Fabrication and optical properties of Y2O3-based ceramics with broad emission bandwidth

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Abstract

In this article, we report on the fabrication and optical properties of highly transparent yttria ceramics for lasers active media with broadband gain profile. Laser synthesis method was used to produce Y2O3-based nanopowders doped with 1 mol.% Nd3+ or Yb3+ for these transparent ceramics. The additives of sesquioxides Lu2O3 and Sc2O3 were used along with ZrO2 to disorder the crystalline structure. The porosity and average grain size decrease with these additives and the emission bandwidths of Nd3+ (4F3/2  4I11/2) and Yb3+ (2F5/2  2F7/2) transitions widen to 40 and 60 nm, respectively. Laser operation with the slope efficiency of 29% was obtained in [(Yb0.01Lu0.24Y0.75)2O3]0.88(ZrO2)0.12 ceramic sample.

Introduction

It is known that lasing of active medium doped with rare-earth ions performs on optical transitions between stark components of electronic states of these ions.1 In turn, energetic position of stark level is defined by crystalline field in position of active ion. Disordering of crystalline structure leads to formation of energy band of stark levels in a crystal and consequently to broadening of gain bandwidth. Such laser media with disordered crystalline structure are promising for ultra-short pulse generation. For that purpose the highly transparent ceramics are most suitable because their sintering temperature much lower than growth temperature of single crystal. It makes possible to fabricate ceramics based on high-temperature sesquioxides.

The goal-directed disordering of crystalline structure of ceramics was first investigated by the authors of Ref. 2 who reported on the synthesis of highly transparent Nd3+-doped Y3ScAl4O12 ceramics. Such laser medium represents an yttrium–aluminum garnet in which one aluminum ion replaced by scandium ion. Emission bandwidth of such ceramics was 5.5 nm. Ultra-short pulses with duration of 10 ps were obtained in Nd3+:Y3ScAl4O12 ceramics. When Nd3+ ion was replaced by Yb3+ ion the emission bandwidth of Yb3+:Y3(Sc0.5Al0.5)2O12 ceramics reached 12.5 nm at the wavelengths of interest and pulses duration was reduced to 580 fs.3

For Nd3+ ion the greatest spectral bandwidth of 30 nm was achieved in Nd3+:Ba(Zr, Mg,Ta)O3 (Nd3+:BZMT) disordered ceramics4 and 1.4 ps pulses were realized.

Lowest pulse duration of 112 fs with disordered ceramics was received in Yb3+:(YGd2)Sc2(GaAl2)O125 at the spectral bandwidth of 14 nm.

To date ultra-short pulse generation was obtained not only in disordered ceramics but in certain Yb3+ doped sesquioxides (e.g. Y2O3, Sc2O3). In this case the emission bandwidth of Yb3+ ion reaches 15 nm without any disordering additives. Thus 188 fs pulses were reported using Yb3+:Y2O3 ceramics.6

The most impressive results in this direction were obtained with combined ceramic medium. Ultra-short 65 fs pulses were realized in Yb:Lu2O3–Y2O3 combined ceramic medium with the total emission bandwidth of 18.9 nm.7 Shorter pulses duration of 53 fs was achieved with another combined ceramic medium of Yb3+:Sc2O3–Yb3+:Y2O3 with the total spectral bandwidth of 27.3 nm.8

The purpose of this paper is to report on fabrication process and characteristics of a highly transparent Y2O3-based ceramics activated with Nd3+ or Yb3+ (CNd/Yb = 1 mol.%). The additives of sesquioxides Lu2O3 and Sc2O3 were used along with ZrO2 to disorder the crystalline structure.

Section snippets

Experimental procedure

Nanopowders serve as an initial material in the fabrication of optical ceramics. The synthesis of nanopowders is one of the most complex, intensive and important stages because the demands such as high purity, small particle sizes, weak agglomeration and complex chemical composition are rather difficult to fulfil. We used laser synthesis of nanopowders, the method that completely satisfies the mentioned requirements; its detailed description is given elsewhere.9

Laser targets were prepared from

Microstructure of synthesized samples

The grain sizes in samples doped with Nd3+ and sintered at 1700 °C for 20 h were as follows. In (Nd0.01Y0.99)2O3 they were largest and equal to 8 μm. Doping such ceramics with 24 mol.% Sc2O3 and 12 mol.% ZrO2 reduced the average grain size to 5 μm. Least average grain size ∼1 μm was found for [(Yb0.01Lu0.24Y0.75)2O3]0.88(ZrO2)0.12 sample sintered at 1700 °C for 20 h. Fig. 1 shows the optical microscopy images of the polished surfaces of (Nd0.01Y0.99)2O3 and [(Yb0.01Lu0.24Y0.75)2O3]0.88(ZrO2)0.12 samples

Laser oscillation

Earlier we have received the laser oscillation in (Nd0.01Y0.99)2O3 optical ceramics with the same characteristics.10 Unfortunately, attempts of lasing realization in the most transparent [(Nd0.01Y0.99)2O3]0.88(ZrO2)0.12 ceramics were unsuccessful. As mentioned above, it is probably connected with accelerated Förster's migration of the 4F3/2 level of Nd3+ at presence of Zr4+ ions. Therefore, the main attention was given to receiving of laser oscillation in [(Yb0.01Lu0.24Y0.75)2O3]0.88(ZrO2)0.12

Conclusion

The fabrication technique of optical ceramics was developed using a known approach and nanopowders prepared with laser synthesis method. Highly transparent ceramic samples of (Nd0.01Y0.99)2O3, [(Nd0.01Y0.99)2O3]0.88(ZrO2)0.12 and [(Yb0.01Lu0.24Y0.75)2O3]0.88(ZrO2)0.12 with the transparency respectively 81.34, 81.4 at 1079 nm and 80.57% at 1064 nm were fabricated.

Doping Nd(Yb):Y2O3 with Sc2O3, Lu2O3 and ZrO2 reduces the grain sizes (in certain cases to ∼1 μm) and pore content up to receiving

Acknowledgements

This work was performed within the projects RFBR №s 11-08-00005-а, 10-08-00078 and 11-02-12164. The authors are grateful to the researchers who have contributed to the production of nanopowders: Platonov V.V. and Podkin A.V.

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