- Email: [email protected]
Fabrication of microlenses in bulk chalcogenide glass
1 July 1998
Optics Communications 152 Ž1998. 215–220
Fabrication of microlenses in bulk chalcogenide glass G. Beadie a b
a,)
, W.S. Rabinovich a , Jas Sanghera b, Ishwar Aggarwal
b
NaÕal Research Laboratory, Code 5642, 4555 OÕerlook AÕe., SW, Washington, DC 20375-5320, USA NaÕal Research Laboratory, Code 5606, 4555 OÕerlook AÕe., SW, Washington, DC 20375-5320, USA Received 14 November 1997; revised 24 March 1998; accepted 25 March 1998
Abstract A new method for fabricating microlenses in bulk, glassy Ge 20 Se 80 is demonstrated. Based upon a laser-induced thermal runaway effect, the process has been used to fabricate lenses with diameters between 70 and 180 mm and numerical apertures of about 0.4. q 1998 Elsevier Science B.V. All rights reserved. PACS: 42.82.Cr; 42.79.Bh; 42.70.Km; 42.62.Cf Keywords: Laser assisted thermal runaway; Microlenses; Chalcogenide glass
Recently, a thermal runaway method was demonstrated for fabricating micro-optical structures in semiconductordoped glasses w1x. This method takes advantage of a thermal nonlinearity in the material which allows low power laser irradiation to melt the substrate locally. The technique has been shown to provide a simple, versatile method for patterning both refractive and diffractive elements on semiconductor-doped glass ŽSDG. substrates, in which the laser acts as a ‘‘pen’’ for writing micro-relief structures w2,3x. Despite the simplicity of the method, microlens arrays with - 1% variation in focal length have been demonstrated w3x. Because the host glass of the SDGs is borosilicate in nature, the transmission window extends towards the infrared only out to 2 mm before absorption features begin to dominate the spectrum. Attention, however, has focused on the infrared regime, particularly in the atmospheric transmission bands of 3–5 mm and 8–12 mm. As a result of this attention, submillimeter-sized elements designed to transmit these wavelengths have become increasingly important. In an effort to extend the thermal runaway process to infrared-transmitting materials, the technique is applied here to bulk chalcogenide glasses. The chalcogenide glass system is comprised of glasses which contain the chalcogenides, e.g. sulfur, selenium, and
)
E-mail: [email protected]
tellurium. It is suited to this work because of the need for a glassy material in the fabrication process and because of the infrared transparency exhibited by these materials, which typically extends out past 10 mm. In addition, chalcogenide glasses have much higher indices of refraction than the borosilicates, ; 2.5 as compared to ; 1.5. Micro-optical structures have been fabricated previously in these materials w4–8x, but utilizing other methods and largely with thin films. The Ge-Se glass used here was produced by batching high purity Ge and Se in quartz ampoules inside a glove box. The amounts of material were chosen to produce Ge 20 Se 80 , which represents a Se-rich material as opposed to the stoichiometric network of GeSe 2 . The ampoules were evacuated to 10y5 torr and sealed using an oxygen-methane flame. The batch was melted at approximately 8508C for 8 hours in a rocking furnace to facilitate mixing. The ampoule was removed from the furnace, quenched in air, and annealed at the glass transition temperature Tg of 1508C. The 1 cm diameter glass rod was removed after cutting the quartz ampoule. From this rod, ; 1r2-mm-thick pieces of Ge 20 Se 80 were cut and polished with grit sizes down to 0.3 mm. To avoid curved surfaces due to tilting of the polishing mount, three samples were mounted and polished simultaneously. Polishing was made difficult by the tendency of the material to chip at the corners, resulting in the presence of visible scratches on the finished samples.
0030-4018r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 1 7 2 - 2
216
G. Beadie et al.r Optics Communications 152 (1998) 215–220
The fabrication setup is shown schematically in Fig. 1. The glan polarizer served as an attenuator, while the electronically-controlled shutter was used to block the beam after a lens formed. The photodiode was used to trigger the shutter. The laser operated at 800 nm because this wavelength is slightly longer than the onset of absorption in Ge 20 Se 80 , in accordance with the requirement of thermal runaway. The sample was placed 3.0 cm beyond the focal plane of the lens. At this point the 1re diameter of the intensity profile was 250 mm, while at the focal plane the 1re diameter was 78 mm. The sample was mounted at near-normal incidence. About 50 cm beyond the sample, a silicon photodiode with a small, sub-mm2 active area was mounted ; 1 cm away from the center of the transmitted laser beam axis. The placement of this photodiode was designed to take advantage of the dynamics that occur in the chalcogenide samples upon laser irradiation. Well before melting occurs in these materials, absorbed, near-bandgap light induces both photostructural and photothermal index changes w9,10x. As a result of these changes, the transmitted laser beam changes its shape. At low laser powers, - 65 mW Ž130 Wrcm2 ., the beam first contracts and then expands to a certain radius at which it remains stable under continued irradiation. At higher powers, 65–75 mW Ž130–150 Wrcm2 ., the transmitted beam expands to a certain radius, holds steady for a variable length of time, and then suddenly begins a quick expansion that signifies imminent surface damage. Surface damage is accompanied by the ejection of an orange vapor, presumably indicative of excess Se coming out of the Se-rich substrate. At incident powers greater than 75 mW, surface damage progresses rapidly, without a metastable state. It has been determined that lens formation occurs during a short time period between the onset of the transmitted beam’s rapid expansion and surface ablation. To determine when the rapid expansion starts, the photodiode is placed just outside of the metastable beam radius formed upon intermediate, 65–75 mW irradiation powers. The signal from this photodiode was set to trigger a timing circuit which, after a specified delay time, closed the shutter. The formation process is presumed to occur via laserinduced thermal runaway. Laser-induced heating causes
Fig. 1. Schematic of the setup used to create microlenses. The laser was a Ti:sapphire laser operated at 800 nm, P represents a glan polarizer, S an electronically controlled shutter, Laq10 cm focal length CaF2 lens, C the chalcogenide glass and PD a Si photodiode.
Fig. 2. Series of eight lenses fabricated using different delay times. The delay times in milliseconds for lenses 1–8 were, respectively: 140, 170, 200, 230, 350, 380, 410, and 440. The lenses are positioned on a 400 mm grid.
the temperature to rise above Tg . Thermal expansion occurs, creating excess volume which wells up out of the substrate. Upon termination of the laser excitation, rapid quenching allows the glass to retain the excess volume at room temperature. Evidence for the high temperatures required by thermal runaway is the substrate decomposition, which does not occur at room temperatures. Moreover, the sharp, threshold behavior of the formation process followed rapidly by ablation argues for a nonlinear, runaway process. Fig. 2 shows a white-light, transmission microscope image of microlenses fabricated successfully using this technique. These lenses were fabricated with delay times between 140 and 440 ms and an incident power of 70 mW. In practice, the relationship between delay times and fabrication progress depends sensitively on the placement of the photodiode with respect to the transmitted beam axis and on the photodiode signal level used to trigger the timing circuit. The time elapsed before the photodiode triggered the delay circuit was typically ; 30 s. In the figure, lenses are positioned on grid points spaced by 400 mm in each direction. Several notable features are visible in the image. First, it is clear that the size of the feature is correlated with the exposure time. The first lens is ; 70 mm in diameter. By 440 ms, the lens Žinner feature. has a diameter of 167 mm. It is also clear that defects in the substrate affect the growth process. Lenses 6 and 7 each terminate at substrate defects, represented by the dark patches in the image. Moreover, the two empty grid points contain substrate defects that scattered the transmitted laser beam enough that the photodiode was unable to properly monitor the formation process. A careful analysis of the dependence of feature size upon exposure time will require a more homogeneous substrate. Another observation drawn from Fig. 2 is that rings are visible around the lenses. In the image, shading makes these areas look like depressions in the substrate. Stylus profiler measurements, however, demonstrate that this is not the case. Therefore, it is assumed that the rings are an optical effect of the transmission of light through regions of the substrate that have become altered by the lens
G. Beadie et al.r Optics Communications 152 (1998) 215–220
fabrication process. It is well known that near-bandgap light induces strong photostructural changes in these materials, resulting in a local change of the bandgap energy w11x. This would, in turn, alter the index of refraction for near-IR light, such as that detected by the CCD detector. It is presumed that the material in the ring areas were exposed to sufficient light to induce this photodarkening effect, but insufficient light to induce melting. This phenomenon: Ži. hinders close-packed array fabrication and Žii. aberrates the transmitted wavefront. Close-packing is made difficult because laser light is refracted nonuniformly when illuminating regions close to a lens. This makes monitoring of the transmitted beam dynamics difficult, as well as depositing light energy unevenly within the substrate. The wavefront aberration is obvious in the image. It is possible that the aberration would be mitigated for wavelengths in the transmission region. Kramers-Kronig relations argue that if the index change D n is due solely to an absorption edge shift, D n for wavelengths near the bandgap is expected to be much greater than for wavelengths in the transmission window. There are reported results, however, which show that D n for at least some chalcogenide glasses extends unchanged throughout the transparency regime w12x. A third observation is that a crescent-shaped crack, which extends into the substrate and under the lens, is starkly outlined around lens 6. This crack demonstrates the strains present near the lens boundaries within the glass material. These cracks were prevalent in the chalcogenide glass but not observed in the previous work on SDGs. This may well be attributed to thermal shock, since the chalcogenide glasses possess coefficients of thermal expansion of about twice that of borosilicate glasses. It should also be mentioned that these cracks generally appeared in conjunction with substrate defects or with lenses exposed for longer times. In certain instances, cracks appeared well after lens fabrication, in response to handling of the substrate. These observations suggest the possibility that cracking would be reduced by better-quality substrate material or by a thermal anneal process which would allow the strains to relax somewhat. To obtain a measure of the surface quality of the microlenses, as well as to verify that the rings surrounding the features are optical rather than structural in nature, profiles of lenses were obtained with a stylus profiler. The data taken from a 150-mm-diameter lens are presented in Fig. 3. The height of this lens is measured to be 6.5 mm, and the radius of curvature R 170 mm. Geometric optics predicts the focal length in air of such a lens to be RrŽ n y 1., which is equal to 113 mm for the index of refraction n s 2.5 for Ge 20 Se 80 wRef. w13xx. To calculate the numerical aperture ŽNA. of the lens, a value must be chosen for the effective radius of the lens. Because the surface profile diverges from the fit at the outer edges of the feature, it would not be accurate to use the full radius of the lens as the lens aperture. To estimate the NA, the
217
Fig. 3. Stylus profile data of a microlens, shown as the solid line. Note the absence of any depression on either side of the lens feature, as appears in the white-light transmission images. The dashed line is a least-squares spherical fit to the curvature of the lens, which gives a value of 170 mm as the radius of curvature.
point at which the fitted curve goes to zero will be used to calculate the effective radius of the lens. Using this radius, 47.3 mm, the NA is calculated to be 0.41. The profiler data can also be used to estimate the maximum temperature achieved in the runaway process. Dilatometer measurements of Ge 20 Se 80 show the thermal expansion coefficient to be 19 ppmrK below Tg and 68 ppmrK above Tg . Assuming that the excess volume of the lens is due to expansion of a volume which extended ; 300 mm into the substrate, one calculates the necessary temperature of 2608C required to produce the lens. As a check on the consistency of this value, the amount of energy required to heat the material to that temperature is estimated to be 1 mJ. This is less than a fourth of the energy deposited into the volume during the 30 s exposure to the 70 mW light. To demonstrate that the structures serve as lenses, a microlens array with an average diameter of 160 mm was fabricated wsee Fig. 4Ža.x. The diameters of these six lenses vary by "5.7%. Fig. 4Žb. depicts the image plane of the array illuminated from the back by a laser diode, captured by an optical relay system on a CCD camera. As is shown in the figure, transmitted light is focused into small spots in the image plane of the lenses. Quantitative measurements were not possible, because the numerical aperture of the optical relay system was smaller than that of the lenses; it was unable to capture all the light focused by the microlenses. What measurements could be made include a crude estimate of the focal length, at 1r6 mm, and an upper limit on the 1re diameter of one of the better-resolved focal spots, 12 mm. A diffraction limited spot for a lens of aperture of 160 mm and focal length 1r6 mm using 850 nm light would be 2.0 mm. If the focal length of the lenses is taken to be 1r6 mm, then the numerical aperture would be 0.43, which is in good agreement with the figure calculated from surface profile measurements. Though it is true that the shape of the profile deviates from a spherical curve, particularly towards the edge of the lens, it should be mentioned that similar profiles in SDG systems empirically demonstrated diffraction-limited focusing capability w1x.
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G. Beadie et al.r Optics Communications 152 (1998) 215–220
Fig. 4. ŽA. White-light transmission image of a 3 = 2 array of microlenses. ŽB. Image plane of the same array illuminated from the back by a laser diode operated at 850 nm. Bright areas in the middle of the features demonstrate light focused due to the microlenses, while the ring structures demonstrate light diffracted as a result of the photoinduced index variations that surround each lens.
We have made preliminary efforts to extend the fabrication parameters. These include using a smaller beam in order to fabricate smaller lenses, changing the wavelength of excitation, fabricating microlenses in vacuum, and using different substrates. Efforts to use a smaller beam have proven difficult because of substrate cracking. So far, each lens attempted with smaller write beams has demonstrated this failure mode. Going to wavelengths longer than 800 nm, while holding the spot size at 250 mm, demonstrates that beyond 820 nm microlens fabrication is no longer possible with incident powers F 100 mW.
The other efforts attempted to address the problem of substrate volatility. In one experiment, the sample was placed in vacuum. By removing air away from the surface of the glass, chemical reactions between gas molecules and heated glass are prevented. In addition, heat removal due to air convection is also removed, further simplifying the dynamic variables. The experiment demonstrated, however, that neither the threshold intensity nor the tendency towards decomposition changed. This suggests that the thermal and chemical effects of air at the surface are negligible in microlens formation.
G. Beadie et al.r Optics Communications 152 (1998) 215–220
We also attempted fabrication of microlenses on a ; 5 mm thick piece of As 2 Se 3 in air. As 2 Se 3 was chosen as the substrate because of its stoichiometric composition and because its bandedge fell within the range of the Ti:sapphire laser. The sample was exposed to radiation at 910 nm, a wavelength determined to be near the onset of absorption. Decomposition was observed to occur even in this stoichiometric material, however, though the ; 2.8 kWrcm2 threshold intensity for the process was over 19 times higher than that for Ge 20 Se 80 . It is instructive to compare this work to that of Hisakuni and Tanaka w4x, in which the authors also report laser-induced microlens formation in a chalcogenide glass, As 2 S 3 , by sub-bandgap illumination. The authors propose that the formation is due to an athermal, photoexpansion process w14x. There are several similarities between that work and this report. Despite these similarities, it is claimed that the process reported here is due to thermal effects, where the elevated temperatures are reached via a thermal runaway process. Empirically, this claim is supported by the observation of substrate decomposition, which does not occur near room temperature. In addition, the temperature dependence of the absorption coefficient at 800 nm is presented in Fig. 5. This demonstrates a clear, monotonic increase of the absorption coefficient as a function of temperature above 358C. It is this signature dependence of the absorption upon temperature that enables thermal runaway. Furthermore, it is not believed that the thermal runaway is reached after and as a result of photoexpansion-induced lens formation. It could be imagined that the lenses formed via the non-thermal photoexpansion process and subsequently served as a focusing agent for the incident laser beam, causing thermal runaway by tightly concentrating light within the substrate. An examination of Figs. 2 and 3 of Ref. w14x, however, reveals that even after 2000 s of illumination at the intensity level and for the geometry reported here, the maximum height is expected to be only 5 nm Ž D LrL ; 10y5 .. This value is three orders of magnitude lower than that observed here. While it is true that photoexpansion, were it to exist in Ge 20 Se 80 , would be expected to have different parameters than for that in As 2 S 3 , it would appear unlikely to account for a ; 10 3 difference in D LrL under similar exposure conditions. Thus, it is assumed that photoexpansion is not responsible for lens formation. It should be pointed out that the authors of Ref. w14x make no mention of sample decomposition, despite their high excitation intensities and long exposure times. There are at least two possible explanations for the absence of thermal runaway: Ži. the chalcogenide films may have been adhered to a thermally conductive substrate and Žii. the runaway threshold in As 2 S 3 is simply greater than 10 4 Wrcm2. Thermal runaway requires that the rate of heat deposition is greater than the rate of heat removal, so the presence of a thermally conductive substrate would increase the threshold laser power. With regards to Žii., it
219
Fig. 5. Plot of Ge 20 Se 80 absorption coefficient at 800 nm versus temperature.
was observed here that the runaway threshold for As 2 Se 3 was ; 10 3 Wrcm2. It may be that the threshold for As 2 S 3 is even greater. It is near-certain, however, that the results reported in Ref. w14x are not due to thermal runaway. Even if the As 2 S 3 were to avoid decomposition while above Tg , as in the case of the doped borosilicate glasses, previous work has shown the runaway process to have a sharp threshold for lens fabrication. This is not consistent with the observations of Ref. w14x, wherein the fabrication of lenses over two orders of magnitude in laser intensity is discussed. In summary, microlenses have been fabricated in bulk Ge 20 Se 80 glass. While acknowledging the existence of other optically-induced processes in this material, it is claimed that microlens fabrication occurs via a laser-induced thermal runaway process. Lenses with diameters between 70 and 170 mm have been observed, with numerical apertures estimated to be 0.4. As opposed to fabrication in semiconductor-doped borosilicate glasses, fabrication in chalcogenide glasses leads directly to destructive sample decomposition. With optically homogeneous glass material, however, the fabrication process can be controlled sufficiently to avoid damage and yield reproducible optical structures for use in the infrared region of the spectrum.
Acknowledgements The authors thank S. Bayya for the dilatometer measurements. The authors also acknowledge the support of the U.S. Office of Naval Research and one of us, G.B., the support of a National Research Council – NRL Research Associateship for this work.
References w1x G. Beadie, N.M. Lawandy, Optics Lett. 20 Ž1995. 2153. w2x A.Y. Smuk, N.M. Lawandy, Optics Lett. 22 Ž1997. 1030. w3x M. Fritze, M.B. Stern, P.W. Wyatt, Optics Lett. 23 Ž1998. 141. w4x H. Hisakuni, K. Tanaka, Optics Lett. 20 Ž1995. 958.
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w5x N.P. Eisenberg, M. Manevich, M. Kebanov, V. Lyubin, S. Shtutina, J. Non-Cryst. Solids 198–200 Ž1996. 766. w6x K. Tanaka, Appl. Phys. Lett. 70 Ž1997. 261. w7x S. Ramachandran, S.G. Bishop, J.P. Guo, D.J. Brady, IEEE Photonics Technol. Lett. 8 Ž1996. 1041. w8x T.V. Galstyan, J.-F. Viens, A. Villeneuve, M.A. Duguay, Proc. Conf. on Lasers and Electro-Optics, Technical Digest Series, Vol. 11, 1997, p. 492. w9x J.P. DeNeufville, in: B.O. Seraphin ŽEd.., Optical Properties of Solids-New Developments, North-Holland, Amsterdam, 1975, p. 437.
w10x A.E. Owen, A.P. Firthg, P.J.S. Ewen, Philos. Mag. B 52 Ž1985. 347. w11x J.P. DeNeufville, S.C. Moss, S.R. Ovshinsky, J. Non-Cryst. Solids 13 Ž1973. 191. w12x Y. Utsugi, Y. Mizushima, Jpn. J. Appl. Phys. 31 Ž1992. 3922. w13x E. Hajto, P.J.S. Ewen, A.E. Owen, J. Non-Cryst. Solids 164–166 Ž1993. 901. w14x H. Hisakuni, K. Tanaka, Appl. Phys. Lett. 65 Ž1994. 2925.
Optics Communications 152 Ž1998. 215–220
Fabrication of microlenses in bulk chalcogenide glass G. Beadie a b
a,)
, W.S. Rabinovich a , Jas Sanghera b, Ishwar Aggarwal
b
NaÕal Research Laboratory, Code 5642, 4555 OÕerlook AÕe., SW, Washington, DC 20375-5320, USA NaÕal Research Laboratory, Code 5606, 4555 OÕerlook AÕe., SW, Washington, DC 20375-5320, USA Received 14 November 1997; revised 24 March 1998; accepted 25 March 1998
Abstract A new method for fabricating microlenses in bulk, glassy Ge 20 Se 80 is demonstrated. Based upon a laser-induced thermal runaway effect, the process has been used to fabricate lenses with diameters between 70 and 180 mm and numerical apertures of about 0.4. q 1998 Elsevier Science B.V. All rights reserved. PACS: 42.82.Cr; 42.79.Bh; 42.70.Km; 42.62.Cf Keywords: Laser assisted thermal runaway; Microlenses; Chalcogenide glass
Recently, a thermal runaway method was demonstrated for fabricating micro-optical structures in semiconductordoped glasses w1x. This method takes advantage of a thermal nonlinearity in the material which allows low power laser irradiation to melt the substrate locally. The technique has been shown to provide a simple, versatile method for patterning both refractive and diffractive elements on semiconductor-doped glass ŽSDG. substrates, in which the laser acts as a ‘‘pen’’ for writing micro-relief structures w2,3x. Despite the simplicity of the method, microlens arrays with - 1% variation in focal length have been demonstrated w3x. Because the host glass of the SDGs is borosilicate in nature, the transmission window extends towards the infrared only out to 2 mm before absorption features begin to dominate the spectrum. Attention, however, has focused on the infrared regime, particularly in the atmospheric transmission bands of 3–5 mm and 8–12 mm. As a result of this attention, submillimeter-sized elements designed to transmit these wavelengths have become increasingly important. In an effort to extend the thermal runaway process to infrared-transmitting materials, the technique is applied here to bulk chalcogenide glasses. The chalcogenide glass system is comprised of glasses which contain the chalcogenides, e.g. sulfur, selenium, and
)
E-mail: [email protected]
tellurium. It is suited to this work because of the need for a glassy material in the fabrication process and because of the infrared transparency exhibited by these materials, which typically extends out past 10 mm. In addition, chalcogenide glasses have much higher indices of refraction than the borosilicates, ; 2.5 as compared to ; 1.5. Micro-optical structures have been fabricated previously in these materials w4–8x, but utilizing other methods and largely with thin films. The Ge-Se glass used here was produced by batching high purity Ge and Se in quartz ampoules inside a glove box. The amounts of material were chosen to produce Ge 20 Se 80 , which represents a Se-rich material as opposed to the stoichiometric network of GeSe 2 . The ampoules were evacuated to 10y5 torr and sealed using an oxygen-methane flame. The batch was melted at approximately 8508C for 8 hours in a rocking furnace to facilitate mixing. The ampoule was removed from the furnace, quenched in air, and annealed at the glass transition temperature Tg of 1508C. The 1 cm diameter glass rod was removed after cutting the quartz ampoule. From this rod, ; 1r2-mm-thick pieces of Ge 20 Se 80 were cut and polished with grit sizes down to 0.3 mm. To avoid curved surfaces due to tilting of the polishing mount, three samples were mounted and polished simultaneously. Polishing was made difficult by the tendency of the material to chip at the corners, resulting in the presence of visible scratches on the finished samples.
0030-4018r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 1 7 2 - 2
216
G. Beadie et al.r Optics Communications 152 (1998) 215–220
The fabrication setup is shown schematically in Fig. 1. The glan polarizer served as an attenuator, while the electronically-controlled shutter was used to block the beam after a lens formed. The photodiode was used to trigger the shutter. The laser operated at 800 nm because this wavelength is slightly longer than the onset of absorption in Ge 20 Se 80 , in accordance with the requirement of thermal runaway. The sample was placed 3.0 cm beyond the focal plane of the lens. At this point the 1re diameter of the intensity profile was 250 mm, while at the focal plane the 1re diameter was 78 mm. The sample was mounted at near-normal incidence. About 50 cm beyond the sample, a silicon photodiode with a small, sub-mm2 active area was mounted ; 1 cm away from the center of the transmitted laser beam axis. The placement of this photodiode was designed to take advantage of the dynamics that occur in the chalcogenide samples upon laser irradiation. Well before melting occurs in these materials, absorbed, near-bandgap light induces both photostructural and photothermal index changes w9,10x. As a result of these changes, the transmitted laser beam changes its shape. At low laser powers, - 65 mW Ž130 Wrcm2 ., the beam first contracts and then expands to a certain radius at which it remains stable under continued irradiation. At higher powers, 65–75 mW Ž130–150 Wrcm2 ., the transmitted beam expands to a certain radius, holds steady for a variable length of time, and then suddenly begins a quick expansion that signifies imminent surface damage. Surface damage is accompanied by the ejection of an orange vapor, presumably indicative of excess Se coming out of the Se-rich substrate. At incident powers greater than 75 mW, surface damage progresses rapidly, without a metastable state. It has been determined that lens formation occurs during a short time period between the onset of the transmitted beam’s rapid expansion and surface ablation. To determine when the rapid expansion starts, the photodiode is placed just outside of the metastable beam radius formed upon intermediate, 65–75 mW irradiation powers. The signal from this photodiode was set to trigger a timing circuit which, after a specified delay time, closed the shutter. The formation process is presumed to occur via laserinduced thermal runaway. Laser-induced heating causes
Fig. 1. Schematic of the setup used to create microlenses. The laser was a Ti:sapphire laser operated at 800 nm, P represents a glan polarizer, S an electronically controlled shutter, Laq10 cm focal length CaF2 lens, C the chalcogenide glass and PD a Si photodiode.
Fig. 2. Series of eight lenses fabricated using different delay times. The delay times in milliseconds for lenses 1–8 were, respectively: 140, 170, 200, 230, 350, 380, 410, and 440. The lenses are positioned on a 400 mm grid.
the temperature to rise above Tg . Thermal expansion occurs, creating excess volume which wells up out of the substrate. Upon termination of the laser excitation, rapid quenching allows the glass to retain the excess volume at room temperature. Evidence for the high temperatures required by thermal runaway is the substrate decomposition, which does not occur at room temperatures. Moreover, the sharp, threshold behavior of the formation process followed rapidly by ablation argues for a nonlinear, runaway process. Fig. 2 shows a white-light, transmission microscope image of microlenses fabricated successfully using this technique. These lenses were fabricated with delay times between 140 and 440 ms and an incident power of 70 mW. In practice, the relationship between delay times and fabrication progress depends sensitively on the placement of the photodiode with respect to the transmitted beam axis and on the photodiode signal level used to trigger the timing circuit. The time elapsed before the photodiode triggered the delay circuit was typically ; 30 s. In the figure, lenses are positioned on grid points spaced by 400 mm in each direction. Several notable features are visible in the image. First, it is clear that the size of the feature is correlated with the exposure time. The first lens is ; 70 mm in diameter. By 440 ms, the lens Žinner feature. has a diameter of 167 mm. It is also clear that defects in the substrate affect the growth process. Lenses 6 and 7 each terminate at substrate defects, represented by the dark patches in the image. Moreover, the two empty grid points contain substrate defects that scattered the transmitted laser beam enough that the photodiode was unable to properly monitor the formation process. A careful analysis of the dependence of feature size upon exposure time will require a more homogeneous substrate. Another observation drawn from Fig. 2 is that rings are visible around the lenses. In the image, shading makes these areas look like depressions in the substrate. Stylus profiler measurements, however, demonstrate that this is not the case. Therefore, it is assumed that the rings are an optical effect of the transmission of light through regions of the substrate that have become altered by the lens
G. Beadie et al.r Optics Communications 152 (1998) 215–220
fabrication process. It is well known that near-bandgap light induces strong photostructural changes in these materials, resulting in a local change of the bandgap energy w11x. This would, in turn, alter the index of refraction for near-IR light, such as that detected by the CCD detector. It is presumed that the material in the ring areas were exposed to sufficient light to induce this photodarkening effect, but insufficient light to induce melting. This phenomenon: Ži. hinders close-packed array fabrication and Žii. aberrates the transmitted wavefront. Close-packing is made difficult because laser light is refracted nonuniformly when illuminating regions close to a lens. This makes monitoring of the transmitted beam dynamics difficult, as well as depositing light energy unevenly within the substrate. The wavefront aberration is obvious in the image. It is possible that the aberration would be mitigated for wavelengths in the transmission region. Kramers-Kronig relations argue that if the index change D n is due solely to an absorption edge shift, D n for wavelengths near the bandgap is expected to be much greater than for wavelengths in the transmission window. There are reported results, however, which show that D n for at least some chalcogenide glasses extends unchanged throughout the transparency regime w12x. A third observation is that a crescent-shaped crack, which extends into the substrate and under the lens, is starkly outlined around lens 6. This crack demonstrates the strains present near the lens boundaries within the glass material. These cracks were prevalent in the chalcogenide glass but not observed in the previous work on SDGs. This may well be attributed to thermal shock, since the chalcogenide glasses possess coefficients of thermal expansion of about twice that of borosilicate glasses. It should also be mentioned that these cracks generally appeared in conjunction with substrate defects or with lenses exposed for longer times. In certain instances, cracks appeared well after lens fabrication, in response to handling of the substrate. These observations suggest the possibility that cracking would be reduced by better-quality substrate material or by a thermal anneal process which would allow the strains to relax somewhat. To obtain a measure of the surface quality of the microlenses, as well as to verify that the rings surrounding the features are optical rather than structural in nature, profiles of lenses were obtained with a stylus profiler. The data taken from a 150-mm-diameter lens are presented in Fig. 3. The height of this lens is measured to be 6.5 mm, and the radius of curvature R 170 mm. Geometric optics predicts the focal length in air of such a lens to be RrŽ n y 1., which is equal to 113 mm for the index of refraction n s 2.5 for Ge 20 Se 80 wRef. w13xx. To calculate the numerical aperture ŽNA. of the lens, a value must be chosen for the effective radius of the lens. Because the surface profile diverges from the fit at the outer edges of the feature, it would not be accurate to use the full radius of the lens as the lens aperture. To estimate the NA, the
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Fig. 3. Stylus profile data of a microlens, shown as the solid line. Note the absence of any depression on either side of the lens feature, as appears in the white-light transmission images. The dashed line is a least-squares spherical fit to the curvature of the lens, which gives a value of 170 mm as the radius of curvature.
point at which the fitted curve goes to zero will be used to calculate the effective radius of the lens. Using this radius, 47.3 mm, the NA is calculated to be 0.41. The profiler data can also be used to estimate the maximum temperature achieved in the runaway process. Dilatometer measurements of Ge 20 Se 80 show the thermal expansion coefficient to be 19 ppmrK below Tg and 68 ppmrK above Tg . Assuming that the excess volume of the lens is due to expansion of a volume which extended ; 300 mm into the substrate, one calculates the necessary temperature of 2608C required to produce the lens. As a check on the consistency of this value, the amount of energy required to heat the material to that temperature is estimated to be 1 mJ. This is less than a fourth of the energy deposited into the volume during the 30 s exposure to the 70 mW light. To demonstrate that the structures serve as lenses, a microlens array with an average diameter of 160 mm was fabricated wsee Fig. 4Ža.x. The diameters of these six lenses vary by "5.7%. Fig. 4Žb. depicts the image plane of the array illuminated from the back by a laser diode, captured by an optical relay system on a CCD camera. As is shown in the figure, transmitted light is focused into small spots in the image plane of the lenses. Quantitative measurements were not possible, because the numerical aperture of the optical relay system was smaller than that of the lenses; it was unable to capture all the light focused by the microlenses. What measurements could be made include a crude estimate of the focal length, at 1r6 mm, and an upper limit on the 1re diameter of one of the better-resolved focal spots, 12 mm. A diffraction limited spot for a lens of aperture of 160 mm and focal length 1r6 mm using 850 nm light would be 2.0 mm. If the focal length of the lenses is taken to be 1r6 mm, then the numerical aperture would be 0.43, which is in good agreement with the figure calculated from surface profile measurements. Though it is true that the shape of the profile deviates from a spherical curve, particularly towards the edge of the lens, it should be mentioned that similar profiles in SDG systems empirically demonstrated diffraction-limited focusing capability w1x.
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Fig. 4. ŽA. White-light transmission image of a 3 = 2 array of microlenses. ŽB. Image plane of the same array illuminated from the back by a laser diode operated at 850 nm. Bright areas in the middle of the features demonstrate light focused due to the microlenses, while the ring structures demonstrate light diffracted as a result of the photoinduced index variations that surround each lens.
We have made preliminary efforts to extend the fabrication parameters. These include using a smaller beam in order to fabricate smaller lenses, changing the wavelength of excitation, fabricating microlenses in vacuum, and using different substrates. Efforts to use a smaller beam have proven difficult because of substrate cracking. So far, each lens attempted with smaller write beams has demonstrated this failure mode. Going to wavelengths longer than 800 nm, while holding the spot size at 250 mm, demonstrates that beyond 820 nm microlens fabrication is no longer possible with incident powers F 100 mW.
The other efforts attempted to address the problem of substrate volatility. In one experiment, the sample was placed in vacuum. By removing air away from the surface of the glass, chemical reactions between gas molecules and heated glass are prevented. In addition, heat removal due to air convection is also removed, further simplifying the dynamic variables. The experiment demonstrated, however, that neither the threshold intensity nor the tendency towards decomposition changed. This suggests that the thermal and chemical effects of air at the surface are negligible in microlens formation.
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We also attempted fabrication of microlenses on a ; 5 mm thick piece of As 2 Se 3 in air. As 2 Se 3 was chosen as the substrate because of its stoichiometric composition and because its bandedge fell within the range of the Ti:sapphire laser. The sample was exposed to radiation at 910 nm, a wavelength determined to be near the onset of absorption. Decomposition was observed to occur even in this stoichiometric material, however, though the ; 2.8 kWrcm2 threshold intensity for the process was over 19 times higher than that for Ge 20 Se 80 . It is instructive to compare this work to that of Hisakuni and Tanaka w4x, in which the authors also report laser-induced microlens formation in a chalcogenide glass, As 2 S 3 , by sub-bandgap illumination. The authors propose that the formation is due to an athermal, photoexpansion process w14x. There are several similarities between that work and this report. Despite these similarities, it is claimed that the process reported here is due to thermal effects, where the elevated temperatures are reached via a thermal runaway process. Empirically, this claim is supported by the observation of substrate decomposition, which does not occur near room temperature. In addition, the temperature dependence of the absorption coefficient at 800 nm is presented in Fig. 5. This demonstrates a clear, monotonic increase of the absorption coefficient as a function of temperature above 358C. It is this signature dependence of the absorption upon temperature that enables thermal runaway. Furthermore, it is not believed that the thermal runaway is reached after and as a result of photoexpansion-induced lens formation. It could be imagined that the lenses formed via the non-thermal photoexpansion process and subsequently served as a focusing agent for the incident laser beam, causing thermal runaway by tightly concentrating light within the substrate. An examination of Figs. 2 and 3 of Ref. w14x, however, reveals that even after 2000 s of illumination at the intensity level and for the geometry reported here, the maximum height is expected to be only 5 nm Ž D LrL ; 10y5 .. This value is three orders of magnitude lower than that observed here. While it is true that photoexpansion, were it to exist in Ge 20 Se 80 , would be expected to have different parameters than for that in As 2 S 3 , it would appear unlikely to account for a ; 10 3 difference in D LrL under similar exposure conditions. Thus, it is assumed that photoexpansion is not responsible for lens formation. It should be pointed out that the authors of Ref. w14x make no mention of sample decomposition, despite their high excitation intensities and long exposure times. There are at least two possible explanations for the absence of thermal runaway: Ži. the chalcogenide films may have been adhered to a thermally conductive substrate and Žii. the runaway threshold in As 2 S 3 is simply greater than 10 4 Wrcm2. Thermal runaway requires that the rate of heat deposition is greater than the rate of heat removal, so the presence of a thermally conductive substrate would increase the threshold laser power. With regards to Žii., it
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Fig. 5. Plot of Ge 20 Se 80 absorption coefficient at 800 nm versus temperature.
was observed here that the runaway threshold for As 2 Se 3 was ; 10 3 Wrcm2. It may be that the threshold for As 2 S 3 is even greater. It is near-certain, however, that the results reported in Ref. w14x are not due to thermal runaway. Even if the As 2 S 3 were to avoid decomposition while above Tg , as in the case of the doped borosilicate glasses, previous work has shown the runaway process to have a sharp threshold for lens fabrication. This is not consistent with the observations of Ref. w14x, wherein the fabrication of lenses over two orders of magnitude in laser intensity is discussed. In summary, microlenses have been fabricated in bulk Ge 20 Se 80 glass. While acknowledging the existence of other optically-induced processes in this material, it is claimed that microlens fabrication occurs via a laser-induced thermal runaway process. Lenses with diameters between 70 and 170 mm have been observed, with numerical apertures estimated to be 0.4. As opposed to fabrication in semiconductor-doped borosilicate glasses, fabrication in chalcogenide glasses leads directly to destructive sample decomposition. With optically homogeneous glass material, however, the fabrication process can be controlled sufficiently to avoid damage and yield reproducible optical structures for use in the infrared region of the spectrum.
Acknowledgements The authors thank S. Bayya for the dilatometer measurements. The authors also acknowledge the support of the U.S. Office of Naval Research and one of us, G.B., the support of a National Research Council – NRL Research Associateship for this work.
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