The effect of particulate density on performance of Nd:Gd3Ga5O12 waveguide lasers grown by pulsed laser deposition

doi:10.1016/S0030-4018(00)00999-8

Optics Communications

Volume 185, Issues 1–3, 1 November 2000, Pages 145-152

https://doi.org/10.1016/S0030-4018(00)00999-8 Get rights and content

Abstract

We have successfully grown a range of waveguiding layers of Nd:Gd₃Ga₅O₁₂ (Nd:GGG) on Y₃Al₅O₁₂ by pulsed laser deposition for purposes of studying the effects of particulates on waveguiding and lasing performance. We have found that particulates have a detrimental effect on lasing threshold for the range of particulate densities studied, and can increase lasing thresholds from as low as 2.5 up to 167 mW. We have also shown that the detrimental effect of particulates in waveguides becomes less significant with increasing waveguide thickness.

Introduction

Pulsed laser deposition (PLD) has always been an attractive method of fabricating waveguides. The ability to quickly and easily grow stoichiometric, epitaxial films of a large variety of laser media makes it an almost ideal fabrication process [1]. PLD has been avoided as a preferred method, however, due to the large number of particulates that can occur on the waveguide surface during growth. These particulates, which originate from the target, are believed to occur, in the growth of dielectric films, through two main processes [1].

The first is due to incomplete vaporisation of the target area by the ablation process itself. The area directly surrounding the vaporised material melts and is subsequently expelled due to the recoil pressure of the plasma plume shock wave. Liquid droplets of target material consequently form particulates on the growing film. Increasing the density of the target material can minimise this process of particulate production. Single crystal targets therefore are the optimum material for good film fabrication compared to alternatives such as compressed ceramic targets [2]. Alternatively, use of a femtosecond ablation source has also been considered as a potential way of avoiding particulates by this route [3]. The ablation process in femtosecond lasers is a non-thermal process, and therefore problems associated with melt expulsion do not occur. To date no multi-component materials (beyond simple oxides and nitrides [4]) have been deposited using femtosecond PLD and it is uncertain whether the stoichiometry of a complex target material can be reproduced in the film. Additionally, deposition rates appear to be very low, so growth of films of typical waveguide dimensions (few μm) within an acceptable time scale may be difficult.

The second process, termed exfoliation is due to poor surface quality of the target. As the ablation laser pulse continually hits the target area, thin structures of μm dimensions can develop on the target, pointing towards the direction of the laser radiation, due to a shadowing effect [5]. These can eventually break off and form particulates on the film surface. Rotating the target and constantly changing the direction of rotation or re-polishing of the target after use can reduce, but not eliminate, this effect. There have also been many other inventive attempts to remove particulates from the plasma plume [6], [7], [8], [9], [10] however these have often involved complicated set-ups or sensitive triggering equipment and may be considered to be generally impractical.

There has been no direct correlation however, between the number of particulates seen on the films and the losses generated when used as a waveguide. In this paper we report on the systematic comparison between lasing threshold observed in waveguides of differing thickness and varying particulate density.

Section snippets

Waveguide fabrication

Using conditions similar to those reported earlier [2], we have grown pulsed laser deposited films from a 1 at.% Nd doped GGG target on a (1 0 0) YAG substrate. The XRD data shown in Fig. 1 confirms the crystallinity of the films and shows growth in the (4 0 0) Nd:GGG direction parallel to the (4 0 0) YAG peak.

Films have been grown at thicknesses of order 2, 4 and 8 μm. A piezoelectric gas valve was used to modify the particulate density in the film during growth [9], [10] as shown in Fig. 2. Opening

Threshold results for waveguide lasing

To confirm spectroscopic properties, the waveguides were pumped using an Ar⁺ pumped Ti:sapphire laser capable of delivering 600 mW at 808 nm, matching the strongest absorption band of Nd:GGG [11]. Light was coupled into the active layer using 25× objectives for the 2 and 4 μm guides and 16× for the 8 μm guide. Fig. 6 shows the fluorescence spectra of the Nd doped waveguides. This agrees with previous results [2], showing peak fluorescence around 1060 nm. Fluorescence lifetime was also measured

Discussion

These results suggest that were particulates to remain an intrinsic problem in PLD fabrication of waveguide devices, then the solution is to move to thicker waveguides where the particulate density is not such a critical factor in the performance of the resultant laser. Although this seems a relatively intuitive deduction it is in contrast to the accepted notion that lower thresholds can be attained with more confined waveguides structures.

The fabrication of thicker waveguides, however, is

Summary

We have shown that the presence of particulates in PLD waveguides increases the losses and lasing thresholds and have analysed this quantitatively. We have also shown that this effect is less pronounced in thicker waveguides. This is believed to be due to surface scattering of light from the particulates being the dominant cause of loss within the guide.

Acknowledgements

We would like to acknowledge the Engineering and Physical Sciences Research Council (EPSRC) for the funding under grant no. GR/L28722. S.J.B. also acknowledges the receipt of EPSRC financial support.

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The effect of particulate density on performance of Nd:Gd3Ga5O12 waveguide lasers grown by pulsed laser deposition

Abstract

Introduction

Section snippets

Waveguide fabrication

Threshold results for waveguide lasing

Discussion

Summary

Acknowledgements

Thin Solid Films

Appl. Surf. Sci.

Appl. Surf. Sci.

Opt. Commun.

Appl. Phys. Lett.

The effect of particulate density on performance of Nd:Gd₃Ga₅O₁₂ waveguide lasers grown by pulsed laser deposition