Fumiyoshi Kuwashima

Highly sensitive electro-optic sampling of terahertz waves using field enhancement in a tapered waveguide structure

We report on increasing the sensitivity of electro-optic sampling detection of terahertz pulses by using the effect of terahertz field enhancement in a tapered metallic waveguide. A thin Si prism, placed in the narrower section of the waveguide, is used to synchronize the enhanced terahertz field with the probe laser pulse propagating in a LiNbO3 crystal attached to the obliquely cut end of the waveguide. A 40-µm-thick MgO-doped layer on the surface of LiNbO3 is used for guiding the probe pulse. The developed structure demonstrates a 20-fold higher detection sensitivity than a no-waveguide, Si-prism-coupled LiNbO3 crystal.

Chaotic Oscillation in a Single-Mode Class A He–Ne Laser (6328 Å) II

Chaotic oscillation in a single-mode He-Ne laser (6328 A), which belongs to a class A laser, is investigated through experiments I which were reported in our previous paper [F. Kuwashima, I. Kitazima and H. Iwasawa: Jpn. J. Appl. Phys. 37 (1998) L325.], and the improved experiments II. The results of experiments II, which were conducted using a neutral density (ND) filter instead of the tilted mirror used in experiments I, demonstrate the effect of the tilted mirror and a pumping parameter on the chaotic behavior of this laser. We also confirm and discuss the threshold value for chaos vs control parameters from the estimated Lyapunov exponents. The route to chaos in this laser is also discussed. Lastly, the laser equation of the class A laser with delayed feedback is introduced and numerical results which reveal chaotic oscillations are reported.

Theory of Chaotic Dynamics on Class A Laser with Optical Delayed Feedback

Chaotic dynamics of a single-mode class A laser is first investigated theoretically. We derived the equation of the electric field for the single-mode class A laser with optical feedback in which the multireflection effect was taken into account. The stationary solution and its stability were considered analytically, and the dynamics were studied by numerical analysis. The results of the simulations were given in time series, phase portraits, bifurcation diagrams, and the Lyapunov exponents, and the threshold value of feedback rate a for chaos, the route to chaos, and also the multireflective effects were discussed. These results agreed qualitatively with the experimental results as shown in our previous paper [Jpn. J. Appl. Phys. 38 (1999) 6321]. It is notable that the time behavior often presented phenomena similar to low-frequency fluctuation (LFF) as already observed in semiconductor lasers.

Chaotic Oscillations in Single-Mode Class A Laser with Long Optical Delayed Feedback

Chaotic oscillations of the single-mode He–Ne (6328 A) class A laser are investigated by optical delayed feedback using a long external optical resonator (3–10 m) instead of a short optical resonator (10–30 cm) used in our previous works. We discuss the round-trip frequency spectrum, low-frequency spectrum and effect of resonator length on chaos threshold.

The Chaotic Oscillation of the Single-Mode He-Ne(6328 A) Class A Laser

The confirmation of chaotic oscillation in a single-mode He-Ne(6328 A) standing-wave class A laser, by means of delayed feedback with an external mirror is reported. Varying the reflectivity of the external mirror, the time series of the laser output power are observed. By constructing the phase portraits and estimating Lyapunov exponents, the transition from a point attractor to a strange attractor is confirmed by the positive Lyapunov exponents in the chaotic region.

Sub-terahertz imaging using time-domain signals obtained with photoconductive spiral antennas

A sub-terahertz time-domain spectroscopic system with spiral antennas, which have high efficiency in the sub-THz region, was applied to imaging. In the time-domain signals, there were several peaks with various widths. Images were obtained by translating samples at the focus of the sub-THz beam with the time delay fixed at certain positions while monitoring the signal. At the edge of the metal patterns, the signal basically showed step-like variations, which were sometimes accompanied by overshoot structures (peaks and dips) depending on the time-delay position. The overshoot can be utilized to make the images sharper. The time-delay position can be adjusted to make the step height zero and produce images that contain only the overshoot, which can be transformed into images of the edge lines of the metal pattern. In addition, an image of the watermark on a 1000-yen bill was obtained. The image contrast caused by the small variation of the bill paper thickness was enhanced by locating the time delay at the position where the signal shows maximum sensitivity to the phase shift.

Superfocusing effect of V-groove metallic structure for terahertz wave

Focusing THz waves into a sub-wavelength area is a crucial issue for THz spectroscopy of minute samples and also for high-spatial resolution THz-imaging. Recently, super-focusing phenomena of THz waves, or more generally, those of electromagnetic waves are under intense investigation with theoretical and experimental studies using tapered metallic waveguiding structures. In this report we present our experimental study on a super-focusing effect of a V-groove metallic structure for THz pulsed waves with an opening angle about 10 degrees. Without coupling substrate lens, a 17% amplitude transmission of THz broadband waves (0∼2.5 THz) through a 2-μm bottom-width was observed without significant waveform distortion and phase delay.

Aperture transmission measurements for characterization of focusing of subterahertz radiation

Aperture-transmission measurements have been investigated as a way to characterize the focus of the subterahertz (sub-THz) beam in a time-domain spectroscopic system. Although measurement methods with a fixed time delay, like the knife-edge measurement, are often used to characterize the focusing of THz or sub-THz beams, such methods sometimes produce data that is difficult to interpret, for example, negative beam intensity. On the other hand, the data obtained by the aperture-transmission measurement is simple; the beam is considered tightly focused if the transmittance is high. However, unintentional effects occur when a metal aperture is used to characterize the focusing and a metal structure is used to assist the beam focusing, because parts of the metal structure are placed adjacent to the metal aperture. Here, we used an insulator aperture and found that it could be used for the characterization of focusing without the unintentional effects.

Identification of Plasmonic Modes in Parabolic Cylinder Geometry by Quasi-Separation of Variables

This paper describes the plasmonic modes in the parabolic cylinder geometry as a theoretical complement to the previous paper (J Phys A 42:185401) that considered the modes in the circular paraboloidal geometry. In order to identify the plasmonic modes in the parabolic cylinder geometry, analytic solutions for surface plasmon polaritons are examined by solving the wave equation for the magnetic field in parabolic cylindrical coordinates using quasi-separation of variables in combination with perturbation methods. The examination of the zeroth-order perturbation equations showed that solutions cannot exist for the parabolic metal wedge but can be obtained for the parabolic metal groove as standing wave solutions indicated by the even and odd symmetries.

Generation of Wide Range THz Waves near to 1 THz Using a Laser Chaos and a High Bias Voltage

Department of Electrical, Electronics and Computer Engineering, Fukui University of Technology, Fukui 910-8505, Japan Research Center for Development of Far-Infrared Region, University of Fukui, Fukui 910-8507, Japan 2 Faculty of Education and Regional Studies, University of Fukui, Fukui 910-8507, Japan Institute of Laser Engineering, Osaka University, Osaka 565-0871, Japan 4 Professor Emeritus, University of Fukui, Fukui 910-8507, Japan E-mail: kuwashima@fukui-ut.ac.jp (Received July 14, 2013)