Photon Factory Activity Report 2019 #37 (2020)

Stimulated emission depletion phenomenon
in luminescence of Ce: LSO excited by soft X-rays

BL-11D/2018G072

EJIMA, Takeo1,*, WAKAYAMA, Toshitaka2, SHINOZAKI, Natsumi3, SHOJI, Misaki3,
HIGASHIGUCHI, Takeshi3
1 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,

2-1-1, Katahira, Aoba-ku, Sendai, 980-8577, Japan

2 Faculty of Health and Medical Care, Saitama Medical University,

1397-1, Yamane, Hidaka, 350-1241, Japan

3 Faculty of Engineering, Utsunomiya University,

7-1-2, Yoto, Utsunomiya, 321-8585, Japan

A stimulated emission depletion (STED) phenomenon occurred in the luminescence of a Ce-doped Lu2SiO5
crystal (Ce:LSO) excited by soft X-rays using BL-11D beamline, when the scintillator was simultaneously
irradiated with azimuthally polarized laser light. The observed spot size of the luminescence was reduced by
the STED phenomenon. Application of this STED scintillator emission to a two-dimensional sensor is one
promising method to reduce the pixel size of SX cameras.
1 Introduction
The spatial resolution of a stimulated emission
depletion (STED) microscope is represented by themodified Abbe diffraction limit [1]. When the spatial
resolution δdN of an ordinary objective lens at awavelength λ is given by the Abbe diffraction limit, δdN= λ/2NA, where NA is the numerical aperture of theobjective lens. The spot diameter δdSTED of theluminescence spot limited by the STED is given by
the modified Abbe diffraction limit is𝛿𝛿𝑑𝑑STED𝛿𝛿𝑑𝑑𝑁𝑁=1

�1+𝐼𝐼STED ⁄𝐼𝐼sat,(1)

where Isat is the fluorescent saturation intensity ofthe phosphor, ISTED is the light intensity of thestimulated emission light at the focal plane, and𝐼𝐼STED ⁄𝐼𝐼sat is referred to as the normalized laser
intensity [1].
Some scintillators have demonstrated the STED
phenomenon under UV light excitation, such as
Tb:LSO scintillators, and the possibility of realising a
super-resolving microscope using a scintillator has
been proposed [2]. If this STED phenomenon can beapplied to detection of the luminescence image of thescintillator plate, the spatial resolution of theluminescence image will decrease and exceed theAbbe diffraction limit [1].
One scintillator, Lu2SiO5 crystal doped with cerium
atoms (Ce:LSO), shows that the luminescenceintensity has a maximum value at a wavelength, 400
nm and shows a tail structure on the long-wavelengthside from the peak [3]. Because the Lu atoms in theLSO crystal are located at two different sites and theCe atoms that are introduced randomly displace Luatoms, the Ce atoms are thus also coordinated to thetwo different sites. The dominant transitions of theluminescence are from the lowest unoccupied Ce 5dstate to the two occupied Ce 4f states: the 2F5/2 and2F7/2 orbitals. Therefore, the Ce:LSO luminescence
spectrum shows four different luminescence peaks.
In this paper, we demonstrate the STED
phenomenon in the luminescence of Ce:LSO that has
been excited by SX light and the area limits of theluminescence region using vector light at the weakluminescent wavelength of Ce:LSO.
2 Experimental
A laser diode (L520P50, Thorlabs Inc.) having awavelength of 520 nm with the power of 50 mW inmaximum used as a STED light source. The lightemitted from the laser was introduced into the vector
light optical system, which is composed of a lightvortex optical system and a vector-vortex opticalsystem, and the vortex phase can be canceled byadding the vortex phases having e+iθ and e-iθ,
respectively (Fig. 1) [4]. The sample Ce: LSO wasoptically polished to a thickness of 0.2 mm and had asize of 5 × 5 mm2.
Beamline BL-11D was used as an SX light source
for the SX excited STED experiment. Since the facility
operated in "Top-Up Mode" at the time of themeasurement, no normalization was performed by
the incident SX light intensity in the STED experiment.
The wavelength resolution of the incident SX lightwas λ / dλ = 1000. For the reduction of a STED lightspot and the enlargement of a scintillator image, aself-made microscope system was used (Fig. 1). The
reflection light of STED light and the luminescencelight of the scintillator were separated from the lightfrom a sample by a dichroic mirror with an optical filter
which was inserted between an infinity-corrected
objective lens (NA = 0.2, 10X) and an imaging lens.

Photon Factory Activity Report 2019 #37 (2020)

Fig. 1: Layout of optics, (a), vector generation optical system, (b), microscope unit for SX excitation.
values of the objective lens was 0.044 at the SX
excitation measurements.
3 Results & Discussion
Figure 2(a) shows the emission point excited at thewavelength 403 nm with the STED beam spot
irradiated. The light intensity profile between thepoints A and B is also plotted in Fig. 2(b). In this case,the diameter of the emission point was 1/16 of theSTED beam diameter. The STED phenomenon of thescintillator excited by an SX light of the photon energy,
800 eV, was also confirmed [4].
4 Summary
A phenomenon of STED occurred in theluminescence of a Ce doped Lu2SiO5 crystal (Ce:LSO) excited by UV or SX light when the scintillator
was irradiated with azimuthally polarized laser lightwith the wavelength 520 nm. The size of theluminescence reduced by the STED phenomenon
and decreased also as the increase of the lightintensity of the STED laser.
Acknowledgments
We are particularly grateful to Prof. K. Mase and Mr.
T. Kikuchi for their assistance in the measurements.
This work was supported by JSPS KAKENHI (grantnumbers 16H03902 and 17H190210).
Fig. 2：Luminescence of the Ce:LSO,(a), andintensity profile at line A-B,(b).
A camera head for detecting images has 2560 × 1920
pixels, and color filters deposited on it operates in thewavelength range of B (390 to 510 nm), G (480 to 600nm), and R (580 to 680 nm). Thus, the camera headcan separate the color of an image, therefore theexcitation light and the luminescence light if the colorsof the lights are different. The obtained lightintensities of the STED light and the scintillator's
luminescence were normalized by the measurement
time and the number of pixels. The effective NAReferences[1] E. Rittweger, K. Y. Han, S. E. Irvine, C. Eggeling,
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* takeo.ejima.e7@tohoku.ac.jp