature of a cloud of cold 85 Rb atoms in a magneto-optical trap using an optical nanofibre. A periodic spatial variation is applied to the magnetic fields generated by the trapping coils and this causes the trap centre to oscillate, which, in turn, causes the cloud of cold atoms to oscillate. The optical nanofibre is used to collect the fluorescence emitted by the cold atoms, and the frequency response between the motion of the centre of the oscillating trap and the cloud of atoms is determined. This allows us to make measurements of cloud temperature both above and below the Doppler limit, thereby paving the way for nanofibres to be integrated with ultracold atoms for hybrid quantum devices.
This work is concerned with density enhancements in a laser-cooled sample of 85Rb using a dark magneto-optical trap (MOT). The density of atoms in a standard MOT is limited to approximately ~1011 atoms/cm3 due to radiation pressure and collisions. The dark MOT (DMOT) circumvents problems associated with radiation pressure in the atom cloud. To create a DMOT, a donut-shaped repump beam is projected on to the cloud of 85Rb. The dark region in the center of the donut is attenuated by ~10^(-4) compared to the bright outer ring. Atoms which encounter this dark region in the repump beam are scattered to the lower hyperfine ground level, F = 2, otherwise known as the dark hyperfine level. Additionally, a depump beam is used to force more atoms into F = 2. Results using free-space spectroscopy are presented to demonstrate the effect of the donut-shaped repump beam and the depump beam on the population distribution of the atoms in the hyperfine ground states. A tapered optical fiber is used to measure the quality of the DMOT via a parameter, p, and a strong population transfer from the bright, F = 3, to the dark, F = 2, hyperfine ground level is demonstrated. The tapered fiber, with a waist of ~ 1 micron, is made from commercially-available optical fiber and is used to detect fluorescence from the cold 85Rb coupled into the fiber guided modes. This work lays the foundation for optical nanofiber absorption spectroscopy with a small number of atoms in an optically-dense system.