Analysis and optimization of propagation losses in LiNbO3 optical waveguides produced by swift heavy-ion irradiation
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- Jubera, M., Villarroel, J., García-Cabañes, A. et al. Appl. Phys. B (2012) 107: 157. doi:10.1007/s00340-012-4897-9
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The propagation losses (PL) of lithium niobate optical planar waveguides fabricated by swift heavy-ion irradiation (SHI), an alternative to conventional ion implantation, have been investigated and optimized. For waveguide fabrication, congruently melting LiNbO3 substrates were irradiated with F ions at 20 MeV or 30 MeV and fluences in the range 1013–1014 cm−2. The influence of the temperature and time of post-irradiation annealing treatments has been systematically studied. Optimum propagation losses lower than 0.5 dB/cm have been obtained for both TE and TM modes, after a two-stage annealing treatment at 350 and 375∘C. Possible loss mechanisms are discussed.
The fabricated waveguides present several relevant advantages over those fabricated by conventional light ion implantation [4–6]. For example, they have step-like and high-jump index profiles (∼0.2 and ∼0.1 for ordinary and extraordinary refractive indexes, respectively, see Fig. 1b) and thick easily programmable amorphous layers which allow for supporting highly confined propagation modes. Moreover, low irradiation fluence is sufficient (≈1014 cm−2) to produce the waveguides, so that the fabrication time may be reduced up to two orders of magnitude in comparison with the implantation case. Finally, good nonlinear optical and photorefractive properties have been recently reported [3, 7, 8]. However, the preliminary reported values for PL yielded values ranging between 1–10 dB/cm [1, 8], which are still high for many applications, leaving much room for improvement and optimization.
The purpose of this work is to systematically address the topic of propagation losses (PL) for the SHI waveguides and look for an optimized response. Suitable post-irradiation annealing treatments were implemented in order to reduce absorption and scattering centers. Moreover, those treatments may improve the quality of the interface between the waveguiding and amorphous layers. However, one has to assure that the amorphous barrier is kept thick enough to avoid a substantial enhancement of the tunnelling losses. The relevance of this latter loss mechanism has been evaluated with the help of a beam propagation method (BPM). As a result of the study, we have demonstrated the feasibility of markedly reduced optical losses reaching values under 0.5 dB/cm for both TE and TM polarizations. This confirms that the novel SHI waveguides are, indeed, very promising candidates for a variety of photonic devices.
2 Experimental techniques
Fabrication parameters and estimated barrier thickness  of the waveguides
Ion, energy, incidence angle
Barrier thickness h (μm)
Type I (guide A)
F, 30 MeV, 70∘
Type II (guide B, C, D)
F, 30 MeV, 70∘
Type III (guide E)
F, 20 MeV, 0∘
The waveguides before and after annealing have been characterized by measuring the refractive index TE (ordinary refractive index no) and TM (extraordinary refractive index ne) profiles using the prism-coupling m-line method with λ=632.8 nm.
The PL were determined through the decay of the light intensity guided mode at λ=632.8 nm, measured via the light scattered along the beam path  recorded by a CCD camera.
3 Waveguide losses: effect of annealing treatments
3.1 Role of temperature for isochronal annealing treatments
3.2 Role of time for isothermal annealing treatments
The above results show that the evolution of the PL with annealing temperature for the novel SHI waveguides presents two different regions. In the initial stage, increasing temperature up to around 300 or 350∘C (depending on the waveguide type) causes a clear reduction in waveguide losses. This stage is very likely related to the removal of radiation induced coloring and defects centers, in accordance with results reported in the literature for waveguides prepared by different ion implantation processes [4, 12–17]. Another source of PL is the scattering at the interface separating the guiding and barrier layers. For our waveguides, it is expected that the interface has some roughness due to the statistical fluctuations caused by the overlapping of the amorphous tracks associated to individual ion trajectories . Since the statistical roughness should decrease with increasing irradiation fluence, one would predict that losses decrease with fluence, in accordance with the observed data. In a second stage at higher temperatures, there is a competing effect due to the narrowing of the optical barrier. The rapid increase of PL observed in Figs. 2a and 2b at higher temperatures suggests an enhanced tunnelling effect. In line with this idea, this fast enhancement of losses is not observed in the isothermal experiment at 300∘C (Fig. 3), where the barrier thickness keeps essentially constant. On the other hand, the isothermal experiment at 350∘C (Fig. 4). shows a similar saturating behaviour of PL for the fundamental mode; whereas the first mode, more sensitive to the tunnelling through the barrier, exhibits a strong increase in PL for t>2.30 h. Therefore, it may be concluded that in previous experiments, the tunnelling mechanism is responsible for the saturation behaviour appearing in the explored route to reduce losses.
A further confirmation of the tunnelling mechanism being responsible for the rise in losses in the second stage (T>300–350∘C), is obtained from calculations using the beam propagation method (BPM) to describe light propagation through the waveguiding structure. The simulations have been performed using the 2-dimension Crank–Nicolson finite differences scheme for beam propagation method . This method considers the waveguide as a set of very thin strips perpendicular to the propagations direction. Prior to the simulation, the guided modes of the waveguide are computed and the selected one is introduced into the waveguide. This initial condition is used to determine the electric field for each cell of the following strip. Calculations show the light intensity of the guided wave decreasing along the propagation direction due to the tunnelling losses. This BPM method has been already used to discuss photorefractive phenomena in LiNbO3 waveguides .
In summary, the analysis performed so far suggests that further optimization of PL might be obtained by an additional rise of the annealing temperature, but avoiding the larger tunnelling losses associated with a reduced barrier thicknesses. This is what has been attempted in the next section.
5 Optimized waveguide
The previous discussion indicates that in the strategy to reduce PL, a key point is to fabricate waveguides with thicker optical barriers. To that end, we have fabricated a waveguide with fluorine irradiation at 20 MeV, a higher fluence of 4×1014 cm−2, and normal ion incidence (guide E, type III). In this case, the optical amorphous barrier is about 3 μm .
Values of propagation losses (PL) and waveguide thickness d, measured in the waveguide of type III after the annealing treatments, which are also indicated in the table. no and ne indicate ordinary and extraordinary polarizations, respectively
It is worthwhile remarking that the PL optimization method proposed in this work relies on two properties of swift-heavy ion irradiated waveguides: (i) the possibility to easily obtain thick amorphous barriers. Note that this is complicated for standard light ion implantation techniques that require multi-energy ion-implantation [23, 24] to enlarge the amorphous barrier and even in that case typical amorphous barriers are about 1 micrometer , (ii) the possibility to modify the barrier thickness by re-crystallization. This is accompanied by a simultaneous reduction of propagation losses for appropriate treatments.
6 Summary and conclusions
A systematic study of the reduction of PL for SHI waveguides through suitable post-irradiation annealing treatments has been carried out. As a result, we have demonstrated the feasibility of markedly reducing PL to values under 0.5 dB/cm for both polarizations.
We can then conclude that waveguide fabrication by swift-heavy ion irradiation has relevant advantages regarding to the conventional implantation method; not only when step-like high-jump index profiles are concerned but also concerning the control and optimization of propagation losses.
This work has been supported by Ministerio de Ciencia e Innovación (MICINN) under grant MAT2008-06794-C03. M. Jubera acknowledges his FPI fellowship from MICINN.