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Xusan Yang


Peking University, No.5 Yiheyuan Road, Haidian District,

Beijing, P. R. China 100871

Phone:  (86) 156-5296-9441

E-mail:  xusanyang@pku.edu.cn

      His research focused on both Stimulated Emission Depletion (STED) microscopy and Localization Microscopy techniques. He is working on technology development to improve the speed, the resolution and the depth penetration of these imaging techniques to expand the application range of super-resolution microscopy to unlock new physics in single nanoparticle (UCNPs, FNDs and Quantum dots) and capture unknown biological process within nanoscale resolution. Xusan is currently working on multiple projects to further improve fluorescence imaging technology and applying these cutting-edge techniques to current biological and physical questions. I also served as OSA, SPIE and OWLS member, and also reviewer of Biomedical Optics Express, Applied Optics, Chinese Optics Letters and reviewer for OSAF fellowship and OSA prize, as well as session chair of some conference scuh as Optofluidics 2016. Xusan have published about 20 peer reviewed journal papers on such as Nature, Nature Photonics, Light: Science & Applications, ACS photonics and so on. He also holds more than 10 patents or pending patents.


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Lecture content:

Photon-avalanche-induced Stimulated Emission in Upconversion Nanoparticles for STED Nanoscopy



Department of Biomedical Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China

Email: xusanyang@pku.edu.cn

        Establishment of population inversion is the key to amplify stimulated emission. Here we discover that upconversion nanoparticles (UCNPs) doped with high concentrations of Tm3+ ions, excited at 980 nm, can easily establish population inversion on their intermediate metastable levels. At high Tm3+ doping concentration, the reduced inter-emitter distance induces intense cross-relaxation in the photon avalanche regime, which consumes the excited Tm3+ at higher levels to quickly populate the intermediate level, resulting in population inversion to the ground level within a single nanoparticle. As a result, a 7-mW laser at 808 nm, matching one of the upconversion bands of Tm3+, 3H4à3H6, can trigger amplified stimulated emission to discharge the intermediate level, and therefore the upconversion pathway to generate blue luminescence can be optically inhibited. We employ this approach to realize a low-power super-resolution stimulated emission depletion (STED) microscopy and achieve 28 nm optical resolution (λ/36) to image the single UCNPs. This work benchmarks the first STED nanoscopy using a low-power, low-cost diode laser, reducing the depletion power by two orders of magnitude.

Key words: super resolution; upconversion; Photon-avalanche; low power

OCIS Codes: □□□□; □□□□; □□□□; □□□□; □□□□

Stimulated emission lays the foundation for a number of phenomenal advancements in physics ranging from lasers and optical fibre amplification2 to quantum cloning3 and super-resolved fluorescence microscopy4. The rare-earth upconversion nanoparticle (UCNP) with particle size of only 40nm, to achieve super-resolution by using its unique feature in energy level, and ultra-low power super-resolution has been demonstrated successfully through the intermediate energy level depletion. Traditionally, STED requires high power laser because the fluorescence has only two energy levels: ground state and excited state. Benefitted from the rich intermediate states of UCNPs, stimulated emission can be induced with very little power. A proper choice of the intermediate state can reach lever effect to effectively deplete the electrons to ground state, prohibiting their further transferring to upper energy level. We also found that such effect can only appear in the highly doped nanoparticles, whereas low-doped nanoparticles cannot be depleted effectively. Through the study of doping concentration and extinction ratio, scientists have revealed the photon avalanche effect in high dopant UCNPs, which reflects a higher nonlinearity than resonant energy transfer.


Figure 1  Super-resolution imaging of the highly-doped UCNPs. (a) Schematics of the upconversion-STED super-resolution imaging of the UCNPs, in which a Gaussian excitation (980 nm) and Gauss-Laguerre mode “doughnut” inhibition (808 nm) at far field are employed. (b) Transmission electron microscopy (TEM) image of the 8% Tm-doped UCNPs, revealing average size of 39.8 nm. Scale bar: 200 nm. (c) The resolution enhancement with the increase of the inhibition light intensity. Dash line indicates the resolution of 50 nm. The 980 nm excitation is kept 4 mW. Pixel dwell time: 4 ms. Scale bars: 500 nm. (d) The intensity profiles along the lines across two UCNPs in the confocal and upconversion-STED images at 808 nm laser power of 7 mW. An FWHM of 73.8 nm (Gaussian fitting) is achieved, corresponding to super-resolution of 62.1 nm after deduction of the bead size. (e) With 808 nm power of 39 mW, an FWHM of 48.3 nm can be attained, corresponding
to super-resolution of 27.4 nm.


Figure 2 Competition between absorption and stimulated emission. (a) The transient response of the 455 nm emission from 8% Tm-doped UCNPs under synchronous 980 nm and 808 nm pulses (1 ms duration). The 980 nm laser power was fixed at 1 mW, while the 808 nm laser power was varied from 0 to 40 mW. (b) Schematics illustrating net absorption (left) and net stimulated emission (right) between 3H4 and 3H6 levels when the UCNPs under 980 nm excitation are probed with the 808 nm laser, which then lead to either inhibition or enhancement of the further upconverted emission. (c) The transient response of the 455 nm emission from 1% Tm-doped UCNPs under measurement conditions identical to those in (a).

  In a combination of the fluorescence properties of UCNPs and the mechanism of intermediate state depletion, researchers have achieved optical resolution as high as 28nm on one single UCNP particle of 40nm and 13nm. Such resolution will help to reveal the structure and function of cells in different life cycles, the virus invasion process, etc. Moreover, as the upconversion of nanoparticles using near-infrared light for excitation, this work has the potential to be used in deep tissue three-dimensional super-resolution imaging.


[1]  Maiman, I. H. Stimulated optical radiation in ruby. Nature 187, 493-494 (1960).

[2]  Mears, R. J., Reekie, L., Jauncey, I. M. & Payne, D. N. Low-noise erbium-doped fibre amplifier operating at 1.54μm. Electronics Letters 23, 1026-1028 (1987).

[3]  Lamas-Linares, A., Simon, C., Howell, J. C. & Bouwmeester, D. Experimental quantum cloning of single photons. Science 296, 712-714 (2002).

[4] Hell, S. W. & Wichmann, J. Breaching the diffraction resolution limit by Stimulated emission depletion fluorescence Microscopy. Optics Letters 19, 780-782 (1994).

[5]  Liu, Y., Lu, Y., Yang, X., Zheng, X., Wen, S., Wang, F., … & Ma, C. (2017). Amplified stimulated emission in upconversion nanoparticles for super-resolution nanoscopy. Nature, 543(7644), 229-233.

[6]  Yang, X., Xie, H., Alonas, E., Liu, Y., Chen, X., Santangelo, P. J., … & Jin, D. (2016). Mirror-enhanced super-resolution microscopy. Light: Science & Applications, 5(6), e16134.

[7] Yu, W., Ji, Z., Dong, D., Yang, X., Xiao, Y., Gong, Q., … & Shi, K. (2016). Super‐resolution deep imaging with hollow Bessel beam STED microscopy. Laser & Photonics Reviews, 10(1), 147-152.