Several groups have demonstrated the possibility of molding chalcogenide glasses against a stamp to make rib waveguides.^[19–25] The process of hot-embossing is conceptually straightforward: chalcogenide film (deposited on silicon wafer) is heated to a temperature above its T_g (glass transition temperature), pressed against a stamp isothermally for a certain time under a prescribed load. After the removal of the load, chalcogenide film sample is cooled to ambient temperature, and then separated from the stamp manually due to tremendous differences in thermal expansion coefficient between the film and stamp material. The stamp imprint is retained in the film surface and then the desired waveguide structure is fabricated. The replication accuracy mainly depends on the quality of the surface finish of the stamp and thus it is repeatable to fabricate successive waveguides with the same stamp.^[26] There are two main stamps used for hot-embossing: hard stamp and soft stamp. Hard stamp which consists of metal (e.g. Ni), silicon, fused silicon, silicon-on-insulator or metal-coated hard polymers, is usually made by e-beam lithography and wet-etching and imprinted on small areas at high loads.^[27] On the other hand, soft stamp can realize multiple and diverse uniform patterning and is more tolerant to the tilt between the stamp and sample than hard stamp.^[28] In addition, soft stamp is easily obtained by casting on a single master mould over large areas with excellent accuracy and reproducibility. Currently, polydimethylsiloxane (PDMS) stamp is widely used because it is sufficiently durable to permit many pattern replications and allows the production of the waveguides with no stamp release issues and good uniformity.^[24,27]

Fig. 2.

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	Fig. 2. (color online) Schematic flow chart of the hot embossing process. (Seddon et al. 2014).[26]

ChGs are metastable and long-range disordered materials, and compared with the crystalline material, their bonded or atomic structures tend to be more prone to change when exposed to illumination with light, thermal treatment, electromagnetic radiation and particle beam bombardment.^[29] That is because (i) chalcogenide glass has a lower spatial mean coordination number (MCN), which means that it has a high degree of freedom and structural flexibility and (ii) chalcogen atoms are two-coordinated and have two high-lying lone-pair electrons on the top of its valence bands. When the chalcogenide glass is exposed to laser light, these nonbonding lone-pair electrons are readily excited across the bandgap by the absorption of light, leading to changes in physical and optical properties of chalcogenide glass. Some distinct photoinduced phenomena are observed in amorphous chalcogenides, including photocrystallization, photovaporization, photodecomposition, photocontraction or expansion, photodarkening or photobleaching, etc. These changes are accompanied by changes in the electronic band gap, refractive index and optical absorption coefficient.^[4] To summarize, ChGs exhibit distinct photosensitivity and devices such as gratings and waveguides can be fabricated in ChGs by using their photoinduced effects.^[30–33] However, the excellent photosensitivity of ChGs is undesirable for fabricating waveguides via hot-embossing. Therefore, it is essential that the core material in the rib waveguide should have good photo stability and high thermal stability (i.e., it remains amorphous and does not devitrify, crystallize in the hot embossing process). Most of selenide-based glass materials have been chosen as the core glasses for fabricating waveguides via hot-embossing due to their excellent glass stability during manufacture under ambient conditions.^[21–24] Generally, the waveguide is based on the chalcogenide film on an oxidized silicon wafer. For evanescent-field-mediated sensing, some groups have also employed a bulk chalcogenide glass with a lower refractive index as both the substrate and optical under-cladding (chalcogenide-on-chalcogenide).^[21,22,24] To select the core and cladding glasses to make a high numerical aperture (NA) single-mode waveguide, some criteria should be meet: i) thermal expansion coefficient mismatching between the core and cladding glasses should be minimum; and ii) the cladding glass should have a higher T_g than the core glass, and this enables a range of temperatures in which the substrate is either solid or much viscous compared with the supercooled liquid thin film in this temperature range.

Several methods have been used to deposit high quality chalcogenide film, including thermal evaporation, radio-frequency (RF)-magnetron sputtering, ultra-fast laser ablation and collosol-gelling method. Each method has its own advantages and drawbacks, and currently thermal evaporation and radio-frequency (RF) sputtering are two main methods to deposit the films. During the thermal evaporation, the source material to be evaporated is placed in a boat and connected to a power supply for heating under vacuum conditions. When heated to a certain temperature, the solid source material is converted into gaseous particles with certain kinetic energy, leaping into high-vacuum space, and finally deposited onto the substrate surface. Thermal evaporation is convenient to prepare large-area thick film with a relatively high deposition speed, but it has problems that the deviation of component between source material and deposited film is relatively large if I) depositing materials have high melting points and II) multi-components and/or composite materials have a large melting point difference between their constituents.^[34] Because non-congruent evaporation occurs, the evaporated particles with high saturated vapor pressure tend to deposit on the substrate preferentially, resulting in nonhomogeneous films. Zakery and Elliot argued that thermal evaporation might not be suitable for the accurate transfer of the stoichiometry of the multicomponent glass films.^[35] Besides, the reproducibility of the film composition is not ensured and the film thickness is difficult to control precisely. On the other hand, RF sputtering is usually performed in an Argon (Ar) gas atmosphere in an evacuated chamber. When the target surface is bombarded with generated Ar⁺ ions, the solid atoms emitted from the sputtered material are condensed and deposited on the surface of the substrate. Unlike thermal evaporation, RF sputtering can ensure the exact reproducibility of the film composition. However, the deposition-rate of RF-sputtering is low and the column-like structures always can be found in the deposited films, resulting in porosity.^[36,37] Balan et al.^[37] demonstrated the refractive indexes of these films were shown to decrease with increasing film thickness because of a change in the morphology (porosity) of the film.

There are two different approaches to hot-emboss the waveguide device: hard and soft-pressing. In the former case, the pressing facility consists of a vacuum chamber and two flat and parallel aluminum plates. The upper aluminum plate is fixed and the lower one can be raised with applied force. After the stamp is placed on the lower plate where the patterned face is up, the sample is placed on the top of the stamp where the sputtered film is faced down to the patterned stamp surface, both aluminium plates are heated to a certain temperature, and then the lower aluminium plate will be raised to accomplish the embossing [see Fig. 3(a)]. The drawback of this approach is the possible misalignment between the stamp and film and uneven loading of the film. Pan et al.^[21] reported ChGs monomode rib waveguides fabricated by means of the hard-pressing with a hard silicon stamp in a one-step ‘fiber-on-glass’ approach. In the latter case, the pressing facility consists of a chamber separated by an elastic membrane. The sample with contacting the stamp sits on a hotplate and in the bottom of the chamber. The hotplate is then heated to a certain temperature, and the bottom of the chamber is evaculated and then the difference in pressure between the up and bottom of the chamber will realize the imprint isobarically [see Fig. 3(b)].^[24] In this way, it is potential to be tolerant to tilt the misalignment between the stamp and sample. The pressing temperature is limited by elastic membrane, which means that the elastic membrane should be heat resisting. Han et al.^[24] first demonstrated a smooth-walled chalcogenide glass optical waveguide molded by soft-pressing approach with a PDMS stamp.

Fig. 3.

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	Fig. 3. (color online) Schematic diagrams of the imprinting chambers of (a) hard-pressing and (b) soft-pressing approach.

Pan et al. (2008)^[21] reported a chalcogenide-glass mono-mode rib waveguide fabricated by hot-embossing through using a hard silicon stamp first in a one-step ‘fiber-on-glass’ approach involving two types of chalcogenide glasses (core/under-clad: As₄₀Se₆₀/Ge₁₇As₁₈Se₆₅). The As₄₀Se₆₀ fibers were hot-pressed between a silicon stamp (the fibers are aligned such that each fiber lay in one channel in the stamp) and a polished and flattened Ge₁₇As₁₈Se₆₅ glass substrate. One of the waveguides was an As–Se rib core with a width of 5-μm and a depth of 1.7 μm and the lowest single-mode waveguide propagation was 2.2 dB/cm for the TE mode at 1550 nm, which is in agreement with numerical modeling. The near-field image and the measured full-width-at-half-power revealed single-mode guiding [see Fig. 4]. Similar waveguides were reported by Lian et al. (2009)^[22] who studied a hot-embossed mono-mode rib waveguide in a thermally-evaporated As₄₀Se₆₀ glass thin film on a Ge₁₇As₁₈Se₆₅ glass substrate (core: 5-μm wide and 1.9-μm high) [see Fig. 5]. Both chambers were heated to 245 ^°C, slightly higher than T_g of the substrate (T_g of the Ge₁₇As₁₈Se₆₅ glass substrate and bulk As₄₀Se₆₀ glass are 236 ± 2 ^°C and 178 ± 2 ^°C, respectively). At this pressing temperature, a substrate became slightly softened and enabled the rib waveguides to be evenly patterned on the thin film surface. The propagation loss was 2.9 ± 0.1 dB/cm, and this value is likely to be overestimated as the end faces of the waveguides are not ideally smooth and flat, resulting in an underestimation of the end-face reflection contribution to the measured round-trip loss.^[22]

Fig. 4.

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	Fig. 4. SEMs of (a) Si mold cross-section and (b) top view of As40Se60 ribs on the Ge17As18Se65 substrate together with a cross-sectional view at higher magnification. (c) Nearfield intensity profile observed at 1.55 μm. (Pan et al. 2008).[21]

Fig. 5.

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	Fig. 5. SEM images of rib waveguides in the chalcogenide glass sample, showing the cross-sectional view of (a) the corresponding embossed rib waveguide and the top view (with a slight angle) of (b) the corresponding embossed glass rib waveguide. (Lian et al. 2009).[22]

Han et al. (2010)^[24] reported low-loss, smooth-walled, chalcogenide glass optical waveguides molded in a single step from As₂₄Se₃₈S₃₈ glass film via hot-embossing with an innovative soft PDMS stamp which could produce waveguides with good surface morphology and low optical losses of 0.26 dB/cm and 0.27 dB/cm for the quasi-TM and quasi-TE polarizations, respectively, at 1550 nm (core: 2 μm–5 μm wide and 1 μm high) [see Fig. 6(a)]. However, due to the relatively low T_g of As₂₄Se₃₈S₃₈ glass (~110 °C) and significantly high imprint temperature (~250 ^°C), it was reported later (Han et al. 2011)^[25] that severe material decomposition and crystallization occurred in the hot-embossing process, and the As₂₄Se₃₈S₃₈ films were found also to exhibit photosensitivity at 1550 nm. To solve these problems, the thermal evaporation of a 50-nm thick Ge_11.5As₂₄Se_64.5 film on the top of the thick As₂S₃ layer was used to protect the surface from degrading during embossing, and this resulted in rib waveguide with better structural stability but slightly higher optical loss of 0.52 dB/cm for the TE and 0.41 dB/cm for the TM polarizations at 1550 nm for a waveguide cross-section dimension of 3.8 × 1 μm [see Fig. 7] (Han et al. 2011).^[25]

Fig. 6.

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	Fig. 6. SEM images of (a) imprinted rib waveguide in As24Se38S38 chalcogenide glass and (b) cross sectional optical microscope view of waveguide. (Han et al. 2010)[24]

Fig. 7.

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	Fig. 7. (color online) SEM images of the embossed As2S3 waveguides: (a) side wall image at 45°; (b) plan view of the embossed waveguide surrounded by depressed trenches; (c) magnified portion from panel (b) showing some roughening at the bottom of the rib. (Han et al. 2011).[25]

Zou et al. (2013)^[38] reported imprinted waveguides from thermally evaporated and solution deposited As₂Se₂ glass films by using hot-embossing. Optical loss in the waveguide fabricated from thermally evaporated film was measured to be 0.8 ± 0.3 dB/cm at 1550 nm, which is far lower than that in the waveguide from solution processed film (8.2 ± 0.6 dB/cm) by cut-back by using the optical fiber end-fire coupling. The higher loss in solution processed film is deemed to result from residual solvent absorption.^[38] Abdel-Moneim et al. (2015)^[23] reported the hot-embossing fabrication of low-loss, single-mode rib optical waveguide in RF-magnetron sputtered-deposited As₂Se₃ chalcogenide-glass thin film (core/under-clad: As₂Se₃/Ge₁₇As₁₈Se₆₅; core: 4 μm–6 μm wide and 1.9 ± 0.1-μm high) [see Fig. 8]. The maximum optical propagation losses measured by the Fabry–Perot technique were smaller than 0.78 dB/cm for the TE mode, and smaller than 0.81 dB/cm for the TM mode, at 1550 nm, respectively. That is because the Fabry–Perot technique is known to over-estimate the loss in most cases. It has been shown that the loss in the sputter-deposited thin film is lower than that in a thermally-evaporated As₄₀Se₆₀ glass thin film on a Ge₁₇As₁₈Se₆₅ glass substrate (Lian et al. 2009).^[22]

Fig. 8.

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Fig. 8. (color online) For the hot-embossing waveguide from RF-sputtered As–Se thin film on the pressed Ge–As–Se substrate: (a) ESEM cross-sectional image of a cleaved edge of an RF-sputtered As–Se thin film on the top of a Ge–As–Se substrate. (b) a near-field intensity profile observed at 1550-nm wavelength in the quasi-TE mode. SEM image of (c) top view of ribs and (d) one of the ribs at higher magnification, with a cross-sectional view sown in the inset. (Abdel-Moneim et al. 2015).[23]

In Table 1 we list the representative hot-embossing experimental results obtained to date.

Table 1.

Table 1.

Table 1. Summary of the lowest recorded optical losses at 1550-nm wavelength for ChGs rib waveguides. The rib waveguides are shaped via hot embossing on either FOG or thermally evaporated, or RF-sputtered deposited, As–Se or As–S–Se films. . Comprising: over-clad/core/under-clad Method of core film deposition (prior to shaping waveguides) Lowest recorded loss of rib waveguide at 1550-nm wavelength Ref. Air/As40Se60/Ge17As18Se65 As–Se fiber (FOG) 2.2 dB/cm for waveguides 5-μm wide × 1.7-μm deep [21] Air/As40Se60/Ge17As18Se65 thermal evaporation 2.9 dB/cm for waveguides 5.5-μm wide × 1.9-μm deep [22] Air/As24S38Se38/oxide-coated Si thermal evaporation 0.26 dB/cm for quasi-TM mode, 0.27 dB/cm for quasi-TE mode, for waveguides 2- to 5-μm wide × 1-μm deep [24] Ge11.5As24Se64.5/As2S3/oxide-coated Si thermal evaporation 0.52 dB/cm for the TE and 0.41 dB/cm for the TM polarizations for waveguides 3.8-μm wide × 1-μm deep [25] Air/As2Se3/oxide-coated Si thermal evaporation 0.8 ± 0.3 dB/cm [38] Air/As2Se3/Ge17As18Se65 RF-magnetron sputtering <0.78 dB/cm for TE mode, <0.81 dB/cm for TM mode, for waveguides 4- to 6-μm wide × 1.9 ± 0.1-μm deep [23]

Table 1. Summary of the lowest recorded optical losses at 1550-nm wavelength for ChGs rib waveguides. The rib waveguides are shaped via hot embossing on either FOG or thermally evaporated, or RF-sputtered deposited, As–Se or As–S–Se films. .