A compact silicon photonic crystal waveguide (PCW) slow-light modulator is presented. The proposed modulator is capable of achieving a 64 Gbps bit-rate in a wide operating spectrum. The slow-light enhances the modulation efficiency in proportion to its group index ng. Two types of 200-µm-long PCW modulators are presented. These are low- and high-dispersion devices, which are implemented using a complementary metal-oxide-insulator process. The lattice-shifted PCW achieved low-dispersion slow-light and exhibited ng ≈ 20 with an operating spectrum Δλ ≈ 20 nm, in which the fluctuation of the extinction ratio is ±0.5 dB. The PCW device without the lattice shift exhibited high-dispersion, for which a large or small value of ng can be set on demand by changing the wavelength. It was found that for a large ng, the frequency response was degraded due to the electro-optic phase mismatch between the RF signals and slow-light even for such small-size modulators. Meander-line electrodes, which bypass and delay the RF signals to compensate for the phase mismatch, are proposed. A high cutoff frequency of 55 GHz was theoretically predicted, whereas the experimentally measured value was 38 GHz. A high-quality open eye pattern for a drive voltage of 1 V at 32 Gbps was observed. The clear eye pattern was maintained for 50-64 Gbps, although the drive voltage increased to 3.5-5.3 V. A preliminary operation of a 2-bits pulse amplitude modulation up to 100 Gbps was also attempted.

We investigate the potential of photonic crystal fiber (PCF) to realize high quality and high-power transmission. We utilize the PCF with a quasi-uniform air-hole structure, and numerically clarify that the quasi-uniform PCF can realize the effective area (Aeff) of about 500µm2 with bending loss comparable with that of a conventional single-mode fiber for telecom use by considering the quasi single-mode transmission. We then apply the quasi-uniform PCF to kW-class high-power beam delivery for the single-mode laser processing. The cross-sectional design of the PCF with the high-power delivery potential of more than 300kW·m is numerically and experimentally revealed. A 10kW single-mode beam at 1070nm is successfully delivered over a 30m-long optical fiber cable containing a fabricated PCF with single-mode class beam quality of M2 =1.7 for the first time.

A single-polarization single-mode (SPSM) photonic crystal fiber (PCF) based on double-hole unit core is proposed in this paper for application to cross-talk free polarization splitter (PS). Birefringence of the PCF is obtained by adopting double-hole unit cells into the core to destroy its symmetry. With an appropriate cladding hole size, single x- or y-polarized PCF can be achieved by arranging the double-hole unit in the core along the x- or y-axis, respectively. Moreover, our proposed SPSM PCF has the potential to be applied to consist a cross-talk free PS. The simulation result by employing a vectorial finite element beam propagation method (FE-BPM) demonstrates that an arbitrary polarized incident light can be completely separated into two orthogonal single-polarized components through the PS. The structural tolerance and wavelength dependence of the PS have also been discussed in detail.

Development of narrowband thermal emitters whose emission wavelengths are dynamically tunable is highly desired for various applications including the sensing of gases and chemical compounds. In this paper, we review our recent demonstration of wavelength-switchable mid-infrared thermal emitters based on multiple quantum wells (MQWs) and photonic crystals (PCs). Through the control of absorptivity by using intersubband transitions in MQWs and optical resonances in PC slabs, we demonstrate novel control of thermal emission, including realization of high-Q (Q>100) thermal emission, dynamic control of thermal emission (∼MHz), and electrical wavelength switching of thermal emission from a single device.

We discuss our recent progress in photonic crystal nanocavity quantum dot lasers. We show how enhanced light matter interactions in the nanocavity lead to diverse and fascinating lasing phenomena that are in general inaccessible by conventional bulky semiconductor lasers. First, we demonstrate thresholdless lasing, in which any clear kink in the output laser curve does not appear. This is a result of near unity coupling of spontaneous emission into the lasing cavity mode, enabled by the strong Purcell effect supported in the nanocavity. Then, we discuss self-frequency conversion nanolasers, in which both near infrared lasing oscillation and nonlinear optical frequency conversion to visible light are simultaneously supported in the individual nanocavity. Owing to the tight optical confinement both in time and space, a high normalized conversion efficiency over a few hundred %/W is demonstrated. We also show that the intracavity nonlinear frequency conversion can be utilized to measure the statistics of the intracavity photons. These novel phenomena will be useful for developing various nano-optoelectronic devices with advanced functionalities.

An ultra-compact InGaAs photodetector (PD) is demonstrated based on a photonic crystal (PhC) waveguide to meet the demand for a photoreceiver for future dense photonic integration. Although the PhC-PD has a length of only 1.7µm and a capacitance of less than 1fF, a high responsivity of 1A/W was observed both theoretically and experimentally. This low capacitance PD allows us to expect a resistor-loaded receiver to be realized that requires no electrical amplifiers. We fabricated a resistor-loaded PhC-PD for light-to-voltage conversion, and demonstrated a kV/W efficiency with a GHz bandwidth without using amplifiers. This will lead to a photoreceiver with an ultralow energy consumption of less than 1fJ/bit, which is a step along the road to achieving a dense photonic network and processor on a chip.

We present the first demonstration of an electro-optic modulator based on a photolithographically fabricated photonic crystal (PhC) nanocavity with a p-i-n junction with SiO2 cladding. We show that the device exhibits an ultrahigh quality factor (Q∼105) and allow us to demonstrate electro-optic modulation through the integrated p-i-n diode structure. We demonstrate an electro-optic modulation based on the carrier injection.

For the future medical diagnostics, high-sensitive, rapid, and cost effective biosensors to detect the biomarkers have been desired. In this study, the polymer-based two-dimensional photonic crystal (2D-PC) was fabricated using nanoimprint lithography (NIL) for biosensing application. In addition, for biosensing application, label-free detection of fibrinogen which is a biomarker to diagnose the chronic obstructive pulmonary disease (COPD) could be achieved using antigen-antibody reaction high-sensitively (detection limit: pg/ml order) and rapidly. Using this polymer-based 2D-PC, optical biosensor can be developed cost effectively. Furthermore, by using polymer as a base material for fabrication of 2D-PC, label-free detection of antigen-antibody reaction can be performed in visible region.

We propose a novel single polarization photonic band gap fiber (SP-PBGF) with an anisotropic air hole lattice in the core. An SP-PBGF with an elliptical air hole lattice in the core recently proposed can easily realize SP guidance utilizing the large difference of cutoff frequency for the x- and y-polarized modes. In this paper, in order to achieve SP guidance based on the same principle of this PBGF, we utilize an anisotropic lattice of circular air holes instead of elliptical air holes to ease the fabrication difficulty. After investigating the influence of the structural parameters on SP guidance, it is numerically demonstrated that the designed SP-PBGF has 381 nm SP operating band.

An efficient sharply bent waveguide with a microcavity constructed by an air-bridge type two-dimensional photonic crystal slab is analyzed. The method of solution is the three-dimensional finite difference time domain (FD-TD) method. The bent waveguide has a microcavity structure that connects to an input and an output waveguide ports. The radius and position of air-holes surrounding the microcavity are modified to adjust the resonant frequency to the single-mode regime of the waveguides. It is confirmed that input optical power is transmitted efficiently to the output waveguide due to resonant tunneling caused by the microcavity.

In this paper, the function expansion based topology optimization is employed to the automatic optimization of the waveguide dispersion property, and the optimum design of low-dispersion slow-light photonic crystal waveguides is demonstrated. In order to realize low-dispersion and large group index, an objective function to be optimized is expressed by the weighted sum of the objective functions for the desired group index and the low-dispersion property, and weighting coefficients are updated through the optimization process.

An efficient 12 optical power splitter constructed by a two-dimensional photonic crystal has been analyzed using the finite difference time domain (FD-TD) method. The power splitter has a microcavity which is coupled to an input and two output waveguides. We have confirmed that all optical power is transmitted into output waveguides due to resonant tunneling caused by the microcavity.

Polarization-maintaining photonic crystal fiber couplers (PM-PCFCs) were fabricated using a CO2 laser irradiation technique. We could control the states of air holes in the tapered region of couplers by adjusting the laser power density in the fusion and the elongation processes. It was demonstrated that the air hole remaining PM-PCFC exhibited polarization-splitting characteristics and that the air hole collapsed PM-PCFC had polarization insensitive coupling characteristics.

Efficient silicon-based light sources are expected to be key devices for applications such as optical interconnection. Huge number of researches has been conducted for realizing silicon-based light sources. Most of them utilized silicon-related materials such as silicon nanostructures or germanium, not crystalline silicon, which has been considered as a poor light emitter because of its indirect electronic bandgap. Light emission properties of materials can be tailored not only by modifying the material properties directly, but also by controlling the electromagnetic environment surrounding the material. Photonic nanostructures are a powerful tool for creating the engineered environment. In this paper, we briefly review the mechanisms for improving the light emission properties of materials by photonic nanostructures and present our recent experimental results showing the enhancement of light emission from silicon by introducing photonic crystal structures.

We propose a built-in planar lens for coupling light to a waveguide on a 2-D photonic crystal (PhC) membrane. A 2-D PhC waveguide with the built-in lens has been fabricated with AlGaAs. Improvement in coupling performance is discussed in comparison to waveguides with straight ends as cleaved.

High-performance and low-cost sensors are critical devices for high-throughput analyses of bio-samples in medical diagnoses and life sciences. In this paper, we demonstrate photonic crystal nanolaser sensor, which detects the adsorption of biomolecules from the lasing wavelength shift. It is a promising device, which balances a high sensitivity, high resolution, small size, easy integration, simple setup and low cost. In particular with a nanoslot structure, it achieves a super-sensitivity in protein sensing whose detection limit is three orders of magnitude lower than that of standard surface-plasmon-resonance sensors. Our investigations indicate that the nanoslot acts as a protein condenser powered by the optical gradient force, which arises from the strong localization of laser mode in the nanoslot.

This paper reviews our recent activities on nanophotonics based on a photonic crystal (PC)/quantum dot (QD)-combined structure for an all-optical device and a metal/semiconductor composite structure using surface plasmon (SP) and negative refractive index material (NIM). The former structure contributes to an ultrafast signal processing component by virtue of new PC design and QD selective-area-growth technologies, while the latter provides a new RGB color filter with a high precision and optical beam-steering device with a wide steering angle.

This paper presents a formulation of two-dimensional photonic crystal waveguide devices formed by circular cylinders. The device structures are considered as cascade connections of straight waveguides. Decomposing the structure into layers of the cylinder arrays, the input/output properties of the devices are obtained using an analysis method of multilayer structure. We introduce periodic boundary conditions in the direction perpendicular to the wave propagation, and the Floquet-modes of each layer are calculated by the Fourier series expansion method with the help of the recursive transition-matrix algorithm. Then, the input/output properties of the devices are obtained by recursive calculation of scattering matrix with each layer. The presented formulation is validated by numerical experiments by comparing with the previous works.

The Fourier series expansion method is a useful tool to approach the problems of discontinuities in optical waveguides, and it can apply to analyze the Floquet-modes of photonic crystal waveguides. However, it has known that the Floquet-mode calculation with large truncation order is limited because of the roundoff errors. This paper proposes a novel formulation of the Floquet-modes propagating in two-dimensional photonic crystal waveguides formed by circular cylinders. We introduce a periodic boundary condition as same with the conventional method, and the fields are expressed in the Fourier series expansions. The present formulation also introduces the cylindrical-wave expansions and uses the recursive transition-matrix algorithm, which is used to analyze the scattering from cylinder array. This makes us possible to obtain very high accuracy without the use of large truncation order for Fourier series expansion. The presented formulation is validated by numerical experiments.

We investigate numerically the applicability of photonic crystal fiber (PCF) with a uniform air hole structure as a wide-band transmission medium. We show that accumulated dispersion over the PCF can be reduced effectively by optimizing the index profile of dispersion compensating fiber (DCF). We also show that a bandwidth of more than 300 nm will be available for 40 Gbit/s NRZ transmission by using the PCF as a transmission medium instead of conventional 1.3 µm zero-dispersion single-mode fiber (SMF).