Achromatic wave plate in THz frequency region based on parallel metal plate waveguides with a pillar array.

We demonstrated an achromatic wave plate based on parallel metal plate waveguides in the high THz frequency region. The metal plates have periodic rough structures on the surface, which allow slow transverse magnetic wave propagation and fast transverse electric wave propagation. A numerical simulation showed that the height of the periodic roughness is important for optimizing the birefringence. We fabricated stacked metal plates containing two types of structures by chemical etching. An array of small pillars on the metal plates allows higher frequency optimization. We experimentally demonstrated an achromatic quarter-wave plate in the frequency region from 2.0 to 3.1 THz.

Nondestructive analysis of structure and components of tablet coated with film by the usage of terahertz time-domain reflection spectroscopy.

Nondestructive analysis of tablet is of great importance from the aspect of productivity and safety. In terahertz (THz) region, however, the analysis of core of coated tablet has not been progressed. In this study, we have measured a flat-surface push-pull osmotic pump tablet, having no orifice, having bilayer core and a coating film layer. The bilayer core was made from the drug layer and the push layer, and acetaminophen was contained in the drug layer as a model drug. To study its structure and components, we have obtained reflection spectra from the drug layer side and the push layer side measurements using THz time-domain reflection spectroscopy (THz-TDRS). From these results, detection of the peak of acetaminophen in the spectrum from the drug layer side measurements was confirmed. We have made ridges approximated toward the peak using a general method of linear regression analysis in both spectra. Two-sample t-test was applied to their gradients, and significant difference between the drug layer and the push layer was shown. These results suggested that THz-TDRS is applicable to the analysis of structure and component of a coated tablet.

Achromatic THz wave plate composed of stacked parallel metal plates.

We propose a simple achromatic terahertz wave plate composed of stacked parallel metal plates with a hole array. It consists of an ensemble of designed parallel plate waveguides; the high and low propagation speeds of waves in TE and TM waveguide modes with the same group velocity cause a constant phase difference over a wide frequency region. Using that wave plate, we obtained intense single- and multi-cycle THz pulses with circular polarization.

Broadband THz-wave generation by satisfying the noncollinear phase-matching condition with a reflected signal beam.

We demonstrated broadband terahertz (THz) wave generation by satisfying the noncollinear phase-matching condition with a reflected signal beam. We constructed a dual-wavelength optical parametric oscillator with two potassium titanium oxide phosphate crystals pumped by a frequency-doubled Nd:YAG laser. The collinear pump and signal waves were irradiated into a lithium niobate crystal. The pump and the signal waves were reflected at the crystal surface. Because the pump and the signal waves have a finite beam diameter, when the reflected signal wave and unreflected pump wave were irradiated at the correct angle, the noncollinear phase-matching condition was satisfied. By changing the incident angle to the crystal, broadband THz-wave generation with a range of over 0.2-7.2 THz was achieved.

High-resolution time-of-flight terahertz tomography using a femtosecond fiber laser.

High-resolution tomographic imaging is demonstrated using a reflection-type terahertz time-domain spectroscopy. To realize a practical system for general use, a robust all-fiber laser was used as the pump light source. Broadband terahertz waves were generated with the combination of optical pulses compressed to 17 fs using optical fibers and a DAST crystal. Using deconvolution signal processing, the wideband spectrum of the generated terahertz waves provided high-axial resolution leading to successful imaging of a multilayered structure containing a 2-microm-thin GaAs layer. To our knowledge, this is the first demonstration of terahertz tomographic imaging of such a thin layer.

Generation and detection of broadband coherent terahertz radiation using 17-fs ultrashort pulse fiber laser.

We describe the generation and detection of broadband terahertz radiation using an all-fiber laser. Optical pulses from a mode-locked fiber laser oscillator are compressed using nonlinear and dispersion effects induced in optical fibers, and 17-fs optical pulses with 170-kW peak power are generated at the wavelength region around 1.5 microm. By injecting these pulses into an organic crystal DAST (4-N, N-dimethylamino-4'-N"-methyl-stilbazolium tosylate), broadband terahertz field is radiated at 0.1-25 THz. The frequency region exceeding 20 THz is achieved with a fiber laser for the first time.

Flatly broadened, wideband and low noise supercontinuum generation in highly nonlinear hybrid fiber.

We present wideband of 1180-2100 nm, flatly broadened supercontinuum (SC) generation using highly nonlinear hybrid fibers and femtosecond fiber laser. Stable and smooth spectra without fine structure are demonstrated. The hybrid fibers are constructed by fusion splicing fibers with different properties. The SC spectra can be properly controlled by the optimal design of the hybrid fiber based on the numerical analysis. The generated SC pulse shows the low relative intensity noise (RIN).