Photon Factory Activity Report 2022 #40 (2023)

軟 X 線励起蛍光用顕微分光器の開発と評価

BL-11D/2020G130

江島丈雄 1,2* 、佐々木英一 21

東北大学 国際放射光イノベーション・スマート研究センター
2
東北大学 多元物質科学研究所〒980-8577 仙台市青葉区片平 2 丁目 1-1

Development and evaluation of micro-spectrometer for SX excited fluorescence
EJIMA, Takeo1,2,* and SASAKI, Eiichi2
1SRIS, Tohoku University
2IMRAM, Tohoku University
2-1-1, Katahira, Aoba-ku, Sendai, 980-8577, Japan
Scintillator, Eu:GGG, is being considered for implementing STED in the soft X-ray (SX) wavelength region, but theoptimal wavelength for inducing fluorescence in SX excited fluorescence is still unknown. To address this issue, a new
microscopic spectrometer has been developed with improvements such as precise pinhole placement and an external
resonator for laser wavelength stabilization. Evaluation results show a decrease in emission intensity of the Eu:GGG
scintillator due to laser irradiation, indicating suppression of induced emission in SX excited fluorescence.
1

Introduction
Certain scintillators can exhibit the stimulated emission
suppression (STED) phenomenon when excited by laser
light [1]. This phenomenon holds promise for further
reducing pixel size in two-dimensional detectors [2]. Twoscintillators, Eu:GGG and Tb:LSO, are particularly
valuable for applying the STED phenomenon due to theirhigh fluorescence intensity in the SX wavelength region[3]. The rare-earth ions in the scintillators demonstratefluorescence levels corresponding to the number of felectrons in their outer shell. However, the optimal
wavelengths for inducing STED phenomenon in thescintillators remain unknown. To explore this, wedeveloped a new microscopic spectrometer (microspectrometer) specifically designed for SX excited
fluorescence and measured the fluorescence intensity [4].
During the STED experiments using the developed
micro-spectrometer, the size of the SX beam, irradiatedsimultaneously with the laser, was approximately 0.2 × 0.2mm2. The beam size was significantly larger than the ϕ1μm spot diameter of the laser beam. As a result, the SX
irradiation area became excessively broad, resulting in an
intense fluorescence that exceeded our measurement
capabilities to assess the laser-induced decrease for STED.
Achieving a matching irradiation area between the SX andthe laser is ideal in the present scenario.
Furthermore, the STED laser employed a laser diodethat matched the fluorescence wavelength of thescintillator. However, it was discovered that the oscillation
wavelength exhibited slight variation of a few nm due toair conditioning of the experimental hall, necessitating
improvements in this aspect. This study addresses theseissues and demonstrates that induced emission at awavelength diminishes the optical intensity of SX excited
fluorescence at a specific wavelength.

Fig. 1：Layout of micro-spectrometer for SX excited
fluorescence.
2

Improvements of a Fluorescence Micro-spectrometer
Figure 1 provides an overview of the developed a microspectrometer for SX excited fluorescence [4]. The figure
illustrates the positioning of the sample at the center of thesample chamber in the micro-spectrometer, which isconnected to the ICF70 nipple of the beamline. The
fluorescence, excited by SX light, is collected andmagnified by an objective lens within the square aperture.
Subsequently, the fluorescence passes through an optical
window, and a dichroic mirror separates it from theirradiation laser light. The separated fluorescence is thenfocused into a pinhole using an imaging lens. Thefluorescence that passes through the pinhole is guided
through an optical fiber to a commercially available
compact spectrometer (BLACK-Comet, StellarNet Inc.).
Additionally, the objective lens functions as theillumination optics for the STED laser beam, reducing thePhoton Factory Activity Report 2022 #40 (2023)

laser beam and other downstream light for sample
illumination.
Two main improvements were implemented. Firstly, ahigh-precision pinhole was introduced in front of thesample to limit the range of SX irradiation and suppress
stray light. The pinhole is designed to be easily replacedand aligned with an accuracy of tens of nanometers,
utilizing a newly developed side-entry vacuum stage(Figure 2). This ensures sub-micrometer alignment of the
optical axes between the SX beam and the optical axis of
the micro-spectrometer.
The second improvement involved the addition of an
external resonator to stabilize the wavelength of the STED
laser [5]. Furthermore, temperature control wasimplemented to ensure a stable light intensity of the laser.
As a result, the wavelength stability was within ±0.3 nm/h,
which corresponds to the minimum measurable step width
for the utilized spectrometer. The normalized variability oflight intensity, expressed as the ratio (%) of the standard
deviation to the mean value of light intensity per hour,
improved from 12.7% to 0.3%.

Fig. 2：The high-precision pinhole manipulator sharesa similar structure to the side-entry manipulators
employed in TEMs. Specifically, inside the vacuumchamber, there is a pinhole attached to an armextending from the left side in the atmosphere. The
pinhole is manipulated by utilizing PicomotorsTM topush and pull the tip of the arm.

3Evaluation Results
We evaluated the improved fluorescence microspectrometer by irradiating the Eu:GGG with 140 eV SXlight using BL11D beamline and obtaining fluorescence
spectra. Additionally, we attempted to suppress the induced
emission of the SX-excited fluorescence by irradiating a708-nm laser beam under the same conditions. We
compared this modified fluorescence spectrum with theoriginal SX-excited fluorescence spectrum. The results
demonstrated a reduction in emission intensity due to laser
irradiation (Figure 4).

Acknowledgments
We are particularly grateful to Prof. T. Higashiguchi andhis students in Utsunomiya Univ. and to Prof. K. Mase in
KEK for their assistance in the measurements. The highprecision pinhole manipulator was manufactured atmachine shop in IMRAM, Tohoku Univ. This work wassupported by JSPS KAKENHI (grant numbers 19H04391).
References[1] M. S. Alekhin, J. Renger, M. Kasperczyk, P.-A.
Douissard, T. Martin, Y. Zorenko, D. A. Vasil'ev, M.
Stiefel, L. Novotny, and M. Stampanoni, Opt. Express25 (2017), pp.1251-1261. doi: 10.1364/OE.25.001251[2] T. Ejima, T. Wakayama, N. Shinozaki, M. Shoji, G.
Hatayama, T. Higashiguchi, Sci. Rep. 10, 5391 (2020).
doi: 10.1038/s41598-020-62100-0[3] T. Ejima, S. Kurosawa, A. Yamaji, T. Hatano, T.
Wakayama, T. Higashiguchi, M. Kitaura, J.
Luminescence,219(2020)
116850.
doi:10.1016/j.jlumin.2019.116850[4] T. Ejima, Photon Factory Activity Report 2020 #39(2021) 109.
[5] Y. Tanaka, Kogaku 41(6) (2012) pp.340-341 (Japanese).
* takeo.ejima.e7@tohoku.ac.jp

Fig. 3 ： Fluorescence of the scintillator when it isirradiated with 140eV SX light passing through a ϕ20μm pinhole positioned in front of it.

Fig. 4：The fluorescence spectra are represented by
the blue and grey curves, corresponding to theexcitation with 140 eV SX light with and without 708
nm wavelength laser irradiation, respectively. The red
curve demonstrates a decrease in spectral intensity
due to the laser irradiation.