We report a novel approach for deposition of amorphous chalcogenide glass films inside the cylindrical air channels of photonic crystal fiber (PCF). In particular, we demonstrate the formation of nanocolloidal solution-based As2S3 films inside the air channels of PCFs of different glass-solvent concentrations for two fibers with cladding-hole diameter 3.5 and 1.3μm. Scanning electron microscopy is used to observe the formed chalcogenide layers and Raman scattering is employed to verify the existence and the structural features of the amorphous As2S3 layers. Optical transmission measurements reveal strong photonic bandgaps over a range covering visible and near-infrared wavelengths. The transmittance spectra and the corresponding losses were recorded in the wavelength range 500–1750 nm. The main advantage of the proposed technique is the simplicity of the deposition of amorphous chalcogenide layers inside the holes of PCF and constitutes an efficient route to the development of fiber-based devices combined with sophisticated glasses for supercontinuum generation as well as other non-linear applications.
© 2012 OSA
Photonic crystal fibers (PCFs) or microstructured optical fibers, consisting of micrometer scaled capillaries extending down the entire length of the fibers, have become a major topic of research over the last years. These fibers are made mainly of glass or polymer [1–4], in precisely designed geometries that can guide light either as index- or bandgap guiding through the refractive index difference between core and cladding [1,5]. Oxide based glasses, such as fused silica, is the main background material of researched PCFs. However, since silicate glasses exhibit limited non-linearity, and lack the ability to guide light in the mid-IR, it appears necessary to consider alternative glasses for mid-IR as well as non-linear applications.
Chalcogenide glasses (ChGs) appear as emerging materials offering great versatility in photonics since they combine high non-linear coefficients  (two to three orders of magnitude larger than silica’s nonlinearity), high refractive indices n = 2-3.5 , high transparency in the infrared  and their hallmark property, i.e. photosensitivity when illuminated with light at wavelength near the band gap edge [9–11]. Fabrication of chalcogenide step-index [12,13], tapers  and microstructured fibers [15,16] has been demonstrated by several groups for mid-IR broadband sources and other applications . However, the synthesis of chalcogenide glasses is more complicated to control compared for example to oxide-based glasses due to their lower chemical stability. Therefore, several difficulties arise during the fabrication of microstructured fibers .
On the other hand, since silica-based PCFs is a well-known and mature technology, several research groups have demonstrated infiltration approaches of materials with desired functionality inside the cladding holes of the PCF, leading to hybrid all-fiber devices. Liquid crystals (or liquids) [18–20], biolayers [21,22], polymers [23,24], ferrofluids , metals , etc. have been used as active materials. However, limited research has been so far carried out on the infiltration of silica PCF’s holes with chalcogenide glasses for the development of highly-nonlinear or bandgap fibers. Very recently, Granzow et al.  reported the infiltration of a chalcogenide glass in the holes of an all-silica PCF using the pressure-assisted melt-filling technique. The fiber was inserted in a high pressure furnace whereas the under pressure molten sulfur-based chalcogenide glass was infused in the holes of PCF at high temperature, i.e. at 665°C. However, such an approach requires specialized and custom-made equipment .
In this paper, we present a new facile and cost effective method for the deposition of thin layers of chalcogenide glasses in the inner surface of the holes of a PCF using nanocolloidal solution of As2S3 glass. Solution-based chalcogenide glass offers several advantages as a route toward realizing tailor made thin films of phase change materials for data storage  as well as integrated fiber devices as demonstrated here. There are several research groups working on the fabrication of planar waveguides for non-linear applications employing spin-coating or sputtering film deposition techniques of solution-processed chalcogenide glasses [28–30]. Fabrication procedures such as micro-molding in capillaries (MIMIC) are directly formed by casting the solution into patterned molds whereas the low processing temperatures (less than 180°C) make hybrid integration with optoelectronic components easily feasible. Another important advantage of the proposed method is that different chalcogenide glass types, can also be processed into a solution in this manner , providing numerous choices for selecting a material with the desired guiding properties.
In experiments presented here, the bulk glass (As2S3) is dissolved in amine solvents at different concentrations. Such solutions are practically nanocolloidal dispersions of As-S clusters, which are nearly monodisperse, whereas the cluster size depends on concentration . Capillary forces are capable of facilitating infiltration of the nanocolloidal solutions several centimeters into the holes of two different commercially available PCFs. Infiltration is followed by mild annealing at low temperature to evaporate the amine solvent, resulting in the deposition of an amorphous thin layer onto the cylindrical silica surface as graphically shown in Fig. 1 . Scanning electronic microscopy (SEM) is employed to identify the amorphous chalcogenide layer. The film thickness depends on the initial solution concentration and is estimated to range form a few to several tens of nanometers. Off-resonant Raman scattering is utilized to characterize the state and the structure of the deposited thin chalcogenide layers inside the air channels of the fiber. In addition, using a broadband supercontinuum source, the optical spectrum of PCF’s infiltrated with different nanocolloidal dispersions were measured indicating clear and well-defined photonic bandgaps that occur mostly in the visible spectrum range. The transmittance spectra and the corresponding losses were recorded in the wavelength range 500–1750 nm.
2.1 Preparation of nanocolloidal chalcogenide solutions
The bulk chalcogenide glass with composition As40S60 (As2S3) was synthesized by mixing appropriate amounts of high purity As and S elements (Alfa Aesar) with purity 99.9999% and 99.9995%, respectively, in evacuated and thoroughly pre-cleaned quartz ampoules. The procedure took place in an Argon-filled glove box to avoid oxidation of Arsenic. The quartz ampoule with the elements was placed in rocking furnace and was heated to 700 °C for 48 hours to ensure sample homogeneity. The glass was prepared by quenching the melt to room temperature. Nanocolloidal solutions of the chalcogenide glass were prepared by dissolving glassy As2S3 into n-butylamine (BA, Acros Organics, 99.5%) and ethylenediamine (EDA, Alfa Aesar, 99%). One solution was prepared in BA with a concentration of c1 = 166 mg/ml and another one in EDA with much denser, c2 = 615 mg/ml (2.5 mol/L). The dissolution of As2S3 was slow in BA; taking almost 3 weeks for complete homogenization. On the other hand, the dissolution in EDA was much faster, lasting for a few hours. The As2S3/ethylenediamine solution was viscous, with a viscosity of about 50 cP . All solutions were clear, with a yellow to amber color depending on the concentration and remained stable for months.
2.2 Fiber preparation
In our experiments, we employed two commercially available PCFs, named as ESM-12-01 and LMA-5, with different pitch Λ (hole-to-hole distance) and air-hole diameter, d. The ESM-12-01 has Λ = 7.7μm and d = 3.5μm, whereas LMA-5 has Λ = 1.4μm and d = 1.3μm. The liquid material was infiltrated inside the holes of PCF using capillary forces in a sealed ampoule. For both concentrations (c1 and c2) several centimeters of the solutions were infused in the holes of both ESM-12-01 and LMA-5. In particular, ~4 cm of the c1 solution was infused into the ESM-12-01 in about 20 min. On the other hand, it required almost 2 hours to infiltrate the same fiber at a length of ~5 cm with the denser concentration (c2). LMA-5 has almost half cladding hole diameter (1.3 μm) compared to ESM-12-01. Consequently, it required ~21 min to fill a length of ~2.5 cm and ~2.5 hours to fill ~3 cm length of the fiber with c1 and c2, solutions respectively. The solution-filled fibers were placed in a fume hood for 24 h and then in an oven at ~50 °C for another 24 h, in order to allow for solvent evaporation and consequently to form an amorphous As2S3 layer in the inner surface of the PCFs’ channels. Based on the aforementioned annealing conditions of the PCFs, the refractive index of the formed As2S3 films inside the holes of the PCFs is expected to be around 2.4-2.6 as was experimentally demonstrated in . In this context, the annealing process provides the ability to control and optimize the refractive index of the infused glass, adjusting the guiding properties of the hybrid fiber with respect the desired application.
3. Results and discussion
3.1 Scanning Electron Microscopy images
A field-emission scanning electron microscopy (Zeiss SUPRA 35VP) was used to identify the amorphous As2S3 layers formed inside the holes of PCF. Figure 2(a) shows the core cross-section of ESM-12-01 filled with c2 concentration. The dark area corresponds to diffused solution appeared at the top surface after cleaving the fiber. Figure 2(b) shows a magnification of a hole around the core, where the As2S3-formed layer can be clearly seen.
In order to confirm the homogeneity of the deposited film, the fiber surface was inclined by 20°, as shown in Fig. 2(c), revealing that amorphous films have been deposited inside all holes around the core. Figure 2(d) displays a magnification of a single hole, showing the formed “bumps” at the smooth surface of the silica channel clearly indicating the nanoscaled thin As2S3 film. It should be mentioned here that for the dilute solution (c1), the formed films were just a few nanometers thick, thus being quite difficult to be quantitatively identified. In order to verify the film uniformity along the fiber, the latter was cleaved into several pieces of few mm in length and SEM images were recorded from all the cross sections. Figures 3(a), (b) and (c) , illustrate the same fiber, after cleaving a part of about 8.5 mm in length, confirming the existence of thin As2S3 layers. Inspecting Fig. 3 (c), one can undoubtedly see the deposited film, which is relatively uniform but contains small bumps (islands). Most probably, these originate from the colloidal nature of the solutions and the way that the solvent evaporates.
Similarly, Fig. 4(a) shows the formed As2S3 layers in the air-holes of the LMA-5 fiber, after evaporating the same concentration (c2). The fact that the cladding holes in this fiber are much smaller than the corresponding ones in the ESM-12-01, makes the solvent evaporation process less efficient with respect concentration c1. Therefore, under the mild annealing conditions followed here, part of the solvent does not evaporate. Indeed, as Fig. 4(a) reveals, some of the air channels are completely filled with a dense EDA/As2S3 solution. In addition, even for holes, where solvent evaporation takes place, we see that the remaining film is thicker than in the case of the ESM-12-01 fibers. The occluded solvent does not practically affect the outcome of the present study, since its main effect is to slightly change the refractive index of the deposited film . This change slightly affects the transmission properties (in our case the position and the bandwidth of the bandgaps) of the fiber, whereas with a controlled evaporation process (i.e. infiltration uniformity, annealing temperature, etc), the layer thickness and the yielding transmission properties can be optimized. This adds an extra advantage, since the bandgaps in transmission (see next section) can be tuned depending on the desired application. Further, with a controlled evaporation process, the layer homogeneity can be improved, avoiding the formation of bumps.
3.2 Raman scattering
Micro-Raman spectroscopy measurements were performed in back-scattering geometry using a He-Ne laser operating at 632.8 nm, i.e. at off-resonance conditions in order to avoid photo-induced effects in the As2S3 and the solutions. A metallurgical microscope (Olympus BHSM-BH2) was used for the delivery of the excitation beam and for the collection of scattered light. The light beam was focused using 100 × and 50 × objectives and the backscattered light (detected by a charge-coupled device cooled to 140 K) was analyzed by a Jobin Yvon T64000 Raman instrument operating at double subtractive mode with resolution of about 3 cm−1. Representative Stokes-side Raman spectra from the bulk glass, the As2S3/amine solutions and the amorphous films formed inside the air channels of the PCF after solvent evaporation are shown in Fig. 5 . The Raman spectra of the As2S3/BA and As2S3/EDA nanocolloidal dispersions were excited at 1064 nm and recorded with a Fourier-transform Raman spectrometer. Figure 5(a) shows the spectra related to c1 concentration (BA solvent), while Fig. 5(b) shows the corresponding spectra for the dense c2 concentration (EDA solvent). The spectra drawn in dashed lines correspond to the neat vibrations of As-S clusters after subtraction of the solvent bands. The bands denoted as A (~328 cm−1) and B (~375 cm−1) represent the molecular vibrations of AsS3/2 pyramidal units of a depolymerized structure . The origin of the dominant Raman band C located at ~435 cm−1 is still controversial [32,35]. Although it has been assigned to S-S vibrations in As-S-S-As units, we know that such disulfide vibrational modes are at much higher wavenumbers, i.e. at ~495 cm−1 . We suggest that the 435 cm−1 vibrational mode arises from new bonding configurations formed between arsenic, sulfur and nitrogen in the presence of the solvent. The gradual fading of the intensity of this band with annealing supports this suggestion.
The Raman spectrum of As2S3 (see bulk glass As2S3 curve in Fig. 5) has been extensively studied and analyzed in the literature. The spectrum is dominated by a broad band in the range 250-420 cm−1 which contains a number of various Raman lines. The latter have been assigned to the symmetric and anti-symmetric bond stretching vibrational modes of AsS3/2 pyramidal units bridged via S atoms to form a quasi-two-dimensional network structure . Dissolution of As2S3 in amines results in nanocolloidal dispersions where small clusters of As2S3 are isolated by amine molecules as envisaged in [32,35]. The hydrodynamic radii of arsenic sulfide clusters in BA was recently determined by dynamic light scattering  and found in the range 1 – 4 nm for concentrations comparable to c1 of the present investigation. Cluster sizes in the dense (c2) dispersion are expected larger than the above values. To this end, as a result of the network degradation in the solution, the Raman spectrum of the dissolved molecular-like species of As2S3 in BA and EDA are characterized by bands, as shown in Fig. 4 (see As2S3/BA(c1) and As2S3/EDA(c2) curves). In general, the spectra of As-S in both the BA and EDA solvents bear a close resemblance showing three main bands at the same energies, indicated by the vertical dashed lines. Their main difference relies to the relative intensity ratio of these bands, which reflects the differences in the As-S/amine interactions. Further, the spectrum bands of As2S3/EDA(c2) are sharper and are in agreement with the corresponding spectrum of a solution with similar concentration reported elsewhere . In contrast, the spectrum bands of As2S3/BA(c1) are broader as BA solvent has several Raman bands in that energy range, where the vibrations of arsenic sulfide clusters in solution also appear (see BA curve in Fig. 5(a)). The situation is more convenient in the case of EDA since only one weak band appears at a frequency somewhat higher than the vibrations of the As-S clusters in solution and the vibrations of the dried amorphous films.
The top curves of Fig. 5(a) and (b) denote the Raman spectra from As2S3/ESM and As2S3/LMA fibers, recorded from the amorphous film remained inside the PCF air-holes, after solvent evaporation. It can be seen that after solvent evaporation, the As2S3 network structure is re-built in both LMA-5 and ESM fibers. In particular, the spectrum of As2S3/ESM is essentially similar to that of the bulk glass apart from a slight broadening at the high energy tail, which reflects contribution from solvent remnants after evaporation at the positions of bands B and C. This finding demonstrates that the dissociated As-S clusters in the solution re-polymerize to produce again the As2S3 network structure in the film deposited inside the air-holes of the PCF. The situation is slightly different for the amorphous films structure produced by evaporation of the EDA solvents, (see As2S3/LMA(c2) curve in Fig. 4(b)). In this case, the occulted solvent that remains after the mild annealing is responsible for the sharp structural features at ~390 and 435 cm−1.
3.2 Transmission spectrum and loss
The transmission spectra of As2S3/ESM and As2S3/LMA-5 fibers for both concentrations (c1,c2) were measured by launching white light from a broadband supercontinuum laser source (500-1700nm). The output beam was collected with a 20 × microscope objective and a multimode fiber, while the transmitted signal was monitored on an optical spectrum analyzer. Any undesired cladding light was blocked by inserting an iris diaphragm into the beam path, such that only light from the waveguide core was recorded. Figure 6 displays the corresponding results. In particular, Fig. 6(a) and (b) display the transmission spectra of As2S3/ESM(c1) and of As2S3/ESM(c2), while Fig. 6(c) and (d) the corresponding normalized losses (dB/cm). From Fig. 6(a), the strong bandgap guidance can be easily observed. The As2S3/ESM(c1) fiber exhibits a cut-off at around 1200 nm, whereas at short wavelengths (500-700nm), it has five narrow guiding windows. At longer wavelengths (750 – 1200nm), there are two clear guiding bands with bandwidth ~150nm (750-900nm) and ~300 nm (900-1200nm), respectively. In contrast the As2S3/ESM(c2) fiber exhibits sharper and clearer bandgaps up to 1750 nm wavelength. At short wavelengths As2S3/ESM(c2) exhibits several bandgaps with relative small bandwidths at short distances. At longer wavelengths, bandgap bandwidth gradually increases along with the spectral distance between them as shown in Fig. 6(b).
The corresponding losses of As2S3/ESM(c1) and As2S3/ESM(c2) were measured using the cut-back technique and are shown in Fig. 6(c) and (d), respectively. For As2S3/ESM(c1), which has a lower refractive index, the overall loss of the infiltrated fiber is several times lower than that of As2S3/ESM(c2). The latter exhibits a maximum loss of 35dB/cm over the 1000-1250nm band (see Fig. 6(d)). The inset of Fig. 6(c) corresponds to the near-field image of the fundamental guiding mode of As2S3/ESM(c2) captured using a CCD camera. It can be seen that light propagates at the high-index layered inclusions of the fiber. However, the overexposed output near field pattern is distorted due to the cleaved end facet of the fiber.
Figures 7(a) to (d) displays the corresponding transmission spectra and losses for the case of As2S3/LMA-5 fiber for both concentrations as well. As2S3/LMA-5(c1) exhibits a cut-off around 1500 nm (see Fig. 7(a)), while As2S3/LMA-5(c2) around 1150 nm (see Fig. 7(b)). From Fig. 7, as well as Fig. 6, it is clear that the denser concentration c2, which results to thicker As2S3 layers, yields sharper and broader transmission bands. For example in the case of As2S3/LMA-5(c2), the maximum bandwidth (~275 nm) of a guiding window was observed over the 875 – 1150 nm wavelength range. The corresponding loss for As2S3/LMA-5(c1) was measured to span from 20 to 3 dB/cm. As2S3/LMA-5(c2) exhibits similar loss levels but with a different variation. The optical image inserted in Fig. 7(b) corresponds to the overexposed near-field output pattern of the fundamental mode of the As2S3/LMA-5(c2) fiber. In comparison to As2S3/LMA-5(c2), the fraction of light in the amorphous chalcogenide layers is clearly higher primarily because of the thicker formed shells in the channels of the PCF.
Generally, in all the aforementioned transmission spectra, the bandgap edges can be theoretically described by employing the anti-resonant reflecting optical waveguide (ARROW) model [38,39]. According to the ARROW model, the locations of the stop bands, which appear as transmission dips in the transmission spectra (see Fig. 6(a),(b) and Fig. 7(a)(b)), are determined by the resonance of an individual high-index layer and coincide with the cutoffs of the guiding modes in the layers. In addition, it was also found that the stop bands are derived, only from specific guiding modes. Numerical analysis has shown that in the weak-guidance approximation, the locations of the transmission dips are determined by the cutoffs of LP0l and LP1l modes [38,39]. However, in this work, it was not possible to determine accurately the position of the bandgap edges as well as the cut-off wavelength, because the nanometer-scaled film thickness was not accurately determined, combined with the fact that the refractive index of the formed layers depends on the annealing process of the solvent. In principle, a small change to these parameters will cause a significant change to the positions of the bandgaps.
In this paper, we presented a novel approach of combining solution-processed As2S3 chalcogenide glasses with the well-grounded technology of silica PCFs. Two different concentrations (155 and 615 mg/ml) of n-butylamine and ethylenediamine-based As2S3 solutions were prepared and infiltrated in two different commercially available PCFs (ESM-12-01 and LMA-5). The evaporation of the amine solvent led to the formation of nanometer thick layers of amorphous As2S3 at the inner surface of the PCFs. Scanning electronic microscopy (SEM) was employed to identify the amorphous chalcogenide layer and Raman scattering to characterize the state and the structure of the deposited thin chalcogenide layers inside the air channels. It was shown that after solvent evaporation, the As2S3 network inside the air-holes of both PCFs is re-built to a large extent, forming the desired thin amorphous film. Transmission spectra indicated strong photonic bandgaps at visible and infrared wavelengths. In the case of the denser concentration c2, thicker layers were formed, yielding sharper and more well-defined bandgaps. Selecting the proper As2S3 concentration and optimizing the annealing process, the layer thickness can be controlled, thus modifying the transmission properties of the fiber.
The proposed cost-effective approach has several advantages against other techniques. Τhe prime one is its simplicity in developing the hybrid fiber, requiring ambient conditions, commercial available fibers and no sophisticated equipment. The glass-solution is infiltrated rather quickly inside the fiber channels and its controlled evaporation can lead to amorphous layers with a varying fraction of the solvent, thus changing the refractive index of the layer. The proposed approach can be used for a number of other solution-processed chalcogenide materials with different linear and non-linear optical properties.
Finally, the proposed method provides the possibility for multi-layer deposition of different materials, which may lead to the development of novel tunable devices for non-linear or sensing applications.
This work was partially supported by the “MEDOYSA” project (09SYN-24-769), which is partly funded by the Hellenic General Secretariat for Research & Technology under the Synergasia programme. The authors would like to thank Dr. G. Kakarantzas (NHRF), Dr. V. Dracopoulos (FORTH/ICE-HT) for the SEM images, Th. Vassiliadis (FORTH/ICE-HT) for his help in the solution preparation and FORTH/ICE-HT for providing the Raman scattering facilities.
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