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A Laser System for Gray-Molasses Cooling on the D1 Transition of an Atomic Gas of 39K

Baraa Shammout

Published Jan 1, 2020• 2 citations
Publisher

Abstract

In this work we present the design and the construction of a laser system for cooling an atomic gas of potassium 39K on the D1-Line by gray molasses technique. The seed laser is an Interference-filter-stabilized external-cavity diode laser (ECDL). The laser frequency is stabilized to one of the atomic features of 39K D1-transition thanks to frequency modulation (FM) spectroscopy. The low seed power is then amplified by means of tapered amplifier (TA). Finally, the cooling and repumping beams are generated and blue-detuned from the atomic transitions |F = 2> → |F'= 2> and |F = 1> → |F'= 2> using two acousto-optic modulators (AOMs).

DOI
10.15488/9417

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•Dec 17, 2019•3 citations
In this work we present the design and the construction of a laser system for cooling an atomic gas of potassium 39K on the D1-Line by gray molasses technique. The seed laser is an Interference-filter-stabilized external-cavity diode laser (ECDL). The laser frequency is stabilized to one of the atomic features of 39K D1-transition thanks to frequency modulation (FM) spectroscopy. The low seed power is then amplified by means of tapered amplifier (TA). Finally, the cooling and repumping beams are generated and blue-detuned from the atomic transitions |F = 2> → |F'= 2> and |F = 1> → |F'= 2> using two acousto-optic modulators (AOMs).
Alkali-metal atoms are the foremost candidates for studies of degenerate quantum gases employing sub-Doppler cooling techniques operating either on the D1 or D2 transition. However, the case of 39K atoms has a narrow excited-state structure of the D2 transition compromising difficulty to achieve below the Doppler limit temperature (TD = hΓ/2kB = 140μK) by employing conventional cooling techniques such as optical molasses or polarization gradient cooling on this transition. In this work, a sub-Doppler cooling known as Gray molasses operating on the ¦42S½} → ¦42P½} 770nm transition D1 is applied to cool a cloud of 39K atoms to sub-Doppler temperatures, which is an important step to reach the quantum degeneracy of the quantum gases. With this approach we reach temperatures below the Doppler limit around 14.3μK by the formation of dark states using the cooling and repumping lasers detuned to the blue side at the Raman resonance (two-photon) in Λ configuration. Then, we ramp down the intensities of each laser beam, in order to cool the atoms to an ultra-low temperature of 8.62μK, getting closer to the photon recoil limit of (Tr = h2k2L/2mkB = 0.4μK). In our results, we present a study of the behaviour of the temperature and the number of atoms by the influence of the relative detuning, and also a particular analysis of the temperature by the influence of the global detuning. In the following, we present the lowest temperature as a function of the final intensities D1 molasses. Finally, we compare the performance of our Gray molasses cloud with other works by using a phase space density analysis.
We present new techniques in cooling 39 K atoms using laser light close to the D1 transition.First, a new compressed-MOT configuration is taking advantage of gray-molasses-type cooling induced by blue-detuned D1 light.It yields an optimized density of atoms.Then, we use pure D1 gray molasses to further cool the atoms to an ultra-low temperature of 6 μK.The resulting phase-space density is 2 × 10 -4 and will ease future experiments with ultracold potassium.As an example, we use it to directly load up to 3 × 10 7 atoms in a far detuned optical trap, a result that opens the way to the all-optical production of potassium degenerate gases.
We have developed a semiconductor laser system used in 40K-87Rb trapping and cooling experiment. The laser diode system is composed of three external cavity diode lasers and four injection diode lasers and a tapered amplifier. Four laser beams used for 40K and 87Rb cooling and repump lights are obtained by using acoustic optic modulators to offset the frequencies from the three external cavity diode lasers. Four light beams with different frequencies are injected into four slave lasers for injection locking. Then Rb cooling light, K cooling light and K repump light are injected into a semiconductor tapered amplifier in a master oscillator-power configuration. This configuration is flexible and stable and can simultaneously produce the cooling and repump light for 40K and 87Rb trapping and cooling experiment.
Sub-Doppler laser cooling of40K with Raman gray molasses on the D2{D}_{2}D2​ line
Gray molasses is a powerful tool for sub-Doppler laser cooling of atoms to low temperatures. For alkaline atoms, this technique is commonly implemented with cooling lasers which are blue-detuned from either the D 1 or D 2 line. Here we show that efficient gray molasses can be implemented on the D 2 line of 40 K with red -detuned lasers. We obtained temperatures of , which enables direct loading of atoms from a magneto-optical trap into an optical dipole trap. We support our findings by a one-dimensional model and three-dimensional numerical simulations of the optical Bloch equations which qualitatively reproduce the experimentally observed cooling effects.
The modulation transfer spectroscopy (MTS) technique used to stabilize two different transition lines of rubidium, required for laser cooling of atoms, is investigated. The laser source is based on a 1560 nm semiconductor laser diode that is amplified by an erbium doped fiber amplifier (EDFA). The laser output is frequency doubled with a fiber coupled periodically poled Lithium Niobate (PPLN) crystal to 780 nm. To address the weak spectral features of repumping laser, a theoretical study is conducted followed by experimental verification. The 87Rb D2 line F = 1 → F' = 0 transition temperature-enhanced MTS signal is observed, which provides a choice to stabilize the repumping laser frquency on the F = 1 → F ' = 2 transition using frequency shifting with acousto-optic modulators (AOMs). Compared with the orignal MTS signal, this method is shown to be more stable and accurate for laser frequency locking. Laer frequency locking is realized by controlling the driving current of the seed laser using digital PID feedback. After locking, the laser linewidth is 35.36 kHz.
We demonstrate a laser frequency stabilization technique for laser cooling of potassium atoms, based on saturated absorption spectroscopy in the C-Band optical telecommunication window, using ro-vibrational transitions of the acetylene molecule (12 C 2 H 2). We identified and characterized several molecular lines, which allow to address each of the potassium D2 (767 nm) and D1 (770 nm) cooling transitions, thanks to a high-power second harmonic generation (SHG) stage. We successfully used this laser system to cool the 41 K isotope of potassium in a 2D-3D Magneto-Optical Traps setup.