Sympathetic cooling of laser-produced doubly charged ions in a few-ion crystal

We present experimental results in which single ${\mathrm{Ca}}^{+}$ ions in a chain of laser cooled ${\mathrm{Ca}}^{+}$ ions are further ionized by means of an intense short pulse laser. The ions are trapped in a linear Paul trap, which is instantaneously loaded by ions from a laser-produced ablation plasma. Due to sympathetic cooling the doubly charged ions are held in place. We study and characterize linear few-ion crystals with mixed charges by applying a radio frequency field, which excites the center of mass (c.m.) and breathing modes of different configurations. From the position shift of laser cooled ions initiated through the higher charge state we can deduce the charge of the nonfluorescing ion. This information might be used as an intensity probe for high intensity lasers.

Figure 1

Shown is the linear ion trap together with an ion detector (MCP), the cooling laser (CL), the pulsed laser for ablation (PL), the femtosecond laser (FL), and the direction of fluorescence observation with a single photon CCD camera (CCD). Shown also are the applied rf and dc trapping voltages, as well as the grounded rods (gnd).

Figure 2

(Color online) Linear three-ion crystal (a) before and (b) after ionization of the inner ion. The higher charge state of the inner ion is indirectly visible in the larger separation of the two outer ions.

Figure 3

(Color online) c.m. resonance frequencies for three different configurations: (a) Three ${\mathrm{Ca}}^{+}$ ions, (b) two ${\mathrm{Ca}}^{+}$ ions and one ${\mathrm{Ca}}^{2+}$ ion, and (c) one ${\mathrm{Ca}}^{+}$ ion and two ${\mathrm{Ca}}^{2+}$ ions. The resolution is $1.788\mu \mathrm{m}∕\text{pixel}$. The ${\mathrm{Ca}}^{2+}$ ion positions are indicated by white lines.

Figure 4

(Color online) c.m. resonance frequencies for four different ion crystals consisting of two ${\mathrm{Ca}}^{+}$ ions and two ${\mathrm{Ca}}^{2+}$ ions. The axial frequency is ${\nu }_z=31.5\mathrm{kHz}$. The ${\mathrm{Ca}}^{2+}$ ion positions are indicated by white lines.