Open Access
Add to BibSonomyAbstract
Development of micro-components for IR integrated optic devices requires the elaboration of IR waveguides. It is shown that amorphous chalcogenide films from the Ge-Se-Te system are well suited to such development. Thermal and optical characteristics of films elaborated by thermal co-evaporation are first measured. The Se-rich (> 60 at. %) region with a Ge content of about 25 at. % comprises films with a vitreous transition temperature, Tg, larger than 400K, a high thermal stability (ΔT > 100K) and a well-controlled refractive index, n, owing to a weak dependence of n with composition in this region. Films in this composition region are then profitably used to develop optical structures, such as straight or S-bend waveguides, spirals, Y-junction or Mach-Zehnder interferometer, by stacking and further etching of the films. The transmission region accessible to these structures lies from telecommunication wavelength up to 16-17 µm. When a higher transmission region is required, the use of pure Ge-Te films is mandatory. A modal filter allowing a light rejection efficiency of 6.10−5 to be a part of a spatial interferometer is then elaborated.
© 2014 Optical Society of America
1. Introduction
Over the last few decades, considerable attention has been focused on amorphous semiconductors, and in particular on chalcogenide glasses which have emerged as multipurpose materials. Their threshold and memory switching behavior, their high Kerr non linearities as well as their remarkable infrared transmission make them potential materials of several industrial applications. For example, they are considered for the development of IR detectors [1–3], resonators [4], photonic crystal cavities [5], electronic and optical switches [6] or optical recording media [7–10]. The main achievements led to commercialized objects, such as lenses (GASIR lenses from Umicore), IR cameras (THALES ELVIR), rewritable CD or DVD (C-RAM: Chalcogenide – Random Access Memory).
More recently, chalcogenide glasses, and telluride glasses in particular, were proposed for the development of devices for other types of infrared applications, such as spatial interferometry [11–15], bio-sensing [16–19] or environmental metrology [20, 21].
Among the possible class of chalcogenide materials that can be used for the development of these devices, Ge-Se-Te glasses are particularly attractive. The presence of both Se and Te in variable quantities makes this class a very versatile one in terms of properties. Se, at the opposite of Te, has a high glass forming ability. The presence of Te in the glass composition can help in enlarging the transmission window towards the far IR region [22]. Ge in turn can be useful to increase the glass transition temperature and therefore the domain of temperatures which the glass can be used in. In the past, the glass forming domain and thermal properties of these ternary glasses were investigated in detail [23]. The work mainly concerned bulk glasses and a few films prepared by RF-sputtering. Later on, few papers reported scarce data, e.g. some characteristic temperatures and optical parameters, on Ge-Se-Te thin films deposited by conventional thermal evaporation [24–28]. In terms of applications, Te-rich Ge-Se glasses were considered for the realization of CO2 laser power transmitting glass fibers [29] and for the realization of CO2 sensors [30]. Kerr non-linearities hundreds of times larger than those observed in silica were obtained in a Ge25Se65Te10 bulk glass [31]. Recently, these glasses were considered for the development of phase change random access memories [32–34].
The above considerations show the interest of the Ge-Se-Te chalcogenide family for the development of IR optical components. The paper focuses on investigations which final goal aims the development of miniaturized components for IR integrated optics.
First, the choice of the most pertinent compositions to develop components for IR integrated optics, was made on the basis of an investigation of a wide range of amorphous Ge-Te-Se films deposited by thermal co-evaporation. Both thermal and optical characterization helped in this task. Preliminary data covering narrow domains of the ternary region were reported in previous papers [35–38].
Infrared channel waveguides e.g. rib waveguides, S-bend or spiral waveguides, Y-junctions and Mach-Zehnder interferometers, were then elaborated by original ways including films stacking, laser lithography and ion beam etching.
Finally, the developed components were optically tested. In particular, the successful elaboration of a Ge-Te based modal filter working in a wide transmission window (6-20 µm) and exhibiting a light rejection efficiency of 6.10−5 was demonstrated.
2. Experimental details
Film deposition. Ge-Se-Te films were deposited by thermal co-evaporation using a PLASSYS MEB 500 device, equipped with two current induced heated sources and an electron beam evaporator which were placed in a specific configuration allowing the deposition of films with uniform composition and thickness over a surface of about 4 cm in diameter [39, 40]. The two current induced heated sources were used to evaporate tellurium and selenium or tellurium alone depending on the deposited film, i.e. a ternary Ge-Se-Te film or a binary Ge-Te one [40]. The ultra-pure powders of Te and Se (Aldrich-99.999%) were placed in dedicated homemade carbon crucibles which were inserted in molybdenum nacelles and covered with perforated molybdenum foils. The ultra-pure chips of Ge (Aldrich-99.999%) were placed in a copper crucible and evaporated by electron beam bombardment. Three types of substrates were used: microscope slides, wafers of silicon and Te75Ge15Ga10 bulk glasses (TGG from the Institut des Sciences Chimiques de Rennes at the Université de Rennes 1, in France). Microscope slides and TGG were cleaned with ethanol and dried with dry air while wafers of silicon were stripped with a solution of 20% HF in order to eliminate native oxide layer. Before deposition, the chamber was evacuated down to around 10−5 Pa in order to avoid ambient contamination. During the deposition process, the substrate holder was rotated and heated at around 70 °C by using hot water circulation. The deposition rates and thicknesses of the three elements were automatically controlled with pre-calibrated quartz crystal monitors. When the elemental deposition rates were stable, the film deposition was initiated. The deposition was then stopped when the film thickness reached at least 5 µm. A typical thermal co-evaporation rate of 4200 Å/min allows the deposition of a 5-µm film in 12 min.
Film characterization. The film thickness was checked with a DEKTAK 3 Veeco mechanical profilometer. The film adhesion was tested by the classical adhesive tape test. The amorphous or crystalline character of the films was controlled by X-ray diffraction using a PANalytical XPERT diffractometer; a Cu (kα) source (λ = 1.5406 Å), an operating voltage of 40 kV and a beam current of 30-40 mA were used for excitation. The film composition was determined by Energy-Dispersive X-ray Spectroscopy using a JEOL JEM2100F instrument. The thermal properties of the films were analyzed by Differential Scanning Calorimetry (DSC) using a METTLER TOLEDO DSC30. A heating ramp of 10°C.min−1 was applied. The optical transmittance spectra of the films, further used to measure the film refractive index (n) and optical band gap energy (Eg), were recorded using a UV-vis-NIR spectrophotometer (VARIAN Cary 5000) in the range 500-2700 nm, through a circular mask (diameter Ø = 5 mm). Finally, a Bruker IFS 66v/s spectrometer was used to determinate the multi-phonon cut-off of the films in the mid-infrared region.
Waveguide fabrication. The Ge-Se-Te waveguides were elaborated by stacking two Ge-Se-Te films onto a silicon wafer. The geometry of the upper film was then modified by ion beam etching after preparation of the surface by a laser lithography step. Bands of different widths from 2 to 12 µm approximately were produced in the film to elaborate the straight rib waveguides. Two lengthes of 6-µm width straight waveguides were joined by two smooth curved waveguides to create S-bend waveguides, Curve radii and angles varied between 0.1 and 50 mm, and 4 and 104.4°, respectively. Two S-bend arms of 6 µm in width and 10 mm in radius and separated by 0.25 mm were used to elaborate Y-junctions. The stretches between the two Y-junctions in Mach-Zehnder interferometers were 15 mm long. In all cases, a negative AZ20.20 photo-resist from Microchemical was spin-coated with a rate of 4000 tr.min−1, annealed 1 min at 110 °C, developed in a commercial AZ726 developer after the laser writing procedure and annealed once again 1 min at 110 °C. Laser writing was performed by using a DILASE 250 set-up from KLOE. Subsequent ion beam etching was performed using a set-up provided by PLASSYS. A mixture of argon and oxygen was used; PO2 and Ptot were set to 5.8.10−5 mTorr and 3.4.10−4 mTorr, respectively. The applied voltage was 400 V.
The Ge-Te rib waveguides were elaborated by depositing a Ge18Te82 film onto a Te75Ge15Ga10 bulk glass substrate. The geometry of the film was then modified by reactive ion etching using a mixture of CHF3/O2/Ar after preparation of the surface by a classical photolithography step with a positive photo-resist S18-18 from Shipley as reported elsewhere [40]. Specific photolithographic masks including bands of various widths were used. The etching durations were chosen in order to achieve the required etching depths according to designs. Etching depths were measured using a DEKTAK 3 VEECO surface profilometer, and the etching profiles were observed by scanning electron microscopy (using a CAMBRIDGE 200 set-up).
Waveguide characterization. The cut-back technique was used to measure the propagation losses of the Ge-Se-Te waveguides at λ = 1.55 µm. The input light was 4.5 mW. The output light was coupled to a photo-detector through a focusing microscope objective, which collected most of the outgoing light. The length of the samples was progressively decreased by cleaving the output facet, in order to maintain the coupling losses as constant as possible.
The intrinsic transmission of the Ge-Te waveguides was obtained by using a dedicated bench. The light coming from a globar source and passing through the interferometer of a Fourier transform spectrometer (Bruker TENSOR 27) was coupled into the waveguide thanks to parabolic optics. The alignment of the waveguide was checked with an IR camera (FLIR SC7000) detecting signal between 2 and 5 µm. The camera was then removed and the transmitted light collected with reflective parabolic optics before being focused on a HgCdTe detector, allowing the measurement of the transmission spectrum of the waveguide between 2 and 22 µm. The performance of the Ge-Te waveguide as a modal filter was tested by building a dedicated laser interferometer bench, powered by a CO2 laser operating at λ = 10.6 µm. A Mach-Zehnder interferometer was inserted in the collimated laser beam, upstream the coupling off-axis parabola. A delay line was adjusted so that a constructive or destructive interference was produced in the beam transmitted to the component, in order to test its interferometric capability.
3. Thermal and optical characteristics of amorphous Ge-Se-Te films
About 40 Ge-Se-Te films of at least 5 µm in thickness were deposited by thermal co-evaporation. Investigation focused on the chalcogen-rich region of the Ge-Se-Te ternary system, excluding compositions that contain more than 50 at.% in Ge. Whatever the composition, the films were passably adherent to microscope slides, and very adherent to silicon substrates and Te75Ge15Ga10 bulk glasses. All the films were amorphous as checked by X-Ray diffraction. Their composition is given in Table 1.
Table 1. Compositions of the thermally co-evaporated Ge-Se-Te films, along with their characteristic temperatures (Tg, Tx, Tm) and their criterion of stability (ΔT = Tx1 – Tg1). “N.O.” means that the corresponding characteristic temperatures were not observed. “N.A.” means that due to a lack of matter, DSC analysis was not performed.
3.1. Thermal analysis
Owing to the thickness of the films, enough matter could be collected to carry out differential scanning calorimetry (DSC) measurements. Depending on the film composition, the DSC curves differ strongly as shown in Fig. 1.
Fig. 1 DSC curves of several Ge-Se-Te films deposited by thermal co-evaporation. Each characteristic temperature is indicated, together with its measurement procedure.
The characteristic temperatures of the films, i.e. their vitreous transition, Tgi, crystallization, Txi, and melting temperatures, Tmi (i = 1, 2,…) are reported in Table 1.The parameter ΔT = Tx1 – Tg1, a characteristics of the thermal stability of amorphous films, is also reported for the compositions exhibiting a single Tg. A Ge-Se-Te projection diagram indicating the investigated compositions, corresponding vitreous transition temperatures Tg1 and proposed isotherms is shown in Fig. 2(a). A second Ge-Se-Te projection diagram giving the ΔT values for the investigated compositions and highlighting a domain of high glass stability (ΔT > 100 K) is shown in Fig. 2(b).
Fig. 2 a) Vitreous transition temperatures Tg1 and possible isotherms. The yellow zone with small dots shows the domain where two Tg were observed. b) Criterion of stability ΔT of the films in the Ge-Se-Te system. The blue shaded zone corresponds to films with ΔT > 100 K, and the hatched zone delimits films which do not present any crystallization peak in their DSC curve. The yellow zone with small dots corresponds to films with two Tg. The red segment shows the liquid-liquid miscibility gap existing in the GeSe2-Te pseudo-binary system according to Bordas et al. [41] and the red dashed area shows the limit of the liquid-liquid miscibility gap in the Ge-Se-Te system according to Borisova [42].
On the whole, the data agree with those reported by Sarrach et al. for several series of Ge-Se-Te bulk and thin films [23].
Few films do not even show Tg. They lie either in a zone of Ge-rich compositions at the limit of the glass-forming domain (Films # 34 and 36 in Table 1) or in a zone identified by Sarrach et al. as a region of compositions with such a low thermal stability that the glasses crystallize at or even before the occurrence of the vitreous transition phenomenon (Film # 5 in Table 1). At the opposite, some films exhibit two vitreous transition phenomena, highlighting the presence of a phase separation. Two examples are given in Fig. 1. The corresponding composition domain is the zone with small dots shown in both Figs. 2(a) and 2(b). The presence of such a domain where a phase separation occurs is consistent with different literature data. First a liquid-liquid miscibility gap, shown as a red segment included in the dotted zone of Fig. 2(b), has been reported in the GeSe2-Te pseudo-binary system [41]. The extension of the liquid-liquid miscibility gap in the whole ternary system has been tentatively reported by Borisova [42], and corresponds to the zone delimited by the dashed red line in Fig. 2(b). The presence of such a liquid-liquid miscibility gap may explain the difficulty in obtaining a homogeneous amorphous phase during quenching. According to Sarrach et al. [23] the gap occurs because strong Ge-Se bonds are energetically favored compared to a random mixture of alternative bonding configurations.
According to Table 1 and Fig. 2(a), the vitreous transition temperature Tg1 of the films globally increases with the atomic percentage in Ge, due to an increase of the mean bonding energy in the films. An exception is observed in the region around the particular composition GeSe2, where Tg1 passes through a maximum. This behavior was already reported by Boolchand et al. for GexSe1-x bulk glasses [43]. The Tg trend was interpreted in terms of a nanoscale phase separation, i.e. GexSe1-x glasses with x > 0.33 would phase-separate in a Se-rich majority phase and a compensating Ge-rich minority phase comprising ethane-like Se3Ge-GeSe3 entities which vibrations were observed in Raman spectra of the glasses. Tg of the Se-enriched majority phase, lower than that of GeSe2 glass, would only be observed by DSC.
A region of compositions of high stability against crystallization, i.e. exhibiting a ΔT > 100 K is shown as a blue zone in Fig. 2(b). No crystallization peak was ever detected in the DSC curves of the more stable compositions found in the central hatched part of the blue zone. Similar findings were already reported for both films and bulk glasses. For example, Afifi et al. reported that no crystallization peak were observed in the DSC curves of thermally evaporated films containing 80 at.% in Se [44]. Similar results were reported by Sarrach et al. [23] and Maurugeon et al. for bulk glasses [30].
This zone of high stability against crystallization corresponds to films which compositions comprise more than 50 at.% in Se. It is consistent with the fact that Se is a good glass-forming element in contrast to Te which presents a strong metallic character.
3.2. Optical properties
Transmission spectra of the films deposited onto microscope slides were recorded between 500 and 2500 nm. The optical band gaps, Eg, measured by using the Tauc’s method [45], were plotted versus the film composition. Figures 3(a) and 3(b) present the 3D evolution of Eg and the corresponding projection diagram, respectively.
Fig. 3 a) Evolution of the optical band gap Eg in the Ge-Se-Te 3D-ternary diagram. b) Corresponding projection diagram. The black squares show the films that were investigated, and which the Eg 2D-tendency versus composition was extrapolated from.
The film refractive index versus wavelength, n(λ), was also extracted from the transmission spectra, thanks to the Swanepoel’s method [46]. The values n(λ) were then fitted by the Cauchy’s law [47]:
Figures 4(a) and 4(b) show the evolution of the refractive index at λ = 1.55 µm, n(1.55), versus composition in the 3D-ternary system and the corresponding projection diagram, respectively.Fig. 4 a) Evolution of the refractive index at 1.55 µm in the Ge-Se-Te 3D-ternary diagram. b) Corresponding projection diagram. The black squares show the films that were investigated, and which the n 2D-tendency versus composition was extrapolated from.
The observed opposite trend in the evolution of Eg and n(1.55) is expected and trivial; it has been reported many times, for example by Moss [48]:
where K is a constant.The replacement of Te by Se in films with a fixed Ge content leads to an increase in the optical band gap Eg and respective decrease in the refractive index n. Eg and n do not evolve monotonically with the addition of germanium. On the whole, a maximum in the optical band gap and a minimum in the refractive index are observed in the composition region lying between 20 and 35 at. % in Ge. Such a trend was already reported for both binary Ge-Te films [38, 49] and Ge-Se bulk glasses [50]. Minima or maxima in the optical properties were also reported for several sulfide and selenide glasses (Ge-Se, Ge-S, As-Se, As-S, Ge-S-Bi, Ge-Sb-Se, Ge-As-Se, Ge-As-S). Different explanations to these trends were proposed, including the presence of phase separation or the existence of network singularities expected in this composition region in the framework of topological models [51–57]. Nevertheless, owing to the scarce number of investigated samples in regard to the explored ternary zone, there is no way to settle the question. Such a task is anyway much beyond the scope of the present work.
4. Relevance to the development of rib waveguides, S-bend waveguides and Y-junctions for infrared applications
The investigation described above helps in getting an insight on the main thermal and optical properties of thick co-evaporated films which can be used for the elaboration of waveguides for the development of IR integrated components. This knowledge is now used to select the optimal compositions for such development.
First the domain of compositions where phase-separated films are obtained is discarded. In order to be on the safe side when using the component, a vitreous transition temperature for the selected films needs to be equal or even larger than 400 K. For example, the vitreous transition temperature of commercialized bulk As2S3 is 481 K (AMTIR-6 glass, from Amorphous Materials) and that of glass Te2As3Se5 used for the development of IR optical fibers is 410 K [58]. The Ge-Se-Te films which composition contains less than 15 at. % in Ge need to be discarded. Films with a high stability against crystallization, i.e. which ΔT parameter is high, are also looked for. This criterion clearly points towards the region where ΔT is larger than 100K and even reaches infinity, i.e. the blue zone in Fig. 2(b).
Finally it can be taken advantage of the non-linear evolution of the refractive index, n, with composition. As a matter of fact, it was recently shown that the unavoidable dispersion in composition during a film deposition has much less impact for compositions lying in the vicinity of a minimum of the refractive index [38]. It can be understood by and it is related to the weaker slope of the curve at this point which leads to a weaker dispersion of n for a similar dispersion in composition. As shown in Fig. 4, these optimal compositions are those that contain an amount of Ge comprised between 20 and 25 at. %.
On the whole, the optimal Ge-Se-Te compositions to elaborate waveguides should contain more than 60 at.% in Se and about 20-25 at. % in Ge. These films are transparent up to about 16-17 µm depending upon the precise composition. For example, a thermally co-evaporated Ge25Se60.3Te14.7 film, measured by Integral Field Spectrometry, exhibited a limit in transparency at ~16 µm.
Whatever the aimed optical component, the first step is the elaboration of more or less complex waveguides. Straight waveguides as well as S-bend waveguides, spirals, Y-junctions and Mach-Zehnder interferometers were thus fabricated. Silicon was chosen as the substrate. A first cladding layer (nclad) was deposited previous to the core layer (with ncore > nclad). The refractive index difference between the cladding layer and the core layer is a critical parameter that impacts the strength of light confinement and hence the aperture of the coupling and collecting optics. As integrated optics devices can stand relatively strong index differences, a refractive index difference Δn = ncore – nclad of 0.1 was chosen for the first implementations. It allows i) to limit the impact of index uncertainty on the guided mode and ii) to increase the component compactness. Rib architectures were chosen. Given the above considerations and on the basis of the dependence of the refractive index with composition shown in Fig. 4, two compositions were selected: Ge25Se65Te10 for the cladding layer (n = 2.55 at λ = 1.55 µm) and Ge25Se55Te20 for the core layer (n = 2.65 at λ = 1.55 µm).
As-deposited Ge25Se65Te10 and Ge25Se55Te20 films were annealed at 458 K (Tg – 25 K) for different periods of time: 0.25 h, 1h, 2h, 3 days, 7 days and 14 days. As expected, owing to the high stability of the compositions against crystallization, no crystallization of the films occurred, whatever the annealing duration. A slight increase of 0.06 eV in the optical band gap occurred after the first period of annealing of 0.25 h, probably due to a decrease in disorder, followed by stabilization for longer annealing. No measurable change in the refractive index of the films was observed during annealing.
Rib waveguides were designed thanks to the Finite Difference and the Beam Propagation methods. The width of the calculation window was 50 times the rib width and the propagation length was fixed to 3 cm. According to simulations, the cladding layer had to have a minimum thickness of 4 µm, the core layer had to be 1.5 ± 0.1 µm thick and be etched to a depth of 0.5 ± 0.1 µm. The rib width had then to be comprised between 2 and 8 µm to observe a single-mode behavior. The cladding and core layers were deposited by thermal co-evaporation. Laser lithography and ion beam etching were then used to create bands of different widths and of about 0.5 µm in depth. A Scanning Electron Microscopy image of a 10 µm-wide rib waveguide is shown in Fig. 5(a).
Fig. 5 a) SEM picture of a 10-µm wide rib waveguide. b) Near-field image at λ = 1.55 µm at the output of a 10 µm-width rib waveguide and corresponding simulated image. c) Near-field image at λ = 1.55 µm at the output of a 6 µm-width rib waveguide and corresponding simulated image. d) Total optical losses at λ = 1.55 µm versus the waveguide length for a 6-µm wide rib waveguide characterized by an initial length of 3.2 cm. The slope of the simulated dotted line, corresponding to propagation losses, is 1.1 dB.cm−1. The intercept of about 15.3 dB gives an order of magnitude of the coupling losses.
Injection efficiency in and guiding properties of the rib waveguides were checked at λ = 1.55 µm. The light was end-coupled into the waveguides by a single-mode optical fibre and the injection spot was centred onto the ribs. Depending on the rib width, multi-mode, single-mode or slab behaviors were observed. In agreement with simulation, a multi-mode behavior was observed when the rib width was higher than 8 µm, as shown in Fig. 5(b) and a single-mode behavior when the rib width was comprised between 5 and 8 µm, (as shown in Fig. 5(c)). Waveguides with rib width comprised between 2 and 4 µm showed a slab behavior. Even though not predicted by simulation, this behavior can be explained by too important coupling losses, due to the size of the injection spot (about 10.4 ± 0.8 µm in diameter) and the coupling of the light energy into the higher order mode supported between two ribs.
Propagation losses of straight rib waveguides were evaluated thanks to the cut-back method for 6 µm-wide rib waveguides. About twenty straight rib waveguides were measured; for each of them we reported the total optical losses versus the sample length, the output facet of the sample having been cleaved progressively. A typical curve is shown in Fig. 5(d). For each sample, the slope of the as-obtained curve allowed estimating the propagation losses and the intercept of the curve permitted to estimate the coupling losses. Average values of the coupling losses and propagation losses of 16 ± 2 dB and 1.0 ± 0.5 dB.cm−1 were obtained, respectively. This value is acceptable for planar waveguides devoted to light propagation along few centimetres length. These propagation losses are indeed comparable to those of other waveguides produced by film stacking and etching. For example, rib waveguides were patterned by dry etching in Ga-Ge-Sb-S with losses of 1.1 ± 0.4 dB.cm−1 at λ = 1.55 µm [59]. Reactive ion etching under CHF3 was used to produce rib waveguides based on Ge20Sb15Se65 film, and the losses were estimated lower than 1 dB.cm−1 at λ = 1.55 µm [60]. Reactive ion etching under CHF3 was also used to fabricate rib waveguides based on Ge11.5As24Se64.5 film. In this case, the losses were below 1 dB.cm−1, the lowest loss being around 0.3 dB.cm−1 at λ = 5 µm [61]. Chemical etching was also used to create structured waveguides in As-S(-Se) glasses which propagation losses were 1 dB.cm−1 at λ = 1.3 µm [62]. Propagation losses of waveguides produced by other techniques were also reported. For example, propagation losses of 2.9 dB.cm−1 at λ = 1.55 µm were reported for waveguides elaborated by hot embossing of an As2S3 film by using a silicon stamp [63]; propagation losses at λ = 1.55 µm dropped down to 0.24 dB.cm−1 for waveguides elaborated by hot embossing of an As24S38Se38 film by using a soft stamp [64]. The lowest propagation losses, i.e. 0.4 dB.cm−1 at λ = 1.064 µm, ever reported for waveguides created by irradiation were observed for a waveguide elaborated in an As-S-Se-Ge film with an Ar laser operating at 514 nm [35]. Inverted-rib As2S3-based waveguides were also recently fabricated by spin-coating and annealing solution-dissolved chalcogenide in pre-etched channels; propagation losses of 1.87 dB.cm−1 at λ = 2.6 µm were measured [65].
S-bend waveguides were then elaborated. Two lengths of 6-µm width straight waveguides were joined by two smooth curved waveguides to create the S-bend waveguides, as illustrated in the inset of Fig. 6(b). The smooth curves are portions of a circle with radius of curvature r. An optical image of a series of 8 S-bend waveguides with different values of r is shown in Fig. 6(a). All the S-bend waveguides were optically tested at λ = 1.55 µm. The light powers at the output of a given S-bend waveguide and at the output of a 6-µm width straight waveguide of similar total length were compared. The difference between the two is clearly due to propagation losses generated by the bending of the waveguide and hereafter called bending losses. The bending losses were measured as a function of the radius of curvature r. The resulting plot is shown in Fig. 6(b).
Fig. 6 a) Optical image of a series of 8 S-bend waveguides of 6-µm in width. r stands for the radius of curvature. For each S-bend waveguide, a distance of 0.25 mm was applied between the two straight branches. b) Bending losses versus the curve radius. The error on the loss values is included in the thickness of the black squares. Inset: schematic diagram of S-bend channel waveguide configuration.
The bending losses exponentially increase as the radius of curvature r becomes smaller. They become significant when the value of r is lower than ~10 mm. At this point, a tradeoff between radius of curvature and length of the curve needs to be found, i.e. a curve with a small curvature radius can only be considered on short distances. Similar results were observed in buried Er3+/Yb3+ co-doped phosphate glass waveguides [66]. Spirals, Y-junctions and Mach-Zehnder interferometers were also fabricated and their optical or SEM pictures are shown in Figs. 7(a)-7(c), respectively. Light observed at the output of the spirals at λ = 1.55 µm is shown in Fig. 7d. Y-junctions comprised two identical branches of 6 µm in width. They play their role of power divider at λ = 1.55 µm, with the light equally divided between the two branches, as illustrated by the near-field image at the output of the two branches shown in Fig. 7€. To end, the re-collection of the light observed at 1.55 µm at the output of the Mach-Zehnder interferometers is shown in Fig. 7(f).
Fig. 7 a) Optical image of a spiral; b) SEM picture of a Y-junction; c) optical image of a Mach-Zehnder interferometer; d, e and f) 1.55 µm near-field images at the output of a spiral, a Y-junction and a Mach-Zehnder interferometer, respectively.
These results allow validating the technology for further use in the fabrication of infrared micro-components working up to 16-17 µm.
When micro-components working at higher wavelength are required, other chalcogenide compositions and eventually other waveguide designs need to be selected. For example, the development of a modal filter to be a part of a spatial interferometer to observe exoplanets requires other compositions to meet the European Space Agency requirement of a device working from 6 to 20 µm [67]. Such a device can still be elaborated with a chalcogenide film but its composition needs to be taken in the Te-rich part of the Ge-Se-Te diagram. The elaboration of the device based on a pure Ge-Te film is now described.
A Te75Ge15Ga10 bulk glass was chosen as a substrate. It is transparent in the whole required wavelength domain, i.e. 6-20 µm [68], so it could be used as the cladding material.
Strong modal filtering efficiency must be achieved after relatively short propagation lengths. The aperture of the coupling optics must then stand within reasonable limits (f # ≤ 1) in the mid-IR. To achieve the limiting value of f # = 1, the refractive index difference between the core and the substrate was set to Δn = ncore − nsubs = 4 × 10−2. The refractive index of the Te75Ge15Ga10 substrate is 3.3960 ± 0.0015 at λ = 10.6 μm [68]. A Ge18Te82 film which refractive index is 3.44 ± 0.02 at λ = 10.6 μm [69] was then chosen as the core layer.
In terms of thermal stability, the vitreous transition temperature, Tg, of the Ge18Te82 film is 432 K and its ΔT value is 40 K. The as-deposited Ge18Te82 films were annealed at 412 K (Tg – 20 K) for different periods of time: 1 h, 8 h, 16 h and 24 h. Despite a rather poor thermal stability, no crystallization of the films occurred, whatever the annealing duration. A slight increase of 0.008 in the refractive index occurred after the first period of annealing of 1 h, followed by stabilization for longer annealing.
An additional test of survival to proton radiations, present in space, was performed on the irradiation site of Louvain-La-Neuve (Université Catholique Libre in Belgique). Ge18Te82 films were deposited onto commercial As2Se3 substrates. The refractive index of the Ge18Te82 films was then measured by the m-lines method at λ = 10.6 µm, before and after exposure to proton doses emitted by a particule accelerator. The proton doses were evaluated based on a typical orbit life of the Darwin mission, i.e. a 5 years life in a L2 orbit, protected by a typical 0.5 mm Aluminium shielding. Numerically speaking, the required radiation doses were 28.5 krad of protons of more than 10 MeV. The applied dose was 25.8 krad with a proton energy of 14.4 Mev. After proton exposure, a slight variation of ± 0.003 in the refractive index of the films was observed.
Owing to satisfying properties, the Ge18Te82 film was definitively selected and two types of rib waveguides were designed using a scalar mode solver (OptTools by Optiwave) [69, 70]. Dimensions were chosen to obtain a single-mode operation in a large spectral range, and a coupling efficiency higher than 50% over the entire band with a diffraction limited beam (f # = 1). A single-mode waveguide operative from 6 to 11 µm was elaborated by depositing a 12 ± 0.5 µm-thick Ge18Te82 film further etched to produce a 15 ± 2 µm wide rib waveguide of 4.5 ± 0.2 µm depth. A second waveguide working in a higher transparency domain from 10 to 20 µm was elaborated by depositing a 24 ± 1 µm-thick Ge18Te82 film further etched to produce a 36 ± 2 µm wide and 9 ± 0.3 µm deep rib waveguide.
First, the transmission spectra of the [6 – 11 µm] and [10 – 20 µm] devices were recorded between λ = 2 µm and λ = 22 µm. Whatever the devices, transmission starts around λ = 6 µm and stops around λ = 19 µm. The overall transmission varies between 3.5% and 4.5% over the 6 µm to 19 µm range, as illustrated by the typical transmission spectrum given in Fig. 8(b). These values encompass at least Fresnel refection on input and output facets (~30% per facet) and coupling efficiencies which were determined by computing the power overlap integral of diffraction limited input beam (~50% to 70% per facet if the beam is diffraction limited, somewhat in practice). The transmission of a [10 – 20 µm] waveguide after correction from the coupling and Fresnel losses was evaluated and ranged between 15 and 35% for a 1 cm-long device.
Fig. 8 a) Modal filtering bench design; b) transmission spectrum of a [10 – 20 µm] rib waveguide; c) and d) SEM images of the [6 – 11 µm] rib waveguide tested as a modal filter.
A modal filtering bench, which diagram is shown in Fig. 8(a), was built to test the rib waveguides. A light rejection efficiency of 6.10−5 was observed for the [6 – 11 µm] device shown in Figs. 8(c) and 8(d), which confirms the potential of Ge-Te-based integrated optics components for the development of modal filters for nulling interferometry.
5. Conclusions
The present investigation helped in identifying compositions of the ternary Ge-Se-Te system with interesting characteristics for the development of IR integrated optic devices. Films were deposited by thermal co-evaporation and their thermal and optical properties were measured by DSC and vis-NIR spectroscopy. A region of phase-separated amorphous films situated in a zone where a liquid-liquid miscibility gap exists had to be discarded. On the other hand, the region comprising Se-rich (Se > 60 at. %) compositions with a Ge content of about 25 at. % had all the required characteristics: a vitreous transition temperature higher than 400K, a high thermal stability with eventually no crystallization observed before melting and an easily controlled refractive index, n, due to a weak dependence of n with composition in this region.
Two compositions, i.e. Ge25Se65Te10 (n = 2.55 at λ = 1.55 µm) and Ge25Se55Te20 (n = 2.65 at λ = 1.55 µm) were selected to fabricate basic elements of infrared micro-components by stacking and further etching of the films.
A series of straight rib waveguides was fabricated by stacking a 4 µm-thick Ge25Se65Te10 film and a 1.5 µm-thick Ge25Se55Te20 film, further etched to a 0.5 µm depth. A single-mode behavior at λ = 1.55 µm was observed when the rib widths were comprised between 5 and 8 µm. Propagation losses of 1.0 ± 0.5 dB/cm were measured by the cut-back method. S-bend rib waveguides allowed measuring the effect of the curve radius on the propagation losses. Even though a trade-off between radius of curvature and length of the curve needed to be found to maintain the propagation losses at a reasonable level, light propagation was large enough to consider the elaboration of more complex components. Y-junctions and Mach-Zehnder interferometers were then elaborated Encouraging results were obtained with the observation of an equally divided light between the two branches of the Y-junctions and a satisfying re-collection of light at the output of the interferometers. The transmission region accessible to these structures lies from telecommunication wavelength up to 16-17 µm. When a higher transmission region is required, the use of pure Ge-Te films is mandatory. Two series of all-telluride rib waveguides were obtained by depositing a Ge18Te82 film onto a Te75Ge15Ga10 substrate The two series were similar but for the geometry of the core layer, which allowed a single mode propagation of light either in [6 – 11 µm] or [10 – 20 µm] transmission window.
A modal filter allowing a light rejection efficiency of 6.10−5 to be a part of a spatial interferometer was then elaborated.
Acknowledgments
The authors thank all the collaborators involved in the “Integrated Optics for the Darwin Mission” project: Jean-Emmanuel Broquin and Lionel Bastard from IMEP-Grenoble, Xianghua Zhang from SCR-Rennes, Gilles Parent from LEMTA-Vandoeuvre Les Nancy. They also thank Thierry Billeton for his help to overcome the challenges of the samples polishing. To end they wish to thank Joël Couve, Frédéric Pichot, Jean Lyonnet, Jean-Marie Peiris, Claude Merlet, Bernard Boyer, Dominique Granier and David Maurin for their help in the film characterization and etching.
References and links
1. S. Chatterjee, P. K. Purkait, and D. Garg, “Athermalization of infra-red camera of projectile weapons,” Appl. Therm. Eng. 29(10), 2106–2112 (2009). [CrossRef]
2. R. Kasztelanic, I. Kujawa, R. Stepien, K. Harasny, D. Pysz, and R. Buczynski, “Fresnel lens fabrication for broadband IR optics using hot embossing process,” Infrared Phys. Technol. 60, 1–6 (2013). [CrossRef]
3. X. H. Zhang, Y. Guimond, and Y. Bellec, “Production of complex chalcogenide glass optics by molding for thermal imaging,” J. Non-Cryst. Solids 326-327, 519–523 (2003). [CrossRef]
4. H. Lin, L. Li, Y. Zou, S. Danto, J. D. Musgraves, K. Richardson, S. Kozacik, M. Murakowski, D. Prather, P. T. Lin, V. Singh, A. Agarwal, L. C. Kimerling, and J. Hu, “Demonstration of high-Q mid-infrared chalcogenide glass-on-silicon resonators,” Opt. Lett. 38(9), 1470–1472 (2013). [CrossRef] [PubMed]
5. H. Lin, L. Li, F. Deng, C. Ni, S. Danto, J. D. Musgraves, K. Richardson, and J. Hu, “Demonstration of mid-infrared waveguide photonic crystal cavities,” Opt. Lett. 38(15), 2779–2782 (2013). [CrossRef] [PubMed]
6. J. A. Savage, Infrared Optical Materials and their Antireflexion Coatings (Adam Hilger, 1985).
7. I. U. Arachchige, R. Soriano, C. D. Malliakas, S. A. Ivanov, and M. C. Kanatzidis, “Amorphous and Crystalline GeTe Nanocrystals,” Adv. Funct. Mater. 21(14), 2737–2743 (2011). [CrossRef]
8. J. Feinleib, J. Deneufvi, S. C. Moss, and S. R. Ovshinsky, “Rapid Reversible Light-induced Crystallization Of Amorphous Semiconductors,” Appl. Phys. Lett. 18(6), 254–257 (1971).
9. D. Kang, D. Lee, H.-M. Kim, S.-W. Nam, M.-H. Kwon, and K.-B. Kim, “Analysis of the electric field induced elemental separation of Ge2Sb2Te5 by transmission electron microscopy,” Appl. Phys. Lett. 95(1), 011904 (2009). [CrossRef]
10. H. B. Lu, E. Thelander, J. W. Gerlach, U. Decker, B. P. Zhu, and B. Rauschenbach, “Single Pulse Laser-Induced Phase Transitions of PLD-Deposited Ge2Sb2Te5 Films,” Adv. Funct. Mater. 23(29), 3621–3627 (2013). [CrossRef]
11. E. Barthélémy, S. Albert, C. Vigreux, and A. Pradel, “Effect of composition on the properties of Te-Ge thick films deposited by co-thermal evaporation,” J. Non-Cryst. Solids 356(41-42), 2175–2180 (2010). [CrossRef]
12. E. Barthélémy, C. Vigreux, G. Parent, M. Barillot, and A. Pradel, “Telluride films and waveguides for IR integrated optics,” Phys. Status Solidi., C Curr. Top. Solid State Phys. 8(9), 2890–2894 (2011). [CrossRef]
13. S. Danto, P. Houizot, C. Boussard-Pledel, X. H. Zhang, F. Smektala, and J. Lucas, “A family of far-infrared-transmitting glasses in the Ga-Ge-Te system for space applications,” Adv. Funct. Mater. 16(14), 1847–1852 (2006). [CrossRef]
14. S. Maurugeon, B. Bureau, C. Boussard-Pledel, A. J. Faber, P. Lucas, X. H. Zhang, and J. Lucas, “Selenium modified GeTe4 based glasses optical fibers for far-infrared sensing,” Opt. Mater. 33(4), 660–663 (2011). [CrossRef]
15. A. A. Wilhelm, C. Boussard-Pledel, Q. Coulombier, J. Lucas, B. Bureau, and P. Lucas, “Development of far-infrared-transmitting te based glasses suitable for carbon dioxide detection and space optics,” Adv. Mater. 19(22), 3796–3800 (2007). [CrossRef]
16. M. L. Anne, J. Keirsse, V. Nazabal, K. Hyodo, S. Inoue, C. Boussard-Pledel, H. Lhermite, J. Charrier, K. Yanakata, O. Loreal, J. Le Person, F. Colas, C. Compère, and B. Bureau, “Chalcogenide Glass Optical Waveguides for Infrared Biosensing,” Sensors (Basel) 9(9), 7398–7411 (2009). [CrossRef] [PubMed]
17. A. Ganjoo, H. Jain, C. Yu, J. Irudayaraj, and C. G. Pantano, “Detection and fingerprinting of pathogens: Mid-IR biosensor using amorphous chalcogenide films,” J. Non-Cryst. Solids 354(19-25), 2757–2762 (2008). [CrossRef]
18. A. Ganjoo, H. Jain, C. Yu, R. Song, J. V. Ryan, J. Irudayaraj, Y. J. Ding, and C. G. Pantano, “Planar chalcogenide glass waveguides for IR evanescent wave sensors,” J. Non-Cryst. Solids 352(6-7), 584–588 (2006). [CrossRef]
19. P. Lucas, M. A. Solis, D. Le Coq, C. Juncker, M. R. Riley, J. Collier, D. E. Boesewetter, C. Boussard-Pledel, and B. Bureau, “Infrared biosensors using hydrophobic chalcogenide fibers sensitized with live cells,” Sens. Actuators B Chem. 119(2), 355–362 (2006). [CrossRef]
20. F. Charpentier, B. Bureau, J. Troles, C. Boussard-Plédel, K. M.-L. Pierrès, F. Smektala, and J. L. Adam, “Infrared monitoring of underground CO2 storage using chalcogenide glass fibers,” Opt. Mater. 31(3), 496–500 (2009). [CrossRef]
21. D. Tsiulyanu, S. Marian, H. D. Liess, and I. Eisele, “Effect of annealing and temperature on the NO2 sensing properties of tellurium based films,” Sens. Actuators B Chem. 100(3), 380–386 (2004). [CrossRef]
22. S. Hocdé, C. Boussard-Pledel, G. Fonteneau, and J. Lucas, “Chalcogens based glasses for IR fiber chemical sensors,” Solid State Sci. 3(3), 279–284 (2001). [CrossRef]
23. D. J. Sarrach, J. P. Deneufville, and W. L. Haworth, “Studies Of Aorphous Ge-Se-Te Alloys. 1. Preparation And Calorimetric Observations,” J. Non-Cryst. Solids 22(2), 245–267 (1976). [CrossRef]
24. S. A. Khan, M. Zulfequar, and M. Husain, “Effects of annealing on crystallization process in amorphous Ge5Se95-xTex thin films,” Physica B 324(1-4), 336–343 (2002). [CrossRef]
25. A. El-Korashy, A. Bakry, M. A. Abdel-Rahim, and M. A. El-Sattar, “Annealing effects on some physical properties of Ge5Se25Te70 chalcogenide glasses,” Physica B 391(2), 266–273 (2007). [CrossRef]
26. P. Sharma and S. C. Katyal, “Thickness dependence of optical parameters for Ge-Se-Te thin films,” Mater. Lett. 61(23-24), 4516–4518 (2007). [CrossRef]
27. P. Sharma and S. C. Katyal, “Optical study of Ge10Se90-xTex glassy semiconductors,” Thin Solid Films 515(20-21), 7966–7970 (2007). [CrossRef]
28. P. Sharma and S. C. Katyal, “Effect of substrate temperature on the optical parameters of thermally evaporated Ge-Se-Te thin films,” Thin Solid Films 517(13), 3813–3816 (2009). [CrossRef]
29. Z. Y. Wang, C. J. Tu, Y. M. Li, and Q. Q. Chen, “The Effects Of Sn and Bi Additions On Properties And Structure In Ge-Se-Te Chalcogenide Glass,” J. Non-Cryst. Solids 191(1-2), 132–137 (1995). [CrossRef]
30. S. Maurugeon, B. Bureau, C. Boussard-Pledel, A. J. Faber, X. H. Zhang, W. Geliesen, and J. Lucas, “Te-rich Ge-Te-Se glass for the CO2 infrared detection at 15 mu m,” J. Non-Cryst. Solids 355(37-42), 2074–2078 (2009). [CrossRef]
31. G. Lenz, J. Zimmermann, T. Katsufuji, M. E. Lines, H. Y. Hwang, S. Spälter, R. E. Slusher, S. W. Cheong, J. S. Sanghera, and I. D. Aggarwal, “Large Kerr effect in bulk Se-based chalcogenide glasses,” Opt. Lett. 25(4), 254–256 (2000). [CrossRef] [PubMed]
32. E.-B. Lee, B.-K. Ju, and Y.-T. Kim, “Phase change and electrical characteristics of Ge–Se–Te alloys,” Microelectron. Eng. 86(7-9), 1950–1953 (2009). [CrossRef]
33. C. Das, M. G. Mahesha, G. M. Rao, and S. Asokan, “Electrical switching and optical studies on amorphous GexSe35-xTe65 thin films,” Thin Solid Films 520(6), 2278–2282 (2012). [CrossRef]
34. E. M. Vinod, A. K. Singh, R. Ganesan, and K. S. Sangunni, “Effect of selenium addition on the GeTe phase change memory alloys,” J. Alloy. Comp. 537, 127–132 (2012). [CrossRef]
35. S. Zembutsu and S. Fukunishi, “Waveguiding properties of (Se,S)-based chalcogenide glass films and some applications to optical waveguide devices,” Appl. Opt. 18(3), 393–399 (1979). [CrossRef] [PubMed]
36. V. T. Mai, R. Escalier, C. Vigreux, and A. Pradel, “Selenium modified Te85Ge15 films elaborated by thermal co-evaporation for infrared applications,” Thin Solid Films 524, 309–315 (2012). [CrossRef]
37. C. Vigreux, M. Vu Thi, R. Escalier, G. Maulion, R. Kribich, and A. Pradel, “Channel waveguides based on thermally co-evaporated Te-Ge-Se films for infrared integrated optics,” J. Non-Cryst. Solids 377, 205–208 (2013). [CrossRef]
38. C. Vigreux, A. Piarristeguy, R. Escalier, S. Ménard, M. Barillot, and A. Pradel, “Evidence of a minimum in refractive indexes of amorphous GexTe100−x films: Relevance to the development of infrared waveguides,” Phys Status Solidi A 211(4), 932–937 (2014). [CrossRef]
39. E. Barthélémy, S. Albert, C. Vigreux, A. Pradel, X. Zhang, S. Zhang, G. Parent, T. Billeton, J.-E. Broquin, S. Ménard, M. Barillot, and V. Kirschner, in Conference Integrated Optics: Devices, Materials, and Technologies XIV (part of Photonics West)(San Francisco, US, 2010), p. 760405.
40. C. Vigreux, S. D. Sousa, V. Foucan, E. Barthélémy, and A. Pradel, “Deep reactive ion etching of thermally co-evaporated Te–Ge films for IR integrated optics components,” Microelectron. Eng. 88(3), 222–227 (2011). [CrossRef]
41. S. Bordas, M. Geli, J. Casas-Vazquez, N. Clavaguera, and M. T. Clavaguera-Mora, “Phase diagram of the ternary system Ge-Te-Se,” Thermochim. Acta 37(2), 197–207 (1980). [CrossRef]
42. Z. Borisova, Glassy Semiconductors (Plenum Press, New York, 1981).
43. P. Boolchand, D. G. Georgiev, T. Qu, F. Wang, L. Cai, and S. Chakravarty, “Nanoscale phase separation effects near r=2.4 and 2.67, and rigidity transitions in chalgonide glasses,” C. R. Chim. 5(11), 713–724 (2002). [CrossRef]
44. M. A. Afifi, N. A. Hegab, H. E. Atyia, and A. S. Farid, “Investigation of DC conductivity and switching phenomenon of Se80Te20−xGex amorphous system,” J. Alloy. Comp. 463(1-2), 10–17 (2008). [CrossRef]
45. J. Tauc, R. Grigorovici, and A. Vancu, “Optical properties and electronic structure of amorphous germanium,” Phys Status Solidi B 15(2), 627–637 (1966). [CrossRef]
46. R. Swanepoel, “Determination of the thickness and optical constants of amorphous silicon,” J Phys E 16(12), 1214–1222 (1983). [CrossRef]
47. T. S. Moss, Optical properties of Semiconductors (Butterworths, London, 1959).
48. T. S. Moss, “A relationship between the refractive index and the infra-red threshold of sensitivity for photoconductors,” Proc. Phys. Soc. B 63(3), 167–176 (1950). [CrossRef]
49. N. El-Kabany, “Compositional dependence of the optical properties of the amorphous GexTe1−x system,” Physica B 403(18), 2949–2955 (2008). [CrossRef]
50. G. Yang, Y. Gueguen, J.-C. Sangleboeuf, T. Rouxel, C. Boussard-Plédel, J. Troles, P. Lucas, and B. Bureau, “Physical properties of the GexSe1-x glasses in the 0<x<0.42 range in correlation with their structure,” J. Non-Cryst. Solids 377, 54–59 (2013). [CrossRef]
51. K. Tanaka, “Structural phase transitions in chalcogenide glasses,” Phys. Rev. B Condens. Matter 39(2), 1270–1279 (1989). [CrossRef] [PubMed]
52. A. Srinivasan, K. N. Madhusoodanan, E. S. R. Gopal, and J. Philip, “Observation of a threshold behavior in the optical band gap and thermal diffusivity of Ge-Sb-Se glasses,” Phys. Rev. B Condens. Matter 45(14), 8112–8115 (1992). [CrossRef] [PubMed]
53. E. Sleeckx, L. Tichý, P. Nagels, and R. Callaerts, “Thermally and photo-induced irreversible changes in the optical properties of amorphous GexSe100-x films,” J. Non-Cryst. Solids 198–200(Part 2), 723–727 (1996). [CrossRef]
54. G. Saffarini, H. Schmitt, H. Shanak, J. Nowoczin, and J. Müller, “Optical band gap in relation to the average coordination number in Ge—S—Bi thin films,” Phys Status Solidi B 239(1), 251–256 (2003). [CrossRef]
55. O. Gamulin, M. Ivanda, V. Mitsa, S. Pasic, and M. Balarin, “Spectroscopy studies of structural phase transitions of chalcogenide glass thin films (Ge2S3)x(As2S3)1− x at coordination number 2.67,” Solid State Commun. 135(11-12), 753–758 (2005). [CrossRef]
56. D. A. P. Bulla, R. P. Wang, A. Prasad, A. V. Rode, S. J. Madden, and B. Luther-Davies, “On the properties and stability of thermally evaporated Ge-As-Se thin films,” Appl Phys Adv Mater 96(3), 615–625 (2009). [CrossRef]
57. P. Jóvári, A. Piarristeguy, R. Escalier, I. Kaban, J. Bednarcik, and A. Pradel, “Short range order and stability of amorphous GexTe100−x alloys (12 ≤ x ≤ 44.6),” J Phys-Condens Matter 25, 195401 (2013).
58. K. Michel, B. Bureau, C. Pouvreau, J. C. Sangleboeuf, C. Boussard-Plédel, T. Jouan, T. Rouxel, J. L. Adam, K. Staubmann, H. Steinner, T. Baumann, A. Katzir, J. Bayona, and W. Konz, “Development of a chalcogenide glass fiber device for in situ pollutant detection,” J. Non-Cryst. Solids 326–327, 434–438 (2003). [CrossRef]
59. V. Nazabal, P. Němec, A. M. Jurdyc, S. Zhang, F. Charpentier, H. Lhermite, J. Charrier, J. P. Guin, A. Moreac, M. Frumar, and J. L. Adam, “Optical waveguide based on amorphous Er3+-doped Ga–Ge–Sb–S(Se) pulsed laser deposited thin films,” Thin Solid Films 518(17), 4941–4947 (2010). [CrossRef]
60. J. Li, X. Shen, J. Sun, K. Vu, D.-Y. Choi, R. Wang, B. Luther-Davies, S. Dai, T. Xu, and Q. Nie, “Fabrication and characterization of Ge20Sb15Se65 chalcogenide glass rib waveguides for telecommunication wavelengths,” Thin Solid Films 545, 462–465 (2013). [CrossRef]
61. P. Ma, D.-Y. Choi, Y. Yu, X. Gai, Z. Yang, S. Debbarma, S. Madden, and B. Luther-Davies, “Low-loss chalcogenide waveguides for chemical sensing in the mid-infrared,” Opt. Express 21(24), 29927–29937 (2013). [CrossRef] [PubMed]
62. J.-F. Viens, C. Meneghini, A. Villeneuve, T. V. Galstian, E. J. Knystautas, M. A. Duguay, K. A. Richardson, and T. Cardinal, “Fabrication and characterization of integrated optical waveguides in sulfide chalcogenide glasses,” J. Lightwave Technol. 17(7), 1184–1191 (1999). [CrossRef]
63. Z. G. Lian, W. J. Pan, D. Furniss, T. M. Benson, A. B. Seddon, T. Kohoutek, J. Orava, and T. Wagner, “Embossing of chalcogenide glasses: monomode rib optical waveguides in evaporated thin films,” Opt. Lett. 34(8), 1234–1236 (2009). [CrossRef] [PubMed]
64. T. Han, S. Madden, D. Bulla, and B. Luther-Davies, “Low loss Chalcogenide glass waveguides by thermal nano-imprint lithography,” Opt. Express 18(18), 19286–19291 (2010). [CrossRef] [PubMed]
65. Y. Zha, P. T. Lin, L. Kimerling, A. Agarwal, and C. B. Arnold, “Inverted-rib chalcogenide waveguides by solution process,” ACS Photonics 1(3), 153–157 (2014). [CrossRef]
66. R. Zhao, M. Wang, B. Chen, K. Liu, E. Y. B. Pun, and H. Lin, “Bent channel design in buried Er3+/Yb3+ codoped phosphate glass waveguide fabricated by field-assisted annealing,” Opt. Eng. 50(4), 044602 (2011). [CrossRef]
67. L. Labadie, P. Kern, P. Labeye, E. LeCoarer, C. Vigreux-Bercovici, A. Pradel, J.-E. Broquin, and V. Kirschner, “Technology challenges for space interferometry: the option of mid-infrared integrated optics,” Adv. Space Res. 41(12), 1975–1982 (2008). [CrossRef]
68. S. Zhang, X. Zhang, M. Barillot, L. Calvez, C. Boussard, B. Bureau, J. Lucas, V. Kirschner, and G. Parent, “Purification of Te75Ga10Ge15 glass for far infrared transmitting optics for space application,” Opt. Mater. 32(9), 1055–1059 (2010). [CrossRef]
69. C. Vigreux, E. Barthélémy, L. Bastard, J. E. Broquin, M. Barillot, S. Ménard, G. Parent, and A. Pradel, “Realization of single-mode telluride rib waveguides for mid-IR applications between 10 and 20 μm,” Opt. Lett. 36(15), 2922–2924 (2011). [CrossRef] [PubMed]
70. C. Vigreux, E. Barthélémy, L. Bastard, J.-E. Broquin, S. Ménard, M. Barillot, G. Parent, and A. Pradel, “Fabrication and testing of all-telluride rib waveguides for nulling interferometry,” Opt. Mater. Express 1(3), 357–364 (2011). [CrossRef]
