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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 14 — Jul. 9, 2007
  • pp: 8805–8811
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Time resolved confocal luminescence investigations on Reverse Proton Exchange Nd:LiNbO3 channel waveguides

E. Martín Rodríguez, D. Jaque, E. Cantelar, F. Cussó, G. Lifante, A.C. Busacca, A.C. Cino, and S. Riva Sanseverino  »View Author Affiliations


Optics Express, Vol. 15, Issue 14, pp. 8805-8811 (2007)
http://dx.doi.org/10.1364/OE.15.008805


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Abstract

In this work we report on the time and spatial resolved fluorescence of Neodymium ions in LiNbO3 channel waveguides fabricated by Reverse Proton Exchange. The analysis of the fluorescence decay curves obtained with a sub-micrometric resolution has evidenced the presence of a relevant fluorescence quenching inside the channel waveguide. From the comparison between diffusion simulations and the spatial dependence of the 4F3/2 fluorescence decay rate we have concluded that the observed fluorescence quenching can be unequivocally related to the presence of H+ ions in the LiNbO3 lattice. Nevertheless, it turns out that Reverse Proton Exchange guarantees a fluorescence quenching level significantly lower than in similar configurations based on Proton Exchange waveguides. This fluorescence quenching has been found to be accompanied by a relevant red-shift of the 4F3/24I9/2 fluorescence band.

© 2007 Optical Society of America

1. Introduction

Reverse Proton Exchange (RPE) has emerged as an alternative method for the fabrication of channel waveguides in Nd3+ doped LiNbO3 waveguides. RPE procedure has been reported to lead to low losses buried and symmetric LiNbO3 channel waveguides able to confine the two principal (orthogonal) polarizations [7–10

7. J. L. Jackel and J. J. Johnson, “Reverse exchange method for burying proton exchanged waveguides,” “Electron. Lett. 27, 1360–1361 (1991). [CrossRef]

]. These facts make RPE LiNbO3 channel waveguides of major interest in the realization of rare earth doped lasing configurations as well as highly efficient one and two dimensional nonlinear devices due to the good matching of the interacting modes field profiles within the waveguide [11

11. K. R. Parameswaran, R. K. Route, J. R. Kurz, R. V. Roussev, M. M. Fejer, and M. Fujimura. “Highly efficient second-harmonic generation in buried waveguides formed by annealed and reverse proton exchange in periodically poled lithium Niobate,” Opt. Lett. 27, 179–181 (2002). [CrossRef]

]. Despite of its interest and potential applications, the actual influence of the RPE process on the spectroscopic properties (especially on the 4F3/2 fluorescence lifetime) of Neodymium ions is still an almost unexplored question [12

12. M. Domenech, G. Lifante, F. Cussó, A. Parisi, A.C. Cino, and S. Riva Sanseverino, “Fabrication and characterisation of reverse proton exchange optical waveguides in Neodymium doped lithium niobate crystals,” Materials Science Forum. 480–481, 429–436 (2005)

]. In this work we have applied the Time Resolved Confocal Luminescence (TRCL) technique to the study of the luminescence properties of Nd3+ ions in RPE channel waveguides fabricated in LiNbO3 crystals. The information extracted in this way is of great relevance from the application point of view (laser performance is determined by the spectroscopic properties of Nd3+ ions) and also from a fundamental point of view if a relation between spectroscopic changes and the proton distribution can be established.

2. Experimental

Z-cut LiNbO3 wafers bulk doped with 0.2 mol% of Nd3+ ions (3.78×1019 ions/cm3) and dimensions of 8.5×10×1 mm3 have been cut and polished up to optical quality. Channels of 17 μm width were defined on a silica mask by using standard photolithographic techniques. Figure 1(a) shows a schematic drawing of the cross section view of the channel mask opening, where the exchange can take place during waveguide fabrication. The reference system used all along this work is also indicated. We have defined the “x” axis as parallel to the diffusion surface, being the origin (x = 0) located at the air-waveguide interface. The “y” axis is then perpendicular to this interface. In this case y = 0 is set to be at the centre of the studied SiO2 channel, whose width is denoted by w. The optical waveguides have been fabricated following the RPE technique. For that purpose, a first PE stage was performed in benzoic acid at 300°C during 14.5 hours with the sealed ampoule technique. In order to produce optical waveguides that support both quasi-TM0,0 and quasi-TE0,0 propagating modes around 1 μm wavelength, a second step (RPE stage) was performed. In this stage, the sample was immersed in a mixture of Li/Na/K nitrides at 350°C during 38.5 hours [7–8

7. J. L. Jackel and J. J. Johnson, “Reverse exchange method for burying proton exchanged waveguides,” “Electron. Lett. 27, 1360–1361 (1991). [CrossRef]

]. In this case, the usual m-lines spectroscopic characterization only can give direct information relative to the ordinary waveguide (TE propagating modes) close to the surface, for example through IWKB calculation. The buried extraordinary waveguide can be modeled with different numerical approach, such as the imaginary distance BPM [13

13. G. Lifante, E. Cantelar, F. Cussó, M. Domenech, A.C. Busacca, A.C. Cino, and S. Riva Sanseverino “Imaginary distance BPM as an efficient tool for modelling optical waveguides fabrication by ion diffusion,” Proc. OWTNM’06, Varese, Italy (2006)

]. Figure 1(b) shows the 2D proton density as obtained by solving the diffusion equations that governs the diffusion processes of the two involved species (H+ and Li+), by this finite difference approach. The calculation of Fig. 1(b) reveals that the initially sharp proton profile characteristic of PE waveguides is smoothed by the RPE technique. Close to the surface, proton out-diffusion mechanisms reduce the proton concentration recovering partially the initial refractive indices of the wafer. On the other hand, the higher temperature used in the RPE gives rise to a redistribution of protons further into the substrate: as consequence a region doped with a moderate proton concentration remains buried in the wafer below the mask opening. The refractive indices of this buried region are such that the extraordinary and ordinary refractive indices are higher and lower, respectively, than those of the initial wafer. Therefore, this region allows the optical confinement of TM propagating modes while it represents an optical barrier of low refractive index for the TE propagating modes, which are confined between the sample surface and this optical barrier.

Fig. 1. (a).- Nd:LiNbO3 sample with silica mask channel opening. Reference axis used in this work are indicated. (b).- Two dimensional calculation of the proton density in the waveguide as obtained by using the imaginary distance beam propagation method

For TRCL experiments we have used an Olympus BX41 fiber coupled microscope. The exciting source was, for all the experiments, a 0.2-mm fibre coupled 808 nm laser diode (LIMO GmbH). The diode was operating in pulsed mode providing 50 μs pulses with a repetition rate of 2.5 kHz. For this excitation wavelength the Nd3+ ions were excited through the 4I9/24F3/2 transition, giving rise to the subsequent 4F3/24I9/2, 4I11/2 and 4I13/2 fluorescence transitions. The 808 nm excitation beam was focused by using a 100X microscope objective. The Nd3+ fluorescence was collected by the same focusing objective and focused into a 50 μm diameter fibre. A variable pinhole was positioned between the microscope objective and the collection fibre to ensure a confocal operation scheme. For lifetime measurements the collection fibre was connected to a photomultiplier tube and the generated signal was averaged and recorded by a 500 MHz Lecroy digital oscilloscope. For the spectral analysis of the Nd3+ luminescence the collection fibre was connected to a CVI spectrometer. The waveguide was mounted onto an XY motorized stage with 0.1 μm spatial resolution. All the experiments were carried out at room temperature. The spatial resolution in the confocal configuration here described and the uncertainty in the lifetime determination was estimated to be 0.7 μm and ± 2%, respectively.

Fig. 2. - Fluorescence decay curve of the 4F3/2 metastable state of Nd3+ ions in the LiNbO3 bulk and RPE channel waveguide (red and blue points, respectively). Solid lines correspond to a single exponential fit. The spatial location at which each curve was measured is indicated in the graph on the left.

3. Results and discussion

Figure 2 shows the 4F3/2 fluorescence decay curves obtained close (exchange region) and far away (no exchange region) from the SiO2 channel opening used to define the RPE channel waveguide. From this Figure a clear reduction in the 4F3/2 fluorescence lifetime (from 90 down to 78 μs) can be observed. In order to get a further understanding on the origin of this lifetime reduction we have measured the 4F3/2 fluorescence lifetime along both the (x, y =3 μm) and (x=0, y) directions (Figures 3(a) and 3(a), respectively). The calculated density of protons along these two directions has been also included as green solid lines in Fig. 3(a) and 3(b). From these Figures a clear connection between the reduction in the Nd3+ fluorescence lifetime and the presence of protons is deduced [11

11. K. R. Parameswaran, R. K. Route, J. R. Kurz, R. V. Roussev, M. M. Fejer, and M. Fujimura. “Highly efficient second-harmonic generation in buried waveguides formed by annealed and reverse proton exchange in periodically poled lithium Niobate,” Opt. Lett. 27, 179–181 (2002). [CrossRef]

]. In Fig. 3(a) the maximum reduction in the 4F3/2 fluorescence lifetime is achieved 6 μm below the sample surface, this being the in depth distance at which the density of protons achieve its maximum value. Furthermore, the reduction in the 4F3/2 fluorescence lifetime extends over 20 μm, in good agreement with the penetration depth calculated for protons. The strong relation between proton presence and the Neodymium fluorescence lifetime reduction is also evidenced from the comparison between experimental data and calculations obtained along the (x = 0, y) scan direction (Fig. 3(b)). In this case the maximum fluorescence lifetime reduction is achieved for x = 0, where the proton density is maximum. Furthermore, the “lateral” proton diffusion length (17 μm) also matches with the spatial extension of the Nd3+ fluorescence quenching.

Fig. 3. - 4F3/2 lifetime as a function of both y and x positions ((a) and (b), respectively). Green line corresponds to the proton density along the y and x scan directions.

In order to get a further understanding on the effects that RPE process has on the spectroscopic properties of Nd3+ ions we have also analyzed in detail the continuous wave 4F3/24I9/2 luminescence spectra. For this purpose the collection fiber was connected to a CVI-240 spectrometer and the 808 nm laser diode was operated in continuous wave mode. A band-pass filter was placed between the microscope objective and the collection fiber in order to block the strong 808 nm pumping scattering. Figure 4(a) shows a typical luminescence spectrum obtained. Note that some of the sub-stark transitions corresponding to the 4F3/24I9/2 luminescence channel are not present due to the spectral response of the filter used. Figure 4(b) shows the 4F3/24I9/2 integrated luminescence obtained along the “y” scan direction, revealing a decrease in the fluorescence efficiency of Nd3+ ions for distances below 20 μm and showing a minimum fluorescence emission at around 6μm where the density of protons reaches its maximum. Data shown in Fig. 4(b) are in good agreement with those included in Fig. 3(a); the reduction in the fluorescence 4F3/2 induced by the proton incorporation leads to a decrease in the storage ability of the metastable state. Therefore, under continuous wave excitation, the steady population of the metastable state is reduced, thus leading to a decrease in the fluorescence intensity.

Fig. 4. (a).- 4F3/24I9/2 micro fluorescence spectrum obtained under continuous wave excitation. (b).- 4F3/24I9/2 emitted intensity as a function of the y position.

Fig. 5. - Energy position of the main luminescence peak within the 4F3/24I9/2 fluorescence band as a function of the y position (red points). Green line is the proton density as obtained from the 2D calculations included in Fig. 1(b).

4. Conclusions

Acknowledgments

This work has been supported by the Spanish Ministerio de Ciencia y Tecnologia MAT2004-03347 and MAT2005-05950, by the UAM-Comunidad de Madrid (project CCG06-UAM/MAT-0347), by the Comunidad Autónoma de Madrid (CAM) under Projects S-0505-TIC-0191 and by the Italian MIUR with a PRIN 2005 project.

References and links

1.

L. Arizmendi, “Photonic applications of Lithium Niobate” Phys. Stat. Solidi A 201, 253–283 (2004) [CrossRef]

2.

P. Baldi, M. De Micheli, K. El Hadi, A. C. Cino, P. Aschieri, and D. B. Ostrowsky, “Proton exchanged waveguides in LiNbO3 and LiTaO3 for integrated lasers and nonlinear frequency converters,” Opt. Eng. 37, 1193–1202 (1998). [CrossRef]

3.

J. L. Jackel, C. E. Rice, and J. J. Veselka, “Proton exchange for high-index waveguides in LiNbO3” Appl. Phys. Lett. 41, 607–608 (1982). [CrossRef]

4.

E. Lallier, J. P. Pocholle, M. Papuchon, C. Grezes-Besset, E. Pelletier, M. De Micheli, M. J. Li, Q. He, and D. B. Ostrowsky, “Laser oscillation of single-mode channel waveguide in Nd: MgO:LiNbO3” Electron. Lett. 25, 1491–1492 (1989). [CrossRef]

5.

J. L. Jackel, C. E. Rice, and J. J. Veselka, “Proton exchange for highindex waveguides in LiNbO3,” Appl. Phys. Lett. 41, 607–608 (1982). [CrossRef]

6.

E. Lallier. “Lasers guides dóndes dans le Niobate de Lithium dope Neodyme,” Universite de Paris-Sud, PhD Thesis (1992).

7.

J. L. Jackel and J. J. Johnson, “Reverse exchange method for burying proton exchanged waveguides,” “Electron. Lett. 27, 1360–1361 (1991). [CrossRef]

8.

Y. N. Korkishko, V. A. Fedorov, T. M. Morozova, F. Caccavale, F. Gonella, and F. Segato, “Riverse proton exchange for buried waveguides in LiNbO3,” J. Opt. Soc. Am. A 15, 1838–1842 (1998). [CrossRef]

9.

A. Di Lallo, C. Conti, A. Cino, and G. Assanto, “Efficient Frequency Doubling in Reverse Proton Exchanged Lithium Niobate waveguides,” IEEE Photon. Technol. Lett. 13, 323–325, (2001). [CrossRef]

10.

J. Olivares and J.M. Cabrera. “Guided modes with ordinary refractive index in proton exchanged LiNbO3 waveguides,” Appl. Phys. Lett. 62, 2468–2470 (1993). [CrossRef]

11.

K. R. Parameswaran, R. K. Route, J. R. Kurz, R. V. Roussev, M. M. Fejer, and M. Fujimura. “Highly efficient second-harmonic generation in buried waveguides formed by annealed and reverse proton exchange in periodically poled lithium Niobate,” Opt. Lett. 27, 179–181 (2002). [CrossRef]

12.

M. Domenech, G. Lifante, F. Cussó, A. Parisi, A.C. Cino, and S. Riva Sanseverino, “Fabrication and characterisation of reverse proton exchange optical waveguides in Neodymium doped lithium niobate crystals,” Materials Science Forum. 480–481, 429–436 (2005)

13.

G. Lifante, E. Cantelar, F. Cussó, M. Domenech, A.C. Busacca, A.C. Cino, and S. Riva Sanseverino “Imaginary distance BPM as an efficient tool for modelling optical waveguides fabrication by ion diffusion,” Proc. OWTNM’06, Varese, Italy (2006)

14.

C. Jacinto, S. L. Oliveira, L. A. O. Nunes, T. Catunda, and M. J. V. Bell. “Thermal lens study of the OH- influence on the fluorescence efficiency of Yb3+-doped phosphate glasses,” Appl. Phys. Lett. 86, 071911 (2005). [CrossRef]

15.

U. R. Rodríguez Mendoza, A. Ródenas, D. Jaque, I. R. Martín, F. Lahoz, and V. Lavín “High pressure luminescence in Nd doped LiNbO3 crystals,” High Press. Res. Journal. 26, 341–343 (2006) [CrossRef]

16.

D. Jaque, E. Cantelar, and G. Lifante “Lattice micro-modifications induced by Zn difussion in Nd:LiNbO3 channel waveguides probed by Nd3+ confocal luminescence,” Appl. Phys. B. DOI: 10.1007/s00340-007-2692-9 (2007).

17.

B. V. Dierold and C. Sandmann. “Inspection of periodically poled waveguide devices by confocal luminescence microscopy,” Appl. Phys. B. 78, 363–366 (2004). [CrossRef]

OCIS Codes
(130.3730) Integrated optics : Lithium niobate
(160.3380) Materials : Laser materials
(230.7370) Optical devices : Waveguides

ToC Category:
Integrated Optics

History
Original Manuscript: May 3, 2007
Revised Manuscript: June 25, 2007
Manuscript Accepted: June 26, 2007
Published: June 28, 2007

Citation
E. M. Rodríguez, D. Jaque, E. Cantelar, F. Cussó, G. Lifante, A.C. Busacca, A. Cino, and S. R. Sanseverino, "Time resolved confocal luminescence investigations on Reverse Proton Exchange Nd:LiNbO3 channel waveguides," Opt. Express 15, 8805-8811 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-14-8805


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References

  1. L. Arizmendi, "Photonic applications of Lithium Niobate," Phys. Status Solidi A 201, 253-283 (2004). [CrossRef]
  2. P. Baldi, M. De Micheli, K. El Hadi, A. C. Cino, P. Aschieri, and D. B. Ostrowsky, "Proton exchanged waveguides in LiNbO3 and LiTaO3 for integrated lasers and nonlinear frequency converters," Opt. Eng. 37, 1193-1202 (1998). [CrossRef]
  3. J. L. Jackel, C. E. Rice, and J. J. Veselka, "Proton exchange for high-index waveguides in LiNbO3," Appl. Phys. Lett. 41, 607-608 (1982). [CrossRef]
  4. E. Lallier, J. P. Pocholle, M. Papuchon, C. Grezes-Besset, E. Pelletier, M. De Micheli, M. J. Li, Q. He and D. B. Ostrowsky, "Laser oscillation of single-mode channel waveguide in Nd: MgO:LiNbO3," Electron. Lett. 25, 1491-1492 (1989). [CrossRef]
  5. J. L. Jackel, C. E. Rice, and J. J. Veselka, "Proton exchange for highindex waveguides in LiNbO3," Appl. Phys. Lett. 41, 607-608 (1982). [CrossRef]
  6. E. Lallier. "Lasers guides dóndes dans le Niobate de Lithium dope Neodyme," Universite de Paris-Sud, PhD Thesis (1992).
  7. J. L. Jackel, and J. J. Johnson, "Reverse exchange method for burying proton exchanged waveguides," Electron. Lett. 27, 1360-1361 (1991). [CrossRef]
  8. Y. N. Korkishko, V. A. Fedorov, T. M. Morozova, F. Caccavale, F. Gonella, and F. Segato, "Riverse proton exchange for buried waveguides in LiNbO3," J. Opt. Soc. Am. A 15, 1838-1842 (1998). [CrossRef]
  9. A. Di Lallo, C. Conti, A. Cino, and G. Assanto, "Efficient Frequency Doubling in Reverse Proton Exchanged Lithium Niobate waveguides," IEEE Photon. Technol. Lett. 13, 323-325, (2001). [CrossRef]
  10. J. Olivares and J. M. Cabrera. "Guided modes with ordinary refractive index in proton exchanged LiNbO3 waveguides," Appl. Phys. Lett. 62, 2468-2470 (1993). [CrossRef]
  11. K. R. Parameswaran, R. K. Route, J. R. Kurz, R. V. Roussev, M. M. Fejer and M. Fujimura. "Highly efficient second-harmonic generation in buried waveguides formed by annealed and reverse proton exchange in periodically poled lithium Niobate," Opt. Lett. 27, 179-181 (2002). [CrossRef]
  12. M. Domenech, G. Lifante and F. Cussó, A. Parisi, A. C. Cino and S. Riva Sanseverino, "Fabrication and characterisation of reverse proton exchange optical waveguides in Neodymium doped lithium niobate crystals," Mater. Sci. Forum 480-481, 429-436 (2005).
  13. G. Lifante, E. Cantelar, F. Cussó, M. Domenech, A. C. Busacca, A. C. Cino and S. Riva Sanseverino "Imaginary distance BPM as an efficient tool for modelling optical waveguides fabrication by ion diffusion," Proc. OWTNM’06, Varese, Italy (2006).
  14. C. Jacinto, S. L. Oliveira, L. A. O. Nunes, T. Catunda, and M. J. V. Bell. "Thermal lens study of the OH-influence on the fluorescence efficiency of Yb3+-doped phosphate glasses," Appl. Phys. Lett. 86, 071911 (2005). [CrossRef]
  15. U. R. Rodríguez Mendoza, A. Ródenas, D. Jaque, I. R. Martín, F. Lahoz and V. Lavín "High pressure luminescence in Nd doped LiNbO3 crystals," High Press. Res. 26, 341-343 (2006). [CrossRef]
  16. D. Jaque, E. Cantelar and G. Lifante "Lattice micro-modifications induced by Zn difussion in Nd:LiNbO3 channel waveguides probed by Nd3+ confocal luminescence," Appl. Phys. B. DOI: 10.1007/s00340-007-2692-9 (2007).
  17. B. V. Dierold and C. Sandmann. "Inspection of periodically poled waveguide devices by confocal luminescence microscopy," Appl. Phys. B. 78, 363-366 (2004). [CrossRef]

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