High-sensitivity liquid refractive-index sensor based ... - OSA Publishing

High-sensitivity liquid refractive-index sensor based on a Mach-Zehnder interferometer with a double-slot hybrid plasmonic waveguide Xu Sun,1,2,* Daoxin Dai,2,3 Lars Thylén,1,2,4,5 and Lech Wosinski1,2 1

School of Information and Communication Technology, KTH Royal institute of Technology, 164 40, Kista, Sweden 2 JORCEP, Joint Research Center of Photonics of the Royal Institute of Technology (Sweden) and Zhejiang University, China 3 Centre for Optical and Electromagnetic research, State Key Laboratory for Modern Optical Instrumentation, Zhejiang University, Zijingang Campus, Hangzhou, 310058, China 4 Dept. of Theoretical Chemistry and Biology, KTH Royal Institute of Technology, 106 91 Stockholm, Sweden 5 Hewlett-Packard Laboratories, Palo Alto, CA 94304, USA * [email protected]

Abstract: A Mach-Zehnder Interferometer (MZI) liquid sensor, employing ultra-compact double-slot hybrid plasmonic (DSHP) waveguide as active sensing arm, is developed. Numerical results show that extremely large optical confinement factor of the tested analytes (as high as 88%) can be obtained by DSHP waveguide with optimized geometrical parameters, which is larger than both, conventional SOI waveguides and plasmonic slot waveguides with same widths. As for MZI sensor with 40μm long DSHP active sensing area, the sensitivity can reach as high value as 1061nm/RIU (refractive index unit). The total loss, excluding the coupling loss of the grating coupler, is around 4.5dB. ©2015 Optical Society of America OCIS codes: (130.0130) Integrated optics; (130.6010) Sensors; (230.7370) Waveguides.

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Received 8 Jul 2015; revised 24 Aug 2015; accepted 16 Sep 2015; published 22 Sep 2015 5 Oct 2015 | Vol. 23, No. 20 | DOI:10.1364/OE.23.025688 | OPTICS EXPRESS 25688

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1. Introduction Integrated optical sensors [1], showing the abilities of high sensitivity, miniaturization and mass production, play increasingly important role in chemical and biomedical analyses. Various optical sensing elements, e. g., Mach-Zehnder interferometers (MZI) [2–6], ring or disk resonators [7, 8], Bragg gratings [9], have been proposed and evaluated during the past years. The changes of the guided-mode effective index (neff), affected either by changing of refractive indices of tested analytes (homogenous sensing) or by thin-layer receptors fixed on the surface of the waveguides (surface sensing), are readout by different optical measurement methods depending on sensor architectures. For most integrated optical sensors, waveguide optimization is the key design task to maximize the sensitivity, e. g. dielectric waveguides [5] and sub-wavelength grating waveguides [6]. Especially sub-wavelength grating waveguides are interesting as the analyte can directly infiltrate the area with high light confinement when the field delocalization is correctly engineered. Slot waveguides, including dielectric slot waveguides [3, 10, 11] and plasmonic slot waveguides [12], have been intensively investigated in order to achieve enhanced optical sensitivity. Differently from conventional Si nanowires, slot waveguides can confine optical mode inside the nano-slot due to high indexcontrast (dielectric slot waveguide) or surface plasmonic enhancement (plasmonic slot waveguide). By optimizing the geometrical parameters, ultra-high sensitivity can be obtained. Compared to dielectric slot waveguides, plasmonic slot waveguides can support subwavelength optical modes, but in the expense of large propagation losses. Hybrid plasmonic (HP) waveguides [13, 14], however, can support a mixture between plasmonic and photonics modes, and allow for sub-wavelength confinement with relative low propagation losses. This has attracted a lot of attention for realizing ultra-compact photonic integrated circuits [15–17]. Additionally, the ultra-high optical confinement factor in the low-index region, due to both, high index contrast and plasmonic enhancement, is also very promising to provide great performances for optical sensing applications. However, in the most often realized HP waveguide geometries [13–18], where the oxide material (e.g. SiO2) rather than open slot is placed between metal and high refractive index material, only few percent of an optical mode is evanescently confined by the covering analyte, which does not show the capacity of high sensitivity. In this paper, we introduce a novel double-slot HP (DSHP) waveguide with two open nano-slots between a high-index layer (Si ridge) and two metal strips (Ag), which is suitable to be filled with test analytes. Similar structures for other applications have been proposed in [18–20], but not yet been realized experimentally. The present DSHP waveguide supports quasi-TE polarization mode and can operate compatibly with conventional SOI optical

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devices. Highly-efficient grating couplers can be adapted at both ends of the device in order to obtain high coupling efficiency between optical fibers and the sensor chip. In this paper, exhaustive investigations have been made to optimize the optical confinement factor of the region filled by the tested liquids, which show the capability of highly-efficient sensing in sub-wavelength scale. Employing such a DSHP waveguide as a sensing element in one arm of an MZI, optical sensors have been realized with high sensitivity when testing the chemical liquid with different concentrations. Further applications, e. g. ultra-compact highly-efficient electro-optics (EO) modulators to realize high-density optical communication chips and photonic interconnects, have been also discussed. 2. Double-slot hybrid Plasmonic waveguide 2.1 Schematic of the DSHP waveguide Figure 1(a) shows the schematic of the DSHP waveguide, which consists of a SOI nanowire located in the middle of the plasmonic slot waveguide (we use Ag as the plasmonic material in this paper). Two narrow slots between Ag strips and Si nanowire are forming the hybrid guided-modes between photonics (Si-slot materials) and plasmonics (slot materials-Ag). Ag tapers are used to perform the photonic to hybrid plasmonic mode conversion. Normally, the width of Si ridge of the DSHP waveguide is narrower than the bus waveguide (SOI waveguide, set to 400nm), hence Si tapers (not shown in this paper) are also applied to connect the bus waveguide to the DSHP waveguide.

Fig. 1. (a) Schematic of the double-slot hybrid plasmonic (DSHP) waveguide coupled to SOI waveguides at both ends. (b) Cross-section view of the DSHP waveguide covered by test liquid. The widths of Si ridge and slots are denoted as wSi and wslot, respectively. The silver pads and Si ridge have identical height hWG. (c) Mode profile of the DSHP waveguide, where wslot = 150nm, wSi = 165nm and hWG = 250nm. The covering material is 100% 2-propanol (IPA).

The cross-section view is shown in Fig. 1(b): SiO2 material is used for the buffer layer, and liquid to be tested is covering the waveguide. The heights of the silver strips and the Si ridge are identical and set as hWG = 250nm. The widths of the nano-slot and the Si ridge are denoted as wslot and wSi, respectively. The mode profile is shown in Fig. 1(c), which is accomplished by finite-element-method (FEM)-based software, COMSOL Multiphysics. The geometric parameters are set to: wSi = 165nm, wslot = 150nm and hWG = 250nm. The

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thicknesses of covering liquids and buffer layers are set as 3μm, so the influences of simulation boundaries can be ignored. In the simulation work, the refractive indices of Si and SiO2 are 3.477 and 1.45, respectively, for the operation wavelength of 1550nm. The permittivity of silver is calculated by Durde model [21]:

ε = ε∞ −

ω p2

(1)

ω 2 + jωγ

where ε ∞ = 3.1 , ω p = 140 × 1014 rad / s and γ = 0.31× 1014 rad / s . In this paper, the liquids to be tested are the aqueous solutions of 2-propanol (IPA), whose refractive indices are taken from [22], as illustrated in Table 1. Table 1. Refractive indices of aqueous solution of 2-propanol [22] C 100% 80% 60% 40% 20% 10% 0 n 1.3776 1.3742 1.3717 1.3642 1.3514 1.3420 1.3330 Due to the high-index contrast (between the Si core region and the IPA cover layer) and plasmonic optical enhancement (Ag-IPA), there is a large interaction between optical mode and the covering material. Next, we will analyze and optimize the optical performance of the described DSHP waveguide with different geometrical parameters. 2.2 Model investigation In order to study the influence of nano-slots, we fixed the wSi with different values (100nm, 200nm, 300nm and 400nm), and gradually increase wslot from 20nm to 200nm. The effective refractive indices (neff) versus wslot are shown in Fig. 2(a). One can see that the neff decreases with increasing of wslot, and gradually tends to the value of SOI waveguides without the influences of plasmonic materials, when wslot is large enough (normally larger than 500nm, not shown in this figure). The larger neff of the DSHP waveguide with narrower slots indicates that the DSHP waveguide provides a larger optical confinement compared with conventional SOI waveguide, however, in the expanse of larger propagation loss, as shown in Fig. 2(b). The trade-off between optical confinement factor and propagation loss in plasmonic and HP waveguides can be chose depending on the applications [14–17]. The optical confinement factor is defined by the power confined in particular area divided by the total power: Γ=



area

E ( x, y ) dxdy /  E ( x, y ) dxdy, 2

2

(2)

∞

which is a key property of optical waveguide in homogeneous sensing applications [23]: the waveguide sensitivity ( ∂neff / ∂nliquid ) is proportional to the optical confinement factor in tested liquids (IPA in this paper), ΓIPA. As shown in Fig. 2(c), ΓIPA increases with narrower slot (smaller wslot) due to the larger plasmonic optical enhancement. While with larger wslot, the DSHP waveguide tends to SOI waveguide with smaller ΓIPA, and simultaneously,ΓSi increases due to lower influence of Ag, as shown in Fig. 2(d). Besides, the value of ΓIPA is generally larger for DSHP waveguides with smaller wSi, which act similarly as SOI waveguide: the optical mode is less confined in Si, but evanescently confined in covered liquids. However, one can observe from the results of DSHP waveguides with 100nm and 200nm wSi: as for wslot larger than 120nm, ΓIPA of waveguide with wSi = 200nm is larger than the one with wSi = 100nm, this indicates that the tendency of ΓIPA is not simply influenced by a photonics mode, but by some more complicated hybrid influences between photonics and plasmonics, which will be discussed in the following sections.

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Fig. 2. Performances (effective refractive index, loss and confinement factors) of double-slot hybrid plasmonic waveguides for different widths of Si ridge and slot. (a) Effective refractive index (neff) changed with slot width (wslot). (b) Loss in the units of dB/μm versus wslot. (c) and (d) Confinement factors in covering IPA and Si with different wslot.

From the analysis above, ΓIPA of DSHP waveguide is larger than for conventional SOI waveguide, which is caused by both, high index contrast and plasmonic optical enhancement, or in other words, the sensitivity DSHP waveguide is larger due to the proportional relationship between optical confinement factor and sensitivity. However, the results above are based on fixed width of Si ridge, and it is difficult to give an optimized ratio between those two modes. Thus, a q factor, which represents the width ratio of Si nanowire to the total width of DSHP waveguide, q = wSi/w, is defined. One needs to note that when q = 0 or 1, the optical waveguide is typically plasmonic slot waveguide with slot material as IPA or Si. The mode profile of DSHP waveguide with q factors of 0, 0.5 and 1 are shown in Fig. 3(a), (b) and (c), respectively. Figure 3(d) shows the neff variations with different q values, and when the total widths of DSHP waveguide w changes from 200nm to 700nm with a step of 100nm. The neff increases with the width ratio of Si ridge (q), which is caused by: (1) the large refractive index of Si causes the high neff of the propagation mode; (2) with a higher q value, wslot is smaller, which results in a larger ΓIPA as we discussed in Fig. 2(c). The former one is the explanation coming from the aspect ratio of photonics modes, while the latter one is from plasmonic optical enhancement. However, due to the interactions between the two modes, the increase of neff is more complicated than simple linear growth. As for the propagation loss, as shown in Fig. 3(e), there exist optimized lowest values for DSHP waveguide (0