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Ion beam modification of electro-optical crystals
1142
Nuclear
Instruments
and Methods in Physics Research B59/60 (1991) 1142-l 146 North-Holland
Ion beam modification of electro-optical
crystals
Ch. Buchal
A short review is given on the role of electro-optical crystals in optical signal processing, on the potential and limitations of ion beam modifwxtion of these materials.
1. Introduction
There is a great deal of scientific, technical and commercial interest in optocommunication and optical signal processing. Today optical fibers compete successfully with high-frequency coaxial cables in local area computer networks (LANs). cross country telephone connections and data links. The necessary laser diodes, glass fibers and photodiodes for light generation, guidance and detection have become very common. Semiconductor laser diodes are marvelous devices, but the wavelength of the generated light is determined by the
applied signals
light in
light out (fiber)
(fiber)
applied
electric
signal
field
effect
Change
function
Index
phase
on some of their structural
detail\,
and
semiconductor bandgap and normally longer than 0.7 pm. Information is transmitted by modulating the light, and direct modulation of the laser cavity is standard technology; but so far the maximum bit rate is achieved in the stabilized continuous operation of the laser with external modulation. The fastest modulators and switches are made from single crystalline electro-optical materials. These amazing materials are not very common in the field yet, but they are standard tools or research objects in the laboratories investigating future technologies. In the following an outline is given of potential and limitations of ion implantation into these crystals. For the details the reader has to refer to the literature. A scheme of the performance of electro-optical (and magneto-optical) crystals is shown in fig. 1 [l-5]. These crystals are oxides, clear as sapphire (AI,O,) or quartz (SiOz) and electrically insulating. So far the workhorse among them remains LiNbO,, which is grown in large boules and sold as high-quality wafers. Those are the starting materials for many components. Their optical performance is a consequence of their single-crystalline structure, and we will look at LiNbOl as the most prominent example. For a recent review on many other materials see ref. [2].
modulation
amplitude
modulation
witching spectral
analysis
diodes)
magnetic
field
rotate
phase
(optical
presrure
wave
change
index
diffrection frequency nodulaticn
change
lnder
change
index
(round) heat
light
(2nd order)
2"d harmonic generation pat-metric amplification
Fig. I. Scheme of the functions of electro-optical and magneto-optical materials. Very different applied signals may he used to modify the propagation of light within the waveguide inside the crystal. 0168-583X/91/$03.50
C’ 1991 - Elsevier Science
2. Structure
and properties
of LiNb03
LiNbO, belongs to the ilmenite structure, similar to sapphire. According to ref. [6], the large O’- ions are arranged in close-packed layers and fill the available volume nearly completely. In the case of AILOlr the A13+ cations find positions between O’- layers, such that they are surrounded by six O’anions in an octahedral position. + of the available octahedral sites are occupied. i remain vacant. These structural vacancies are ordered to minimize the Coulomb energy. Along the hexagonal c-axis the Al-sublattice shows well ordered structural vacancies: “A1~AI-vac-Al~Al~vac-
Publishers B.V. (North-Holland)
Ch. Buchal / Electra-optical crystals
. . “. LiNbO, is composed according to the same principles, but the cations Li+ and Nd+ carry a distinctly different charge and have different ionic radii (A13+: 0.50 A, Li+: 0.60 A, Nd+: 0.70 A). In the ferroelectric state, these cations are also ordered along the c-axis: “Nd+-Li+-vat-Nd+-Li’-vac. . . “. Obviously excess positive charge is concentrated at the Nd+ end of the unit cell and this constitutes the crystal’s electric dipolar moment, see fig 2. [7,8]. LiNbO, crystals are grown in this well ordered single-domain ferroelectric state, if a small electric field is applied during the growth process [9]. Although the atomic density of LiNbO, (31.8 cm3/mole)-’ is lower than that of Al,O, (25.6 cm3/mole)-‘, its optical indices are higher (Al,O,: no= ne= 1.77; LiNbO,: no = 2.29, ne = 2.20). The reason lies mainly in the higher polarizability of the NbO, octahedron compared to the AlO, configuration. We add the observation that rutile (TiO,) consists of cleverly stacked TiO, octahedra and displays even higher indices: n, = 2.9, ne = 2.6. For many applications, light is coupled from fibers into the crystal and forward into other fibers (fig. 1). This is achieved by optical waveguides, small stripes of an enhanced index. They confine the optical intensity,
POSITIVE DIPOLE
EN0
+E
AXIS
4
a
FERROELECTRIC
Fig. 2. Atomic arrangement of LiNbO, in the ferroelectric phase. On the right, the oxygen layers are denoted as lines. The Nb” ions (hatched) reside near the middle of an octahedron of six 02- ions, whereas the Li+ ions (black) move close to an oxygen layer (from ref. [7]).
1143
thereby permitting efficient operation. The fabrication of low-loss waveguides is a prerequisite for essentially all integrated optical devices [l-5]. An index change of 1% is already sufficient for guiding. Normally polarized light is used and the E-vector of the light interacts with electronic orbitals within the crystal, which shows strong optical anisotropy. Externally applied signals (fig. 1) change atomic positions and modify polarizabilities, thus changing the index. Strong intensities of light may perturb some orbitals so much that a nonlinear response is observed. This is the basis for frequency doubling or parametric amplification.
3. Utilization of the damage generated by ion implantation The detailed crystalline order is responsible for the usefulness of electro-optical crystals. The collision events of ion implantation generate disorder. Fig. 3 shows an illustrative example. Crystalline quartz (SiO,) is damaged by implantation; the density and the index decrease. In contrast, SiO, glass is somewhat compacted by implantation and accordingly the index is increased somewhat [lo]. This increase is exceptional, whereas the index decrease due to disorder generation is very common for crystalline materials. It can be used to define barriers for light. Townsend and coworkers have studied this type of waveguide formation and communicated it in several reviews [11,12]. It is important to recall that low-mass (He) ion implantation generates the maximum nuclear recoil damage near the end of the ion’s range [13], while the near-surface regime remains relatively undamaged. Therefore MeV He implantation can be used to generate a buried layer of reduced optical index within essentially all optical crystals. By changing the ion’s energy, the layer depth is adjustable and multiple barriers or barriers separating two guiding regions can be fabricated [12]. This has considerable relevance for applications, as many electro-optical crystals do not lend themselves to diffusion or ion exchange techniques [1,5] for index modification. One example is KNbO,, which is useful for second-harmonic generation. Under certain conditions it doubles the frequency of light from 860 nm (AlGaAs laser diode) to 430 nm. This is desirable for higher-density optical data storage. For efficient operation, strong light intensities guided along specific crystallographic directions are necessary. Therefore several groups recently studied the properties of He-implanted waveguides in KNbO, [14-161. It turns out that KNbO, is very sensitive to radiation damage, and a dose of 5 X lOI He/cm2 at 2 MeV is already sufficient to fabricate a planar guide with losses of less than 4 dB/cm [15]. LiNbO, guides generally are implanted IX. INSULATORS/CERAMICS/POLYMERS
Ch.
1144
0 Ar’/50.
a 8’
Buchul
/ Electra-opiical
150keVJ
1
[70keV!
1 N’ llOQkeVl + Ne: Ar: Kr ’
Fig. 3. Change of index of SiOz as a function of ion implantation dose. here normalized to the deposited nuclear recoil energy. Crystalline quartz is disordered and its index is lowered; quartz glass is compacted and the index is raised (from ref.
[101).
with lOI to 5 x lOI He/cm*, and the best guide published showed losses of less than 0.1 dB/cm [ll].
4. Beyond the implantation damage: annealing and solid phase epitaxy It is a well established technology for waveguide fabrication to diffuse Ti into LiNbO, at high temperatures [17,5]. Its microscopic background is the introduction of Tt“+ into TiO, octahedral positions within the LiNbO, crystal in order to enhance the optical index.
1400
ctytuls
Motivated by the desire to raise the Ti dopant concentration above the limits imposed by diffusion kinetics, the Oak Ridge group has developed and perfected the concept of direct Ti implantation into LiNbO, [l&19]. Details of the technological process and the optical characterizations of the fabricated devices are published [20]. The typical dose of 2.5 X 10” Ti/cm’ provides a huge concentration of Ti in the as-implanted sample, but the LiNbO, crystal is entirely amorphized over the implanted depth. An additional annealing step at typically 1000 ’ C in an oxygen atmosphere causes the underlying undamaged crystal to regrow towards the surface. The details of this process are complex, since they involve diffusion of implanted Ti, diffusion of oxygen from the annealing atmosphere and Li diffusion out of the bulk. The process has been identified as solid phase epitaxy (SPE) and is most clearly demonstrated at a low-dose implant, see fig. 4 [21]. The SPE is performed at temperatures below the ferroelectric Curie temperature, and it is amazing that the Ti-doped crystal regrows into a single ferroelectric domain without additional poling. Lattice location studies show the substitutionality of the Ti ions [22,23]. The maximum Ti concentration for good guides in LiNbO, is 18 mole%, independent of implantation target temperature (620 to 77 K) [24]. It remains an open issue how the crystal accommodates the charge inbalance of a four-valent impurity (Ti4+) into a substitutional site. Two Ti4+@ NbSt substitutions can be compensated by one uncharged vacancy in the oxygen sublattice, or by two extra Li+ ions or by two protons from the water vapor of the annealing atmosphere. Presently we are investigating the co-implantation of charge-compensating cations, especially Mg2+ for Ti4+. Implanted Mg and Ti to-
1
T 1000
-
z 2 s
800
-
9 e
600400
-
-500
0
500
1000
1500
2000
2500
3000
3500
4000
4500
DEPTH (A) Fig. 4. RBS spectra
of SPE regrowth of implanted LiNbO, at 400 o C. The arrows represent the approximate positions of the interface at various annealing times. The Ti dose was lOI Ti/cm2. which is only a small disturbance of the stoichiometry (from ref. [211X
1145
Ch. Buchul / Electra-optical ctytals
gether with oxygen, introduced during annealing offers a good chance of forming stable (LiNb), _,(MgTi),O,, which should result in a waveguide with reduced susceptibility to the so-called “optical damage”. The virtue of doping the LiNbO, melt with up to 5% MgO has been demonstrated [25-271. Actually the He-implanted guides, as described in the preceding section. seem to be much less susceptible to optical damage than the Tidoped (diffused) guides. A stabilization of the Fe’+ impurity by the Ti4+ dopant is discussed as a possible reason. and annealing An extensive review of implantation of many crystalline oxides has been published recently
IW 5.Lasers and amplifiers Semiconductor diode lasers designed to match waveguide geometries are standard technology. It is known that the electro-optical crystals constitute good laser ion hosts, too. For laser action in these materials, one has to mtroduce 4f or 3d transition elements into the crystals and optically pump electrons to energy levels permitting stimulated emission. Two examples for oxide solid state lasers are the very popular Nd: YAG laser and the relatively new Ti : sapphire laser. In the case of the rare-earth doped lasers, a deep-lying 4f electron is laser active. Its orbitals are somewhat shielded from the surrounding crystalline lattice, and the corresponding energy levels remain undisturbed. In contrast, the 3d laser ions’ electrons populate 3d orbitals which are spatially further extended and are affected and broadened by phonons, forming “vibronic energy bands”. Photon emission is stimulated over a wider frequency range and therefore the Ti:Al,O, laser is tunable from 0.6 to 1.06 l.trn wavelength [29]. Note that the substitutional Ti’+ ion in Al,O, retains one d electron, which is responsible for color (absorption) and laser action. In contrast, a Ti4+ ion has given all outer electrons into bonds. It is used for index enhancement (TiO, octahedron) and shows no color. This remark emphasizes the necessity to control the charge state of implanted ions, which may be adjusted by an oxidizing or reducing atmosphere during post-implantation heat treatment, e.g., refs. [28,30-321. Experiments with coherent emission from rare-earth ions in electro-optical crystals have been started 25 years ago [33-351. An optical waveguide in doped Nd: LiNbO, was considered by ref. [35]. Although the measured Nd optical gain was good, the LiNbO, crystal suffered damage so rapidly that the experiments were limited to a few minutes. The subject was revived in 1985 with much greater success, because MgO-doped LiNbO, crystals proved to be less susceptible to this optical damage (photorefractive effect). The Nd : MgO : LiNbO, lasers permitted continuous operation, in-
tracavity Q-switching and frequency doubling, thereby taking full advantage of the electro-optical potential of LiNbO, [27,36,37]. Recently results from the first Nd: MgO: LiNbO, waveguide laser have been published. A melt-doped substrate was the starting material and several singlemode optical waveguides were fabricated on it by standard ion exchange techniques [38]. The Nd spectroscopic properties were practically unchanged by the waveguide environment. The optical pump operated at 0.814 pm and the laser oscillations started above a pump power threshold of 1.5 mW. The output power efficiency was 14%. The laser operated at a wavelength of 1.08 pm. The laser cavity consisted of a single waveguide of 8 urn width and 12 mm length with polished and coated end mirrors. The experiment has to be considered as a “proof of concept”. The potential of the approach lies in the future integration of a laser element together with other functions on one chip. or in the intracavity use of the electro-optical properties of the host (see above). The Sussex group has taken their well proven approach to tackle the problem of constructing a waveguide on a laser crystal also. They started from a Nd-doped YAG crystal and used 7 x lOi He/cm’ at up to 2.8 MeV energy to construct a planar waveguide of 6 pm depth and 10 mm length on one surface of the crystal. The unannealed guide showed losses of 4 dB/cm. Mirrors were attached to both ends of the guide. The Nd fluorescence showed no significant change, if compared to the bulk measurements. The optical pump operated at 0.59 urn. The laser threshold was reached at approximately 50 mW pump power inside the cavity, and the slope efficiency was around 1.7%. With that experiment the first laser action inside an ion-implanted waveguide in a dielectric crystal has been demonstrated also. Lower-loss stripe waveguides certainly will improve its performance [39]. These authors also studied the index profiles of He-implanted LiNbO, melt-doped with Nd, Cr or MgO and showed that the necessary low-level doping has very little effect on the efficiency of waveguide production by ion implantation [40]. We have been working along another road. As in semiconductor technology, we start from undoped substrates and investigate ion implantation doping of LiNbO, with rare-earth ions. The ultimate objective is a local doping for laser or amplifier action by selectively implanting and/or diffusing rare-earth ions into those parts of future integrated optical circuits, where it is desired, but leaving the main part of the substrate undoped to provide space for additional functions on the same wafer. The ion implantation, diffusion and annealing characteristics of Nd and Er in LiNbO, have been investigated [41]. After implantation and annealing at 1300 K 0.5 mole% of Er3+ could be introduced IX. INSULATORS/CERAMICS/POLYMERS
1146
Ch. Buchaf / Electra-optical
into substitutional lattice sites of LiNbO,. This is the same dopant concentration as used in bulk lasers. Thereafter the first Er-implanted single mode channel guides in LiNbO, have been fabricated and the Er fluorescence spectra measured [42]. They were compared to spectra excited in bulk Er: MgO: LiNbO, crystals and, as hoped, showed only very small changes of their characteristics. Therefore Er-doped waveguides are promising candidates for optical amplifiers and lasers in the 1.5 to 1.6 pm wavelength range.
P31
[I41 P51 U61 1171
6. Summary Light-ion (He) implantation with MeV energy into electro-optical crystals performs very well. The optical index is modified by irradiation damage in a controllable way and index barriers are easily constructed. Many waveguide geometries have been studied in numerous hosts, including the first planar waveguide laser in a Nd: YAG crystal. Ion implantation of transition elements or rare-earth ions generally needs additional processing, since a well defined lattice location requires annealing steps or even epitaxial regrowth. Implantation doping LiNbO, with laser active ions has been demonstrated and opens perspectives for more complex optical integrated circuits.
WI u91 WI 1211 P21 1231 ~41 ~51 WI ~71
Acknowledgement WI
Cooperation and discussions with B.R. Appleton, C.W. White, P.R. Ashley, P. Giinter, W. Sohler, T. Bremer and all the other Jiilich and Oak Ridge colleagues are gratefully acknowledged.
1291 [301
References 111 H. Nishihara, M. Haruna and T. Suhara. Optical Integrated Circuits (McGraw-Hill, New York, 1989). 121 P. Gtinter (ed.), Electra-optic and Photorefractive Materials (Springer, Berlin, 1987). 131 R.G. Hunsperger, Integrated Optics: Theory and Technology (Springer, Berlin, 1985). [41 T. Tamir (ed.), Integrated Optics (Springer, Berlin, 1979). I51 L.H. Hutcheson, Integrated Optical Circuits and Components (Dekker, New York. 1987). [61 L. Pauling, The Nature of the Chemical Bond (Cornell Univ. Press, Ithaca, NY, 1969). Appl. Phys. A37 (1985) I71 R.W. Weis and T.K. Gaylord, 191. PI A. Rluber, in: Current Topics in Materials Science vol. 1. ed. K. Keldis (North-Holland, 1978) p. 481. [91 K. Nassau. H.J. Levinstein and G.M. Loiacono, J. Phys. Chem. Solids 27 (1966) 983. of Insulators, eds. P. [lOI G. G&z, in: Ion Beam Modification Mazzoldi and G.W. Arnold (Elsevier, Amsterdam, 1987) p. 412. Rep. Progr. Phys. 5 (1987) 501: Cryst. 1111 P.D. Townsend, Latt. Def. and Amorph. Mat. 18 (1989) 377. Effects [=I P.D. Townsend, Proc. 5th Int. Conf. on Radiation
[311 [32] (331 [34] [35] [36] [37] [3X] [39]
1401 [41] [42]
crvstals
in Insulators, Hamilton, Canada, 1989. Nucf. Instr. and Meth. B46 (1990) 18. J.P. Biersack. in: Ion Beam Modification of Insulators. eds. P. Mazzoldi and G.W. Arnold (Elsevier. Amsterdam, 1987) p. 1 and 648. T. Bremer, W. Heiland, B. Hellerman. P. Hertel. E. Kratzig and D. Kollewe, Ferroelectr. Lett. 9 (1988) II. F.P. Strohkendl, P. Gtinter. Ch. Buchal and R. Irmscher, J. Appl. Phys. 69 (1991) 84. L. Zhang, P.J. Chandler and P.D. Townsend. Ferroelectr. Lett. 11 (1990) 89. R.V. Schmidt and I.P. Kaminow, Appl. Phys. Lett. 25 (1974) 458. Ch. Buchal, P.R. Ashley and B.R. Appleton. J. Mater. Res. 2 (1987) 222. Ch. Buchal, P.R. Ashley, D.K. Thomas and B.R. Appleton, Mater. Res. Sot. Symp. Proc. 88 (7987) 93. P.R. Ashley, W.S.C. Chang. Ch. Buchal and D.K. Thomas, IEEE J. Lightwave Technol. 7 (1989) 855. D.B. Poker, Mater. Res. Sot. Symp. Proc. 93 (1987) 293. P. Skeath, W. Elam. W. Burns. F. Stevie and T. Briggs, Phys. Rev. Lett. 59 (1987) 1950. Ch. Buchal, S. Mantl and D.K. Thomas, Mater. Res. Sot. Symp. Proc. 100 (1988) 317. T. Bremer. W. Heiland, Ch. Buchal, R. Irmscher and B. Stritzker. J. Appl. Phys. 67 (1990) 1183. D.A. Bryan, R. Gerson and H.E. Tomaschke. Appl. Phys. Lett. 44 (1984) 847. H. Wang, J. Wen, J. Li and H. Wang, Appl. Phys. Lett. 57 (1990) 344. T.Y. Fan, A. Cordova-Plaza, M. Digonnet. R.L. Byer and H.J. Shaw. J. Opt. Sot. Am. B3 (1986) 140. C.W. White. C.J. McHargue, P.S. Sklad. LA Boatner and G.C. Farlow. Mater. Sci. Rep. 4 (1989) 41, and references therein. J.C. Walling in: Tunable Lasers. eds. L.F. Mollenauer and J.C. White (Springer. Berlin, 1987) p. 331. A.J. Bourdillon, S.J. Bull, P.J. Burnett and T.F. Page. J. Mater. Sci. 21 (1986) 1547. G.C. Farlow. C.W. White, C.J. McHargue. P.S. Sklad and B.R. Appleton, Nucl. Instr. and Meth. B7/8 (1985) 541. G.C. Farlow, P.S. Sklad, C.W. White and C.J. McHargue. J. Mater. Res. 5 (1990) 7502. G. Burns, D.F. O’Kane and R.S. Title, Phys. Lett. 23 (1966) 56. L.F. Johnson and A.A. Ballman, J. Appl. Phys. 40 (1969) 297. I.P. Kaminow and L.W. Stulz, IEEE J. Quantum Electron. QE-11 (1975) 306. A. Cordova-Plaza, J.F. Digonnet and H.J. Shaw, IEEE J. Quantum Electron. 23 (1987) 262. A. Cordova-Plaza, T.Y. Fan, J.F. Digonnet, R.L. Byer and H.J. Shaw. Opt. Lett. 13 (1988) 209. E. Lallier et al., Electron. Lett. 25 (1989) 1491. P.J. Chandler, S.J. Field, D.C. Field, D.C. Hanna, D.P. Shepherd, P.D. Townsend, A.C. Tropper and L. Zhang, Electron. Lett. 25 (1989) 985. P.J. Chandler, L. Zhang and P.D. Townsend. Electron. Lett. 26 (1990) 332. Ch. Buchal and S. Mohr, J. Mater. Res. 6 (1991) 134. R. Brinkman. Ch. Buchal, St. Mohr. W. Sohler and H. Suche, Tech. Dig. on Integr. Photonics Res. vol. 5 (Opt. Sot. Amer., Washington. DC. 1990) post-deadline paper PDI.
Nuclear
Instruments
and Methods in Physics Research B59/60 (1991) 1142-l 146 North-Holland
Ion beam modification of electro-optical
crystals
Ch. Buchal
A short review is given on the role of electro-optical crystals in optical signal processing, on the potential and limitations of ion beam modifwxtion of these materials.
1. Introduction
There is a great deal of scientific, technical and commercial interest in optocommunication and optical signal processing. Today optical fibers compete successfully with high-frequency coaxial cables in local area computer networks (LANs). cross country telephone connections and data links. The necessary laser diodes, glass fibers and photodiodes for light generation, guidance and detection have become very common. Semiconductor laser diodes are marvelous devices, but the wavelength of the generated light is determined by the
applied signals
light in
light out (fiber)
(fiber)
applied
electric
signal
field
effect
Change
function
Index
phase
on some of their structural
detail\,
and
semiconductor bandgap and normally longer than 0.7 pm. Information is transmitted by modulating the light, and direct modulation of the laser cavity is standard technology; but so far the maximum bit rate is achieved in the stabilized continuous operation of the laser with external modulation. The fastest modulators and switches are made from single crystalline electro-optical materials. These amazing materials are not very common in the field yet, but they are standard tools or research objects in the laboratories investigating future technologies. In the following an outline is given of potential and limitations of ion implantation into these crystals. For the details the reader has to refer to the literature. A scheme of the performance of electro-optical (and magneto-optical) crystals is shown in fig. 1 [l-5]. These crystals are oxides, clear as sapphire (AI,O,) or quartz (SiOz) and electrically insulating. So far the workhorse among them remains LiNbO,, which is grown in large boules and sold as high-quality wafers. Those are the starting materials for many components. Their optical performance is a consequence of their single-crystalline structure, and we will look at LiNbOl as the most prominent example. For a recent review on many other materials see ref. [2].
modulation
amplitude
modulation
witching spectral
analysis
diodes)
magnetic
field
rotate
phase
(optical
presrure
wave
change
index
diffrection frequency nodulaticn
change
lnder
change
index
(round) heat
light
(2nd order)
2"d harmonic generation pat-metric amplification
Fig. I. Scheme of the functions of electro-optical and magneto-optical materials. Very different applied signals may he used to modify the propagation of light within the waveguide inside the crystal. 0168-583X/91/$03.50
C’ 1991 - Elsevier Science
2. Structure
and properties
of LiNb03
LiNbO, belongs to the ilmenite structure, similar to sapphire. According to ref. [6], the large O’- ions are arranged in close-packed layers and fill the available volume nearly completely. In the case of AILOlr the A13+ cations find positions between O’- layers, such that they are surrounded by six O’anions in an octahedral position. + of the available octahedral sites are occupied. i remain vacant. These structural vacancies are ordered to minimize the Coulomb energy. Along the hexagonal c-axis the Al-sublattice shows well ordered structural vacancies: “A1~AI-vac-Al~Al~vac-
Publishers B.V. (North-Holland)
Ch. Buchal / Electra-optical crystals
. . “. LiNbO, is composed according to the same principles, but the cations Li+ and Nd+ carry a distinctly different charge and have different ionic radii (A13+: 0.50 A, Li+: 0.60 A, Nd+: 0.70 A). In the ferroelectric state, these cations are also ordered along the c-axis: “Nd+-Li+-vat-Nd+-Li’-vac. . . “. Obviously excess positive charge is concentrated at the Nd+ end of the unit cell and this constitutes the crystal’s electric dipolar moment, see fig 2. [7,8]. LiNbO, crystals are grown in this well ordered single-domain ferroelectric state, if a small electric field is applied during the growth process [9]. Although the atomic density of LiNbO, (31.8 cm3/mole)-’ is lower than that of Al,O, (25.6 cm3/mole)-‘, its optical indices are higher (Al,O,: no= ne= 1.77; LiNbO,: no = 2.29, ne = 2.20). The reason lies mainly in the higher polarizability of the NbO, octahedron compared to the AlO, configuration. We add the observation that rutile (TiO,) consists of cleverly stacked TiO, octahedra and displays even higher indices: n, = 2.9, ne = 2.6. For many applications, light is coupled from fibers into the crystal and forward into other fibers (fig. 1). This is achieved by optical waveguides, small stripes of an enhanced index. They confine the optical intensity,
POSITIVE DIPOLE
EN0
+E
AXIS
4
a
FERROELECTRIC
Fig. 2. Atomic arrangement of LiNbO, in the ferroelectric phase. On the right, the oxygen layers are denoted as lines. The Nb” ions (hatched) reside near the middle of an octahedron of six 02- ions, whereas the Li+ ions (black) move close to an oxygen layer (from ref. [7]).
1143
thereby permitting efficient operation. The fabrication of low-loss waveguides is a prerequisite for essentially all integrated optical devices [l-5]. An index change of 1% is already sufficient for guiding. Normally polarized light is used and the E-vector of the light interacts with electronic orbitals within the crystal, which shows strong optical anisotropy. Externally applied signals (fig. 1) change atomic positions and modify polarizabilities, thus changing the index. Strong intensities of light may perturb some orbitals so much that a nonlinear response is observed. This is the basis for frequency doubling or parametric amplification.
3. Utilization of the damage generated by ion implantation The detailed crystalline order is responsible for the usefulness of electro-optical crystals. The collision events of ion implantation generate disorder. Fig. 3 shows an illustrative example. Crystalline quartz (SiO,) is damaged by implantation; the density and the index decrease. In contrast, SiO, glass is somewhat compacted by implantation and accordingly the index is increased somewhat [lo]. This increase is exceptional, whereas the index decrease due to disorder generation is very common for crystalline materials. It can be used to define barriers for light. Townsend and coworkers have studied this type of waveguide formation and communicated it in several reviews [11,12]. It is important to recall that low-mass (He) ion implantation generates the maximum nuclear recoil damage near the end of the ion’s range [13], while the near-surface regime remains relatively undamaged. Therefore MeV He implantation can be used to generate a buried layer of reduced optical index within essentially all optical crystals. By changing the ion’s energy, the layer depth is adjustable and multiple barriers or barriers separating two guiding regions can be fabricated [12]. This has considerable relevance for applications, as many electro-optical crystals do not lend themselves to diffusion or ion exchange techniques [1,5] for index modification. One example is KNbO,, which is useful for second-harmonic generation. Under certain conditions it doubles the frequency of light from 860 nm (AlGaAs laser diode) to 430 nm. This is desirable for higher-density optical data storage. For efficient operation, strong light intensities guided along specific crystallographic directions are necessary. Therefore several groups recently studied the properties of He-implanted waveguides in KNbO, [14-161. It turns out that KNbO, is very sensitive to radiation damage, and a dose of 5 X lOI He/cm2 at 2 MeV is already sufficient to fabricate a planar guide with losses of less than 4 dB/cm [15]. LiNbO, guides generally are implanted IX. INSULATORS/CERAMICS/POLYMERS
Ch.
1144
0 Ar’/50.
a 8’
Buchul
/ Electra-opiical
150keVJ
1
[70keV!
1 N’ llOQkeVl + Ne: Ar: Kr ’
Fig. 3. Change of index of SiOz as a function of ion implantation dose. here normalized to the deposited nuclear recoil energy. Crystalline quartz is disordered and its index is lowered; quartz glass is compacted and the index is raised (from ref.
[101).
with lOI to 5 x lOI He/cm*, and the best guide published showed losses of less than 0.1 dB/cm [ll].
4. Beyond the implantation damage: annealing and solid phase epitaxy It is a well established technology for waveguide fabrication to diffuse Ti into LiNbO, at high temperatures [17,5]. Its microscopic background is the introduction of Tt“+ into TiO, octahedral positions within the LiNbO, crystal in order to enhance the optical index.
1400
ctytuls
Motivated by the desire to raise the Ti dopant concentration above the limits imposed by diffusion kinetics, the Oak Ridge group has developed and perfected the concept of direct Ti implantation into LiNbO, [l&19]. Details of the technological process and the optical characterizations of the fabricated devices are published [20]. The typical dose of 2.5 X 10” Ti/cm’ provides a huge concentration of Ti in the as-implanted sample, but the LiNbO, crystal is entirely amorphized over the implanted depth. An additional annealing step at typically 1000 ’ C in an oxygen atmosphere causes the underlying undamaged crystal to regrow towards the surface. The details of this process are complex, since they involve diffusion of implanted Ti, diffusion of oxygen from the annealing atmosphere and Li diffusion out of the bulk. The process has been identified as solid phase epitaxy (SPE) and is most clearly demonstrated at a low-dose implant, see fig. 4 [21]. The SPE is performed at temperatures below the ferroelectric Curie temperature, and it is amazing that the Ti-doped crystal regrows into a single ferroelectric domain without additional poling. Lattice location studies show the substitutionality of the Ti ions [22,23]. The maximum Ti concentration for good guides in LiNbO, is 18 mole%, independent of implantation target temperature (620 to 77 K) [24]. It remains an open issue how the crystal accommodates the charge inbalance of a four-valent impurity (Ti4+) into a substitutional site. Two Ti4+@ NbSt substitutions can be compensated by one uncharged vacancy in the oxygen sublattice, or by two extra Li+ ions or by two protons from the water vapor of the annealing atmosphere. Presently we are investigating the co-implantation of charge-compensating cations, especially Mg2+ for Ti4+. Implanted Mg and Ti to-
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DEPTH (A) Fig. 4. RBS spectra
of SPE regrowth of implanted LiNbO, at 400 o C. The arrows represent the approximate positions of the interface at various annealing times. The Ti dose was lOI Ti/cm2. which is only a small disturbance of the stoichiometry (from ref. [211X
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Ch. Buchul / Electra-optical ctytals
gether with oxygen, introduced during annealing offers a good chance of forming stable (LiNb), _,(MgTi),O,, which should result in a waveguide with reduced susceptibility to the so-called “optical damage”. The virtue of doping the LiNbO, melt with up to 5% MgO has been demonstrated [25-271. Actually the He-implanted guides, as described in the preceding section. seem to be much less susceptible to optical damage than the Tidoped (diffused) guides. A stabilization of the Fe’+ impurity by the Ti4+ dopant is discussed as a possible reason. and annealing An extensive review of implantation of many crystalline oxides has been published recently
IW 5.Lasers and amplifiers Semiconductor diode lasers designed to match waveguide geometries are standard technology. It is known that the electro-optical crystals constitute good laser ion hosts, too. For laser action in these materials, one has to mtroduce 4f or 3d transition elements into the crystals and optically pump electrons to energy levels permitting stimulated emission. Two examples for oxide solid state lasers are the very popular Nd: YAG laser and the relatively new Ti : sapphire laser. In the case of the rare-earth doped lasers, a deep-lying 4f electron is laser active. Its orbitals are somewhat shielded from the surrounding crystalline lattice, and the corresponding energy levels remain undisturbed. In contrast, the 3d laser ions’ electrons populate 3d orbitals which are spatially further extended and are affected and broadened by phonons, forming “vibronic energy bands”. Photon emission is stimulated over a wider frequency range and therefore the Ti:Al,O, laser is tunable from 0.6 to 1.06 l.trn wavelength [29]. Note that the substitutional Ti’+ ion in Al,O, retains one d electron, which is responsible for color (absorption) and laser action. In contrast, a Ti4+ ion has given all outer electrons into bonds. It is used for index enhancement (TiO, octahedron) and shows no color. This remark emphasizes the necessity to control the charge state of implanted ions, which may be adjusted by an oxidizing or reducing atmosphere during post-implantation heat treatment, e.g., refs. [28,30-321. Experiments with coherent emission from rare-earth ions in electro-optical crystals have been started 25 years ago [33-351. An optical waveguide in doped Nd: LiNbO, was considered by ref. [35]. Although the measured Nd optical gain was good, the LiNbO, crystal suffered damage so rapidly that the experiments were limited to a few minutes. The subject was revived in 1985 with much greater success, because MgO-doped LiNbO, crystals proved to be less susceptible to this optical damage (photorefractive effect). The Nd : MgO : LiNbO, lasers permitted continuous operation, in-
tracavity Q-switching and frequency doubling, thereby taking full advantage of the electro-optical potential of LiNbO, [27,36,37]. Recently results from the first Nd: MgO: LiNbO, waveguide laser have been published. A melt-doped substrate was the starting material and several singlemode optical waveguides were fabricated on it by standard ion exchange techniques [38]. The Nd spectroscopic properties were practically unchanged by the waveguide environment. The optical pump operated at 0.814 pm and the laser oscillations started above a pump power threshold of 1.5 mW. The output power efficiency was 14%. The laser operated at a wavelength of 1.08 pm. The laser cavity consisted of a single waveguide of 8 urn width and 12 mm length with polished and coated end mirrors. The experiment has to be considered as a “proof of concept”. The potential of the approach lies in the future integration of a laser element together with other functions on one chip. or in the intracavity use of the electro-optical properties of the host (see above). The Sussex group has taken their well proven approach to tackle the problem of constructing a waveguide on a laser crystal also. They started from a Nd-doped YAG crystal and used 7 x lOi He/cm’ at up to 2.8 MeV energy to construct a planar waveguide of 6 pm depth and 10 mm length on one surface of the crystal. The unannealed guide showed losses of 4 dB/cm. Mirrors were attached to both ends of the guide. The Nd fluorescence showed no significant change, if compared to the bulk measurements. The optical pump operated at 0.59 urn. The laser threshold was reached at approximately 50 mW pump power inside the cavity, and the slope efficiency was around 1.7%. With that experiment the first laser action inside an ion-implanted waveguide in a dielectric crystal has been demonstrated also. Lower-loss stripe waveguides certainly will improve its performance [39]. These authors also studied the index profiles of He-implanted LiNbO, melt-doped with Nd, Cr or MgO and showed that the necessary low-level doping has very little effect on the efficiency of waveguide production by ion implantation [40]. We have been working along another road. As in semiconductor technology, we start from undoped substrates and investigate ion implantation doping of LiNbO, with rare-earth ions. The ultimate objective is a local doping for laser or amplifier action by selectively implanting and/or diffusing rare-earth ions into those parts of future integrated optical circuits, where it is desired, but leaving the main part of the substrate undoped to provide space for additional functions on the same wafer. The ion implantation, diffusion and annealing characteristics of Nd and Er in LiNbO, have been investigated [41]. After implantation and annealing at 1300 K 0.5 mole% of Er3+ could be introduced IX. INSULATORS/CERAMICS/POLYMERS
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Ch. Buchaf / Electra-optical
into substitutional lattice sites of LiNbO,. This is the same dopant concentration as used in bulk lasers. Thereafter the first Er-implanted single mode channel guides in LiNbO, have been fabricated and the Er fluorescence spectra measured [42]. They were compared to spectra excited in bulk Er: MgO: LiNbO, crystals and, as hoped, showed only very small changes of their characteristics. Therefore Er-doped waveguides are promising candidates for optical amplifiers and lasers in the 1.5 to 1.6 pm wavelength range.
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[I41 P51 U61 1171
6. Summary Light-ion (He) implantation with MeV energy into electro-optical crystals performs very well. The optical index is modified by irradiation damage in a controllable way and index barriers are easily constructed. Many waveguide geometries have been studied in numerous hosts, including the first planar waveguide laser in a Nd: YAG crystal. Ion implantation of transition elements or rare-earth ions generally needs additional processing, since a well defined lattice location requires annealing steps or even epitaxial regrowth. Implantation doping LiNbO, with laser active ions has been demonstrated and opens perspectives for more complex optical integrated circuits.
WI u91 WI 1211 P21 1231 ~41 ~51 WI ~71
Acknowledgement WI
Cooperation and discussions with B.R. Appleton, C.W. White, P.R. Ashley, P. Giinter, W. Sohler, T. Bremer and all the other Jiilich and Oak Ridge colleagues are gratefully acknowledged.
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