Ja-Yu Lu

Terahertz air-core microstructure fiber

A low-loss terahertz air-core microstructure fiber is demonstrated for terahertz waveguiding. Substantially low attenuation constant less than 0.01cm−1 has been achieved and the guiding wavelength is found to be tunable by linear scaling the fiber size. The experimental results well agree with the simulation based on the finite-difference frequency-domain method, which interprets the guiding mechanism as the antiresonant reflecting waveguiding. The simulated modal pattern shows that most terahertz field is concentrated inside the central hollow air core and is guided without outside interference, which has high potential for guiding intense terahertz waves with minimized loss.

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.

Investigation on spectral loss characteristics of subwavelength terahertz fibers

By measuring the spectral loss characteristics of subwavelength-diameter terahertz fibers, our study supports the recent theory proposed by M. Sumetsky [Opt. Lett. 31, 870 (2006)] that diameter-variation-induced radiation is a dominant loss mechanism for subwavelength fibers in the low- (<1%) core-fraction-power regime. This physical mechanism limits the lowest guidable frequency in a subwavelength fiber.

THz interferometric imaging using subwavelength plastic fiber based THz endoscopes.

We demonstrate a new reflective imaging technique using continuous-wave THz fiber-endoscopy, in which the sample is placed behind the output of a THz subwavelength plastic fiber and the Fabry Perot interference is formed therein. 3D THz reflective images with a reasonable SNR as well as high lateral and subwavelength axial resolutions are acquired by moving the sample along the axial (z) direction and by 2D scanning the output end of the subwavelength plastic fiber without any focusing medium. By analyzing the axial-position dependent THz signals backward collected by the subwavelength plastic fiber, the THz reflection amplitudes and phases on the sample surface can be successfully reconstructed.

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.

Terahertz scanning imaging with a subwavelength plastic fiber

The feasibility to perform fiber-scanning terahertz imaging utilizing a terahertz subwavelength plastic fiber is investigated. Our study shows that, with a low (<1%) fractional power inside the fiber core, the bending loss of the terahertz subwavelength fiber is acceptable to enable large area scanning without seriously sacrificing the signal-to-noise ratio of the acquired image. With a transmission geometry, this feasibility is demonstrated by direct two-dimensional scanning of the terahertz subwavelength fiber output end to image different biological samples and concealed substances.

Separated-transport-recombination p-i-n photodiode for high-speed and high-power performance

We demonstrate a novel p-i-n photodiode (PD) structure, the separated-transport-recombination PD, which can greatly relieve the tradeoffs among the resistance-capacitance bandwidth limitation, responsivity, and output saturation power performance. Incorporating a short carrier lifetime (less than 1 ps) epitaxial layer to serve as a recombination center, this device exhibits superior speed and power performance to a control PD that has a pure intrinsic photoabsorption layer. Our demonstrated structure can also eliminate the bandwidth degradation problem of the high-speed photodetector, whose active photoabsorption layer is fully composed of short lifetime (/spl sim/1 ps) materials, under high dc bias voltages.

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.

Device saturation behavior of submillimeter-wave membrane photonic transmitters

Ultrahigh-speed photodetectors and printed-circuit antennas construct photonic transmitters. In this letter, we studied the saturation behaviors of an edge-coupled membrane photonic transmitter based on low-temperature-grown GaAs. The saturation behaviors determine the optimized operation condition of photonic transmitters. Ultrahigh external light-terahertz (THz) conversion efficiency of 0.11% was achieved with 645-GHz radiation. According to our knowledge, this value is the highest reported external conversion efficiency of all photonic transmitters with radiation higher than 500 GHz. The high conversion efficiency and the edge-coupled structure of our devices release the power burden imposed on tunable semiconductor laser sources and imply their applications as compact all-solid-state THz radiation sources.