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Chemical mechanical polishing (CMP) of bulk AlN was performed with colloidal silica slurry at pH 9 for different times. The result shows that colloidal silica slurry at pH 9, which has the relatively high surface charge of-50.7 mV is most stable. Thus, it was selected as chemically optimum condition in this study. The ultrasmooth surface was shown in CMP 90 min with the roughness average (Ra) value of 0.172 nm. It was demonstrated that the damaged layers including subsurface defects and micro scratches in the whole machining process were successfully removed and atomically flat surface can be shown. With increasing process time, the zeta potential and mean particle size of the colloidal silica decreased and increased to 35.07 mV and 143.4 nm, respectively. While the silica particles agglomerated and densely packed slurry particles were formed by mechanical shearing, these increased the Ra value to above 0.5 nm of AlN substrate and generated additional surface damages. In terms of the surface chemistry, the carbon compounds and organic impurities adsorbed on the substrate during mechanical polishing (MP) can be removed and aluminum oxide-hydroxide; AlOOH and Al(OH) 3 were observed during the CMP. It was determined that the chemically polished AlN substrate was continuously hydrated with generating the AlOOH and Al(OH) 3 on the surface.

We employed spark plasma sintering (SPS) in the presence of sintering aids to fabricate highquality Ho 3+ ,Yb 3+-co-doped Y 2 O 3 transparent ceramics suitable for use as a laser gain medium, and analyzed their microstructure and optical properties. Ho 3+ ,Yb 3+-co-doped Y 2 O 3 transparent ceramics with different contents of Yb 3+ were fabricated by SPS and exhibited high transparency in both near-infrared and visible regions. All specimens showed high transparency in both the near-infrared and visible-light regions, and sample microstructure was almost unaffected by the amount of doped Yb 3+. The transmission spectra of 1 mol% Yb 3+ doped specimens exhibited transmissivities of 73.4 and 81.0% at wavelengths of 700 and 1500 nm, respectively. Relatively high transmissivity was quickly achieved through the addition of a sintering aid (1 mol% La 2 O 3) which accelerated mass transport during sintering. The effect of Yb 3+ doping on the upconversion photoluminescence of Y 2 O 3 :Ho 3+ ,Yb 3+ was examined. Irradiation at 980 nm resulted in strong green photoluminescence (552 nm) and weak red photoluminescence (669 nm). Excitation at 447 and 980 nm resulted in strong green emission (5 F 4 , 5 S 2 → 5 I 8) from Ho 3+. The effects of Yb 3+ content and laser input power on emission intensity and luminescence decay were investigated in detail. Under excitation at 980 nm, the presence of Yb 3+ increased emission intensity over the entire range, while the opposite behavior was observed under excitation at 447 nm.

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