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Abstract
Programmable integrated photonics is an emerging research field due to its range of applications, from data processing to computing. Phase change materials (PCMs) on waveguides provide enormous flexibility for programmable integrated photonics. These materials show a large contrast in the optical properties (such as refractive index and optical loss) between the two stable states (i.e., amorphous to crystalline) of the PCM. These states are reversible and reproducible with an external stimuli which could be optical, thermal or electrical. Their non-volatile behavior allows PCMs to serve as an active layer for programmable photonics. In this work, we investigate hybrid device architectures utilizing the non-volatile properties of PCMs for integrated programmable photonics in a Si3N4 platform. FDTD modeling was carried out to design two configurations of non-volatile reconfigurable switches: (1) A non-volatile reconfigurable reflector switch consisting of a photonic crystal (PhC) slab waveguide and having a thin layer of Sb2Se3 on top of the waveguide and (2) 1×2 mode converter switch. In the case of a non-volatile reconfigurable reflector switch, the reflectivity of the waveguides is controlled by shifting the mode-gap of the photonic crystal slab waveguide by changing the phase of the PCM. It has applications as a reconfigurable reflective filter in the optical communication system and on-chip smart Bragg mirror. Non-volatile broadband directional switches provide a new paradigm for designing programmable multifunctional nanophotonics, which works in the same way as electronic field-programmable gate arrays. A 1×2 mode converter switch — based on Sb2Se3 as a clad layer on a coupling waveguide of a directional coupler — is proposed. The optical switches proposed in this paper offer very low optical insertion loss (∼ 0.5 dB), low coupling length (∼ 12 µm), broadband operation (∼ 80 nm), small cross talk (∼ 16 dB), and zero static power consumption.
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1. Introduction
Photonic integrated circuits (PICs) based on silicon nitride platform have received significant attention in several applications such as on-chip sensing [1,2], optical transceiver circuit in datacom and telecom [3,4], photonic neuromorphic computation [5], AI accelerator [6], in-memory computation [7,8], integrated quantum computing [9,10], and LIDAR/optical beamforming [11,12]. Silicon nitride based waveguides offer very low propagation loss (∼ 5-7 times lower than silicon waveguides) depending on the geometry of the waveguides, unprecedented transparency range (400-4000 nm), high optical power handling capability (weaker Kerr non-linearity with no two-photon absorption in the C-band), complementary metal oxide semiconductor (CMOS) compatibility, high tolerance against temperature variations, and the possibility of multi-layer integration with SOI wafer [13]. On the other hand, Si3N4 PIC technology shows larger footprints because of their lower relative index compared with silicon-on-insulator (SOI) devices, thus there is a trade-off between footprint and optical losses [13,14]. In the last few years, several passive photonic components such as microring resonator, microdisk resonator, Mach-Zehnder interferometer (MZI), crossbar switching, and filter have been developed and experimentally demonstrated using silicon nitride waveguides [14–21]. Reconfigurable photonic devices are the next-generation optical technology. Several recent studies describe integration of materials exhibiting non-linear effects — materials employing thermo-optic effects, poled polymers, ferroelectric materials, and 2D materials such as graphene and transition metal di-chalcogenides [22–30] — with silicon nitride waveguides. These non-linearities have weak electro-refractivity effects (Δn < 10−3), leading to a large interaction length for effective light-matter interaction. Moreover, it also needs continuous power to operate in the non-linear effect.
In recent times, silicon nitride waveguides in conjunction with phase change materials (PCMs) have been used to make modulators and switches in the application of optical neuromorphic computing, optical memories, logic devices, and meta-surfaces [31–38]. PCMs offer changes in properties (both in the electrical and optical domain) in different stable states. These states are reversible and reproducible using optical, thermal, or electrical stimuli [39,40]. The switching speeds of these states are fast (in the range of µs to ns), depending on the material as well as the type of stimulus. PCM can be triggered either electrically (through thermal conduction heating or electrical threshold switching) or optically. In the case of electrical triggering, a voltage is provided across electrodes connected to both sides of a PCM film (electrical threshold switching) or an external heater (thermal conduction heating) is utilized to change the phase of the PCM. Furthermore, in the case of optical triggering, either an on-chip high-intensity laser pulse coupled to the waveguide with a PCM film (on-chip all-optical switching) or a laser beam focused on the PCM film (free space all-optical switching) is used to heat up the material [41]. These materials consume power only during the phase transition, and there is no power consumption to hold the states of the materials. Several well-known optical PCMs such as GST, GSST, and GeTe have been extensively studied in photonic applications [42]. These materials show high index contrast between the two states and high optical losses. Sb2Se3 and Sb2S3 have recently been demonstrated as a new type of PCM (wide bandgap) with moderate index contrast and ultra-low optical losses in the near-infrared range for both the phases [43,44]. The crystallization temperatures of the Sb2Se3 and Sb2S3 PCMs are 200 °C and 300 °C [45,46], respectively, which are comparable (and in some cases slightly higher) to those of GST, GeTe, and GSST which have crystallization temperatures of 140 °C, 200 °C, and 277 °C, respectively. Similarly, the amorphization temperatures of the Sb2Se3 and Sb2S3 PCMs are 620 °C and 550 °C, respectively, which are comparable to those of GST, GeTe, and GSST which have a crystallization temperatures of 550 °C, 700 °C, and 617 °C, respectively. The crystallization and amorphization temperature of these PCMs are also depends on the material thickness and encapsulation. In the process of crystallization of the PCMs, optical or electrical pulses are applied to heat up the PCM above the crystallization temperature but below the melting temperature and cool it down slowly. However, a high intensity pulse is applied to achieve a temperature above the melting point for the amorphization process and cool it down very fast. The phase of the Sb2Se3 can be changed optically and electrically. In optical triggering, optical pulses from a diode laser operating at a visible wavelength (638 nm) can be used for photothermal heating [47]. The photothermal heating mechanisms have been used for all-optical applications that involve a high-intensity free space laser pulse focused onto the PCM. PCM absorbs the light intensity and therefore gets heated up to the desired temperature. An on-chip high-intensity laser pulse coupled to the waveguide can also be used. But the implementation of a large scale network using optical triggering is very difficult, as multiple laser sources are needed. On the other hand, for the electrical triggering, micro-heaters are used to heat up the PCM. In the silicon nitride platform, metal heaters and ITO based heaters are frequently used [48]. A graphene monolayer can also be used as a heater by applying voltage to the graphene [49]. But it introduces extra optical losses. The optical losses in graphene can be reduced by chemically or electro-statistically doping the graphene so that it works in the Pauli blocking region. Moreover, several applications of these new low loss materials in photonic devices have been demonstrated in the last two years [47,50–52].
In this paper, we have used Sb2Se3 PCM as an active material as it has no intrinsic absorption losses (k < 10−5) in the telecommunication band. It shows an index contrast (Δn) of ∼ 0.77 at 1550 nm between the amorphous and crystalline phases. To design a reflector switch, photonic crystal (PhC) slab waveguide structure has been used with Sb2Se3 thin film in the waveguide cladding section to take advantage of a tunable mode-gap effect, as shown in Fig. 1(a). Due to the contrast in refractive index between the two states, the mode-gap of the proposed photonic crystal slab waveguide can be modulated by changing the phase of the PCM. The silicon nitride photonic crystal slab waveguides show higher optical losses compared to the silicon nitride strip waveguides due to the intrinsic diffraction losses in photonic crystal slab waveguides with line defects [53]. Moreover, we have used an asymmetric directional coupler to design a mode converter switch, as shown in Fig. 1(b) and Fig. 1(c). The index matching conditions in the directional coupler can be changed by changing the states of the PCM for switching the path of the light propagation between waveguides of two different modes. Although there are some works based on the low loss PCMs in the SiN waveguide based integrated platforms [45,54], to the best of our knowledge, there is no previous report of either a reflector switch or a mode-convertor switch in a SiN waveguide platform that employs Sb2Se3 as the phase change material. Moreover, the properties of the switches being proposed in this paper are significantly better than those proposed earlier for either a reconfigurable on-chip periodic waveguide or a reconfigurable mode converter based on phase change materials. The optical switches proposed in this paper offer the very low optical insertion loss (∼ 0.5 dB), low coupling length (∼ 12 µm), broadband operation (∼ 80 nm), small cross talk (∼ 16 dB), and zero static power consumption.
Fig. 1. (a) Schematic of the proposed non-volatile optical reflector switch. The inset illustrates the cross-sectional view of the PhC slab waveguide with a thin layer of Sb2Se3. (b) Schematic of the proposed non-volatile mode converter switch showing the TE0 mode in the output waveguide for the crystalline (Cr) phase of Sb2Se3 and TEn mode in the output waveguide for the amorphous (Am) mode of Sb2Se3. (c) Cross-sectional view of the hybrid structure in the YZ plane.
2. Design and analysis
In order to design the devices, Three-dimensional Finite Difference Time Domain (3D FDTD) solver (using Lumerical FDTD) has been used to calculate the optical transmission, photonic band diagram and optical field profiles. Moreover, we have also used a 2D Finite Difference Eigenmode (FDE) solver (Lumerical MODE) for waveguide design parameters such as effective index and mode profile calculations. We have taken perfectly matched layers (PML) in all boundaries in our simulations. The mesh size convergence test is carried out to ensure the stability of the simulation results. Both the devices are designed for the Transverse Electrical (TE) polarization of light. The following approaches have been utilized to design the optical reflector switch (shown in Fig. 1(a)) and the mode converter switch (shown in Fig. 1(b) and Fig. 1(c)).
2.1. Non-volatile on-chip photonic reflector switch
The non-volatile optical reflector switch being proposed in this paper consists of a Si3N4 based PhC slab waveguide with PCM as an active cladding material on the surface of the waveguide. Figure 1(a) shows the structure of the proposed tunable reflector in a silicon nitride platform. The PhC is formed by hexagonally arranged air holes in a Si3N4 medium. The structure has three main sections, the first section being the input waveguide consisting of a silicon nitride strip waveguide on a silica substrate. The second section is a PhC slab waveguide coated with a thin layer of Sb2Se3 where the reflectivity and tunability occur, and the third section is a silicon nitride strip output waveguide. The right top inset of Fig. 1(a) exhibits the cross-sectional view (in the y-z plane) of the Sb2Se3 coated PhC slab waveguide. The Sb2Se3 is coated with a thin layer of SiO2 to prevent oxidation. We have chosen the following parameters of the non-volatile tunable reflector: the thickness of the silicon nitride film ‘t’ = 450 nm, lattice constant ‘a’ = 490 nm, radius ‘r’ = 0.33×a, the thickness of the Sb2Se3 layer ‘tpcm’ = 40 nm, and the thickness of the SiO2 coating layer ‘tSiO2’ = 10 nm. The parameters were chosen to obtain an effective band shifting as well as to operate at the telecom wavelength of 1550 nm. The experimental work by Matthew Delaney et al. has provided the complex refractive indices of the amorphous and crystalline phases of Sb2Se3 [43]. We have employed these refractive indices of Sb2Se3 in this paper. The Palik model has been used to model the refractive index of SiO2. The Refractive index of Si3N4 was taken as (nSi3N4) 2.01.
In order to investigate the characteristics of a PhC based waveguide, it is important to analyze the photonic band diagram and dispersion characteristics. Figure 2(a) shows the TE band structure of the PhC slab (without the defect and the Sb2Se3 layer — see Fig. 2(b)) by analyzing the out of plane electric field components. There is no bandgap for TE-like mode (even mode). We created a defect onto the 2D PhC slab to make a tunable waveguide with reflector property by removing a row of air holes and placing thin films of Sb2Se3 and SiO2 on top of the PhC waveguide section only, as shown in Fig. 2(d). We have found a mode-gap by analyzing the dispersion characteristics of the PhC slab waveguide. Figure 2(c) and Fig. 2(e) show the dispersion characteristics for the amorphous phase and crystalline phase, respectively. This mode-gap is present between two slab modes from the frequencies 0.304(c/a) to 0.324(c/a) in the case of the amorphous phase. Due to a change in the effective refractive index for the crystalline phase, the photonic bands are shifted to a lower value. The TE mode-gap exists from the frequencies 0.298(c/a) to 0.316(c/a). The blue shaded regions correspond to the projected band structure of the perfect PhC structure (without waveguide).
Fig. 2. (a) Photonic band diagram of a hexagonal array of air holes in a silicon nitride film. (b) Schematic of the silicon nitride based PhC slab. Dispersion diagram of the proposed PhC slab waveguide for the (c) amorphous phase and (e) crystalline phase. The zoom section shows the mode-gap region with shift in the guided modes. While the purple colored dotted lines indicate the guided modes between mode-gap for the amorphous phase, red colored connecting lines indicate the guided modes between mode-gap for the crystalline phase. (d) Schematic of the silicon nitride based PhC slab waveguide with thin film of Sb2Se3. The band diagrams were calculated using FDTD modeling for TE polarization of light. The following optimized geometrical parameters of the PhC slab waveguide were chosen to calculate the band diagrams—the lattice constant ‘a’ was taken to be 490 nm, the radius of air holes ‘r’ was taken to be 0.33×a, the thickness of the Sb2Se3 layer ‘hpcm’ was taken to be 40 nm, the thickness of the coating layer SiO2 layer ‘tSiO2’ was taken to be 40 nm and thickness of the slab waveguide H was taken to be 450 nm.
3D FDTD simulations of the PhC structure based non-volatile reflector switch (shown in Fig. 3(a)) were also carried out to verify the performance (in terms of optical transmission) of the reflector switch. Normalized reflection spectra for both the states are shown in Fig. 3(b). It can be observed from the figure that the patterns of the reflection spectra for both states are almost the same. It is only shifted towards higher wavelengths in the crystalline state as compared to the amorphous state, which is expected from the frequency shift of the band structures shown in Fig. 2(c) and Fig. 2(e). Around 1550 nm, the mode-gap appears in the amorphous state, whereas the guided mode appears in the crystalline state. Therefore, the optical field propagates through the PhC waveguide in the crystalline phase, which leads to the minimum reflectivity [see in Fig. 3(d)]. Once the Sb2Se3 layer is triggered to transition from the crystalline state to the amorphous state, the optical field cannot propagate through the PhC waveguide, and the optical mode is reflected within the first few periods [see in Fig. 3(c)]. Therefore, by changing the phase of the active Sb2Se3 segment, we can effectively modulate the reflectivity of the optical mode guided through the PhC waveguide for a broad range of wavelengths.
Fig. 3. (a) Top view of the 3D FDTD simulation region of the proposed non-volatile tunable optical reflector. (b) Reflectance spectra of the proposed non-volatile reflector switch. (c)–(d) The optical electric field distribution of the PhC slab waveguide for the amorphous and crystalline states at the wavelength of 1550 nm.
The operating band of the frequencies of the reflector switch depends on the periodicity of the air holes [i.e., the lattice constant a shown in Fig. 1(a)] in the PhC slab waveguide. We have analyzed the effect of the air hole periodicity on reflectivity spectra. Figure 4(a) shows the redshift of the reflectivity spectra as the periodicity increases. With an increase in the periodicity, the position of the mode gap changes to a lower frequency in the band diagram shown in Fig. 2(c). The effect of the number of rows on either side of the waveguide on the normalized reflectance spectra is shown in Fig. 4(b). As we increase the number of rows, the mode-gap becomes more pronounced, leading to an increase in the value of reflectance from the non-volatile reflector switch. The partial crystallization of PCM also offers the intermediate modulation of the mode-gap. The optical permittivity approximations (ɛeff) of the Sb2Se3 with a different fraction of the crystallization (f) are calculated using Lorentz-Lorenz effective medium expression as defined by Eq. (1) [55,56]:
Fig. 4. (a) Effect of varying the air holes periodicity, i.e., the lattice constant ‘a’ of the PhC slab waveguide (in the non-volatile reflector switch) on the reflectance spectra, considered only in the main mode-gap region. (b) Effect of the number of rows of the air holes on the reflection spectra. Please note that the PhC slab waveguide with a thin layer of Sb2Se3.
Fig. 5. (a) Effect of the Sb2Se3 crystallization fraction on the reflectivity spectra, (b) Effect of the number of air holes in the optical mode propagation direction on the transmission spectra.
Table 1. Comparison of reconfigurable on-chip periodic waveguide based on phase change material
The optical mode coupling from strip waveguides to PhC slab waveguide shows significant insertion losses (some reflection) due to strong impedance mismatch. The strong impedance mismatch occurs due to large difference in the group index at the interface of PhC slab waveguide. Different approaches have been proposed to overcome the coupling issues, which reduces the reflection from the PhC waveguide interface [57]. It was proven that increased mode matching, which can be obtained by choosing an optimum cut position within the basic period of PhC waveguide, can improve the coupling efficiency dramatically. Adiabatic coupling technique has a good potential for coupling the entire spectrum of the guided modes. The technique utilizes smooth variation in the group indices as the mode propagates in PhC waveguide coupling interface. Different types of tapered couplers using channel width modulation and a gradual change in hole dimensions can be used [57]. Moreover, step couplers with lattice shifting can also be employed.
2.2. Non-volatile mode converter switch
We extend the non-volatile tunability scheme to design a mode converter switch. Figure 1(b) shows the schematic diagram of the proposed broadband switch based on the two-waveguide based directional coupler design with an asymmetric coupling region. The working principle of the proposed broadband switch is based on the tunability in index matching conditions of the different waveguides due to the phase change of the Sb2Se3 thin film. The proposed non-volatile mode converter switch consists of an asymmetric directional coupler with a low loss PCM (Sb2Se3) as an active material. Figure 1(b) shows the structure of the proposed asymmetric directional coupler that uses a Si3N4 strip waveguide and a PCM coated hybrid waveguide. The directional coupler has the following sections, the input section, which is just a Si3N4 strip waveguide and designed for the fundamental mode (TE0). The coupling section is an active non-volatile tunable segment in a two waveguide system. In this section, the optical modes couple from the input waveguide to the hybrid waveguide in the amorphous phase on the basis of matching of the effective indices between the fundamental mode of the input waveguide and higher-order modes (TEn) of the hybrid waveguide. There is an index mismatch between the two waveguides in the crystalline phase. The output section is comprised of two output waveguides: one that carries optical power when the phase of Sb2Se3 is crystalline and the other when the phase of Sb2Se3 is amorphous. The first output waveguide operates in the TE0 mode (for the crystalline phase of Sb2Se3) and the second output waveguide in the TEn mode (for the amorphous phase of Sb2Se3). Figure 1(c) exhibits the cross-sectional view (in the YZ plane) of the coupling section (two-waveguide based system). The width of the Si3N4 waveguide decides the existence of the higher-order modes. Thus, we have calculated the range of waveguide widths for the higher-order modes using a 2D Finite Difference Eigenmode (FDE) solver (Lumerical MODE). Figure 6(c) demonstrates the change in effective refractive index with the variation in the width ‘wsw’ of the Si3N4 strip waveguide (shown in Fig. 6(a) and Fig. 6(b)) for fundamental and higher-order Transverse Electric (TE) modes calculated at 1550 nm wavelength. The normalized field profiles for all these modes are shown in Fig. 6(d). The waveguide width is an important parameter for designing an arbitrary mode converter switch (which can convert one mode to any higher-order selected mode). The effective refractive index calculations provide flexibility to make an index matching between the waveguides and to choose the corresponding input waveguide width. In this configuration, the proposed mode converter switch is based on converting the optical TE0 mode to TE1 mode, but other higher-order mode conversions can also be possible.
Fig. 6. (a) Schematic showing an input strip waveguide. (b) Cross-sectional view of the input strip waveguide with a width ‘wsw’. (c) Effective indices of the fundamental and higher-order TE modes in a Si3N4 strip waveguide as a function of width of the strip waveguide (wsw). (d) Normalized field profiles of the TE0, TE1, TE2, TE3, TE4, and TE5 modes. All the effective indices and optical modes are calculated at a wavelength of 1550 nm.
Figure 7(a) shows the index matching in the amorphous phase between the strip waveguide and hybrid waveguide in a non-volatile mode converter switch shown in Fig. 1(c). We have chosen the optimal value of the strip waveguide (wsw) and the hybrid waveguide (wHW) as 850 nm and 1400 nm, respectively, to equal the effective indices. We have also verified this by super mode calculations of the two waveguide systems. One can observe the strong coupling between these two waveguides in the amorphous phase (See in Fig. 7(b) and Fig. 7(c)). Moreover, there is a large index contrast between the strip waveguide and the hybrid waveguide in the crystalline phase. As a result, the spatial field profiles of the super modes in Fig. 7(d) and Fig. 7(e) show the isolated modes of both waveguides. We have calculated the coupling length of the hybrid waveguide as 12 µm, and it is given by:
Fig. 7. (a) Effective refractive indices of the fundamental modes of the Si3N4 strip waveguide and the hybrid waveguide for the amorphous and crystalline phase of the Sb2Se3. The spatial field profiles of the super-modes in the (b)-(c) amorphous phase and (d)-(e) crystalline phase.
The normalized transmission spectra of the proposed non-volatile mode converter switch for both the output ports are shown in Fig. 8(a) and Fig. 8(b). In the amorphous phase, the optical mode from the input strip waveguide is coupled to the hybrid waveguide from the input strip waveguide because of good matching of the refractive indices of the strip waveguide and the hybrid waveguide. This leads to a strong coupling. As a result, we achieved a low insertion loss of ∼ 0.52 dB and a high extinction ratio of ∼ 13 dB at 1550 nm for the amorphous phase. Moreover, the insertion loss and extinction ratio in the crystalline phase are ∼ 0.31 dB and 13.58 dB, respectively. The proposed non-volatile mode converter switch shows a broadband nature in the normalized transmission spectra. The optical mode propagation for the amorphous phase at a wavelength of 1550 nm is shown in Fig. 8(c). Moreover, after the phase change from amorphous to crystalline on applying an external stimulus, the optical modes show minimal coupling and allow almost all the power to pass through the strip waveguide. Figure 8(d) demonstrates the fundamental mode propagation in the crystalline phase at 1550 nm. The key performance metrics of the proposed non-volatile mode converter switch are compared to those of previously reported architectures in Table 2. It can be observed from Table 2 that the non-volatile mode converter switch proposed in this paper is superior to the previously reported architectures in terms of having much lower insertion loss when compared to that in the previously reported architectures.
Fig. 8. The normalized transmission spectra in both the outputs for the (a) amorphous phase and (b) crystalline phase, respectively. The optical field propagation at a wavelength of 1550 nm for the (c) amorphous phase and (d) crystalline phase.
Table 2. Comparison of reconfigurable mode converter based on phase change material
Accurate and efficient fabrication, of submicron-scale photonic crystal (PhC) structures on a large scale, is challenging. The conventional photolithography cannot achieve the submicron-scale photonic crystal (PhC) structures, thereby necessitating the use of nanolithography processes to fabricate these structures. As E-beam lithography can achieve high resolution fabrication (sub-20 nm resolution) of nanostructures, photonic crystals at telecom wavelengths can be easily fabricated using E-beam lithography. Hence, the majority of planar photonic crystal structures are fabricated by the E-beam lithography technique. But, E-beam lithography lacks the potential for large scale fabrication of planar photonic crystals due to the long writing times. Another issue is the proximity effects caused by electron backscattering in E-beam lithography that restrict the structural accuracy. This type of structure requires deep etching using dry-etched technologies, which produces significant sidewall roughness. As a result, scattering losses increase dramatically. Other techniques used for PhC structure fabrication are Deep UV lithography and Focused Ion Beam (FIB). FIB can remove the proximity effect as the substrate is etched directly. But, the fabrication of a perfect vertical wall is difficult using FIB due to the re-deposition of etched material. Moreover, the fabrication of the mode converter switch is more robust using E-beam lithography and Deep UV lithography techniques.
Several impressive works on silicon-based monolithic integrated optoelectronic circuits for on-chip optical interconnects have been reported using the standard commercial CMOS technology [67,68]. This monolithic optoelectronic integration can be implemented in two ways. The first approach is to integrate silicon photonic devices into the front-end CMOS process, which occupies valuable transistor space due to the large footprint of the photonic devices. As a result, the cost increases. The second approach is the backend integration of silicon photonic devices using new layers (3D integration of electronics and photonics) in the CMOS process, which provides the high integration density and low cost. The Si3N4 photonic devices can be implemented directly on top of the CMOS IC. The photonic circuits can be tuned by providing the necessary electrical signals from the CMOS IC, such as voltage to the micro-heater for thermal tuning. The backend fabrication processes need to strictly maintain the thermal limit of the CMOS ICs to avoid performance degradation. At 450°C, the aluminum metallization begins to deteriorate. As a result, the CMOS backend process should be kept below 450°C. Deposition, lithography, etching, and chemical mechanical polishing are the steps in the backend CMOS process for fabricating Si3N4-based photonic devices. Thus, it started with the deposition of SiO2 and Si3N4 layers. Plasma-enhanced chemical vapor deposition (PECVD) is one of the most commonly used low-temperature deposition techniques. It is used to meet the strict thermal limits and compatibility of the COMS process. Furthermore, Si3N4 is the material utilized in the CMOS backend passivation layer, resulting in fewer changes to typical CMOS technology. First, a thick layer of SiO2 is deposited on the top surface of the CMOS IC for optical isolation, followed by CMP for the planarization of the SiO2 layer. Then Si3N4 layer is deposited and patterned for different photonic devices using EBL and inductively coupled plasma (ICP) etch. All of the back-end deposition processes necessitate a temperature less than 400°C. The silicon nitride is deposited with the gas mixture of silane (SiH4) and ammonia (NH3) in the presence of a nitrogen gas environment. The ratio of NH3 to SiH4 affects the chemical composition of the deposited silicon nitride film, making the silicon nitride layer either nitrogen-rich or silicon-rich, based on the requirement. The temperature of a typical back-end-of-the-line PECVD nitride process ranges between 300 °C and 400 °C
3. Conclusions
In this paper, a low loss broadband compact non-volatile reflector switch is proposed in a photonic crystal slab waveguide using a low loss phase change material Sb2Se3. The proposed switch exhibits high reflectivity for a broad range of frequencies, large tunable range and high extinction ratio. We have also designed a mode converter switch based on a directional coupler using the active PCM Sb2Se3 thin layer as a cladding material. It was demonstrated that the mode converter shows an excellent performance in different parameters such as low insertion loss (∼ 0.52 dB), high extinction ratio (> 13 dB), high bandwidth (> 80 nm), and compact size (∼ 12 µm length of the active section).
Funding
Defence Research and Development Organisation (RP03356G, RP03436G, RP03437G); Science and Engineering Research Board (RP03932G); Ministry of Education, India (RP03246G: UAY program, RP03417G: IMPRINT program).
Acknowledgments
Above all, A. D. would like to thank Lord Jesus Christ for blessing this work. We would also like to thank the Digital India Corporation. This publication is an outcome of the R&D work undertaken in the project under the Visvesvaraya PhD Scheme of Ministry of Electronics & Information Technology, Government of India, being implemented by Digital India Corporation (formerly Media Lab Asia).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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