Extensive Q-factor tuning for leaky modes with minimal frequency variation in asymmetric slab grating structures.

We investigated an asymmetric slab grating structure to achieve significant tuning of the quality (Q) factor for a leaky mode while minimizing frequency variation. This structure comprises two identical gratings placed on the top and bottom of a slab waveguide, with one grating laterally shifted to introduce asymmetry. Simulations demonstrate that lateral shifting of one grating induces extensive changes in the Q-factor with minimal frequency variation, particularly near the band-flip filling fraction because the band-flip filling fraction remains unaffected by the shifting.

Broadband terahertz guided-mode resonance filter using cyclic olefin copolymer.

We propose an all-dielectric single-layer guided-mode resonance filter (GMRF) operating in the high-frequency terahertz (THz) region. For the fabrication of thin gratings to achieve strong resonance in the high-frequency region, the refractive index and absorption must be small, while the tensile strength must be high. Cyclic olefin copolymer (COC) films have a lower refractive index and absorption than polyethylene terephthalate (PET) films and a higher tensile yield strength than polytetrafluoroethylene (PTFE) films.

Polarization-independent all-dielectric guided-mode resonance filter according to binary grating and slab waveguide dimensions.

All-dielectric binary gratings, with and without slab waveguides, are designed to generate polarization-independent guided-mode resonance filters (GMRFs) operating in the THz frequency region using the rigorous coupled-wave analysis (RCWA) method. The filling factor and thickness of the grating were adjusted to have equal resonance frequencies of transverse electric (TE)- and transverse magnetic (TM)-polarized THz beams. The single polarization-independent resonance for a binary grating without a slab waveguide was obtained at 0.

THz guided-mode resonance notch filter with variable filtering strength.

In this paper, we propose a terahertz (THz) guided-mode resonance (GMR) notch filter made of a monolithic polyethylene terephthalate (PET) film, which has a monolayer grating structure. The proposed configuration shows both polarization-dependent and polarization-independent notch filter characteristics for the incident THz wave depending on the rotation angle of the second grating film. When the rotation angle is 0°, the filtering strength (transmittance) at resonance frequency changes from 0.

Enhancement of THz resonance using a multilayer slab waveguide for a guided-mode resonance filter.

We propose a multilayer slab waveguide (SWG) to enhance the resonance of the transmittance with a guided-mode resonance (GMR) filter. The resonance characteristics of the GMR filter were studied in three types according to the method of attaching the grating film to a SWG, which consists of 25 µm thick polyethylene terephthalate (PET) film layers separated by 25 µm air layers. The resonance depth with the multilayer SWG was improved over that of the monolayer SWG because the refractive index and absorption of the multilayer SWG were reduced.

Remote NO gas sensing by enhanced 910-m propagation of THz pulses.

We modified our 910-m long path THz system to increase the signal-to-noise ratio (S/N) with a nanostructure plasmonic THz transmitter (Tx) chip and a seven-mirror array reflector with 1 m diameter. When the THz pulse propagates the 910-m distance in the atmosphere, the S/N is up to 1170:1, which made the THz pulse measurable at a high water vapor density (WVD) of up to 25.2 g/m.

Tunable terahertz guided-mode resonance filter with a variable grating period.

A variable grating period made of quartz has been applied to fabricate a tunable guided mode resonance (TGMR) filter with transverse-electric (TE) and -magnetic (TM) modes in the terahertz (THz) region. We prepared three TGMR filters with grating periods of 5.0, 3.

Transmission characteristics of all-dielectric guided-mode resonance filter in the THz region.

In this study we report the first on the terahertz (THz) transmission characteristics of a guided-mode resonance (GMR) filter made of all-dielectric material. Two strong transverse electric (TE) resonance modes, TE and TE, and one strong transverse magnetic (TM) resonance mode, TM, were detected. The measured resonances can be explained by diffraction from the grating surface of the GMR filter, and by guiding along the inside of the filter (slab waveguide).

910-m propagation of THz ps pulses through the Atmosphere.

We measured the atmospheric propagation of ps THz pulses with a 0.4-THz bandwidth through a 910-m distance; the pulse delay corresponded to 255 pulses down the pulse train of the mode-locked ring laser excitation pulses. The complexity of the atmosphere requires the use of the complete theory of Essen and Froome to compare the measured time shifts due to both the dry atmosphere and water vapor with theoretical calculations.

Improvement of Terahertz Wave Radiation for InAs Nanowires by Simple Dipping into Tap Water.

We report improvement of terahertz (THz) wave radiation for Si-based catalyst-free InAs nanowires (NWs) by simple dipping into tap water (DTW). In addition, the possibility of using InAs NWs as a cost-effective method for biomedical applications is discussed by comparison to bulk InAs. The peak-to-peak current signals (PPCSs) of InAs NWs measured from THz time-domain spectroscopy increased with increasing NW height.

Terahertz otoscope and potential for diagnosing otitis media.

We designed and fabricated a novel terahertz (THz) otoscope to help physicians to diagnose otitis media (OM) with both THz diagnostics and conventional optical diagnostics. We verified the potential of this tool for diagnosing OM using mouse skin tissue and a human tympanic membrane samples prior to clinical application.

Enhanced THz guiding properties of curved two-wire lines.

We present experimental and simulation studies of enhanced terahertz (THz) guiding properties of curved two-wire lines for several surface conditions. When a THz-wave propagates through curved two-wire lines, a rough wire surface with dielectric coating contributes to a lower bending loss compared to a smooth or rough wire surface without coating. Dielectric coating and rough surface confine the THz field to the wire surface making the bending loss low.

Feasibility of terahertz reflectometry for discrimination of human early gastric cancers.

We have investigated the feasibility of THz time-domain reflectometry for the discrimination of human early gastric cancer (EGC) from the normal gastric region. Eight fresh EGC tissues, which were resected by endoscopic submucosal dissection, were studied. Of them, six lesions were well discriminated on THz images and the regions well correlated with tumor regions on pathologically mapped images.

Terahertz spectroscopic imaging and properties of gastrointestinal tract in a rat model.

We have investigated basic properties of normal gastrointestinal (GI) tract tissues, including glandular stomach (GS), fore stomach (FS), large intestine (LI), small intestine (SI), and esophagus (ESO), from a rat model using terahertz (THz) reflection imaging and spectroscopy. The THz images collected from stratified squamous epithelia (SSE) of FS and ESO show a lower peak-to-peak value compared to those from columnar epithelia (CE) of GS, LI, or SI because the SSE contains less water than CE. The refractive index and absorption coefficient of FS were less than those of GS or LI, both having values similar to those of water.

Thin layer terahertz sensing using two-channel parallel-plate waveguides.

We report on the highly sensitive terahertz measurement of a thin, dielectric layer using two channels formed by inserting a single slit sheet in the parallel-plate waveguides (PPWGs). When a thin layer is applied to coat the upper surface of the channel, the single resonance frequency caused by the two-channel PPWGs is shifted as a result of the layer's properties, including length, thickness, and refractive index. The measured frequency tuning sensitivities (FTS) throughout the 20-mm layer length are 2.

Tunable THz notch filter with a single groove inside parallel-plate waveguides.

A single groove in a parallel-plate waveguide (PPWG) has been applied to a tunable terahertz (THz) notch filter with a transverse-electromagnetic (TEM) mode. When the air gap between the metal plates of the PPWG is controlled from 60 to 240 μm using a motor controlled translation stage or a piezo-actuator, the resonant frequency of the notch filter is changed from 1.75 up to 0.

Terahertz band gaps induced by metal grooves inside parallel-plate waveguides.

We report experimental and finite-difference time-domain simulation studies on terahertz (THz) characteristics of band gaps by using metal grooves which are located inside the flare parallel-plate waveguide. The vertically localized standing-wave cavity mode (SWCM) between the upper waveguide surface and groove bottom, and the horizontally localized SWCM between two groove side walls (groove cavity) are observed. The E field intensity of the horizontally localized SWCM in grooves is very strongly enchanced which is three order higher than that of the input THz.

Terahertz notch and low-pass filters based on band gaps properties by using metal slits in tapered parallel-plate waveguides.

We present a tunable notch filter having a wide terahertz (THz) frequency range and a low-pass filter (LPF) having a 0.78 THz cutoff frequency. Single slit and multiple slits are positioned at the center of air gaps in tapered parallel-plate waveguides (TPPWG) to obtain the notch filter and LPF, respectively.

Probing structural transition and guest dynamics of hydroquinone clathrates by temperature-dependent terahertz time-domain spectroscopy.

The structural transition from hydroquinone clathrates to crystalline α-form hydroquinone was observed up to the range of 3 THz frequency as a function of temperatures. We found that all three hydroquinone clathrates, CO(2)-, CH(4)-, and CO(2)/CH(4)-loaded hydroquinone clathrates, transform into the α-form hydroquinone at around 102 ± 7 °C. The resonance peak of the CO(2)-loaded hydroquinone clathrate at 2.

Improvement of THz coupling using a tapered parallel-plate waveguide.

This paper reports an experimental and simulation study of a tapered parallel-plate waveguide (TPPWG) to improve THz coupling to the plate separation gap. The flat- and round-type TPPWG without any silicon lens is compared to the parallel-plate waveguide (PPWG) with a plano-cylindrical silicon lens. The spectrum amplitudes of the input-side TPPWG and the input- and output-side TPPWG both having a 3 degrees slop angle increased about 56% and 103% at 1 THz when compared to that of the PPWG.

THz surface wave collapse on coated metal surfaces.

The Zenneck THz surface wave (Z-TSW) on metals is discussed with respect to its difficulty in generation and measurement. The spatial collapse of the extent of the Z-TSW evanescent field, upon the addition of a sub-wavelength dielectric layer on the metal surface, is explained by a simple model, in good agreement with exact analytical theory. Experimental measurements of the THz surface wave on an aluminum surface covered with a 12.

A miniaturized fiber-coupled terahertz endoscope system.

In this study, we have designed, fabricated, and characterized a miniaturized optical fiber-coupled terahertz (THz) endoscope system. The endoscopic system utilized a photoconductive generator and detector driven by a mode-locked Ti:sapphire laser. In reflection mode, the endoscope showed a high signal-to-noise ratio and a wide frequency spectrum similar to the conventional THz time-domain spectroscopic system.

Bragg reflection of terahertz waves in plasmonic crystals.

We present experimental and theoretical studies on terahertz surface plasmon (TSP) propagation on slit and rectangular aperture arrays in an aluminum sheet. Terahertz waves are coupled onto the plasmonic structures via a parallel plate waveguide. Long-lasting oscillations are observed in the temporal pulse shape after propagating through the periodic structure, whose Fourier transformation into the frequency domain results in Bragg-resonance spectral features.

Enhancement of the detection of THz Sommerfeld wave using a conical wire waveguide.

In this study, the coupling properties of a conical copper wire waveguide were investigated in the terahertz (THz) frequency range using theoretical simulations and experiments. Because a conical wire tip has a smaller tip diameter than a cylindrical wire tip, it has a greater THz field density than a cylindrical wire tip. The measured THz pulse increased 4.