1. Introduction

Glasses and other dielectrics containing metal nanoparticles are of interest due to their unique linear and nonlinear optical properties. These properties are dominated by the strong surface plasmon resonances (SPRs) of the metal nanoparticles. The spectral position and shape of these SPRs can be designed within a wide spectral range throughout the visible and near-infrared spectrum by choice of the metal and the dielectric matrix [1,2] or by manipulation of the size [3], shape [4] and spatial distribution [5] of the metal clusters. Therefore, these compound materials are promising candidates for many applications in optoelectronics [6–12].

Over the past few years [9, 13–18], the possibility of either global or selective modification of glass with embedded metallic nanoparticles was demonstrated. This technique is based on the application of an intense direct-current (dc) electric field at moderately elevated temperatures resulting in the dissolution of metallic nanoparticles in the glass matrix. In particular [9], studied the feasibility of this technique for fine and selective structuring of metal-doped nanocomposite glasses.

In this paper, we introduce the importance of the process parameters, the applied voltage and temperature, by introducing the current-time dynamics of the structuring process. We will demonstrate how a correct combination of these parameters leads to the large-area fabrication of embedded diffractive optical element in glass containing silver nanoparticles.

2. Experimental methods

The silver-doped nanocomposite glass samples were prepared from a 1 mm thick soda-lime float glass (comprising in wt.-%: 72.5 SiO₂, 14.4 Na₂O, 6.1 CaO, 0.7 K₂O, 4.0 MgO, 1.5 Al₂O₃, 0.1 Fe₂O₃, 0.1 MnO, 0.4 SO₃) by Ag⁺-Na⁺ ion exchange and subsequent annealing at 400°C in H₂ reduction atmosphere [19]. This resulted in the formation of spherical silver nanoparticles of 30-40 nm mean diameter in a thin surface layer of ~10 μm on both sides of the glass substrate. The nanoparticle-containing layers were formed some 20-30 nm beneath the surface of the glass. Single-sided samples were used in our experiments. These were made by removing ~10 μm thick layer from one side of the samples by etching in 12% HF acid.

A scanning electron microscope (SEM) image of the sample is shown in Fig. 1(a) . It can be seen that the volume-filling factor (f = V_metal / V_total) of the nanoparticles near the surface of the sample is very high. This value decreases exponentially with depth and drops to zero in a few microns as can be seen from Fig. 1(b). In order to visualize the depth profile of the silver particle-containing layer, one of the samples was cut and a thin slice was prepared. For this, we embedded the sample in an epoxy resin (Specifix-20, Struers Limited) to prevent chipping of the glass and to make it physically manageable for grinding, polishing etc. The resin cures at room temperature. The section has been polished on both sides and was ~30 μm thick.

 

Fig. 1 (a) SEM image of the glass with embedded spherical silver nanoparticles of 30-40 nm mean diameter. The nanoparticle-containing layer is 20-30 nm beneath the surface of the glass. (b) A thin slice showing the cross-section of the nanoparticle-containing layer. The volume-filling factor of the layer reduces to zero within a few microns and has an exponential profile with the maximum just beneath the surface of the sample. The red arrow indicates the surface.

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Three pieces of such silver-doped nanocomposite glass samples were used for our experiments. Each sample was sandwiched between two metallic electrodes (rectangular area of 100 mm²), with the anode facing the nanoparticle-containing layer. The anode was a metallic mesh with a lattice constant of 2 μm. The cathode was a flat piece of stainless steel and in order to improve the contact a piece of graphite foil was inserted between the sample and the negative electrode. This has also the advantage that the substances coming out of the glass do not pollute the electrodes. Additionally, graphite forms a non-blocking cathode since it accepts alkali ions.

We performed the experiments by placing the samples inside an oven and connecting the electrodes to a high-voltage power supply. The samples were treated under the following three conditions: (i) 0.6 kV at 200°C for 30 min, (ii) 0.4 kV at 200°C for 30 min, and (iii) 0.4 kV at 300°C for 30 min.

The characterizations of the sample were performed using a JASCO V-670 UV/VIS/NIR Spectrophotometer, KEYENCE Digital Microscope VHX-1000, JEOL JSM-7400F scanning electron microscope and a He-Ne laser.

3. Results and discussion

The absorbance spectrum of the original sample is shown in Fig. 2 (black line). The original sample shows a strong and broad SPR band corresponding to the embedded spherical silver nanoparticles with high volume fill factor.

 

Fig. 2 Measured extinction spectra of the original sample (black line), and samples after modification. Each modification was performed for 30 min.

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As can be seen from Fig. 2, the structuring experiments (described in the Experimental methods section) resulted in narrowing of the SPR band and its blue shift. This is well described by the Maxwell-Garnett effective medium theory [2,8,15,16], which predicts narrowing of the SPR band and its blue shift with the reduction in the filling factor of the inclusions [8,16]. Here, the reduction in filling factor is due to the dissolution of the nanoparticles in the glass matrix owing to the combined action of the applied electric filed and temperature [13,14]. Figure 2 also shows that the structuring experiments at (0.4 kV, 300°C) - green line and (0.6 kV, 200°C) – red line are approximately the same, in terms of their effects on the narrowing and blue shift of the SPR band compared with the original sample (black line).

Current as a function of time during the dissolution processes and its effect on the structuring are shown in Fig. 3 . As it can be seen from Fig. 3(a), for the sample modified at (0.4 kV, 300°C), the current starts at ~3500 μA and then reduces exponentially below 100 μA in 10 min and then falls to zero in 30 min. Integrating the current over time gives a total charge transfer of ~0.13 A⋅s⋅cm⁻². This resulted in the formation of the structures shown in Fig. 3(b). The structures are ~500 nm high and formed some 500 nm beneath the surface of the glass.

 

Fig. 3 (a) Current-time dynamic of the structured sample modified at (0.4 kV, 300°C). (b) SEM image of the cross-section of this sample showing the effect of structuring. Here, the white color is indicative of silver and black represents the dielectric matrix. (c) Current-time dynamics of the structured samples modified at (0.4 kV, 200°C) and (0.6 kV, 200°C). (d) SEM image of the cross-section of the sample modified at (0.6 kV, 200°C). Modification of the sample at (0.4 kV, 200°C) led to the formation of similar structures.

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The current-time dynamic in Fig. 3(a) is in stark contrast to that observed for the samples structured at (0.4 kV, 200°C) and (0.6 kV, 200°C) – Fig. 3(c); for instance, in the latter case (0.6 kV at 200°C), the current first rose to ~140 μA and then slowly decreased to below ~20 μA in 30 min. Such a process results in the formation of the structures shown in Fig. 3(d). Here the structures are just beneath the glass surface and the imprinted “scalloped” profile of the remaining “un-dissolved” nanoparticles is indicative of a slow process. This arises from the orthogonal electric field distribution. Modification of the sample at (0.4 kV, 200°C) led to the formation of similar structures. Integrating the current over time gives total charge transfers of ~0.08 A⋅s⋅cm⁻² and ~0.12 A⋅s⋅cm⁻² respectively.

From the graph presented in Fig. 3(a) and the imprinted “castellated” shape of the cross-section in Fig. 3(b), one can conclude that in this case (0.4 kV, 300°C) a fast process initiated the observed space-selective dissolution process. Reducing the temperature by 100°C results in a very different current-time dynamic behavior – Fig. 3(c).

The samples used for our experiments have a high filling factor of silver nanoparticles in the very near surface layer. Therefore, the potential barrier between two neighboring metal clusters is low enough to make the thermally activated tunneling process possible [13]. This triggers the process and leads to the observed rise of conductivity and large current in Fig. 3(a) - for the sample structured at (0.4 kV, 300°C). Further contribution to the process comes from the fact that at the applied temperature (300°C) more cations are available and contribute to the conduction mechanism. Therefore, the increased number cations in the glass, mainly Na⁺, K⁺, Ca²⁺, have a more pronounced contribution to the conduction mechanism (sodium is particularly known to be mobile at elevated temperatures) [20, 21]. The applied dc electric field also leads to an ionic current flow and depletion of alkali and alkaline ions under the anode (beneath the nanoparticle-containing layer). This results in a space-charge region with a strong electric field falling across the nanoparticle-containing layer. The process leads to an electronic current from the nanoparticles towards anode where there is a contact with the anode (the mesh electrode). This makes the nanoparticles unstable and results in their selective dissolution in the matrix [13–15].

At the reduced temperature (200°C) - structuring processes presented in Fig. 3(c) - the much lower current is due to the much-reduced mobility of the cations at this temperature. Here, the ionic nature of the observed current with no sign of a sudden rise due to electronic current is pronounced. This leads to a much slower dissolution process [15].

It is worth pointing out that, looking back at Fig. 2 one could see that the amount of dissolved silver nanoparticles is the same for the samples structured at (0.4 kV, 300°C) - green line, and (0.6 kV, 200°C) – red line. In both cases, the SPR bands are narrowed down and blue shifted approximately by the same amount as compared to the original sample (Fig. 2 - black line). This is consistent with the very similar total charge transfer values for each of these processes. However, the results of the structuring (Figs. 3(b) and 3(d)), in terms of their size depth profile and shape, are very different.

Figure 4 represents a diffraction pattern of the structured sample whose cross-section was shown in Fig. 3(d). This is a typical “clean” diffraction pattern showing the ability of this technique to fabricate diffractive optical elements in glass with embedded metallic nanoparticles. The zero, first and second diffraction orders can easily be recognized. Similar diffraction patterns were observed from the samples structured at (0.4 kV, 200°C) and (0.4 kV, 300°C). Importantly for their application to optical elements, because the fabricated structures are beneath a thin glass layer, they are robust and environmentally stable.

 

Fig. 4 Diffraction pattern (in transmission) of the sample structured at (0.6 kV, 200°C, 30 min) upon He-Ne laser illumination. Similar diffraction patterns were observed from the samples structured at (0.4 kV, 200°C) and (0.4 kV, 300°C).

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4. Conclusion

In summary, a diffractive optical element has been fabricated in a medium that has proven to be stable over centuries – glass containing metallic nanoparticles. The fabrication technique was based on the dc electric field-assisted dissolution of the metallic nanoparticles in glass.

The experiments presented here offer the results for the most optimized structuring conditions: performing the experiment at higher voltages or temperatures leads to the full dissolution of the nanoparticles. We have also presented a cross-section image of the samples after structuring showing the depth profile of the metallic nanoparticles with an unprecedented clarity. These images allowed optical analysis of the embedded layers and enabled our studies to uniquely show the shape of the nanoparticle-containing layer after application of the electric field.

Work is currently in progress to fabricate complex diffractive optical elements in silver-doped nanocomposite glasses. These will involve a combination of dc electric-field-assisted techniques with laser-assisted space-selective modification processes.