Yoshihiro Akahane

High-Q photonic nanocavity in a two-dimensional photonic crystal

Photonic cavities that strongly confine light are finding applications in many areas of physics and engineering, including coherent electron–photon interactions, ultra-small filters, low-threshold lasers, photonic chips, nonlinear optics and quantum information processing. Critical for these applications is the realization of a cavity with both high quality factor, Q, and small modal volume, V. The ratio Q/V determines the strength of the various cavity interactions, and an ultra-small cavity enables large-scale integration and single-mode operation for a broad range of wavelengths. However, a high-Q cavity of optical wavelength size is difficult to fabricate, as radiation loss increases in inverse proportion to cavity size. With the exception of a few recent theoretical studies, definitive theories and experiments for creating high-Q nanocavities have not been extensively investigated. Here we use a silicon-based two-dimensional photonic-crystal slab to fabricate a nanocavity with Q = 45,000 and V = 7.0 × 10-14 cm3; the value of Q/V is 10–100 times larger than in previous studies. Underlying this development is the realization that light should be confined gently in order to be confined strongly. Integration with other photonic elements is straightforward, and a large free spectral range of 100 nm has been demonstrated.

Fine-tuned high-Q photonic-crystal nanocavity

A photonic nanocavity with a high Q factor of 100,000 and a modal volume V of 0.71 cubic wavelengths, is demonstrated. According to the cavity design rule that we discovered recently, we further improve a point-defect cavity in a two-dimensional (2D) photonic crystal (PC) slab, where the arrangement of six air holes near the cavity edges is fine-tuned. We demonstrate that the measured Q factor for the designed cavity increases by a factor of 20 relative to that for a cavity without displaced air holes, while the calculated modal volume remains almost constant.

In-plane-type channel drop filter in a two-dimensional photonic crystal slab

An in-plane-type channel-drop filtering device with input/output waveguides and a point defect cavity in a two-dimensional photonic crystal slab is investigated. The in-plane operation becomes possible by employing a point defect cavity with an extremely high Q factor to suppress the radiation loss for the out-of-plane direction. 60° bends are also introduced in the output waveguide to avoid interference between the input/output waveguides. The transmission frequency range of the output waveguide with bends is tuned by changing the size of the air holes at the apex of the corner so that the resonant frequency of the point defect cavity is within the transmission band. A channel drop operation with a very high resolution of 0.12 nm is successfully observed at 1.55 μm wavelengths.

Theoretical investigation of a two-dimensional photonic crystal slab with truncated cone air holes

The effects of truncated cone air holes on propagation losses from line defect waveguides in two-dimensional (2D) photonic crystal (PC) slabs are investigated. It is shown that coupling between TE-like waveguide modes and TM-like slab modes due to out-of-plane structural asymmetries can result in large propagation losses. It is also shown that coupling, and therefore propagation loss, does not occur in a frequency range where wave vectors of TE-like waveguide modes do not match projections of those of TM-like slab modes. The results are thought to be applicable to other structures exhibiting out-of-plane asymmetries, such as 2D PC slabs attached to silicon on insulator substrates.

Investigation of high-Q channel drop filters using donor-type defects in two-dimensional photonic crystal slabs

This letter describes experimental investigations of surface-emitting channel drop filters using donor-type point defect cavities and line-defect waveguides in two-dimensional photonic crystal slabs. By using donor-type defect cavities with three and four linearly aligned missing air holes, filter quality factors of around 2600 and 6400, respectively, are achieved experimentally, compared to the quality factor of 400 of previous acceptor-type defect cavities. Radiation patterns and polarization properties of light emitted from the defects are also discussed. The results indicate that these donor-type defects are very useful for the development of ultrasmall high-performance channel add/drop filters.

Ultrahigh-

In this paper, we discuss methods to suppress the radiation loss of ultrasmall cavities, of the size of the optical wavelength, in two-dimensional photonic crystal slabs. An important design concept to suppress radiation loss is introduced: The envelope of the cavity mode field should have no abrupt changes and should ideally follow a Gaussian function. Cubic wavelength order cavities, with experimental Q factors of 100 000 and nearly 1 000 000 are obtained by tailoring the envelope functions using air-hole shifts and multistep heterostructures, respectively. In addition, the experimental Q factors of the latest cavities are shown to be determined by the imperfections in the fabricated structures and not by the cavity design. The differences between the experimental and the theoretical Q factors are investigated in order to demonstrate how higher Q factors could be realized in the future

Q

We report a design of the surface-emitting-type channel drop filters based on point defect cavities and line defect waveguides in two-dimensional photonic crystal slabs, which aim to improve the filtering resolution and light emission characteristics. Since the filters are passive, the mode volume size of the defects needs not be minimized, but the interaction between the defect cavity and the line defect waveguide must be considered. By adopting a donor-type point defect with three missing holes of linear shape, the quality factor of the filter theoretically increases to values as high as 2900 while it reached only 500 in the previously utilized acceptor-type defect. The results suggest that this donor-type defect is very useful for the development of ultrasmall channel add/drop devices.

Nanocavities in Two-Dimensional Photonic Crystal Slabs

We report a theoretical and experimental study of a channel drop filter with two cascaded point-defects between two line-defects in a two-dimensional photonic-crystal slab. Using coupled-mode analysis and a three-dimensional finite-difference time-domain method, we design a filter to engineer the line shape of the drop spectrum. A flat-top and sharp roll-off response is theoretically and experimentally achieved by the designed and fabricated filters. Furthermore, we theoretically demonstrate that drop efficiency is increased dramatically, up to 93%, by introducing hetero-photonic-crystals. We also describe a method to modify the bandwidth of the spectrum.

Design of a channel drop filter by using a donor-type cavity with high-quality factor in a two-dimensional photonic crystal slab

We theoretically and experimentally investigate the characteristics of a multichannel add/drop filter which utilizes in-plane hetero photonic crystals (IP-HPCs). The structure consists of an in-plane array of photonic crystals with different lattice-constants. Finite-difference time-domain calculations reveal that optimal performance in terms of wavelength resolution and efficiency can be kept almost constant at different wavelengths even though slab thickness is not changed. The multichannel add/drop device fabricated consists of seven photonic crystals, with a lattice parameter difference of 1.25 nm between neighboring regions. This device demonstrated wavelength spacing /spl sim/5nm with almost constant Q factor and efficiency. It is also shown that the boundary between different PCs (the heterointerface), plays an important role not only in improving add/drop efficiency but also in performing directional filtering.

Two-dimensional photonic-crystal-slab channel-drop filter with flat-top response

We report transmission and reflection characteristics of in-plane hetero-photonic crystals (IP-HPCs) consisting of two serially connected photonic crystal waveguides with differing lattice constants. We show experimentally and theoretically that the transmission spectrum of the structure corresponds to the transmission frequency range common to both waveguides. Also, there exists a frequency gap where the structure has a guiding mode for one waveguide but not for the neighboring one. Our calculated results reveal that light within the common frequency range is transmitted with approximately 100% efficiency through the waveguides while light within the gap is almost perfectly reflected.