Periodically poled lithium niobate structures grown by the off-center Czochralski technique for backward and forward second harmonic generation
Introduction
Today efforts made in optoelectronics and photonics are mainly focused to develop faster, more efficient ways of acquiring, storing and transmitting information and, consequently, there is a drive to obtain better nonlinear materials. When second-order nonlinear effects are considered, however, efficient nonlinear interactions require that the phase matching conditions are fulfilled between two interacting waves. To this purpose, periodic structures have been widely used to overcome the dispersion of the refractive index. In particular, it has been demonstrated that an efficient second harmonic generation (SHG) can be achieved in periodically poled lithium niobate (PPLN) crystals via the quasi-phase matching (QPM) method [1]. Since QPM was first introduced, the attention was mostly concentrated on forward SHG, where a pump beam propagating in a second-order nonlinear medium can generate a second-harmonic (SH) beam propagating along the same direction. On the other hand, it was shown [2], [3] that a backward (BW) SHG is possible. In this case a pump beam can generate a SH beam propagating along the opposite direction. There are certain potential advantages of BW SHG [4]. Among the most promising applications, we mention the remote sensing via SHG or optical parametric oscillation since the backward SH or parametric waves will follow the optical path and propagate back to the pump beam [5]. However, it was shown that for the first-order phase matching at a fundamental wavelength of a period close to is needed [6]. This fact can represent a strong limitation for the experimental techniques used to prepare PPLN structures. As a matter of fact, periodically poled structures are usually prepared by the electric-field poling technique by applying an electric field either during the material growth (about 0.4 V/cm) or after that, even at room temperature [7], [8]. In this case, when small periods are required complications can arise from such very narrow electrode deposition. However, it was demonstrated that with the intentional displacement of the growth axis from the symmetry axis of the temperature field, periodic fluctuations of temperature occur leading to the periodic domains reversal [9]. The result is a PPLN structure ranging through the whole crystal, typically few centimeters long. This technique can be considered a suitable alternative to prepare tailored periodic structures. Up to our knowledge, the off-center Czochralski technique has not been tested to grow PPLN structures with period as short as required by the QPM condition for the BW SHG. Unfortunately during the PPLN growth imperfections in the structure periodicity can occur, such as random errors or periodic modulation in the domain width [10]. Depending on their magnitude, these imperfections lead to a reduction of the SHG efficiency and, therefore, should be deeply investigated. The control in the structure periodicity is one of the key points to optimize the device performances, especially when small periods are concerned. The aim of this work is to predict the SHG efficiency resulting by the propagation of the fundamental beam in “real”, and therefore imperfect, PPLN samples in function of the process parameters. We intend to investigate the conditions needed to obtain an efficient SHG both in FW and BW configurations. For this reason, we adopt a numerical tool (a recently proposed nonlinear bidirectional beam propagation method, NL Bi BPM) to predict the amount of energy conversion in ideal PPLN structures. Subsequently we exploit the method of Fejer et al. to estimate the effect of border domain errors. To this goal we perform a map of the periodic domain structures using a standard profilometer after a chemical etching process. This analysis was carried out on PPLN samples with periods ranging between and .
Section snippets
Experimental
Periodically poled single crystals were grown by the off-center Czochralski technique [9], [11] starting from a melt containing 0.5–0.7 mol% of . The growth conditions were chosen to tailor the PPLN periods, as reported in Table 1, ranging between and , respectively. In particular, the pulling rate was varied from 2 to 3 mm/h, the rotation rate from 4 to 16.7 rpm while the off-center was ranged between 7 and 9 mm. The crystals containing the PPLN structure were
Results
In Fig. 1 we report the scan of the positive and negative domain widths across the Z-cut slices for crystals with off-center equal to 7 mm. Similar scans were performed on the other PPLN structures grown under the experimental conditions summarized in Table 1. The dips refer to the negative domains which, as previously mentioned, are etched faster than the positive ones. By processing the data given by the profilometer scan, it is possible to investigate the domain width distribution along the
Second harmonic generation
In this section we will first treat the SHG efficiency expected by a perfect PPLN structure with period equal to the average value given by the profilometer scans. The effect of the border domain errors will consider a further correction.
Conclusions
In this work we presented the characterization of PPLN structures grown by the off-center Czochralski technique with period ranging from 2 to . The study of the domain distribution along the crystal, performed by a profilometer scanning of the etched structures followed by a suitable data processing, allows to check the periodicity of the structure. We observed that at higher rotational rates the PPLN domain distribution is sharper. When higher off-center values are exploited, a period as
Acknowledgements
This work has been partially supported by FIRB RBNE01KZ94F and FIRB RBAU01XEEEM.
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