1. Introduction

Relief gratings and photonic crystals (PCs) are periodic structures of high refractive index contrast, providing enhanced spectral, temporal and spatial dispersion; operational characteristics that render them attractive candidates for potential applications in sensing and optical communications [1]. Such relief structures that scatter in the infrared and visible bands, extending in two- or three-dimensions have been fabricated in several optical materials, including photoresits [2, 3], organic-inorganic matrixes [4], crystals [5], or glasses [6], by employing a variety of different laser based approaches. Specifically, laser based techniques such those of inverse microscopy beam-scanning [4] or holography [7] have been straightforwardly applied for the fabrication of PCs in soft matrixes by photo-polymerisation. Even though these polymeric stratified structures are of high quality and scatter strongly at the resonance bands that being designed, they lack in terms of thermal stability and dopand solubility, in case they are considered for active applications. On the other hand, “hard” optical materials such as glasses or crystals may not suffer from the above drawbacks, providing improved mechanical properties and versatility in terms of doping and active performance, nonetheless, their surface nano-processing processing can be complex and laborious. Moreover, even though direct laser ablation techniques have been applied for the inscription of relief lattice structures in “hard” optical materials [5, 6], the products (debris deposition, optical loss increase and stitching errors) usually related with such violent interaction can significantly degrade the overall scattering performance. Thus, the investigation of a laser based periodic structuring method for the high yield fabrication of relief gratings and PCs in “hard” optical materials, without suffering from the aforementioned drawbacks, can be of high importance for the easier design and development of devices that use such lattices and provide improved performance characteristics.

The two-dimensional periodic patterning of a hybrid rare-earth doped phosphate glass is presented here. The glass patterned is the aluminophosphate glass IOG1, fabricated by Schott USA and codoped with Er and Yb, for exhibiting high absorbance at the 980nm wavelength. IOG1 phosphate glass is an optimized phosphate glass, for sustaining high solubility of rare earths ions and being ion-exchangeable using K⁺ and Ag⁺ ions, for forming low loss channel waveguides. The above material characteristics render IOG1 phosphate glass a promising host for the development of compact, and low-pump, high power lasers [8] and amplifiers [9] emitting at the 1μm or 1.5μm bands. The two step selective etching method [10] which is adopted here, has been analysed before for its specific operational characteristics [11]. Initially, high damage is induced into the glass volume by means of colour centres [11, 12] and structural changes [13], after exposure to intense ultraviolet (UV) laser radiation. Subsequently, a chemical development in an alkali solution, selectively etches the exposed areas, revealing the relief pattern. Namely, the glass matrix is treated as a “hard positive resist”, where the exposed areas are removed by the selective chemical development. Additional advantages of the presented method are the mask-less imprinting; and the potential for monitoring the relief pattern growth during the wet etching process.

2. Experiment

The experimental setup used for inscribing the two-dimensional gratings in the phosphate glass slabs is presented in Fig. 1. A quintupled 213nm, 150ps Nd:YAG laser, of high temporal coherence (2.5cm) and 13mJ output per pulse, was used for the interferometric exposures. For improving the beam quality the laser output at the fundamental frequency was spatially filtered, before frequency multiplied and combined in the non-linear crystals unit. The UV laser beam was directed in a beam splitting composite element, constituted of two fused silica phase masks (IBSEN) being positioned face-to-face in contact mode, with periodicities of 1080nm and 1072nm, respectively. The two phase masks were placed with their k-vector being perpendicularly with respect to each other, for constituting a four beam splitting element. The four 1^st diffracted orders were refolded onto the sample surface using Al-coated mirrors (M1-4) with improved reflectivity at 213nm.

 

Fig. 1. Four-beam interferometric setup used for exposing the two-dimensional periodic pattern. A composite beam splitting element is employed to split the beam emitted from the 213nm, 150ps Nd:YAG laser into multiple beams. A beam selecting aperture (BS) allows only the ±1 first diffraction orders to propagate. (L) 60cm focal length lens, (BSE) beam splitting element, (BS) beam selector, (M1, M2, M3, M4) metal coated mirrors, (S) sample.

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Before the diffracted beams reach the folding mirrors, a screening aperture was used for blocking 0^th and higher diffraction orders emerged from the beam splitting element. The four beams were precisely aligned onto the sample surface for attaining maximum spatial overlap and minimum temporal mismatches. Before the interferometric cavity a fused silica spherical lens of 60cm focal length was placed, for adjusting the energy density of the exposure.

The samples used were 1mm thick, and contained 2.3% wt. Er₂O₃ and 3.6% wt. Yb₂O₃. After exposure the glass samples were developed into 1M KOH/0.4M ethylene-diamine-tetra-acetic acid (EDTA) aqueous solution, kept at 40°C. We chose 1M KOH alkali solution because it exhibited high selectivity, being aggressive mostly to the damaged areas instead of the non-exposed, while the etching rate is substantial slower in comparison to the rate measured in higher concentrations [11]. Alkaline solutions are aggressive to phosphate glasses, providing high etching rates even at low molar concentrations, thus, the selectivity of the process is limited [14, 15]. EDTA agent was used as a complexing agent for preventing leach contamination of the etched surface [15]. During the development process diffraction efficiency of the two-dimensional gratings were performed in parallel with the AFM measurements, using the 496.5nm output of an Ar⁺-ion laser, probing the sample at Bragg angles. For avoiding photorefractivity problems during the diffraction efficiency measurements the output of the probing laser was kept below 1mW.

3. Results and discussion

The diffraction efficiency data versus the chemical developing time are presented in Fig.2.

 

Fig. 2 Diffraction efficiency as a function of etching time for a 2D periodic structure inscribed in IOG-1 glass with 136mJ/cm² and 36000 pulses. The diffraction efficiency values that refer to the Y-scattering orders have been given negative values solely for assisting observation.

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The measured diffraction efficiencies correspond to the 1^st scattered orders along the x- and y-axis (X and Y order labels, respectively). Their diffraction efficiency increases for the first 40min of developing time, while for longer immersion in the KOH solution, reaches a plateau. This specific feature may be correlated with the accumulation of the radiation induced defects in a limited volume close to the sample surface. Colouration imaging measurements preformed on 1mm thick cleaved and polished samples exposed to 133mJ/cm² energy density, provided a visible damage depth of the order of 270μm [Fig.3(a)]. Optical density spectroscopy performed in 0.65mm thick pristine glass samples led to an optical absorption factor a of the order of ≈100cm^-1 (≈100μm), for the 213nm wavelength [see Fig.3 (b)].

 

Fig. 3. (a) Cross-section image of an IOG1 glass sample exposed to 133mJ/cm² energy density and 36000 pulses. (b) Absorption spectrum of the pristine Er/Yb-codoped IOG1 glass.

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The radiation penetration lengths given above are substantially greater than the etching depths observed for this glass. We believe that extensive bond cleaving and glass de-polymerization [11], adequate for supporting differential etching rates, takes place in significantly shallower depths of the order of few microns or less, as the etching results depict.

Prolonged immersion in the alkaline solution results in overetching of the periodic features, leading to lower diffraction efficiencies. This overetching mechanism has been observed before and affects both the depth and the shape of the grating grooves [11].

 

Fig. 4. AFM scans of the same sample inscribed with 136mJ/cm² and 36000 pulses. The scans were performed at the 5^th min, 27^th min and during the saturation (36^th and 45^th min) of the chemical etching process.

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The above mechanism has also been verified by employing atomic force microscope (AFM) scans taken at different instances of the developing of the relief periodic structures (see Fig.4). The periodicities along x and y axes were measured to be ≈510nm and ≈550nm, respectively. During the early stages of the selective chemical etching, shallow gratings are formed of increased surface roughness, as this is shown in the upper left AFM scan of Fig.4. However, during later phases of the chemical development this roughness is eliminated, while deeper and smoother two dimensional grating grooves are formed. Furthermore, the grating groove shape, transforms to different forms during the chemical etching, being of “sharp-notch-type” after 27min of development; and “round-like” for the later development stages.

In general, the relief profile obtained is a combination of two discrete processes; that of UV induced volume damage, and that of selective chemical etching/dissolution. The nature of the former process is strongly indicated to be single photon absorption [11, 13], thus, it can be described by employing Beer absorption law, which has been widely used for describing ablation experiments. Furthermore, for multi-beam interference the volume damage effects become more prominent at the bright fringes of the periodic pattern, were the local intensities are sixteen times higher than those occur in the individual beams. We believe that for the wavelength and the maximum intensities (≈3.5GW/cm²) used herein, the nature of the volume damage is primarily that of colour centres. No macroscopic structural modifications were detected for the samples exposed by means of volume dilation, by employing polarization rotation measurements. Volume dilation effects have been reported by Yliniemi, et al, [13] for the same glass under exposure to 193nm excimer laser radiation. Such structural effects would be directly associated with extensive changes in the microscopic glass coordination over greater depths, due to the ultraviolet exposure. Additional, inspection of the samples using atomic force microscopy after the 213nm exposure, did not reveal any surface structural deformation related with either volume modification or amorphisation effects. Nonetheless, we cannot exclude the case where glass matrix modification effects (such as PO bond cleaving) are confined in a thin volume close to the irradiated surface, assertion which can be supported from the shallow etching depth results. Moreover, the temperature increase ∆T ($\mathrm{\Delta T}=\frac{{\mathrm{\alpha F}}_{\mathrm{laser}}}{\rho C_p},$ , a absorption coefficient at 213nm, F_laser energy density, ρ glass density and C_p heat capacity) in the bright fringes due to the irradiation process, is calculated to be ≈25°C, approximately. Therefore, no thermal effects above the transformation point (T_g=474°C) of the glass take place during the irradiation.

The nature of the selective chemical etching for phosphate glasses in alkaline solutions, is still under consideration, since more than one individual processes are possibly involved, such as those of acid-base interaction [16], ion-exchange between sodium and potassium ions and long-polymeric chain dissolution by hydration [17]. As the spectral measurements depicted, the exposure of the IOG1 glass to 213nm radiation generates a number of defects (mainly P-O hole centres, PO₄ electron centres and PO₃ electron centres) that are of electron donor nature [18]. These defects increase locally the glass acidity, and therefore, an acid-base interaction is expected to contribute significantly to the selective glass dissolution. Similarly, in chemical etching experiments performed in phosphorus doped and pure silicate glasses after nanosecond [19], femtosecond [20, 21] or neutron [22] irradiation, and subsequent selective etching in diluted HF solution, colour-centre-rich or compacted areas exhibited an accelerated chemical solubility, simultaneously with an increase of the glass Lewis basicity [20]. In addition to the above mechanism, the radiation induced cleaving of the long polymeric P-O bond, can further assist the hydration detachment process [16, 17], which also contributes to the glass dissolution. Experiments carried out for the inscription of one dimensional Bragg reflectors using a total energy density for the two beams of 124mJ/cm², and 1M KOH/0.4M EDTA solution, led to an etching rate that is 2.4 times faster for the exposed glass (≈4.1nm/min) compared to the one for the pristine glass (1.7nm/min) [11]. The energy density in the bright fringes of the 4-beam interference that is employed here is of the order of 532mJ/cm², hence, a substantially greater etching rate and depth can occur in the exposed areas.

Based on the above experimental results, we believe that precise choice of the exposure and chemical etching conditions may provide additional control over the filling ratio (namely, the duty cycle for 1D gratings) of the two dimensional periodic structures, allowing a precise tailoring over their reflectivity and dispersion properties.

 

Fig. 5. Large area AFM scans of two-dimensional Bragg gratings inscribed in Er/Yb-codoped IOG1 phosphate glass, obtained at different instances of the chemical etching process.

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Wide area AFM scans of the fabricated two-dimensional periodic lattices which were obtained at different instances of the wet etching, are shown in Fig. 5. During the early stages of development, the periodic structure appears to be of high homogeneity on large-area inspection (20μm × 20μm), without exhibiting observable stitching errors which can degrade the scattering behaviour of the crystal [23]. After 45min of development, a long-range one-dimensional wavy pattern is observed onto the high spatial frequency of the crystal. This parasitic wavy structure superimposes with the high-spatial frequency periodicity, producing a complex relief pattern of variable depth and shape. Such parasitic periodicity becomes more prominent after long immersion in the alkaline solution, since its existence may be associated with low-contrast fringe pattern formation. Low-contrast fringe patterns do not favor selective etching processes, since the difference in terms of volume damage between the highly and under- exposed areas may be marginal. Simulations performed on the multi-beam interference pattern, have shown that this long-range artifact may be attributed to imperfect alignment of the interfering beams in opposite pairs. Such misalignment -which is simultaneously translated to angular deviations and beam path phase shifts- can lead to a long range amplitude modulation, similar to the one presented here. This parasitic modulation affects also the diffraction behavior of the crystal, splitting the diffraction orders in smaller components scattered within a narrow angular cone. The above effect was verified by employing FFT of wide range AFM scans, but also during the diffraction efficiency measurements, using the Ar⁺-ion laser beam.

 

Fig. 6. SEM images of the photonic crystal inscribed with 134mJ/cm² and developed in 1M KOH aqueous solution for 45 minutes.

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Scanning electron microscopy (SEM) pictures of the two-dimensional crystal after 45min of chemical development are presented in Fig. 6(a) and (b). As it is shown in Fig.6(a) the long-range inscribed modulation, affects the shape and filling ratio of the high-spatial frequency periodic structure, resulting in a smooth transition from “spike-like” features in the valleys, to “cell-like” features in the peaks. Both of the aforementioned relief features exhibit high aspect ratios, which are estimated to be greater than a factor of two for the case of the spikes (see Fig.6(b)); being not easily measurable using the AFM scans. The last finding related to the high aspect ratio of the individual groove features, strengthens the point about the good etching selectivity of the process for low molar concentration solutions of KOH [11]. We believe that the topological SEM results presented in Fig.6, prompt the application of selective wet etching process for the fabrication of three-dimensional photonic crystals in suitable glass optical matrices (as the one used here), in combination with free-space multi-beam or phase mask holography [24].

4. Conclusions

The fabrication of two dimensional photonic crystal structures in Er/Yb-codoped Shott USA IOG1 phosphate glass by employing ultraviolet laser-assisted, selective chemical etching in combination with multi-beam interference, was presented here. Two-dimensional, rhombic lattice, of submicron period was patterned in the glass substrates, by employing the output of a high coherence 213nm, 150ps Nd:YAG laser and a 4-beam interfererometric setup. Issues related to the wet etching process and the morphology of the Bragg reflectors were investigated by using AFM and SEM scans. We are working towards the optimization of the laser assisted-wet-etching method using laser wavelengths of longer optical penetration depth; and the investigation of inscribing three dimensional photonic crystals in soft glasses.