Masahiro Imada

Trapping and emission of photons by a single defect in a photonic bandgap structure

By introducing artificial defects and/or light-emitters into photonic bandgap structures, it should be possible to manipulate photons. For example, it has been predicted that strong localization (or trapping) of photons should occur in structures with single defects, and that the propagation of photons should be controllable using arrays of defects. But there has been little experimental progress in this regard, with the exception of a laser based on a single-defect photonic crystal. Here we demonstrate photon trapping by a single defect that has been created artificially inside a two-dimensional photonic bandgap structure. Photons propagating through a linear waveguide are trapped by the defect, which then emits them to free space. We envisage that this phenomenon may be used in ultra-small optical devices whose function is to selectively drop (or add) photons with various energies from (or to) optical communication traffic. More generally, our work should facilitate the development of all-optical circuits incorporating photonic bandgap waveguides and resonators.

Coherent two-dimensional lasing action in surface-emitting laser with triangular-lattice photonic crystal structure

Lasing action of a surface-emitting laser with a two-dimensional photonic crystal structure is investigated. The photonic crystal has a triangular-lattice structure composed of InP and air holes, which is integrated with an InGaAsP/InP multiple-quantum-well active layer by a wafer fusion technique. Uniform two-dimensional lasing oscillation based on the coupling of light propagating in six equivalent Γ−X directions is successfully observed, where the wavelength of the active layer is designed to match the folded (second-order) Γ point of the Γ−X direction. The very narrow divergence angle of far field pattern and/or the lasing spectrum, which is considered to reflect the two-dimensional stop band, also indicate that the lasing oscillation occurs coherently.

Room temperature continuous wave operation of a surface-emitting two-dimensional photonic crystal diode laser

We achieved room temperature continuous wave operation of a surface-emitting two-dimensional photonic crystal diode laser by current injection. This is the first time ever that room temperature continuous wave operation of a photonic crystal diode laser has been realized. This laser features single mode oscillation over a large area, which is impossible for conventional lasers. In this work, we optimized the epitaxial layer composition for better carrier confinement and clarified the relationship between the diameter of the air holes in the photonic crystal and the threshold current of the laser in order to estimate the optimized threshold current.

Surface-emitting channel drop filters using single defects in two-dimensional photonic crystal slabs

We report a theoretical analysis of trapping and emission of photons by a single defect in a two-dimensional photonic crystal slab. We show that the tuning of emission wavelength is possible by selecting appropriate geometry of structure while keeping the emission efficiency maximal. The results suggest the possibility of applying it to ultra-small channel add/drop device in wavelength division multiplexed optical communications.

Direct creation of three-dimensional photonic crystals by a top-down approach

Three-dimensional (3D) photonic crystals can block photons in any direction and are expected to make possible their ultimate control. However, creating 3D crystals without any unintentional defects over large areas at optical wavelengths has been challenging. For example, opal-based crystals inevitably contain unintentional defects, it is difficult to increase the sizes of micro-manipulated crystals over approximately 6 microm and producing stacked 3D crystals with thin 2D layers requires complicated and time-consuming processes. So far, these difficulties have hindered 3D photonic-crystal research. Here, we demonstrate a novel top-down approach to creating 3D crystals that overcomes these difficulties and significantly simplifies the process. We have developed a double-angled deep-etching method, which enables the direct creation of 3D woodpile crystals in single-crystalline silicon. A strong photonic bandgap effect with >20 dB attenuation in all directions has been achieved. Furthermore, bonding a light emitter onto or between 3D crystals created in this way has been shown to enhance or suppress spontaneous emission.

Alignment and stacking of semiconductor photonic bandgaps by wafer-fusion

Semiconductor-based three-dimensional (3D) photonic crystals are developed by utilizing a wafer fusion and alignment technique. A clear and considerable bandgap effect is successfully demonstrated in infrared to near-infrared wavelengths. It is pointed out that the introduction of arbitrary defect states and/or efficient light emitters can be possible in this method. For the example of the introduction of a light-emitting element, a surface-emitting laser with a two-dimensional (2D) photonic crystal structure is fabricated, and very unique lasing characteristics are demonstrated. These results encourage us very much for the development of various quantum optical devices and circuits including not only passive, but also active devices.

Channel drop filter using a single defect in a 2-D photonic crystal slab waveguide

This paper describes a theoretical and experimental analysis of the channel drop filter using a single defect formed near the two-dimensional (2-D) photonic crystal slab waveguide. First, we calculate the transmission spectrum of a 2-D photonic crystal waveguide and show that high transmittance for a wide wavelength range (/spl sim/60 nm) is obtained in the 1.55-/spl mu/m region. We also show that a defect state having a wavelength within the high transmission wavelength range can be formed in the photonic bandgap by introducing a single defect of appropriate radius. Next, we fabricate several devices and show that the emission wavelength from each defect can be tuned by changing the defect radius. The measured tuning characteristics coincide well with the calculated results. From the near-field pattern of the device, we estimate the emission efficiency of the present device at almost a few tens percent. We clarify the structural condition in order to obtain the maximum output efficiency and show that tuning of emission wavelength while maintaining high output efficiency is possible by selecting appropriate defect radius and position. Based on these results, we propose an ultrasmall channel drop filter for a wavelength-division-multiplex optical communication system.

Semiconductor three-dimensional and two-dimensional photonic crystals and devices

Semiconductor three-dimensional (3-D) and two-dimensional (2-D) photonic crystals and their effects on the control of photons are investigated for possible applications to optical chip and functional devices. First we review our approaches creating full 3-D photonic bandgap crystals at near-infrared wavelengths, and also functional devices based on 2-D photonic crystals where the focus is on surface-emitting-type channel-drop filtering devices utilizing single defects in 2-D photonic crystal slabs. Then, we describe the recent progress on 3- and 2-D crystals. On 3-D crystals, the effect of the introduction of a light emitter into the 3-D photonic crystal is investigated, and the design of a single defect cavity is performed. On the 2-D photonic crystals, the photonic states are investigated from the perspective of their polarization properties.

Development of three-dimensional photonic-crystal waveguides at optical-communication wavelengths

Photonic crystals have a photonic band gap (PBG) in which light propagation and emission is prohibited. In particular, three-dimensional (3D) photonic crystals have a complete PBG in all directions, which might allow the complete control of light emission and propagation in devices. Here, we report the first demonstration of light propagation in a 3D photonic-crystal waveguide at optical communication wavelengths. A line defect is introduced into a 3D photonic crystal composed of nine stacked layers, having a complete PBG in the 1.55μm wavelength region. Light incident on the waveguide edge successfully propagates along the line-defect waveguide. The propagation characteristics agree with the calculated photonic band diagram of the structure. The calculated results indicate that lossless propagation becomes possible by increasing the number of layers in the device. These results are an important step toward the realization of multifunctional 3D photonic chips integrated within a small region.

A channel drop filter using a single defect in a 2-D photonic crystal slab-defect engineering with respect to polarization mode and ratio of emissions from upper and lower sides

A detailed theoretical analysis of defect engineering in a channel drop filter consisting of a single point defect near a waveguide in a two-dimensional (2-D) photonic crystal (PC) slab is presented. Initially, engineering of the point defect to control the polarization modes of emitted light is examined. By introducing an elliptical defect laterally shifted from the PC lattice, a single linearly polarized light mode can be selected to emit the majority of light, whereas light emitted from the original circular defect is made up of a range of linearly polarized modes. It is also shown that the ratio of light emitted from the top and bottom side of the defect can be improved considerably by introducing a defect with a stepped section along the vertical axis, thereby increasing the net efficiency of the device.