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arXiv:1110.4067 · Physical Review A

Low-temperature high-density magneto-optical trapping of potassium using the open 4S→5P transition at 405 nm

David C. McKay, Dylan Jervis, Daniel Fine, John W. Simpson-Porco, Graham J. A. Edge, Joseph H. Thywissen

Published Dec 21, 2011• 68 citations
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Abstract

We report the laser cooling and trapping of neutral potassium on an open transition. Fermionic ${}^{40}$K is captured using a magneto-optical trap (MOT) on the closed $4{S}_{1/2}\ensuremath{\rightarrow}4{P}_{3/2}$ transition at 767 nm and then transferred, with high efficiency, to a MOT on the open $4{S}_{1/2}\ensuremath{\rightarrow}5{P}_{3/2}$ transition at 405 nm. Because the $5{P}_{3/2}$ state has a smaller linewidth than the $4{P}_{3/2}$ state, the Doppler limit is reduced from 145 \ensuremath{\mu}K to 24 \ensuremath{\mu}K, and we observe temperatures as low as 63(6) $\ensuremath{\mu}$K. The density of trapped atoms also increases, due to reduced temperature and reduced expulsive light forces. We measure a two-body loss coefficient of $\ensuremath{\beta}=1.4(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm${}^{3}$/s near saturation intensity, and estimate an upper bound of $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$ cm${}^{2}$ for the ionization cross section of the $5P$ state at 405 nm. The combined temperature and density improvement in the 405 nm MOT is a twenty-fold increase in phase-space density over our 767 nm MOT, showing enhanced precooling for quantum gas experiments. A qualitatively similar enhancement is observed in a 405 nm MOT of bosonic ${}^{41}$K.

DOI
10.1103/physreva.84.063420

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We report the laser cooling and trapping of neutral potassium on an open transition. Fermionic 40K is captured using a magneto-optical trap (MOT) on the closed 4S1/2 → 4P3/2 transition at 767 nm and then transferred, with high efficiency, to a MOT on the open 4S1/2 → 5P3/2 transition at 405 nm. Because the 5P3/2 state has a smaller linewidth than the 4P3/2 state, the Doppler limit is reduced from 145 μK to 24 μK, and we observe temperatures as low as 63(6) μK. The density of trapped atoms also increases, due to reduced temperature and reduced expulsive light forces. We measure a two-body loss coefficient of β = 1.4(1) × 10−10 cm3/s near saturation intensity, and estimate an upper bound of 8 × 10−18 cm2 for the ionization cross section of the 5P state at 405 nm. The combined temperature and density improvement in the 405 nm MOT is a twenty-fold increase in phase-space density over our 767 nm MOT, showing enhanced precooling for quantum gas experiments. A qualitatively similar enhancement is observed in a 405 nm MOT of bosonic 41K.

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Ultracold quantum gases have become a frequently used tool for the investigation of a variety of physical issues within the last decade, ranging from general questions in quantum physics up to special topics, e.g. quantum simulations of the physical origin of high temperature superconductivity in condensed matter physics. In this thesis, experiments are presented on the creation of an ultracold quantum gas of the fermionic potassium isotope, 40K, by all-optical means. For this purpose, a new experimental setup was designed, which was started from the scratch, with the long term goal of creating a supersolid in an optical lattice. 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In this way, a spatially inhomogeneous ac Stark shift of the cooling transitions due to the spatially inhomogeneous intensity distribution of the dipole trapping beam was avoided and again light-assisted losses were reduced during the loading phase of atoms from the compressed magneto-optical trap into the optical dipole trap. The loading rate was measured to be approximately 2.8x108 atoms/s. The light-assisted losses during loading of the optical dipole trap were determined to be β ≈ 4.8x10-10 cm3/s. In the final step of the experiment, the temperature of the atomic ensemble confined in the optical dipole trap was further reduced by evaporative cooling. After an evaporation time of about 30 s, temperatures of 140 nK were reached at typical atom numbers of 12000. The corresponding Fermi temperature is TF = 110 nK yielding a ratio T/TF ≈ 1.3 which indicates that the fermionic potassium atoms are cooled close to the onset of quantum degeneracy. In the future, an additional trapping and evaporative cooling step within a magnetic quadrupole trap should allow for an improved loading efficiency into the far detuned optical dipole trap and finally allow for a subsequent evaporative cooling to a quantum degenerate Fermi gas.

•Aug 3, 2012•0 citations
We report the laser cooling and trapping of neutral potassium on an open transition. Fermionic 40{}^{40}40K is captured using a magneto-optical trap (MOT) on the closed 4S1/2\ensuremath→4P3/24{S}_{1/2}\ensuremath{\rightarrow}4{P}_{3/2}4S1/2​\ensuremath→4P3/2​ transition at 767 nm and then transferred, with high efficiency, to a MOT on the open 4S1/2\ensuremath→5P3/24{S}_{1/2}\ensuremath{\rightarrow}5{P}_{3/2}4S1/2​\ensuremath→5P3/2​ transition at 405 nm. Because the 5P3/25{P}_{3/2}5P3/2​ state has a smaller linewidth than the 4P3/24{P}_{3/2}4P3/2​ state, the Doppler limit is reduced from 145 \ensuremath{\mu}K to 24 \ensuremath{\mu}K, and we observe temperatures as low as 63(6) \ensuremathμ\ensuremath{\mu}\ensuremathμK. The density of trapped atoms also increases, due to reduced temperature and reduced expulsive light forces. We measure a two-body loss coefficient of \ensuremathβ=1.4(1)\ifmmode×\else\texttimes\fi10\ensuremath−10\ensuremath{\beta}=1.4(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\ensuremathβ=1.4(1)\ifmmode×\else\texttimes\fi10\ensuremath−10 cm3{}^{3}3/s near saturation intensity, and estimate an upper bound of 8\ifmmode×\else\texttimes\fi10\ensuremath−188\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}8\ifmmode×\else\texttimes\fi10\ensuremath−18 cm2{}^{2}2 for the ionization cross section of the 5P5P5P state at 405 nm. The combined temperature and density improvement in the 405 nm MOT is a twenty-fold increase in phase-space density over our 767 nm MOT, showing enhanced precooling for quantum gas experiments. A qualitatively similar enhancement is observed in a 405 nm MOT of bosonic 41{}^{41}41K.
Low-temperature, high-density magneto-optical trapping of potassium using the open 4S →\rightarrow→ 5P transition at 405\,nm
We report the laser cooling and trapping of neutral potassium on an open transition. Fermionic 40K is captured using a magneto-optical trap (MOT) on the closed 4S1/2 → 4P3/2 transition at 767 nm and then transferred, with high efficiency, to a MOT on the open 4S1/2 → 5P3/2 transition at 405 nm. Because the 5P3/2 state has a smaller linewidth than the 4P3/2 state, the Doppler limit is reduced from 145 μK to 24 μK, and we observe temperatures as low as 63(6) μK. The density of trapped atoms also increases, due to reduced temperature and reduced expulsive light forces. We measure a two-body loss coefficient of β = 1.4(1) × 10−10 cm3/s near saturation intensity, and estimate an upper bound of 8 × 10−18 cm2 for the ionization cross section of the 5P state at 405 nm. The combined temperature and density improvement in the 405 nm MOT is a twenty-fold increase in phase-space density over our 767 nm MOT, showing enhanced precooling for quantum gas experiments. A qualitatively similar enhancement is observed in a 405 nm MOT of bosonic 41K.
We demonstrate a dual magneto-optical trap (MOT) of 109Ag^{109}\mathrm{Ag}109Ag and 39K^{39}\mathrm{K}39K. For silver, a decreasing-field Zeeman slower loads a MOT of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms at a temperature of 0.74(5) mK, with laser cooling occurring primarily on the D2{D}_{2}D2​ line of $4{d}^{10}5s\phantom{\rule{0.28em}{0ex}}^{2}S_{1/2}\ensuremath{\rightarrow}5p\phantom{\rule{0.28em}{0ex}}^{2}P_{3/2}$ at 328 nm. We create a Ag ``dark spot MOT,'' where shelving the atoms in a dark state enhances the captured atom number by a factor of 2 and the lifetime by a factor of 4. For potassium, we obtain $2\ifmmode\times\else\texttimes\fi{}{10}^{8}$ trapped atoms, and further cooling on the D1{D}_{1}D1​ transition via gray molasses results in a cloud of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms at $7(1)\phantom{\rule{0.28em}{0ex}}\textmu{}\mathrm{K}$. We observe evidence of photoionization loss of the K MOT in the presence of Ag laser-cooling light, necessitating sequential rather than simultaneous loading for the two species. Our results on Ag point to simple and general laser cooling strategies for other coinage metals (Au, Cu). Furthermore, this work lays the foundation for the production of alkali-metal--coinage-metal degenerate quantum mixtures and highly polar molecules.
We report an experimental study of peak and phase-space density of a two-stage magneto-optical trap (MOT) of 6Li^{6}\mathrm{Li}6Li atoms, which exploits the narrower $2{S}_{1/2}\ensuremath{\rightarrow}3{P}_{3/2}$ ultraviolet (UV) transition at 323 nm following trapping and cooling on the more common D2D2D2 transition at 671 nm. The UV MOT is loaded from a red MOT and is compressed to give a high phase-space density up to $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. Temperatures as low as $33\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$ are achieved on the UV transition. We study the density limiting factors and in particular find a value for the light-assisted collisional loss coefficient of $1.3\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm3{\mathrm{cm}}^{3}cm3/s for low repumping intensity.
We have constructed a magneto-optical trap (MOT) for metastable triplet helium atoms utilizing the $2{}^{3}{S}_{1}\ensuremath{\rightarrow}3{}^{3}{P}_{2}$ line at 389 nm as the trapping and cooling transition. The far-red-detuned MOT (detuning $\ensuremath{\Delta}=\ensuremath{-}41\mathrm{MHz})$ typically contains few times 107{10}^{7}107 atoms at a relatively high $(\ensuremath{\sim}{10}^{9}{\mathrm{cm}}^{\ensuremath{-}3})$ density, which is a consequence of the large momentum transfer per photon at 389 nm and a small two-body loss rate coefficient $(2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}/\mathrm{s}<\ensuremath{\beta}<1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}).$ The two-body loss rate is more than five times smaller than in a MOT on the commonly used $2{}^{3}{S}_{1}\ensuremath{\rightarrow}2{}^{3}{P}_{2}$ line at 1083 nm. Furthermore, laser cooling at 389 nm results in temperatures somewhat lower than those achieved using 1083 nm. The 389-nm MOT exhibits small losses due to two-photon ionization, which have been investigated as well.
We realize a magneto-optical trap (MOT) of titanium (Ti) atoms, performing laser cooling on the 498 nm transition between the long-lived 3 d 3 ( F 4 ) 4 s a 5 F 5 metastable state and the 3 d 3 ( F 4 ) 4 p y 5 G 6 o excited state. Without the addition of any repumping light, we observe MOTs of the three stable, I = 0 bosonic isotopes, Ti 46 , Ti 48 , and Ti 50 . Up to 8.30 ( 26 ) × 10 5 Ti 48 atoms are trapped at a maximum density of 1.3 ( 4 ) × 10 11 cm − 3 and at a temperature of 90 ( 15 ) µ K . By measuring the decay of the MOT, we constrain the leakage branching ratio of the cooling transition ( ≤ 2.5 × 10 − 6 ) and the two-body loss coefficient ( ≤ 2 × 10 − 10 cm 3 s − 1 ). Our approach to laser cooling Ti can be applied to other transition metals, enabling a significant expansion of the elements that can be laser cooled.
Ultracold quantum gases have become a frequently used tool for the investigation of a variety of physical issues within the last decade, ranging from general questions in quantum physics up to special topics, e.g. quantum simulations of the physical origin of high temperature superconductivity in condensed matter physics. In this thesis, experiments are presented on the creation of an ultracold quantum gas of the fermionic potassium isotope, 40K, by all-optical means. For this purpose, a new experimental setup was designed, which was started from the scratch, with the long term goal of creating a supersolid in an optical lattice. One focus of this work lies in identifying light-assisted losses as a main restriction for achieving higher 40K atom numbers in a magneto-optical trap and in identifying light-assisted losses as a main reason for a low atomic transfer efficiency from a magneto-optical trap into an optical, quasi-electrostatic dipole trap (further called optical dipole trap) for 40K atoms. Naturally, another focus of this work lies in the application of different techniques to reduce these from light-assisted losses dependent restrictions. A two-dimensional magneto-optical trap, located in the prechamber of a vacuum system, creates a continuous beam of 40K atoms. The atoms of this beam are recaptured in the main chamber of the vacuum system by a spatial dark spot magneto-optical trap. Here, a dark magneto-optical trap is implemented instead of a standard magneto-optical trap in order to reduce light-assisted losses. Up to 1x109 atoms at temperatures around 240 μK are trapped in the dark magneto-optical trap. This is a factor of ten higher compared to atom numbers of a 40K standard magneto-optical trap. The light-assisted losses in the 40K magneto-optical trap were measured as a function of the total trapping intensity and as a function of the repumping intensity. In both cases, the smallest observed loss coefficient was β ≈ 2.5x10-10 cm3/s. Compared to the smallest values of β of all other alkali elements (apart from the bosonic potassium isotopes), the light-assisted losses with 40K atoms are approximately two orders of magnitude larger. Up to 6x105 atoms were transferred from the spatial dark magneto-optical trap into the quasi-electrostatic dipole trap, which was created by using the trapping potential induced by radiation emitted by a CO2 laser operating at a wavelength near 10.6 μm. In order to reach the here reported atom numbers in the optical dipole trap, its trapping volume had to be increased in comparison to a simple dipole trap arrangement as induced with a single focused dipole trapping beam. Simultaneously, the strong confinement, as present in a tigthly focused single beam trap and as required for a subsequent evaporative cooling, had to be maintained. For this purpose, the trapping laser beam of the optical dipole trap was retroreflected in such a way that an overall optical dipole trap with a nearly doubled waist and a slightly deeper potential depth was created. By this trapping arrangement, the number of atoms confined in the optical dipole trap could be nearly doubled compared to a simple dipole trapping arrangement consisting of a single tightly focused trapping beam. The atomic transfer process itself, from the magneto-optical trap into the optical dipole trap, was organized in two steps. In a first step, the dark magneto-optical trap was transformed into a compressed magneto-optical trap by increasing the field gradient and bringing the cooling laser closer to resonance as compared to the detuning of the usual magneto-optical trap. In contrast to other experiments, the repumping laser of the magneto-optical trap was detuned to the blue of the used repumping transition. As a consequence, atoms were optically pumped into the upper hyperfine ground state |S1/2, F=7/2〉, realizing a temporal dark, compressed magneto-optical trap in analogy to the operation of a temporal dark magneto-optical trap. By this approach, light-assisted losses were reduced. In the second step of the transfer process, the light fields of the magneto-optical trap and the optical dipole trap were alternately switched on and off. In this way, a spatially inhomogeneous ac Stark shift of the cooling transitions due to the spatially inhomogeneous intensity distribution of the dipole trapping beam was avoided and again light-assisted losses were reduced during the loading phase of atoms from the compressed magneto-optical trap into the optical dipole trap. The loading rate was measured to be approximately 2.8x108 atoms/s. The light-assisted losses during loading of the optical dipole trap were determined to be β ≈ 4.8x10-10 cm3/s. In the final step of the experiment, the temperature of the atomic ensemble confined in the optical dipole trap was further reduced by evaporative cooling. After an evaporation time of about 30 s, temperatures of 140 nK were reached at typical atom numbers of 12000. The corresponding Fermi temperature is TF = 110 nK yielding a ratio T/TF ≈ 1.3 which indicates that the fermionic potassium atoms are cooled close to the onset of quantum degeneracy. In the future, an additional trapping and evaporative cooling step within a magnetic quadrupole trap should allow for an improved loading efficiency into the far detuned optical dipole trap and finally allow for a subsequent evaporative cooling to a quantum degenerate Fermi gas.