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, 211105 (2012) Ashida lab Tomoki Nagao

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

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.

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

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

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

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。 （http://www.g5-hakuto.jp/mass/lalibs/laser_ablation.html)

Experimental setup

Lasing characteristics of ZnO

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

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

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 =−0.0054 𝑛𝑚 −1

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.

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

参考スライド

六角柱

ワイヤー

ラマン分光

半値全幅

Crystal structure of ZnO spheres X-ray diffraction Wurtzite structure (https://ja.wikipedia.org/wiki/結晶構造)

Mode spacing