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It is shown that chalcogenide glasses with suitably underconstrained network can undergo reversible giant photocontractions up to a micron depth. These effects result from the combination of two attributes particular to these glasses, (i) the high photosensitivity characteristic of low coordination floppy networks and (ii) the wide window of structural configuration characteristic of fragile glass former. Interestingly these effects are reversible and subsequent irradiation with high intensity results in giant photoexpansion in the same glass. The combination of subsequent photocontraction and photoexpansion on the same glass surface has good potential for the design of complex optical elements.
©2009 Optical Society of America
1. Introduction
The recent development of efficient laser sources in the mid-IR has generated a regain of interest in infrared sensing technologies [1]. In that respect, chalcogenide glasses have come out as the material of choice for the development of infrared devices due to their large optical transparency as well as their good photosensitivity. These glasses exhibit unique photostructural changes upon irradiation with sub-bangap light which permit direct-writing of many optical components such as waveguides and gratings [2–6]. The mechanism of these effects has received extensive attention over the years and is still being actively investigated [7–11]
Recently, it was shown that the magnitude of photostructural changes in chalcogenide glass is highly dependent on the covalent network connectivity [12]. A drastic increase in photosensitivity was observed for glasses exhibiting low bond density and low structural constraints, or so-called floppy networks [13]. Photo-activated bond scission generates a high density of zero-frequency modes [14] and high degrees of freedom in this type of networks due to notable structural openness and steric freedom. This allows for large local structural rearrangements which are then revealed by considerable photoexpansion and photodarkening [12].
Another property of underconstrained network glasses is related to their visocosity-temperature dependence and is defined as “fragility” according to the Angell’s classification [15–17]. It is shown that the structure of these glasses collapse rapidly at temperature above Tg and that they rapidly become fluid [18]. The thermodynamic parallel to that behavior is a sharp variation of the liquid excess entropy with temperature [16]. The consequence for the corresponding glass is a large departure from the equilibrium liquid line when the glass falls out of equilibrium and solidifies below Tg [19,20] (Fig. 1 ). At ambient temperature these “fragile” glasses therefore have a large propensity for relaxation (Fig. 1) and a wide range of structural sates to explore (volume or enthalpy).
Fig. 1 Temperature dependence of the molar volume of a strong glassformer compared with a fragile glassformer. Fragile glassforming liquids have a sharper volume variation with temperature. As a result, fragile glasses frozen-in at fast cooling rate have a wide potential for volume relaxation and well-annealed fragile glasses have wide potential for photoexpansion
In this paper we demonstrate that the combination of large photosensitivity and large relaxation window in underconstrained glasses allows for photocontractions of unprecedented depth and that these processes are nonetheless reversible. These giant photocontraction effects offer the potential for production of complex diffractive optics.
2. Experimental
High purity Ge-As-Se samples were synthesized in silica ampoules using 5N starting elements purified in situ under 10−6 Torr vacuum. Elements were purified by sublimation of high vapor pressure impurities then mixed and sealed under vacuum. Sealed ampoules were heated to 750°C overnight in a rocking furnace and quenched in water. The resulting glass rods were annealed overnight near Tg. The glass composition chosen for this study was a low coordination GeAsSe13 glass with an average coordination of <r> = 2.2. All samples investigated in this study were homogeneous melt-quenched bulk glasses, not films. The samples were prepared into polished discs with a thickness of 0.6 mm.
Calorimetric measurements were performed using a TA Q1000 modulated differential scanning calorimeter. The enthalpy measurements were obtained by integrating heat capacity curves of annealed samples and subtracting from that of a reference heated and cooled at 10°C/min following a procedure described elsewhere [21]. Alternatively the enthalpy was estimated by measuring the fictive temperature using Moynihan’s method [22].
Photoinduced volume change experiments were performed with a CW tunable Ti-Sapphire laser which wavelength was adjusted in order to irradiate the glass in the band tail, at the limit but within the transparency domain. Photocontraction and photoexpansion were induced in GeAsSe13 polished glass samples with 825 nm light focused with a 5X microscope objective. Combined photocontraction and photoexpansion were produced by varying the focus of the laser through the 5X microscope. Photoinduced volume changes were characterized in 2-D with a Dektat 6M stylus profilometer and in 3-D with a Wyko optical profilometer. Photobleaching and photodarkening were induced with an 825 nm free beam and characterized with a Perkin-Elmer Lambda 9 UV-VIS spectrometer.
Irradiation effects were tested on annealed and quenched glass samples. The annealed glasses were heated near Tg for several hours and left to cool in the oven resulting in a fictive temperature Tf = 348 K. The quenched glasses were rapidly cooled between two aluminum plates resulting in a fictive temperature Tf = 374 K. The cooling rate associated with this procedure was estimated to be 55 K/min from a measured activation energy Ea = 233.5 kJ/mol.
3. Results
3.1 Photoinduced volume changes
The use of photoinduced volume change for producing various microlenses shapes is illustrated in Fig. 2 . A convex microlenses produced by photoexpansion during high intensity irradiation (3.2 W/cm2) of an annealed glass is shown in Fig. 2a. The formation of convex lenses is relatively conventional and has been reported previously in chalcogenide thin films [3,23]. On the other hand Fig. 2b shows that low intensity (0.4W/cm2) irradiation of the same glass cooled rapidly results in concave lenses formation due to giant photocontraction. Such large contractions in the micron range have not been previously reached. Finally Fig. 2c shows that this effect is reversible and that it is possible to induce subsequent photoexpansion of the center through simple refocalization of the 5X microscope objective used for irradiation. It can be shown that further irradiation at high intensity results in complete disappearance of the photocontraction. This demonstrates that it is possible to selectively induce photocontraction and photoexpansion in different areas of the glass surface by simply varying the light intensity. This also suggests that these effects are very localized in the area of irradiation and should permit to create complex 3D shapes. In particular this could be applicable to the development of photoinduced patterns by holographic methods and could be used for the fabrication of gratings with high contrast.
Fig. 2 (Color online) Interferometric profilometer images of photoinduced microlenses in a GeAsSe13 glass. a) Convex lenses produced by photoexpansion on a 2 mm thick annealed glass disc. Height is ~3.8 microns (5W/cm2, 20 min). b) Concave lenses produced by photocontraction on a 2 mm thick quenched glass disc. Depth is ~800 nm (0.4W/cm2, 20 min). c) Combined concave-convex lenses produced on a 2 mm thick quenched glass disc. The depth is ~600 nm after 8 min irradiation with a low intensity of 0.4 W/cm2, followed by a reversible expansion of ~400 nm upon high intensity irradiation of 3.5 W/cm2 for 2 min with a focused beam. Complete reversibility of the contraction is observed for longer irradiation time at high intensity.
3.2 Effect of irradiation time
The results of Fig. 2 imply that photoinduced volume changes evolve from a fine interplay between the initial glass state, the light intensity and the irradiation time. Figure 3 shows the effect of irradiation time. Both photocontraction and photoexpansion reach a saturated state after sufficient illumination dosage. The annealed glass (Fig. 3a) irradiated with high intensity (4.5 W/cm2) displays photoexpansion similar to the giant expansion previously observed in As2S3 and As-Se glasses [8,24]. This effect appears to saturate after approximately 30 min. In contrast the quenched glass (Fig. 3b) displays the opposite behavior and shows photocontraction upon irradiation with lower intensity light (0.5 W/cm2). This effect is also shown to saturate after a long irradiation time (Fig. 3d). The observation of such plateaus after long irradiation times suggests that the glass structure reaches a photo-saturated state for a given light intensity. The molar volume therefore reaches a final value representative of the irradiated glass state shown as the thick dashed line in Fig. 1. This final state appears to be the result of a balance between photorelaxation and photoexcitation and should therefore be highly dependent on the light intensity.
Fig. 3 Effect of irradiation time on a volume change in GeAsSe13 glass upon irradiation with 825 nm laser light through a 5X microscope objective; a) Photoexpansion of an annealed glass upon irradiation with 4.5 W/cm2 intensity for 5 min. b) Photocontraction of a quenched glass upon irradiation with 0.5 W/cm2 intensity for 2 min. c) Saturation of the photoexpansion effect with irradiation time in the annealed glass. d) Saturation of the photocontraction effect with irradiation time in the quenched glass. Lines in figure c) & d) are guides to the eye
3.3 Effect of irradiation intensity
Figure 4 compares the effect of light intensity on the photoinduced volume change of a quenched and annealed GeAsSe13 glass. It is shown that the well-annealed glass does not exhibit any contraction at low intensity and only undergoes photoexpansion at higher light intensity. As shown in Fig. 1, the well annealed glass state is close to the equilibrium line and therefore does not possess significant driving force for photorelaxation and photocontraction. Instead this glass only undergoes photoexpansion when the photon flux is sufficiently high to introduce large structural rearrangement into the glass network. As expected, the greater the intensity the greater the structural change after a constant irradiation time of 10 min (Fig. 4).
Fig. 4 (Color online) Effect of light intensity on the photoinduced volume change in GeAsSe13 glass upon irradiation of a 0.5mm2 area with an 825 nm laser for 10 min. The annealed glass (black dots) shows photoexpansion, while the quenched glass (red dots) shows photocontraction. For large irradiation intensity, the quenched glass eventually develops expansion.
In contrast, the quenched glass is trapped in a high molar volume state that is out of equilibrium far above the liquid line. In this case, low intensity irradiation introduces some degrees of freedom into the network that allows the glass to relax towards the equilibrium liquid line. This produces contraction in the area of irradiation. Increasing photon fluxes first facilitate this process and accelerate the kinetic of contraction, but for higher intensities the system reaches a balance between photorelaxation and photoexcitation and the contraction comes to a maximum. For even higher intensities, photoexcitation is prevalent and the glass starts to expand.
3.4 Role of the structural enthalpy
The photostructural changes observed in this study appear to be consistent with a competitive process between photoexcitation and photorelaxation. It is useful to note that while the photoexcitation process is purely optically driven, the photorelaxation process is only optically activated but mostly driven by thermodynamic relaxation. In that respect it is important to characterize the thermodynamic state of the system before and during irradiation in order to estimate its contribution to the photostructural changes and in particular to the photocontraction. Figure 5 shows the evolution of the glass enthalpy with irradiation time for a quenched and an annealed GeAsSe13 glass. It is shown that the annealed glass goes from a low initial enthalpy to a higher enthalpy state during high intensity irradiation (3.2 W/cm2). This indicates that the photoexpansion observed in Fig. 2a is visibly associated with an increase in structural enthalpy. Conversely, the quenched glass irradiated with lower intensity (1.6 W/cm2) shows a large decrease in enthalpy concomitant with the giant photocontraction observed in Fig. 2b. These results confirm the correlation between the thermodynamic state of the glass and the photoinduced volume change. Additionally, it should be expected that glasses with higher initial enthalpy would have a greater capacity for contraction as illustrated in Fig. 1 due to the larger departure from equilibrium and the larger driving force for relaxation. Indeed, an experiment performed with glasses quenched at various rates reveals an almost linear relationship between the initial enthalpy and the magnitude of contraction in glasses irradiated for 10 min (Fig. 6 ). This further confirms that the main driving force for photocontraction is enthalpic as opposed to chemical effects such as photopolymerization observed in as-deposited films [25,26] or mesoscopic effects such as the collapse of columnar structures observed in obliquely deposited films [27,28].
Fig. 5 (Color online) Variation of the enthalpy of GeAsSe13 glass estimated by measuring the fictive temperature of the glass upon irradiation with 825 nm laser light; Upper curve: decrease in enthalpy of a glass quenched in water and irradiated at 1.6 W/cm2. Lower curve: Increase in enthalpy of an annealed glass irradiated at 3.2 W/cm2
Fig. 6 Magnitude of photocontraction as a function of the initial glass enthalpy in a GeAsSe13 glass irradiated for 10 min with a 3 W/cm2 laser at 825nm. The greater the initial enthalpy the greater the magnitude of photocontraction. The glasses used in this experiment are obtained by quenching the melts in liquid nitrogen, ice water and air, respectively. As described in Fig. 1, the fastest cooled glasses have a larger propensity for volume contraction.
3.5 Photobleaching
It is well known that irradiation of chalcogenide glass results in a red shift of the absorption edge called photodarkening [29]. This effect is associated with a reduction in interband energy induced by the formation of defects electronic states [30] or the occurrence of bond twisting [31]. Figure 7a&c show that this behavior is present as expected in annealed glass irradiated with sub-gap 825nm light. The effect shows a plateau after long irradiation time in agreement with previously reported studies [32]. However it is shown that irradiation of a quenched GeAsSe13 glass actually results in photobleaching or a blue shift in absorption edge (Fig. 7b&d). Previously reported cases of photocontraction have normally been associated with a red shift in band-edge [25,33,34]. A red shift is consistent with the positive volume coefficient of the absorption edge in these glasses V(δE/δV)T which predicts a decrease in bandgap energy with contraction [35]. In the present case however the large volume decrease is associated with a blue shift opposite to expectation based on an elastic change in density. This can be explained by considering that defects states have a contribution towards the optical edge shift that is far larger that the effect of density as recently suggested [26]. In the present glass, the large photoinduced relaxation process actually results in “healing” of structural defects and consequently leads to widening of the bandgap as defect electronic states are being erased from the bandtail. This unusual behavior further emphasizes the importance of the wide domain of structural configurations that is uniquely accessible in low coordination fragile glasses.
Fig. 7 (Color online) Band edge shift upon irradiation of GeAsSe13 glass: a) Annealed glass irradiated at 825 nm and 3.2 W/cm2; b) Quenched glass irradiated at 825 nm and 1.6 W/cm2; c) Saturation of photodarkening with time in annealed glass. d) Saturation of photobleaching with time in quenched glass. Lines in figure c) & d) are guides to the eye.
4. Discussion
Giant photocontraction has been previously reported in as-deposited thin films [34,36,37] as well as obliquely deposited thin films [38,39]. However in both cases the mechanism of the contraction process is fundamentally different from the phenomenon reported in this study. In particular the effect presented here is shown to be intrinsically reversible. In the case of as-deposited film the contraction is associated with photopolymerisation of molecular units present in the film structure [25,40]. As-deposited films typically have a molecular structure composed of As4S4 and Sn fragments [41], and it is shown that irradiation of these films induces a decrease in the number of homopolar bonds that corresponds to the irreversible crosslinking of these units [42]. In the case of obliquely deposited films the contraction is associated with the collapse of the columnar microstructure characteristic of these samples [27,28]. The photo-structural process induces the collapse of the film porosity and leads to irreversible photo-densification. Both processes are therefore non-reversible unlike the giant photocontraction observed in underconstrained fragile glasses. It must be noted that reversibility of the photocontraction is desirable in order to permit subsequent photoexpansion and to open the possibility for the design of optical structures with large contrast or lenses with complex shape. In addition, the design of optical devices requires glasses with optimal transparency. Melt quenched glasses have another advantage over the previously mentioned films in that they have a homogeneous structure that results in low scattering losses. In contrast, porous films and as-deposited films would display high attenuations due to scattering losses and would therefore be a poor choice for the design of quality optics.
Finally it should be emphasized that the low coordination of underconstrained glasses is key in allowing for large photo-structural effects. The floppy network can easily rearrange during irradiation and result in pronounced change in physical property on a local scale. The fragile nature of the glass accentuates that potential further by providing a wider range of stable configuration to explore. In terms of energy landscape each of this configuration correspond to mechanically stable “inherent structure” that generate a well defined minima on the energy surface [43,44]. Fragile glasses posses a higher density of these minima while strong glasses have fewer stable configurations and fewer minima [45,46]. In terms of covalent structure the strong glass correspond to an optimally constrained network with average coordination number <r> = 2.4. These glasses therefore undergo very moderate photostructural changes [12]. It is then interesting to point out that overconstrained glass with <r> > 2.4 are also expected to be fragile in nature and possess a wide density of potential configurations [18]. However these glasses also possess a high density of covalent bonds that cannot be easily photoexcited in sufficiently large fraction to introduce structural degrees of freedom [12]. Hence these glasses do not allow easy exploration of the energy landscape by photoexcitation and exhibit low photosensitivity. This is consistent with previous results of reversible photocontraction observed on an overconstrained GeAs4Se5 glass with <r> = 2.6 [33]. This glass is fragile and therefore posses a large driving force for relaxation that should result in contraction as observed. However the high bond density only allows for very little photostructural rearranagement and the photocontraction is limited to about 50Å as opposed to the micron scale contraction observed in underconstrained glasses. While both are reversible, only the underconstrained glass can undergo giant photocontraction.
5. Conclusion
It is shown that an appropriate choice of glass composition and irradiation conditions permits to induce giant photocontraction and giant photoexpansion selectively and reversibly in the same glass. This permits to combine expansion and contraction for the processing of complex optics on chalcogenide glass surfaces. Proper control of irradiation dosage in a fragile glassformer allows good contrast in photostructural changes. These effects result from optically tuning the local structural enthalpy; effectively altering the local glass density. The photostructural changes are the result of competing contributions from photoexcitation and photorelaxation. Since low coordination fragile glass former have wide propensity for relaxation, it is possible to induce large volume changes in these network glasses. Irradiation with low intensity of a glass with high enthalpy then results in large photocontraction while high intensity irradiation generates large photoexpansion.
Acknowledgement
This work was supported by DOE grant DE-FG52-06NA27501.
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