Abstract-Terahertz biosensing with a graphene-metamaterial heterostructure platform

Wendao Xu, Lijuan Xie,Jianfei Zhu, Longhua Tang, Ranjan Singh, Chen Wang, Yungui Ma, Hou-Tong Chen, Yibin Ying,

https://www.sciencedirect.com/science/article/pii/S0008622318308662

Terahertz (THz) radiation attracted great interest in the fields of material characterization, nondestructive security screening, clinical diagnostics, and identification of chemicals and molecules. Label-free THz sensing of trace amount of targets including biomolecules is promising because of their rich spectral fingerprint in this electromagnetic region; however, improving the sensitivity remains to be a challenge, partially due to the limitations of THz sources and detectors. The resonantly enhanced electromagnetic fields in metamaterialsand metasurfaces offer a potentially viable solution, although highly complicated decoration process is still needed for biosensing on the surface of metamaterials. Here we demonstrate a simple biosensing platform by integrating a monolayer graphene on a THz metamaterial absorber cavity, where the introduction of sensing targets results in a large change of the metamaterial resonant absorption (or reflection) because of their strong interaction with graphene. We experimentally show its ultrahigh sensitivity through detecting trace amount of chlorpyrifos methyl down to 0.2 ng. Using simple decoration steps and utilizing DNA to capture thrombin, we further show the feasibility of this platform serving as a sensitive biosensor.

Abstract-Broadband terahertz polarization conversion using metasurfaces (Conference Presentation)

Hou-Tong Chen,  Chun-Chieh Chang,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10756/107560C/Broadband-terahertz-polarization-conversion-using-metasurfaces-Conference-Presentation/10.1117/12.2322496.short

Two-dimensional metamaterials - metasurfaces - offer tremendous opportunities in realizing exotic optical phenomena and functionalities to address the technological challenges encountered in the terahertz frequency regime. By tailoring the resonant response of basic building blocks as well as their mutual interactions, we are able to effectively control of amplitude, phase, and polarization state of terahertz waves. Here we report the realization of highly efficient polarization conversions including: (1) Reflective linear polarization rotation using an array of anisotropic resonators backed with a ground plane; (2) Transmissive linear polarization rotation using an array of anisotropic resonator array sandwiched by two orthogonal gratings; and (3) Reflective linear-to-circular polarization conversion using two cascading arrays of complementary resonators. They operate over a broad bandwidth more than one octave and approaching two octaves in some cases. We further show that the linear polarization rotation is accompanied by a tunable phase discontinuity, which allows us to demonstrate an ultrathin terahertz flat lens enabling diffraction-limited focusing. The broadband linear-to-circular polarization may also find applications including terahertz circular dichroism spectroscopy and excitation of valley polarization in 2D materials.

Abstract-Broadband terahertz polarization conversion using metasurfaces (Conference Presentation)

Hou-Tong Chen,  Chun-Chieh Chang,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10756/107560C/Broadband-terahertz-polarization-conversion-using-metasurfaces-Conference-Presentation/10.1117/12.2322496.short

Two-dimensional metamaterials - metasurfaces - offer tremendous opportunities in realizing exotic optical phenomena and functionalities to address the technological challenges encountered in the terahertz frequency regime. By tailoring the resonant response of basic building blocks as well as their mutual interactions, we are able to effectively control of amplitude, phase, and polarization state of terahertz waves. Here we report the realization of highly efficient polarization conversions including: (1) Reflective linear polarization rotation using an array of anisotropic resonators backed with a ground plane; (2) Transmissive linear polarization rotation using an array of anisotropic resonator array sandwiched by two orthogonal gratings; and (3) Reflective linear-to-circular polarization conversion using two cascading arrays of complementary resonators. They operate over a broad bandwidth more than one octave and approaching two octaves in some cases. We further show that the linear polarization rotation is accompanied by a tunable phase discontinuity, which allows us to demonstrate an ultrathin terahertz flat lens enabling diffraction-limited focusing. The broadband linear-to-circular polarization may also find applications including terahertz circular dichroism spectroscopy and excitation of valley polarization in 2D materials.

Abstract-Metasurfaces for broadband terahertz linear polarization rotation and linear-to-circular polarization conversion

Hou-Tong Chen

https://www.osapublishing.org/abstract.cfm?uri=ISUPTW-2018-TuI2

We show metasurfaces consisting of few-layer anisotropic structures that allow for highly efficient and broadband terahertz linear polarization rotation and linear-to-circular polarization conversion, operating either in reflection or transmission.  We show metasurfaces consisting of few-layer anisotropic structures that allow for highly efficient and broadband terahertz linear polarization rotation and linear-to-circular polarization conversion, operating either in reflection or transmission.

© 2018 OSA

Abstract-Metasurfaces for broadband terahertz linear polarization rotation and linear-to-circular polarization conversion

Hou-Tong Chen

https://www.osapublishing.org/abstract.cfm?uri=ISUPTW-2018-TuI2

We show metasurfaces consisting of few-layer anisotropic structures that allow for highly efficient and broadband terahertz linear polarization rotation and linear-to-circular polarization conversion, operating either in reflection or transmission.  We show metasurfaces consisting of few-layer anisotropic structures that allow for highly efficient and broadband terahertz linear polarization rotation and linear-to-circular polarization conversion, operating either in reflection or transmission.

© 2018 OSA

Abstract-Highly efficient terahertz metasurface flat lenses (Conference Presentation)

Hou-Tong Chen

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10209/1020903/Highly-efficient-terahertz-metasurface-flat-lenses-Conference-Presentation/10.1117/12.2266427.pdf?SSO=1

Conventional optical lenses focus electromagnetic waves by imparting position-dependent phase delay through shaping their geometry. This poses difficulties in eliminating the geometric aberrations in high numerical aperture lenses, in addition to the fabrication challenges when operating at short wavelengths (e.g. visible light), and bulky devices operating at long wavelengths (e.g. microwaves). In contrast, metasurfaces realize full control of phase through tailoring the subwavelength resonant structures, allowing for the demonstration ultrathin flat lens, although the efficiency is still rather low using single-layer metasurfaces. Here we report the demonstration of high-performance flat lens in the terahertz frequency range using few-layer metasurfaces. The three-layer metasurface structure is capable of rotating the incident linear polarization by 90° with a very high efficiency over a bandwidth of two octaves. More importantly, the phase of the output light can be tuned over the entire 2π range with subwavelength resolution through simply tailoring the structure geometry of the basic building blocks. Based on this success, we design, fabricate, and characterize a metasurface lens operating at 0.4 THz. With a lens diameter and focal length both 5 cm, we realize a high numerical aperture of 0.5 and diffraction-limited terahertz beam focusing. Terahertz time-domain spectroscopy measurements show that the metasurface lens is capable of achieving the same signal intensity as compared to a bulk TPX lens of the same size and focal length.

Abstract-Characterization of an active metasurface using terahertz ellipsometry

Nicholas Karl, Martin S. Heimbeck,   Henry O. Everitt,   Hou-Tong Chen, Antoinette J. Taylor, Igal Brener, Alexander Benz, John L. Reno, Rajind Mendis,  Daniel M. Mittleman,

http://aip.scitation.org/doi/abs/10.1063/1.5004194

Switchable metasurfaces fabricated on a doped epi-layer have become an important platform for developing techniques to control terahertz (THz) radiation, as a DC bias can modulate the transmission characteristics of the metasurface. To model and understand this performance in new device configurations accurately, a quantitative understanding of the bias-dependent surface characteristics is required. We perform THz variable angle spectroscopic ellipsometry on a switchable metasurface as a function of DC bias. By comparing these data with numerical simulations, we extract a model for the response of the metasurface at any bias value. Using this model, we predict a giant bias-induced phase modulation in a guided wave configuration. These predictions are in qualitative agreement with our measurements, offering a route to efficient modulation of THz signals.

Abstract- Efficient terahertz metasurface-based flat lens

Daniel Headland.   Chun-Chieh Chang, Derek Abbott,  Withawat Withayachumnankul,  Hou-Tong Chen,

http://ieeexplore.ieee.org/document/8067113/

We present a nonuniform metasurface that operates as a flat lens at 400 GHz. Efficiency is enhanced by the use of a novel, tri-layer polarization-converting nonuniform meta-surface structure. The device is fabricated and experimentally characterized. The results confirm that the flat lens is indeed capable of focusing radiation to a focal spot with ∼68% efficiency. Constrained by spatial dispersion, the −3 dB bandwidth is 150 GHz.

Abstract-Resonance coupling and polarization conversion in terahertz metasurfaces with twisted split-ring resonator pairs

Chenyu Li, Chun-Chieh Chang, Qingli Zhou, Cunlin Zhang, and Hou-Tong Chen

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-21-25842

We investigate edge-coupling of twisted split-ring resonator (SRR) pairs in the terahertz (THz) frequency range. Using a simple coupled-resonator model we show that such a system exhibits resonance splitting and cross-polarization conversion. Numerical simulations and experimental measurements agree well with theoretical calculations, verifying the resonance splitting as a function of the coupling strength given by the SRR separation. We further show that a metal ground plane can be integrated to significantly enhance the resonance coupling, which enables the effective control of resonance splitting and the efficiency and bandwidth of the cross-polarization conversion. Our findings improve the fundamental understanding of metamaterials with a view of accomplishing metamaterial functionalities with enhanced performance, which is of great interest in realizing THz functional devices required in a variety of applications.

© 2017 Optical Society of America

Abstract-Demonstration of a highly efficient terahertz flat lens employing tri-layer metasurfaces

Chun-Chieh Chang, Daniel Headland, Derek Abbott, Withawat Withayachumnankul, and Hou-Tong Chen

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-42-9-1867&origin=search

We demonstrate a terahertz flat lens based on tri-layer metasurfaces allowing for broadband linear polarization conversion, where the phase can be tuned through a full 2𝜋$2\pi$ range by tailoring the geometry of the subwavelength resonators. The lens functionality is realized by arranging these resonators to create a parabolic spatial phase profile. The fabricated 124-μm-thick device is characterized by scanning the beam profile and cross section, showing diffraction-limited focusing and ∼68%$\sim 68\%$ overall efficiency at the operating frequency of 400 GHz. This device has potential for applications in terahertz imaging and communications, as well as beam control in general.

© 2017 Optical Society of America

Abstract-Bilayer Metasurfaces for Dual- and Broadband Optical Antireflection

Li Huang, Chun-Chieh Chang , Beibei Zeng, John Nogan, Sheng-Nian Luo,  Antoinette J. Taylor,  Abul K. Azad, Hou-Tong Chen,

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.7b00471?journalCode=apchd5

Optical antireflection has long been pursued for a wide range of applications, but existing approaches encounter issues in the performance, bandwidth, and structure complexity, particularly in the long-wavelength infrared regime. Here we present the demonstration of bilayer metasurfaces that accomplish dual- and broadband optical antireflection in the terahertz and mid-infrared spectral ranges. By simply tailoring the structural geometry and dimensions, we show that subwavelength metal/dielectric structures enable dramatic reduction of Fresnel reflection and significant enhancement of transmission at a substrate surface, operating either at two discrete narrow bands or over a broad bandwidth up to 28%. We also use a semianalytical interference model to interpret the obtained results, in which we find that the dispersion of the constituent structures plays a critical role in achieving the observed broadband optical antireflection.

Abstract-Single-layer plasmonic metasurface half-wave plates with wavelength-independent polarization conversion angle

Zhaocheng Liu, Zhancheng Li, Zhe Liu, Hua Cheng, Wenwei Liu, Chengchun Tang, Changzhi Gu, Junjie Li, Hou-Tong Chen, Shuqi Chen, and Jianguo Tian

ACS Photonics, Just Accepted Manuscript

DOI: 10.1021/acsphotonics.7b00491

Publication Date (Web): June 30, 2017

Copyright © 2017 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.7b00491?journalCode=apchd5

Manipulation of polarization state is of great fundamental importance and plays a crucial role in modern photonic applications such as optical communication, imaging and sensing. Metamaterials and metasurfaces have attracted increasing interest in this area because they facilitate designer optical response through engineering the composite subwavelength structures. Here we propose a general methods of designing half-wave plate, and demonstrate in the near-infrared wavelength range an optically thin plasmonic metasurface half-wave plates that rotate the polarization direction of the linearly polarized incident light with a high degree of linear polarization. The half-wave plate functionality is realized through arranging the orientation of the nanoantennas to form an appropriate spatial distribution profile, which behave exactly as in classical half-wave plates but over in a wavelength-independent way.

Abstract -Metasurfaces for THz antireflection coatings, polarization rotators, flat lenses, and modulators

Presenter:

Dr. Hou-Tong Chen

http://bit.do/160926_ee_chen

Abstract: The control of electromagnetic waves lies at the core of many modern technologies. Naturally occurring materials provide only limited electromagnetic response, particularly in the challenging terahertz (THz) frequency range, which is insufficient for emerging technologies with increasingly demanding requirements. Metamaterials are a class of effective media that allow for exotic electromagnetic properties by tailoring their composite metallic/dielectric subwavelength structures. Metasurfaces, the two-dimensional equivalent of metamaterials, can provide desirable functionalities suitable for device applications and, at the same time, address the loss and fabrication issues. In this talk I will present our recent advances in metasurfaces for manipulating the amplitude, phase, polarization, and propagation of THz waves. These include antireflection, polarization conversion, beam focusing, and signal modulation. The metasurface approach shows significant advantages in performance and device simplicity, as compared to conventional THz devices and components, and can be also scaled to operate at other wavelength ranges.

Abstract-Experimental demonstration of terahertz metamaterial absorbers with a broad and flat high absorption band

Li Huang, Dibakar Roy Chowdhury, Suchitra Ramani, Matthew T. Reiten, Sheng-Nian Luo, Antoinette J. Taylor, and Hou-Tong Chen
https://www.osapublishing.org/ol/abstract.cfm?URI=ol-37-2-154

We present the design, numerical simulations and experimental measurements of terahertz metamaterial absorbers with a broad and flat absorption top over a wide incidence angle range for either transverse electric or transverse magnetic polarization depending on the incident direction. The metamaterial absorber unit cell consists of two sets of structures resonating at different but close frequencies. The overall absorption spectrum is the superposition of individual components and becomes flat at the top over a significant bandwidth. The experimental results are in excellent agreement with numerical simulations.

© 2012 Optical Society of America

Full Article  |  PDF Article

Abstract-Anomalous Terahertz Reflection and Scattering by Flexible and Conformal Coding Metamaterials

1. Lanju Liang¹, 
2. Meiqing Qi², 
3. Jing Yang³,
4. Xiaopeng Shen², 
5. Jiquan Zhai¹, 
6. Weizong Xu⁴, 
7. Biaobing Jin^1,5,*, 
8. Weiwei Liu^3,5,*,
9. Yijun Feng⁴, 
10. Caihong Zhang¹, 
11. Hai Lu⁴,
12. Hou-Tong Chen⁶, 
13. Lin Kang¹, 
14. Weiwei Xu¹,
15. Jian Chen^1,5, 
16. Tie Jun Cui^2,5,*, 
17. Peiheng Wu¹ and
18. Shenggang Liu^5,7

Article first published online: 30 JUN 2015

http://onlinelibrary.wiley.com/doi/10.1002/adom.201500206/abstract;jsessionid=75D9087BA4464D14DC4C0EA2F158CD85.f03t01?userIsAuthenticated=false&deniedAccessCustomisedMessage=

DOI: 10.1002/adom.201500206

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Arbitrary control of terahertz (THz) waves remains a significant challenge although it promises many important applications. Here, a method to tailor the reflection and scattering of THz waves in an anomalous manner by using 1-bit coding metamaterials is presented. Specific coding sequences result in various THz far-field reflection and scattering patterns, ranging from a single beam to two, three, and numerous beams, which depart obviously from the ordinary Snell's law of reflection. By optimizing the coding sequences, a wideband THz thin film metamaterial with extremely low specular reflection, due to the scattering of the incident wave into various directions, is demonstrated. As a result, the reflection from a flat and flexible metamaterial can be nearly uniformly distributed in the half space with small intensity at each specific direction, manifesting a diffuse reflection from a rough surface. Both simulation and experimental results show that a reflectivity less than −10 dB is achieved over a wide frequency range from 0.8 to 1.4 THz, and it is insensitive to the polarization of the incident wave. This work reveals new opportunities arising from coding metamaterials in effective manipulation of THz wave propagation and may offer widespread applications.

Abstract-Terahertz Surface Wave Modulation in a Dielectric Slab Metasurface

Nicholas Karl, Hou-Tong Chen, Antoinette J. Taylor, Igal Brener, Alexander Benz, John Reno, Rajind Mendis, and Daniel Mittleman
https://www.osapublishing.org/abstract.cfm?uri=CLEO_SI-2015-STu1H.6

We experimentally and numerically investigate a switchable dielectric-slab-waveguide metasurface device. We use an active metasurface to manipulate the interaction with a propagating THz surface wave, giving us dynamic control of the wave at 0.3 THz.

© 2015 OSA

PDF Article

Abstract-Semiconductor activated terahertz metamaterials

• Hou-Tong Chen

http://link.springer.com/article/10.1007%2Fs12200-014-0436-0

Metamaterials have been developed as a new class of artificial effective media realizing many exotic phenomena and unique properties not normally found in nature. Metamaterials enable functionality through structure design, facilitating applications by addressing the severe material issues in the terahertz frequency range. Consequently, prototype functional terahertz devices have been demonstrated, including filters, antireflection coatings, perfect absorbers, polarization converters, and arbitrary wavefront shaping devices. Further integration of functional materials into metamaterial structures have enabled actively and dynamically switchable and frequency tunable terahertz metamaterials through the application of external stimuli. The enhanced light-matter interactions in active terahertz metamaterials may result in unprecedented control and manipulation of terahertz radiation, forming the foundation of many terahertz applications. In this paper, we review the progress during the past few years in this rapidly growing research field. We particularly focus on the design principles and realization of functionalities using single-layer and fewlayer terahertz planar metamaterials, and active terahertz metamaterials through the integration of semiconductors to achieve switchable and frequency-tunable response.

Abstract-Nonlinear high-temperature superconducting terahertz metamaterials

Nathaniel K Grady¹, Bradford G Perkins Jr^2,3, Harold Y Hwang², Nathaniel C Brandt², Darius Torchinsky², Ranjan Singh¹, Li Yan¹, Daniel Trugman¹, Stuart A Trugman¹, Q X Jia¹, Antoinette J Taylor¹, Keith A Nelson² and Hou-Tong Chen^1,4

http://iopscience.iop.org/1367-2630/15/10/105016

We report the observation of a nonlinear terahertz response of split-ring resonator arrays made of high-temperature superconducting films. Intensity-dependent transmission measurements indicate that the resonance strength decreases dramatically (i.e. transient bleaching) and the resonance frequency shifts as the intensity is increased. Pump–probe measurements confirm this behaviour and reveal dynamics on the few-picosecond timescale.

Abstract-Terahertz metamaterials for linear polarization conversion and anomalous refraction

Nathaniel K. Grady, Jane E. Heyes, Dibakar Roy Chowdhury, Yong Zeng, Matthew T. Reiten, Abul K. Azad, Antoinette J. Taylor, Diego A. R. Dalvit, Hou-Tong Chen
Polarization is one of the basic properties of electromagnetic waves conveying valuable information in signal transmission and sensitive measurements. Conventional methods for advanced polarization control impose demanding requirements on material properties and attain only limited performance. Here, we demonstrate ultrathin, broadband, and highly efficient metamaterial-based terahertz polarization converters that are capable of rotating a linear polarization state into its orthogonal one. Based on these results we create metamaterial structures capable of realizing near-perfect anomalous refraction. Our work opens new opportunities for creating high performance photonic devices and enables emergent metamaterial functionalities for applications in the technologically difficult terahertz frequency regime.

Abstract-Terahertz Metamaterials for Linear Polarization Conversion and Anomalous Refraction

1. 
   

1. Nathaniel K. Grady¹, 
2. Jane E. Heyes¹, 
3. Dibakar Roy Chowdhury¹, 
4. Yong Zeng², 
5. Matthew T. Reiten¹, 
6. Abul K. Azad¹,
7. Antoinette J. Taylor¹, 
8. Diego A. R. Dalvit², 
9. Hou-Tong Chen¹,*

+Author Affiliations

http://www.sciencemag.org/content/early/2013/05/15/science.1235399.abstract
Polarization is one of the basic properties of electromagnetic waves conveying valuable information in signal transmission and sensitive measurements. Conventional methods for advanced polarization control impose demanding requirements on material properties and attain only limited performance. Here, we demonstrate ultrathin, broadband, and highly efficient metamaterial-based terahertz polarization converters that are capable of rotating a linear polarization state into its orthogonal one. Based on these results, we create metamaterial structures capable of realizing near-perfect anomalous refraction. Our work opens new opportunities for creating high-performance photonic devices and enables emergent metamaterial functionalities for applications in the technologically difficult terahertz frequency regime.