1. Introduction

Photonic crystals based on glassy materials with high refractive index have been attracted recently a substantial interest because of the large range of promising photonic applications, such as optical band-gap devices and omni-directional reflectors, e.g. in [1–8] and refs therein. Principally, the most important requirement for a material, which could be used for the fabrication of the photonic crystal with a complete photonic band-gap, is the high refractive index, which desirably should be higher than 2.7, e.g. [1–8] and refs therein. This criterion therefore motivates the choice of the material, which, in addition, should afford fabrication as a large bulk of an arbitrary shape and/or waveguide; therefore, the material ought to be of glassy origin [1–8]. Chalcogenide glasses have been attracted recently a substantial interest as they have highest linear and non-linear refractive index amongst glasses resulting in highest non-linear properties [9 and refs therein] and allow fabrication of rib and fibre waveguides, which can be structured by photolithographic techniques for photonic crystal applications, e.g. [10–13 and refs therein]. Hence, one of the most promising materials for these purposes is a kind of the chalcogenide glass, the Te-enriched Ge-As-Se-Te glass (TeGAST), which has the highest refractive index, up to 3.5, available amongst glasses [1].

Here it is proposed that the use of the TeGAST glass opens substantial advantages for fabrication of photonic crystals. In addition to the highest refractive index available for glasses, it also benefits from: a) a broadest transmission window from about 1.5 to 23 μm available for glasses in the infrared [1], b) an excellent glass-stability [1], that enables applications of the surface moulding and hot pressing techniques, which achieve a best glass surface finish available to date by means of a conventional polishing [1], c) possibility to manufacture the photonic crystal structures by means of a fs laser writing with highest photosensitivity available to date, to the best of our knowledge.

2. Experimental

The experimental for preparation of bulk glass samples Ge₂₀As₂₀Se₁₄Te₄₆, typically of 10 mm diameter and several milimeters/cantimeters length, has been described in [1] and it has been also used elsewhere [6,7] confirming the reproduceability of the preparation procedure. The moulding of the opposite parallel surfaces of these glass samples has been carried out at a temperature slightly above the glass transition temperature Tg (Tg=200°C), resulting in glass samples with strictly flat surfaces. The deviation from the flattness was within ±0.1 μm and the surface roughness varied within the range of ±1 nm in the areas of 1 × 1 mm² on the glass surface. Such flattnes is certainly important for applying direct laser writing on the glass surface by means of interference pattern of interfering femtosecond laser beams. Similar results have been obtained using standard diamond polishing techniques, however in the latter case the surface preparation procedure requires several subsequent changes of the diamond grain and careful cleaning of the sample surface prior to each use of the smaller diamond grain resulting in longer time consumption and eventually in more expensive preparation process.

The fs Ti:sapphire laser beam (100 fs pulse duration, 50 Hz repetition rate and 100 μJ/pulse/cm² irradiance), has been split using the diffractive optics either in four (serving as two couples) or two (serving as one couple) coherent phase-controlled beams of the similar intensity, as described in [14,15]. The beams have been interfering on the sample surface producing the interference patterns for writing either the array of the holes in the case of the four interfering beams (orthogonal-cross grid pattern) and grooves in the case of the two interfering beams (parallel grid pattern), respectively. The respective interference patterns have been formed just on the sample surface using an imaging system described in [16]. The interference half-angle was equal to 20° for each of the couples of the interfering beams.

3. Results and discussion

Figure 1(a) (holes) and Fig. 1(b) (grooves) show the surface relief, which has been prescribed on the glass surface after a 1 second irradiation (or with 50 fs pulses) with four (a) and two (b) interfering beams, respectively, as described above. The depth of the holes and grooves was not less than 0.8 μm, as estimated by means of the atomic force microscope (AFM). The depth in fact may be substantially larger because the tip of the AFM did not reach the deepest point of the structures due to the finite size of the tip. Also, some ablated and precipitated material is seen on the surface in Fig.1 indicated that an ablated material could also be trapped within the depths. Therefore we believe that further optimisation of the writing conditions, e.g. applying the flowing air above the sample for removing of ablated material, may result in faster writing of the further deeper relief.

 

Fig. 1. Scanning electron microscope (SEM) images of the surface relief of the Ge₂₀As₂₀Se₁₄Te₄₆ glass consisting of air holes (a) or air grooves (b) written in 1 second by means of four (a) and two (b) interfering beams of the fs Ti-sapphire laser, as described in Experimental. The dark spots in (a) and wide dark bands in (b) are the depths of the relief. Some ablated and precipitated powder material is seen as bright irregular shapes between the holes in (a) and as bright sparkling points in (b). The laser parameters were: λ,=800 nm, pulse duration 100 fs, repetition rate 50 Hz (i.e. 50 fs pulses in 1 second of this writing), irradiance 100 μJ/pulse/cm². The interference half-angle equals to 20° for each of the couples of the interfering beams in (a) and for one couple of the interfering beams in (b).

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The difference in the refractive index of the holes/grooves (n=1.0 of the air) and of the Ge₂₀As₂₀Se₁₄Te₄₆ glass (n=3.3) [1]) equals to about 2.3, therefore this structure may serve as a 2-dimensional photonic crystal. Such crystal may provide complete photonic band gap at some wavelengths in the infrared because it was shown already in [4] that a combination of the chalcogenide glass As₂S₃ with a lower refractive index (n=2.4) with air holes provides the full photonic band gap at certain wavelengths. In particular, the structure in Fig.1 is a kind of a “moth-eye” structure [17], which may serve as an anti-reflection coating, which is especially important for optical transmission applications of this Ge₂₀As₂₀Se₁₄Te₄₆ glass with such high, n=3.3, refractive index. The structure in Fig. 1(b) may serve as a parallel rib waveguides [18] for the infrared light.

 

Fig. 2. The relief patterns on the surface on the Ge₂₀As₂₀Se₁₄Te₄₆ glass consisting of air grooves (wide dark parallel lines) and glass ribs (bright parallel lines) perforated by air holes (small dark spots). The laser writing parameters are as in Fig.1, but an irradiance of the source Ti:sapphire laser beam was at 150 μ/pulse/cm².

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Figure 2 shows an unusual example of the surface relief written by means of the four interfering beams, in the conditions applied to Fig.1(a), but the irradiance of the laser beam was increased to the value above 100 μJ/pulse/cm². This kind of quasi-periodic structure, Fig.2, could be written only when the laser irradiance was set in a very narrow range between 150 to 155 μJ/pulse/cm². At present we do not completely understand the mechanism for writing of such unusual structure, Fig.2, but it may be related to a kind of-self-organisation of the glass surface relief in the intense electric field of the incident femtosecond laser pulses, which produces surface plasmon waves. A kind of self-organisation of the surface relief in the intense field of the interfering femtosecond laser beams has been reported for other semiconductors [19], however we note, in addition, that neither period nor the size of the small dark spots (dips) in Fig.2 does correspond to the period/size of the respective cross-grid pattern of the incident beams. The mechanism for writing of such unusual structures is currently under investigation, but a small diameter of the small dark spots (dips), at about 100 nm, may indicate an effect of the surface plasmon waves generated in the course of writing with very intense laser beam.

Figure 3 shows a magnified image of the grooves written in the same conditions as in Fig.1(b), while the precipitated ablation deposit (bright sparkling spots) can be visualized. It indicates that an extract of the ablated material may be required to achieve smoother structures.

 

Fig. 3. A magnified image of the grooves (dark wide parallel lines) written on the surface on the Ge₂₀As₂₀Se₁₄Te₄₆ glass by means of two interfering beams; writing conditions as in Fig.1(b). The sparkling bright spots represent the ablated powder precipitated on the surface, as in Fig. 1(b).

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The Ge₂₀As₂₀Se₁₄Te₄₆ is a narrow-band semiconductor [1 and refs herein] and its conductivity is strongly temperature dependent already at about room temperature [6]. Therefore, even minor heating (for several tens of degree) of the irradiated area of the sample by the laser beam results in creation of free electrons. These electrons cause the free-electron absorption and further more efficient heating of the irradiated area; therefore photosensitivity of this glass, especially to femto-second radiation, becomes comparable and perhaps even higher than the photosensitivity of metals, where the free-electron absorption dominates in the mechanism of laser writing [14].

Remarkably, the structures in Fig.1 have been written in 1 second time with as small number of fs pulses as only 50, suggesting that this writing process may be practically important. This time scale is comparable with the similar writing process in metals, where also the 1 second time scale has been indicated [14], however a substantially higher repetition rate of fs pulses was used. Therefore we believe that the Ge₂₀As₂₀Se₁₄Te₄₆ glass is more photosensitive to fs pulses than metals, perhaps because it has lower softening/melting points.

The work on writing the 3-dimensional photonic crystals in this glass is in progress, in particular our computations indicate that the full photonic band gap of groove-type 3-dimensional structure is expected to be light polarisation sensitive resulting in a fully dichroic photonic crystal (polaroid).

4. Conclusion

We have shown that surface of the high refractive index chalcogenide glass, in particular the Ge₂₀As₂₀Se₁₄Te₄₆ glass (n=3.3), can be patterned by ablation using four- and two-beam interference femto-second laser setups operating at 800 nm. The regular arrays of typical size of 1×1 mm² consisting of 0.8 μm diameter and more than 0.8 μm depth holes and/or grooves have been written on the surface of the glass in a time-scale of 1 second with 50 femtosecond pulses.