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Characteristics of optical waveguides formed in LiNbO3 by He+ implantation
Vacuum/volume 37/numbers Printed in Great Britain
3/4/pages
0042-207X/87$3.00+.00 Pergamon Journals Ltd
261 to 264/l 987
Characteristics of optical waveguides LiNbO, by He+ implantation
formed
in
B L Weiss, Department of Electronic and Electrical Engineering, University of Surrey, Guildford, Surrey GU2 5XH, UK
This paper outlines the waveguide structures obtained by He + ion implantation into LiNbO, and the characteristics of interest, i.e. number of modes, modal attenuation and effective waveguide refractive index (modal propagation constant). The effects of ion energy, ion dose and material orientation are shown to affect the characteristics of the resultant devices so that waveguides with predictable properties may be fabricated.
1. Introduction is now receiving growing The process of He + implantation interest for the fabrication of integrated optical waveguides in LiNbO,, due to the flexibility of the process and the superior process control offered by ion implantation, as compared to the more conventional process of Ti indiffusion into LiNbO, (refs l-3). The characteristics of planar optical waveguides formed in lithium niobate by the implantation of He+ are dependent upon many process parameters of which the important ones include the ion energy, the ion dose and the orientation of the LiNbO, sample. In this paper these effects are reviewed and are shown to have a significant effect on the refractive index profile which determines the properties of the resultant optical waveguides. The properties of planar optical waveguides which are of interest are the number of modes supported by the waveguide, the modal propagation constant and the modal attenuation.
lncldenl
Isolation
Implanted Refractive
Substrate Refractive Index
Figure 1. Configuration LiNbO,.
layer
wdth
region Index
= n -An
= ”
of waveguide formed by ion implantation
in
Fundamental E Field Profile
2. Waveguide structure The implantation of He+ ions into LiNbO, produces a buried damage layer which has a reduced density, an increased volume and, therefore, a reduced refractive index. The depth of this damage layer is determined by the energy of the incident ion4 whilst the change of the refractive index (An) is determined by the ion dose up to saturation’, as shown in Figures 1 and 2 respectively. Consequently, the sample surface, which has the highest refractive index, is surrounded by the low refractive index buried damage layer on one side and by air on the other, so that a planar waveguide with the light confined to the sample surface is produced. The waveguide depth and An determine the number of modes propagating in the waveguide whilst the damage in both the guiding layer and the buried layer determine the propagation constant of each mode. Attenuation of the propagating mode is due to scattering of the propagating beam by the damage in the guiding layer, since the presence of the damage produces a
10”s
I
LOSS
MECHANISM
Figure 2. Loss mechanism.
localized change of refractive index and is determined by the overlap integral of the modal field and the damage profile. One effect of the implantation process is the decomposition of the surface layers of the samples by the energetic incident ions. This decomposition results in a loss of Li from the sample which 261
8 f Weiss: Optical waveguides formed in LiNbO, by He+ implantation
selectively affects only the extraordinary refractive index (n,) of the material, so that n0 is 2.29 whilst n, is increased above 2.20, its value for as grown material. Consequently the different polarizations (TE and TM) see different values of refractive index. The damage profile is also seen to be a function of the crystallographic orientation of the sample5. Whilst the depth of the peak damage level remains the same for all orientations, the damage profile is not the same in all samples so that the refractive index depth profile is different for each orientation. Also different orientations access different refractive indices (n, and n,).
3. Experimental method The material was prepared by cutting large slices of LiNbO, into samples 2.5 x 1 x 0.05 cm (thick) using a diamond saw lubricated with oil containing 240 grade grit. To achieve good smoothness the slices were mounted between glass plates using wax. After cutting, the samples were separated from the glass plates and cleaned in warm toluene, trichloroethylene and isopropanol. The He+ implantation employed a 2 MeV Van de Graaff accelerator used to generate a collimated beam of He+ ions which was scanned over the sample surface to ensure that the sample received a uniform ion dose. The samples were mounted in the chamber which was pumped to - 10e6 torr using an oil diffusion pump with a liquid nitrogen trap. To ensure that their temperature did not rise significantly above room temperature, the samples were mounted on a stainless steel plate using conducting epoxy, thus providing a high thermal conductivity path between the sample and its holder. Beryllium copper clips were used to hold the sample on the plate and to provide a source of secondary electrons to neutralize any surface charge build up due to the insulating nature of LiNbO,. Following implantation the samples were annealed in an oxygen atmosphere at 200°C for 30 min to reduce the damage and, thus, the scattering in the surface (guiding) layer. An oxygen atmosphere was required during annealing to prevent further decomposition of the sample and to aid partial recovery which has been found to take place during annealing. Optical characterization of the resultant waveguide consists of measurement of the propagation constant and the attenuation of each mode supported by the waveguide. These measurements are made using a prism coupler6, see Figure 3, which uses a pair of
rutile prisms to couple light into and out of the waveguide. Variation of the angle of incidence enables each mode supported by the waveguide to be excited selectively. By measuring the angle of incidence of each mode the propagation constant may be calculated. Similarly varying the prism separation and measuring the change in the intensity in the central spot of the output mode line enables the modal attenuation to be calculated.
4. Results and discussion 4.1. Ion energy. As the ion energy is increased the depth of the waveguide increases and therefore the number of modes supported by the waveguide increases, as shown in Table 1. However for low ion energies (< 1 MeV) the Li loss dominates the TE behaviour of these waveguides so that an initial decrease of the number of modes is seen until the behaviour of the waveguide is determined by the buried damage layer when the number of modes increases. For the TM excitation the waveguide supports one mode at an energy of 0.8 MeV which rises to four modes at 2 MeV, for a dose of 1016 He+ cm-*. The propagation constant is seen to increase towards the refractive index of LiNbO, with increasing ion energy, due to the lower average damage concentration in the guiding layer for the higher energies, for the TM modes. In contrast the TE mode behaviour is complicated by the Li loss’. The attenuation of the TM modes is seen to decrease with ion energy due to smaller overlap between the E field of the propagating mode and the damage profile of the waveguide
RUTILE
PRISM
I
SUBSTRATE
PRISM COUPLER
Figure 3. Prism coupler.
Table 1. Variation of TM and TE properties of the waveguides with ion energy
Ordinary (TM) modes Ion energy (MW 0.8 1.0 1.2 1.5 2.0
262
Mode No.
Propagation (B/k)
1 2 1 1 2 1 2 1 2 3 4
constant
Extraordinary
(TE) modes
Attenuation a(db cm-‘)
Propagation (Blk)
2.261 -
37.0 -
2.270 2.215 2.265 2.273 2.265 2.216 2.213 2.266 2.258
32.5 22.0 < 50.0 6.0 15.0 4.5 6.3 10.5 22.0
2.215 2.205 2.209 2.209 -
4.5 9.5 15.25 9.0 -
2.213 -
-
2.211 2.206 -
constant
Attenuation a(dB cm-‘)
6.6 8.5 9.25
B L Weiss: Optical waveguides
formed in LiNbO, by He+ implantation
resulting in less light being scattered. The higher order modes have an increasingly higher attenuation as the overlap between their E field and the damage profile increases with mode order. For the TE modes, their behaviour is again complicated by the presence of this Li loss, with a minimum value of -6 dB cm- ’ being obtained with an ion energy of 1.6 MeV, in contrast the lowest TM attenuation of -3 dB cm-‘, which occurs at an ion energy of 2.0 MeV. 4.2. Ion dose. The variations of waveguide properties with ion dose are given in Table 2. An increase in the ion dose produces an increase in the value of An in the buried layer and, thus, the number of modes supported by the waveguide increases. At lOi He+ cme2 the waveguide supports only one TM mode whilst above 5 x 10” He+ ions cm-’ it supports two TM modes and above 2 x 1Ol6 He+ cm-’ there are three TM modes. However for the TE polarization the number of modes is increased from one to two modes for doses above 1016 He+ cm-‘. These differences in the behaviour of the TE and TM polarizations are due to the different refractive index profiles seen by the TE and TM modes due to the selective effects of the Li loss affecting only the TE modes. The variation of attenuation with ion dose is seen to have a minimum value at 8 x 1015 He+ cm-’ and about 12x 1Ol5 He+ cm- ’ for TE and TM modes respectively, with the value of attenuation being slightly higher for TE modes which is thought to be due to the influence of the surface decomposition. 4.3. Material crystallographic orientation. Consider the results given in Table 3. For the orientations considered, Y-cut and Zcut, the Z-cut samples were always found to have a lower loss
while it supports three ordinary modes due to n, and one extraordinary mode due to nc. However, its extraordinary modal propagation constant is always greater than that of LiNbO, (i.e. 2.20) indicating the presence of Li loss from the sample surface, with this loss being lower in Z-cut samples, due to the lower level of damage in the Z-cut material. Propagation along the short (1 cm) width of the samples enabled these propagation constants to be measured. The results obtained are as expected with the Y-cut Z propagating sample accessing n, for both input polarizations. 4.4. Discussion of results. By fitting the results of the effect of ion energy and dose on the propagation constant into the transverse resonance equation, good agreement with the results is obtained. The one exception is the effect of ion energy on the (b/k) results where the initial drop in the number of TE modes supported by the structure appears to be incorrect. However, by inclusion of the surface Li loss in the TE refractive index depth profile these effects can be modelled7. The waveguide losses are effectively a measure of the overlap between the modal field and the damage depth profile. The variation of the latter with ion energy and dose and the former with waveguide depth and An show good agreement with the results obtained. Thus, the modal field at the guiding layer buried layer decreases with increasing waveguide depth whilst the gradient of the damage profile at that point decreases with increasing ion energy and dose. For the material orientation variations the only significant variation is the level of damage in the guiding layer which is the cause of the lower attenuation of the waveguides in the Z-cut material.
Table 2. Variation of the TM and TE properties of the waveguides with ion dose
Ordinary (TM) modes Ion dose (lOI cm-‘) 0.5 0.8 1.0 1.5 2.2
Extraordinary (TE) mode
Mode No.
Propagation constant (B/k)
Attenuation a(dB cm-‘)
Propagation constant (B/k)
1 1 2 1 2 1 2 1 2 3
2.280 2.273 2.265 2.213 2.265 2.273 2.264 2.271 2.262 2.251
20.5 10.5 15.75 6.0 15.0 6.25 16.25 19.0 20.5 36.0
2.206 2.211
Attenuation a(dB cm-‘) 9.75 6.0 -
2.213
-
2.212 2.205 2.212 2.205 -
6.5
10.0 8.5 14.75 > 50.0 -
Table 3. Values of B/k and a for each TE and TM mode in Z-cut and Y-cut LiNbO, samples implanted with 1.5 MeV He+ and 1Ol6 ions cm-* dose Z-cut X propagation Mode No TE TM
Y-cut X propagation
Blk
a(dB cm-‘)
Blk
a(dB cm-‘)
2.2780 2.2700 2.2575 2.2030
2.5kO.26 7.9kO.5 Extremely faint 5.2k2.0
2.209
6.7kO.8
2.2745 2.2665
3.2* 1.33 10.4k1.19
B L Weiss:
Optical waveguides formed in LiNbO, by He+ implantation
5. Conclusions
References
The above results show that both the ion energy and the ion dose determine the number of modes supported by the waveguide whilst scattering of light by damage in the guiding layer determines the attenuation of the waveguide. The extraordinary mode propagation constants show an increase above the value of n, for LiNbO, which indicates that a Li loss was found in all samples. A comparison of Y- and Z-cut samples shows a lower loss for the Z-cut material.
’ G D H King et al, Electron Letts, 17, 897 (1981). J M Naden and B L Weiss, J Lightwave Technol, LT-3, 855 (1985). ’ G L Destefanis et al, J Appl Phys, 50, 7898 (1979). 3 G Gotz and H Karge, Nucl Instrum Meth, 209/210, 1079 (1983). ’ K M Barfoot and B L Weiss, J Phys D: Appl Phys, 17, L47 (1984). ’ B L Weiss and J L Flint .I Appl Phys, 60, 464 (1986). 6 T Tamir (ed), Topics in Applied Physics, Vol 7, 2nd edition, Integrated Optics, p 223, Springer, Berlin (1979). ’ I M Skinner et al, The modelling of Li outdiffusion in He+ implanted optical waveguides in LiNbO,, Solid State Electron (submitted for publication).
Acknowledgement The author acknowledges the assistance of Mr J M Naden with the experimental work and the SERC for financial support.
264
3/4/pages
0042-207X/87$3.00+.00 Pergamon Journals Ltd
261 to 264/l 987
Characteristics of optical waveguides LiNbO, by He+ implantation
formed
in
B L Weiss, Department of Electronic and Electrical Engineering, University of Surrey, Guildford, Surrey GU2 5XH, UK
This paper outlines the waveguide structures obtained by He + ion implantation into LiNbO, and the characteristics of interest, i.e. number of modes, modal attenuation and effective waveguide refractive index (modal propagation constant). The effects of ion energy, ion dose and material orientation are shown to affect the characteristics of the resultant devices so that waveguides with predictable properties may be fabricated.
1. Introduction is now receiving growing The process of He + implantation interest for the fabrication of integrated optical waveguides in LiNbO,, due to the flexibility of the process and the superior process control offered by ion implantation, as compared to the more conventional process of Ti indiffusion into LiNbO, (refs l-3). The characteristics of planar optical waveguides formed in lithium niobate by the implantation of He+ are dependent upon many process parameters of which the important ones include the ion energy, the ion dose and the orientation of the LiNbO, sample. In this paper these effects are reviewed and are shown to have a significant effect on the refractive index profile which determines the properties of the resultant optical waveguides. The properties of planar optical waveguides which are of interest are the number of modes supported by the waveguide, the modal propagation constant and the modal attenuation.
lncldenl
Isolation
Implanted Refractive
Substrate Refractive Index
Figure 1. Configuration LiNbO,.
layer
wdth
region Index
= n -An
= ”
of waveguide formed by ion implantation
in
Fundamental E Field Profile
2. Waveguide structure The implantation of He+ ions into LiNbO, produces a buried damage layer which has a reduced density, an increased volume and, therefore, a reduced refractive index. The depth of this damage layer is determined by the energy of the incident ion4 whilst the change of the refractive index (An) is determined by the ion dose up to saturation’, as shown in Figures 1 and 2 respectively. Consequently, the sample surface, which has the highest refractive index, is surrounded by the low refractive index buried damage layer on one side and by air on the other, so that a planar waveguide with the light confined to the sample surface is produced. The waveguide depth and An determine the number of modes propagating in the waveguide whilst the damage in both the guiding layer and the buried layer determine the propagation constant of each mode. Attenuation of the propagating mode is due to scattering of the propagating beam by the damage in the guiding layer, since the presence of the damage produces a
10”s
I
LOSS
MECHANISM
Figure 2. Loss mechanism.
localized change of refractive index and is determined by the overlap integral of the modal field and the damage profile. One effect of the implantation process is the decomposition of the surface layers of the samples by the energetic incident ions. This decomposition results in a loss of Li from the sample which 261
8 f Weiss: Optical waveguides formed in LiNbO, by He+ implantation
selectively affects only the extraordinary refractive index (n,) of the material, so that n0 is 2.29 whilst n, is increased above 2.20, its value for as grown material. Consequently the different polarizations (TE and TM) see different values of refractive index. The damage profile is also seen to be a function of the crystallographic orientation of the sample5. Whilst the depth of the peak damage level remains the same for all orientations, the damage profile is not the same in all samples so that the refractive index depth profile is different for each orientation. Also different orientations access different refractive indices (n, and n,).
3. Experimental method The material was prepared by cutting large slices of LiNbO, into samples 2.5 x 1 x 0.05 cm (thick) using a diamond saw lubricated with oil containing 240 grade grit. To achieve good smoothness the slices were mounted between glass plates using wax. After cutting, the samples were separated from the glass plates and cleaned in warm toluene, trichloroethylene and isopropanol. The He+ implantation employed a 2 MeV Van de Graaff accelerator used to generate a collimated beam of He+ ions which was scanned over the sample surface to ensure that the sample received a uniform ion dose. The samples were mounted in the chamber which was pumped to - 10e6 torr using an oil diffusion pump with a liquid nitrogen trap. To ensure that their temperature did not rise significantly above room temperature, the samples were mounted on a stainless steel plate using conducting epoxy, thus providing a high thermal conductivity path between the sample and its holder. Beryllium copper clips were used to hold the sample on the plate and to provide a source of secondary electrons to neutralize any surface charge build up due to the insulating nature of LiNbO,. Following implantation the samples were annealed in an oxygen atmosphere at 200°C for 30 min to reduce the damage and, thus, the scattering in the surface (guiding) layer. An oxygen atmosphere was required during annealing to prevent further decomposition of the sample and to aid partial recovery which has been found to take place during annealing. Optical characterization of the resultant waveguide consists of measurement of the propagation constant and the attenuation of each mode supported by the waveguide. These measurements are made using a prism coupler6, see Figure 3, which uses a pair of
rutile prisms to couple light into and out of the waveguide. Variation of the angle of incidence enables each mode supported by the waveguide to be excited selectively. By measuring the angle of incidence of each mode the propagation constant may be calculated. Similarly varying the prism separation and measuring the change in the intensity in the central spot of the output mode line enables the modal attenuation to be calculated.
4. Results and discussion 4.1. Ion energy. As the ion energy is increased the depth of the waveguide increases and therefore the number of modes supported by the waveguide increases, as shown in Table 1. However for low ion energies (< 1 MeV) the Li loss dominates the TE behaviour of these waveguides so that an initial decrease of the number of modes is seen until the behaviour of the waveguide is determined by the buried damage layer when the number of modes increases. For the TM excitation the waveguide supports one mode at an energy of 0.8 MeV which rises to four modes at 2 MeV, for a dose of 1016 He+ cm-*. The propagation constant is seen to increase towards the refractive index of LiNbO, with increasing ion energy, due to the lower average damage concentration in the guiding layer for the higher energies, for the TM modes. In contrast the TE mode behaviour is complicated by the Li loss’. The attenuation of the TM modes is seen to decrease with ion energy due to smaller overlap between the E field of the propagating mode and the damage profile of the waveguide
RUTILE
PRISM
I
SUBSTRATE
PRISM COUPLER
Figure 3. Prism coupler.
Table 1. Variation of TM and TE properties of the waveguides with ion energy
Ordinary (TM) modes Ion energy (MW 0.8 1.0 1.2 1.5 2.0
262
Mode No.
Propagation (B/k)
1 2 1 1 2 1 2 1 2 3 4
constant
Extraordinary
(TE) modes
Attenuation a(db cm-‘)
Propagation (Blk)
2.261 -
37.0 -
2.270 2.215 2.265 2.273 2.265 2.216 2.213 2.266 2.258
32.5 22.0 < 50.0 6.0 15.0 4.5 6.3 10.5 22.0
2.215 2.205 2.209 2.209 -
4.5 9.5 15.25 9.0 -
2.213 -
-
2.211 2.206 -
constant
Attenuation a(dB cm-‘)
6.6 8.5 9.25
B L Weiss: Optical waveguides
formed in LiNbO, by He+ implantation
resulting in less light being scattered. The higher order modes have an increasingly higher attenuation as the overlap between their E field and the damage profile increases with mode order. For the TE modes, their behaviour is again complicated by the presence of this Li loss, with a minimum value of -6 dB cm- ’ being obtained with an ion energy of 1.6 MeV, in contrast the lowest TM attenuation of -3 dB cm-‘, which occurs at an ion energy of 2.0 MeV. 4.2. Ion dose. The variations of waveguide properties with ion dose are given in Table 2. An increase in the ion dose produces an increase in the value of An in the buried layer and, thus, the number of modes supported by the waveguide increases. At lOi He+ cme2 the waveguide supports only one TM mode whilst above 5 x 10” He+ ions cm-’ it supports two TM modes and above 2 x 1Ol6 He+ cm-’ there are three TM modes. However for the TE polarization the number of modes is increased from one to two modes for doses above 1016 He+ cm-‘. These differences in the behaviour of the TE and TM polarizations are due to the different refractive index profiles seen by the TE and TM modes due to the selective effects of the Li loss affecting only the TE modes. The variation of attenuation with ion dose is seen to have a minimum value at 8 x 1015 He+ cm-’ and about 12x 1Ol5 He+ cm- ’ for TE and TM modes respectively, with the value of attenuation being slightly higher for TE modes which is thought to be due to the influence of the surface decomposition. 4.3. Material crystallographic orientation. Consider the results given in Table 3. For the orientations considered, Y-cut and Zcut, the Z-cut samples were always found to have a lower loss
while it supports three ordinary modes due to n, and one extraordinary mode due to nc. However, its extraordinary modal propagation constant is always greater than that of LiNbO, (i.e. 2.20) indicating the presence of Li loss from the sample surface, with this loss being lower in Z-cut samples, due to the lower level of damage in the Z-cut material. Propagation along the short (1 cm) width of the samples enabled these propagation constants to be measured. The results obtained are as expected with the Y-cut Z propagating sample accessing n, for both input polarizations. 4.4. Discussion of results. By fitting the results of the effect of ion energy and dose on the propagation constant into the transverse resonance equation, good agreement with the results is obtained. The one exception is the effect of ion energy on the (b/k) results where the initial drop in the number of TE modes supported by the structure appears to be incorrect. However, by inclusion of the surface Li loss in the TE refractive index depth profile these effects can be modelled7. The waveguide losses are effectively a measure of the overlap between the modal field and the damage depth profile. The variation of the latter with ion energy and dose and the former with waveguide depth and An show good agreement with the results obtained. Thus, the modal field at the guiding layer buried layer decreases with increasing waveguide depth whilst the gradient of the damage profile at that point decreases with increasing ion energy and dose. For the material orientation variations the only significant variation is the level of damage in the guiding layer which is the cause of the lower attenuation of the waveguides in the Z-cut material.
Table 2. Variation of the TM and TE properties of the waveguides with ion dose
Ordinary (TM) modes Ion dose (lOI cm-‘) 0.5 0.8 1.0 1.5 2.2
Extraordinary (TE) mode
Mode No.
Propagation constant (B/k)
Attenuation a(dB cm-‘)
Propagation constant (B/k)
1 1 2 1 2 1 2 1 2 3
2.280 2.273 2.265 2.213 2.265 2.273 2.264 2.271 2.262 2.251
20.5 10.5 15.75 6.0 15.0 6.25 16.25 19.0 20.5 36.0
2.206 2.211
Attenuation a(dB cm-‘) 9.75 6.0 -
2.213
-
2.212 2.205 2.212 2.205 -
6.5
10.0 8.5 14.75 > 50.0 -
Table 3. Values of B/k and a for each TE and TM mode in Z-cut and Y-cut LiNbO, samples implanted with 1.5 MeV He+ and 1Ol6 ions cm-* dose Z-cut X propagation Mode No TE TM
Y-cut X propagation
Blk
a(dB cm-‘)
Blk
a(dB cm-‘)
2.2780 2.2700 2.2575 2.2030
2.5kO.26 7.9kO.5 Extremely faint 5.2k2.0
2.209
6.7kO.8
2.2745 2.2665
3.2* 1.33 10.4k1.19
B L Weiss:
Optical waveguides formed in LiNbO, by He+ implantation
5. Conclusions
References
The above results show that both the ion energy and the ion dose determine the number of modes supported by the waveguide whilst scattering of light by damage in the guiding layer determines the attenuation of the waveguide. The extraordinary mode propagation constants show an increase above the value of n, for LiNbO, which indicates that a Li loss was found in all samples. A comparison of Y- and Z-cut samples shows a lower loss for the Z-cut material.
’ G D H King et al, Electron Letts, 17, 897 (1981). J M Naden and B L Weiss, J Lightwave Technol, LT-3, 855 (1985). ’ G L Destefanis et al, J Appl Phys, 50, 7898 (1979). 3 G Gotz and H Karge, Nucl Instrum Meth, 209/210, 1079 (1983). ’ K M Barfoot and B L Weiss, J Phys D: Appl Phys, 17, L47 (1984). ’ B L Weiss and J L Flint .I Appl Phys, 60, 464 (1986). 6 T Tamir (ed), Topics in Applied Physics, Vol 7, 2nd edition, Integrated Optics, p 223, Springer, Berlin (1979). ’ I M Skinner et al, The modelling of Li outdiffusion in He+ implanted optical waveguides in LiNbO,, Solid State Electron (submitted for publication).
Acknowledgement The author acknowledges the assistance of Mr J M Naden with the experimental work and the SERC for financial support.
264