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
Integrated optical sensors , 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 , 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  and sub-wavelength grating waveguides . 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 , 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 index-contrast (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 sub-wavelength 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 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.
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 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 :22], as illustrated in Table 1.
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:23]: the waveguide sensitivity () 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.
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<q<1), when w is larger than 200nm, which is lower than purely plasmonic slot waveguide (q = 0 or 1). Together with the results shown in Fig. 2(b), we can conclude that the propagation loss of DSHP waveguide is higher than conventional SOI waveguide and lower than plasmonic slot waveguide, which indicates that the mode confined in DSHP waveguide is a mixture between loss-less photonics and lossy plasmonic modes. Additionally, similarly to other types of plasmonic waveguides, with a wider width of the waveguide, the propagation loss is lower, which can be explained by the fact that the influence of plasmonic material (silver in this paper) is significantly reduced, when the effective area of confined mode is larger. When w decreases down to 200nm, the conventional Si nanowire hardly supports photonics mode due to diffraction limit, and the DSHP waveguide acts similarly to plasmonic slot waveguide (plasmonic mode dominates the propagation), without optimized value for loss.
Figure 3(f) shows the optical confinement factors of covering IPA material. Similarly to propagation loss, optimized ΓIPA values can be obtained, which can reach as high as 88% confinement for the DSHP waveguide with the width of 200nm. Special attention needs to be paid, when the width of the DSHP goes down to 200nm, the optimized q factor still exists, which is different from the analysis of propagation loss in Fig. 3(d). That is to say that even though photonic mode disappears when the waveguide dimension decreases below diffraction limit, the high-index of Si material can also play a role in increasing the ΓIPA.
Moreover, the confinement factors of the nano-slot (Γslot) (the optical power concentrated in the slot), shown in Fig. 3(g), are also investigated, which are about 10% lower than respective ΓIPA, that includes, both slot and top IPA layer. This means that the most of optical field is concentrated in the nano-slot and the remaining 10% is leaking to the top IPA layer (similar amount is also leaking to the SiO2 substrate). In the further discussion about the electro-optics (EO) modulator we will consider the values for Γslot. The confinement factors of Si (ΓSi) changes inversely proportional compared to ΓIPA and Γslot, as shown in Fig. 3(h).
Summarizing the analysis above, we can conclude that DSHP waveguide has better performance in the aspects of propagation loss and sensitivity than pure plasmonic slot waveguide. The optimized q factors for propagation and sensitivity are different, which need careful design to satisfy the requirement of particular applications.
3. Experimental results
As we discussed above, the guided-mode of the DSHP waveguide is extremely sensitive to the refractive index change of the tested liquids, which results in a phase change. In order to transfer the phase change into measureable intensity signals, resonant cavities (like ring/disk resonators or MZIs), can be used. In this paper, we design and fabricate an MZI with a DSHP waveguide as an active sensing arm to experimentally evaluate the performance of the DSHP waveguide for homogenous sensing.
3.1 MZI sensor with a DSHP waveguide
The designed MZI employing a DSHP waveguide in one arm as an active sensing area is shown in Fig. 4. Due to the notable propagation loss introduced by the sensing arm, an asymmetric Y-splitter is applied to improve extinction ratio. The Y-splitter is composed of a straight and S-shaped bent wires with bending radius of 5μm (denoted as rref), which are connected at the input/output ends. Compared to typical 3dB splitter, the designed Y-splitter is estimated to give more optical power to the sensing arm, which can compensate the coupling and propagation losses of the DSHP waveguide sensing area. The length of the reference arm, Lref, is 67μm with a width of 400nm (wref = 400nm). As for the sensing arm, the DSHP sensing area is inserted between the input and output SOI waveguides, which have the same widths as the reference arm. The lengths of the input and output SOI waveguides, Linput and Loutput, are both 20μm. Si tapers are used as the couplers between the SOI waveguides and the DSHP waveguide sensing area, as shown in the sub-figure. As for the sensing area, two Ag pads are put aside the Si ridge to form the DSHP waveguide, and the tapers of Ag pads are used to decrease the coupling loss from SOI waveguide to DSHP waveguide. Taking into account optical confinement factor, propagation loss and fabrication difficulties, the parameters for the DSHP waveguide are chosen as: wSi = 165nm and wslot = 150nm (w = 465nm and q = 0.355), which will give about 78% ΓIPA and 0.01dB/μm propagation loss according to the simulation results shown in Fig. 3. The length of the DSHP waveguide is varied from 20μm to 40μm with a step of 10μm, which is denoted as LDSHP. One should notice that when the length of the DSHP waveguide decreases, a part of Si ridge with 165nm width will be without metal on both sides, which will also support optical propagation with a relative large loss (near cut-off width). The length of this part is denoted as Lridge, which ranges from 15μm to 5μm depending to the length of the DSHP waveguide.
The total effective length difference between the two arms is then:
3.2 Fabrication and measurement setup
The fabrication process starts from commercial Silicon-on-insulator (SOI) wafer with 250nm crystalline Si on top of 3μm thick SiO2 buffer layer. After patterning the Si structure with e-beam lithography (EBL), Inductively Coupled Plasma (ICP) dry etching with 10% over etch is performed, which is processed by C4F8-SF6 gas mixture under low temperature (~10 °C). Then, after removal of e-beam resist, the second E-beam exposure and etching process are performed to fabricate the highly efficient non-uniform grating couplers . The etching depth is around 80nm. Finally, the pattern of silver pads is introduced by the third E-beam exposure. After patterning, 20nm Ti and 230nm Ag layers are evaporated by metal evaporation tool, where the Ti layer is used to increase the adhesive strength between silver and substrate material (SiO2). Then, metal lift-off process is used to open the silver pads.
The optical characterization is carried out by grating coupler setup: an input optical fiber connected to continuous wave (CW) tunable laser with wavelength range of 1460-1580nm is placed above the grating coupler at one end of a tested device; another optical fiber connected to optical spectrum analyzer (OSA) is placed at the other end of the device. Polarization controller is also used before input into the device, to achieve the characterization with only TE mode. The test liquids (different concentrations of IPA in water) are directly dropped onto the surface of the tested sample. The transmission response of the tested device is then obtained after adjusting the positions of input and output optical fibers.
3.3 Characterization results
The scanning electron microscope (SEM) top-view of the MZI sensor is shown in Fig. 5. False monochrome color is added to the picture in order to enhance contrast. The lower red-wire is Si reference arm, with a width of 400nm, and the upper narrower one is Si ridge of DSHP waveguide (wSi = 165nm). The white bright elements with tapers are Ag pads (wAg = 1µm). The black substrate is SiO2 buffer layer. Grating couplers (not shown in the figure) are placed at each end of the sensor and give around 50% coupling efficiency (estimated result) between optical fiber and the device.
The measurement is accomplished by infiltrating the fabricated sample with different concentrations of IPA. Figure 6 (a) shows the measured transmission response of MZI sensor with 30μm long DSHP waveguide covered by 100% (black curves) and 60% IPA in water (red curves). The refractive indices of 100% and 60% IPA are 1.3773 and 1.3717 , respectively. The reference level is the transmission response of the straight waveguide with input/output grating couplers, which is normalized to 0dB (green dashed line). Sinusoidal fitting curves are applied in order to clearly show the transmission properties. One can see that the resonant wavelength (λres) of MZI sensor decreases with the refractive indices of the covering material (from 100% to 60% IPA), which can be easily understood by the basic principles of MZI sensor, where the resonant wavelength, λres = ∆leff/m, is proportional to the effective length difference. When the refractive index of the covering material decreases, the effective refractive indices of the elements in sensing arm will decrease more than those in the reference arm (due to higher sensitivity of the DSHP waveguide), which will consequently lead to the decreasing of the effective length difference, according to Eq. (3). Hence, the resonant wavelength λres will shift to the left (shorter wavelengths) when covered with lower refractive index material. The free-spectral-range (FSR) of the interferometer is about 10nm, and the extinction ratio is around 5dB, when infiltrated with 100% IPA. Compared to the reference level (grating coupler, 0dB), the total loss, excluding the grating coupler loss, is about −4.5dB.
Figure 6(b) shows the normalized output powers of the MZI sensor with 30μm long DSHP waveguide. The different color curves are the measured results for various concentrations of IPA in water (ranges from 100% to 10%). From the fitted sinusoidal curves for the MZI's responses, one can easily read the resonant wavelengths for the tested liquid with different concentrations and the sensitivity is given by
Figures 7(a)-7(c) show the transmission responses of the MZI sensors with 20μm, 30μm and 40μm-long DSHP waveguides respectively covered with 100% (black curves) and 60% IPA (red curves). One can clearly observe that the wavelength shifts are increased with the length of DSHP waveguide, which ranges from 1.2nm (for the design with a 20μm-long DSHP waveguide) to 6.4nm (for the design with a 40μm-long DSHP waveguide). Figure 7(d) shows the wavelength shifts versus refractive index of covering liquids (100% to 10% IPA). The corresponding sensitivities read from fitting lines are 153nm/RIU, 406nm/RIU and 1061nm/RIU. By simulating the neff differences of Si ridge and DSHP waveguide merged in 100% and 60% IPA (∆neff,ridge = 0.0007 and ∆neff, DSHP = 0.0047), without consideration of the phase-shift provided by Si and DSHP tapers, the phase change of the sensing arm with and without 40μm DSHP waveguide is −0.23 (2π) and −0.04 (2π), respectively. In the experimental results, the large difference of phase changes is demonstrated. However, the phase changes are not only provided by the neff differences of the sensing arm, but also provided by: the phase shifts caused by DSHP and Si tapers, the dispersion of MZI sensor, fabrication roughness and alignment error of Ag pads, which altogether result in a larger phase-shift, about −0.55 (2π) for 40μm DSHP waveguide, than the estimated results.
4. Potential application of the DSHP waveguide for an EO modulator
Besides the optical liquid sensor application, the present structure is also a promising solution for an ultra-compact highly-efficient optical modulator when the nano-slots are filled with an active nonlinear material. Particularly, for EO modulators, the plasmonic material (silver or gold) pads can be also used as electrodes, which will provide a strong electric field even with low bias voltage because of the narrow distance between two metal pads. Differently from optical sensors, the modulation efficiency of an EO modulator depends on the interaction between optical and modulating radiofrequency (RF) signals. Normally, only the optical confinement factor of materials filled into the nano-slot (Γslot) dominants the performance of an EO modulator. As we discussed in Fig. 3, the Γslot can reach as high value as 75% (w = 200nm, q = 0.75), which is about 10% higher than plasmonic slot waveguide, and 25% higher than Si slot waveguides .
Additionally, the adjustable performances of the DSHP waveguide (propagation loss and sensitivity) provide solutions for different qualities of EO polymers. For example, if the EO polymer has large nonlinear coefficient (r33), a relative compact DSHP waveguide can be designed in order to increase the eficiency, reduce the power consumption and the device size. Vice versa, one can also get enough phase modulation by prolonging the DSHP waveguide with acceptable propagation loss, when low nonlinear coefficient (r33) EO polymers are used.
In this paper, we have theoretically and experimentally evaluated the performances of the DSHP waveguide in the application to homogeneous sensing. First, we have done numerical simulation and shown that the present DSHP waveguide has better sensitivity than SOI waveguide with the same width of Si ridge. The optical confinement factor for the region with the covering material (proportional to waveguide sensitivity), ΓIPA, increases with narrower wslot. The factor ΓIPA can reach as high as 90%, when wslot = 20nm and wSi = 100nm, however, in the expense of large propagation loss (0.1 dB/μm).
Further optimizations have been made by tuning the Si ridge ratio, q = wSi/w, from 0 to 1, as shown in Fig. 3 (d)-3(h). Results show that the DSHP waveguide has lower propagation loss as well as higher optical confinement factor ΓIPA than a pure plasmonic slot waveguide, when the waveguide width is larger than 200nm, where hybrid photonic-plasmonic modes can be supported. We have shown that high sensitivity waveguide with acceptable propagation loss can be obtained by careful design. Generally, within the width ranges from 200nm to 700nm, the propagation loss can be as low as 0.001dB/μm (w = 700nm, q = 0.65), while the optical confinement factor of covering material can reach as high value as 85% (w = 200nm, q = 0.5), depending on the requirements of the device size and sensitivity.
After the theoretical evaluation of the DSHP waveguide, we have designed and fabricated an MZI architecture employing a DSHP waveguide as an active sensing arm. The geometrical parameters of the DSHP waveguide are: wSi = 165nm and wslot = 150nm (ΓIPA≈78% and Loss≈0.01dB/μm according to simulation results). Grating couplers are used to couple light from optical fiber to the device and back to fiber, which have a coupling efficiency around 50%. Experimental results show that the sensitivity increases dramatically with the length of the DSHP waveguide: as for MZI with 40μm DSHP waveguide, the sensitivity can reach as high value as 1061nm/RIU.
Further discussions on optical modulators, especially on EO modulator, have been done, where we discussed the advantages of the DSHP waveguide in the applications for optical modulator: (1) large sensitivity to slot materials (large Γslot); (2) adjustable waveguide performance for different quality of nonlinear polymer; (3) good interaction between RF mode and optical mode of EO modulators (plasmonic material pads can be used for electrodes).
This work was supported by “the Swedish Research Council (VR) through its Linnæus Center of Excellence ADOPT”. Xu Sun acknowledges China Scholarship Council (CSC) for the financial support.
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