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An He-implanted optical planar waveguide in an Nd:YGG laser crystal preserving fluorescence properties
Applied Surface Science 257 (2011) 7310–7313
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
An He-implanted optical planar waveguide in an Nd:YGG laser crystal preserving fluorescence properties Jin-Hua Zhao, Qing Huang, Peng Liu, Xue-Lin Wang ∗ School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
a r t i c l e
i n f o
Article history: Received 11 February 2011 Received in revised form 17 March 2011 Accepted 18 March 2011 Available online 26 March 2011 PACS: 33.50.Dq 42.70.Hj 42.82.Et 61.80.Jh
a b s t r a c t We report the formation of a planar waveguide in an Nd:YGG laser crystal by low-energy He-ion implantation at liquid nitrogen temperature (77 K). The optical properties are measured by the prism coupling and end-face coupling methods, the absorption properties the waveguide and Nd:YGG substrate are obtained. The fluorescence spectrums are investigated by confocal methods. The experimental results revealed that the planar waveguide preserved the absorption and fluorescence properties of the Nd:YGG laser crystal. Thus, the planar waveguide formed by the ion implantation method is a promising candidate in waveguide lasers. © 2011 Elsevier B.V. All rights reserved.
Keywords: Nd:YGG waveguide ion implantation fluorescence
1. Introduction Rare-earth garnet single-crystals are promising for optical applications. Nd-doped garnet crystals have been identified as excellent laser materials. Representative of this class of crystals, Nd:YAG has become a commercial success. Neodymium-doped yttrium aluminum garnet (Nd:Y3 Al5 O12 or Nd:YAG) is one of the most frequently used gain media for the generation of high-power solidstate lasers due to its excellent optical and physical properties. It has been shown that waveguide lasers have lower pumping thresholds and improved efficiency relative to bulk lasers due to the much higher optical intracavity intensities achieved in reduced active volumes [1]. The investigation of waveguide lasers has been reported using Nd:YAG crystals or ceramics in conjunction with several techniques, for instance, ultrafast laser inscription [2], proton implantation [3], and swift-heavy ion irradiation [4]. By replacing aluminum with gallium in the YAG crystal, another garnet material, yttrium gallium garnet (YGG), can be generated. Previous studies on this crystal have revealed that it has typical spectral and thermal properties [5,6]. Similar to YAG, Y3 Ga5 O12 (YGG) has
∗ Corresponding author. Tel.: +86 531 88362385; fax: +86 531 88363350. E-mail address: [email protected] (X.-L. Wang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.03.110
many desirable advantages for laser materials: stability, hardness, optical isotropy, good thermal conductivity (9 W/mK), and accepting of substitutional trivalent ions of both rare-earth metal and iron groups [7,8]. Compared to other laser materials, Nd:YGG has many advantages; for instance its luminescence lifetime of the 4 F3/2 metastable state was almost unchanged with temperature variation, which indicates the quantum efficiency of this state was close to unity [6], and it has the potential to obtain a Nd:YGG laser with high efficiency. In addition, Nd:YGG has comparable emission cross-sections at different wavelengths, approximately 1.06 m [9]. Therefore, it has the potential to achieve an efficient Nd:YGG laser at multiple wavelengths [9]. As the most fundamental and integral part of integrated optics circuits, the optical waveguide structure is of great importance in modern optical communication. The research on Nd:YAG is relatively mature, and the waveguide has been fabricated by several techniques [3,4,10]. However, to our knowledge, there have been no reports on the waveguide on an Nd:YGG laser crystal. Ion implantation is one of the effective methods to fabricate optical waveguides, and due to it unique properties, it has been applied on many materials for fabricating the waveguide structure [11]. In our work, we fabricate the waveguide structure using the ion implantation method for the first time.
J.-H. Zhao et al. / Applied Surface Science 257 (2011) 7310–7313
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2. Experiment details The Nd:YGG samples were grown by an optical floating zone method provided by the State Key Laboratory of Crystal Materials, Shandong University. The doping concentration of Nd is approximately 1 at.%, and the density is = 5.7 g/cm3 . The as-grown single crystal was cut to the dimensions of 3 mm × 2 mm × 1.5 mm, and its large faces were optically polished. The substrate refractive index of the sample is 1.9353 (nsub ), as measured by the prism coupling method at a wavelength of 633 nm. The sample was implanted with 500-keV helium ions at a dose of 3 × 1016 ions/cm2 using the implanter at the Institute of Semiconductors, Chinese Academy of Science. The implantation process was performed at a temperature of 77 K, and the beam direction was tilted by 7◦ to minimize the channeling effect. After the implantation process, the sample was annealed twice at 260 ◦ C for 30 min and 300 ◦ C for 30 min, respectively, to improve the waveguide quality and thermal stabilities. We measured the effective index (neff ) of the guided mode using the prism coupling method [12] with a Model 2010 prism coupler (Metricon, USA). In the arrangement, a silicon photodetector was used to detect the reflected light intensity from the sample. When the light was coupled with the waveguide, a lack of reflected light resulted in a dip in intensity, and the corresponding value of this dip was the neff of the guided mode. To verify that the light wave was truly propagating inside the waveguide region, the optical properties were investigated by the end-face coupling method, and the near-field light intensity profile was collected by a CCD camera, which was connected to a computer. The laser source was an He–Ne laser at a wavelength of 633 nm. The absorption spectra of the waveguide and substrate of the Nd:YGG crystal were measured by a Jasco U570 spectrophotometer to investigate the absorption properties. The measurement method of the absorption spectra is as follows: firstly, the light source shines through a monochromator; then, an output wavelength is selected and beamed at the sample surface; finally, a fraction of the monochromatic light is transmitted through the x-axis direction of the sample and to the photodetector. A confocal microluminescence experiment was performed with a JY-T64000 laser-Raman spectrum system with an excitation wavelength of 532 nm. The experiments were completed at the Institute of Physics, Chinese Academy of Sciences. The microluminescence experiments were conducted using a confocal microscope in combination with an XY motorized stage. Continuous-wave 532nm radiation was focused on the sample surface using a 100× microscope objective. In this configuration, the 532-nm laser radiation excites the Nd3+ ions from their ground state (4 I9/2 ) to the 2G 4 4 3/2 excited state. Then, the subsequent F3/2 → I9/2 emission band from the Nd3+ ions is back-collected by the same microscope objective and analyzed on a high-resolution spectrometer.
3. Results and discussion To obtain the energy deposition versus the penetration depth of the He ions at 500 keV into the YGG crystal, we used SRIM 2006 to simulate the implantation process. Fig. 1 shows the normalized electronic and nuclear energy deposition. The location of the peak value of the nuclear energy deposition was at a depth of 1.2 m. This is the depth of the optical barrier (D) in the refractive index profile. We carried out the prism coupling method (via a prism coupler, Model 2010, Metricon, USA) to investigate the guided mode of the planar waveguide at the wavelength of 633 nm. Fig. 2 shows the measured relative intensity of the TM polarized light at a wavelength of 633 nm reflected from the prism formed by the Nd:YGG planar waveguides before and after annealing at 260 ◦ C
Fig. 1. Normalized nuclear and electronic energy deposition as a function of the penetration depth of the 500-keV He ions in the Nd:YGG crystal.
Fig. 2. Measured relative intensity of the light (TM polarized) reflected from a prism formed by the Nd:YGG planar waveguides formed by 500-keV He ions implanted with a fluence of 3 × 1016 ions/cm2 at the temperature of 77 K at a wavelength of 633 nm before (solid line) and after the annealing treatment at 260 ◦ C for 30 min (dashed line).
for a 30-min treatment, respectively. We observed that the surface refractive index (nsur ) of the Nd:YGG planar waveguide was higher than the bulk index (nsub ). This phenomenon was discovered in an Nd:YAG waveguide formed by the ion implantation technique by Tan and Chen [3]. They reported that the refractive index profile had an index-enhanced well in the waveguide region and an index-decreased barrier at the end of the ion track. In our work, it seems reasonable to assume that the He-ion-implanted Nd:YGG waveguide has a similar index profile. To determine the refractive index profiles of the planar waveguides, we assumed that it could be depicted by two half-Gaussian curves. We reconstructed the RIP of the Nd:YGG planar waveguide by adjusting five parameters: nsur (the refractive index of the waveguide surface), W1 and W2 (the FWHM of the two half-Gaussian curves), D (the depth of the optical barrier), and nb (the index value of optical barrier); a detailed discussion is provided in Ref. [13]. The reconstructed result of the Nd:YGG planar waveguide after annealing at 260 ◦ C for 30 min is shown in Fig. 3. We can see that the index profile consists of an enhanced index well (nw = 0.005) in the waveguide region and an optical barrier (nb = −0.06) at a depth of ∼1.2 m inside the crystal. Fig. 4(a) shows the near-field intensity profile of the Nd:YGG planar waveguide, which was measured by the end-face coupling method. We can say that the Nd:YGG planar waveguide formed by the He-ion implantation can propagate the 633-nm light successfully. We simulated the light propagation process by BPM (beam propagation method) software [14], and the simulation result is shown in Fig. 4(b). Comparing this result with the experimental
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Fig. 3. The refractive index profile of the He-implanted Nd:YGG planar waveguide after annealing at 260 ◦ C for 30 min.
results indicates that our simulation process was reasonable. The refractive index profile with the “well + optical barrier” is appropriate for our planar waveguide. We will demonstrate this in the following section. Our simulation method confirms the ability to design an Nd:YGG waveguide by the ion implantation method. There have been no studies on the mechanism of waveguide formation in YGG by the ion implantation procedure. The details are usually complex, but we will provide a possible explanation in the following section. The refractive index profile in the heliumimplanted Nd:YGG waveguide had an index-enhanced well and an index-decreased barrier. The optical barrier may be induced by a defect suffered as a consequence of the nuclear stopping of the implanted ions at the end of the range; this point of view on the optical barrier has been accepted by an increasing number of researchers [11,13,15,16]. We suggest that the same mechanism occurs in the Nd:YGG planar waveguide. The implanted helium ions
concentrated at the end of the range led to a volume expansion such that a decrease in the physical density corresponded with this barrier. This explains why the location of the peak value of the nuclear energy deposition was the same as the depth of the optical barrier (D). The positive change in the refractive index of the waveguide region may be induced by the compaction effect during the process of the implantation [13], which was usually accompanied by an increase in the physical density in the corresponding region. The waveguide layer was thus surrounded by the low indices of air and the optical barrier. Such a “well + barrier” structure confined the light in a narrow “well” layer with a relatively high refractive index, which formed an optical waveguide between the barrier and the crystal surface [3,13,17,18]. To investigate the effect of the process of He implantation on the light absorption characteristics of the Nd:YGG crystal, the absorption spectra were collected and are shown in Fig. 5. The dashed line is the substrate, and the solid line is the Nd:YGG waveguide after annealing at 260 ◦ C for 30 min and 300 ◦ C for 30 min; we labeled the absorption peaks with their eigen states in the figure, respectively, and the relative ground state was 4 I9/2 . In the following discussion, we abbreviated wavelength as w for clarity. Fig. 5(a) indicates that the absorption spectra of the annealed waveguide sample showed a lower intensity in the UV–visible range (w < 550 nm), which was due to a decrease in the absorption by defects. As shown in Fig. 5(b), the spectra of the Nd:YGG waveguide were very similar to that of the unimplanted sample, including the shape, peak position, and intensity. In conclusion, there were almost no changes between the Nd:YGG substrate and the waveguides in Fig. 5(b), and we can say that the implantation processes had almost no influence on the absorption properties at the wavelength scale for w > 550 nm. To explore the fluorescence emission bands of the Nd3+ ions that may have been affected by the implantation, the confocal micro-photoluminescence properties of the Nd:YGG waveguide
Fig. 4. Near-field mode profiles of the planar waveguide in Nd:YGG (a) collected by the CCD camera and (b) simulated by the BPM software.
Fig. 5. Absorption spectra of the Nd:YGG samples, wavelength scale: (a) 280 nm < w < 550 nm and (b) 550 nm < w < 950 nm. The red dashed line represents the substrate; the dark solid line represents the 500-keV He-implanted waveguide with a dose of 3 × 1016 ions/cm2 after annealing at 260 ◦ C for 30 min + 300 ◦ C for 30 min.
J.-H. Zhao et al. / Applied Surface Science 257 (2011) 7310–7313
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Fig. 6. Comparison of the room temperature microluminescence emission spectra correlated to the Nd3+ ions at (a) 4 F3/2 → 4 I9/2 and (b) 4 F3/2 → 4 I9/2 transition obtained from the planar waveguide (after annealing at 260 ◦ C for 30 min + 300 ◦ C for 30 min, solid line) and the bulk (dashed line).
were measured by a confocal microscope, as already described elsewhere [3,4,18,19]. Fig. 6 shows the room temperature microluminescence spectrum obtained from the Nd:YGG crystal when the 532-nm excitation spot was positioned on the waveguide (after annealing at 260 ◦ C for 30 min and 300 ◦ C for 30 min) and the bulk, respectively. The spectra in Fig. 6(a) and (b) correspond to the 4 F5/2 → 4 I9/2 and 4 F3/2 → 4 I9/2 laser transitions, respectively. As shown in the figure, the emission spectra obtained from the waveguide and the bulk are very similar in shape and peak position. Ref. [3] reported that the analysis of the Nd3+ fluorescence provided information about the local presence of disorder (through the line width), damages and defects (monitored by a fluorescence intensity reduction), and changes in the unit cell volume (revealed by a spectral shift of the luminescence lines). This suggests the absence of any relevant disorder and a lack of change in the unit cell volume. These properties are crucial because they indicate that the intrinsic fluorescence properties of the Nd3+ ions did not deteriorate due to the waveguide fabrication procedure. In addition, because the He-ion implantation induced a slight decrease only in the Nd3+ fluorescence intensity, one may conclude that there was no quenching of the emission performances of the Nd3+ ions at the waveguide region as a consequence of the helium implantation process, and the quantum efficiency of the 4 F3/2 and 4 F5/2 metastable states were almost unaffected during the procedure. In the most basic sense, the observed, unaffected fluorescence efficiency reveals that the waveguide region was virtually free from disorder or volume expansion; the damage and defects formed by the He implantation process were also minor due to the small change in the fluorescence intensity. 4. Conclusions The planar waveguide formed by the ion implantation method on the Nd:YGG crystal was investigated for the first time to our knowledge. The “well + barrier” type refractive index profile was obtained by a 500-keV helium-ion implantation at a dose of 3 × 1016 ions/cm2 at the low temperature of 77 K. The implantation processes had almost no influence on the absorption properties at the wavelength scale of w > 550 nm on account of the absorption spectrum. The microluminescence investigation reveals that, in the waveguide, the fluorescence properties of the Nd3+ ions and the energy transfer efficiency were not deteriorated by the helium implantation. Therefore, we believe that this Nd:YGG waveguide has potential applications in the study of waveguide lasers. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 10975094 and 10735070), National Basic
Research Program of China (Grant No. 2010CB832906), University and a Foundation for the Author of National Excellent Doctoral Dissertation of China, Independent Innovation Foundation of Shandong University, and the State Key Laboratory of Nuclear Physics and Technology, Peking University. References [1] J.I. Mackenzie, Dielectric solid-state planar waveguide lasers: a review, IEEE J. Sel. Top. Quantum Electron. 13 (2007) 626–637. [2] G.A. Torchia, A. Rodenas, A. Benayas, E. Cantelar, L. Roso, D. Jaque, Highly efficient laser action in femtosecond-written Nd:yttrium aluminum garnet ceramic waveguides, Appl. Phys. Lett. 92 (2008) 111103. [3] Y. Tan, F. Chen, Proton-implanted optical channel waveguides in Nd:YAG laser ceramics, J. Phys. D: Appl. Phys. 43 (2010) 075105. [4] Y. Ren, N. Dong, F. Chen, A. Benayas, D. Jaque, F. Qiu, T. Narusawa, Swift heavyion irradiated active waveguides in Nd:YAG crystals: fabrication and laser generation, Opt. Lett. 35 (2010) 3276–3278. [5] A.A. Kaminskii, G.A. Bogomolova, D.N. Vylegzhanin, K.S. Bagdasarov, A.M. Kevorkov, M.M. Gritsenko, Spectroscopic properties of Nd3+ ions in garnet compounds forming in the Y2 O3 → Ga2 O3 system, Phys. Status Solidi A 38 (1976) 409–422. [6] P.A. Giesting, A.M. Hofmeister, Thermal conductivity of disordered garnets from infrared spectroscopy, Phys. Rev. B 65 (2002) 144305. [7] J.E. Geusic, H.M. Marcos, L.G. Van Uitert, Laser oscillations in Nd-doped yttrium aluminum gallium and gadolinium garnets, Appl. Phys. Lett. 4 (1964) 182–184. [8] Y. Zhang, Z. Wei, B. Zhou, C. Xu, Y. Zou, D. Li, Z. Zhang, H. Zhang, J. Wang, H. Yu, K. Wu, B. Yao, J. Wang, Diode-pumped passively mode-locked Yb:Y3 Al5 O12 laser, Opt. Lett. 34 (2009) 3316–3318. [9] H. Yu, K. Wu, B. Yao, H. Zhang, Z. Wang, J. Wang, X. Zhang, M. Jiang, Efficient triwavelength laser with a Nd:YGG garnet crystal, Opt. Lett. 35 (2010) 1801–1803. [10] M. Ezaki, M. Obara, H. Kumagai, K. Toyoda, Characterization of Nd:Y3 Al5 O12 thin films grown on various substrates by pulsed laser deposition, Appl. Phys. Lett. 69 (1996) 2977–2979. [11] P.D. Townsend, P.J. Chandler, L. Zhang, Optical Effects of Ion Implantation, Cambridge University Press, Cambridge, 1994. [12] P.K. Tien, R. Ulrich, R.J. Martin, Modes of propagating light waves in thin deposited semiconductor films, Appl. Phys. Lett. 14 (1969) 291–294. [13] L. Wang, F. Chen, X.L. Wang, L.L. Wang, K.M. Wang, L. Gao, H.J. Ma, R. Nie, Si2+ ion implanted into stoichiometric lithium niobate crystals: Waveguide characterization and lattice disorder analysis, Nucl. Instrum. Meth. B 251 (2006) 104–108. [14] Rsoft Design Group, Computer software BEAMPROP, http://www. rsoftdesign.com. [15] X.L. Wang, K.M. Wang, F. Chen, G. Fu, S.L. Li, H. Liu, L. Gao, D.Y. Shen, H.J. Ma, R. Nie, Optical properties of stoichiometric LiNbO3 waveguides formed by lowdose oxygen ion implantation, Appl. Phys. Lett. 86 (2005) 041103. [16] D. Kip, S. Aulkemeyer, P. Moretti, Low-loss planar optical waveguides in strontium barium niobate crystals formed by ion-beam implantation, Opt. Lett. 20 (1995) 1256–1258. [17] G.G. Bentini, M. Bianconi, L. Correra, M. Chiarini, P. Mazzoldi, C. Sada, N. Argiolas, M. Bazzan, R. Guzzi, Damage effects produced in the near-surface region of xcut LiNbO3 by low dose, high energy implantation of nitrogen, oxygen, and fluorine ions, J. Appl. Phys. 96 (2004) 242–247. [18] Y.X. Kong, F. Chen, D. Jaque, Q.M. Lu, H.J. Ma, Optical channel waveguide in Nd/Ce codoped YAG laser crystal produced by carbon ion implantation, Appl. Opt. 48 (2009) 4514–4518. [19] W.F. Silva, C. Jacinto, A. Benayas, J.R.V. de Aldana, G.A. Torchia, F. Chen, Y. Tan, D. Jaque, Femtosecond-laser-written, stress-induced Nd: YVO4 waveguides preserving fluorescence and Raman gain, Opt. Lett. 35 (2010) 916–918.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
An He-implanted optical planar waveguide in an Nd:YGG laser crystal preserving fluorescence properties Jin-Hua Zhao, Qing Huang, Peng Liu, Xue-Lin Wang ∗ School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
a r t i c l e
i n f o
Article history: Received 11 February 2011 Received in revised form 17 March 2011 Accepted 18 March 2011 Available online 26 March 2011 PACS: 33.50.Dq 42.70.Hj 42.82.Et 61.80.Jh
a b s t r a c t We report the formation of a planar waveguide in an Nd:YGG laser crystal by low-energy He-ion implantation at liquid nitrogen temperature (77 K). The optical properties are measured by the prism coupling and end-face coupling methods, the absorption properties the waveguide and Nd:YGG substrate are obtained. The fluorescence spectrums are investigated by confocal methods. The experimental results revealed that the planar waveguide preserved the absorption and fluorescence properties of the Nd:YGG laser crystal. Thus, the planar waveguide formed by the ion implantation method is a promising candidate in waveguide lasers. © 2011 Elsevier B.V. All rights reserved.
Keywords: Nd:YGG waveguide ion implantation fluorescence
1. Introduction Rare-earth garnet single-crystals are promising for optical applications. Nd-doped garnet crystals have been identified as excellent laser materials. Representative of this class of crystals, Nd:YAG has become a commercial success. Neodymium-doped yttrium aluminum garnet (Nd:Y3 Al5 O12 or Nd:YAG) is one of the most frequently used gain media for the generation of high-power solidstate lasers due to its excellent optical and physical properties. It has been shown that waveguide lasers have lower pumping thresholds and improved efficiency relative to bulk lasers due to the much higher optical intracavity intensities achieved in reduced active volumes [1]. The investigation of waveguide lasers has been reported using Nd:YAG crystals or ceramics in conjunction with several techniques, for instance, ultrafast laser inscription [2], proton implantation [3], and swift-heavy ion irradiation [4]. By replacing aluminum with gallium in the YAG crystal, another garnet material, yttrium gallium garnet (YGG), can be generated. Previous studies on this crystal have revealed that it has typical spectral and thermal properties [5,6]. Similar to YAG, Y3 Ga5 O12 (YGG) has
∗ Corresponding author. Tel.: +86 531 88362385; fax: +86 531 88363350. E-mail address: [email protected] (X.-L. Wang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.03.110
many desirable advantages for laser materials: stability, hardness, optical isotropy, good thermal conductivity (9 W/mK), and accepting of substitutional trivalent ions of both rare-earth metal and iron groups [7,8]. Compared to other laser materials, Nd:YGG has many advantages; for instance its luminescence lifetime of the 4 F3/2 metastable state was almost unchanged with temperature variation, which indicates the quantum efficiency of this state was close to unity [6], and it has the potential to obtain a Nd:YGG laser with high efficiency. In addition, Nd:YGG has comparable emission cross-sections at different wavelengths, approximately 1.06 m [9]. Therefore, it has the potential to achieve an efficient Nd:YGG laser at multiple wavelengths [9]. As the most fundamental and integral part of integrated optics circuits, the optical waveguide structure is of great importance in modern optical communication. The research on Nd:YAG is relatively mature, and the waveguide has been fabricated by several techniques [3,4,10]. However, to our knowledge, there have been no reports on the waveguide on an Nd:YGG laser crystal. Ion implantation is one of the effective methods to fabricate optical waveguides, and due to it unique properties, it has been applied on many materials for fabricating the waveguide structure [11]. In our work, we fabricate the waveguide structure using the ion implantation method for the first time.
J.-H. Zhao et al. / Applied Surface Science 257 (2011) 7310–7313
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2. Experiment details The Nd:YGG samples were grown by an optical floating zone method provided by the State Key Laboratory of Crystal Materials, Shandong University. The doping concentration of Nd is approximately 1 at.%, and the density is = 5.7 g/cm3 . The as-grown single crystal was cut to the dimensions of 3 mm × 2 mm × 1.5 mm, and its large faces were optically polished. The substrate refractive index of the sample is 1.9353 (nsub ), as measured by the prism coupling method at a wavelength of 633 nm. The sample was implanted with 500-keV helium ions at a dose of 3 × 1016 ions/cm2 using the implanter at the Institute of Semiconductors, Chinese Academy of Science. The implantation process was performed at a temperature of 77 K, and the beam direction was tilted by 7◦ to minimize the channeling effect. After the implantation process, the sample was annealed twice at 260 ◦ C for 30 min and 300 ◦ C for 30 min, respectively, to improve the waveguide quality and thermal stabilities. We measured the effective index (neff ) of the guided mode using the prism coupling method [12] with a Model 2010 prism coupler (Metricon, USA). In the arrangement, a silicon photodetector was used to detect the reflected light intensity from the sample. When the light was coupled with the waveguide, a lack of reflected light resulted in a dip in intensity, and the corresponding value of this dip was the neff of the guided mode. To verify that the light wave was truly propagating inside the waveguide region, the optical properties were investigated by the end-face coupling method, and the near-field light intensity profile was collected by a CCD camera, which was connected to a computer. The laser source was an He–Ne laser at a wavelength of 633 nm. The absorption spectra of the waveguide and substrate of the Nd:YGG crystal were measured by a Jasco U570 spectrophotometer to investigate the absorption properties. The measurement method of the absorption spectra is as follows: firstly, the light source shines through a monochromator; then, an output wavelength is selected and beamed at the sample surface; finally, a fraction of the monochromatic light is transmitted through the x-axis direction of the sample and to the photodetector. A confocal microluminescence experiment was performed with a JY-T64000 laser-Raman spectrum system with an excitation wavelength of 532 nm. The experiments were completed at the Institute of Physics, Chinese Academy of Sciences. The microluminescence experiments were conducted using a confocal microscope in combination with an XY motorized stage. Continuous-wave 532nm radiation was focused on the sample surface using a 100× microscope objective. In this configuration, the 532-nm laser radiation excites the Nd3+ ions from their ground state (4 I9/2 ) to the 2G 4 4 3/2 excited state. Then, the subsequent F3/2 → I9/2 emission band from the Nd3+ ions is back-collected by the same microscope objective and analyzed on a high-resolution spectrometer.
3. Results and discussion To obtain the energy deposition versus the penetration depth of the He ions at 500 keV into the YGG crystal, we used SRIM 2006 to simulate the implantation process. Fig. 1 shows the normalized electronic and nuclear energy deposition. The location of the peak value of the nuclear energy deposition was at a depth of 1.2 m. This is the depth of the optical barrier (D) in the refractive index profile. We carried out the prism coupling method (via a prism coupler, Model 2010, Metricon, USA) to investigate the guided mode of the planar waveguide at the wavelength of 633 nm. Fig. 2 shows the measured relative intensity of the TM polarized light at a wavelength of 633 nm reflected from the prism formed by the Nd:YGG planar waveguides before and after annealing at 260 ◦ C
Fig. 1. Normalized nuclear and electronic energy deposition as a function of the penetration depth of the 500-keV He ions in the Nd:YGG crystal.
Fig. 2. Measured relative intensity of the light (TM polarized) reflected from a prism formed by the Nd:YGG planar waveguides formed by 500-keV He ions implanted with a fluence of 3 × 1016 ions/cm2 at the temperature of 77 K at a wavelength of 633 nm before (solid line) and after the annealing treatment at 260 ◦ C for 30 min (dashed line).
for a 30-min treatment, respectively. We observed that the surface refractive index (nsur ) of the Nd:YGG planar waveguide was higher than the bulk index (nsub ). This phenomenon was discovered in an Nd:YAG waveguide formed by the ion implantation technique by Tan and Chen [3]. They reported that the refractive index profile had an index-enhanced well in the waveguide region and an index-decreased barrier at the end of the ion track. In our work, it seems reasonable to assume that the He-ion-implanted Nd:YGG waveguide has a similar index profile. To determine the refractive index profiles of the planar waveguides, we assumed that it could be depicted by two half-Gaussian curves. We reconstructed the RIP of the Nd:YGG planar waveguide by adjusting five parameters: nsur (the refractive index of the waveguide surface), W1 and W2 (the FWHM of the two half-Gaussian curves), D (the depth of the optical barrier), and nb (the index value of optical barrier); a detailed discussion is provided in Ref. [13]. The reconstructed result of the Nd:YGG planar waveguide after annealing at 260 ◦ C for 30 min is shown in Fig. 3. We can see that the index profile consists of an enhanced index well (nw = 0.005) in the waveguide region and an optical barrier (nb = −0.06) at a depth of ∼1.2 m inside the crystal. Fig. 4(a) shows the near-field intensity profile of the Nd:YGG planar waveguide, which was measured by the end-face coupling method. We can say that the Nd:YGG planar waveguide formed by the He-ion implantation can propagate the 633-nm light successfully. We simulated the light propagation process by BPM (beam propagation method) software [14], and the simulation result is shown in Fig. 4(b). Comparing this result with the experimental
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Fig. 3. The refractive index profile of the He-implanted Nd:YGG planar waveguide after annealing at 260 ◦ C for 30 min.
results indicates that our simulation process was reasonable. The refractive index profile with the “well + optical barrier” is appropriate for our planar waveguide. We will demonstrate this in the following section. Our simulation method confirms the ability to design an Nd:YGG waveguide by the ion implantation method. There have been no studies on the mechanism of waveguide formation in YGG by the ion implantation procedure. The details are usually complex, but we will provide a possible explanation in the following section. The refractive index profile in the heliumimplanted Nd:YGG waveguide had an index-enhanced well and an index-decreased barrier. The optical barrier may be induced by a defect suffered as a consequence of the nuclear stopping of the implanted ions at the end of the range; this point of view on the optical barrier has been accepted by an increasing number of researchers [11,13,15,16]. We suggest that the same mechanism occurs in the Nd:YGG planar waveguide. The implanted helium ions
concentrated at the end of the range led to a volume expansion such that a decrease in the physical density corresponded with this barrier. This explains why the location of the peak value of the nuclear energy deposition was the same as the depth of the optical barrier (D). The positive change in the refractive index of the waveguide region may be induced by the compaction effect during the process of the implantation [13], which was usually accompanied by an increase in the physical density in the corresponding region. The waveguide layer was thus surrounded by the low indices of air and the optical barrier. Such a “well + barrier” structure confined the light in a narrow “well” layer with a relatively high refractive index, which formed an optical waveguide between the barrier and the crystal surface [3,13,17,18]. To investigate the effect of the process of He implantation on the light absorption characteristics of the Nd:YGG crystal, the absorption spectra were collected and are shown in Fig. 5. The dashed line is the substrate, and the solid line is the Nd:YGG waveguide after annealing at 260 ◦ C for 30 min and 300 ◦ C for 30 min; we labeled the absorption peaks with their eigen states in the figure, respectively, and the relative ground state was 4 I9/2 . In the following discussion, we abbreviated wavelength as w for clarity. Fig. 5(a) indicates that the absorption spectra of the annealed waveguide sample showed a lower intensity in the UV–visible range (w < 550 nm), which was due to a decrease in the absorption by defects. As shown in Fig. 5(b), the spectra of the Nd:YGG waveguide were very similar to that of the unimplanted sample, including the shape, peak position, and intensity. In conclusion, there were almost no changes between the Nd:YGG substrate and the waveguides in Fig. 5(b), and we can say that the implantation processes had almost no influence on the absorption properties at the wavelength scale for w > 550 nm. To explore the fluorescence emission bands of the Nd3+ ions that may have been affected by the implantation, the confocal micro-photoluminescence properties of the Nd:YGG waveguide
Fig. 4. Near-field mode profiles of the planar waveguide in Nd:YGG (a) collected by the CCD camera and (b) simulated by the BPM software.
Fig. 5. Absorption spectra of the Nd:YGG samples, wavelength scale: (a) 280 nm < w < 550 nm and (b) 550 nm < w < 950 nm. The red dashed line represents the substrate; the dark solid line represents the 500-keV He-implanted waveguide with a dose of 3 × 1016 ions/cm2 after annealing at 260 ◦ C for 30 min + 300 ◦ C for 30 min.
J.-H. Zhao et al. / Applied Surface Science 257 (2011) 7310–7313
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Fig. 6. Comparison of the room temperature microluminescence emission spectra correlated to the Nd3+ ions at (a) 4 F3/2 → 4 I9/2 and (b) 4 F3/2 → 4 I9/2 transition obtained from the planar waveguide (after annealing at 260 ◦ C for 30 min + 300 ◦ C for 30 min, solid line) and the bulk (dashed line).
were measured by a confocal microscope, as already described elsewhere [3,4,18,19]. Fig. 6 shows the room temperature microluminescence spectrum obtained from the Nd:YGG crystal when the 532-nm excitation spot was positioned on the waveguide (after annealing at 260 ◦ C for 30 min and 300 ◦ C for 30 min) and the bulk, respectively. The spectra in Fig. 6(a) and (b) correspond to the 4 F5/2 → 4 I9/2 and 4 F3/2 → 4 I9/2 laser transitions, respectively. As shown in the figure, the emission spectra obtained from the waveguide and the bulk are very similar in shape and peak position. Ref. [3] reported that the analysis of the Nd3+ fluorescence provided information about the local presence of disorder (through the line width), damages and defects (monitored by a fluorescence intensity reduction), and changes in the unit cell volume (revealed by a spectral shift of the luminescence lines). This suggests the absence of any relevant disorder and a lack of change in the unit cell volume. These properties are crucial because they indicate that the intrinsic fluorescence properties of the Nd3+ ions did not deteriorate due to the waveguide fabrication procedure. In addition, because the He-ion implantation induced a slight decrease only in the Nd3+ fluorescence intensity, one may conclude that there was no quenching of the emission performances of the Nd3+ ions at the waveguide region as a consequence of the helium implantation process, and the quantum efficiency of the 4 F3/2 and 4 F5/2 metastable states were almost unaffected during the procedure. In the most basic sense, the observed, unaffected fluorescence efficiency reveals that the waveguide region was virtually free from disorder or volume expansion; the damage and defects formed by the He implantation process were also minor due to the small change in the fluorescence intensity. 4. Conclusions The planar waveguide formed by the ion implantation method on the Nd:YGG crystal was investigated for the first time to our knowledge. The “well + barrier” type refractive index profile was obtained by a 500-keV helium-ion implantation at a dose of 3 × 1016 ions/cm2 at the low temperature of 77 K. The implantation processes had almost no influence on the absorption properties at the wavelength scale of w > 550 nm on account of the absorption spectrum. The microluminescence investigation reveals that, in the waveguide, the fluorescence properties of the Nd3+ ions and the energy transfer efficiency were not deteriorated by the helium implantation. Therefore, we believe that this Nd:YGG waveguide has potential applications in the study of waveguide lasers. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 10975094 and 10735070), National Basic
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