Projects:2019s2-21501 Terahertz Waveplates

The project is to learn knowledge of terahertz technology, wave plate, CST software. And in this project we aim to design, simulate and test a wave plate operating at a frequency between 0.1 to 10 THz.

Introduction

The terahertz range spans 0.1 and 10 THz. It defines a transition between the electronics and photonics domains—the frequency range is at the upper bound of electronics and the lower bound of photonics. For this very reason, in the past the band has been perceived as a terahertz gap due to the lack of efficient generation and detection approaches. Over a few decades, a myriad of sources and detectors have become mature to tap into unique opportunities in this frequency range. Much has yet to be done in this area towards integrated high-performance platforms, envisioned for post-5G applications. Core components that underpin any integrated platform are interconnects, i.e., waveguiding structures. A large number of terahertz waveguides have been proposed with different trade-offs related to bandwidth, dispersion, confinement, losses, and fabrication complexity. Here we will investigate a waveguide design that could carry terahertz waves with practical performance. This project will tackle on different waveguide designs and related components. The students will gain experience on full-wave electromagnetic simulation, theoretical analysis of waveguides, and a terahertz measurement system.

Project team

Project students

• Fan Zhang
• Yingzhe Guo

Supervisors

• Dr. Withawat Withayachumnankul
• Dr. Wendy Suk Ling Lee

Objectives

To design, simulate and test a terahertz wave plate. we need to achieve:

• learn the background of terahertz technology and wave plate
• design a half-wave plate and a quarter-wave plate
• test the half-wave plate and half wave plate

Background

Terahertz technology

The terahertz range spans from 100 GHz to 10 THz which is between microwaves and infrared waves.The Photon energy is from 1.2 meV to 12.4 meV.The advantage of the terahertz technology is non-ionisation. The limitation of the terahertz is hardly penetrate fog and clouds, and it is cannot penetrate water or metal ,so the terahertz technology can only use indoor.The property the terahertz is Far-infrared, sub-millimetre wave,Penetrate conventionally opaque materials and high chemical sensitivity.Some of the applications are used in Information and communication technology areas,Biological; medical and pharmaceutical sciences areas; security areas;Earth and space science.

Wave plates

Wave plate is a optical device that changes the polarisation state of the transmitted wave.Wave plate can be divided into two types: half-wave plate and quarter-wave plate. In Lee’s (2018) study, as for the half-wave plate, a 45 degrees linearly polarized light going through the wave plate will be resolved into a vertical component and a horizontal component. And there will be a phase difference of  radian between these two components, then the two components will compose a linearly polarized light but being rotated by 90 degrees than the original light. As for the quarter-wave plate, if a 45 degrees linearly polarized light goes through the wave plate, it will also be resolved into a vertical component and a horizontal component.There will be a phase difference of pi/2 radian between these two components. And the two components will compose a circularly polarized light.

Design

Experiments in MATLAB

The experiment of MATLAB is use different formulas to determine the correct MATLAB code to calculate the refractive index and delta n.The main MATLAB function be used in the project is phase unwrapping and extrapolation.There is a picture shows these two functions.

Experiments in CST

Before the half wave plate and the quarter wave plate are constructed, the dimension of the slab needs to be determined. Note that this dimension is about the widths of the COC bars and air grooves, but not the thickness of the slab. The thickness of the wave plate is determined by the difference between the horizontal and vertical refractive index which is called delta n. As the thickness of the wave plate is unknown, an initial thickness can be assumed as 1 mm. Then, the widths of the COC bars and air grooves are initially set as 0.3 mm periodic which means the COC bars and air grooves are all 0.3 mm in width. The slab of this dimension can be considered as a sample. After the dimension is set, the model of the sample in the CST can be changed into this dimension and simulated. The figure below is the model of the sample in the CST.

,

Results

Simulation results for the half wave plate and quarter wave plate

After the half wave plate and quarter wave plate are simulated, the S21 parameter data are imported into the MATLAB code and the delta n for the half wave plate and quarter wave plate is the same with the delta n of the sample. To be more specific, the phase difference of the wave plate is shown has already be verified.For the phase difference of the half wave plate is 179.86 degrees and the phase difference of the quarter wave plate should be 90.15 degrees.

Conclusion

Based on the relevant background, our project is aimed at designing a half wave plate and a quarter wave plate at terahertz frequency. Through the simulation in the CST, calculation in the MATLAB and modifying, the final design meets the expected requirement. As for the half wave plate which has 0.3 mm periodic widths for the COC bars and air grooves and 5.75 mm thickness, the phase difference can reach 179.86 degrees and it is very close to the needed 180 degrees. For the quarter wave plate, it has 0.3 mm periodic widths for the COC bars and air grooves and 2.84 mm thickness, the phase difference can reach 90.15 degrees which is also close to the needed 90 degrees.

For the future work, as we cannot go to the lab due to some constraints, we would like to print out our design to test it in the terahertz laboratory in the future. And we could gather the measured S parameter data and validate our calculations and simulations.

References

[1] Tonouchi, M 2007, Cutting-edge THz technology, Nature Photonics, vol. 1, pp.97–105 .

[2] Ferguson, B & Xi-Cheng Zhang 2002, Materials for terahertz science and technology, Nature Materials, vol. 1, pp.26–33.

[3] P.H. Siegel 2002, Terahertz Technology, IEEE Transactions on Microwave Theory and Techniques, vol. 50, no. 3.

[4] Ornik, J, Gomell, L, Busch, SF, Hermans, M, & Koch, M 2018, High quality terahertz glass wave plates, Optics Express, vol. 26, no. 25, pp. 32631-32639.

[5]Lee, WSL 2018, Terahertz metasurfaces for wideband polarization control.