Design of terahertz metal-dielectric-metal waveguide with microfluidic sensing stub

doi:10.1016/j.optcom.2015.10.007

Optics Communications

Volume 361, 15 February 2016, Pages 130-137

https://doi.org/10.1016/j.optcom.2015.10.007 Get rights and content

Highlights

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We design a terahertz metal-dielectric-metal waveguide sensor with embedded microfluidic channel suitable for sensing the refractive index variations in liquid.
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The transmission properties are described using transmission line model and numerically simulated using finite-difference time domain method.
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The THz-MDM sensor has a theoretical value of the refractive index detection sensitivity at 0.457 THz/RIU for a 20 $μm \times$ 24 $μm$ cross-section channel.

Abstract

We design a terahertz (THz) metal-dielectric-metal (MDM) waveguide sensor with embedded microfluidic channel suitable for sensing the refractive index variations in liquid. The transmission properties are described using transmission line model (TLM) and numerically simulated using finite-difference time domain (FDTD) method. The sensing characteristics of the structure are systematically analyzed through the examination of the transmission spectrum. The results reveal a series of pronounced resonance peaks in the transmission spectrum, which has linear relationship with the refractive index variation of the material under investigation. For detecting the presence of various cancer cells flowing through the microfluidic channel, we designed and optimized the structural parameters of the THz-MDM sensor and achieved a theoretical value of the refractive index detection sensitivity as high as 0.457 THz/RIU for a 20 $μm \times$ 24 $μm$ cross-section channel. This work shows great promise toward realizing a compact THz refractive index sensor with high sensitivity for identifying the signatures of biological samples in liquid.

Introduction

Terahertz spectroscopy, which covers radiation band ranging 100 GHz to 10 THz, has intrigued tremendous interests in the biological, chemical sciences as well as clinical research communities. Studies have shown that terahertz waves could identify intermolecular and intramolecular hydrogen bonds in biological materials such as amino acids [1], polypeptides [2], DNA [3], protein [4], sugars [5], pharmaceuticals [6]. Applications such as pathological examinations of tissues [7] and identification of drugs or explosives in postal packages can also be realized with this real-time, marker-free and non-ionizing technique 8, 9.

However, there are a few limitations for applying THz spectroscopy in biological detection in water solutions. Firstly, water has huge absorption of THz waves due to the excitation of both water dipolar moments and the hydrogen bond network. The method of using attenuated total reflection can partially solve the problem, but it will encounter the large signal loss from the in and out of the coupling prism [10]. Second, relatively larger volume of bio-samples would be needed for THz analyses based on transmission, since the wavelength of 1 THz is about 300 µm and the wrist of the correspondent Gaussian beam is therefore above 1 mm. Large quantities of samples to be studied may be hard to collect in reality. To overcome such difficulties in sample volume, microfluidic systems have been proposed for accurate volume control of the solutions to perform THz spectroscopy 11, 12, 13, 14.

Alternatively, to address the above limitations, different THz wave manipulation structures such as parallel plate waveguides (PPWGs) or similarly metal-dielectric-metal (MDM) waveguides were proposed, since they can support modes with deep sub-wavelength scale and high group velocity over a very wide range of frequencies extending from DC to visible scale. This is due to the formation of the dispersion-less transverse electro-magnetic (TEM) mode or the similarly MDM fundamental mode (TM0) considering surface plasmon polaritons(SPP) [15]. In THz domain, the highly localized field inside the PPWGs has been used to increase the sensitivity of a measurement 16, 17, 18. Furthermore the stub like resonator has been integrated in the THz PPWG as a refractive index sensor for liquids in a microfluidic platform 19, 20, but they did not use the TM0 mode and the influence of THz SPP at metal-dielectric interfaces had not been explored though the metals behave almost like a perfect conductor (PEC).

In this paper, we proposed a compact (several tens of micrometers width and several hundreds of micrometers length) and highly sensitive liquid refractive index sensor based on a two layers THz MDM stub structure. To make optimized designs, we not only employed the numerical technique such as the finite difference-time domain method (FDTD), but also generated the transmission line model (TLM) for the proposed sensing structure. TLM is a fast and reliable analytical approach investigated for optical bands 21, 22, 23, 24, but few applications are reported in the THz band. The characterization of the impedance of the MDM by TLM can be useful for a better understanding of the MDM based THz components. TLM analytical modal also provides a design tool to generate complex plasmonic structures since the amount of calculation will dramatically drop when the optimization is necessary compared with EM popular full wave numerical methods such as FDTD and FEM. The model allows us to rapidly and precisely simulate the transmission spectra of THz MDM stub sensor considering SPP. The sensing characteristics of the proposed THz MDM stub structure were analyzed in detail. The result show that THz MDM stub integrated with a microfluidic channel can be a very promising for both molecular and cellular analyses towards point-of-care type of biological sensing.

Section snippets

Structure and method

The proposed THz MDM wave guide with a two layers stub structure is schematically shown in Fig. 1(a) and (b), in which the stub includes a solid spacing dielectric layer and a liquid biomaterial sample layer. Consider the possible applied scenario in Fig. 1(a) where the thickness t of the structure is thick enough to simplify the sensor into a 2D scheme in Fig. 1(b). The channel ( $ε_{air}$ ) width is d. The two layers stub with width w includes a dielectric spacing layer ( $ε_{d}$ ) with height h₁ and a

Results and discussion

To illustrate the developed transmission line model by comparing the metal models by PEC and Drude, we considered the THz stub waveguide with the following set of parameters L=200 $μ m$ , h₂=w=d=20 $μm$ , and h₁=10 $μm$ . The metal is gold, the channel is filled with air, the spacing dielectric is Si, and the sample is water by the model in Eq. (8). The geometry parameters are typical in applications such as PCR [30] and circulation tumor cell (CTC) detection 31, 32, 33, 34, 35. We examined the accuracy of

Conclusion

In summary, we introduce the design of a compact THz MDM waveguide-coupled microfluidic stub structure for the detection of refractive index variations in liquid. TLM method is proved as an efficient and reliable analytical approach to design the sensor structure on deep THz sub-wavelength MDM waveguide. The resonant wavelength of the sensor has a linear relationship with the refractive index of the sample under investigation. The sensitivity of the sensor can be further optimized by narrowing

Acknowledgments

Our work has been financially supported by the Nonprofit Technology Research Programs of Zhejiang Province of China under Grant no. 2013C31088 (Li), National Science Foundation CAREER Award under Grant no. 0846313 and DARPA Young Faculty Award under N66001-10-1-4049 (Zhang).

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Citation Excerpt :
Finally, the details of the geometrical and electrical parameters of the proposed structure are reported in Table 1. The sensitivity of the proposed RI sensor is compared with the previous works [66–81] and presented in Table 2. As seen the proposed sensor has the highest sensitivity and the widest operational range.
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View full text

Design of terahertz metal-dielectric-metal waveguide with microfluidic sensing stub

Highlights

Abstract

Introduction

Section snippets

Structure and method

Results and discussion

Conclusion

Acknowledgments

Chem. Phys. Lett.

Chem. Phys. Lett.

Opt. Commun.

Chem. Phys. Lett.

Chem. Phys. Lett.

Trends Biotechnol.

Quantitative measurements of amino acids by terahertz time-domain transmission spectroscopy

Anal. Chem.

Terahertz underdamped vibrational motion governs protein-ligand binding in solution

Nat. Commun.

Investigation of aqueous alcohol and sugar solutions with reflection terahertz time-domain spectroscopy

Opt. Express

Application of London-type dispersion corrections to the solid-state density functional theory simulation of the terahertz spectra of crystalline pharmaceuticals

Phys. Chem. Chem. Phys.

Characterization of Dielectric Responses of Human Cancer Cells in the Terahertz Region

J. Infrared Millim. Terahertz Waves

Microfluidic devices for terahertz spectroscopy of biomolecules

Opt. Express

Integrated waveguide structure for highly sensitive THz spectroscopy of nano-liter liquids in capillary tubes

Prog. Electromagn. Res.

Highly sensitive terahertz spectroscopy in microsystem

RSC Adv.

Metal-dielectric-metal plasmonic waveguide devices for manipulating light at the nanoscale

Chin. Opt. Lett.

Line narrowing of terahertz vibrational modes for organic thin polycrystalline films within a parallel plate waveguide

Appl. Phys. Lett.

7 GHz resolution waveguide THz spectroscopy of explosives related solids showing new features

Opt. Express