Chalcogenide glass materials exhibit a variety of optical properties that make them desirable for near- and mid-infrared communications and sensing applications. However, processing limitations for these photorefractive materials have made the direct integration of waveguides with sources or detectors challenging. Here we demonstrate the viability of two complementary soft lithography methods for patterning and integrating chalcogenide glass waveguides from solution. One method, micro-molding in capillaries (MIMIC), is shown to fabricate multi-mode As2S3 waveguides which are directly integrated with quantum cascade lasers (QCLs). In a second method, we demonstrate the ability of micro-transfer molding (µTM), to produce arrays of single mode rib waveguides (2.5µm wide and 4.5µm high) over areas larger than 6 cm2 while maintaining edge roughness below 5.1 nm. These methods form a suite of processes that can be applied to chalcogenide solutions to create a diverse array of mid-IR optical and photonic structures ranging from <5 to 10’s of µm in dimension.
©2010 Optical Society of America
1.1 Chalcogenide glass waveguides for the mid-IR
The mid-infrared wavelength region (3-20µm) is the ideal operating regime for spectroscopic trace chemical detection due to the presence of fundamental molecular transitions . Chalcogenide glasses are an excellent dielectric material platform for constructing thin film photonic components for such sensors. They possess high mid-IR transparency, compared to oxide glasses, with low absorption into the 9-20µm wavelength range. In addition, as amorphous glasses, they have good formability compared to crystalline materials for low temperature processing .
The development of low loss chalcogenide glass waveguides as well as quantum cascade lasers (QCLs), mid-IR sources with the form factor of a semiconductor chip , offers the opportunity for chip-scale integration of waveguides with optoelectronic components. With miniaturized device footprints, portable mid-IR photonic chips would find use in a wide variety of field monitoring and remote sensing applications, from atmospheric greenhouse gas monitoring, to manufacturing process control, or from non-invasive detection of biomarkers for medical diagnosis, to free-space communication systems [4,5].
In this paper, we describe the development of chalcogenide glass waveguides for integration and single mode propagation, two important milestones for realizing chip-scale photonic circuits. For instance, Fig. 1 shows schematically how a waveguiding element, such as a y-splitter, can be integrated with a QCL chip. This functionality would be useful in a sensor, where the QCL emission would be split into a reference arm and a sensing arm. Moreover, with direct integration, light emitted by the laser is captured by the waveguide right at the facet, eliminating the need for collimating optics and improving the energy efficiency of the system.
To achieve this, design and fabrication processes to make chalcogenide glass waveguides for the mid-IR must be established. While there are many examples of chalcogenide glass waveguides developed for telecom wavelengths [6–12], there are fewer examples of mid-IR waveguides. Recently, mid-IR waveguides have been demonstrated using several different fabrication approaches. For example, laser writing is used to produce single mode channel waveguides for λ = 8.4µm in As2Se3 films deposited by thermal evaporation , while reactive-ion etching is used to pattern strip waveguides in sputter-deposited As2S3 films . In our prior work, a solution-casting and molding process using a soft lithography method called micro-molding in capillaries (MIMIC) [15,16], is employed to form multi-mode As2S3 strip waveguides .
Some special considerations should be taken to account in designing waveguides for the mid-IR. Substrate material choice is important since the absorptiveness of the substrate will have a large effect on waveguide loss . Low loss substrate possibilities include lithium niobate (LiNbO3)  or sapphire wafers for λ < 5µm, and salt crystals  or other chalcogenide materials [13,18] for longer wavelengths. In addition, compared to waveguides for shorter wavelengths, mid-IR waveguide dimensions will be in the 2-10µm range, which can make mechanical stress a concern in thin film deposition processes. While thin film deposition techniques like thermal evaporation or sputtering can reliably produce films a couple microns thick, deposition times may be cumbersome when thicknesses approach 10µm. Solution-processing chalcogenide glasses such as As2S3 thus becomes an attractive option.
1.2 Solution processing and patterning by soft lithography
Solution-processing chalcogenide glass offers several advantages as a route toward realizing integrated waveguide devices. Processes like MIMIC in which waveguides are formed by casting the solution into patterned molds are purely additive and etch-free. This, combined with low processing temperatures (below 185°C), makes hybrid integration with optoelectronic components feasible. The flexibility and variety of applicable processes means that depositing and patterning films thicker than 2µm is relatively straightforward, which is important for mid-IR applications.
In general, As2S3 can be made into a liquid solution by dissolving bulk glass pieces in propylamine or other appropriate solvent at a concentration of approximately 2g/10mL . Other chalcogenide glass types, such as Ge23Sb7S70, can also be processed into a solution in this manner , providing numerous choices for selecting a material with the desired properties. In solution form, the material can be deposited onto substrates or existing optoelectronic structures by a variety of methods, including spin-coating, ink-jet printing, drop casting, or mold-casting. For instance, drop-casting is used to apply an As2S3 film on top of a distributed feedback (DFB) grating on a QCL , which allows the QCL to be tuned by photo-modifying the refractive index of the As2S3.
The quality of the material produced from solution is sufficient for waveguide production. Studies of the solvent evolution during baking and the final material composition of solution-processed films show that resulting films have little absorption due to residual solvent . With post-process heat treatment (at temperatures between 120°C and the glass transition temperature ~185°C), the refractive index of the material and coordination number of the glass network will approach bulk values . However, as a result of this densification there can be a substantial volume change associated with heat treatment.
Applying soft lithography methods like MIMIC allows for the formation of waveguide microstructures on the order of 10s of microns in dimension. In this process, shown in Fig. 2 , As2S3 solution is forced into micro-channels created by a placing a relief-patterned poly-dimthylsiloxane (PDMS) mold onto a substrate. The As2S3 solution is pipetted to the channel inlet and fills the PDMS micro-channels by capillarity, and therefore the flow rate and incursion length is determined by solution viscosity and the dimensions of the channel . The sample is baked to remove the solvent and solidify the As2S3 structure, and in the final step, the PDMS mold is removed. As in typical soft lithography processes, the PDMS mold is created by casting PDMS pre-cursor on photolithographically patterned relief structures, curing the polymer, and peeling away the soft mold . In recent work, it has been shown that 40µm wide by 10µm high multi-mode As2S3 waveguides fabricated by MIMIC on NaCl substrates after heat treatment at 135°C for 6 hours have a measured propagation loss of 4.5 dB/cm at λ = 4.8µm .
A complementary process to MIMIC is micro-transfer molding (µTM), which can overcome the size and pattern limitations presented by relying on capillarity. µTM was originally developed to make optical waveguide structures using liquid prepolymer [25,26]. Plastic laser resonators  as well as integrated polymer waveguide devices  have been demonstrated using this technique. In µTM, as in MIMIC, a PDMS mold with a pre-patterned surface is employed to pattern a liquid pre-cursor. In this process, however, the PDMS mold is not used to form enclosed channels. Instead, the solution is deposited as a film either on the PDMS mold or a substrate (in this case, the PDMS mold), and sandwiched between mold and substrate (Fig. 3 ). As a result, µTM is more suited to produce large area patterns (at least 3cm2) , and smaller features without being limited by solution viscosity or effects of capillarity.
In this paper, we review the use of the MIMIC process to achieve an integrated QCL/waveguide chip  and introduce the use of µTM for fabricating integrated As2S3 waveguides. Propagation loss and edge roughness of these waveguides are characterized, showing low loss over large areas.
2. Experimental methods
2.1 Waveguide-QCL integration 
A 40µm wide by 20µm high PDMS channel is used to form the waveguide mold for the MIMIC process. We use a glass microscope cover slip as a waveguide substrate. This substrate is glued in front of the QCL bar (a cleaved chip containing an array of QCL ridges) in order to account for the location of the active region above the surface. The PDMS mold is aligned to the selected laser ridge and placed on the glass substrate. To ensure coupling between the laser and the waveguide, the channel overlaps the front facet of the laser. The laser end of the channel is widened from 40µm to 200µm to ease the alignment. After the As2S3 solution (2g/10mL propylamine) is cast into the channel, the sample is baked under vacuum, reaching a high temperature of 95°C, which is compatible with the QCL and the packaging materials. The solution is prepared following the process outlined in .
In the optical measurement of the integrated waveguide-QCL, the laser is operated at room temperature in pulsed mode (λ ≈5µm, 0.8% duty cycle). The output is collimated and focused by a series of two 2” (5.1cm) diameter ZnSe lenses (f = 1.5”, 3.8cm) and collected by a liquid nitrogen cooled HgCdTe (MCT) detector. The signal is amplified and read out on a lock-in amplifier.
2.2 Waveguides by µTM
To generate the waveguides by µTM, the As2S3 solution is spin-coated on a PDMS mold with grooves 3.5 µm deep, at speed 1000rpm for 10 seconds (Fig. 3b). A LiNbO3 substrate (x-cut, 0.5mm thick) is pressed down onto the spun-coat film (Fig. 3c). Capillary forces attract the substrate and film mold such that little downward pressure is needed to create adequate adhesion between the two. The substrate-film-mold assembly is then baked under vacuum (~8 x 10−3 Torr) by slowly ramping the oven temperature from 45°C to 100°C, at a ramp rate of 15°C/hr, and the bake is held at 100°C for 1 hr. The PDMS mold is then removed from the solidified film (Fig. 3d), and the LiNbO3 is cleaved to produce smooth waveguide end facets. To ensure crack-free films by spin coating, several modifications are made to the process. To combat the cracking caused by the rapid evolution of solvent from the thin film, the As2S3-propylamine solution is diluted with ethanol (1 part ethanol to 3 parts As2S3-propylamine by volume), a solvent with lower vapor pressure. In addition, the spin-coating is done in an enclosed box saturated with propylamine vapor.
For cut-back measurement of the µTM waveguides, one sample is used containing arrays of waveguides of widths 2.5, 5, 7.5, and 10µm. The waveguides are aligned and end-fired coupled to a QCL emitting TM-polarized light at λ = 4.8µm . The sample is cleaved to shorter lengths (every 2-4mm) from the output end of the waveguides, so that the same input facet conditions are present at each length. For each length, at least 5 waveguides of each width are measured, and the results averaged. The QCL, operating at room temperature, is biased at 50V and pulsed at 0.8% duty cycle. Two CaF2 lenses (1” diameter, f = 1”, 2.5cm) collimate and focus the waveguide emission into a cooled MCT detector.
3. Integrated waveguide and QCL by MIMIC
Straight As2S3 waveguides were previously fabricated by MIMIC and measured by cut-back resulting in loss as low as 4.5 dB/cm or 6.7 dB/cm when appropriately heat-treated on a NaCl or LiNbO3 substrate respectively . Here, we describe a waveguide with a 90° bend directly fabricated on a QCL, as shown in Fig. 4 . A bar of QCLs in Fabry-Perot configuration consists of ridges etched from epitaxially-grown heterostructure layers on an InP substrate. Ridges are typically 15-25µm wide and about 8µm high, and these ridge dimensions determine the size of the QCL modes . The integrated waveguide, 40 µm wide by 20 µm high, with a bend of radius 1mm, and an overall length of 7mm, overlaps the output facet of the laser ridge. Light emitted from the waveguide (λ ≈5 µm) is measured as the laser current is increased (Fig. 4b). The presence of the As2S3 waveguide (n ~2.3) on the as-cleaved laser facet (neff ~3.2), has the effect of reducing the reflectance of the laser facet by a factor of 10, however, good coupling efficiency is achieved, and light is effectively guided around the 90° bend .
The key to the hybrid integration of the As2S3 waveguide with a Fabry-Perot configured QCL is the MIMIC solution molding process. In one additive step, the material is deposited and patterned into a structure with dimensions matching the width and height of the laser ridge. However, the capillary forces that drive the filling process limit the dimensions of the structures that can be produced. In order to either increase the length or decrease the width and height, either the solution viscosity or channel surface properties must be modified, or external pumps must be used to aid flow .
4. Single mode waveguides by micro-transfer molding
In order to overcome the length and size limitations of the MIMIC process, we turn our attention to the µTM method to produce crack-free and uniform films with imprinted As2S3 waveguide patterns over a 6cm2 area as shown in Fig. 5 . The main difference in the resulting waveguide structure is that in the case of µTM, there is always a thin (< 1µm) As2S3 film surrounding the waveguides as shown in Fig. 3d. Waveguides with heights ranging 1-5µm and widths ranging 2.5-10µm are fabricated with good fidelity, as waveguide height and widths match those of the PDMS mold. Figure 5a shows an array of 7.5µm wide waveguides. Patterns are not limited to straight waveguides. Figure 5b shows a patterned y-splitter waveguide, with guiding arms 5µm wide.
Waveguides with dimensions for single mode propagation of λ = 5µm light are produced. Figure 5c shows an SEM image of a 2.5µm wide by 4.5µm high single mode waveguide, with a 1µm thick surrounding film. To verify that a rib waveguide with those dimensions is single mode, the structure is modeled in BeamPROP simulation software, using the optical properties (n, k) of the materials and an input Gaussian beam matching the QCL dimensions. The calculated mode field profile shown in Fig. 5d clearly demonstrates the single mode predicted operation expected from a waveguide of these dimensions.
A cut-back measurement is performed to measure the propagation loss of the µTM-generated waveguides. In this case, the waveguides are not heat treated following the µTM process. From a starting length of 16mm, waveguides of width 2.5, 5, 7.5, and 10µm, and height 4.5µm, and surrounding slab height of 1 µm, are measured (Fig. 6 ). The 2.5µm wide waveguides, which are single mode, have the lowest total loss. However, the losses per unit length are similar across the waveguide widths. Propagation loss of the 2.5µm x 4.5µm single mode waveguides as measured by cut-back is 4.52 ± 0.07 dB/cm on the LiNbO3 substrate, a 33% improvement over the MIMIC produced waveguides .
Factors contributing to absorption or scattering loss are examined. In this demonstration, we do not perform any post-process heat treatment of the waveguides to prevent volume change and to keep the designed dimensions. We expect that heat treatment at temperatures above 120°C will reduce absorption loss due to residual solvent and low glass density . In future designs, the PDMS mold must be fabricated with larger feature sizes to accommodate the volume reduction incurred by heat treatment.
In this demonstration, we observe formation of particulates that are likely crystallites on the surface of the film (Fig. 7a ) over the course of several hours exposure to ambient air, humidity and lighting illumination during optical measurements. These particles, which form by photo-decomposition and photo-oxidation [7,32], are likely contributors to increased scattering loss. Studies show that heat treatment  and applying a protective cladding such as poly-methyl methacrylate (PMMA) , forestalls the particle formation. Examination of IR transmission spectra of PMMA  shows that there is a transmission window at 3.6-5.5µm, which makes it a suitable cladding layer for the wavelengths used in this study.
Surface and edge roughness is characterized to determine scattering effects. Characterization of the film roughness is conducted by AFM. Figure 7b shows an AFM tapping mode scan of the top surface of a waveguide in a 2µm x 2µm area. The RMS roughness of the As2S3 film is found to be 0.75nm. This surface roughness value compares favorably to other examples of chalcogenide glass waveguides, thermally evaporated and dry etched which show a 1.5nm roughness , and thermally evaporated and patterned by lift-off reporting a roughness of 1.6nm .
Line edge roughness is measured by analyzing top down SEM images using edge detection algorithms  implemented in MATLAB. Figure 7c shows an edge trace taken from an image with 9.8nm pixel resolution. The two standard measures of line edge roughness are the standard deviation of edge roughness, σr, and roughness autocorrelation length, Lc. In the waveguide sample analyzed (w = 2.5µm), σr = 5.1nm and Lc = 441nm. These values are incorporated into the BeamPROP model as sidewall perturbations to calculate the loss dependence on σr, plotted in Fig. 7d. At these dimensions, even a modest increase in edge roughness would lead to increased scattering loss. The edge roughness of µTM-fabricated waveguides is comparable to the reported roughness of chalcogenide glass waveguides patterned by lift-off [10,35]. In addition, compared to MIMIC, µTM offers better control over edge roughness, yielding lines that show good fidelity with the mold, whereas heat treated MIMIC waveguides, with σr = 0.57µm and Lc = 2.9µm, display edge variations largely due to surface tension effects . In both cases, the correlation lengths are longer than typical Lc of plasma-etched waveguides . The short spatial frequency components of the sidewall variation are suppressed, likely because of smoothening due to surface tension.
Chalcogenide glass waveguides play an important role in the development of low cost and portable mid-IR sensing technologies. By developing processes to implement chip-scale integration and form large-area, single mode patterns, strides can be made toward realizing miniature sensors, such as micro-chip spectrometers , in mid-IR materials. Moreover, the availability of planar chalcogenide glass waveguides for the mid-IR opens up possibilities for integration with microfluidics , and even bio-functionalization  for sensing.
We demonstrate the viability of two complementary processes for patterning chalcogenide glass waveguides from solution. The MIMIC method reliably produces structures 10-40µm in dimension directly on the substrate, and is used to great effect for integrating mid-IR lasers. The µTM method extends the repertoire of solution-processing techniques and allows for a wider range of geometries that can be longer and narrower than those produced in the MIMIC process, including the fabrication of single-mode waveguides below 10 µm in dimension with arbitrary increases in length. Single mode As2S3 waveguides made by µTM display low surface and line edge roughness, demonstrating great potential for low loss applications in near- to mid-IR wavelengths. As demonstrated by the application of different soft lithography techniques, solution processing can be applied broadly, taking advantage of the diverse chalcogenide optical properties to lead the way in further study and development of much-needed components such as splitters, couplers, or resonators for use in emerging near- or mid-infrared applications.
This work is supported by NSF grant EEC-0540832 through the Mid-Infrared and Technologies for Health and Environment (MIRTHE) center. The authors acknowledge the use of the PRISM Imaging and Analysis Center, which is supported in part by the Princeton Center for Complex Materials, NSF grant DMR-0819860. The authors are thankful for the contributions of C. Gmachl, C. Madsen, F. Toor, and E. Mujagić in support of this work.
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