Design and Simulation of Single-Mode and Polarization Independent

Deeply Etched Amorphous Silicon on SOI Waveguides

Babak Hashemi

, Sandro Rao

, Maurizio Casalino

2 c

and Francesco Della Corte

Department of Information Engineering Infrastructures and Sustainable Energy (DIIES),

“Mediterranea” University Reggio Calabria, Italy

Institute of Applied Sciences and Intelligent Systems (ISASI) National Research Council (CNR) Naples, Italy

Department of Electrical Engineering and Information Technologies, Università degli Studi di Napoli Federico II,

80125 Naples, Italy

Keywords: Polarization-Independent, Single-Mode, SOI Waveguides, Amorphous Silicon.

Abstract: The conditions for simulating both single-mode behavior and polarization independence in deeply etched

amorphous silicon (a-Si) on Silicon-On-Insulator (SOI) rib waveguides are presented and discussed. The

paper aims to provide valuable insights and guidance for the design and optimization of waveguide-integrated

electro-optic devices based on deeply etched hydrogenated amorphous silicon (a-Si:H)/SOI waveguides

operating in a broad spectrum of wavelengths. The top layer of the waveguide consists of a-Si:H, whose

deposition can be performed at low temperatures with no impact at all for previously fabricated CMOS-based

electro-photonic integrated circuit.

1 INTRODUCTION

The rapid evolution of photonic technologies has

ushered in a new era of high-speed data

communication, sensing, and signal processing

(Marpaung, Yao, & Capmany, 2019). Among the

many facets of integrated-waveguide design,

achieving both single-mode behavior and

polarization independence stands as a complex

challenge, however pivotal to a variety of photonic

applications (Aalto, et al., 2019), (Lim, Eng Png,

Ong, & Ang, 2007). To date, many studies have been

conducted; e.g., Neslihan and Kurt (Neslihan & Kurt,

2016) analyzed the mode characteristics and light

confinement properties of different geometries of rib

waveguides, Seong et al. (Seong P., Eng Png, & Lim,

2004) investigated polarization-independent SOI

waveguides giving valuable insights into the

requirements for achieving such condition. More

recently, SM (single-mode) and PI (polarization-

independent) waveguides play a pivotal role in

different devices. A proposed power splitter utilizes

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these features with three waveguides (Samanta, Dey,

Banerji, & Ganguly, 2018) , while (Zhang, et al.,

2020) explores the design of a hybrid-plasmonic-

waveguide directional coupler.

In this paper, we explore the design and

simulation of single-mode (SM) and polarization-

independent (PI) large cross-section waveguides,

focusing on deeply etched a-Si:H/SOI rib

waveguides. The introduction of a-Si:H material

deposited as a back-end process of the standard

CMOS-based fabrication, ensures, in principle, a full

compatibility with existing microelectronics and

photonics leading to new integrated photonic

architecture with new functionalities in a wide range

of applications. a-Si:H can be, in fact, deposited at

relatively low temperatures (T<150°C), a property

that has opened up the way for a true integration of

photonics and electronics without any risks to

existing CMOS devices (Della Corte & Rao, 2013)

(Spear & Le Comber, 1975).

Hashemi, B., Rao, S., Casalino, M. and Corte, F.

Design and Simulation of Single-Mode and Polarization Independent Deeply Etched Amorphous Silicon on SOI Waveguides.

DOI: 10.5220/0012584200003651

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 12th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2024), pages 94-98

ISBN: 978-989-758-686-6; ISSN: 2184-4364

Moreover, the requirement of waveguides,

satisfying both SM and PI behavior, has significant

relevance in the design of photonic integrated circuits

(PICs) as electro-optic (EO) modulators (Ma, Li,

Han, Maeda, & Pištora, 2021), passive and active

switches for telecommunications, and similar.

2 THEORETICAL SCHEME

A waveguide propagating mode has an effective

refractive index that can be defined as follows (Jia-

Ming, 2009):

zeff

(1)

Where

is value (phase constant) of the

waveguide,

eff

is the effective refractive index and λ is the

wavelength.

The effective refractive index provides insight into

how tightly the mode is confined within the

waveguide core. For guided modes, the effective

index should satisfy the following condition:

𝑛



𝑛



𝑛



For unguided or radiating modes (Butt & Kozlova,

2017):

𝑛



𝑛



Therefore, modes are supported to the waveguide-

core propagation only if their effective index is larger

than the refractive index of the slab. Moreover, the

effective index calculation can also be explored to

determine the waveguide geometries for designing PI

waveguides. To achieve this last condition, the

effective index values of fundamental TE- and TM-

polarized light must be equal . (Dai, Liu, Gao, Xu, &

He, 2013)

3 SM AND PI WAVEGUIDE

DESIGN

The structure under consideration is a rib waveguide

consisting of three different layers: SiO2 (cladding

layer), crystal silicon (c-Si) and a-Si:H (core layers),

as showed schematically in Fig. 1.

Figure 1: Schematic cross-section waveguide structure

Table 1: Layer geometries and physical parameters.

Height Width Length

Refractive

Index

Sio2 3 µm 3 µm 50 µm 1.44

Crystal

Silicon

220nm 3 µm 50 µm 3.47

amorphous

Silicon

h w 50 µm 3.57

Ansys Lumerical Photonic Multiphysics

Simulation tool (Bachorec & Sedlář, 2018) has been

used for numerical calculations, providing geometries

and physical material parameters for each layer

considered, as reported in Table 1. First, the

waveguide length was fixed to L=50 µm.

The optimal performance of semiconductor

devices is considered while determining the 220 nm

thickness of crystalline silicon (c-Si) in CMOS wafers

(Bellutti, Boscardin, Soncini, Zen, & Zorzi, 1995).

This thickness was chosen after thorough

consideration of all the factors of the optical device,

aiming to achieve optimal performance in both the

optical and electrical domains. The aim of this first

run of parametric simulations is to calculate the ideal

height (h) and width (w) for the a-Si:H layer to

achieve both criteria of SM behavior and PI.

4 SIMULATION RESULT

4.1 Single-Mode Condition

Parametric simulations have been performed to

calculate the difference between the effective index

of the modes propagating into the a-Si:H waveguide

core and the refractive index of the thin layer of c-Si

(nc-Si = 3.47). Fig. 2 shows the corresponding results,

at the wavelength of 1.55 µm, as a function of the a-

Si:H height. If a waveguide width of W=1.5 µm is

considered, the SM condition can be achieved for an

a-Si:H height ranging from 0.8 µm up to 2 µm.

Beyond these values, high-order modes appear.

Design and Simulation of Single-Mode and Polarization Independent Deeply Etched Amorphous Silicon on SOI Waveguides

Figure 2: Effective refractive index difference between the waveguide-core propagating optical modes and refractive index

of the c-Si as a function of the amorphous silicon height.

4.2 Polarization Independent (PI)

Waveguide

In Fig. 3, the effective refractive index of the

fundamental TE-polarized optical field is subtracted

from the effective index of the fundamental TM mode

as a function of the a-Si:H height, always considering

a constant value for the a-Si:H width (W = 1.5 µm) at

the wavelength of 1550 nm.

The a-Si:H height (h) that results in Δn equal to

zero is determined to be 1.22 µm about.

Figure 3: Difference between the effective refractive index

of TE and TM modes (Δn) as a function of the amorphous

silicon height.

4.3 Polarization Independent and

Single Mode Waveguide

To achieve both PI and SM conditions for different

cross-section waveguide geometries, parametric

simulations were performed for all of the

combinations of height (h) and width (w) of a-Si:H at

a fixed wavelength, as shown in Fig. 4. The achieved

results were used to identify the overlapping region

that meets both criteria.

Figure 4: a-Si:H height as a function of width. Both PI and

SM conditions are achieved in the range 1.4 µm<Wa-

Si:H<2 µm at the wavelength of 1.6 µm.

In order to extend the operating wavelengths beyond

the fiber-optic window, parametric simulations were

performed for wavelengths ranging from λ= 1.2 µm

to 1.6 µm. Fig. 5 shows heights and widths of a-Si:H

that simultaneously satisfy both PI and SM

conditions. It is worth noting that if the width of a-

Si:H is W=1.46 µm and the a-Si:H height falls in

between 1.19 µm and 1.15 µm, a SM and PI

waveguide can be achieved throughout the

considered wide wavelength window. The outcomes

of this study not only offer valuable insights into the

optimal geometric parameters of a-Si:H waveguides

for specific operational conditions but also provide a

pathway for enhancing their versatility across A

wider range of wavelengths. These findings

contribute to the ongoing efforts to advance the

development of efficient and adaptable photonic

devices for diverse applications in optical

communication and beyond.

PHOTOPTICS 2024 - 12th International Conference on Photonics, Optics and Laser Technology

(a)

(b)

Figure 5: a-Si:H height (h) as a function of width for different wavelengths (a), in the range 0.9 µm<Wa-Si:H<2 µm (b) in

the range 1.35 µm<Wa-Si:H<1.5 µm.

(a) (b) (c)

Figure 6: e-Field amplitude at the input (a), along the propagation direction (b) and at the waveguide output (c) for the PI and

SM waveguide 3 mm-length at λ= 1.5 µm.

4.4 Waveguide with Longer Length

In Fig. 6, finite-difference time-domain (FDTD)-

based simulation results about the propagation of the

fundamental TE/TM mode along a SM and PI

waveguide, L=3 mm long, are reported at various

positions along the propagation direction.

Based on Figure 6, despite a substantial increase

in the waveguide length, the guided mode

propagating through it shows no significant losses.

Consequently, it is reasonable to assume that the

simulated single-mode (SM) and polarization-

independent (PI) waveguide, characterized by a large

cross-sectional area, can be efficiently applied in the

development of both passive and active devices even

with extended length.

5 CONCLUSION

In this study, the design of single-mode and

polarization-independent large cross-section

waveguides have been investigated using commercial

software. We successfully identified the optimal

geometries to achieve both conditions in a wide

operating wavelength range, from 1.2 up to 1.6 um.

Design and Simulation of Single-Mode and Polarization Independent Deeply Etched Amorphous Silicon on SOI Waveguides

These results provide valuable insights to support the

ongoing advancement of optical communications by

developing new integrated electro-optic integrated

devices to be explored in many application fields,

from sensing to telecommunications.

ACKNOWLEDGMENT

Graphics project (F5) under the RESTART research

program (PE-14) (MUR PE00000001) is

acknowledged.

REFERENCES

Aalto, T., Cherchi, M., Harjanne, M., Bhat, S., Heimala, P.,

Sun, F., . . . Vehmas, T. (2019). Open-access 3-μm SOI

waveguide platform for dense photonic integrated

circuits. IEEE Journal of selected topics in quantum

electronics, 25(5), 1-9.

Bachorec, T., & Sedlář, T. (2018). A new era in simulation.

IEEE. Mikulov, Czech Republic.

Bellutti, P., Boscardin, M., Soncini, G., Zen, M., & Zorzi,

N. (1995). On the choice of the optimum silicon

substrate for CCD/CMOS technology. In Proceedings

of 4th International Conference on Solid-State and IC

Technology, 176-178.

Butt, M., & Kozlova, E. (2017). Single mode ZnO/Al2O3

Strip loaded waveguide at 633 nm visible wavelength.

Information technology and nanotechnology.

Dai, D., Liu, L., Gao, S., Xu, D.‐X., & He, S. (2013).

Polarization management for silicon photonic

integrated circuits. Laser & Photonics Reviews, 7(3),

303-328.

Della Corte, F. G., & Rao, S. (2013). Use of Amorphous

Silicon for Active Photonic Devices. IEEE

Transactions on Electron Devices, 1495-1505.

Jia-Ming, L. (2009). Photonic devices. Cambridge

University Press.

Lim, S., Eng Png, C., Ong, E., & Ang, Y. (2007). Single

mode, polarization-independent submicron silicon

waveguides based on geometrical adjustments. Optics

Express, 15(18), 11061-11072.

Ma, Y., Li, J., Han, Z., Maeda, H., & Pištora, J. (2021). All-

dielectric graphene-induced T-slot waveguide electro-

optic modulator with polarization-independent

operation. IEEE Journal of Selected Topics in Quantum

Electronics, 27(3), 1-8.

Marpaung, D., Yao, J., & Capmany, J. (2019). Integrated

microwave photonics. Nature photonics, 13(2), 80-90.

Neslihan, E., & Kurt, H. (2016). Model analysis of ridge

and rib types of silicon waveguides with void

compositions. IEEE Journal of Quantum Electronics,

52(10), 1-7.

Samanta, S., Dey, P. K., Banerji, P., & Ganguly, P. (2018).

A 1 × 2 polarization-independent power splitter using

three-coupled silicon rib waveguides. Journal of

Optics, 20(9), 095801.

Seong P., C., Eng Png, C., & Lim, S. (2004). polarisation

independent waveguides in silicon-on-insulator. Ist

IEEE International Conference on Group Iv Photonics,

115-117.

Spear, W., & Le Comber, P. (1975). Substitutional doping

of amorphous silicon. Solid state communications,

17(9), 1193-1196.

Zhang, L., Pan, C., Zeng, D., Yang, Y., Yang, Y., & Junxian

, M. (2020). A Hybrid-Plasmonic-Waveguide-Based

Polarization-Independent Directional Coupler. IEEE

Access , 8, 134268-134275.

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