Abstract-Terahertz Spoof Surface Plasmon Polariton Waveguides: A Comprehensive Model with Experimental Verification

Muhammed Abdullah Unutmaz, Mehmet Unlu,

https://www.nature.com/articles/s41598-019-44029-1

Spoof surface plasmon polariton waveguides are perfect candidates to enable novel, miniaturized terahertz integrated systems, which will expedite the next-generation ultra-wideband communications, high-resolution imaging and spectroscopy applications. In this paper, we introduce, for the first time, a model for the effective dielectric constant, which is the most fundamental design parameter, of the terahertz spoof surface plasmon polariton waveguides. To verify the proposed model, we design, fabricate and measure several waveguides with different physical parameters for 0.25 to 0.3 THz band. The measurement results show very good agreement with the simulations, having an average and a maximum error of 2.6% and 8.8%, respectively, achieving 10-to-30 times better accuracy than the previous approaches presented in the literature. To the best of our knowledge, this is the first-time investigation of the effective dielectric constant of the terahertz spoof surface plasmon polariton waveguides, enabling accurate design of any passive component for the terahertz band.

Abstract-Miniature multi-contact MEMS switch for broadband terahertz modulation

Miniature multi-contact MEMS switch for broadband terahertz modulation Mehmet Unlu and Mona Jarrahi  »View Author Affiliations

Optics Express, Vol. 22, Issue 26, pp. 32245-32260 (2014)
http://dx.doi.org/10.1364/OE.22.032245

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-22-26-32245

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Abstract

A miniature MEMS switch is designed, fabricated, and incorporated in a reconfigurable metallic mesh filter for broadband terahertz modulation. The mechanical, electrical, and geometrical properties of the MEMS switch are set to enable broadband terahertz modulation with relatively low modulation voltage, high modulation speed, and high device reliability. The implemented miniature MEMS switch exhibits an actuation voltage of 30 V, a fundamental mechanical resonance frequency of 272 kHz, and an actuation time of 1.23 μs, enabling terahertz modulation with a record high modulation depth of more than 70% over a terahertz band of 0.1-1.5 THz, with a modulation voltage of 30 V and modulation speeds exceeding 20 kHz.

© 2014 Optical Society of America

Fully integrated broadband terahertz modulators

Mehmet Unlu, Mohammad Hashemi, Christopher Berry, Shenglin Li, Shang-Hua Yang and Mona Jarrahi,

http://spie.org/x110776.xml?highlight=x2406&ArticleID=x110776

Metasurfaces that can be reconfigured by microelectromechanical systems allow extreme switching in scattering parameters.

19 November 2014, SPIE Newsroom. DOI: 10.1117/2.1201410.005657

Recent advances in terahertz sources and detectors have made terahertz radiation much more accessible for many practical imaging and sensing applications, such as identifying chemicals, characterizing materials, biological sensing, and biomedical imaging, increasing demand for high-performance terahertz passive components. Among these, terahertz modulators play a key role in single-pixel compressive sensing and imaging systems,1, 2 especially when high-sensitivity detector arrays with a large number of pixels are not available.

It is challenging to construct high-performance terahertz modulators based on conventional optical and IR modulation schemes3–8 due to a lack of materials with the desired properties at terahertz frequencies and practical difficulties in scaling device dimensions to operate efficiently in this regime.9 In contrast, the spectral response of reconfigurable metamaterials (constructed by assembling so-called meta-molecule building blocks in a specific arrangement) can be engineered by their geometry, rather than being limited by the characteristics of natural materials at terahertz frequencies. As a result, they offer a very promising platform for manipulating terahertz waves.10, 11

However, until now the operational bandwidth of terahertz modulators based on metamaterials has been limited by the resonant nature of the device configurations employed. There is a good reason for using resonant structures in previously demonstrated reconfigurable metamaterials: the response is highly sensitive to small changes in the metamaterial's constituent parts as a result of external stimuli at resonance frequency.

To enable broadband modulation, we have developed a reconfigurable metasurface that, unlike previous attempts, offers extreme switching in the scattering parameters without use of any resonant enhancement within the device operation frequency range.12 Therefore, it enables the broadband modulation required in time-domain and frequency-tunable terahertz spectroscopy and imaging systems. Moreover, we achieved the demonstrated broadband switching of the scattering parameters through a fully integrated, electrically controlled, room-temperature platform, making it suitable for real-world, practical wave manipulation applications, such as portable sensors and imaging systems.

The metasurface consists of an array of microscale metallic islands integrated with an array of microelectromechanical systems (MEMS) switches (see Figure 1). Activating the MEMS switches brings the previously separate metallic islands into contact with each other. Their electromagnetic properties then change from a low pass (capacitive) filter to a high-pass (inductive) filter over a range of wavelengths that are much larger than the feature size and periodicity of the islands.12, 13 As a result, by shifting the MEMS switches between the contact and non-contact modes, their response to an incident electromagnetic wave changes from transmission to reflection, leading to efficient terahertz modulation over a broad range of frequencies determined by the meta-surface geometry.

 

Figure 1. Scanning electron microscopy images (above) and schematic diagram (below) of a microelectromechanical systems (MEMS)-reconfigurable metasurface that offers broadband terahertz modulation. E, H, KTHz: Electric field, magnetic field, and wave vector of the incident terahertz wave. Au: Gold. Si: Silicon. SiO2: Silicon dioxide.

We used the extraordinary switching of the scattering parameters of the presented metasurface to develop a fully integrated room-temperature terahertz intensity modulator. The modulator was fabricated on a high-resistivity silicon substrate and characterized in a time-domain terahertz spectroscopy setup exhibiting terahertz intensity modulation with record high modulation depths (>70%) and modulation bandwidths (>1.5THz). The high modulation depth of the presented modulator offers higher signal-to-noise ratio levels in single-pixel compressive sensing and imaging systems. Additionally, its broad modulation bandwidth makes it compatible with time-domain and frequency-tunable terahertz spectroscopy and imaging systems. We are now working on miniaturizing the reconfigurable metasurface to extend the terahertz modulator's bandwidth beyond 1.5THz. We are also working on improving the yield of the MEMS switch arrays to further increase the device modulation depth beyond 70%.

We gratefully acknowledge financial support from the National Science Foundation Sensor and Sensing Systems division (1030270) and Army Research Office Young Investigator Award (W911NF-12-1-0253).

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Mehmet Unlu, Mohammad Hashemi, Christopher Berry, Shenglin Li, Shang-Hua Yang, Mona Jarrahi

Electrical Engineering and Computer Science Department University of Michigan

Ann Arbor,

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References:

1. W. L. Chan, M. L. Moravec, R. G. Baraniuk, D. M. Mittleman, Terahertz imaging with compressed sensing and phase retrieval, Opt. Lett. 33, p. 974-976, 2008.

2. W. L. Chan, K. Charan, D. Takhar, K. F. Kelly, R. G. Baraniuk, D. M. Mittleman, A single-pixel terahertz imaging system based on compressed sensing, Appl. Phys. Lett. 93, p. 121105, 2008.

3. A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, M. Paniccia, A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor, Nature 427, p. 615-618, 2004.

4. M. Liu, X. Yin, E. Ulin-Avila, B. Geng, T. Zentgraf, L. Ju, F. Wang, X. Zhang, A graphene-based broadband optical modulator, Nature 474, p. 64-67, 2011.

5. M. Jarrahi, T. H. Lee, D. A. B. Miller, Wideband, low driving voltage traveling wave Mach-Zehnder modulator for RF photonics, Photon. Technol. Lett. 20, p. 517, 2008.

6. W. E. Ross, D. Psaltis, R. H. Anderson, Two-dimensional magneto-optic spatial light modulator for signal processing, Opt. Eng. 22, p. 485-490, 1983.

7. O. Solgaard, F. S. A. Sandejas, D. M. Bloom, Deformable grating optical modulator, Opt. Lett. 17, p. 688-690, 1992.

8. M. Jarrahi, R. F. W. Pease, D. A. B. Miller, T. H. Lee, Optical switching based on high-speed phased-array optical beam steering, Appl. Phys. Lett. 92, p. 014106, 2008.

9. M. Rahm, J.-S. Li, W. J. Padilla, THz wave modulators: a brief review on different modulation techniques, J. Infrared Milli. Terahz. Waves 34, p. 1-127, 2012.

10. H. Tao, W. J. Padilla, X. Zhang, R. D. Averitt, Recent progress in electromagnetic metamaterial devices for terahertz applications, IEEE J. Sel. Top. Quant. Electron. 17, p. 92-101, 2011.

11. H. Tao, A. C. Strikwerda, K. Fan, W. J. Padilla, X. Zhang, R. D. Averitt, Reconfigurable terahertz metamaterials, Phys. Rev. Lett. 103, p. 147401, 2009.

12. M. Unlu, M. R. Hashemi, C. W. Berry, S. Li, S.-H. Yang, M. Jarrahi, Switchable scattering meta-surfaces for broadband terahertz modulation, Nat. Sci. Rep. 4, p. 5708, 2014.

13. C. W. Berry, J. Moore, M. Jarrahi, Design of reconfigurable metallic slits for terahertz beam modulation, Opt. Exp. 19, p. 1236-1245, 2011.

Breakthrough Terahertz Modulator Holds Promise for Advanced Medical and Security Imaging Systems

Electron microscope image showing the metasurface for a terahertz modulator developed by a group led by UCLA professor Mona Jarrahi. Source: UCLA Engineering

A UCLA Henry Samueli School of Engineering and Applied Science research team has developed a breakthrough broadband modulator that could eventually lead to more advanced medical and security imaging systems.

Modulators manipulate the intensity of electromagnetic waves. For example, modulators in cell phones convert radio waves into digital signals that the devices can use and understand. In terahertz-based communication and imaging systems, they modify the intensity of terahertz waves.

Today's technologies take advantage of many parts of the electromagnetic spectrum — notably light waves and radio waves — but they rarely operate in the terahertz band, which lies between infrared and microwave on the spectrum.

Led by Mona Jarrahi, UCLA associate professor of electrical engineering, the group developed a terahertz modulator that performs across a wide range of the terahertz band with very high efficiency and signal clarity. Among the device’s advantages are that it could easily be incorporated into existing integrated circuit manufacturing processes, can operate at room temperature and does not require an external light source to operate.

The terahertz band has been the subject of extensive research, in large part because of its potential for medical imaging and chemical sensing technologies. For example, terahertz waves could be used to examine human tissue for signs for disease without damaging cells or the other health risks posed by X-rays. They also could be used in security screenings to penetrate fabric or plastics that conceal weapons.

Current optical modulators that use naturally existing materials, such as silicon or liquid crystals, to manipulate the intensity of light waves have proven to be very inefficient in terahertz frequencies. And modulators based on artificial materials, so-called metamaterials, thus far have a limited use because they only operate in a narrow band of the terahertz range.

The new modulator is based on an innovative artificial metasurface — a type of surface with unique properties that is defined by the geometry of its individual building blocks, and their arrangement. The metasurface developed by Jarrahi's team is composed of an array of micro-electromechanical units that can be opened and closed using electric voltage. Opening or closing the metasurface encodes the incoming terahertz wave into a corresponding series of zeroes or ones, which are then transformed into images.

"Our new metasurface broadens the realm of metamaterials to broadband operation for the first time, and it diminishes many of the fundamental physical constraints in routing and manipulating terahertz waves, especially in terahertz imaging and spectroscopy systems," Jarrahi said. "Our device geometry can switch from an array of microscale metallic islands to an array of interconnected metallic loops, altering its electromagnetic properties from a transparent surface to a reflecting surface, which manipulates the intensity of terahertz waves passing through over a broad range of frequencies."

The research was published July 16 in the journal Nature Scientific Reports.

The study's lead authors are Mehmet Unlu and Mohammed Reza Hashemi, who were postdoctoral scholars in Jarrahi's group when she was a member of the faculty at the University of Michigan. Other authors are Christopher Berry and Shenglin Li, former students in Jarrahi's group, and Shang Hua Yang, a current UCLA graduate student.

The research was funded by the National Science Foundation's Sensor and Sensing Systems Division and an Army Research Office Young Investigator award.

Source: http://www.ucla.edu/