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Nd3+ doped Sc2O3 waveguiding film produced by pulsed laser deposition
Optical Materials 28 (2006) 883–887 www.elsevier.com/locate/optmat
Nd3+ doped Sc2O3 waveguiding film produced by pulsed laser deposition Y. Kuzminykh *, A. Kahn, G. Huber Institut fu¨r Laser-Physik, Luruper Chaussee 149, 22761 Hamburg, Germany Available online 2 November 2005
Abstract We report here for the first time on growth and characterization of a neodymium (Nd3+) doped scandium oxide (Sc2O3) waveguiding film. The pulsed laser deposition technique (PLD) was used to deposit a 3 lm thick film on a sapphire (a-Al2O3) substrate, which provides almost lattice matched growth conditions. A 1.5 lm thick Al2O3 superstrate was deposited on top of the scandia film. Crystalline structure of the film was investigated by X-ray diffraction. The film exhibits high crystalline quality and has been proved to be highly textured in h1 1 1i direction. An x-scan has been performed and the deviation in crystallites orientation has been determined to be 1.07. Emission and excitation spectra of the Nd3+ dopant in the film are very similar to those in the scandia bulk crystal. The spectral lines exhibit no broadening and high emission cross-sections are observed. The lifetime of metastable 4F3/2-multiplet of Nd3+ has been measured to be 180 ls. Waveguiding of the Ti:Sapphire laser emission in the Nd:Sc2O3 film has been demonstrated and propagation losses have been estimated. 2005 Elsevier B.V. All rights reserved.
1. Introduction In recent time, compact elements for integrated optical devices acquire much attention, as they are very promising for communication, data processing and sensing applications. Though variety of individual elements for integrated optics have been already reported (e.g. [1–3]), a wide range of individual passive and active elements is still necessary to develop. Active optical elements like light emitters or amplifiers require high gain materials, so that their size can be reduced. Although semiconductor materials are often used, oxide materials could be advantageous in certain cases, where lower refractive indices (1.8–2.0), low electrical conductivity, wide transparency range (UV to mid-IR) and the ability to host rare-earth-ions are essential. Neodymium doped materials, which are well known for high gain and effective laser operation, seem
*
Corresponding author. E-mail address: [email protected] (Y. Kuzminykh). 0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.09.051
to fulfill the requirements. The following neodymium doped waveguiding films have been already reported: Nd:YAG [4], Nd:GGG [5,6], Nd:YAP [7], Nd:KGW [8]. Up to now only in garnet films laser action has been achieved. In this paper we present a neodymium doped scandium oxide waveguiding film grown by pulsed laser deposition (PLD). Scandia is a promising host material, as it exhibits a higher thermal conductivity than YAG (17 W/mK and 11 W/mK, respectively) and also exerts a very high crystal field on rare earth ions. Strong crystal field causes neodymium ions to provide spectral lines on wavelengths that are not achievable in other host materials [9]. This property of Nd:Sc2O3 can be favorable for sensing applications. Although effective laser action has been demonstrated by Fornasiero et al. [9], the high melting point of scandia hinders the growth of high quality bulk crystals and special techniques need to be utilized [10]. Thus, deposition methods such as PLD can be an attractive alternative. Also molecular beam epitaxy (MBE) grown undoped Sc2O3 films by have been reported recently [11,12].
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Y. Kuzminykh et al. / Optical Materials 28 (2006) 883–887
2. Film fabrication The Nd:Sc2O3 film has been grown by pulsed laser deposition technique using a KrF excimer laser operating at a wavelength of 248 nm, at a repetition rate of 37 Hz and with a pulse duration of 25 ns. The radiation density on the ceramic target was estimated to be 4 J/cm2. The target was prepared by pressing and sintering (at 1600 C) Sc2O3 powder with some amount of Nd2O3 powder added to provide the 0.2 mol % concentration of Nd3+ ions in the scandia crystal structure. The substrate was a 10 · 10 · 0.5 mm (0 0 0 1)-oriented sapphire (a-Al2O3) plate polished with epitaxial quality. During the deposition the substrate was heated up to 700 C. The background gas in the deposition chamber was molecular oxygen at a pressure of 0.33 Pa, the target-substrate distance was 8 cm. The oscillation of red laser diode beam intensity reflected from the substrate was used to control film thickness during the deposition. On top of the 3 lm thick Nd:Sc2O3 film a 1.5 lm thick Al2O3 superstrate was deposited to reduce scattering on the film/air interface and to make the refractive index profile of the waveguide more symmetric. The superstrate also serves as mechanical protection during the subsequent endfaces polishing. The ceramic Al2O3 target was prepared in the same way as the Sc2O3 one. 3. Structural characterization The crystalline quality of the deposited film is a important factor for active integrated optics components doped with rare-earth ions, as the peak emission and absorption cross-sections of the dopant strongly depend on quality of the crystalline structure of the host material. Since a high gain material is required in order to reduce the size of active elements, crystalline materials having high peak cross-sections are preferred. Moreover, an oriented crystal-
line growth is desired in order to reduce scattering on the grain boundaries. In spite of the fact that the crystal structure of Sc2O3 under normal conditions is cubic (space group Ia3, a = 0.9846 nm) and the one of a-Al2O3 is hexagonal (space group R3c, a = 0.4759 nm, c = 1.2989 nm), the (1 1 1)plane of Sc2O2 and (0 0 0 1)-plane of a-Al2O3 have the same hexagonal symmetry with the lattice mismatch pffiffiffi of only 2.5%. (Corresponding relation is 3 aAl2 O3 2 aSc2 O3 Þ. Thus (0 0 0 1)-oriented sapphire substrate provide good conditions for quasi lattice matched heteroepitaxy. The crystalline structure of the Sc2O3 film was investigated by X-ray diffraction (XRD) using a Siemens Kristalloflex 810 diffractometer, which provides radiation from a copper X-ray tube. The diffraction pattern of the sample is presented in Fig. 1. Only the {2 2 2}-peak from the Sc2O3 film and the {0 0 0 6}-peak from the sapphire substrate are visible. This indicates that the film is highly textured in h1 1 1i direction and the portion of crystallites with other orientations is negligible. The width of the {2 2 2}peak is approximately 0.14. Taking into account the distance between Cu-Ka1 and Cu-Ka2 lines of 0.08 and using the Scherrer equation, the size of crystallites in the film was estimated to be 150 nm. To determine the orientation deviation of crystallites in the film an x-scan (rocking curve) was performed (Fig. 2). The width of the rocking curve peak is 1.07 for the PLD-film and 0.3 for a Sc2O3 bulk crystal measured for comparison. 4. Spectroscopic characterization The spectroscopic properties of Nd3+ dopants in the Sc2O3 film have been determined, by means of emission and excitation spectroscopy as well as lifetime measurements of the 4F3/2-multiplet. PLD technique is well known to preserve the stochiometry of deposited material, e.g. Ba¨r
1.1
1.1
(a)
I [a.u.]
1.0
{222} Sc2O3
(b)
{222} Sc2O3
1.0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3 0.2 0.1
0.3
{0001} Al2O3
0.2 0.1
{222} Sc2O3 Cu Kβ
0.0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 31.0
31.2
31.4
31.6
0.0 31.8
2Θ [deg] Fig. 1. X-ray diffraction pattern of Nd:Sc2O3 3 lm thick film deposited on a (0 0 0 1)-oriented a-Al2O3 substrate: (a) low resolution scan (slit 0.15), 2H from 20 to 50; (b) high resolution scan (slit 0.05), 2H from 31.0 to 31.8.
Y. Kuzminykh et al. / Optical Materials 28 (2006) 883–887 1.1
2Θ=31.35˚
1.0
1.0
0.9
0.9
0.8
0.8
0.7
0.7
I [a.u.]
I [a.u.]
1.1
0.6
FWHM= 1.07˚
0.5
885
λ em = 1082 nm
4
4
4
4
I9/2-> F5/2
I9/2-> F7/2
0.6 0.5 0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0.0 720 730 740 750 760 770 780 790 800 810 820 830 840 850
0.0 13
14
15
16
17
18
λ [nm]
19
Θ [deg]
Fig. 4. Excitation spectra of Nd3+-ion in Sc2O3 film.
Fig. 2. x-Scan (rocking curve) of {2 2 2} diffraction peak of Nd:Sc2O3 film.
et al. demonstrated this for Eu3+:Y2O3 system [13], which is very similar to Nd:Sc2O3. Thus, the concentration of neodymium in the film can be assumed to be the same as in the ceramic target (0.2 mol.%). The emission and excitation spectra have been recorded using a 1 m SPEX monochromator, an InGaAs detector and a Ti:Sapphire laser as an excitation source. Fig. 3 shows a spectrum of stimulated emission crosssection for the 4F3/2 ! 4I11/2 transition (kex = 826 nm), resulting from the Fu¨chtbauer–Ladenburg equation. This transition is most often utilized for laser action, and in the case of scandia host matrix lasing can be achieved either at 1082 nm or at 1087 nm. The spectrum of the Nd:Sc2O3 film resembles the one of the bulk crystal very closely [14]. The emission cross-section values exceed the ones presented in [15] by about 10%. This effect is caused by shorter lifetimes obtained in our measurements.
11 10
4
4
F3/2-> I11/2
Unfortunately it was not possible to measure an absorption spectrum of neodymium by conventional methods. The signal was too weak because of the quite thin active layer (3 lm). So the excitation spectrum (kem = 1082 nm) in the range of 720–850 nm has been measured (Fig. 4), in order to estimate the best wavelength for pumping. The 826 nm line could the most beneficial, but also the 808 nm line, where commercial laser diodes are available, can be utilized. The luminescence decay curves for the 4F3/2 ! 4I9/2 transition (966 nm) of Nd3+ for the film and for the ceramic target are presented in Fig. 5 for comparison. The excitation wavelength was 826 nm. It can be seen that the decay has a nonexponential behavior. This is caused by nonlinear processes (up-conversion, cross-relaxation and energy migration) taking place in spite of the low neodymium concentration. The similar effect was observed by Fornasiero [14] for neodymium doped bulk sesquioxide crystals. Therefore only the later part of the decay curve was used
λ ex=826 nm
e
Measurement Fit
0
9
e-1
7
e-2
6
I [a.u.]
σem [10-20cm2]
8
5
(a)
e-3
4
(b) e-4
3 2
e-5
1 0 1050 1060 1070 1080 1090 1100 1110 1120 1130 1140 1150 1160
λ [nm] Fig. 3. Emission cross-section spectrum for 4F3/2 ! 4I11/2 transition of Nd3+-ion in Sc2O3 film.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
t [ms] Fig. 5. Luminescence decay curves for 4F3/2 ! 4I9/2 transition of Nd3+ in: (a) Sc2O3 ceramic target; (b) 3 lm Sc2O3 PLD-film.
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to estimate the exponential decay component. The fluorescence decay lifetime of 4F3/2 multiplet for the film was measured to be 180 ls and 230 ls for the ceramic target used as a reference. Imperfect crystalline structure of the film is believed to cause a faster nonradiative depopulation of the 4F3/2 excited state of Nd3+-ion in the film. Only few reports of lifetime measurements for Nd:Sc2O3 can be found in the literature for comparison: 260 ls (0.5% Nd) [15], 224 ls (0.7% Nd) [9], 285 ls (0.12% Nd) [14]. Strong dependence on the neodymium concentration is observed, hence it can be concluded that processes other than spontaneous emission noticeably affect luminescence decay of the 4F3/2 multiplet. Thus, a complex fitting model will be required to determine the intrinsic radiative lifetime of 4 F3/2(Nd3+) in Sc2O3. Sharp spectral lines obtained in emission and excitation spectra confirm the high crystallinity of the scandia film. High peak cross-section values make Nd:Sc2O3 films suitable for high gain active components. The reduced lifetime values in comparison with that observed in ceramic and bulk crystal can result in a slightly higher threshold for laser operation. 5. Waveguiding experiments Due to the fact that the refractive index of scandia is larger than that of sapphire ðnAl2 O3 ;od ¼ 1:763; nAl2 O3 ;ext ¼ 1:754; nSc2 O3 ¼ 1:981 at 720 nmÞ it was possible to achieve guiding of light in the film. Radiation of a Ti:Sapphire laser, operating at a wavelength of 720 nm, has been coupled into the film at a polished end face using a microscope objective (50·, NA = 0.5). Another objective and a CCD camera have been used to record the image of the opposite end face of the sample (Fig. 6). The waveguide can be easily recognized. However inhomogeneity of the light intensity is observed. We explain this phenomenon by interference of light scattered on the crystallites inside the film. Luminescence of the neodymium dopant has been observed with the same CCD-camera from the side of the
Fig. 6. Image of Nd:Sc2O3 3 lm thick PLD waveguide. Ti:Sapphire laser emission at 720 nm coupled in.
sample, when Ti:Sapphire laser was tuned to the absorption line of Nd3+ (826 nm). A low-pass filter was placed in front of the camera in order to block the pump light. The emission was not detected, if the pump wavelength was detuned from the absorption peak. The fluorescence intensity decay along the sample was recorded with the camera and utilized to estimate propagation losses in the film. Attenuation due to scattering (8.5 cm1/37 dB cm1) was determined as difference between the measured attenuation and calculated neodymium absorption (5 cm1). The propagation losses for Nd:Sc2O3 film are much higher than, the loss values achieved for other neodymium doped waveguides (e.g. [6]). The cause of such high losses is still not clear. We suppose this can originate from scattering on grain borders or particulates. Thus further optimization of the deposition process is still required. 6. Conclusion In this work we presented Nd:Sc2O3 waveguiding film doped with neodymium grown by pulsed laser deposition method. The X-ray diffraction analysis showed that film is highly textured in h1 1 1i direction and confirmed high crystalline quality. Emission and excitation spectra of neodymium ions in the film resemble closely the spectra in the bulk crystal. The spectral lines do not exhibit broadening and have high peak cross-sections. The lifetime of the 4 F3/2-level of Nd3+-ion, which is important for potential laser operation, was shorter in the film than in the ceramic target (180 ls and 230 ls, respectively). Waveguiding properties of the film were also demonstrated using emission of a Ti:Sapphire laser. However, further research need to be carried out in order to reduce propagation losses. References [1] T.A. Ibrahim, K. Amarnath, L.C. Kuo, R. Grover, V. Van, P.-T. Ho, Opt. Lett. 29 (2004) 2779. [2] P.G. Kika, A. Polman, J. Appl. Phys. 93 (2003) 5008. [3] A. Driessen, D.H. Geuzebroek, H.J.W.M. Hoekstra, H. Kelderman, E.J. Klein, D.J.W. Klunder, C.G.H. Roeloffzen, F.S. Tan, E. Krioukov, C. Otto, H. Gersen, N.F. van Hulst, L. Kuipers, in: AIP Conference Proceedings, 709 (1) (2004) 1. [4] I. Chartier, B. Ferrand, D. Pelenc, S.J. Field, D.C. Hanna, A.C. Large, D.P. Shepherd, A.C. Tropper, Opt. Lett. 17 (1992) 11. [5] D.S. Gill, A.A. Anderson, R.W. Eason, T.J. Warburton, D.P. Shepherd, Appl. Phys. Lett. 69 (1996) 1. [6] C. Grivas, T.C. May-Smith, D.P. Shepherd, R.W. Eason, Opt. Commun. 229 (2004) 355. [7] J. Sonsky, J. Lancok, M. Jelinek, J. Oswald, V. Studnicka, Appl. Phys. A 66 (1998) 5. [8] P.A. Atanasov, R.I. Tomov, J. Perriere, R.W. Eason, N. Vainos, A. Klini, A. Zherikhin, E. Millon, Appl. Phys. Lett. 76 (2000) 18. [9] L. Fornasiero, E. Mix, V. Peters, E. Heumann, K. Petermann, G. Huber, in: Advanced Solid-State Lasers, OSA Trend in Optics and Photonics Series, vol. 26, 1999, p. 249. [10] V. Peters, A. Bolz, K. Petermann, G. Huber, J. Cryst. Growth 237– 239 (2002) 879. [11] A.R. Kortan, M. Hong, J. Kwo, P. Chang, C.P. Chen, J.P. Mannaerts, S.H. Liou, in: J. Morais, D. Kumar, M. Houssa, R.K. Singh, D. Landheer, R. Ramesh, R.M. Wallace, S. Guha, H.
Y. Kuzminykh et al. / Optical Materials 28 (2006) 883–887 Koinuma (Eds.), Integration of Advanced Micro- and Nanoelectronic Devices—Critical Issues and Solutions, Mater. Res. Soc. Symp. Proc. 811, Warrendale, PA, 2004, p. 329. [12] D.O. Klenov, L.F. Edge, D.G. Schlom, S. Stemmer, Appl. Phys. Lett. 86 (2005) 5.
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[13] S. Ba¨r, G. Huber, J. Gonzalo, A. Perea, M. Munz, Appl. Phys. A 80 (2005) 209. [14] L. Fornasiero, Nd3+ and Tm3+ dotierte Sesquioxide, Thesis, University of Hamburg, 1999. [15] G. Zverev, G. Kolodnyi, A. Smirnov, Opt. Spectra 23 (1967) 325.
Nd3+ doped Sc2O3 waveguiding film produced by pulsed laser deposition Y. Kuzminykh *, A. Kahn, G. Huber Institut fu¨r Laser-Physik, Luruper Chaussee 149, 22761 Hamburg, Germany Available online 2 November 2005
Abstract We report here for the first time on growth and characterization of a neodymium (Nd3+) doped scandium oxide (Sc2O3) waveguiding film. The pulsed laser deposition technique (PLD) was used to deposit a 3 lm thick film on a sapphire (a-Al2O3) substrate, which provides almost lattice matched growth conditions. A 1.5 lm thick Al2O3 superstrate was deposited on top of the scandia film. Crystalline structure of the film was investigated by X-ray diffraction. The film exhibits high crystalline quality and has been proved to be highly textured in h1 1 1i direction. An x-scan has been performed and the deviation in crystallites orientation has been determined to be 1.07. Emission and excitation spectra of the Nd3+ dopant in the film are very similar to those in the scandia bulk crystal. The spectral lines exhibit no broadening and high emission cross-sections are observed. The lifetime of metastable 4F3/2-multiplet of Nd3+ has been measured to be 180 ls. Waveguiding of the Ti:Sapphire laser emission in the Nd:Sc2O3 film has been demonstrated and propagation losses have been estimated. 2005 Elsevier B.V. All rights reserved.
1. Introduction In recent time, compact elements for integrated optical devices acquire much attention, as they are very promising for communication, data processing and sensing applications. Though variety of individual elements for integrated optics have been already reported (e.g. [1–3]), a wide range of individual passive and active elements is still necessary to develop. Active optical elements like light emitters or amplifiers require high gain materials, so that their size can be reduced. Although semiconductor materials are often used, oxide materials could be advantageous in certain cases, where lower refractive indices (1.8–2.0), low electrical conductivity, wide transparency range (UV to mid-IR) and the ability to host rare-earth-ions are essential. Neodymium doped materials, which are well known for high gain and effective laser operation, seem
*
Corresponding author. E-mail address: [email protected] (Y. Kuzminykh). 0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.09.051
to fulfill the requirements. The following neodymium doped waveguiding films have been already reported: Nd:YAG [4], Nd:GGG [5,6], Nd:YAP [7], Nd:KGW [8]. Up to now only in garnet films laser action has been achieved. In this paper we present a neodymium doped scandium oxide waveguiding film grown by pulsed laser deposition (PLD). Scandia is a promising host material, as it exhibits a higher thermal conductivity than YAG (17 W/mK and 11 W/mK, respectively) and also exerts a very high crystal field on rare earth ions. Strong crystal field causes neodymium ions to provide spectral lines on wavelengths that are not achievable in other host materials [9]. This property of Nd:Sc2O3 can be favorable for sensing applications. Although effective laser action has been demonstrated by Fornasiero et al. [9], the high melting point of scandia hinders the growth of high quality bulk crystals and special techniques need to be utilized [10]. Thus, deposition methods such as PLD can be an attractive alternative. Also molecular beam epitaxy (MBE) grown undoped Sc2O3 films by have been reported recently [11,12].
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Y. Kuzminykh et al. / Optical Materials 28 (2006) 883–887
2. Film fabrication The Nd:Sc2O3 film has been grown by pulsed laser deposition technique using a KrF excimer laser operating at a wavelength of 248 nm, at a repetition rate of 37 Hz and with a pulse duration of 25 ns. The radiation density on the ceramic target was estimated to be 4 J/cm2. The target was prepared by pressing and sintering (at 1600 C) Sc2O3 powder with some amount of Nd2O3 powder added to provide the 0.2 mol % concentration of Nd3+ ions in the scandia crystal structure. The substrate was a 10 · 10 · 0.5 mm (0 0 0 1)-oriented sapphire (a-Al2O3) plate polished with epitaxial quality. During the deposition the substrate was heated up to 700 C. The background gas in the deposition chamber was molecular oxygen at a pressure of 0.33 Pa, the target-substrate distance was 8 cm. The oscillation of red laser diode beam intensity reflected from the substrate was used to control film thickness during the deposition. On top of the 3 lm thick Nd:Sc2O3 film a 1.5 lm thick Al2O3 superstrate was deposited to reduce scattering on the film/air interface and to make the refractive index profile of the waveguide more symmetric. The superstrate also serves as mechanical protection during the subsequent endfaces polishing. The ceramic Al2O3 target was prepared in the same way as the Sc2O3 one. 3. Structural characterization The crystalline quality of the deposited film is a important factor for active integrated optics components doped with rare-earth ions, as the peak emission and absorption cross-sections of the dopant strongly depend on quality of the crystalline structure of the host material. Since a high gain material is required in order to reduce the size of active elements, crystalline materials having high peak cross-sections are preferred. Moreover, an oriented crystal-
line growth is desired in order to reduce scattering on the grain boundaries. In spite of the fact that the crystal structure of Sc2O3 under normal conditions is cubic (space group Ia3, a = 0.9846 nm) and the one of a-Al2O3 is hexagonal (space group R3c, a = 0.4759 nm, c = 1.2989 nm), the (1 1 1)plane of Sc2O2 and (0 0 0 1)-plane of a-Al2O3 have the same hexagonal symmetry with the lattice mismatch pffiffiffi of only 2.5%. (Corresponding relation is 3 aAl2 O3 2 aSc2 O3 Þ. Thus (0 0 0 1)-oriented sapphire substrate provide good conditions for quasi lattice matched heteroepitaxy. The crystalline structure of the Sc2O3 film was investigated by X-ray diffraction (XRD) using a Siemens Kristalloflex 810 diffractometer, which provides radiation from a copper X-ray tube. The diffraction pattern of the sample is presented in Fig. 1. Only the {2 2 2}-peak from the Sc2O3 film and the {0 0 0 6}-peak from the sapphire substrate are visible. This indicates that the film is highly textured in h1 1 1i direction and the portion of crystallites with other orientations is negligible. The width of the {2 2 2}peak is approximately 0.14. Taking into account the distance between Cu-Ka1 and Cu-Ka2 lines of 0.08 and using the Scherrer equation, the size of crystallites in the film was estimated to be 150 nm. To determine the orientation deviation of crystallites in the film an x-scan (rocking curve) was performed (Fig. 2). The width of the rocking curve peak is 1.07 for the PLD-film and 0.3 for a Sc2O3 bulk crystal measured for comparison. 4. Spectroscopic characterization The spectroscopic properties of Nd3+ dopants in the Sc2O3 film have been determined, by means of emission and excitation spectroscopy as well as lifetime measurements of the 4F3/2-multiplet. PLD technique is well known to preserve the stochiometry of deposited material, e.g. Ba¨r
1.1
1.1
(a)
I [a.u.]
1.0
{222} Sc2O3
(b)
{222} Sc2O3
1.0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3 0.2 0.1
0.3
{0001} Al2O3
0.2 0.1
{222} Sc2O3 Cu Kβ
0.0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 31.0
31.2
31.4
31.6
0.0 31.8
2Θ [deg] Fig. 1. X-ray diffraction pattern of Nd:Sc2O3 3 lm thick film deposited on a (0 0 0 1)-oriented a-Al2O3 substrate: (a) low resolution scan (slit 0.15), 2H from 20 to 50; (b) high resolution scan (slit 0.05), 2H from 31.0 to 31.8.
Y. Kuzminykh et al. / Optical Materials 28 (2006) 883–887 1.1
2Θ=31.35˚
1.0
1.0
0.9
0.9
0.8
0.8
0.7
0.7
I [a.u.]
I [a.u.]
1.1
0.6
FWHM= 1.07˚
0.5
885
λ em = 1082 nm
4
4
4
4
I9/2-> F5/2
I9/2-> F7/2
0.6 0.5 0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0.0 720 730 740 750 760 770 780 790 800 810 820 830 840 850
0.0 13
14
15
16
17
18
λ [nm]
19
Θ [deg]
Fig. 4. Excitation spectra of Nd3+-ion in Sc2O3 film.
Fig. 2. x-Scan (rocking curve) of {2 2 2} diffraction peak of Nd:Sc2O3 film.
et al. demonstrated this for Eu3+:Y2O3 system [13], which is very similar to Nd:Sc2O3. Thus, the concentration of neodymium in the film can be assumed to be the same as in the ceramic target (0.2 mol.%). The emission and excitation spectra have been recorded using a 1 m SPEX monochromator, an InGaAs detector and a Ti:Sapphire laser as an excitation source. Fig. 3 shows a spectrum of stimulated emission crosssection for the 4F3/2 ! 4I11/2 transition (kex = 826 nm), resulting from the Fu¨chtbauer–Ladenburg equation. This transition is most often utilized for laser action, and in the case of scandia host matrix lasing can be achieved either at 1082 nm or at 1087 nm. The spectrum of the Nd:Sc2O3 film resembles the one of the bulk crystal very closely [14]. The emission cross-section values exceed the ones presented in [15] by about 10%. This effect is caused by shorter lifetimes obtained in our measurements.
11 10
4
4
F3/2-> I11/2
Unfortunately it was not possible to measure an absorption spectrum of neodymium by conventional methods. The signal was too weak because of the quite thin active layer (3 lm). So the excitation spectrum (kem = 1082 nm) in the range of 720–850 nm has been measured (Fig. 4), in order to estimate the best wavelength for pumping. The 826 nm line could the most beneficial, but also the 808 nm line, where commercial laser diodes are available, can be utilized. The luminescence decay curves for the 4F3/2 ! 4I9/2 transition (966 nm) of Nd3+ for the film and for the ceramic target are presented in Fig. 5 for comparison. The excitation wavelength was 826 nm. It can be seen that the decay has a nonexponential behavior. This is caused by nonlinear processes (up-conversion, cross-relaxation and energy migration) taking place in spite of the low neodymium concentration. The similar effect was observed by Fornasiero [14] for neodymium doped bulk sesquioxide crystals. Therefore only the later part of the decay curve was used
λ ex=826 nm
e
Measurement Fit
0
9
e-1
7
e-2
6
I [a.u.]
σem [10-20cm2]
8
5
(a)
e-3
4
(b) e-4
3 2
e-5
1 0 1050 1060 1070 1080 1090 1100 1110 1120 1130 1140 1150 1160
λ [nm] Fig. 3. Emission cross-section spectrum for 4F3/2 ! 4I11/2 transition of Nd3+-ion in Sc2O3 film.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
t [ms] Fig. 5. Luminescence decay curves for 4F3/2 ! 4I9/2 transition of Nd3+ in: (a) Sc2O3 ceramic target; (b) 3 lm Sc2O3 PLD-film.
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to estimate the exponential decay component. The fluorescence decay lifetime of 4F3/2 multiplet for the film was measured to be 180 ls and 230 ls for the ceramic target used as a reference. Imperfect crystalline structure of the film is believed to cause a faster nonradiative depopulation of the 4F3/2 excited state of Nd3+-ion in the film. Only few reports of lifetime measurements for Nd:Sc2O3 can be found in the literature for comparison: 260 ls (0.5% Nd) [15], 224 ls (0.7% Nd) [9], 285 ls (0.12% Nd) [14]. Strong dependence on the neodymium concentration is observed, hence it can be concluded that processes other than spontaneous emission noticeably affect luminescence decay of the 4F3/2 multiplet. Thus, a complex fitting model will be required to determine the intrinsic radiative lifetime of 4 F3/2(Nd3+) in Sc2O3. Sharp spectral lines obtained in emission and excitation spectra confirm the high crystallinity of the scandia film. High peak cross-section values make Nd:Sc2O3 films suitable for high gain active components. The reduced lifetime values in comparison with that observed in ceramic and bulk crystal can result in a slightly higher threshold for laser operation. 5. Waveguiding experiments Due to the fact that the refractive index of scandia is larger than that of sapphire ðnAl2 O3 ;od ¼ 1:763; nAl2 O3 ;ext ¼ 1:754; nSc2 O3 ¼ 1:981 at 720 nmÞ it was possible to achieve guiding of light in the film. Radiation of a Ti:Sapphire laser, operating at a wavelength of 720 nm, has been coupled into the film at a polished end face using a microscope objective (50·, NA = 0.5). Another objective and a CCD camera have been used to record the image of the opposite end face of the sample (Fig. 6). The waveguide can be easily recognized. However inhomogeneity of the light intensity is observed. We explain this phenomenon by interference of light scattered on the crystallites inside the film. Luminescence of the neodymium dopant has been observed with the same CCD-camera from the side of the
Fig. 6. Image of Nd:Sc2O3 3 lm thick PLD waveguide. Ti:Sapphire laser emission at 720 nm coupled in.
sample, when Ti:Sapphire laser was tuned to the absorption line of Nd3+ (826 nm). A low-pass filter was placed in front of the camera in order to block the pump light. The emission was not detected, if the pump wavelength was detuned from the absorption peak. The fluorescence intensity decay along the sample was recorded with the camera and utilized to estimate propagation losses in the film. Attenuation due to scattering (8.5 cm1/37 dB cm1) was determined as difference between the measured attenuation and calculated neodymium absorption (5 cm1). The propagation losses for Nd:Sc2O3 film are much higher than, the loss values achieved for other neodymium doped waveguides (e.g. [6]). The cause of such high losses is still not clear. We suppose this can originate from scattering on grain borders or particulates. Thus further optimization of the deposition process is still required. 6. Conclusion In this work we presented Nd:Sc2O3 waveguiding film doped with neodymium grown by pulsed laser deposition method. The X-ray diffraction analysis showed that film is highly textured in h1 1 1i direction and confirmed high crystalline quality. Emission and excitation spectra of neodymium ions in the film resemble closely the spectra in the bulk crystal. The spectral lines do not exhibit broadening and have high peak cross-sections. The lifetime of the 4 F3/2-level of Nd3+-ion, which is important for potential laser operation, was shorter in the film than in the ceramic target (180 ls and 230 ls, respectively). Waveguiding properties of the film were also demonstrated using emission of a Ti:Sapphire laser. However, further research need to be carried out in order to reduce propagation losses. References [1] T.A. Ibrahim, K. Amarnath, L.C. Kuo, R. Grover, V. Van, P.-T. Ho, Opt. Lett. 29 (2004) 2779. [2] P.G. Kika, A. Polman, J. Appl. Phys. 93 (2003) 5008. [3] A. Driessen, D.H. Geuzebroek, H.J.W.M. Hoekstra, H. Kelderman, E.J. Klein, D.J.W. Klunder, C.G.H. Roeloffzen, F.S. Tan, E. Krioukov, C. Otto, H. Gersen, N.F. van Hulst, L. Kuipers, in: AIP Conference Proceedings, 709 (1) (2004) 1. [4] I. Chartier, B. Ferrand, D. Pelenc, S.J. Field, D.C. Hanna, A.C. Large, D.P. Shepherd, A.C. Tropper, Opt. Lett. 17 (1992) 11. [5] D.S. Gill, A.A. Anderson, R.W. Eason, T.J. Warburton, D.P. Shepherd, Appl. Phys. Lett. 69 (1996) 1. [6] C. Grivas, T.C. May-Smith, D.P. Shepherd, R.W. Eason, Opt. Commun. 229 (2004) 355. [7] J. Sonsky, J. Lancok, M. Jelinek, J. Oswald, V. Studnicka, Appl. Phys. A 66 (1998) 5. [8] P.A. Atanasov, R.I. Tomov, J. Perriere, R.W. Eason, N. Vainos, A. Klini, A. Zherikhin, E. Millon, Appl. Phys. Lett. 76 (2000) 18. [9] L. Fornasiero, E. Mix, V. Peters, E. Heumann, K. Petermann, G. Huber, in: Advanced Solid-State Lasers, OSA Trend in Optics and Photonics Series, vol. 26, 1999, p. 249. [10] V. Peters, A. Bolz, K. Petermann, G. Huber, J. Cryst. Growth 237– 239 (2002) 879. [11] A.R. Kortan, M. Hong, J. Kwo, P. Chang, C.P. Chen, J.P. Mannaerts, S.H. Liou, in: J. Morais, D. Kumar, M. Houssa, R.K. Singh, D. Landheer, R. Ramesh, R.M. Wallace, S. Guha, H.
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