Comparative growth study of garnet crystal films fabricated by pulsed laser deposition
Introduction
A combination of fabrication technique and a range of materials supporting epitaxial growth suitable for making multilayer planar waveguide structures is highly desirable for utilising high-power diode arrays that are now routinely available and affordable as pump sources [1]. Suitable multilayer cladding-pumped planar waveguide structures analogous to double-clad high-numerical-aperture [2] and large-mode-area fibres [3] would enable low-brightness non-diffraction-limited diode array pump light to be efficiently converted into more useful wavelengths and a higher-brightness single spatial-mode beam with a relatively compact, cheap and simple device layout [1].
The range of garnet crystal compositions is highly suitable for the fabrication of advanced multilayer planar waveguide structures. The wide transparency range, isotropic crystal structure and good thermal conductivity of garnets make them ideal laser hosts, and the availability of different compositions with slightly different refractive indices, but similar thermal expansion coefficients and lattice constants, allows epitaxial multilayers to be built up with custom-designed refractive index profiles. Some properties of the range of garnets under investigation for this work are shown in Table 1 where they have been arranged in order of decreasing refractive index. Compositions at and near the top would be candidate materials for doped cores, whereas compositions further down would be suitable for inner cladding layers and then outer cladding layers, with the exception of Yb3Al5O12 (YbAG) and Yb3Ga5O12 (YbGG), which are of interest for use as a highly doped (100% ytterbium laser-ion substituted for yttrium) ultra-thin cores and as thin-disk laser materials.
The fabrication of crystal multilayers presents numerous challenges such as precise layer thickness control, compatibility with the different desired layer compositions and achieving clean and optically flat layer interfaces. Liquid phase epitaxy (LPE) is capable of producing very high-quality crystal films of thicknesses exceeding 200 μm and intrinsic losses as low as in bulk crystal [4], and single-clad waveguide structures based on Y3Al5O12 (YAG) substrates with Nd:YAG cores have been fabricated [5]. Although thickness control is an issue, layers could simply be polished before subsequent overgrowth of the next layer. However, the growth of complicated structures using different garnet compositions would necessitate the creation of several different flux compositions and require undesirably time-consuming optimisation experiments for each composition. There is no such material limitation for the technique of direct bonding and structures based on Nd:YAG [6] and Yb:YAG [7] cores with YAG inner cladding layers and sapphire outer cladding layers have been reported. However, the fabrication process is technically difficult and is also undesirably time consuming and, significantly, layers can separate due to thermal expansion mismatch when being pumped with very high powers (hundreds of watts).
The motivation behind this work is to test the feasibility of using the technique of pulsed laser deposition (PLD) to grow such multilayer structures with different garnet crystal compositions. PLD offers several advantages such as fast growth rates up to 10 μm h−1, simple layer thickness control by changing deposition time or laser repetition rate, and easy extension to different materials by using pieces of bulk crystals as targets. The combination of PLD and garnet crystals also creates some further potential enhancements. Layer interfaces can be graded instead of being stepped, potentially increasing the layer bonding strength and reducing the chances of layer separation. Also, since film layers are grown at ∼800 °C, the high temperatures involved with high-power diode pumping that would normally lead to stress induced by thermal expansion mismatch should instead lead to relaxation of PLD-fabricated devices.
A problem with PLD that must be overcome is the occurrence of particulates which form scattering centres at layer interfaces and contribute significantly to the overall propagation loss. This is a secondary motivation for cladding layers which have been shown to reduce the effect of particulates by burying them [8]. However, previous reports of laser operation in Nd:Gd3Ga5O12 (GGG) [9] PLD grown planar laser waveguides with a low loss of 0.1 dB cm−1 [10] show that their effect can also be minimised by using simple experimental procedures such as reconditioning target surfaces and performing thick film depositions in multiple runs. More recently, the trial growth of a multilayer double-clad planar waveguide structure consisting of a core of Nd,Cr:Gd3Sc2Ga3O12 (GSGG); inner cladding layers of Y3Ga5O12 (YGG) and outer cladding layers of YAG has been reported [11], proving that epitaxial multilayer growth of different garnet compositions is possible.
Achieving precise control over film stoichiometry is critical if stress-free films and structures are to be produced. In our experience the stresses in gallium deficient GGG films can cause fracturing at the polishing stage of waveguide preparation due to gadolinium ions occupying the gallium lattice sites left open by the missing gallium. An indication of garnet constituent relative ‘deposition volatility’ is therefore of interest to evaluate the relative ease of growth of different garnet compositions. Films of Nd,Cr:GSGG; Cr:Gd3Sc2Al3O12 (GSAG); Nd,Cr:Y3Sc2Al3O12 (YSAG); Nd:GGG; YGG; YAG and YbAG have all been grown on YAG substrates by PLD to allow such an evaluation to be made.
Section snippets
Experimental procedure
Depositions were performed inside a stainless steel vacuum chamber with a base pressure of 10−4 Pa. The ablating laser used was a 248 nm KrF excimer set to an output of about 175 mJ per pulse (∼20 ns pulse duration) operating at a repetition rate of 10 Hz and focussed to produce a fluence of approximately 2.5 J cm−2 at the target surface. Single crystal off-cuts of Nd,Cr:GSGG; Cr:GSAG; Nd,Cr:YSAG; Nd:GGG; YGG; YAG and YbAG were used as targets, and these were rotated to maximise their utilisation by
Results and discussion
XRD spectra of the garnet crystal films with different compositions are shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7. XRD spectra of the thicker film of YbAG (18 μm in thickness) are shown in Fig. 8. Fig. 1(a) shows a spectrum from a full 10–80° 2θ scan and it can be seen that only peaks for the expected epitaxial (4 0 0) and (8 0 0) orientations have been detected. Since this was typical of all the films analysed, only expanded detail views of XRD spectra have been shown for the
Conclusion
The technique of PLD has been used to grow films of Nd,Cr:GSGG; Cr:GSAG; Nd,Cr:YSAG; Nd:GGG; YGG; YAG and YbAG all on YAG substrates. XRD results from the films are in line with the occurrence of single crystal epitaxial growth on the YAG (1 0 0)-oriented substrates. Comparison of the FWHM from the different film XRD spectra suggests that films with quaternary compositions grow with a higher degree of crystal order than ternary compositions. XRD analysis of a thick (18 μm) YbAG film indicates that
Acknowledgements
The authors would like to acknowledge the support of the Engineering and Physical Sciences Research Council (EPSRC) for funding under grant nos. GR/R74154/01 and EP/C515668. The authors would also like to acknowledge the services of the EPSRC National Crystallography Service based in the Chemical Crystallography Laboratory at the School of Chemistry, University of Southampton. T.C. May-Smith also acknowledges the receipt of an EPSRC studentship.
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