STED properties of Ce 3 + , Tb 3 + , and Eu 3 + doped inorganic scintillators

Scintillator-based X-ray imaging is a powerful technique for noninvasive realspace microscopic structural investigation such as synchrotron-based computed tomography. The resolution of an optical image formed by scintillation emission is fundamentally diffraction limited. To overcome this limit, stimulated scintillation emission depletion (SSED) X-ray imaging, based on stimulated emission depletion (STED) microscopy, has been recently developed. This technique imposes new requirements on the scintillator material: efficient de-excitation by the STED-laser and negligible STED-laser excited luminescence. In this work, luminescence depletion was measured in several commonly-used Ce, Tb, and Eu doped scintillators using various STED lasers. The depletion of Tb and Eu via 4f-4f transitions was more efficient (Ps = 8...19 mW) than Ce depletion via 5d-4f transitions (Ps = 43...45 mW). 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d probing a s
mple, is absorbed by a scintillator screen where it forms an optical image via spontaneous emission of the X-ray excited luminescence centers [1].This image is projected on a camera with a standard diffraction limited microscope.To overcome this limit, stimulated scintillation emission depletion (SSED) X-ray imaging technique was recently proposed [2].Its idea was adopted from stimulated emission depletion (STED) microscopy [3,4].A doughnut-shaped STED-laser beam is applied to the scintillator screen simultaneously with X-ray excitation.The STED-beam instantly de-excites the excited luminescence centers via stimulated emission, confining the excited scintillator region to the very center of the doughnut.The scintillation signal from this region is registered in a raster-scan mode.

Crucial scintillator requirements for high-resolution X-ray imaging are high density, h gh effective atomic number (Z eff ), and high X-ray-to-optical light conversion efficiency [5].SSED method imposes two additional requirements: efficient de-excitation of the excited luminescence centers by the STED-laser, and sufficiently weak luminescence excited by the STED-laser itself.To our knowledge, no systematic studies on the depletion properties of scintillation materials exist, although some useful information can be obtained from solidstate lasers [6] or up-conversion nano-probes [7,8] of a similar class of compounds.Therefore, the aim of this work is experimental investigation of the depletion properties of several commonly used in X-ray microtomography scintillators, in particular the Ce 3+ , Tb 3+ , or Eu 3+ doped compounds.Furthermore, lanthanide luminescence centers can themselves be potentially used as nano-probes for STED-microscopy, advantaging in narrow multi-color emission bands, non-blinking, and non-photobleacing [7][8][9].

The resolution in STED-microscopy is determined by the ratio P STED /P s , where P STED is he STED-laser power applied to a specimen, and P s -is the STED laser power at which the probability of the excited luminescence center to be de-excited via stimulated emission is 50%.P s is given by [10]
s STED STED fl Ahc P λ σ τ =(1)
where h, c, A, σ STED , and τ fl are the Planck constant, the ea, the stimulated emission cross-section and the fluorescence lifetime of the luminescence centers respectively.Large σ STED and long τ fl are therefore required for the efficient de-excitation.Ce 3+ emission is caused by 5d-4f electronic transitions with typical τ fl = 15-100 ns [11], an order of magnitude longer than 1-10 ns of STED organic dyes [12,13].σ STED of Ce 3+ is 10 −18 …10 −17 cm 2 [14][15][16], an order of magnitude smaller than 10 −17 …10 −16 cm 2 of STED organic dyes [17,18].Thus, Ce 3+ -doped scintillators are expected to be depleted at similar STED-laser powers as STED-organic dyes.Tb 3+ and Eu 3+ emission is caused by 4f-4f electronic transitions with typical τ fl > 1 ms [19,20], and σ STED ≈10 −20 …10 −22 cm 2 [21][22][23], which might relax the STED-power requirement up to two orders of magnitude.The paper is structured as follows.In section 2, we introduce the investigated scintillators and the setup for the depletion measurements.In section 3, we present luminescence depletion measurements as a function of STED-laser power.From this, we establish the values of P s , evaluate the intensities of unwanted STED-laser excited luminescence, and discuss its origins.


Experiment

Based on high density, high Z eff , and high X-ray-to-optical light conversion efficienc

requirement
[2], the following scintillators were selected for the depletion measurements.[27].Gd 3 Ga 5 O 12 doped with 2.5% at.Eu 3+ (GGG:Eu) 2 μm thick single-crystalline film (SCF) was grown on a 170 μm GGG substrate with LPE method by Martin et al [27].The scintillation properties of these screens are compiled in Table 1, and their emission spectra are shown in Fig. 1.The experimental setup for the luminescence depletion measurements is shown in Fig. 2. Excitation and STED Gaussian laser beams were spatially overlapped into the same pathway using a 30:70 and a 90:10 R:T non-polarizing beam-splitters, and focused with a microscope objective (Nikon Plan Apo 40x/0.95NAair) into the same spot on a scintillator screen.Scintillation emission was collected by the same objective and measured by a photo-detector (Micro Photon Devices, SPAD 50 μm, grade C), which also acted as a confocal pinhole.Scattered and reflected excitation and STED photons as well as Raman-scattered STED-laser photons [36][37][38] were blocked by detection optical filters placed in front of the photodetector.The filters were tilted at 10 0 relative to each other and separated by 2 cm to maximize their optical density (OD) at the excitation and STED laser wavelengths.Emission spectra were measured with a spectrometer (Princeton instruments, Pixis 100B Excelon CCD coupled to a 600 l/mm with 750 nm blaze grating).


Table 1. Scintillation properties of the screens used in the luminescence depletion measurements. Columns from left to right: the thickness of the active layer, the light yield expressed in optical photons per absorbed X-ray energy and corrected for the nonlinearity of the X-ray response at 10-20 keV [27-32], and the scintillation decay time

Each scintillator was tested with several STED lasers in the following combinations.LuAG:Ce excitation was depleted with 568 nm an 647 nm lines of a Kr-Ar cw-laser (Innova 70C, Coherent inc), and with a 628 nm cw-laser (MPB Communications).GGAG:Ce excitation was depleted with the 628 nm laser.LSO:Ce excitation was depleted with a 488 nm line of the Kr-Ar laser and with a 532 nm cw-laser (Verdi V10, Coherent inc).LSO:Tb excitation was depleted with a 542 nm and the 628 nm cw-lasers (MPB Communications), and with 601 and 621 nm lines of a 76.1 MHz fs-pulsed Ti:sapphire pumped OPO laser (Mira-900, Coherent inc).GGG:Eu excitation was depleted with the 628 nm laser.LuAG:Ce, GGAG:Ce, LSO:Ce, and GGG:Eu were excited by a 405 nm cw-laser (CrystaLaser).LSO:Tb was excited by the 488 nm line of the Kr-Ar cw-laser.The scintillator was always positioned with its active layer towards the objective, and the laser was focused through the air inside the scintillator just below the scintillator-air surface.The goal was to have the same spherical aberrations for all the scintillators.This configuration could however enhance the aberrations decreasing actual laser intensity in the confocal volume.The luminescence intensity was measured as a function of STED-laser power multiple times for each combination of scintillator and STED laser.The photo-stability of the scintillators and reproducibility of the results were ensured for several-hour long exposures to < 100 mW STED-lasers.


Results and discussion


Ce 3+ doped scintillators

The emission of Ce 3+ in LuAG, GGAG, and LSO consists of two bands due to transitions fro

e 4f 1 levels split by 0.2
eV [39][40][41].To minimize the excitation by the STED-laser, we adjusted the STED wavelengths to the lower-energy transitions (Fig. 1).The results of LuAG:Ce luminescence depletion measurements are presented in Fig. 3 and Table 2.The depletion was clearly demonstrated with all the STED-lasers.LuAG:Ce luminescence intensity decreased by 20% with 5 mW @ 568 nm, by 65% with 104 mW @ 628 nm, and by 30% with 12.5 mW @ 647 nm.P s of the 628 nm laser was determined as 45 mW.The depletion curve of the 647 nm STED-laser is not shown in Fig. 3 as it coincides with that of the 628 nm at the studied STED-laser powers.All the lasers excited the scintillator as well.LuAG:Ce emission spectra excited by the 568 nm STED-laser and measured in the 494/41 nm detection window revealed Ce 3+ emission similar to that in Fig. 1.Its intensity linearly increases with 568 nm laser power, see bottom plot of Fig. 3, which we ascribe therefore to one-photon excitation (OPE).It can be either direct phonon-assisted excitation of Ce 3+ [42], or excitation of impurities with subsequent energy transfer to Ce 3+ [43].LuAG:Ce emission spectra excited by the 628 nm and 647 nm lasers revealed only 4f-4f transitions of unwanted Re 3+ impurities.GGAG:Ce depletion measurements are shown in Fig. 4. The depletion curve is essentially the same as for LuAG:Ce.The luminescence intensity decreased by 64% with 104 mW @ 628 nm STED-laser, and P s is 43 mW.GGAG:Ce emission spectra excited by the 628 nm STED-laser and measured in the 494/41 and 586/20 detection windows revealed Ce 3+ emission similar to that in Fig. 1. Figure 5 shows luminescence depletion measurements in LSO:Ce.The luminescence intensity decreased by 2% with 6 mW @ 488 nm of STED-laser, and by 20% -with 38 mW @ 532 nm.The depletion is less efficient compared to LuAG:Ce and GGAG:Ce.We ascribe this to detected out-of-focus luminescence that cannot be depleted [2].Due to the 100 μm scintillator thickness, its relative fraction in LSO:Ce is the largest among the studied scintillators.The STED-laser excited luminescence signal is 1-3 orders of magnitude lower compared to that of LuAG:Ce and GGAG:Ce.The lowest energy excitation band of LuAG:Ce [39] is separated from the 568, 628, and 647 nm STED-laser lines by 0.57, 0.78, and 0.84 eV respectively.The lowest energy excitation band of LSO:Ce [40] is separated from the 532 nm STED-laser line by 0.98 eV.This might explain the observed one-photon excitation in the first case, and its absence in the latter cases.The scintillation mechanism of LSO:Tb includes emission from the 4f[ 5 D 3 ] and 4f[ 5 D 4 ] levels of Tb 3+ .Both levels should therefore be de-excited for efficient SSED X-ray imaging.If Tb 3+ concentration in the crystal is high enough (e.g.> 8% in the melt [19]), spontaneous emission rate from the 5 D 3 level becomes negligible compared to the 5 D 3 de-excitation rate via crossrelaxation to the 5 D 4 level [33].Almost all the scintillation is then due to 5 D 4 → 7 F x emission transitions, and only 5 D 4 level of Tb 3+ must be de-excited with a STED laser for the efficient depletion.


Tb 3+ and Eu 3+ doped scintillators

The depletion measurements were performed in two configurations of STED-lasers and emission filters (Fig. 1).In one, we employed the

D 4 → 7 F 4 (601 nm) and 5 D 4 → 7 F
3 (621 nm, 628 nm) transitions for stimulated emission depletion of Tb 3+ excitation, and the 5 D 4 → 7 F 5 transition for luminescence detection.In another, the 5 D 4 → 7 F 5 (542 nm) transition was used for STED, and lower-energy transitions -for luminescence detection.The results of the luminescence depletion measurements of LSO:Tb are shown in Fig. 6.The depletion was observed with all the STED-lasers.LSO:Tb luminescence intensity decreased by 91% with 97 mW @ 542 nm, by 51% with 22 mW @ 601 nm, by 10% with 8 mW @ 621 nm, and by 86% with 104 mW @ 628 nm.P s of LSO:Tb is 8 mW, 19 mW, and 17 mW for the 542 nm, 601 nm, and 628 nm STED-lasers respectively.P s for 542 nm is twice smaller than P s for 601 nm and 628 nm apparently due to larger σ STED for the 5 D 4 → 7 F 5 transition compared to the 5 D 4 → 7 F 3,4 transitions [44].

542 nm and 601 nm STED-lasers cause strong Tb 3+ luminescence signal due to one-and two-photon excitation (TPE), confirmed by linear and quadratic dependences of the emission intensity on the STED-laser power (Fig. 6).TPE can be avoided by using a cw-laser instead of a fs-pulsed.The 628 nm STED-laser produced one of the weakest luminescence signals among all the studied combinations.The results of GGG:Eu luminescence depletion measurements are shown in Fig. 7. Its luminescence intensity decreased by 89% with 104 mW @ 628 nm STED-laser.Its P s = 11 mW is similar to those of LSO:Tb.The STED-laser excited Eu 3+ luminescence signal is caused by OPE as confirmed by a linear fit of the luminesce intensity.In GGG:Eu, the ratio of the luminescence signal caused by the STED-laser to that caused by the excitation laser is the highest among the studied scintillators.Figure 7 then shows both measured and corrected data to visualize the effect of the STED-laser excited luminescence discussed in Appendix A. The depletion of the luminescence has been demonstrated in all the studied scintillators.Due to larger product σ STED τ fl , the depletion of Tb 3+ and Eu 3+ require smaller STED-laser powers (P s ≈10…20 mW) compared to Ce 3+ (P s ≈45 mW).Both are similar to P s ≈10 mW of STED-nanoscopy fluorescent dyes [10] and P s ≈50 mW of quantum dots [45].Due to spherical aberrations, the STED-laser intensity in the confocal volume can be several-fold

Fig. 1 .Fig. 2 .
12
Fig. 1.Emission spectra of LuAG:Ce, GGAG:Ce, LSO:Ce, LSO:Tb, and GGG:Eu scintillators.The spectra were measured with the methods described in the corresponding reports [24, 26, 31, 35].The vertical lines represent the STED laser wavelengths, and the hatched areas designate the transmission windows of the detection filters used in the depletion measurements.


Fig. 3 .
3
Fig. 3. (Top) LuAG:Ce luminescence intensity excited by the 405 nm laser as a function of 568 nm and 628 nm STED-laser power.(Bottom) LuAG:Ce luminescence intensity excited by the 568 nm and 628 nm STED lasers and normalized to luminescence intensity excited by the 405 nm laser as shown in Appendix A. All laser powers were measured before the objective and corrected for the objective transmittance.


Fig. 4 .
4
Fig. 4. (Top) GGAG:Ce luminescence intensity excited by the 405 nm laser as a function of 628 nm STED-laser power.(Bottom) GGAG:Ce luminescence

tensity excit
d by the 628 nm STED lasers and normalized to the 405 nm excited luminescence intensity as shown in Appendix A.


Fig. 5 .
5
Fig. 5. LSO:Ce luminescence intensity excited by the 405 nm laser as a function of 488 nm and 532 nm STED laser power.The luminescence excited by the STED-laser is negligible.


Fig. 6 .
6
Fig. 6. (Top curves) LSO:Tb luminescence intensity excited by the 488 nm laser as a function of time-averaged power of the 601 nm and 621 nm pulsed STED-lasers and as a function of power of the 542 nm and 628 nm cw STED-lasers.(Bottom curves) LSO:Tb luminescence intensity excited by the 542 nm and 601 nm STED-lasers and normalized to the 488 nm excited luminescence intensity as shown in Appendix A.


Fig. 7 .
7
Fig. 7. (Top curves) GGG:E

.6 21.690 9 4915 1700 5000OPE + Tb 3+ emission TPE +
b 3+ emission628-cw171041410GGG:Eu628-cw11104133500OPE + Eu 3+ emission
AcknowledgmentsLuAG:Ce single crystalline films were grown in a frame of Polish NCN 2016/21/B/ST8/03200 project.The growth of GGAG:Ce was supported by European Social Fund's Doctoral Studies and Internationalisation Programme DoRa.We thank Natalia Vasil'eva and Stefan Stutz for scintillator preparation, Pablo Villanueva for critical comments and Zachary Lapin for help with lasers.FundingERC grant ERC-2012-StG 310005-PhaseX.lower than the actual one.In any case, about 50-150 mW of STED-laser power focused into a single doughnut would be required for decent SSED X-ray imaging resolution[2].To relax the power requirement, one can consider to use e. g.Nd 3+ transitions[46], or depletion via upconversion[8].In synchrotron-based SSED X-ray imaging,