We proposed and designed a compact unbalanced Mach-Zehnder interferometer (MZI) based on metal/insulator/metal (MIM) plasmonic waveguides for ultrafast optical signal processing. The MZI was fabricated by a lithography technique and we provide, for the first time experimental evaluation of the transmission performance of the MZI using MIM PWGs. The experimental results were in good agreement with the numerical simulations. The proposed structure could be considered as a key device for on-chip optical integrated circuits.
© 2016 Optical Society of America
The integration of nanoscale optoelectronic devices is rapidly approaching limitations below the diffraction limit of light. Diffraction limit of light does not allow the confinement of light into nanoscale regions much smaller than the wavelength of light. Plasmonic waveguides (PWGs) are able to concentrate and manipulate light below the diffraction limit. Plasmonic devices have attracted a great attention for developing next generation of integrated optical systems. Plasmonic devices provide the facility for ultra-compact optical integrated circuits with the bandwidth of photonic networks [1–3]. Metal/Insulator/Metal (MIM) PWGs for propagating surface plasmon polariton (SPP) have recently been proposed [4–10].
Optical Modulators are one of the most effective and important parts in optical signal processing of the optical integrated circuits. Many plasmonic modulators based on PWGs have been reported [11–16]. In particular, Mach-Zehnder interferometers (MZIs) are typical structures used in electro-optic modulators. MZIs based on gap PWGs [17–20], ridge PWGs  and V-groove PWGs  have been investigated. Gap PWGs allow plasmonic mode to be confined and propagates in the middle of the gap. For MZIs based on gap PWGs recently reported, the SPP is propagating not in the center but the edges of the gap . In such cases SPP will be subject to some scattering loss due to waveguide curvature (i.e. bending losses) or some defects and surface roughness of the fabricated structure . In the multilayer MIM PWGs, there are no such the problems [1,9,10]. In this paper, we propose MIM PWGs based on deposition of a metal thin film and a dielectric layer. The proposed structure possesses a strong stability to confine light  and low loss in the bending waveguides . Our proposed structure combining a simple design and rabid fabrication process. A number of researchers have numerically investigated the optical properties of MZI using MIM PWGs [26–28]. To the best of our knowledge, no experimental investigations on MZIs have been reported by using multilayer MIM PWGs.
Furthermore, the propagation length of plasmon mode in MIM PWGs, defined as the distance for SPP intensity to decrease by a factor 1/e is short (~10 μm) , compared with those of guided modes in the dielectric waveguide. To realize a large change in the light intensity at a modulator using the MZI, it is necessary to have interference in the optical path length which can apply a π phase shift. However, the length of the mode has to be short to be compatible with the short propagation length of SPP. Hence, it is expected that we cannot obtain enough change in light intensity for a typical MZI structure if an electro optic material has lower electro-optic coefficients r33 ~32 pm/V . Therefore, in our study we have focused on a normally OFF type unbalanced MZI which provides theoretically a high ON/OFF ratio. In this study we have experimentally confirmed the interference of SPPs propagating for each propagation path of the proposed structure. The proposed structure could be used for the modulator replace to electro-optic materials from PSSNa. In this paper, we report a passive unbalanced MZI as a first step for the development of a compact electro-optic modulator.
2. Design and analysis of the unbalanced MZI
At first, the configuration and the performance of the MZI structure has been analyzed and discussed by numerical simulations. To be able to evaluate the optical properties of the unbalanced MZI, 2D finite difference time domain (FDTD) method has been employed to stimulate the transmission characteristics . The spatial cell size in the 2D FDTD method has been set to be 2 nm. Figure 1 shows a typical schematic diagram of the MZI based on MIM PWGs used in the FDTD simulations. Silver has been used as the upper metal layer and as the lower metal layer of the MIM structure where the dielectric constant is adapted for a previous work . The dielectric layer of the MIM structure is comprised of sodium p-styrenesulfonate homo-polymer (PSSNa) having refractive index n = 1.395. Thicknesses of PSSNa are d0 = d1 = d2 = 100 nm. The input port has been illuminated by light with Ez component to excite the plasmon. The input light will be confined in the dielectric layer and will propagate along the waveguides. SPPs waves will be obtained and guided along the branch waveguide then divided into path P1 and path P2. Transmittances have been calculated from the ratio (EOut/EIn) of the electric field intensity in the input port EIn and in the output port EOut. A different path length ΔL between P1 and P2 has three different values of Ag thickness d3 to be 400 nm, 600 nm and 800 nm as shown in Fig. 1. To confirm a constructive and a destructive interference at the wavelength from 1100nm to 1600nm, different path length ΔL were chosen 400 nm, 600nm and 800nm.
By the results of the FDTD simulations, propagation loss in bends and splitters PWGs were evaluated. The transmittance in 90° bends of PWGs is 80% compared with straight PWGs. The ratio of the transmittance between straight and 90° bends direction of splitters PWGs were 55% and 45%. Figure 2(a) is showing the results of our calculations of the transmittance spectra. Transmittances have been found to vary with the wavelength of input light λ0. The transmittance at λ0 = 1420nm has been found to be a maximum for ΔL = 800nm. The maximum transmittance for ΔL = 800nm as shown Fig. 2 is 79%. This transmittance is similar to the theoretical transmittance in straight PWGs of same path length. The transmittance at λ0 = 1300nm has been found to be a minimum for ΔL = 400nm. Figure 2(b) showing the transmission spectra of path length difference of 400nm, 600nm and 800nm deduced by the analytical solution. The spectra shapes by numerical solution are quite similar to the spectra shapes by analytical solution. That is an evidence that for SPPs propagating in each path of MZI at λ0 = 1300nm for ΔL = 400nm undergo a destructive interference. For SPPs having wavelength λSPP propagating in the MIM PWGs, the condition of a destructive interference can be expressed according to the following equation.
The wavelength λSPP of SPPs propagating in the waveguide as shown Fig. 1 is 750 nm at λ0 = 1300nm, which has been calculated from the MIM SPPs dispersion relation . Therefore, according to Eq. (1) we may conclude that for a destructive interference Eq. (1) approximately tends to (λSPP/2 = 375nm). So by increasing the path length difference ΔL, the wavelength of input light λ0 for maximum (minimum) transmission will be shifted to longer wavelengths. Therefore, the phase difference between P1 and P2 leads to a change in transmittance which is clearly shown as in Fig. 2.
3. Fabrication of MZI based on MIM PWGs
Figure 3 shows a schematic diagrams of the whole fabrication process of the our MZI sructure. As shown in the proposed design the device structure has been started by a thin film deposition of Ni (about 10 nm) on a Si substrate using sputtering method to improve adhesion. Then Ag thin film (100 nm) has been deposited by a thermal evaporation method. Sodium p-styrenesulfonate Homo-Polymer (PSSNa) has been used as a dielectric layer which has been deposited using a spin coating method. The Ag thin film of a middle layer (300 nm) has been deposited. Finally a second Ag thin film (about 72 nm) has been also deposited by a thermal evaporation method at an incident angle 45° and −45° degrees. V-like channels have been fabricated by focus ion beam (FIB) milling to serve as input and output ports. For using a modulator in this structure, the PSSNa will be replaced to electro-optic materials. The structure using electro-optic materials can be fabricated by the same fabrication process as shown Fig. 3.
Figure 4 shows scanning electron microscope (SEM) images of the fabricated unbalanced MZI based on MIM PWGs. Figure 4(a) shows SEM of top-view of fabricated structure. Figure 4(b) shows a cross section of the fabricated structure along the region from A to B as shown Fig. 4(a) fabricated at the same substrate. As shown in Fig. 4(b), the thickness of PSSNa film before a branch (d0) is about 350 nm. The thicknesses of the PSSNa film after a branch (d1, d2) are the same and were about 130 nm. The SPPs can be confined and propagating within those thicknesses PSSNa with Ag cladding. A Propagation length of the fabricated MIM PWGs constructed by Ag/PSSNa/Ag films was evaluated. As a result, propagation length at λ0 = 1500nm is 13.5μm.
Ag-1 and Ag-2 films as shown Fig. 4(b) were in a conductive state. If we apply an electric field between Ag-1 (Ag-2) and Ag-3, the structure can be used as a modulator due to phase shift.
4. Results and discussions
Figure 5 shows a schematic diagram of the experimental setup to be able to measure in/output light properties of the fabricated MZI. Tunable laser source (wavelength λ0) by optical parametric oscillator (OPO) pumped Ti: Sapphire laser (wavelength = 835 nm) has been utilized as an incident light source. A λ/4 plate was used to turn plane-polarized light into circularly polarized light. The laser polarization has been controlled by a polarizer. A laser light has been illuminated at the input port of the unbalanced MZI through an objective lens. The intensities of the scattered light at the output port of the unbalanced MZI have been recorded by an infrared charge coupled device (IR-CCD) camera through an objective lens and a half mirror.
CCD images of the unbalanced MZI under excitation of a laser with a wavelength λ0 = 1300nm are shown in Fig. 6. Scattered lights at the output port as shown Fig. 6(b) have been observed whereas scattered lights at the output port as shown Fig. 6(c) could not be observed. Incident lights which illuminate the input port are subject to be reflected to an edge of the MIM PWG. In this case, when the incident light has TM or TE polarization, incident light which illuminate to edge of the MIM PWG has polarization direction of the vertical or parallel direction of a substrate, respectively. In general, polarization direction of SPPs to propagate in layered MIM PWGs will be the vertical direction of metal-insulator interface . When the incident light in Fig. 6(b) leads to an observed scattered light at the output port having a polarization direction in the vertical direction of the MIM PWGs, a condition of propagation of the SPPs are satisfied. Therefore, the propagation of the SPPs in the fabricated unbalanced MZI has been verified.
The spectrum of light intensities at the output port of the unbalanced MZIs as shown Fig. 4 has been measured by using the proposed experimental setup as shown in Fig. 5. The wavelength of the incident light was varied from 1150 nm to 1550 nm (50nm step) by the OPO.
Figure 7 is showing the experimental results of the spectrum of light intensities at the output port as a function of wavelength of incident light λ0. Transmittance spectrum in a simulation model based on the different parameters of fabricated structure is also shown in Fig. 7. The experimental results have been deduced as average values from 15 data of 5 times measurement of three samples at the same substrate and same fabrication condition. The error bars as shown in Fig. 7 express standard deviations. Light intensities at the output port have been normalized by the intensities of incident light. Normalized light intensity Io at the output port has been found to change as a result of changing of the wavelength of incident light as shown in Fig. 7. Io is maximum at λ0 = 1250 nm and gradually decreases for longer wavelengths to reach its minimum at λ0 = 1500nm. In the fabricated structure, it has been assumed that constructive and destructive interference has been achieved at λ0 = 1250nm and 1500nm, respectively.
As shown previously in Fig. 2, the experimental results of the spectrum of light intensities at the output port are not in agreement with the numerical simulated results. This may related to the variation of PSSNa for branch, incoming and outgoing waveguides. The film thickness variation has been a result of a spin coating of PSSNa twice. To investigate the effect of thickness d0, transmittance spectra of the unbalanced MZI have been calculated by the 2D FDTD simulation by changing the parameter from a SEM image as shown in Fig. 4. The thickness of the PSSNa before a branch d0 has been found to have different thicknesses as 130nm, 240nm and 350nm. Another sizes of the unbalanced MZIs are d1 = d2 = 130 nm, l = 6 μm and d3 = 300 nm based on a SEM image as shown in Fig. 4(b).
Transmittance spectra for different thicknesses are shown in Fig. 8. Transmittance spectra have been found to change as a result of changing the thickness d0. The transmittance at a wavelength of input light λ0 = 1350 nm has been found to be a maximum for thickness d0 = 130 nm. By increasing the thickness d0, transmittance spectra have been found to be shifted to shorter wavelength.
If the thickness of the PSSNa d0 before a branch increased, SPPs quickly reach to upper waveguide. Therefore, transmittance spectra will be subject to be shifted to shorter wavelengths due to the change of the condition of the interference.
The thickness of the PSSNa before a branch at the fabricated structure is close to d0 = 350nm. Therefore, the transmittance spectrum by the 2D FDTD simulation at such thickness is shown as in Fig. 7. The experimental results and the numerical simulated results have been found to be in good agreement and the fabricated structure behaves as MZI.
In this study, we proposed and designed unbalanced MZIs based on the MIM PWGs. The configuration, the numerical simulations and the performance of the proposed structure have been analyzed and discussed. The proposed structure has been fabricated by combining electron beam lithography with some lift off techniques. As a result of optical measurement, the normalized light intensity at the output port has been found to change as a result of change in incident light wavelength. The normalized light intensity at the output port has been found to be a maximum and a minimum at λ0 = 1200nm and 1500nm, respectively. The experimental results and the numerical simulated results have been found to be in good agreement and the fabricated structure behaves as MZI. The proposed structure is expected to realize an ultra-compact modulator by replacing a part of insulator to electric-optic polymer.
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