1. Introduction

The development of compact, versatile and multifunctional systems capable of controlling light by ordering structures in 1D, 2D or 3D is currently one of the most challenging goals in the field of Photonics [1–4]. Indeed, light control at reduced dimensions (micro and/or nanometer scale) has shown a large variety of practical applications such as chemical sensing on patterned structures [5], surface plasmon enhanced light emitters [6], and other areas involving photonic technology [7–10].

In this context, among the different approaches adopted to manipulate and control light in patterned structures we have recently proposed the use of alternate ferroelectric domain patterns as micrometric templates on which high refractive index nanoparticles, optically activated with trivalent rare earth ions (RE³⁺), are assembled [11]. On one hand, the use of ferroelectric materials provides functional substrates capable to generate electromagnetic radiation through nonlinear processes, as well as laser action, when those systems are doped with optically active ions. On the other hand, the production of organized micro- and submicro- objects involving optically active RE³⁺ ions has been rarely attempted [12]. The richness of their optical spectra, high quantum efficiency and good thermal stability make the development of RE³⁺ based microstructure materials particularly attractive in the development of advanced functional materials for photonic applications.

Lithium niobate, LiNbO₃, is one of the most widely studied ferroelectric materials in photonic technology due to its excellent electro-optic and nonlinear properties, among others. When doped with optically active rare earth ions, laser action and self-frequency doubling have been demonstrated in bulk and waveguide configurations [13,14]. It has 3m point group symmetry with two possible domain orientations with the spontaneous polarization (Ps) pointing along ± z axis. The direction of Ps in these domains can be switched by the application of an external field. Relative to optical devices, lens shaped ferroelectric domains [15], periodically poled structures [16], or two-dimensional (2D) periodic or a-periodic domain arrangements in LiNbO₃ have been successfully produced to achieve new and exciting properties in the field of non-linear optics [17,8].

In this work, two dimensional ferroelectric domain patterns in Nd³⁺ doped LiNbO₃ are prepared and used to fabricate artificial templates that act as micrometer cavities in which it is possible to incorporate optically active nanoparticles. The ferroelectric patterning is achieved by direct electron beam writing (EBW) and the micro-cavities are obtained by means of controlled selective chemical etching, which produce an engraving on the inverted domain regions. Specifically, we show the possibility of preparing two-dimensional heterostructures of micro-composites by embedding high refractive-index Er³⁺ doped CaTiO₃ nanoparticles into a Nd³⁺ doped LiNbO₃ substrate pre-patterned with an alternate ferroelectric domain structure. This procedure enables, not only to control the spatial location of the emitters at the micrometer scale, but also the formation of a new luminescent compound around the micro-cavities which results from the chemical reaction between the starting CaTiO₃ and LiNbO₃ oxides at high temperature. New geometries of micrometric luminescent compounds in two dimensional arrays are achieved.

2. Fabrication process and experimental details

A 1 mm thick plate from a single domain LiNbO₃ crystal doped with MgO (5%) and Nd³⁺ (0.1 wt %) was cut and polished with its main faces oriented perpendicular to the ferroelectric c-axis. To fabricate the polarization reversed ferroelectric micro-structures EBW was used without any masking process. Prior to the electron irradiation, a 100 nm thin film of Al was evaporated onto the c⁺ face of the sample, which acted as a ground electrode during the electron beam bombardment. The sample was irradiated on the c^- face of the crystal by means of a Philips XL30 Schottky field emission gun scanning electron microscope (SEM) driven by an Elphy Raith nanolithography software. The main irradiation parameters were 10 keV of incident electron energy and 1000 μC/cm² of electronic dose. Additional information on the procedure is reported elsewhere [18].

Once the ferroelectric pattern was created, selective chemical etching was employed to produce artificial templates in which the optically active nanoparticles could be incorporated. HF:HNO₃ (1:2 in volume) chemical solution at room temperature was used, the etching rate was about 0.5μm/h. Upon these experimental conditions an arrangement of well defined etched-microcavities (micro-voids) on the c + face of the LiNbO₃ crystal was obtained. The micro-cavities showed a high flatness with a surface area corresponding to the originally inverted domain regions. 2D patterns with a spatial extension of 2x2 mm² were achieved. Figure 1 (a) and (b) show a scheme of the fabrication process, as well as the SEM image of a single hexagonal cavity obtained after the selective chemical etching.

 

Fig. 1 (a) Schematic picture of the fabrication process to obtain the micro-void structure in this work; (b) SEM image of a single hexagonal micro-cavity obtained after selective chemical etching; (c) SEM image of a Er³⁺:CaTiO₃ filled micro-cavity after annealing at 1000°C during 1h; (d) Enlarge view of the same image showed in (c) to illustrate the absence of additional compounds in the proximities of the walls; (e) Detailed view of the ≈1µm thick shell of CaNb₂O₆ compound formed around the filled micro-cavity after annealing at 1000 °C during 2h; (f) Optical image of a 2D filled micro-void pattern.

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To optically activate the micro-voids, Er³⁺ doped CaTiO₃ nanoparticles were prepared by sol-gel method by using stoichiometric amounts of [Ca(NO₃)₂·4H₂O], [Ti(OC₄H₉)₄] [Er₆O₁₁] as reagents. The average size of the nanocrystals was around 50 nm. More details about the preparation procedure can be found elsewhere [19].

The nanoparticles were deposited into the voids by coating the LiNbO₃ substrate with CaTiO₃:Er³⁺ nanoparticles and sweeping them across the sample using a scraper. The Er³⁺ doped CaTiO₃ nanocrystals were consolidated into the voids by means of a thermal treatment at 1000°C. Figure 1(c) shows the SEM image of a filled micro-cavity after annealing during 1 hour. A detailed view of two filled micro-cavities comparing the results after thermal annealing during one and two hours is shown in Fig. 1(d) and 1(e). In both cases, the fluorescent nanoparticles are uniformly consolidated into the cavities and preserve their original optical and structural properties. However, while at shorter annealing times the boundary Er³⁺:CaTiO₃/Nd³⁺:LiNbO₃ is clean and does not show any secondary phase (see Fig. 1 (d)), for annealing times close to two hours, a ring-shaped shell surrounding the micro-cavity is clearly observed (Fig. 1 (e)). This “ring” spreads over 1 µm and, as shown next, it is obtained from the reaction of the CaTiO₃ nanoparticles and the LiNbO₃ substrate, which gives rise to a new compound (CaNb₂O₆) with innovative morphology. Finally, Fig. 1(f) shows the optical image of a portion of the 2D microstructure pattern formed by hexagonal shape motif distributed in a square lattice obtained after the whole process.

Micro-Raman and micro-photoluminescence experiments were performed at room temperature in a laser scanning confocal microscope. An Ar⁺ laser at 488 nm or a laser diode tuned at 808 nm were used as excitation sources. The laser beam was focused onto the sample by a microscope objective. The photoluminescence was collected in backscattering geometry with the same objective (100x and 50x magnification) and focused into a multi-mode fiber. The end of the fiber was directly connected to the spectrometer and the signal was detected with a Peltier-cooled charge coupled device camera. A beam splitter and notch filter were used to attenuate the pump laser line. The sample was placed on a 2-axis XY motorized stage with 0.1μm spatial resolution, thus precise positioning of the sample under the laser spot was achieved. For the 488 nm excitation wavelength, the laser was focused to a spot size less than 1μm.

3. Results and discussion

To determine the nature of the ring-shaped shell, X ray diffraction (XRD) experiments at glazing incidence and spatially resolved confocal micro-Raman spectroscopy were employed. XRD at glazing incidence is a useful technique to identify polycrystalline compounds onto a crystalline substrate since the number of the diffraction peaks in the spectrum associated with the substrate can be strongly reduced due to its monocrystalline character. As an inconvenient a weaker signal from the surface may lead to the absence of certain diffraction peaks in the spectrum, as well as to different relative intensities from those expected. Figure 2(a) shows the XRD spectrum obtained from a two dimensional pattern with a spatial extension of 2x2 mm². It is composed of several diffraction peaks most of them located at the same values than those expected for CaNb₂O₆. Indeed, they can be associated with that niobate compound excepting for those peaks at 46.83° and 46.91°, which correspond to CaTiO₃ nanocrystals. Thus, on the basis of the XRD spectra, the formation of CaNb₂O as new a compound can be inferred. To confirm the assignment of CaNb₂O₆ as the new polycrystalline material that surrounds the micro-cavities, spatially resolved confocal micro-Raman spectroscopy has been employed as a complementary tool. Figure 2(b) shows two unpolarized Raman spectra collected when the laser source is located right on the top of the shell (red line) and when the laser source is focused on the LiNbO₃ substrate, far away of the fabricated micro-composite (black line). The two spectra were obtained under similar experimental conditions and have been plotted together for the sake of comparison. Note that, even when confocal spectroscopy allows discriminating the optical response that arises from the focal plane, the thickness of the shell and the in depth spatial resolution of our confocal microscope prevent the total annihilation of the substrate contribution. Thus, in order to identify the energy position associated with the Raman modes of the shell, a comparison between both spectra is needed.

 

Fig. 2 (a) XRD spectrum of the system obtained at glazing incidence; (b) Raman spectra obtained in confocal geometry when the excitation laser is focused on the LiNbO₃ substrate (black line) and on the ribbon shaped shell (red line). The peaks corresponding to the CaNb₂O₆ shell have been labeled.

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As seen, up to seven Raman modes can be easily identified and associated with the ring-shaped compound, their energy values peaking at 196, 290, 383, 494, 541, 628 and 904 cm⁻¹. These energy positions are in excellent agreement with those reported for CaNb₂O₆ in previous works [20], which corroborates our assignment.

Additional information concerning the possible incorporation of RE³⁺ ions into the CaNb₂O₆ shell was obtained by means of scanning confocal micro-fluorescence experiments. Figure 3 shows the emission spectra obtained under excitation at 488 nm at the sideband of the ⁴G_9/2 and ²G_9/2 states of Nd³⁺ ions. The same results were obtained under excitation at 808 nm at the ⁴F_5/2 state. The spectra are obtained for two different spatial regions: under excitation on the CaNb₂O₆ shell and on the Nd³⁺:LiNbO₃ substrate far enough from the micro-composite.

 

Fig. 3 Confocal fluorescence spectra obtained under excitation at 488 nm. (a) (S)pectral region associated with the ⁴F_3/2→⁴I_9/2 transitions of Nd³⁺ ions and (b) spectral region associated with the ⁴F_3/2→⁴I_11/2 transitions of Nd³⁺. The emission spectra were collected from the CaNb₂O₆ shell region (red line) and from the Nd³⁺:LiNbO₃ substrate (black line). Similar results were obtained under laser diode excitation at 808 nm.

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The spectral region associated with the ⁴F_3/2→⁴I_9/2 electronic transitions of Nd³⁺ ions in LiNbO₃ is shown in black line on Fig. 3(a). The presence of extra peaks (marked with arrows on the figure), can be clearly observed when the excitation laser is focused onto the shell. Similarly, additional spectral features to that of Nd³⁺:LiNbO₃ can also be distinguished when the emission spectra associated with ⁴F_3/2→⁴I_11/2 transition is analyzed (Fig. 3 (b)). Accordingly, the obtained results show that Nd³⁺ ions incorporate into the shell, leading to a ribbon shaped Nd³⁺ doped CaNb₂O₆ composite. This result can be of interest since CaNb₂O₆ was the seventh host-crystal where laser action from RE³⁺ ions was demonstrated [21]. Moreover, laser action from Nd³⁺:CaNb₂O₆ in micrometer size (single crystal fiber) has also been reported [22].

Figure 4 shows three fluorescence spectra in the spectral region 500-950 nm, obtained when the emission is collected, i) inside the filled micro-cavity (Er³⁺:CaTiO₃), ii) at the shell (Nd³⁺:CaNb₂O₆) and iii) outside the micro-composite (Nd³⁺:LiNbO₃). In the first case, inside the filled micro-cavity, the obtained spectrum is composed by four main groups of emission lines centered at 520-550 nm, 660 nm and 850 nm which correspond to the radiative de-excitation ²H_11/2, ⁴S_3/2 → ⁴I_15/2, ⁴F_9/2 →⁴I_15/2 and ⁴S_3/2 →⁴I_13/2, respectively. We would like to remark that under excitation with laser diode at 808 nm the same spectral shape for Er³⁺ doped CaTiO₃ nanoparticles was obtained. In that case, the Er³⁺ emission was produced by means of two-photon mechanisms involving excited state absorption and energy transfer upconversion as previously reported [23]. In this case a quadratic power dependence of Er emission was obtained. The emission spectrum obtained outside the filled micro-structure, i.e. from the Nd³⁺:LiNbO₃ substrate consists mainly of two groups of lines associated with the Stark transitions from the ⁴F_3/2 metastable state (centered at 880 nm) and from the ²H_9/2, ⁴F_5/2 states (≈815 nm) to the ⁴I_9/2 ground state. Finally, the emission lines of Nd³⁺ ions in CaNb₂O₆ can be detected in the spectrum collecting the luminescence from the shell.

 

Fig. 4 Fluorescence spectra obtained when the emitted intensity was collected inside the filled micro-cavity (Er³⁺:CaTiO₃) (blue line), at the border (Nd³⁺:CaNb₂O₆) (red line) and outside the micro-composite (Nd³⁺:LiNbO₃) (green line). The excitation wavelength was 488 nm.

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A visual example of the multicolor character of the fabricated luminescent micro-composite can be found in Fig. 5 , where we show the spatial fluorescent maps obtained from a single hexagonal motif after selecting different spectral regions. The optical image of the filled hexagonal micro-cavity is displayed on top of the figure and the luminescent maps obtained for the green, red and near infrared spectral regions are plotted bellow. Each pixel in the fluorescence image corresponds to the integrated emission spectra shown below. The relative emitted intensities obtained by integrating the spectra are 67, 11.5 and 21.5 for the green, read and near infrared regions, respectively. In the excitation power range used in this work (up to 150KW/cm²) the emission intensities showed a linear behavior versus excitation power. As observed, there is a good spatial correlation between the optical and the fluorescence images.

 

Fig. 5 Green, red and near infrared fluorescent maps obtained for a single filled micro-cavity when plotting the emission area associated with ²H_11/2, ⁴S_3/2 →⁴I_15/2, ⁴F_9/2 →⁴I_15/2 and ⁴S_3/2→⁴I_13/2 optical transitions of Er³⁺ ions in CaTiO₃. The emitted spectra are also shown in the figure as well as the optical image of the single microstructure.

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Therefore, since the optical properties in both systems remain unaltered after the whole synthesis process the viability of preparing fluorescent heterogeneous micro-structures out of rare earth based optically active materials is demonstrated. Moreover, the results displayed in Fig. 4 clearly show the possibility of chromatic switching among the three different compounds at the micrometer scale, even when the excitation wavelength is fixed. Here, it is important to note that the good variety of excited energy levels of both, Nd³⁺ and Er³⁺ ions, along with their characteristic sharp lines transitions arising from their f electronic configuration_, allows switching the pump (excitation) wavelength in such a way that one can selectively excite the filled micro-voids, the optically active host matrix, or the Nd³⁺ doped active shell. Additionally, the excitation can also be performed on a wide spatial area which would allow the simultaneous excitation of the whole micro-composite.

The collected emission can be tuned to other spectral ranges in such a way that the luminescence arising from the substrate or even from the optically active ribbon shaped shell can be selected. Thus, the emitted light from our composite can be spatially and spectrally switched by exciting and/or collecting only the fluorescence arising from a single compound. Figure 6 shows three confocal micro-fluorescence maps obtained under excitation at 488 nm and collecting the luminescence produced from different compounds forming the hetero-structure, namely, Nd³⁺:LiNbO₃, Nd³⁺:CaNb₂O₆ and Er³⁺:CaTiO₃. As seen, the obtained fluorescence images are different. In particular, while the Nd³⁺ doped LiNbO₃ leads to the negative image (color reversed) to that obtained from the filled micro-cavities (Er³⁺:CaTiO₃), the confocal fluorescence image obtained from Nd³⁺:CaNb₂O₆ shows a well defined ring shaped geometry. That is, the annular shape of the formed calcium niobate compound is reproduced in the luminescence map. The results reveal the possibility to reach new geometries that can be useful in the development of new micro-composite rare earth based photonic devices, such as multi-color emission displays or pixelated color structures.

 

Fig. 6 Spatial luminescent maps obtained from a single micro-composite when selecting: (left) the green fluorescence from Er³⁺ ions in the CaTiO₃ nanoparticles in the filled voids; (center) the emission from Nd³⁺ in the CaNb₂O₆ ring-shaped compound ₍peaking at 878.8 nm); (right) the emission from Nd³⁺ ions in the LiNbO₃ substrate.

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4. Conclusion

We have shown the feasibility of fabricating a new type of fluorescent arrays that allow the combination of different RE³⁺ ions with the optical properties of patterned ferroelectric substrates. By embedding high refractive index Er³⁺ doped CaTiO₃ nanoparticles into Nd³⁺ doped LiNbO₃ substrates pre-patterned with a two dimensional arrays of micro-voids we can manipulate the spatial location of the RE³⁺ emitters in the micrometer range, hence leading to a simultaneous spatial and spectral control of the electromagnetic radiation. The heterostructures have been characterized by scanning confocal micro-fluorescence spectroscopy showing the different multicolor emission arrangements that can be obtained. Additionally, the possibility of chromatic switching among the three different compounds at the micrometer scale has been also demonstrated. The incorporation of high refractive index optically active nanoparticles into the alternate domain structures could lead to a more complex behavior of the system allowing exploring the possibilities for new functionalities in the field of Photonics. For instance, the spatial control of high refractive index nanoparticles may lead to confinement effects and/or optical enhancement in both, the linear and non linear optical response of the global heterostructure, which could be obtained with innovative morphologies (shapes and sizes).