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Spectroscopy and microscopy of single ions in the solid state

Mid-Term Report Summary - SINGLEION (Spectroscopy and microscopy of single ions in the solid state)

In this project we aim to optically address single rare earth ions that are embedded in a solid-state crystal. These emitters comprise many advantages over other quantum systems in the gas phase and in the solid state. In particular, they provide long optical and spin coherence times on the order of tens seconds and well-defined magnetic sublevels. However, the long optical lifetimes lead to weak fluorescence and pose a major challenge in detecting the signal of a single ion. This had hampered the spectroscopy of rare earth ions for nearly three decades.
We started the project with high-resolution spectroscopy on Praseodymium ion ensembles doped into Y2SiO5 (YSO) crystals. For the optical excitation, the transition from the ground state (3H4) to the excited state (3P0) is favorable since 3P0 has a relatively short lifetime (2µs). We measured the absorption of the transition by using a frequency doubled cw Ti:sapphire laser which we tuned across the 3H4 --> 3P0 resonance. The light was coupled into a confocal microscope while fluorescence and extinction measurements were used for detection. The crystal was kept at T = 4.3K in a liquid-helium flow cryostat to reduce thermal broadening. The spectrum shows a transition linewidth of a few GHz, which results from the inhomogeneous broadening of the individual ions in the crystal.
The lifetimes of the 3P0 and 1D2 level were measured in time-resolved fluorescence experiments. Based on these, we could estimate a homogeneous linewidth of 80kHz for the 3P0 state. The lifetime of the intermediate 1D2 level together with the collection and detection efficiency of the confocal microscope setup let us assess the expected fluorescence signal of a single ion to be about 50 counts per second only.
In early 2013, under two years after the beginning of our ERC funding, we succeeded to isolate single ions spectrally from an ensemble in a microcrystal. The major challenge turned out to be the reduction of the background signal from other Pr ions. To address this issue, we milled YSO crystals to obtain micro- and nanocrystals which we deposited on the flat side of a solid-immersion lens (SIL). To image individual crystallites we used interferometric scattering (iSCAT) detection on the reflected excitation light. This signal was further used to actively track the laser spot on a single crystallite. By scanning the laser and its sidebands over several GHz we observe isolated narrow peaks in the fluorescence spectrum, which we could attribute to individual ions. Each ion leads to a peak in the spectrum of approximately 60 counts per second on a background of 20 counts per second.
In a series of careful experiments, we examined the obtained signal. In particular, we studied the temperature dependence of the transition linewidth and determined the saturation intensity of the transition. We also examined the effect of optical pumping and population trapping in the long-living hyperfine levels of the electronic ground state. These features were used to prepare well-defined ground states, which will be of central importance for future experiments. Finally, we demonstrated superresolution microscopy of close-lying ions. Here, the exquisite spectral selectivity of the system allows us to localize the position of various neighboring individual ions independently of others within a microcrystal with a spatial precision of 10nm. Our results set a milestone in the spectroscopy of rare earth ions and nano-optics and will be published in Nature Communications in mid-April 2014.
In parallel to the above-mentioned efforts, we investigated techniques to enhance the spontaneous emission rate of a single emitter. We were able to enhance the fluorescence of a single molecule experimentally by a factor of 64 by using a dimer antenna (Optics Express 20, 23331 (2012)). Furthermore, our theoretical calculations have shown that conical nanoantennas should be able to enhanced spontaneous emission by several thousand times (Phys. Rev. Lett. 108, 233001 (2012)). We plan to apply these methods to increase the fluorescence signal of an ion from a few tens per second to few thousands per second.