Tze An Liu

Modal characteristics of antiresonant reflecting pipe waveguides for terahertz waveguiding

Modal characteristics of the THz pipe waveguide, which is a thin pipe consisting of a large air core and a thin dielectric layer with uniform but low index, are investigated. Modal indices and attenuation constants are calculated for various core diameters, cladding thicknesses, and cladding refractive indices. Numerical results reveal that the guiding mechanism of the leaky core modes, which transmit most of the power in the air-core region, is that of the antiresonant reflecting guiding. Moreover, modal patterns including modal intensity distributions and electric field vector distributions are shown for the fundamental and higher order modes. Experiments using time-domain spectroscopy with PMMA pipes also confirm the antiresonant reflecting guiding mechanism.

Terahertz refractive index sensors using dielectric pipe waveguides

A dielectric pipe waveguide is successfully demonstrated as a terahertz refractive index sensor for powder and liquid-vapor sensing. Without additional engineered structures, a simple pipe waveguide can act as a terahertz resonator based on anti-resonant reflecting guidance, forming multiple resonant transmission-dips. Loading various powders in the ring-cladding or inserting different vapors into the hollow core of the pipe waveguide leads to a significant shift of resonant frequency, and the spectral shift is related to the refractive-index change. The proven detection limit of molecular density could be reduced to 1.6nano-mole/mm3 and the highest sensitivity is demonstrated at around 22.2GHz/refractive-index-unit (RIU), which is comparable to the best THz molecular sensor [Appl. Phys. Lett. 95, 171113 (2009)].

Subwavelength film sensing based on terahertz anti-resonant reflecting hollow waveguides

A simple dielectric hollow-tube has been experimentally demonstrated at terahertz range for bio-molecular layer sensing based on the anti-resonant reflecting wave-guidance mechanism. We experimentally study the dependence of thin-film detection sensitivity on the optical geometrical parameters of tubes, different thicknesses and tube wall refractive indices, and on different resonant frequencies. A polypropylene hollow-tube with optimized sensitivity of 0.003 mm/μm is used to sense a subwavelength-thick (λ/225) carboxypolymethylene molecular overlayer on the tubes inner surface, and the minimum detectable quantity of molecules could be down to 1.22 picomole/mm(2). A double-layered Fabry-Pérot model is proposed for calculating the overlayer thicknesses, which agrees well with the experimental results.

Hybrid terahertz plasmonic waveguide for sensing applications

The suitability of a terahertz plasmonic sensor for sensing applications is successfully demonstrated using a hybrid planar waveguide composed of a subwavelength plastic ribbon waveguide and a diffraction metal grating. The subwavelength-confined terahertz plasmons on the hybrid waveguide resonantly reflect from the periodic metal structure under phase-matched conditions and perform resonant transmission dips. The resonant plasmonic frequencies are found to be strongly dependent on the refractive indices and thicknesses of analytes laid on the hybrid planar waveguide. Both plastic films with varying thicknesses and granular analytes in different quantities are successfully identified according to the spectral shifts of resonant dips. An optimal refractive index sensitivity of 261 GHz per refractive index unit is achieved. Within localized and enhanced terahertz plasmonic fields, the minimum detectable optical path difference can be reduced to 2.7 μm corresponding to λ/289, and the minimum detectable amount of analytes in powdered form reaches 17.3 nano-mole/mm(2). The sensing technique can be used to detect particles in a chemical reaction or monitor pollutants.

Terahertz plasmonic waveguide based on metal rod arrays for nanofilm sensing

A high-aspect-ratio metallic rod array is demonstrated to generate and propagate highly confined terahertz (THz) surface plasmonic waves under end-fire excitation. The transverse modal power distribution and spectral properties of the bound THz plasmonic wave are characterized in two metallic rod arrays with different periods and in two configurations with and without attaching a subwavelength superstrate. The integrated metallic rod array-based waveguide can be used to sense the various thin films deposited on the polypropylene superstrate based on the phase-sensitive mechanism. The sensor exhibits different phase detection sensitivities depending on the modal power immersed in the air gaps between the metallic rods. Deep-subwavelength SiO(2) and ZnO nanofilms with an optical path difference of 252 nm, which is equivalent to λ/3968 at 0.300 THz, are used as analytes to test the integrated plasmonic waveguide. Analysis of the refractive index and thickness of molecular membranes indicates that the metallic rod array-based THz waveguide can integrate various biochip platforms for minute molecular detection, which is extremely less than the coherent length of THz waves.

Subwavelength plastic wire terahertz time-domain spectroscopy

This work demonstrates the feasibility of a terahertz time-domain spectrometer based on a subwavelength-diameter plastic wire (SPW) for sensing applications. The dispersion property of the SPW is experimentally and theoretically studied. The SPW exhibits a low and controllable waveguide dispersion, which can be engineered by changing the core diameter, the core index, and the cladding index of the wire. Two white powders, tryptophan and polyethylene, deposited on the bottom of the wire can be successfully distinguished based on the waveguide dispersion of SPW. The SPW would be a promising candidate for combination with biochips for sensing minute molecules.

Application of metal-clad antiresonant reflecting hollow waveguides to tunable terahertz notch filter

A continuously tunable terahertz notch filter is demonstrated by using antiresonant reflecting hollow waveguides. The maximum frequency-tuning-range approached 50% of the bandwidth, and a 20dB notch-depth with a linewidth of 6GHz was successfully achieved.

A terahertz plastic wire based evanescent field sensor for high sensitivity liquid detection

A highly sensitive detection method based on the evanescent wave of a terahertz subwavelength plastic wire was demonstrated for liquid sensing. A 20ppm melamine alcohol solution is successfully identified with refractive-index sensitivity of 0.01.

Subwavelength confined terahertz waves on planar waveguides using metallic gratings.

Subwavelength confined terahertz waves propagating along a planar waveguide is successfully demonstrated via the integration between a diffraction metal grating and a plastic ribbon. The longest propagation distance and minimum confined range are, respectively, 50 mm and λ/5, improving the deliverable range of spoof-surface-plasmon-polaritons.

Terahertz artificial material based on integrated metal-rod-array for phase sensitive fluid detection

Transverse-electric (TE) polarized terahertz (THz) waves along a three-layered metal-rod-array (MRA) are demonstrated to identify liquid molecular dipole moments based on the extended optical-path-Iengths. The minute sensing ability is less than 0.1 mmol, equaling