Superresolution Microscopy of Single Rare-Earth Emitters in YAG and $H3$ Centers in Diamond

We demonstrate superresolution imaging of single rare-earth emitting centers, namely, trivalent cerium, in yttrium aluminum garnet crystals by means of stimulated emission depletion (STED) microscopy. The achieved all-optical resolution is $\approx 50\mathrm{nm}$. Similar results were obtained on $H3$ color centers in diamond. In both cases, STED resolution is improving slower than the conventional inverse square-root dependence on the depletion beam intensity. In the proposed model of this effect, the anomalous behavior is caused by excited state absorption and the interaction of the emitter with nonfluorescing crystal defects in its local surrounding.

Figure 1

Emission spectra of a (a) ${\mathrm{Ce}}^{3+}$ ion in a YAG crystal and (b) $H3$ center in diamond (single emitters). Both species can be efficiently excited by 470 nm radiation and depleted by a 600 nm laser. In both cases, a quite narrow detection window was used in order to efficiently reject the depletion laser. Slight spectral wiggles are introduced by interference in the optical path and are unphysical.

Figure 2

(a) Superresolution image of individual ${\mathrm{Ce}}^{3+}$ centers in YAG. (b) Image of the same centers obtained with confocal microscopy. (c) Enlarged image of a single ${\mathrm{Ce}}^{3+}$ center representing the PSF of our STED microscope. An ellipsoid shows the section of the PSF at its half maximum. The axes of the ellipsoid are 51 and 68 nm. (d) Photon antibunching shows that the observed ${\mathrm{Ce}}^{3+}$ center is indeed single. The experimental photon correlation data were corrected for background according to the standard correction procedure [11]. (e) Superresolution image of $H3$ centers in diamond. (f) Image of the same centers obtained with confocal microscopy. (g) Enlarged image of a single $H3$ center representing the PSF of our STED microscope. An ellipsoid shows the section of PSF at its half maximum. The axes of the ellipsoid are 41 and 53 nm. (h) The second-order photon correlation function shows that the observed $H3$ center is indeed single.

Figure 3

Double logarithmic plot of the FWHM of superresolution PSF versus depletion beam power. The dependencies are $d\propto P^{-0.25}$ in the case of ${\mathrm{Ce}}^{3+}$:YAG and $d\propto P^{-0.2}$ in the case of a $H3$ center in diamond.

Figure 4

(a) Proposed diagram of ${\mathrm{Ce}}^{3+}$ and electron trap levels in YAG accompanied by possible electronic transitions. $P$ stands for pumping at 470 nm, $D$ for the depletion donut beam, $\mathrm{\Gamma }$ for fluorescence, and the ${\gamma }_i$ for nonradiative decays. The indices to $P$ and $D$ correspond to the appropriated cross sections used. (b) Simulated behavior of the single emitter fluorescence, which was taken to be the maximum value of the PSF for three different distances from the single emitter to the electron trap. The global simulation parameters were $\mathrm{\Gamma }=1$, ${\gamma }_{\mathrm{CBCe}}=400$, ${\gamma }_{\mathrm{CBT}}=400$, and ${\gamma }_{\mathrm{CBVB}}=0.1$. (c) Simulated STED exponents for different distances between the cerium and the electron trap and for different ${\gamma }_{\mathrm{TVB}}$: 0.04 (purple), 0.2 (blue), 1 (red), and 5 (black). The global simulation parameters were the same as in (b). (d) Measured PSF maximum versus STED power of a single ${\mathrm{Ce}}^{3+}$ ion in YAG. (e) Measured FWHM of the PSF versus STED power (black) and power law fit giving the exponential $e=-0.24$ (red).