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A selenide integrated platform working in the mid-infrared was designed, fabricated and optically characterized at 7.7 µm. Ge-Sb-Se multilayered structures were deposited by RF magnetron sputtering. Using i-line photolithography and fluorine-based reactive ion etching, ridge waveguides were processed as Y-junction, spiral and S-shape waveguides. Single-mode optical propagation at 7.7 µm was observed by optical near-field imaging and optical propagation losses of 2.5dB/cm are measured. Limits of detection of 14.2 ppm and 1.6 ppm for methane and nitrous oxide, respectively, could be potentially measured by using this platform as an evanescent field sensor. Hence, these technological, experimental and theoretical results represent a first step towards the development of an integrated optical sensor operating in the mid-infrared wavelength range.
© 2016 Optical Society of America
1. Introduction
The Mid-Infrared (mid-IR) is a spectral range (2-20 µm) of great scientific and technological interest. Indeed, the atmospheric transmission windows (3-5 µm and 8-13µm) and the strong vibrational absorption bands of numerous gases, liquids or solids overlap this wavelength range [1, 2]. Recent progress in fabrication of low-loss optical platforms and quantum cascade laser sources (operating from 3.7 µm to 24 µm) have enabled the development of mid-IR optical sensors [3, 4]. Integrated optical sensors could allow quantitative, sensitive and selective detection of numerous molecules for health, defense and environmental applications. Moreover, they provide several advantages over other kind of sensors, such as high integration of elements in a compact device, immunity to electromagnetic interference and low fabrication cost by an easy-going to mass production. .
Photonic integrated circuits operating in the mid-IR have been implemented using the mature technology offered by III-IV and group IV semiconductors. These devices have taken advantage of the mid-IR transparency of silicon (2–8 µm), germanium (2-14 µm) and gallium arsenide (2-16 µm) [5] to integrate passive components at chip level [6–8]. Chalcogenide glasses (ChGs) are amorphous semiconductors containing elements such as tellurium, selenium or sulphur covalently bonded with As, Ge, Sb, Ga [9]. They can be doped with rare-earth ions and processed as fibers, thin films or integrated waveguides displaying a broad transparency extending towards the mid-IR, respectively up to 12 µm, 15 µm and 20 µm for sulphide, selenide and telluride glasses [10]. Mid-IR supercontinuum generation [11] and sensing have been demonstrated using chalcogenides fibers [12,13].
Integrated mid-IR chalcogenide devices have been fabricated on different substrates such as silicon (Si) [14], calcium fluoride (CaF2) [15], sulphide glass (As2S3) [16], sodium chloride (NaCl) [17], lithium niobate (LiNbO3) [18] or directly integrated on QCL’s chips [19]. The feasibility of chemical-biological sensing in gas and liquids environment have been shown by employing mid-IR luminescence with doped chalcogenide platforms [20, 21].
In this paper, single-mode selenide ridge waveguides in multiple configurations (spiral, Y-junction, S-bend) are fabricated through RF magnetron sputtering, photolithography and dry etching. This chalcogenide platform is designed for sensing applications in the mid-IR. Light propagation in the waveguides is demonstrated at λ = 7.7 µm and optical propagation losses are measured at the same wavelength which overlaps the absorption bands of several substances (CH4, N2O) [1]. A further analysis of gas detection at 7.7 µm is carried out in order to calculate the lowest concentration potentially measurable by the optical evanescent integrated sensor. Except for numbering and titling of sections, which may not be desirable for short articles, the express journal style and layout rules have been followed in this guide. There is a checklist available in Section 8 that summarizes the style specifications.
2. Design and fabrication
Selenide glass targets, named Se6 and Se2, with respective nominal compositions of Ge12.5Sb25Se62.5 and Ge28.1Sb6.3Se65.6 were synthesized [22] by conventional melting and quenching techniques from commercial elemental precursors of high purity (5N). The chalcogenide thin films were subsequently deposited onto silicon substrates by RF magnetron sputtering under argon pressure of 10−2 mbar. The multilayered structure was made of a 5 µm-thick lower confinement layer (Se2) and a 1.7 µm-thick guiding layer (Se6). Ridge waveguides of different widths were then fabricated using a classical i-line photolithographic process (MJB4 Suss Microtech mask aligner) followed by a dry etching procedure at low pressure (Corial 200IL) combining reactive ion etching (RIE) and inductively coupled plasma (ICP) etching (5 sccm CHF3, 5 mTorr, 75 W ICP, 25 W RF). Under these optimized etching conditions, no lateral undercut was observed and the etching rate of the chalcogenide film was 330 nm.min−1. Scanning electron microscopy shows that vertical waveguide sidewalls are achieved in Fig. 1(a). Optical characterizations were performed by coupling a QCL emitting at 7.7 µm (1290 cm−1) from Alpes Lasers into the ridge waveguides through a ZnSe microscope objective (Innovation Photonics) and imaging the output facet on a microbolometer-based focal plane array (FPA, Optris PI400) using a second ZnSe microscope objective.
Fig. 1 (a) Cross section of ridge waveguide (SEM image) made of two different compositions of chalcogenide glasses: Ge12.5Sb25Se62.5 (Se6) as guiding layer, Ge28.1Sb6.3Se65.6 (Se2) as confinement layer and the silicon substrate. (b) Schematic of cross-section of chalcogenide ridge waveguide showing the dimensions and refractive index of the different layers. (c) Evolution of the evanescent power factor η as a function of width (w) and height (h) in the detection gases at λ = 7.7 µm. The color area represents the dimensions where the effective index of guided mode is a value amongst the refractive index of confinement and guiding layer respectively. (d) Simulation of TM fundamental mode in a ridge waveguide (h = 1.7 µm, w = 10 µm).
The ridge waveguide structures were designed using a commercial software (FIMMWAVE, Photon Design) to obtain the geometrical dimensions, width (w) and height (h), which fulfill the condition: nconfinement < Neff < nguiding at 7.7 µm, while maximizing the evanescent power factor in the superstrate. This evanescent power is given by the part of optical field non-confined in the waveguide guiding layer [1] and it takes into account the high-index-contrast structure [23]. The sputtered chalcogenide confinement layer was intentionally restrained in the range of 4-6 µm in order to limit optical losses generated at the interface confinement-guiding layer as RMS roughness is usually increasing for PVD films proportionally to the film thickness [24]. For Se6 guiding layer, the thickness has been set between 1.5 and 2 µm, single-mode propagation at a wavelength of 7.7 µm was shown in Fig. 1(c) for ridge waveguide widths ranging from 8 to 15 µm. An optimal evanescent power factor of 5% is obtained for a 10 x 1.7 µm2 ridge waveguide (width x height), this calculation is based in the Effective Index Method.
In the context of integrated optical evanescent sensors, the devices based on chalcogenide glasses could take advantage over the alternative semiconductor technologies such as Si and Ge materials, because they present a weaker refractive index contrast (Δn) at the interface between waveguide and the sample medium. Then, this work presents a Δn between air and Se6 of ≈1.77 (air-Se2) leading an evanescent power factor around 5% for the optimal dimensions. For GaAs, Ge and Si, the larger Δn between guiding layer and air (≈2.3-2.5) lowers the evanescent power factor below 2% [8, 25, 26]. In the case of sensors made of As2Se3 [27], the index contrast is ≈1.7 but the structure implemented limits the evanescent power factor to 0.75%. The stronger evanescent optical field in the superstrate generated in the presented chalcogenide structure could enable a higher sensitivity for mid-IR sensors. Furthermore, selenides waveguides are hydrophobic and therefore relatively resistant compared to immersion in a liquid or a gas. But they present a photosensitivity that can eventually degrade them [28, 29].
A numerical analysis based on the model developed by Marcatili and Miller [30] was performed to estimate the bending losses for the single mode ridge waveguides. The aforementioned model is used to quantify the radiative losses on mode confinement as shown in Fig. 2(a) and it assumes that the bending loss implies an exponential dependence on the radius of curvature. The evolution of bending losses as a function of the radius of curvature for guiding layer widths equals to 10 µm of selenide ridge waveguide is presented below in Fig. 2(b).
Fig. 2 (a) Simulation of mode profile in a bend waveguide. (b) Bending losses at 90° as a function of the radius of curvature for guiding layer widths equal to 10 µm of selenide ridge waveguide.
The simulation indicates negligible bending loss in selenide ridge waveguides for radius of curvature larger than 150 µm and widths of 10 µm. The loss tends to increase drastically as the bend radius decreases. The waveguides (spiral, S-shape and Y-junction) designed for this work, present an internal radius in this order of magnitude, thus the optical bending loss may be neglected here.
Ridge selenide waveguides, width from 8 to 14 µm and height about 1.7 µm, were obtained on Si substrate [Fig. 1(a)]. Spiral ridge, Y-junction and S-shape waveguides were also fabricated and optically characterized to evaluate the optical losses at 7.7 µm. The widths of these transducers were 10 µm, 12 µm and 14 µm. The spiral waveguide presents a total length of 22.45 mm, an internal radius of 200 µm and a separation gap of 50 µm [Fig. 3(a)]. Several lengths of ridge waveguide have been obtained by the S-shape waveguide the range of 12 to 37 mm, the radius at S-shaped curve is 150 µm, then the gap separation is 300 µm [Fig. 3(b)]. With regard to Y-junction waveguide, the length of arms is 500µm, the gap between them is 400 µm and the internal radius 200 µm [Fig. 3(c)].
Fig. 3 (a) Near field of propagated light at λ = 7.7 µm and microscopic images of single mode ridge waveguides made of chalcogenide glasses (Ge-Sb-Se) in configuration of spiral, (b) S-shape and (c) Y-junction.
3. Optical characterization
Optical propagation losses for single mode ChGs waveguides was measured using a cut back method from S-shape waveguides by using two guiding structure widths of 10 µm and 14 µm. Different S-shape waveguide lengths (1.95 cm, 2.2 cm, 2.45 cm, 2.7 cm, 3.2 cm and 3.7 cm) having the same number of bends of 150 µm radius were fabricated. The intensity of guided mode, observed at the output of the waveguide by near-field at λ = 7.7 µm, was measured by the thermographic camera as a function of the waveguide lengths by using the S-shape and the experimental configuration previously detailed. A similar technique has been proved for surface scattering loss [31]. At initial stage, a calibration shows the linear performance of the imaged intensity to respect of QCL’s output power. The reproducibility was confirmed by three series of measurements for each waveguide length. From the cut-back measurements are reported Fig. 4, propagation losses of selenide ridge S-shape waveguides have been estimated to 2.6 ± 0.1 dB/cm for waveguides of width 14µm, and α = 2.5 ± 0.1 for widths 10 µm and 12 µm respectively.
Fig. 4 Optical propagation loss at λ = 7.7 µm measured by mode profile imaging method for selenide ridge waveguides with height equal to 1.7 µm. Each point represents averaged data for 3 experimental measures. Propagation loss of 2.6 ± 0.1 dB/cm and 2.5 ± 0.1 dB/cm are given for widths of 14 µm and for 10 µm, 12 µm respectively.
The losses in selenide waveguides arise from three sources namely (a) the defects located at selenides confinement-guiding layer interface, (b) the side wall roughness and (c) volume scattering of selenide guiding layer [24, 32]. Beside around 7.7 µm, it has been reported an increase in the propagation loss due to fluorocarbon contamination of the waveguide surfaces given by the fluorine-based etching [24, 33]. At wavelengths around 7.4 µm, optical losses could rise due to fluorocarbon absorption. An alternative solution to etch the guiding layer is to use argon RF plasma [34]. Nevertheless, these waveguide losses are sufficiently low for cm scale photonic integrated circuits and are comparable to the values obtained in others waveguide platforms in this wavelength range. For instance, optical losses measured on SiGe graded index waveguides at 7.4 µm were equal to 2 dB/cm [35] or on ridge and planar ChGs waveguides and they were inferior to 1 dB/cm for a wavelength range of 3.0-4.2 µm and 6.2-7.4 µm [33] and also at 8.4 µm [27]. For wavelengths in the range of 3.7–5.8 µm, other low loss propagation devices have been proposed: As2S3 waveguides on a LiNbO3 have been demonstrated with propagation loss equal to 0.33 dB/cm at 4.8 µm [36], ring resonators made of GeAsSe on GeAsS [37] and As2S3 on GeSbS [14] have reported propagation loss equal to 0.84 dB/cm and 0.7 dB/cm at 5.2 µm respectively. Moreover, some mid-IR waveguides have been fabricated in Si and Ge exhibiting low propagation loss, for example silicon nitride ridge waveguides [38] working at 3.75 µm with propagation loss of 2.1 dB/cm, Si3N4 on Ge ridge waveguides [7] of 2.5 dB/cm at 5.8 µm and air-clad pedestal Si waveguides [6] of 2.7 dB/cm at 3.7 µm. Silicon on sapphire waveguides presenting loss close to 1dB/cm around 4 µm have been also reported [39, 40].
4. Study of gas sensing based on evanescent field absorption at 7.7 µm
Providing an optimized evanescent field power factor, the chalcogenide integrated platform has been designed for sensing gas applications in the Mid-IR by using the evanescent field absorption. The wavelength range between 7.0µm and 8.0µm contains absorption peaks of trace gases, particularly the methane (CH4) and nitrous oxide (N2O) at 7.7 µm (Fig. 5(a)). The optical absorption is totally dependent on the concentration of the target substance, which may be associated with the optical power transmitted P through the waveguide with the Lambert-Beer’s law [41] as follows:
Where P0 (W) is the optical power at the waveguide input, η (%) is the evanescent power factor, α (cm−1) is the waveguide propagation optical loss, L (cm) is the waveguide length (optical path), C is the concentration of target substance and ε the molar absorption. For the detection, the output optical signal power is monitored and it varies as a function of the concentration C (mol L−1). Indeed, the power attenuation increases with higher levels of the solute concentration, but also it is directly dependent of η.Fig. 5 (a) Molar absorption of methane and nitrous oxide in the Mid-IR under normal conditions of pressure and temperature (1 atm, T = 273 K) [42, 43]. (b) Evolution of the sensitivity S as a function of the waveguide length L for α = 2.5 dB/cm.
It is noteworthy to consider that for an optimal integrated sensor design, there is a compromise between a high evanescent power factor and a low propagation loss value. Lastly, the ridge selenide waveguides presented in this paper have an evanescent power factor three times higher than As2Se3, GaAs, Ge and Si devices, and a similar value of propagation loss as those measured for waveguides in Si or Ge in the mid-IR [6, 7, 35, 38].
The sensitivity S is given by the variation of output optical power as a function of C, it could be calculated differentiating Eq. (1) with respect to C [41]:
For this expression, the condition is accomplish. Thus, the lower are the propagation losses the higher is the sensitivity.The sensitivity of the integrated sensor is calculated as a function of the waveguide length L taking into account the parameters previously obtained: η = 5%, α = 2.5 dB/cm, P0 = 1 mW. The results are showed in Fig. 5(b).
The maximum of the function S allows to determine the optimum waveguide length Lopt, which is 1.7 cm for 2.5 dB/cm of propagation loss. For this purpose, we have fabricated the spiral guide.
The limit of detection is physically represented by the lowest concentration Cmin that the evanescent field sensor could be detect, nevertheless it is limited by the performance of the photodetector employed in the measurement. Afterwards some parameters of the photodetector must be considered to calculate Cmin, such as the signal to noise ratio SNR (including the coupling losses), the bandwidth B and the noise equivalent power NEP. The expression to calculate Cmin [41] is given by the following expression:
It is possible to estimate Cmin in the detection of CH4 and N2O at 7.7 µm by using the Eq. (3). For this analysis we propose as photodetector the DSS-MCT14 020L from Horiba, for which the operation spectral range is from 3 to 12 µm, NEP = 5x10−12 W Hz-1/2, B = 5 KHz and the SNR is fixed equal to 10 considering the coupling loss. The molar absorption for methane and nitrous oxide are εCH4 = 174 (L mol−1 cm−1) and εN2O = 1484 (L mol−1 cm−1) respectively, these coefficients are obtained from the NIST Chemistry Web Book absorption spectra [44] and they were normalized with the reported absorption cross sections [42, 43], the calculation involves: Absorbance = Cε (cm−1).The lowest concentrations are Cmin-CH4 = 6.35x10−7 mol L−1 = 14.2 ppm and Cmin-N2O = 7.44x10−8 mol L−1 = 1.6 ppm. The occupational exposure limits recommended by the international environmental standards to methane and nitrous oxide are 1000 ppm [45] and 25 ppm [46] respectively, then Cmin-CH4 and Cmin-N2O are lower than these values. Likewise, the limits of detection presented in this paper are at least three magnitude orders lower than the most recent measurements of CH4 in the Mid-IR achieved with an integrated setup [47]. Hence, we consider that the integrated optical sensor may be employed as an alarm tool for environment monitoring.
5. Conclusions
In summary, the optical design, the technological processing and the optical characterization of mid-IR selenide ridge waveguides have been presented. Light propagation through the spiral, Y-junction and S-shape waveguides was observed at 7.7 µm. The bending losses were subsequently neglected by a numerical estimation for internal radius equal or higher than 150 µm. Intrinsic propagation losses of 2.5 dB/cm and 2.6 dB/cm were measured by near filed-imaging method for ridge S-shape waveguides with height equal to 1.7 µm and widths of 10 µm and 14 µm respectively. Furthermore 2.5 dB/cm is typical value reported in silicon and germanium waveguides in the mid-IR, nevertheless this chalcogenide platform present a higher evanescent power factor (η = 5%) allowing to enable higher sensitive detections and also to achieve theoretical limits of detection of Cmin-CH4 = 14.2 ppm and Cmin-N2O = 1.6 ppm at 7.7 µm, which are lower than the limit occupational exposures determined by the international environmental standards. Hence, these theoretical and experimental results are the first stage in the development of a chalcogenide optical integrated sensor for the mid-IR range.
Funding
Consejo Nacional de Ciencia y Tecnología (OptiMIR PhD grant No. 421002); Région Bretagne (PONANT) IFREMER (Institut Français de Recherche pour l'Exploitation de la Mer)/BRGM (Bureau de Recherches Géologiques et Minières).
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