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A diffractive optical element is fabricated in soda-lime float glass using a simple and inexpensive process. The glass is sandwiched between a mesh anode (lattice constant 2 µm) and a flat metal cathode. Applying a direct current while at a moderately elevated temperature of 553 K induces thermal poling of the glass. The result is that the structured pattern of the electrode is imprinted on the glass as the electric field causes ion depleted regions where there is contact between the glass and electrode. The current-time dynamics of the structuring process along with X-ray element analysis and conductivity measurements are presented. Optical analyses of the resultant diffraction patterns of samples suggest that large-scale and complex patterns can be fabricated.
© 2015 Optical Society of America
1. Introduction
Diffractive optical elements (DOEs) are an essential component in the field of optics. Due to the fragile nature of most of these elements, they can be easily damaged calling for the need for a robust and inexpensive fabrication technique. Coatings can be used to protect the surface of DOEs, however these can also become damaged with time.
Thermal poling has been extensively investigated due to its ability to induce second-order nonlinearity in various types of glass as a reliable and reproducible technique [1–6]. Parallel plate thermal poling of glass involves sandwiching the glass between two electrodes and applying a dc electric field across the sample while at elevated temperatures. This forms subsurface layers that exhibit nonlinear optical properties (such as the generation of second harmonics) which are depleted of highly mobile positive ions. This is because during poling the applied electric field forces them to move from the near-anode space into the bulk of the material, towards the cathode [7–9]. These depleted layers are highly useful as they present a different refractive index than that of the original glass; thermal poling has been used, for this reason, in the fabrication of waveguides [10,11].
Thermal poling has also been used with composite glasses (here the technique was referred to as electric-field-assisted dissolution, EFAD) [12,13] and was shown to successfully produce nanostructures in silver-doped nanocomposite glass [14,15]. The structures produced during EFAD have been used in the production of efficient DOEs in glass embedded with silver nanoparticles [16], where a patterned anode is used to selectively dissolve the silver nanoparticles creating a periodic pattern (dictated by the relief of the anode pattern), which produces a diffraction pattern upon laser illumination.
A similar process involving glass containing silver ions showed that by using the difference in refractive index between the original glass and areas depleted of the positive silver ions it was possible to fabricate diffractive phase gratings [17,18]. However, by using an alkali-rich borosilicate glass (BO73) it was shown that the addition of silver ions was not necessarily needed for the production of this type of DOE. The near-surface depletion of highly mobile sodium ions can produce a sufficient difference in the refractive index that is needed to create a diffractive phase grating (although the difference in refractive index in silver ion diffraction phase gratings is much higher) [19].
The necessity of DOEs requires that they be as robust and inexpensive as possible and so in answer to this demand we propose using soda-lime glass; the benefits of using this type of glass being that, while still alkali-rich, soda-lime glasses are perhaps the least expensive and most widely used of all the glasses made commercially. The glass will be thermally poled, the migration of the positive ions forming a periodic pattern which will act as the diffracting medium. As the poled regions are beneath the surface of the glass the fabricated DOEs are more durable over time and more hardwearing. To the best of our knowledge there are no reports describing the fabrication of diffractive optical elements using this combination of technique and material – in particular formation of a 2-D grating using double poling of soda-lime glass. We will describe the process of fabricating a DOE in soda-lime float glass while investigating the importance of the process parameters, mainly the applied voltage. The distribution of ions post thermal poling is investigated as well as any change in conductivity of the material. We will show how the redistribution of positive ions can be used to fabricate large area, effective and, most importantly, robust diffractive elements.
2. Experimental methods
For the experiments pieces of soda-lime float glass (72.2 SiO2, 14.2 Na2O, 0.71 K2O, 6.5 CaO, 4.42 MgO, 1.49 Al2O3, 0.13 Fe2O3, 0.4 SO3, in wt.-%) were sandwiched between two electrodes. The anode was a mesh (periodically structured silicon sputtered with 30 nm chromium film to avoid anodic bonding during the experiments – the fabrication technique of one such mesh was described by authors in [15]) 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 cathode and also between the positive electrode and the mesh. An added advantage of having graphite at the negative electrode is 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 temperature was kept constant at 553 K for all samples while the effect of using different peak voltages on the sample was investigated. For all samples the voltage was applied in small amplitude steps, either 0.2 kV or 0.1 kV, in a way that during each step the current never exceeded a few hundred microamperes, typically < 150 µA. The characterizations of the samples were performed using a KEYENCE Digital Microscope VHX-1000, JEOL JSM-7400F scanning electron microscope, Atomic Force Microscopy and a 632 nm He-Ne laser.
Pre- and post-structuring the surface conductivity of selected samples was measured as a function of increasing temperature. Three samples were prepared; original soda-lime glass, a thermally poled sample of the same glass (using the previously described grid-structured electrode) and a thermally poled sample (of the same glass) using a plain anode. Two gold electrodes in a gap-cell configuration (width w = 0.1 cm, length l = 0.5 cm, separation s = 0.1 cm) were deposited on the surface of each sample using a thermal evaporation unit (Edwards model 306) operating at a base pressure of around 4 × 10−5 mbar.
The electrical resistance (R) of the samples was then determined by performing current-voltage measurements using a source-measurement unit (Keithley Model 236). Resistance data was obtained for temperatures (T) ranging from 335 K to 455 K by attaching samples to a ceramic heating element in a sealed air enclosure. The gap-cell current response over this temperature range exhibited excellent Ohmic behaviour for applied voltages up to 100 V which allowed for the associated conductivity (σ) for samples of thickness d = 0.1 cm to be calculated as;
In Eq. (1), a small correction factor κ was applied to account for the non-uniform electric field distribution that arises from the surface gap-cell geometry as the sample thickness becomes comparable to the electrode separation. Electric field calculations confirm that κ = 1 for d << s, and indicate that κ progressively increases to attain a value of 1.6 when d = s in the present samples. Conductivity data were generally recorded as the sample temperature was reduced following an initial short annealing period at 455 K to remove any surface contamination such as water-vapour. Good reproducibility of σ(T) sample data was observed following this procedure.3. Results and discussion
Four samples were thermally poled using different maximum voltages, the post poling samples under microscope (Fig. 1) show a clear structured pattern, with a lattice constant of 2 µm in agreement with the structured electrode (Fig. 1(i)) used, implying that the pattern of the anode has been imprinted onto the surface of the glass.
Fig. 1 The microscope images are taken of the sample surfaces post poling (ii - v). The image in (i) shows surface of the structured electrode for comparison. The maximum applied poling electric potentials were (ii) 1.0 kV (in 0.2 kV steps), (iii) 0.8 kV (0.2 kV steps), (iv) 0.6 kV (0.2 kV steps), and (v) 0.3 kV (0.1 kV steps).
The pattern produced is the result of structures formed near the surface of the glass where there has been migration of highly mobile positive ions, mainly Na+ and K+. These ions are mobile at the elevated temperatures used during the experiment and are repelled away from the anode by the electric field. The positive ions are pushed into the bulk of the glass or into the relief areas where the glass is not in contact with the electrode, which produces the structures seen under microscope in Fig. 1. The depletion regions between structures have a lower refractive index than the relief ‘structures’ due to the reduced ionic content.
The displacement of positive ions, which are mainly sodium and potassium ions, was shown using X-ray elemental analysis of a sample before and after poling at 553 K to a maximum voltage of 0.3 kV (0.1 kV steps) - Fig. 2. Previously reported in literature [9, 11] and shown in Fig. 2(b), post poling sodium and potassium content has decreased close to the surface and increased in the depth of around 6 µm. Since it is not possible to distinguish between atoms and ions using this process, it is an obvious conclusion that these changes are due to a field-driven displacement of Na+ and K+ ions. It is therefore reasonable to take the depth where the Na+ and K+ ion concentration begins to increase as the thickness of the poled region which would be approximately 6 µm.
Fig. 2 Distributions of key elements (sodium, potassium and silicon) as a function of depth measured by local X-ray element analysis of vertical cross section samples (a) before and (b) after poling at a maximum electric potential of 0.3 kV (0.1 kV steps). The red line indicates the surface of the glass.
Additionally the ion displacement was further investigated by measuring the conductivity. Although similar measurements have previously been reported in [19] for pre- and post-poling of soda lime glasses, detailed here are the conductivity results at various stages of poling. For this the conductivity was measured for three different samples; original soda-lime glass, a thermally poled glass using the grid-structured electrode and a thermally poled glass using a plain electrode. Both the poled samples were fabricated at 553 K with a maximum voltage of 0.3 kV reached in stepped increases of 0.1 kV. The conductivity, calculated according to Eq. (1), for each sample is plotted in Fig. 3 from where it is observed that, over the experimentally accessible temperature range, σ exhibits Arrhenius behaviour according to:
In Eq. (2) kB is the Boltzmann constant and σ0 and ΔE respectively represent a pre-exponential conductivity factor and the activation energy that is characteristic of the underlying ionic transport mechanism within the glass. From Fig. 3, it is evident that the conductivity of the poled samples has decreased in comparison to the unaltered reference sample; the reduction in conductivity that is induced by poling is accompanied by a systematic increase in the associated activation energy.Fig. 3 Comparison of the conductivity of unaltered soda-lime glass (“Unaltered”), a sample post poling using grid structured electrode (“Grid Structured”), and a sample post poling using a plain electrode (“Plain Poled”). Both poled samples were fabricated under the same conditions (oven temperature was 553 K with a maximum voltage of 0.3 kV, using a step increase of 0.1 kV). The solid lines represent the best fit of Eq. (2) to the data points (using the calculated ΔE values given in the key).
After poling the deficit of positive charge caused by the displacement of the positive ions is compensated by hydronium ions (H3O+) which are formed on the surface from the ambient air [20, 21]. The mobility of H3O+ ions is much lower than the alkali ions (sodium, potassium) from the glass and as the conductivity of the glass is proportional to the mobility of positive ions it is subsequently reduced post-poling. As grid structured poling only selectively displaces the positive ions within the glass, some high mobility ions still remain, while a maximum number of these ions have been displaced by using the plain electrode and hence this procedure produces the lowest conductivity.
The current-time dynamics of the thermal poling processes are shown in Fig. 4. For the sample poled with a maximum voltage of 1.0 kV (Fig. 4(i)) the application of the first voltage step of 0.2 kV causes a current peak of ~60 µA which decreases exponentially below ~20 µA in 40 min. The second voltage step induces a current peak of ~105 µA which again decreases exponentially, this time to ~30 µA, before the third voltage step was applied. After this point the current increases to ~105 – 115 µA with each voltage step and decreases exponentially to around 30 µA. Integrating the current over time gives a total charge transfer of 0.18 A·s·cm−2. A similar pattern is seen in the current time dynamics for the other samples; the first current peak is significantly lower than those corresponding to the proceeding voltage increase steps. The total charge transfer decreased, as would be expected, with decreasing maximum voltage. The charge transfer for the samples modified with maximum voltage 0.8 kV, 0.6 kV, and 0.3 kV was 0.12 A·s·cm−2, 0.09 A·s·cm−2, and 0.04 A·s·cm−2 respectively.
Fig. 4 Current-time dynamics recorded during thermal poling. Maximum voltage; (i) 1.0 kV (0.2 kV steps), (ii) 0.8 kV (0.2 kV steps), (iii) 0.6 kV (0.2 kV steps), and (iv) 0.3 kV (0.1 kV steps). The peaks in the current coincide with the increase stepped in voltage.
Figure 5(a) displays the diffraction patterns from four samples poled at 553K with maximum applied poling electric potentials of: 1.0 kV, 0.8 kV, 0.6 kV, and 0.3 kV. Comparing the patterns for the different samples shows a variation in the intensity of the patterns and the number of orders that are clearly visible. In order to better analyse these patterns the diffraction efficiency for each of the samples was measured and the results are plotted in Fig. 5(b). The sample with the lowest diffraction efficiency was produced using a maximum voltage of 1.0 kV, while the highest was for the sample poled at a maximum voltage of 0.3 kV. Higher voltage increases the electric field strength between the two plates and at the edge of the areas of glass in contact with the electrode. With increased electric field strength the mobility of the hydronium ions increases resulting in increase in the depth profile of the structures and a smearing of the lateral surface profile [22]. The smearing of the surface profile results in a decrease in the phase modulation index and therefore with increased voltage the diffraction efficiency decreases. It is worth mentioning that the formation of surface reliefs and their heights as a result of thermal poling would also influence the diffraction efficiency. Surface relief height is a function of the poling condition and duration (Fig. 4) and was measured using atomic force microscopy. The highest measured value for surface relief height was ~30 nm for the sample poled at 0.6 kV while the measured value for the sample poled at 0.3 kV exhibiting the highest diffraction efficiency was ~7 nm.
Fig. 5 (a) Diffraction pattern (in transmission) of the samples structured at a maximum voltage of (i) 1.0 kV (0.2 kV steps), (ii) 0.8 kV (0.2 kV steps), (iii) 0.6 kV (0.2kV steps), and (iv) 0.3 kV (0.1 kV steps) upon illumination by a 632 nm He-Ne laser. The intensity of the zero order has been reduced by a small piece of black felt in order to take these images. (b) Log plot displaying the values of diffraction efficiency (η) for the visible orders (zero, first and second) of the diffraction patterns shown in Fig. 5(a).
As the highest diffraction efficiency was found for the sample produced following poling with a maximum voltage of 0.3 kV, these parameters were used in an attempt to produce a more complex diffraction pattern. For this, a sample was thermally poled at 553 K using a maximum voltage of 0.3 kV (0.1 kV steps) using the grid structured electrode (Fig. 1(i)) previously described, the experiment was then repeated using the structured electrode at 45° to its original position. By poling twice at different angles a star-like diffraction pattern was successfully produced, as shown Fig. 6.
Fig. 6 Diffraction pattern from a sample which was poled twice using different angles for the grid structuring; the effect is a star-like unlike the grid-like pattern shown in Fig. 5(a) .
4. Conclusions
In summary, a series of diffraction gratings have been fabricated in an inexpensive and readily available material, soda-lime float glass, using a well-known fabrication process, thermal poling. The samples produced are effective as gratings; this is shown by the diffraction patterns produced and the results allowed for the inference of optimal structuring conditions.
It was shown that the fabrication method used results in the re-structuring of the glass itself and therefore alters its refractive index. By selectively structuring the material with the use of a structured electrode the refractive index hence varies periodically throughout the final product and is responsible for the diffraction patterns shown. The diffraction pattern produced is governed by the electrode used and so complex, large-scale patterns can easily be fabricated using this process.
Acknowledgements
This work was conducted under the aegis of the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom (EP/I004173/1). The authors would like to thank Dr. Yongchang Fan for his assistance with the X-ray analysis and atomic force microscopy measurements and Dr. Svetlana Zolotovskaya for her help and guidance (both of the University of Dundee). AA is an EPSRC CAF at the University of Dundee (DOI: 10.15132/10000101).
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