This paper provides a review on the optimal design of photonic bandgap structures by inverse problem techniques. An overview of inverse problems techniques is given, with a special focus on topology design methods. A review of first applications of inverse problems techniques to photonic bandgap structures and waveguides is given, as well as some model problems, which provide a deeper insight into the structure of the optimal design problems.

We describe new architectures for micro-fabrication of large-scale PBG materials. A universal approach to embedding optical circuitry within a planar defect layer is illustrated for the square spiral and inverse opal PBG materials.

Colloidal crystallization is one of the most promising approaches to the fabrication of photonic crystals with periodicity at the submicron length scale. Several approaches have been explored to enhance the optical quality of these materials and, at the same time, to integrate these materials in substrates of interest in current technology. In this paper we review some of the most promising advances recently made in this direction, as well as some achievements towards the creation of new colloidal structures.

Autocloning is a method for fabricating multi-dimensional structures by stacking the corrugated films while keeping the shape. Its productivity, robustness against perturbation, and flexibility regarding materials and lattice types make autocloning suitable for mass production of photonic crystals. Therefore we aim to industrialize autocloned photonic crystals. Recently, we are starting to market polarization beam splitters for optical telecommunication by using 2D photonic crystals, and are developing some devices using the splitters, such as isolators or beam combiners. The applications of the splitters are also extending to multi-section type of devices and to visible range devices. Meanwhile, development of optical integrated circuits by utilizing autocloned photonic crystals is in progress. Low loss propagation and some functions have been demonstrated.

We have designed, fabricated and characterized efficient optical resonators and low-threshold lasers based on planar photonic crystal concept. Lasers with InGaAsP quantum well active material emitting at 1550 nm were optically pumped, and room temperature lasing was observed at threshold powers below 220 µW. Porous high quality factor cavity that we have developed confines light in the air region and therefore our lasers are ideally suited for investigation of interaction between light and matter on a nanoscale level. We have demonstrated the operation of photonic crystal lasers in different ambient organic solutions, and we have showed that planar photonic crystal lasers can be used to perform spectroscopic tests on femtoliter volumes of analyte.

We have demonstrated low-threshold two-dimensional photonic crystal lasers with self-assembled InAs/GaAs quantum dots. Coupled cavity designs of whispering gallery modes are defined in square lattice photonic crystal slabs. Our lasers showed a small 120 µW input pumping power threshold. Actual absorption power is evaluated to be less than 20 µW. Our lasers show high spontaneous emission coupling (β) factors0.1. The mode volumes are expected to be 0.7-1.2 times cubed wavelength by our modelling. Based on threshold analysis, 80 QDs are the effective number of QDs defined as the number of QDs needed to make PC cavities transparent if they are on maximum optical field points. Using the same analysis we found that single quantum dot lasing is likely to occur both by proper alignment of the single quantum dot relative to geometries of photonic crystals and by using sharp QD emission lines in high-Q localized modes.

Free-standing 2D slab photonic band-edge lasers based on square lattice and triangular lattice are realized by optical pumping at room-temperature. Both in-plane-emission and surface-emission photonic band-edge lasers are observed and compared. Analyses on optical loss mechanisms for finite-size photonic band-edge lasers are also discussed.

We have fabricated several two-dimensional photonic-crystal (2DPC) slab waveguides by using fine electron beam lithography and dry etching. The 2DPC waveguides include straight, bend, Y-branch, directional coupler, and coupled-cavity waveguides on the GaAs/AlGaAs substrate as an application to the ultra-small and ultra-fast all-optical switching device. Transmission spectra and near field patterns were characterized in a wide wavelength range from 850 to 1600 nm with the sample finished to the air-bridge type 2DPC slab. These waveguides appear to be suitable for achieving the waveguide platform in the symmetrical-Mach-Zehnder device.

Low-loss optical coupling structures between photonic crystal waveguides and channel waveguides were investigated. It was emphasized that impedance matching of guided modes of those waveguides, as well as field-profile matching, was essential to achieving the low-loss optical coupling. We developed an impedance matching theory for Bloch waves, and applied it to designing the low-loss optical coupling structures. It was demonstrated that the optical coupling loss between a photonic crystal waveguide and a Si-channel waveguide was reduced to as low as 0.7 dB by introducing an interface structure for impedance matching between the two waveguides.

Recent progress on photonic crystal fibers is reviewed aiming at their application to high performance networks. A photonic crystal fiber has an array of air holes surrounding the silica core region. Light is confined to the core by the refractive index difference between the core and the array of air holes. Photonic crystal fibers have special characteristics compared with conventional single mode fibers. One is that the dispersion characteristics can be designed. Another characteristic, that strong birefringence can be established by sizing and/or arranging the air holes, is expected to realize a polarization maintaining fiber with high birefringence of the order of 110^-3. This paper will describe the characteristics of dispersion controlled PCFs and polarization maintaining PCFs that include supercontinuum generation and absolute single polarization characteristics for various types of optical devices in high performance network systems.

We discuss photonic crystals (PhCs) with advanced micro/nano-structres which are semiconductor quantum dots (QDs) and micro electro-mechanical systems (MEMS) for the purpose of realizing novel classes of PhC devices in future photonic network system. After brief introduction on advantages to implement QDs and MEMS with PhCs, we discuss optical characterization of PhC microcavity containing self-assembled InAs QDs. Modification of emission spectrum of a QD ensemble due to the resonant cavity modes is demonstrated. We also point out the feasibility of low-threshold PhC lasers with QD active media in numerical analysis. A very low threshold current of 10 µA is numerically obtained for lasing action in the multi dimensional distributed feedback mode by using realistic material parameters. Then, the basic concept for MEMS-controlled PhC slab devices is described. We show numerical results that demonstrate some of interesting functions such as the intensity modulation and the tuning of resonant frequency of cavity mode. Finally, a preliminary experiment of MEMS-based switching operation in a PhC line-defect waveguide is demonstrated.

A silicon (Si) wire waveguiding system fabricated on silicon-on-insulator (SOI) substrates is one of the most promising platforms for highly-integrated, ultra-small optical circuits, or microphotonics devices. The cross-section of the waveguide's core is about 300-nm-square, and the minimum bending radius are a few micrometers. Recently, crucial problems involving propagation losses and in coupling with external circuits have been resolved. Functional devices using silicon wire waveguides are now being tested. In this paper, we describe our recent progress and future prospects on the microphotonics devices based on the silicon-wire waveguiding system.

Artificial electromagnetic structures have significantly broadened the range of wave propagation phenomena available. In particular, it has been shown that metamaterials can be constructed for which the index-of-refraction is negative over a finite band of frequencies. In this paper, we present the design, fabrication and characterization of a metamaterial that exhibits negative refraction. The metamaterial design we explore is anisotropic in the plane of propagation. Based on our analysis and supporting simulations and measurements, we demonstrate that for the geometry considered, the anisotropic metamaterial has the identical negative refraction properties as would an isotropic negative index metamaterial.

We present an overview of our work on the application of scanning near-field optical microscopy (SNOM) to photonic crystal structures. Our results show that SNOM can be used to map the subwavelength confinement of light to a point-defect in a 2D photonic crystal microresonator. Comparison with numerical modelling shows that SNOM is able to resolve patterns in the intensity distribution that are due to the slight non-uniformity in the crystal structure. We also discuss the future possibilities for applications of different modes of SNOM to photonic crystal devices.

Lasers have been established as a unique nanoprocessing tool due to its intrinsic three-dimensional (3D) fabrication capability and the excellent compatibility to various functional materials. Here we report two methods that have been proved particularly promising for tailoring 3D photonic crystals (PhCs): pinpoint writing via two-photon photopolymerization and multibeam interferential patterning. In the two-photon fabrication, a finely quantified pixel writing scheme and a method of pre-compensation to the shrinkage induced by polymerization enable high-reproducibility and high-fidelity prototyping; well-defined diamond-lattice PhCs prove the arbitrary 3D processing capability of the two-photon technology. In the interference patterning method, we proposed and utilized a two-step exposure approach, which not only increases the number of achievable lattice types, but also expands the freedom in tuning lattice constant.

By means of the three-dimensional (3D) finite-difference time domain (FDTD) method, we have investigated in detail the optical properties of a two-dimensional photonic crystal (PC) surface-emitting laser having a square-lattice structure. The 3D-FDTD calculation is carried out for the finite size PC slab structure. The device is based on band-edge resonance, and plural band edges are present at the corresponding band edge point. For these band edges, we calculate the mode profile in the PC slab, far field pattern (FFP) and polarization mode of the surface-emitted component, and photon lifetime. FFPs are shown to be influenced by the finiteness of the structure. Quality (Q) factor, which is a dimensionless quantity representing photon lifetime, is introduced. The out-plane radiation loss in the direction normal to the PC plane greatly influences the total Q factor of resonant mode and is closely related with the band structure. As a result, Q factors clearly differ among these band edges. These results suggest that these band edges include resonant modes that are easy to lase and resonant modes that are difficult to lase.

We theoretically investigated the resolution of the photonic crystal (PC) K-vector superprism, which utilized the wavelength-dependent refraction of light at an angled output end as a narrow band filter at 1.55 µm wavelength range. Similarly to the case of the conventional S-vector prism, we defined the equi-incident-angle curve against the dispersion surface, and calculated the beam collimation, wavelength sensitivity and resolution parameters for light propagation in the PC. We estimated that the resolution of the K-vector prism is the same as or higher than that of the S-vector prism and the PC can be significantly miniaturized. In addition, we clarified the relation of the S-vector prism phenomenon and the position of the output end in the K-vector prism, and different results for the reduced and repeated zone schemes, which are important for the detailed design. We also confirmed that the light propagation simulated by the FDTD method well agreed with the results of the dispersion surface analysis.

The design, fabrication, and measurement of photonic-band-gap (PBG) waveguides and resonators in two-dimensional photonic crystal slabs have been investigated. Although photonic crystal slabs have only partial gaps, efficient waveguides and resonators can be realized by appropriate design. As regards PBG waveguides, we show various designs for efficient single-mode waveguides in PhC slabs with SiO₂ cladding, we report group dispersion measurements of PBG waveguides in PhC slabs, and describe the successful fabrication of PBG waveguides with adiabatic connectors that enable us to couple the light from single-mode fibers efficiently to PBG waveguides. As regards PBG resonators, we show how to realize very high-Q and small volume resonators in hexagonal PhC slabs, and report the fabrication of resonant tunneling filters that consist of PBG resonators coupled with PBG waveguides. We also describe the successful fabrication of resonant tunneling mode-gap filters with adiabatic mode connectors.

We propose a novel optical signal processing using an optically pumped vertical-cavity surface-emitting laser (VCSEL) with an external light input. The mode transition between a fundamental and a 1st-high-order transverse mode is induced by an external light injection. Since a single mode fiber (SMF) spatially selects a fundamental transverse mode as an output signal, we are able to realize a nonlinear transfer function, which will be useful in future photonic networks. The mode transition characteristic of a 1.55 µm optically pumped two-mode VCSEL has been simulated by using a two-mode rate equation, which includes the effects of spatial hole burning and spectral hole burning as gain saturation coefficients. We focus on the detuning effect in the injection locking. When the wavelength of an input light with a fundamental mode is slightly longer than that of a VCSEL operating in a 1st-high-order transverse mode, the transverse mode of the VCSEL is switched to a fundamental mode at a critical input power level. This gives us an ideal transfer function for 2R (reamplification and reshaping) regeneration. Also, the proposed scheme may enable polarization insensitive signal processing, which is a unique feature in surface emitting lasers.

This paper introduces a power-efficient on-chip DC-DC converter, which produces a 1.0 V output by being stepped-down from a 3.6 V input, utilizes a 10 µH external inductor, and realizes more than 80% power-efficiency. In order to realize a 1.0 V output without decreasing power-efficiency, a synchronous-type rectifier scheme with a reverse current protection circuit is adopted and a reference voltage of less than 1.0 V is developed. The external inductor value is reduced by applying the PWM control scheme and a new low-power 1 MHz triangular waveform oscillator. High-value resistors are used in analog circuits including a voltage reference, a triangular waveform oscillator, an error amplifier, and a comparator to have the ultra-low power characteristics. A chip is actually designed and fabricated by using the 2 µm CMOS process. As a result, a 1 MHz, synchronous, step-down from 3.6 V to 1 V, PWM DC-DC converter has been realized with a power efficiency of more than 80% in the output current range from 40 to 70 mA.

In this letter, we present the new type parallel-coupled band-pass filter (BPF) which uses the dielectric guide in coupled sections with finite metallization thickness. A mode-matching method has been used to analyze this new structure and the simulation results are shown and validated through comparison with other available data. The results in this letter show that the dielectric guide of coupled lines with finite metal strips can be newly added to the design parameters of the parallel-coupled BPF structure and other microwave applications.