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We present the results of a theoretic study of the electromagnetic field imbalance in surface plasmon polaritons (SPPs), which reveal that the magnetic field components induced by the electric fields normal and parallel to a metal surface cancel each other in SPPs, resulting in an imbalance. A group velocity analysis shows that this imbalance contributes significantly to the slow propagation of SPPs. We also analyze the enhanced spontaneous emission and nonlinearity in plasmonic cavities, and the results indicate that the electromagnetic field imbalance must be considered to correctly estimate the interaction strength.

We proposed and demonstrated a wavelength-division multiplexing (WDM) optical beam-steering device consisting of a thermally controlled doubly periodic Si two-dimensional bulk photonic crystal waveguide and coupled microring multiplexers. Beam forming and steering while maintaining a sharp profile is much easier in this device than with optical phased arrays which need the fine phase control. By dividing the range of beam-steering angles into different wavelength channels, it is possible to cover a wide range of angles, even when each angle is small. In this study, we fabricated a device with four wavelength channels, each of which showed beam steering of 4°-5° as a result of heating, resulting in a total of 16°. Two-dimensional steering is also achieved by loading a collimator lens and selecting one waveguide from those arrayed. We evaluated 112 resolution points with four wavelengths and 448 points in total by switching four waveguides. If this WDM concept is introduced into light detection and ranging and the number of wavelengths is increased, it will be possible to increase the sensing throughput, which is usually constrained by the round-trip time of light, by simultaneous parallel operation.

We demonstrate a Si photonic crystal waveguide Mach-Zehnder modulator that incorporates meander-line electrodes to compensate for the phase mismatch between slow light and RF signals. We first employed commonized ground electrodes in the modulator to suppress undesired fluctuations in the electro-optic (EO) response due to coupled slot-line modes of RF signals. Then, we theoretically and experimentally investigated the effect of the phase mismatch on the EO response. We confirmed that meander-line electrodes improve the EO response, particularly in the absence of internal reflection of the RF signals. The cut-off frequency of this device can reach 27 GHz, which allows high-speed modulation up to 50 Gbps.

We report an on-chip autocorrelator comprising a Si rib waveguide and two-photon-absorption photodiodes in which two pulses simultaneously propagate in opposite directions, and their correlation is acquired. The rib waveguide's broadband guiding mode and moderately low group index allow the device to operate over a wide wavelength range with high resolution. We confirm that the waveguide has a transmission band of at least 1300-1630 nm and observed correlation waveforms for pulses of the order of several hundred femtoseconds to several picoseconds, with a range of at least the entire C band region.

The slow-light effect in silicon lattice-shifted photonic crystal waveguide (LSPCW) Mach–Zehnder modulators allows compact phase shifters, while limiting the working spectrum Δλ. We optimized the structural parameters of the LSPCW and extended Δλ to cover the full C-band in exchange for moderately decreasing the group index ng. We obtained Δλ=42 nm with ng=8−9 in a fabricated device and observed 25 Gbps eye opening in the 200 μm modulator in the full C-band.

In general, biochemical sensors based on photonic cavities are used to detect changes in the refractive index of the environment. In this study, however, a GaInAsP semiconductor photonic-crystal nanolaser sensor that we recently developed was found to detect not only the environmental refractive index but also the surface charge. In contrast to the pH sensitivity we reported previously, this is an ultra-sensitive detection mechanism capable of identifying proteins and deoxyribonucleic acids (DNA) at a femtomolar-order or lower concentrations. When the device is exposed to plasma or DNA solutions, the laser wavelength simultaneously changes with the zeta potential and the flat-band potential of the semiconductor surface. This indicates that the charged functional groups on the surface, which are formed by these treatments, modify the Schottky barrier near the semiconductor surface, trap the excited carriers in the barrier, and change the refractive index of the semiconductor via the carrier effects. These findings also suggest that some other photonic sensors may also exhibit similar electrochemical and optoelectronic effects.

We propose an ultra-compact solid-state autocorrelator fabricated using Si photonics technology that obtains correlation by detecting the overlap of two slow light pulses counter-propagating in a photonic crystal waveguide integrated with a two-photon absorption photodiode array. As the device does not require a mechanical delay scanner, it can be integrated onto a chip and operated without alignment. Using the correlator, we successfully acquired the autocorrelation of picosecond pulses. The nonlinear enhancement of slow light improves sensitivity, resulting in an evaluated detection limit in terms of the product of the peak and average power on the order of 10−7 W2, which is equal to or even better than that of commercial scanning autocorrelators and 107-fold better than that of conventional single-shot autocorrelators.

We have developed compact Si Mach-Zehnder modulators that are assisted by wideband low-dispersion slow light in lattice-shifted photonic crystal waveguides. We have also developed Si triangular-shaped coupled-microring multiplexers that allow a box-like spectrum, a wide free spectral range, and an efficient thermal tuning. In this study, we integrated three sets of these devices in a small footprint of 2.0 × 0.7 mm(2) and achieved their 25 Gbps/ch operation as a wavelength division multiplexing transmitter. Moreover, we demonstrated hitless wavelength tuning using thermo-optic switches loaded in the bus waveguide.

We proposed and demonstrated a label-free imaging method for living cells using a GaInAsP H0-type photonic crystal nanolaser array. We integrated 441 nanolasers in an arrayed configuration and achieved photopumped lasing with a 100% yield. Then, we attached HeLa cells on it, measured the wavelengths of all nanolasers and used them as pixel information. We acquired cell images, which partially corresponds to optical micrographs and probably reflects the attachment condition of the cells. We improved the refractive index resolution from ~10(-2) to 2 × 10(-3) by incorporating a nanoslot into each nanolaser and compensating the nonuniformity of each index sensitivity.

Biosensors selectively detecting a very small amount of biomarker protein in human blood are desired for early and reliable diagnoses of severe diseases. This paper reports the detection of protein (streptavidin: SA) in ultra-low concentration, with an ultra-high selectivity against contaminants, using photonic crystal nanolasers. For biotin-modified nanolasers in pure water with SA, an extremely-low detection limit of 16 zM is evaluated. Even in a mixture with 1 μM bovine serum albumin as the contaminant, 100 zM SA is detected, meaning a selectivity of 10(13). These are remarkable capabilities that are promising for practical biosensing in the medical applications mentioned above.

We demonstrate the first sub-100 μm silicon Mach-Zehnder modulators (MZMs) that operate at >10 Gb/s, by exploiting low-dispersion slow-light in lattice-shifted photonic crystal waveguides (LSPCWs). We use two LSPCW-MZM structures, one with LSPCWs in both arms of the MZM, and the other with an LSPCW in only one of the arms. Using the first structure we demonstrate 10 Gb/s operation with a operating bandwidth of 12.5 nm, in a device with a phase-shifter length of only 50 μm. Using the second structure, owing to a larger group index as well as lower spectral noise, we demonstrate 40 Gb/s operation with a phase-shifter length of only 90 μm, which is more than an order-of-magnitude shorter than most 40 Gb/s MZMs.

Photonic crystal slab line defect waveguides with slightly small innermost holes are theoretically expected to show light transmission with low-group-velocity and low-dispersion (LVLD) characteristics owing to a linear and almost flat photonic band. In this study, the LVLD characteristics of such waveguides were experimentally confirmed by using modulation phase shift measurement and transmission of ultrashort optical pulses. These results will be applicable to buffering and nonlinearity enhancement of optical signals.

The dispersion characteristics and radiation loss characteristics of ARROW (antiresonant reflecting optical waveguide), a novel single-mode slab waveguide, are analyzed in detail. The mechanism by which the waveguide produces a large amount of light radiation from the waveguide core to the semiconductor substrate is explained and some approximate formulas of propagation characteristics are obtained, which are useful for the design of ARROW. Two novel integrated structures of ARROW and photodetector are proposed to realize high coupling efficiency from waveguide to photodetector, small device size, and easy fabrication process. In these structures, an antireflecting (AR) layer is introduced to greatly enhance coupling efficiency. Their performance is discussed compared with the conventional waveguide structure. It is shown theoretically and experimentally that a high coupling efficiency (>90%) is achievable with short detector length in the ARROW structures.