Kenji Ishizaki

Manipulation of photons at the surface of three-dimensional photonic crystals

In three-dimensional (3D) photonic crystals, refractive-index variations with a periodicity comparable to the wavelength of the light passing through the crystal give rise to so-called photonic bandgaps, which are analogous to electronic bandgaps for electrons moving in the periodic electrostatic potential of a material’s crystal structure. Such 3D photonic bandgap crystals are envisioned to become fundamental building blocks for the control and manipulation of photons in optical circuits. So far, such schemes have been pursued by embedding artificial defects and light emitters inside the crystals, making use of 3D bandgap directional effects. Here we show experimentally that photons can be controlled and manipulated even at the ‘surface’ of 3D photonic crystals, where 3D periodicity is terminated, establishing a new and versatile route for photon manipulation. By making use of an evanescent-mode coupling technique, we demonstrate that 3D photonic crystals possess two-dimensional surface states, and we map their band structure. We show that photons can be confined and propagate through these two-dimensional surface states, and we realize their localization at arbitrary surface points by designing artificial surface-defect structures through the formation of a surface-mode gap. Surprisingly, the quality factors of the surface-defect mode are the largest reported for 3D photonic crystal nanocavities (Q up to ∼9,000). In addition to providing a new approach for photon manipulation by photonic crystals, our findings are relevant for the generation and control of plasmon-polaritons in metals and the related surface photon physics. The absorption-free nature of the 3D photonic crystal surface may enable new sensing applications and provide routes for the realization of efficient light–matter interactions.

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.

Light propagation in three-dimensional photonic crystals

We demonstrate the operation of two types of waveguides formed in three-dimensional (3D) photonic crystals (PCs). We first created a vertical waveguide by stacking acceptor-type defects, in which near-infrared light propagates in the stacking direction. Light is transmitted independent of polarization in this waveguide because electromagnetic waves couple to a degenerate mode derived from the structural symmetry of the defects. We then connected horizontal and vertical waveguides to form an L-shaped waveguide, which is able to guide near-infrared light from the horizontal to vertical direction in the 3D PC. We envisage the realization of more complex 3D optical interconnections by optimizing the waveguide structures and increasing the PC period in the vertical direction.

High-Precision Alignment and Bonding System for the Fabrication of 3-D Nanostructures

We successfully developed a high-precision wafer alignment and bonding system for the fabrication of a variety of 3-D nanostructures. To control the wafer positions with high accuracy during the wafer-bonding process, we improved upon a design of the conventional mask-alignment stage. A stress sensor was incorporated to measure the load between the two wafers. In addition, the parallelism of the wafers was monitored by an optical interferometry system. To determine alignment errors in both the and directions simultaneously, we devised an alignment method consisting of crossed vernier scales. We demonstrated that the new alignment and bonding system allowed us to realize precise 3-D photonic crystals with the alignment inaccuracy of < 100 nm at most, and we show that the best experimental error achieved to date was < 25 nm. As this system has the benefit of more readily and intuitively determining the absolute positions of the two wafers, it can be applied to the fabrication of a wide variety of nanoscale multilayer devices.

Three-dimensional coupled-wave analysis for triangular-lattice photonic-crystal surface-emitting lasers with transverse-electric polarization

Three-dimensional coupled-wave theory is extended to model triangular-lattice photonic-crystal surface-emitting lasers with transverse-electric polarization. A generalized coupled-wave equation is derived to describe the sixfold symmetry of the eigenmodes in a triangular lattice. The extended theory includes the effects of both surface radiation and in-plane losses in a finite-size laser structure. Modal properties of interest including the band structure, radiation constant, threshold gain, field intensity profile, and far-field pattern (FFP) are calculated. The calculated band structure and FFP, as well as the predicted lasing mode, agree well with experimental observations. The effect of air-hole size on mode selection is also studied and confirmed by experiment.

Improved efficiency of ultra-thin µc-Si solar cells with photonic-crystal structures

We investigate the improvement of the conversion efficiency of ultra-thin (~500nm-thick) microcrystalline silicon (μc-Si) solar cells incorporating photonic-crystal structures, where light absorption is strongly enhanced by the multiple resonant modes in the photonic crystal. We focus on the quality of the intrinsic μc-Si layer deposited on the substrate, which is structured to form a photonic crystal at its upper surface with a period of several hundred nanometers. We first study the crystalline quality from the viewpoint of the crystalline fraction and show that the efficiency can be improved when the deposition conditions for the μc-Si layer are tuned to give an almost constant crystalline fraction of ~50% across the entire film. We then study the influence of the photonic-crystal structure on the crystalline quality. From transmission-electron microscope images, we show that the collision of μc-Si grains growing at different angles occurs when a photonic-crystal structure with an angular surface is used; this can be suppressed by introducing a rounded surface structure. As a result, we demonstrate an efficiency of 8.7% in a ~500-nm thick, homo-junction μc-Si solar cell, which has only ~1/4 the thickness of typical μc-Si solar cells. We also discuss the possibility of further improving the efficiency by performing calculations that focus on the absorption characteristics of the fabricated cell structure.

Experimental Demonstration of Quasi-resonant Absorption in Silicon Thin Films for Enhanced Solar Light Trapping

We experimentally demonstrate that the addition of partial lattice disorder to a thin-film microcrystalline silicon photonic crystal results in the controlled spectral broadening of its absorption peaks to form quasi resonances: increasing light trapping over a wide bandwidth while also reducing sensitivity to the angle of incident radiation. Accurate finite-difference time-domain simulations are used to design the active-layer photonic crystal so as to maximize the number of its absorption resonances over the broadband interval where microcrystalline silicon is weakly absorbing before lattice disorder augmented with fabrication-induced imperfections is applied to further boost performance. Such a design strategy may find practical use for increasing the efficiency of thin-film silicon photovoltaics.

Mode stability in photonic-crystal surface-emitting lasers with large κ1DL

We study mode stability in photonic-crystal surface-emitting lasers (PCSELs) with large coupling-coefficient-length product κ1DL(>6). We observe that mode competition occurs at high current levels above threshold. Our combined experimental and theoretical study provides the first evidence of the mode competition originating from the high-order band-edge modes. The decreased threshold margin between these competing high-order modes and the main lasing mode with increasing cavity length as well as the spatial hole burning effect may deteriorate the single-mode stability. Our finding is essential for designing single-mode high-power PCSELs for which the strategy to suppress the high-order modes must be considered.

Numerical investigation of emission in finite-sized, three-dimensional photonic crystals with structural fluctuations

We investigate the inhibited emission of light-emitters embedded in finite, three-dimensional woodpile (or stacked-striped) photonic crystals (PCs) that exhibit structural fluctuations. Finite-difference time-domain calculations reveal that the influence of the positional fluctuations of stacked rods of dielectric material depends on the total number of structural periods. For PCs with up to nine stacked layers, the inhibition of emission is principally determined by intruding free-space modes resulting from the finite nature of the structure, with little influence from fluctuations. In PCs with more periods, the modes resulting from fluctuations dominate the emission properties. The influence of these fluctuations is strongly suppressed when the positional shifts are less than +/-20%.

Nanocavities at the surface of three-dimensional photonic crystals

We investigate nanocavities at the surface of three-dimensional (3D) photonic crystals, where the polarization-independent surface-mode gap can be utilized. We consider the formation of various nanocavities by introducing artificial defects utilizing the 3D structures around the surface and discuss the possibilities for increasing the Q-factors of the surface nanocavities with TE-like polarization based on the advanced designs of donor-type defects. We also introduce the design of acceptor-type defects and show that TM-like nanocavities are obtained. We then fabricate the designed nanocavities and examine their resonant characteristics; we successfully demonstrate TE-like nanocavities with Q-factors of ~40,000, which is four-times higher than previous surface cavities and as high as that of the cavities embedded inside 3D photonic crystals. TM-like nanocavities with Q-factors of ~22,000 are also demonstrated for the first time.