1. Ultraviolet whispering-gallery-mode lasing in ZnO micro/nano sphere crystal
Kota Okazaki, Tetsuya Shimogaki, Koshi Fusazaki, Mitsuhiro Higashihata, Daisuke Nakamura, Naoto Koshizaki, and Tatsuo Okada
Appl. Phys. Lett. 101, (2012)
Ashida lab
Tomoki Nagao

2. Why this paper?
To study Whispering Gallery Mode (WGM) and Zinc Oxide(ZnO).

3. Abstract
They report ultraviolet (UV) whispering-gallery- mode (WGM) lasing in a zinc oxide (ZnO) micro/ nanosphere crystal fabricated by simply ablating a ZnO sintered target.
The lasing spectral mode spacing was controlled by changing the diameters, and single-mode lasing was realized from a ZnO nanosphere.

4. cavity cavity・・・confines light 2𝑑=𝑚 λ 𝑛
mirror d
2𝑑=𝑚 λ 𝑛
( λ : wavelength, 𝑛 : refractive index, 𝑚 =1,2,3,…)
→ wave strengthen

5. Whispering Gallery Mode(WGM)
If total reflection is repeated, the light goes around the sphere.
WGM
WGM spectrum
𝐿≈𝜋𝐷=𝑚 λ 𝑛
𝐿 : cavity length, 𝐷 : diameter
→ wave strengthen

6. Motivation
ZnO is one of the most prospective materials in ultraviolet (UV) region.
The spherical-cavity lasing in UV region from any materials including ZnO has not been reported, so far.
Conduction band
3.37 eV
Valence band

7. ZnO micro/nanosphere Laser ablation Irradiate laser on surface
→ Constituent of sample explosively released
(for example atoms, molecules, fine particles etc.)
物質をエッチングする技術。レンズで集光照射された固体表面は局所的に高温になり、プラズマの発生、衝撃波の発生組成原子や分子、クラスターや光の爆発的な放出、様々な物理現象が生じる。 （

8. Experimental setup

9. Lasing characteristics of ZnO

10. Q factor Q= λ 𝑤 D=3.6μm 𝑄= 399.4 0.69 ≅579 D=0.65μm 𝑄= 395.4 1.4 ≅282
λ : the peak wavelength
𝑤: linewidth(FWHM)
D=3.6μm
永井先生に言われた、何周期分滞在するかの表記も検討！
D=0.65μm

11. The WGM-cavity Q factor
𝑄= 𝜋𝑎𝑛𝐷 𝑅 𝑎 4 2λ(1− 𝑅 𝑎 2 ) sin 2𝜋 𝑎 ⁡
𝑎 : internal reflection times 𝑛 :refractive index 𝐷 :spherical diameter 𝑅 :reflectivity at the boundaries
𝐷=3.6 𝜇𝑚, n≈2.3, λ=399.4 𝑛𝑚, 𝑄=579
∴𝑅>0.89
Similarly, 𝐷=0.65 𝜇𝑚,, λ=395.4 𝑛𝑚
𝑅>0.95

12. Mode spacing ∆λ= λ 𝑚 2 𝐿 𝑛 𝑚 − λ 𝑚 𝑑𝑛 𝑑λ + λ 𝑚 𝐿 −1 λ 𝑚 ≒400 𝑛𝑚
∆λ= λ 𝑚 2 𝐿 𝑛 𝑚 − λ 𝑚 𝑑𝑛 𝑑λ + λ 𝑚 𝐿 −1
∆λ : mode spacing
𝑚 : modal number
𝐿 : cavity length =𝐷𝑎∗𝑠𝑖𝑛 𝜋 𝑎
λ 𝑚 ≒400 𝑛𝑚
𝑛 𝑚 =2.3
𝑑𝑛 𝑑λ =− 𝑛𝑚 −1

13. Summary Superior UV lasing could be observed from a ZnO sphere.
Experimental results were in good agreement with WGM theories in terms of Q factor and mode spacing.

14. My Work
In air
My purpose is fabricating ZnO microspheres with higher Q values in superfluid helium. Therefore, working on elucidation of the detailed mechanism of laser ablation in superfluid helium.
In superfluid helium

15. 参考スライド

16. 六角柱

17. ワイヤー

18. ラマン分光

19. 半値全幅

20. Crystal structure of ZnO spheres
X-ray diffraction
Wurtzite structure (

21. Mode spacing