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Conference on Lasers and Electro-Optics

System for high-resolution spectroscopy on laser-cooled calcium atoms

Flávio C. Cruz, Germano Woehl, G.A. Garcia, Edjar Martins Telles, Ronaldo Celso Viscovini, D. Pereira, Artemio Scalabrin

Published Nov 27, 2002• 0 citations
Publisher

Abstract

Summary form only given.High-resolution spectroscopy of calcium is attractive for several reasons and has been performed by a number of groups. A strong dipole-allowed /sup 1/S/sub 0/-/sup 3/P/sub 1/ transition at 423 nm allows laser cooling and trapping of calcium and can be accessed by diode lasers. The very narrow intercombination transition at 657 nm can be used for developing an optical frequency standard.

DOI
10.1109/iqec.1998.680393

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Summary form only given.High-resolution spectroscopy of calcium is attractive for several reasons and has been performed by a number of groups. A strong dipole-allowed /sup 1/S/sub 0/-/sup 3/P/sub 1/ transition at 423 nm allows laser cooling and trapping of calcium and can be accessed by diode lasers. The very narrow intercombination transition at 657 nm can be used for developing an optical frequency standard.
An absolutely stabilized laser diode system for high-resolution spectroscopy of the 4 2 S 1/2 –3 2 D 5/2 electric-quadrupole-allowed transition in calcium ions at 729 nm is developed. Both its short-term and long-term stabilities are examined. A system for laser cooling of single calcium ions using an ultraviolet laser diode for excitation of the 4 2 S 1/2 –4 2 P 1/2 transition is also constructed, and by using it we have observed quantum jumps of single calcium ions with a high stability and a high signal-to-background ratio.
The 657 nm calcium intercombination line is an attractive optical frequency/length reference due to its high line Q (10/sup 12/) and insensitivity to external perturbations. In this paper we describe progress towards an all-diode-laser-based reference using laser-trapped Ca. To generate 35 mW of 423 nm trap light, we double the frequency of a 150 mW, 846 nm diode laser with a nonlinear crystal (KNbO/sub 3/) inside a resonant build-up cavity. We use this light to trap 2/spl times/10/sup 7/ atoms in a compact, magneto-optic trap. Spectroscopy of the narrow 657 nm transition is performed with a frequency-stabilized diode laser system. With the trapped atoms we have measured optical Ramsey fringes with a linewidth of 6 kHz. We discuss future improvements as well as potential performance which could achieve a fractional frequency instability of <3/spl times/10/sup -15/.
Calcium atoms could be trapped and locked directly in a small cell (about 20 cm long). A high power injection lock diode laser being frequency doubled to 422.5 nm by through a nonlinear crystal KNbO/sub 3/, is used to trap calcium atoms in the "heart" of the cell. The trapped atoms will be probed by a narrow linewidth extended-cavity diode laser. The diode laser then will be locked to the calcium /sup 3/P/sub 1/-/sup 1/S/sub 0/ intercombination line at 657 nm. The direct cell trapping and locking method makes it become possible to establish a compact high precision optical frequency standard.
Optical frequency standards with the reference to a narrow electronic transition of a laser-cooled collection of neutral atomic particles are becoming essential tools of research in modern precision physics experiments. In the core of a building block of an optical frequency standard is the optical continuous wave laser that has a good spectral purity of the emitted light. Such a stable optical oscillator is highly desirable in high resolution spectroscopy, if it emits in a good quality beam at a short visible wavelength. This Master thesis explores efficient techniques for building such an optical frequency source intended for use in the cooling and trapping of Calcium atoms scheme. The strong dipole transition at the blue wavelength in the atomic Calcium is needed to reduce the kinetic energy of atoms by nearly six orders of magnitude. A further reduction in the thermal energy of the laser cooled atoms is required to locate with ultra-high precision the 400 Hz narrow clock transition of the stable 40Ca isotope. The experimental methods that achieve this and approach sub-microkelvin temperature of the laser cooled bosonic isotopes of alkaline earths are inspected. The blue laser with a uniform intensity distribution in the beam is useful to maintain the trapped number of cold atoms during these experiments. The spectroscopic properties of the relative transitions in Calcium atom are also reviewed following relevant publications in the area. The constructed blue laser can be used as a primary wavelength source in the lasers network for cooling and trapping of Calcium atoms. These experiments will constitute part of the project to build an optical atom clock referenced to 40Ca narrow linewidth transition. The blue laser is constructed by generating second harmonic in a Potassium Niobate crystal, which is temperature controlled to use a type-I noncritical phase-matching of the optical nonlinear process. The power of the intracavity-generated second harmonic depends on the resonance properties of the optical resonator where this nonlinear crystal is placed. The study is aimed at characterising the designed optical resonator and the experimental parameters that describe it. The formula is derived that relates the resonance power enhancement coefficient with finesse and the power coupling contrast of a passive optical cavity. The obtained relationship is verfied during the experiments. The produced efficiency of the intracavity second harmonic generation is approx. 0.0023 mWblue/(mWred)2. The research work also examines the noise characteristics of the infrared diode laser that is used as a pump source for the intracavity generated second harmonic and determines the spectroscopic precision of the produced blue light. The frequency locking experiment is analysed using the unbalanced scheme of the polarisation stabilisation technique. The designed optical buildup cavity became a part of the unbalanced frequency discriminator in such a scheme. The results demonstrate high gain of frequency noise suppression of the stabilised laser. The unbalanced arrangement of the H}ansch-Couillaud technique has been possible due to a very low amplitude noise of semiconductor lasers.
Single ions trapped and laser cooled provide ideal objects for high resolution spectroscopy and frequency standards. For the latter purpose we are investigating the 5s2 1S0 → 5s5p 3P0 transition of In+. With a natural linewidth of only 1.1 Hz this resonance offers very high resolution and is highly immune to frequency shifts due to external electromagnetic fields, because it connects two levels with vanishing electronic magnetic momenta [1]. The wavelength of this clock transition is 236.5 nm and is technically very convenient, since it coincides with the fourth harmonic of the 946 nm Nd:YAG laser line. So this intrinsically frequency stable solid-state laser can be used to excite the transition [2]. For laser cooling and fluorescence detection of the indium ion the 5s2 1So -> 5s5p 3P1 transition at 230.6 nm can be employed. The use of the relatively narrow intercombination line for laser cooling allows us to study optical sideband cooling in the strong binding regime, where the oscillation frequencies of the ion in the trap (around 1 MHz) are larger than the optical linewidth of 360 kHz. In this parameter range laser cooling is possible to the quantum ground state of the vibrational motion in the trap corresponding to a temperature of roughly 100 μK. The same low temperature was obtained with sideband cooling of a small Coulomb crystal consisting of two ions, creating an interesting new quantum few-particle system.