Laser ablation and two-step photo-ionization for the generation of 40Ca+

In this paper, we present a detailed method for rapidly and effectively loading a single 40Ca+ ion into a miniature linear Paul trap. Calcium atoms are generated by laser ablation, and the single ion is loaded and specific isotopes are selected by a two-step photo-ionization method. Compared to the traditional photo-ionization method in which atoms are typically emitted from a resistively heated oven, the advantages of laser ablation are that it can be precisely controlled, it can effectively avoid the problem of fluxed calcium deposition on the trap electrodes, and it restricts the generation of dark ions, which can influence atomic-precision spectroscopy measurements. With its short loading time, it is possible to load an ion in few seconds with the appropriate parameters. The technique of laser ablation has been applied to generate several species of atoms and ions at present, it will potentially benefit in the studies of optical frequency standards, quantum simulation and quantum information processing in an ultra-high vacuum condition and cryogenic system.


Introduction
Trapped and laser-cooled ions are proven to be ideal candidates for optical frequency standards [1,2] and quantum information processing [3,4], due to their environmental isolation and long interrogation time. There are similar applications for the measurement of lifetimes and lifetime ratio [5][6][7][8][9], branching fractions [10,11], and transition matrix elements [12,13]. As detrimental destruction to vacuum conditions is inappreciable through accurate control by laser ablation, an ideal vacuum condition is beneficial to the experimental accuracy for limiting the effects of collisions and J mixing [14,15]. In the method of electron impact ionization with a resistively heated oven, dark ions are often produced, which affecting vacuum pressure, causing collisions between the ultra-cold ions and background gases, results in trapped ion depopulation [8]; reactions that generate molecule-ions [16], leading to the loss of individual ions. The situation is similar to the pulse laser ablation (PLA) at high intensity, without requiring other lasers for photo-ionization, ablation pulses can directly create ions. Thus some multiple dark ions can be loaded into the trap simultaneously, and undergo sympathetic cooling by calcium ions. The heating effect of dark ions not only affects the cooling efficiency of the target ions, but also further affects measurements taken with atomic precision spectroscopy. The laser ablation method, combined with use of ionization lasers, gives rise to accurate control and high efficiency during ion loading. The similar method of 40 Ca + ion loading will benefit the study of the characteristics of many kinds of trapped ions such as 37 Al + , 88 Sr + , 138 Ba + , 159 Hg + , etc.
To our knowledge, up until now the methods of electron impact ionization [17][18][19] and photo-ionization [20][21][22][23][24][25][26][27][28][29][30][31] have been the universal methods for producing ions in an ion trap. The traditional method of electron impact ionization is an effective tool that was first used for the generation of ions decades ago. The disadvantages of this method are the lack of isotope selectivity, the lack of control over the number of loading ions, and that the Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.
Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. residual gas in the vacuum can also be ionized. The fluxed sample also deposits onto the trap electrodes during the loading process, which causes patch potentials and increases the heating rate of trapped ions from the motional ground state when compared with the case of clean electrode surfaces [17][18][19]. Later, in order to overcome the shortcomings of electron impact ionization, the method of using a resistively heated oven with photo-ionization was universally adopted. Using photo-ionization to load a single calcium ion into a Paul trap was first demonstrated by Kjaergaard et al [21], who adopted the 4 s 2 1 S 0 -4s5p 1 P 1 transition at a wavelength of 272 nm for excitation far above the ionization limit. Later, Gulde et al [22] and Schuck et al [23] demonstrated another scheme with the 4 s 2 1 S 0 -4s4p 1 P 1 transition at 423 nm followed by excitation close to the continuum by ultraviolet radiation (from an LED) near 390 nm. Following the success of this transition, Lucas et al [24] and Tanaka et al [25,26] performed 40 Ca + loading as well as the rare isotopes of 43 Ca + , 46 Ca + and 48 Ca + . Recently, a different scheme demonstrated by Zhang et al [27] used a 423 nm-wavelength laser combined with another of 732 nm for the realization of 40 Ca + loading. By adjusting the temperature of the atomic oven and using the appropriate-wavelength ionizing lasers, this method allowed isotopic selectivity and avoided ionization of the residual gases, but the problems of electrode contamination and uncontrollable loading rate persisted. Recently, with the development of laser techniques, PLA as a simple and versatile experimental method with efficient, clean and controllable advantages has been adopted. Due to experimental requirements, Zimmermann et al [28] adopted this scheme for the direct generation of Th + ions, and Leibrandt et al [29] for 88 Sr + ions, during the ablation process. Hendricks et al [30] adopted this scheme for generating calcium ions with an ablation beam at a wavelength of 1064 nm, and a photo-ionization laser at a wavelength of 272 nm, in a linear Paul trap. In that paper, the ablation depth was studied as a function of pulse energy, and the influence of vacuum pressure was also monitored. The relationship between properties of Rydberg atoms and laser power was also investigated. Later, Sheridan et al [31] performed a detailed investigation of the difference between this all-optical ion generation technique and the resistively heated oven method.
In this paper, in order to accurately load a single 40 Ca + ion with maximum probability and avoid the dark ion problem, we use 532 nm-wavelength laser pulses to generate calcium atoms and a two-photon resonanceenhanced photo-ionization scheme with ultraviolet laser sources of 423 nm and 375 nm wavelengths to acquire a single ion. This method can avoid heat load during loading process, which is especially important for cryogenic systems [32][33][34], and increase the rate at which ions can be reloaded after they are lost. The laser ablation technique can also govern the shape of the heated area, and the ability to target small areas makes it possible to avoid large pressure rises during loading and obtain a faster recovery time for returning to the initial level of pressure [30]. Ion-trapping experiments typically aim to load either a single ion or small numbers of ions, so a large continuous flux of atoms passing through the trap structure is undesirable. Therefore, the aforementioned advantages will provide important benefits for optical frequency standards. The effects of intensity and frequency of the three lasers are studied carefully, with results showing that the short loading time makes it easier to produce a single ion within a few seconds. With optimal parameters, only one ion is generated in the trap in each loading process in a controllable and clean manner, which can successfully reduce electrode contamination and avoids the issue of dark ions affecting atomic precision spectroscopy. The method could potentially be used to generate ions in miniature ion traps, especially chip-scale traps [35,36], with only tens of microns between the trapped ions and the electrode surface.
The ablation and photo-ionization schematic is shown in figure 1. First, calcium atoms are generated by the laser pulses when the energy density exposure on the surface of calcium target exceeds the threshold value of relaxation processes such as thermal conduction, which requires the laser pulse duration to be smaller than the relaxation time ( figure 1(a)). The dipole transition 4s4s 1 S 0 ↔4s4p 1 P 1 of neutral calcium is then excited by a laser operating at a wavelength of 423 nm, and followed by excitation to the continuum by a laser at a wavelength of 375 nm ( figure 1(b)). Lastly, the newly-ionized ion is trapped in a linear trap and Doppler cooled by 397 nmwavelength and 866 nm-wavelength lasers (figure 1(c)).

Experimental apparatus
In our experiment, three lasers are used for loading the ion into the trap. The experimental apparatus is shown in figure 2. First, a 532 nm-wavelength ablation laser is used for producing the atoms, this wavelength was chosen due to the requirements of nanosecond pulse duration and high pulse energy for ablation experiments. The laser has a pulse width of approximately 20 ns at full-width half-maximum (FWHM), and an effective output power of 128 mW under a 1 kHz repetition rate. Short duration pulses allow the energy density in the region to exceed the ablation limit and facilitate the removal of material from the target. For conveniently aligning the laser beams, 532 nm was selected as the visible wavelength, also this wavelength will be more strongly absorbed by metals than infrared radiation. The three lenses in figure 2 are used to keep the beam waist on the calcium target below 30 μm. A signal generator combined with an acousto-optical-modulator (AOM) controls the ablation laser power and exposure time of the calcium surface while randomly changing the ablation region by changing its frequency in the range of 180-220 MHz. The 423 nm-wavelength photo-ionization laser is used for driving the dipole transition from the ground 4s4s 1 P 1 state to the intermediate 4s4p 1 P 1 state, while another laser with a wavelength of 375 nm is used for driving the transition from the 4s4p 1 P 1 state to the continuum. The two photo-ionization lasers are coupled to one fiber and controlled by a shutter, with a beam waist of ∼100 μm in the trap center. By this process, the ion is generated. The ion trap is mounted in a vacuum chamber with a vacuum pressure of approximately 2.0×10 −9 Pa to maintain a stable environment for long-lifetime trapping. An ideal vacuum environment is not only beneficial for reducing the effect of collisions between trapped ions and background gases, but also for preventing the loss of ions through reactions with those gases that generate new molecule-ions. The chambers with relevant attachments are constituted of titanium metal, a material that possesses nonmagnetic characteristics, which is beneficial for magnetic control. Once the ion is successfully loaded, the 397 nm-wavelength laser with a power of 35 μW and beam waist of 40 μm and 866 nm-wavelength laser with a power of 47 μW and beam waist of 86 μm are used to cool the newly loaded ions to the Lamb-Dicke regime. The powers of 397 and 866 nm under their beam waist are found to be the optimum values for a single ion in our trap. As the lifetime of the 4p 2 P 1/2 state in the 40 Ca + ion is approximately 7 ns [13], it is very suitable for Doppler cooling at 397 nm. The laser beam is red-detuned by 100 MHz from the resonant frequency of the dipole transition 4 s 2 S 1/2 ↔4p 2 P 1/2 . As there is a probability that the ion could decay from the 4p 2 P 1/2 state to  Experimental apparatus for calcium ion loading. All of the lasers used are diode lasers, and their wavelengths are monitored with a wavemeter. The ablation pulse laser at 532 nm is used for the generation of atoms; the combined 423 nm and 375 nm lasers are used for two-step photo-ionization; the 397 nm and 866 nm lasers are used for Doppler cooling, which are stabilized by a ULE cavity with a linewidth of about 100 kHz. The frequency and power of the 532-nm laser are precisely controlled by a computer, and measurements are performed in an ultrahigh vacuum chamber with a pressure of 2.0×10 −9 Pa. the 3d 2 D 3/2 state [10] resulting in the cooling being interrupted, an 866 nm-wavelength laser is applied to depopulate the 3d 2 D 3/2 state. The 866 nm-wavelength laser frequency is locked in the position of the 4p 2 P 1/2 -3d 2 D 3/2 resonance. The cooling and detection lasers at 397 nm and 866 nm are frequency-locked to a high-finesse ULE (ultra-low-expansion) cavity using the Pound-Drever-Hall scheme, with a typical linewidth (FWHM) of ∼100 kHz. The power of the 423 nm and 375 nm lasers can be adjusted from 0 to 3.5 mW and 1 mW by using separate attenuators, while the 532 nm-wavelength laser can be adjusted from 0 to 128 mW. The fluorescence of the 4p 2 P 1/2 →4 s 2 S 1/2 dipole transition is recorded at 397 nm. The signal detection is carried out by combining an Electron-Multiplying Charge-coupled Device (EMCCD) and a photomultiplier tube (PMT) with the 397 nm and 866 nm lasers switched on. A narrow-band-pass filter and a pinhole with a diameter of 50 μm are placed in front of the PMT and EMCCD to block the background scattering light. The fluorescence directly into PMT or EMCCD is controlled by a high reflectivity mirror (shown in figure 2). All vacuum windows are coated with antireflection film to ensure high transmittance while reducing background light which may affect signal detection. A computer recorded all experimental data and simultaneously controlled the photon counter, AOMs, and shutters via a PIC-6733 DAQ card with microsecond accuracy.

Ion loading scheme
A simplified sequence for the measurement of loading a single 40 Ca + ion is shown in figure 3. This sequence consists of three major steps for a complete round of measurement. In the first step (t 1 ), the 397 nm and 866 nm lasers are employed to measure the background level as the state with no ions. After confirming the background level, the ion loading process is performed in the second step in a process where all five lasers are all switched on simultaneously for an interval. The atoms are generated by 532 nm-wavelength ablation pulses to the calcium metal surface while photo-ionization is performed with 423 nm and 375 nm ultraviolet lasers in the center of the trap. If the ion is successfully produced, the detected signal will appear greater than background level. In this instance, the computer sends a command to the AOM and shutter in the 523 nm, 423 nm, and 375 nm light routes simultaneously to switch those lasers off. The loading time is denoted as t. The delay time between ion production and ionization lasers switch off is about 200 ms. In order to distinguish the ion level from the background level, an optimal threshold value is needed. In the last step (t 2 ), the 397 nm and 866 nm lasers are combined with the photon counter for the recording of the ion's fluorescence. When the 866 nm-wavelength laser is blocked, the fluorescence drops to the background level instantly, demonstrating that the ions have been generated. The ion is then released and the cycle is repeated until six of the same processes are completed. In figure 3, the high steps represent the time when the lasers or counter are switched on, and the low steps represent periods when they are off.
Before carrying out the study, the PLA is first performed in an auxiliary system where a mass spectrometer is utilized. The beam waist and power are consistent with the system previously described, except that the vacuum pressure is inferior, in the order of 10 −6 Pa. The red and blue lines in figure 4 show mass scans with the atomic beam production by PLA turned off and on, respectively. From figure 4 we can see that a peak appears at a mass of 40 atomic units relative to the background level when the ablation laser of 532 nm is incident on the calcium target surface, which demonstrates that the calcium atoms can be generated through this form of PLA. Figure 5 shows the single trapped ion that was created by the pulse laser ablation and two-step photoionization scheme. The black points and corresponding images on the right, represent photon counts and EMCCD signals, respectively, from one to four loaded ions. The photon counts are measured in 200 ms recording intervals. The signal of a single ion varying with a 397 nm-wavelength detuning frequency as detected by PMT is shown in the top left corner. The maximum photon count value at the resonant frequency can reach 11, which is the accumulative total number detected in 0.15 ms recording intervals (for 200 ms recording intervals, the resonant fluorescence of a single ion can reach 15 000). The signal is optimized in several ways, by carefully aligning the laser beams for better Doppler cooling, by micromotion compensation through voltages in the electrode, by a radio-frequency field, and by adjusting for the Hanle effect.

Loading time measurements
A single calcium ion can be successfully loaded by PLA and the two-step photoionization scheme, but which is affected by many factors such as laser frequency and power. Maintaining a short loading time and avoiding dark ion generation are the key reasons to be considered in optical frequency standard. In this section, the loading time is measured as a function of the power and detuning frequency of the 532 nm, 423 nm and 375 nm lasers.
In the first experiment, the loading time as a function of the 532 nm-wavelength pulse laser power is studied. We used a model of power function to display the tendency of loading time with 532 nm laser' power, the results are shown in figure 6. Each point represents the average loading time of six independent experiments, while the error bars in the figure indicate the standard deviation of the mean. From the red data points of figure 6 we can see that the loading time is improved from 10 s to 2 s when the power is varied from 50 mW to 88 mW. However, when the laser power is set below 50 mW, we cannot directly detect ion generation by the 532 nm-wavelength laser alone. These results demonstrate that an ion can be directly produced by the ablation laser at high intensity. The loading time is verified to be further improved by experimentally introducing the 423 nm and 375 nm lasers. Here, the 423 nm and 375 nm lasers are operated at a power of 3 mW and 1 mW, repectively, with a waist of 100 μm, and the 423 nm-wavelength laser frequency is 709.077 80 THz. As shown in the blue data points of figure 6, there is a significant decrease in loading time for each successive point with the inclusion of the photoionization laser beams, especially when the 532 nm-wavelength laser power is low. It is important to note that the enhancement in this case is not obvious when the 532 nm-wavelength laser power is greater than 30 mW. Therefore, short loading time can be reached with high ablation and photoionization laser power.
In the second experiment, the loading time as a function of the 423 nm-wavelength laser detuning frequency is studied. The results are shown in figure 7. Again, each point represents the average loading time of six independent experiments, and the error bars in the figure indicate the standard deviation of the mean. From figure 7, we can see that the loading time is decreased from 10 s to 2 s with the detuning frequency set close to the resonant frequency of the 4s4s 1 S 0 ↔4s4p 1 P 1 dipole transition. Fitting a Gaussian to the loading times measured as a function of the laser frequency yields a value of 709.077 74 THz for the centroid of this transition.
Aside from the frequency of the 423 nm-wavelength laser, the loading time may also be affected by the laser power. In the third experiment, the loading time as a function of the 423 nm and 375 nm laser powers are investigated independently. The ablation laser with a wavelength of 532 nm is set to a power of 30 mW in 1 kHz pulses as only atoms are created at this power without the direct generation of ions, allowing the change in loading time with the power of the 423 nm and 375 nm lasers to be observed. Figure 8 shows the results of plotting the loading time as a function of the power of the photo-ionization lasers. We used a model of power function to display the tendency of loading time with 423 nm and 375 nm lasers' power. The blue points represent the loading time with varying 423 nm-wavelength laser power, performed with a power of 1.0 mW for the 375 nm-wavelength laser, while the red points represent the loading time with varying 375 nm-wavelength laser power, with a power of 3.0 mW for the 423 nm-wavelength laser. The results show that at lower PLA intensities the loading times are improved with increasing powers of both the 423 nm and 375 nm lasers, but as the powers increase further the effect saturates. When the power of the 375 nm and 423 nm lasers reach 1.0 mW and 2.0 mW, respectively, the mean loading time can stably reach 4 s, but does not appear to improve further with increasing laser power. From experimental results, we conclude that powers of 30 mW at 523 nm, 1 mW at 375 nm, and 2 mW at 423 nm are the optimal parameters for producing a single calcium ion when combined with the 423 nm-wavelength laser frequency of 709.077 74 (20) THz. With these parameters selected, we find that the single ion can be loaded with a short loading time of approximately 4 s in each loading process. The single ion loading probability was briefly investigated by 30 independent experiments with the appropriate parameters and detected the single ion loading of 24. Moreover, with these parameters, we can successfully avoid generated dark ions affecting atomic precision spectroscopy measurements.

Conclusion
In this paper, we present a scheme that utilizes pulse ablation and two ultraviolet lasers for the photo-ionization of calcium in a miniature linear Paul trap, and report the appropriate and critical parameters for accurately and effectively loading a single 40 Ca + ion in optical frequency standard. The atoms are generated by the method of Figure 7. The loading time plotted against the 423 nm-wavelength laser frequency. The ions were produced by PLA of atoms with a wavelength of 532 nm, power set to 40 mW, and a waist of 28 μm. The photo-ionization lasers' power was 1 mW at a wavelength of 375 nm, and 3 mW at 423 nm, respectively. The frequency detuning of the 423 nm-wavelength laser is varied from 709.0768 THz to 709.0790 THz. The blue points show the measured loading times and illustrate its dependence on the frequency detune. Given error bars are the standard deviations of the mean from six independent measurements. The red line is a Gaussian fit to the data to extract the centroid.