Bong-Shik Song

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.

Analysis of the experimental Q factors (~1 million) of photonic crystal nanocavities

In this letter, we show that the Q factors of the latest high-Q cavities in two dimensional photonic crystals, measured experimentally to be ~1000000, are determined by losses due to imperfections in the fabricated structures, and not by the cavity design. Quantitative analysis shows that the dominant sources of loss include the tilt of air-holes within the cavity, the roughness of the inner walls of the air-holes, variation in the radii of the air-holes, and optical absorption by adsorbed material. We believe that cavities with experimental Q factors of the order of several millions will be obtained in the future by reducing the losses due to imperfections through improved fabrication techniques.

High-Q nanocavity with a 2-ns photon lifetime.

We have succeeded in fabricating a photonic crystal nanocavity with a photon lifetime of 2.1 ns, which corresponds to a quality factor of 2.5 x 10(6). This lifetime is the longest recorded thus far in photonic crystal cavities, and was brought about by improvements in the fabrication process. Comparing our experimental quality factor with the results of calculations shows that we have suppressed variations in the radii and positions of the air holes composing a nanocavity such that their standard deviations are less than 1 nm.

Highly efficient multi-channel drop filter in a two-dimensional hetero photonic crystal.

An in-plane, multi-channel drop filter with high efficiency, in a two-dimensional photonic crystal (PC) slab, is experimentally demonstrated. Based on the concept of heterostructure photonic crystals proposed previously, the device consists of multiple simply connected, PC-based filter units, in which each unit has a structure proportional to an optimized basic unit and operates at a different wavelength. Four-channel drop operation was successfully obtained, with high efficiencies of almost 100%, and equal quality factors, across all channels.

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 investigated the characteristics of an ultra-high-Q photonic nanocavity (Q = ~230,000 and modal volume = ~1.2 cubic wavelengths) at various input light powers. The cavity characteristics were red-shifted as the input power increased. This nonlinearity could be explained by coupled-mode theory, taking into account two-photon absorption, the associated free-carrier absorption, consequent free-carrier absorption, plasma effect, thermo-optic effect, and a Kerr effect. Nonlinear cavity characteristics were observed at an extremely low input light power of 10 muW. We confirmed that these low-power nonlinear optical effects could be attributed to the ultra-high Q factor of the nanocavity.

Nanocavities in Two-Dimensional Photonic Crystal Slabs

Improvements in the channel drop efficiency of an in-plane drop filter in a two-dimensional photonic crystal slab are presented, using a device consisting of two photonic crystal slabs with different lattice constants. It is theoretically shown that drop efficiencies much higher than the maximum of 25% for a conventional configuration are achievable when utilizing reflections at the photonic crystal heterostructure interface. Additionally, the higher drop efficiency is found to be less sensitive to structural fluctuations. Drop operations with efficiencies of more than 80% are experimentally demonstrated by the fabricated devices.