Optical system for Doppler cooling of trapped calcium ions

Abstract

Abstract-In the experiments involving trapped ions, the application of a cooling procedure is required. The optical Doppler scheme is one of the most commonly used methods for slowing down ions motion. In this paper we present an optical system sufficient for cooling calcium ions in such a scheme. The system allows also for optical detection of trapped ions.Experiments with trapped ions require an efficient method for cooling and detecting the ions inside the trap [1]. In the presented experimental set-up [2] it is intended to investigate various collisional effects involving ions of different species, including atomic and molecular ones, for example: further ionization, dissociation, chargetransfer collisions, chemical reactions, elastic scattering energy transfer, etc.The trapped ions will be slowed down using a sympathetic cooling scheme, which involves direct optical quenching of at least one species of ions forming the ensemble. For numerous reasons (atomic mass 40 close to many other ion species, easy evaporating, easy optical access) calcium ions were chosen as the cooling medium. To ensure efficient calcium cooling, two laser systems are necessary [3].The application of two lasers is explained in Fig. 1, where a simplified scheme of energy levels of calcium ions is shown. Doppler cooling is ensured by the UV laser driving 4 2 S 1/2 -4 2 P 1/2 transition, while the NIR laser is necessary to avoid trapping ions in the dark 3 2 D 3/2 state.The apparatus consists of a linear segmented Paul trap placed in a vacuum chamber. The trap is equipped with an electron gun, oven providing a calcium beam, and source of a molecular beam (not used for calcium cooling). The optical part of the apparatus contains two laser systems (397 and 866nm) used for Doppler cooling and detection of calcium ions.Both systems are external cavity diode lasers. As the lasers wavelengths tend to drift in time, stabilization schemes must be applied to provide efficient cooling.As the UV photon momentum is higher than NIR and the S-P transition is much stronger than that of P-D, the fine tuning of a 397nm beam is more important than a 866nm one. This way, for a UV beam a more precise * E-mail: lklos@izyka.umk.pl method of Pound-Drever-Hall [4] locking was used while for NIR, a simpler FM lock [5] scheme was applied. Both stabilization schemes use vacuum optical cavities made of ultra-low expansion glass (ULE), 150mm long each, providing a free spectral range of 2GHz and finesse of 1000.The Doppler cooling efficiency depends on the UV laser detuning as presented in Fig. 2. The details of derivation of the curve can be found in ref. [6]. The efficiency can be understood as the rate of energy loss by the ion ensemble during the interaction with the laser. Positive values mean cooling the system (red-detuned laser) and negative stand for heating up the ions (bluedetuned laser). As the ions gain energy from the RF electric field, collisions with the residual gas, etc., the equilibrium energy of the ions depends on this rate. The efficiency also ...

Energy transfer in elastic collisions between electrons and trapped ions

“…The apparatus used in experiment was described in our previous works [4,7,15,16]. It consists of a linear, segmented Paul trap inside a vacuum chamber.…”

“…The temperatures are calculated from mean kinetic energy of the ions, averaged the same way. The color lines refer to equation (7) fitted to numerical data for various numbers of ions. The ion numbers are marked with colors as labeled in the image.…”

“…As the ions (calcium in this case) are produced in electron-atom collisions, trapped, and optically cooled down [7], it is possible to observe directly the reactions of ion ensembles to presence of other particles, such as electrons. In the experiment it was observed, that captured ion ensemble is temporarily expanded in space when it is bombarded with electrons, while the total number of trapped ions remains unchanged.…”

Nonlinear resonances in linear segmented Paul trap of short central segment

“…The apparatus consists of a linear Paul trap (described in detail in Section 2) placed in the vacuum, equipped with resistively heated oven for calcium beam production and low‐energy, pulsed electron gun for ionization. Calcium ions were Doppler cooled using 397 and 866 nm laser beams and detected by capturing 397‐nm fluorescence images with an image‐intensified charge‐coupled device camera …”

“…The trap is placed in a magnetic field of the order of 0.1 mT. Its role is to improve the optical cooling of the trapped ions and performance of the ionization electron gun . The force of such magnetic field acting an ion at room temperature is equal to an electric force from electric field below 1 mV/cm.…”