Propagation losses were measured for surface plasmon-polariton (SPP) modes at metal waveguides on semiconductor substrates. The waveguides are simple strips of Au or Al deposited on InP substrates or 300-nm-thick SiO2 film covering the InP substrates. We used a direct method that can clearly discriminate SPP modes in vidicon-camera images, thereby allowing quantitative measurements. The loss coefficients measured at a wavelength of 1.55 μm were, as predicted by theory, in the range of 8.5–17 dB/mm, which shows the waveguides are feasible for practical applications.
A supersmall optically switched laser (OSL) head is proposed. A laser diode attached to an air bearing slider forms a complex cavity together with a recording medium, and its light output is detected by a photodetector placed at the other end of the laser facet. Data signals and track error signals read from the sampled servo marks are successfully detected. The signal amplitude variation caused by the flying height change is much reduced, and the SNR is increased to 36 dB (40 kHz to 20 MHz for a phase change medium) by decreasing the reflectivity of the laser facet facing the medium to <5%.
A composite-cavity laser diode is used to monitor the reflectivity or the displacement of the external-cavity mirror for micromechanical photonics devices. Optical disk bits are read out in the near field from the difference in medium reflectivity with an antireflection-coated laser diode and a photodiode. Microbeam vibration is also detected in the near field from the phase difference with an uncoated laser diode and a photodiode. In both cases the carrier-to-noise ratio is very high (more than 45 dB) because of the lack of mode-hopping noise resulting from the extremely short (less than 3 µm) external-cavity length and strong light feedback. These composite-cavity laser diode microdevices are fabricated on a gallium arsenide substrate to eliminate the need for optical alignment.
Increased spatial frequency is experimentally observed in the interferential light-output undulation of coupled cavity lasers that use a Fabry-Perot laser diode. The frequencies correspond to undulation periods of λ/4, λ/6, λ/8, and λ/10, which are extremely short compared with the normal period of λ/2. This increase is explained theoretically with a multiple-mode model in which one of the longitudinal modes of a coupled cavity laser with the lowest lasing threshold is selected as a lasing mode. This theoretical explanation is confirmed through experiments with a distributed feedback laser that shows a strong single-mode oscillation and yields light-output undulations with a spatial period of λ/2.
A compact scanning-probe microscope (SPM) using an ultrasmall distortion sensor has been developed for versatile implementation. The sensor is based on a coupled-cavity laser (CCL) consisting of a Fabry-Perot laser diode monolithically integrated with a photodiode, an external mirror on a cantilever, and a small ball lens for laser-beam collimation. With the CCL stabilized by a simple mechanically controlled negative-feedback loop, the temporary distortion of the cantilever's sharp tip in response to the surface structure is measured with a vertical accuracy of 0.8 nm. The SPM was verified by imaging tracking grooves of an optical disk.
There is a growing interest in the diagnosis and treatment of patients with dementia and cognitive impairment at an early stage. Recent imaging studies have explored neural mechanisms underlying cognitive dysfunction based on brain network architecture and functioning. The dorsal anterior cingulate cortex (dACC) is thought to regulate large-scale intrinsic brain networks, and plays a primary role in cognitive processing with the anterior insular cortex (aIC), thus providing salience functions. Although neural mechanisms have been elucidated at the connectivity level by imaging studies, their understanding at the activity level still remains unclear because of limited time-based resolution of conventional imaging techniques. In this study, we investigated temporal activity of the dACC during word (verb) generation tasks based on our newly developed event-related deep brain activity (ER-DBA) method using occipital electroencephalogram (EEG) alpha-2 powers with a time resolution of a few hundred milliseconds. The dACC exhibited dip-like temporal waveforms indicating deactivation in an initial stage of each trial when appropriate verbs were successfully generated. By contrast, monotonous increase was observed for incorrect responses and a decrease was detected for no responses. The dip depth was correlated with the percentage of success. Additionally, the dip depth linearly increased with increasing slow component of the DBA index at rest across all subjects. These findings suggest that dACC deactivation is essential for cognitive processing, whereas its activation is required for goal-oriented behavioral outputs, such as cued speech. Such dACC functioning, represented by the dip depth, is supported by the activity of the upper brainstem region including monoaminergic neural systems.
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