Tahito Aida

Strain-driven self-positioning of micromachined structures

We introduce a method to make self-positioned micromachined structures by using the strain in a pair of lattice-mismatched epitaxial layers. This method allows the fabrication of simple and robust hinges for movable parts, and it can be applied to any pair of lattice-mismatched epitaxial layers, in semiconductors or metals. As an application example, a standing mirror was fabricated. A multilayer structure including an AlGaAs/GaAs dielectric mirror and an InGaAs strained layer was grown by molecular-beam epitaxy on a GaAs substrate. After releasing the multilayer structure from the substrate by selective etching, it moved to its final position powered by the strain release in the InGaAs layer.

High-frequency broad-band signal generation using a semiconductor laser with a chaotic optical injection

Chaotic signals with a flat power spectrum over 20 GHz have been generated using two commercially available semiconductor lasers coupled in a unidirectional master-slave scheme. The master laser has an external optical feedback that induces optical chaos in the laser output. A part of the chaotic light output from the master laser is injected into the slave laser. We experimentally demonstrated the generation of broad-band signals up to 22 GHz using lasers whose relaxation oscillation frequency in the free-running state is only around 6.4 GHz. We also show that the experimental results can be well reproduced by numerical simulations using two coupled rate equations. The numerical investigation shows that the high-frequency broad-band signal generation is owing to two key effects: high-frequency oscillations as a result of beating between the master and slave laser lights, and spectrum flattening due to the injection of the chaotic signal. The flatness, stability, and tunability of the power spectra demonstrated in our experiments suggests that the proposed system can be potentially useful for generation of high-frequency broad-band random signals.

Plasma mechanism of terahertz photomixing in high-electron mobility transistor under interband photoexcitation

We show that modulated near-infrared radiation can generate terahertz plasma oscillations in the channel of a high-electron mobility transistor. This effect is associated with a temporarily periodic injection of the electrons photoexcited by modulated near-infrared radiation into the transistor channel. The excitation of the plasma oscillations has the resonant character. It results in the pertinent excitation of the electric current in the external circuit that can be used for generation of terahertz electromagnetic radiation.

Experimental observation of complete chaos synchronization in semiconductor lasers

We experimentally demonstrate the complete synchronization of a semiconductor laser to the injection of a chaotic oscillating optical signal that is generated by a similar semiconductor laser with external optical feedback. The synchronization is characterized by sensitive dependencies on frequency detuning and injection strength and a time lag that varies reversely with the variation of the delay time in the external optical feedback of the master laser.

Axi-Vision Camera (real-time distance-mapping camera)

The camera described here makes color TV images that include information about the distance between the camera and the objects in the images. This range information is obtained from two images of the same scene taken under different illumination conditions. The camera does not require scanning, multiple camera units, or complicated computation. Range information for each pixel is acquired fast enough to keep up with the video rate of a TV camera. We describe various operational features and technical specifications such as ranging errors as well as the results of experimental investigations of the dependence on the color and reflectivity of the objects, of the sensitivity to interference from external light, and of the effects of the movement of the objects.

Injection locking and synchronization of periodic and chaotic signals in semiconductor lasers

We experimentally investigate the synchronous response of a semiconductor laser to the injection of a periodic or chaotic oscillating optical signal that is generated by a similar semiconductor laser with optical feedback. We show that there are two different types of synchronous response, appearing in separate regimes of laser frequency detuning and injection strength. They are distinguished by the time lag of the slave-laser response with respect to the injection signal from the output of the master laser. The experimental observations are well described by a numerical model consisting of a set of rate equations. It is revealed that the first type of synchronous response corresponds to the complete synchronization solution of the equations and the second type of response is the result of strong driving. The relevance of these two types of synchronous behavior to a number of recent experiments on chaos synchronization and their implications for data encoding/recovery using chaotic carriers are discussed.

High-definition real-time depth-mapping TV camera: HDTV Axi-Vision Camera.

We have developed a field-worthy, high-definition, real-time depth-mapping television camera called the HDTV Axi-Vision Camera. The camera can simultaneously capture both an ordinary HDTV color image and a depth image of objects on more than 1280 x 720 pixels at a frame rate of 29.97 Hz, or on 853 x 480 pixels at a frame rate of 59.94 Hz. The number of detectable pixels per unit time was increased by about 5 times that of the prototype camera by improving the sensitivity and resolution of the depthmapping camera. Short video clips demonstrate how depth information from the camera can be used to create a virtual image in actual television program production.

Optical actuation of micromirrors fabricated by the micro-origami technique

Micromirrors were fabricated by the micro-origami technique. This technique allows the fabrication of simple and robust hinges for movable parts, and it can be applied to any pair of lattice mismatched epitaxial layers, in semiconductors or metals. A multilayer structure, including AlGaAs/GaAs component layers and an InGaAs strained layer, was grown by molecular beam epitaxy on a GaAs substrate. After definition of the hinge and mirror’s shape by photolithography, the micromirrors were released from the substrate by selective etching. They moved to their final position powered by the strain release in the InGaAs layer. Optical actuation was achieved by irradiation with the 488 nm line of an argon laser, and the mirror’s position was measured by sensing the reflection of a He–Ne laser. Continuous wave irradiation with a power density of 450 mW/mm2 produced an angular deflection of the mirror of around 0.5°. The frequency response of the mirrors shows a resonance at 25 kHz.

Quantum-well microtube constructed from a freestanding thin quantum-well layer

We fabricated and experimentally investigated a nanostructure known as a quantum-well (QW) microtube, which is a fine tube with a micron- or nanometer-order diameter fabricated by rolling a semiconductor GaAs QW. Although the wall thickness is only 40 nm, the system retains the quantum properties of a QW, and photoluminescence from the QW subband can be clearly observed. Even though the QW width is sufficiently small to make the QW subband type-II band-aligned, a type-II to type-I transition caused by uniaxial strain in the microtube allows for optical emission.

Oscillation mode selection using bifurcation of chaotic mode transitions in a nonlinear ring resonator

This paper demonstrates the effectiveness of an adaptive parametric control method for searching and switching among a large number of multistable oscillation modes using chaotic mode transitions. The adaptive control is used to select nonlinear oscillation modes in an electro-optic ring resonator. In the adaptive control scheme, the result of a simple test of resonator output is fed back to a single parameter, pump laser power, governing bifurcation to and from chaos. The test is the presence or absence of a target code in the oscillation waveform of the resonator output. Chaotic mode transition phenomena, called chaotic itinerancy, are investigated in terms of code dynamics, and the results are used to determine the optimal parameters for the adaptive control. >